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Low-relief shield volcano origin for the South Kaua‘i Swell Ito, Garrett; Garcia, Michael O.; Smith, John R.; Taylor, Brian; Flinders, Ashton; Jicha, Brian; Yamasaki, Seiko; Weis, Dominique; Swinnard, Lisa; Blay, Chuck Jul 29, 2013

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A low-relief shield volcano origin for the SouthKaua?i SwellGarrett ItoDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USA (gito@hawaii.edu)Michael O. GarciaDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAJohn R. SmithHawaii Undersea Research Laboratory, SOEST, University of Hawai?i, Honolulu, Hawai?i, USABrian TaylorDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAAshton FlindersGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USABrian JichaDepartment of Geoscience, University of Wisconsin, Madison, Wisconsin, USASeiko YamasakiTono Geoscience Center, Japan Atomic Energy Agency, Gifu, JapanDominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth Ocean Sciences,University of British Columbia, Vancouver, British Columbia, CanadaLisa SwinnardDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAChuck BlayTEOK, Kaua?i, Hawai?i, USA[1] The South Kaua?i Swell (SKS) is a 110 km x 80 km ovoid bathymetric feature that stands >2 kmhigh and abuts the southern flank of the island of Kaua?i. The origin of the SKS was investigated usingmultibeam bathymetry and acoustic backscatter, gravity data, radiometric ages, and geochemistry of rocksamples. Most of the SKS rock samples are tholeiitic in composition with ages of 3.9?5.4 Ma indicatingthey were derived from shield volcanism. The ages and compositions of the SKS rocks partially overlapwith those of the nearby Ni?ihau, Kaua?i and West Ka?ena volcano complexes. The SKS was originallydescribed as a landslide; however, this interpretation is problematic given the ovoid shape of SKS, itsrelatively smooth, flat-to-convex surface, and the lack of an obvious source region that couldaccommodate what would be one of Earth?s most voluminous (6 x 103 km3) landslides. The morphology,? 2013. American Geophysical Union. All Rights Reserved. 2328ArticleVolume 14, Number 729 July 2013doi: 10.1002/ggge.20159ISSN: 1525-2027size, and the surrounding gravity anomaly are more consistent with the SKS being a low-relief shieldvolcano, which was partially covered with a small volume of landside debris from south Kaua?i and laterwith some secondary volcanic seamounts. A shield origin would imply that Hawaiian and possibly otherhotspot shield volcanoes can take on a wider variety of forms than is commonly thought, ranging fromtall island-building shields, to smaller edifices such as Ka?ena Ridge and Mahukona, to even lower-reliefvolcanoes represented by the SKS and possibly the South West O?ahu Volcanic Field.Components: 13,156 words, 10 figures, 3 tables.Keywords: South Kauai Swell ; submarine landslides; hotspot volcanism; shield volcanism; secondary volcanism.Index Terms: 3037 Oceanic hotspots and intraplate volcanism: Marine Geology and Geophysics; 3070 Submarine land-slides: Marine Geology and Geophysics; 3075 Submarine tectonics and volcanism: Marine Geology and Geophysics; 8137Hotspots, large igneous provinces, and flood basalt volcanism: Tectonophysics;8415 Intra-plate processes: Volcanology;1033 Intra-plate processes: Geochemistry; 3615 Intra-plate processes: Mineralogy and Petrology.Received 30 March 2013; Revised 24 April 2013; Accepted 25 April 2013; Published 29 July 2013.Ito, G., M. Garcia, J. Smith, B. Taylor, A. Flinders, B. Jicha, S. Yamasaki, D. Weis, L. Swinnard, and C. Blay (2013), A low-relief shield volcano origin for the South Kaua?i Swell, Geochem. Geophys. Geosyst., 14, 2328?2348, doi:10.1002/ggge.20159.1. Introduction[2] The Hawaiian-Emperor Chain is one of the mostextensively studied in the world and has greatlyinfluenced our knowledge of how oceanic islandsform and evolve (Figure 1). The Hawaiian islandsare perched on broader subsided platforms (pinkareas in Figure 1) from which emanate elongateridges that are, or once were volcanic rift zones [For-nari, 1987]. The platform flanks are mantled withthe products of mass wasting, which range in sizeand character from localized turbidite channels tomassive landslides [Moore et al., 1989; Smith et al.,2002; Morgan et al., 2007]. Some of the latter formsare the remnants of huge island sector collapses rep-resenting the most dramatic landslides on Earth[Moore et al., 1989, 1994; Hampton and Lee, 1996].[3] Other submarine features along the Hawaiian-Emperor Chain include products of secondary vol-canism, forming extensive lava fields on the flex-ural arches north of O?ahu (200 km wide) andsouth of Hawai?i (50 km wide) [Lipman et al.,1989; Clague et al., 1990], as well as on the north-east flank of Ni?ihau (100 km wide) [Clagueet al., 2000; Dixon et al., 2008] and around Ka?ulaIsland [Garcia et al., 2008]. Another lava field sitson a seafloor bulge southwest of O?ahu (100 kmwide) [Moore et al., 1989]. One interpretation ofthe SW O?ahu Volcanic Field (SWOVF) is that itis a veneer of secondary volcanism overlying thebulging deposits of a landslide from O?ahu[Coombs et al., 2004]. Alternatively, the lava andseafloor bulge together, could be part of a verylow-relief shield volcano [Takahashi et al., 2001;Noguchi and Nakagawa, 2003]. The causes of theabove forms of volcanism far from the hotspotcenter are poorly known.[4] Another class of submarine construct that ispoorly understood is represented by the large edificesprotruding northwest from the Big Island of Hawai?i(Mahukona) and northwest from O?ahu (Ka?enaRidge). Mahukona is interpreted as a separate low-relief shield volcano as evidenced by geophysical,geochronological, and geochemical data [Garciaet al., 1990; Clague and Moore, 1991; Garcia et al.,2012]. Ka?ena Ridge, however, is not as well studied.It has been proposed as being the submerged exten-sion of the Wai?anae Rift Zone or an entirely sepa-rate, volcanic system [Smith, 2002; Coombs et al.,2004], and has been compared to the large submarineHana Ridge east of Maui. Thus, Hawaiian construc-tional volcanism is manifested in a variety of formsfrom small, but extensive lava fields, to more volu-minous ridges, to massive shield volcanoes.[5] A prominent feature that is particularly enig-matic is the large bathymetric swell south of theIsland of Kaua?i (Figures 1 and 2). The SouthKaua?i Swell (SKS) was originally described as asubmarine landslide based on the U.S. GeologicalSurvey?s, GLORIA side-scan surveys, whichlacked swath bathymetry data [Moore et al.,ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.2015923291994; Groome et al., 1997; Holcomb and Robin-son, 2004]. Subsequent reconnaissance bathymet-ric mapping showed that the gross morphology isquite unlike other large Hawaiian landslides. Forexample, many of the features that were consid-ered as debris blocks are actually cone-shapedseamounts, some with acoustically reflective lavaflows (Figure 2). These morphological featuresled us to consider other hypotheses for the originof the SKS: (1) widespread secondary volcanismsuch as the vast lava field northwest of Ni?ihau,and (2) a low-relief shield volcano, intermediate inrelief between Mahukona and the SW O?ahu Vol-canic Field. To test these and the landslide hypoth-eses, we undertook a marine expedition of the SKSin 2007 on the R/V Kilo Moana, operated by theUniversity of Hawaii, during which we performedgeophysical surveys and sampled rocks usingWoods Hole Oceanographic Institution?s JASONROV. The results from this expedition are com-bined with those from three dredge hauls madefrom the R/V Kilo Moana in 2005, and samplestaken during a JASON test dive in 2006. Here wereport on the findings and interpretations of themultibeam bathymetry and acoustic imagery, thegravity surveys combined with an extensive localgravity dataset, and the lava geochemistry and geo-chronology analyses.2. Bathymetric Data and AcousticImagery[6] The South Kaua?i Swell (SKS) is a SSE-trending, ovoid feature (110 km x 80 km) thatspans an area of 6.7 x 103 km2 adjacent to thesouth flank of Kaua?i (Figures 1?3). It stands a max-imum height of 2?2.5 km above the deep abyssalseafloor and merges with the steep-sided southernmargin of Kaua?i at a depth of 2 km. The overallsurface of SKS is slightly convex, having minimalslope near a depth of 2.5 km and sloping slightlysteeper at greater depths toward the surroundingabyssal seafloor (Figures 3 and 4). The swell has acentral axis extending SSE, flanked by slopes dip-ping to the SSW and E. A few low-relief (10?30 kmwide) lobes, or spur-like features, splay SSW andFigure 1. Multibeam bathymetry map of Hawaiian Islands, illuminated from the NW (grid available athttp://www. soest.hawaii.edu/HMRG/cms). Areas discussed in the text are outlined (black dashed). Whitelines mark locations of profiles labeled as shown in Figure 4.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592330Figure 2. ContinuedITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592331SE away from the central axis, producing a subtle,large-scale pinnate structure. Numerous seamounts(>400 with height> 60 m and ellipticity< 3, me-dian and standard deviation of the major axis lengthis 6726 562 m) are present; a few are circular andsmooth-topped (e.g., at 15935?W, 2144N),although most are more irregular in shape.[7] The acoustic backscatter of SKS is relativelylow in amplitude over large areas (light gray towhite; Figure 2b), which indicates that much ofthe SKS is relatively thickly sedimented as wasconfirmed by inspection during JASON dives.Higher acoustic backscatter is found along alluvialchannels extending south from Kaua?i and onmany of the seamounts (Figure 2b). Three stronglyreflective areas just east of SKS (Figure 2b) wereconfirmed to be lava flows [Greene et al., 2010].Video from the JASON dives confirmed that themore reflective escarpments and seamounts, whereFigure 2. (a) Multibeam bathymetry data around Kaua?i and Ni?ihau from our survey (KM0718) and from the Hawai?i Multi-beam Synthesis (http://www.soest.Hawai?i.edu/HMRG/cms), with 111 m grid spacing, colored and illuminated from the NW.Contours are every 0.5 km from depths of 4.5?2 km. JASON dive numbers are labeled; sample locations are marked with redcircles; dredge numbers (KS1?3) are labeled with locations marked by triangles. SKS is outlined with dashed line and its centralaxis is marked by large arrows. Gravity anomaly highs ( ?80 mGal) overly the inferred magmatic centers of the two shield vol-canoes are marked by translucent yellow patches [Flinders et al., 2010]. White symbols mark locations of identified slope breaks,interpreted as the paleo-shorelines of Kaua?i (triangles) and Ni?ihau (circles) [Flinders et al., 2010]. Dotted lines indicate the sec-tions where the slope break continues between the white symbols. Small black arrow marks where a slope-break of Kaua?i?s ter-race lies above a slope-break of Ni?ihau?s terrace. (b) Mosaic of acoustic backscatter from KM0718 and the Hawai?i MultibeamSynthesis. Dark is high return, light is low return. Contours are the same as in Figure 2a. Small red arrows point to acousticallyreflective areas confirmed to be young (< 0.4 Ma) alkalic lava flows as reported in Greene et al. [2010].Figure 3. Perspective views of South Kaua?i Swell bathymetry (color scale in km) from (top) azimuth of210, illuminated from 300, and (bottom) azimuth of 110, illuminated from 30. Color bar indicates depthin km. Vertical exaggeration is about 5:1.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592332we focused our sampling efforts, are generallymore sparsely sedimented. The areas sampled con-tain loose to partially lithified sediments, fracturedbasement rock, talus, breccia, and few insitu pillowflows. The samples recovered from the pillowbasalts were found to be young (<2 Ma) andalkalic, not tholeiitic (discussed below). The overalllow and sporadic backscatter of SKS clearly con-trast with the more uniformly high backscattersurrounding Ni?ihau and Ka?ula (small islandWSW of Ni?ihau), which denotes extensive areasof flows and volcanic cones produced by secondaryvolcanism [Dixon et al., 2008; Garcia et al., 2008].3. Gravity Data[8] Hawaiian volcanoes typically have gravityhighs over their magmatic centers [e.g., Krivoyand Easton, 1961; Kinoshita et al., 1963]; thus,gravity may be used to test the shield volcano hy-pothesis for the SKS. A regional compilation ofgravity data [Flinders et al., in press] was madefor the northern Hawaiian Islands from our andother R/V Kilo Moana cruises, the National Geo-physical Data Center (www.ngdc.noaa.gov), aswell as onshore gravity [Flinders et al., 2010].Details of the gravity data processing and reduc-tion are given in Flinders et al. [2010]; only theessential points are summarized here. Free-airgravity data from individual ship survey lineswere hand-edited for noisy sections (typicallyassociated with course and speed changes), brokeninto straight line segments, and then corrected fordiscrepancies between line crossings [Prince andForsyth, 1984]. The standard deviation of thereduced crossing errors was 2 mGal over theFigure 4. Profiles of various offshore features along the lines shown and labeled in Figure 1. Vertical exag-geration is 5:1. Labeled horizontal axes bound each profile triplet (for SKS) or pair of the labeled features.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592333area of free-air gravity shown in Figure 5. As iscommon, the free-air gravity anomaly shows astrong correlation with topography.[9] We produced a complete Bouguer anomaly(CBA) by subtracting from the free-air gravityanomaly the gravitational attraction of the submar-ine and subaerial topography (Figure 5b). The den-sity of water, the crust above, and the crust belowsea level were taken to be 1000, 2400, and 2700kg/m3, respectively [Flinders et al., 2010]. Varia-tions in the CBA reflect density structure that dif-fers from the above reference densities. Forexample, short wavelength (50 km wide) highsprobably mark dense cumulate-rich crust at thecenters of the Ni?ihau, Kaua?i [Flinders et al.,2010], Wai?anae, and Ko?olau shields (Figure 5b).From the southwest to northeast across the islandchain, the CBA is relatively high (> 300 mGal),decreases near the center of the chain (170 mGaljust west of Maui), and then increases again to thenortheast. This long wavelength variation iscaused by the crust-mantle interface taking theshape of an elastic lithospheric plate, which hasbeen flexed downward by the weight of the vol-cano chain [Watts and ten Brink, 1989].[10] To isolate the shorter-wavelength signals dueto local density variations beneath the SKS and theother volcanic features and landslides, weremoved wavelengths >150 km from the completeBouguer anomaly (using the Gaussian tapered fil-ter of ??grdfilter?? in the GMT software package[Wessel and Smith, 1995]). This filtering producesthe residual gravity anomaly (Figure 5c). Over thecentral portion of SKS, the residual anomaly flat-tens to neutral or low-amplitude positive values(5 mGal, yellow to orange in Figure 5c), similarto that over the SW O?ahu Volcanic Field(SWOVF). This low-amplitude high is interruptedto the south by a linear residual gravity low (5 to10 mGal, light green) that trends ENE toward theisland of Moloka?i and overlies the northern branchof the Moloka?i Fracture Zone (the southern branchis evident as a similar linear low projecting towardMaui). The slightly high-residual gravity over theSKS is bounded to the north by an area of verylow-residual gravity. This low-gravity anomalyresembles the low gravity on the other flanks sur-rounding most of Kaua?i and is most negative onKaua?i south shelf area (20 mGal). This promi-nent low clearly separates the strong positive grav-ity signature on the Island of Kaua?i from theneutral-to-small positive gravity over the SKS.[11] The gravity signatures over the SKS andSWOVF contrast with the larger-amplitude gravityhighs over the Wai?anae rift zone (10?40 mGal)and the southern part of Mahukona (10?20 mGal)[Garcia et al., 2012], as well as the negative resid-ual gravity low over the Wai?anae Slump (westflank of O?ahu). The Nu?uanu (NE of O?ahu) andFigure 5. (a) Free-air gravity anomaly is colored along shiptracks. (b) Complete Bouguer anomaly is free-air anomalyminus the effects of submarine and subareal topography. (c)The residual gravity anomaly is the complete Bouguer anom-aly with wavelengths >150 km removed. Depth contours areevery 1 km and islands are outlined in black. Relevant fea-tures are outlined with bold dashed lines.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592334Wailau (N of Moloka?i) landslide deposits have nosignificant residual gravity anomalies relative tothe areas immediately adjacent to them. Thus,although the gravity signatures of the SKS andSWOVF can distinguish them from the Wai?anaeSlump, their gravity anomalies alone are not dis-tinctive with respect to the Nu?uanu and Wailaulandslide deposits.4. Lava Samples[12] Samples from the South Kaua?i Swell were col-lected during four JASON dives and three dredge-hauls (Figure 1) to determine rock type,vesicularity, and chemical and isotopic composi-tions. The dives traversed many kilometers of sea-floor and thus enabled numerous samples frommultiple seamounts or morphologically distinctlocations (see Appendix for detailed maps of divesand sample locations; supporting information).1 Atotal of 110 samples were obtained from 19 sea-mounts at depths 2970?4170 m below sea level.Most of the 74 lava samples are breccias, consistingof lithologically identical (monomict), angularclasts. The other SKS samples include mudstones,sandstones, pebbly conglomerates with rounded,matrix-supported clasts of diverse lithologies (poly-mict) suggesting the clasts have been transported.For comparison, monomict as well as polymict vol-canoclastic rocks were found interbedded in the sub-marine portion of Mauna Kea?s deep drill core(3100 m) (HSDP2; [Garcia et al., 2007]). TheHSDP2 monomict fragmental deposits were inter-preted to be of local origin, formed as lavas eruptedon steep submarine slopes, whereas the polymictbreccias were thought to have formed by mass wast-ing on the unstable slopes of the active volcano[Garcia et al., 2007]. Mn coatings on the SKS sam-ples vary from <1 to 14 mm in thickness. Glass wasfound on only a few samples and was mostlyaltered. The interiors of the SKS rocks show varyingdegrees of alteration from unaltered with pristine ol-ivine, to moderate levels of alteration with partialreplacement of olivine by iddingsite and, in rarecases, clay and or zeolite in the vesicles.[13] Vesicularity shows no systematic variationwith sample location or rock composition but dis-plays a somewhat bimodal distribution with mostsamples having <5 vol. % or >20 vol. % vesicles.By comparison, subaerial HSDP2 lavas have meanvesicularities of 9, 11 and 18 vol.% for MaunaLoa, Mauna Kea and Kilauea volcanoes respec-tively, and a larger total variation (<1 to >30vol.%) for each volcano. The HSDP2 submarinelavas tend to have lower vesicularity, averaging<3 vol.%, but ranging widely (0.1 to 19 vol.%).Offshore Hawai?i, highly vesicular submarinelavas are found near submarine vents (e.g., up to33 vol.% in a tholeiitic lava from Loihi seamount)and tend to decrease with eruption depth [Moore,1965; Garcia et al., 1995]. Thus, the highly vesic-ular SKS lavas (>20 vol.%) probably erupted inshallow water depths (< 1000 m) and some per-haps subaerially. Because all of the SKS sampleswere extracted from water depths of >2500 m, thehighly vesicular lavas were probably transportedfrom shallower depths. These lavas were collectedfrom the western side (dive 297, water depths3330?3770 m) and central part (dives 252 and 299water depths 3260?3322 m) of SKS, but not fromthe site (298) furthest southeast from Kaua?i. Thissoutheastern-most dive (298) recovered rocks withvesicularities all <0.1 %. Thus, the highly vesicu-lar lavas from SKS could be landslide debris.5. Petrology and Geochemistry[14] The primary objective of the petrological andgeochemical components of the investigation wasto identify the type of lava (tholeiitic versusalkalic) and to determine whether the SKS lavasare compositionally similar to or distinct fromKaua?i lavas. Fifty SKS samples were analyzed byXRF for whole-rock major and trace element com-positions. Nineteen of these samples were also an-alyzed by ICP-MS for trace elements, and byTIMS and MC-ICP-MS for Pb, Sr, Nd and Hf iso-topes. Only an overview of the geochemicalresults is presented here to provide a basic charac-terization and address the origin of the swell ; thegeochemistry data are reported in Swinnard[2008], and the detailed examination of these datawill be presented in a separate study [Garciaet al., in preparation].[15] The SKS rocks range in composition fromalkalic basalt and basanite to tholeiitic basalts andpicrites (Figure 6). Only seven alkali rocks werecollected, and they were from the western flank ofthe SKS (dive 297 and dredge KS2, Figure 2) inareas of locally high-acoustic backscatter near andon relatively smooth-topped and broad seamounts.Those from dive 297 came from an outcrop of pil-low basalts. These rocks are similar in major and1Additional supporting information may be found in the onlineversion of this article.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592335trace element composition to the rejuvenated lavason Kauai?i [Garcia et al., 2010](Figure 6).[16] Tholeiites are the most abundant rock typesampled and range widely in composition (Figure6, e.g., 6?25 wt.% MgO, 0.16?0.62 K2O wt.%,and 0.73?0.91 CaO/Al2O3) in the least alteredsamples [Swinnard, 2008]. The rocks show widecompositional variations at the same MgO content,which cannot be attributed to olivine addition oralteration; they must reflect distinct parental mag-mas. Some seamounts show a small range in com-position (e.g., Zr/Nb, which is resistant toalteration, ranges 11.5?12.5 among six samplesfrom one seamount), whereas others have a largerrange (e.g., Zr/Nb ranges 10.1?13.3 among threesamples from another seamount). Compositiondoes not appear to vary systematically with posi-tion within individual dives sites or among theseven sample locations. The SKS tholeiitic com-positions generally overlap with those for Kaua?ishield lavas, although some have higherNa2O?K2O contents and Zr/Nb ratios (Figure 6),as well as somewhat lower CaO/Al2O3 at a givenMgO (not shown) content than Kaua?i lavas, sug-gesting slightly distinct sources.[17] The isotopic data for SKS lavas form twogroups that correspond with the rock types: thealkalic lavas have distinctly lower 87Sr/86Sr thanthe tholeiites and plot close to the compositionalfield of the Koloa (rejuvenated) lavas on Kaua?i(Figure 7). The tholeiites show overlap in87Sr/86Sr and 206Pb/204Pb with tholeiitic lavasfrom Kaua?i and Wai?anae, as well as some fromWest Ka?ena.6. Geochronology[18] A two-pronged approach was used in datingthe SKS lavas. Initially, 12 samples spanning awide range in rock composition were analyzed bythe unspiked K-Ar method at Kyoto University(see Table 1 and Appendix for details on methodsused; supporting information). Two samples wereanalyzed also at Japan Atomic Energy Agency(JAEA) by similar method. These K-Ar methodshave proven useful for dating young Hawaiianbasalts [e.g., Ozawa et al., 2005; Garcia et al.,2012]. In addition, sixteen tholeiitic samples wereanalyzed by 40Ar/39Ar methods at the Universityof Wisconsin-Madison (Tables 2, A1, and Appen-dix; supporting information). The plateau ages ofthese samples represent a high percentage of the39Ar (all greater than 93% and 100% for 14 of 15analyses), have low MSWD values (all <1.3 andmost <0.75), and are used here as the preferredages.[19] Among the three samples analyzed with bothK-Ar and Ar-Ar methods, two yielded remarkablyconsistent results: sample 297-09 gave ages4.246 0.46 Ma by K-Ar vs. 4.146 0.14 Ma and4.226 0.12 Ma by Ar-Ar (all errors are 2) ; sam-ple 299-29 gave ages of 4.036 0.12 Ma by K-Arand 4.026 0.13 Ma by Ar-Ar (Tables 1 and 2).For K-Ar dating of sample 299-29, the weightedmean uncorrected age is used because 38Ar/36Arratio was not well determined and no technicalcause was identified for this problem. In anotherstudy on rocks collected from seamounts east ofKaua?i [Greene et al., 2010], 40Ar/39Ar andunspiked K-Ar ages produced consistent resultsfor young alkalic lavas (0.37 vs. 0.24 Ma) as wellas for older tholeiitic lavas (3.6?4.9 vs. 4.3?4.7Ma). Thus, we think both geochronology methodsprovide useful constraints for the eruption ages ofthe least altered SKS lavas.Figure 6. Major element compositions of SKS samplescompared with compositions of the shield and rejuvenated(Koloa) stages on Kaua?i and tholeiitic lavas from westKa?ena [Garcia et al., 2010; Greene et al., 2010]. Red sym-bols denote tholeiites; blue denotes alkalic lavas.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592336[20] The 21 new ages for tholeiitic lavas rangefrom 3.9 to 5.4 Ma (Figure 8). We see no clear ge-ographic pattern to the distribution of ages (seeAppendix for detailed maps of sample locationsand ages; supporting information). For examplefrom Dive 252, we obtained two samples from ad-jacent seamounts separated by 2 km that yieldedidentical ages (both 4.29 Ma), and from dive 297,two samples from adjacent seamounts yielded dis-tinct but similar ages (4.03?4.34 Ma). Whereasfrom the two other dives, samples from the sameseamount yielded appreciably different ages (4.00and 4.29 Ma from Dive 298, and 4.02, 4.44, 5.11Ma from Dive 299).[21] The frequency distributions of the ages of thetholeiites show a large cluster (16 samples) withages 3.9?4.5 Ma, and a smaller cluster (3 samples)spans ages of 4.75?5.4 Ma (Figure 8). The oldestages (>5 Ma) were obtained for samples fromdives 297 and 299 on the central and western partof SKS (Figure 2). These areas are south anddownslope of the gap in Kaua?i?s ancient shoreline(Figure 2), and thus could well contain debrisfrom Kaua?i. The oldest rock (5.40 Ma, dive 297,sample 26) was sampled from a loosely stratified,alluvial deposit near the end of a submarine chan-nel that originated at the outlet of the WaimeaRiver. This sample is therefore our most-likelycandidate for being from Kaua?i. The SKS tholei-ite ages overlap with the ages for the Kaua?i shield(3.6?5.1 Ma [McDougall, 1979; Garcia et al.,2010]), Ni?ihau (4.3?6.3Ma) and W. Ka?ena (2.9?4.9 Ma), but extend to younger ages than Ni?ihauand older ages than W. Ka?ena.[22] The SKS alkalic lavas are much younger(0.08?1.9 Ma, Figure 8), indicating a 1.8 Myr hi-atus in volcanism after the tholeiites. This age gapand the alkalic compositions, together, are consist-ent with these rocks representing a secondary vol-canic phase.7. South Kaua?i Swell Volume[23] Size is a critical characteristic of the SouthKaua?i Swell. The volume of SKS was computedbased on the border of the swell shown in Figure 2using two methods. Method 1 is most appropriatefor landslides and method 2 for shield volcanoes.We also estimated the volumes of large Hawaiianlandslides and near-by shield volcanoes (bordersshown in Figure 1) for comparison.[24] Method 1 is appropriate for landslides thatformed on top of sediments infilling the flexuralmoat [Watts and ten Brink, 1989] surrounding theoriginal shield volcanoes. We estimated the vol-ume between the seafloor and a flat base at a depthof 4.6 km, which corresponds to the depth of theabyssal seafloor just outside of the flexural moatsof Kaua?i. Method 2 is more appropriate for ashield volcano that grew directly on top of the pre-existing seafloor. Following Robinson and Eakins[2006], we approximated the surface of the pre-existing seafloor as that of an elastic lithosphericplate with an effective thickness of 35 km beingflexed downward by the weight of the island chain.We used a submarine crustal density of 2700 kg/m3 and a subaerial crustal density of 2400 kg/m3(same as those used for computing the completeBouguer anomaly). This model predicts a flexedsurface that closely matches the shape of the base-ment of the pre-existing oceanic crust on the flanksof the island of O?ahu as was modeled and seismi-cally imaged by Watts and ten Brink [1989] andused by Robinson and Eakins [2006]. Then, weremoved 0.5 km [Robinson and Eakins, 2006]above the modeled pre-existing basement toaccount for the pre-existing pelagic sediments thatare not part of the volcanoes.[25] To verify our methods, we compared our vol-ume estimates of other edifices in the area withFigure 7. Isotope compositions of lavas from SKS (trian-gles) compared to of tholeittic basalts from near-by volcanicprovinces. Data sources: SKS [Swinnard, 2008]; West ofKa?ena [Greene et al., 2010]; Wai?anae [Coombs et al.,2004]; Kaua?i [Garcia et al., 2010]. The portion of Koloafield that lies within the plot is outlined (black); the wholefield spans 18.0895-18.6136 and 0.702959-0.703248 in206Pb/204Pb and 87Sr/86Sr, respectively [Garcia et al., 2010].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592337Table1.K-ArDatingResultsSamplenameSamplesiteRocktypeAlterationaK2O/P 2O5LOIK2O(wt.%)LabSamplewt.(g)40Ar/36Ar38Ar/36Ar40Ar/36ArInit.b40Arrad.(108cm3STP/g)40Arair(%)Uncorrectedagec(Ma)62Correctedage(Ma)62297-9coneBtholeiiteB21.880.160.50Kyoto0.703359.761.00.186160.0022293.067.06.960.881.44.0860.164.2460.46297-18coneCtholeiiteB21.520.070.32Kyoto0.615332.160.80.185660.0017291.565.74.660.787.83.9160.164.3460.64297-20coneDbasaniteA22.710.610.74Kyoto0.606304.963.50.189360.0106302.7634.80.162.099.20.2360.080.0560.84JAEA0.597300.660.70.186260.0011293.463.70.560.397.60.1660.020.2360.12weightedmeancorrectedagefromtwoexperiments:0.2360.12297-21coneDbasaniteA22.430.600.73Kyoto0.602299.860.80.186660.0016294.565.30.660.698.20.2060.040.2560.25Kyoto0.627298.960.70.187360.0017296.865.70.360.899.30.2160.040.1360.36Kyoto0.705298.860.70.187160.0017296.065.40.460.899.10.1960.040.1660.32weightedmeancorrectedagefromthreeexperiments:0.1960.18297-24coneDbasaniteA21.411.200.50Kyoto0.614297.661.00.187860.0030298.269.80.1262.4100.20.3260.140.08615.4JAEA0.128297.760.80.187460.0016297.065.40.161.099.80.2760.100.0860.70297-26AreaEtholeiiteB21.590.120.63Kyoto0.202332.361.00.186860.0022295.267.411.162.288.85.3460.225.4061.10299-29SiteCtholeiiteB21.790.170.50Kyoto0.550334.261.8295.55.960.388.43.6560.20Kyoto0.455333.060.6295.56.960.286.74.2860.16weightedmeanuncorrectedagefromtwoexperiments:4.0360.12KS-2dredgebasaniteA13.551.631.67Kyoto0.731384.561.60.186960.0016295.665.010.160.676.91.8660.061.8660.12KS-3-1dredgetholeiiteB11.810.500.48Kyoto0.740410.161.60.188760.0018301.065.66.160.473.44.0560.123.8660.24A1A2B1B2FreshnessofolivinephenocrystfreshfreshminordminordSecondarymineralsinvesiclesnopresentnopresentFreshnessofgroundmassolivinefreshfreshfreshfreshPreferredageinbolda Alterationcriteriabythinsectionobservation.bInitial40Ar/36Arcalculatedfrom38Ar/36Arassumingmassfractionation.c Massfractionationuncorrected.dMinoroxidationatthemargin(<10%ofthediameter).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592338previously published estimates. Method 1 pro-duces a volume for the Wailau debris avalanche of1.1 x 103 km3 (Table 3), which is consistent withestimates by Satake et al. [2002] (1.56 0.5 x 103km3), Robinson and Eakins [2006] (1.6 x 103km3), and Moore et al. [1989] (1 x 103 km3). Forthe Nu?uanu debris avalanche, our estimate of 2.7x 103 km3 compares favorably with estimates bySatake et al. [2002] (3.56 0.5 x 103 km3) andRobinson and Eakins [2006] (2.4 x 103 km3), butis less than that by Moore et al. [1989] (5 x 103km3). An important difference is that Moore et al.[1989] attributed a 0.9 km thick layer within thesediments that infill O?ahu?s flexural moat as partof the Nu?uanu debris avalanche (based on seismicevidence [Watts et al., 1985; Brocher and tenBrink, 1987]), whereas Satake et al. [2002] and wedo not. For the Mahukona, Ko?olau, Wai?anae,and Kaua?i shield volcanoes, method 2 producesvolumes that match those of Robinson andEakins?s [2006] (Table 3) within 8%. The aboveconsistencies lend confidence to our methods ofestimating volumes of landslides (method 1) andshield volcanoes (method 2).[26] For the SKS, method 1 yields a volume of 6 x103 km3. This volume is greater than our estimatesTable 2. Summary of 40Ar/39Ar Dating ResultsaSample no. Location K/Ca totalTotal fusion IsochronNPlateauAge (Ma)6 2 40Ar/36Ari6 2 MSWD Age (Ma)6 239Ar % MSWD Age (Ma)6 2252-04 Cone A 0.13 4.286 0.14 296.46 2.1 0.65 4.236 0.14 8 of 8 100.0 0.59 4.296 0.12252-10 Cone B 0.18 4.336 0.16 297.66 2.9 0.64 4.246 0.10 8 of 8 100.0 0.62 4.296 0.12297-04 Cone B 0.18 4.056 0.24 297.06 4.0 0.13 3.786 0.66 8 of 8 100.0 0.14 4.036 0.22297-09 Cone B 0.15 4.066 0.18 287.06 6.5 0.85 4.596 0.34 8 of 8 100.0 1.00 4.146 0.140.18 4.126 0.14 292.36 4.4 0.42 4.406 0.25 7 of 8 97.2 0.43 4.226 0.12Weighted mean plateau age from two experiments: 4.196 0.09298-12 Cone D 0.20 4.016 0.15 297.16 3.7 0.83 3.796 0.46 8 of 8 100.0 0.74 4.006 0.13298-20 Cone D 0.13 4.606 0.18 295.66 1.3 1.28 4.746 0.10 8 of 9 96.7 1.10 4.756 0.110.11 4.836 0.16 296.76 0.9 0.94 4.756 0.09 10 of 10 100.0 1.04 4.846 0.13Weighted mean plateau age from two experiments: 4.796 0.08299-04 Site A 0.08 4.536 0.32 295.06 0.9 0.61 4.666 0.17 10 of 10 100.0 0.58 4.606 0.260.08 4.396 0.30 295.76 1.0 1.42 4.446 0.16 9 of 9 100.0 1.25 4.466 0.22Weighted mean plateau age from two experiments: 4.516 0.16299-08 Site A 0.07 4.346 0.35 295.16 2.9 0.95 4.496 0.50 10 of 10 100.0 0.85 4.426 0.280.13 4.376 0.29 296.76 1.9 0.72 4.216 0.35 9 of 9 100.0 0.69 4.426 0.25Weighted mean plateau age from two experiments: 4.426 0.18299-15 Site A 0.10 4.116 0.20 294.16 5.8 1.29 4.286 0.43 10 of 10 100.0 1.16 4.176 0.16299-20 Site B 0.13 5.266 0.15 296.26 2.0 0.35 5.086 0.11 6 of 8 93.7 0.30 5.126 0.120.18 5.196 0.17 295.96 1.8 0.16 5.096 0.12 6 of 8 96.8 0.14 5.116 0.14Weighted mean plateau age from two experiments: 5.116 0.09299-21 Site B 0.09 4.486 0.17 296.36 0.4 0.79 4.406 0.07 10 of 10 100.0 1.04 4.446 0.13299-23 Site B 0.13 4.026 0.16 296.36 3.3 0.28 3.996 0.13 8 of 8 100.0 0.25 4.026 0.12299-29 Site C 0.17 4.036 0.15 295.06 2.3 0.22 4.086 0.27 10 of 10 100.0 0.20 4.026 0.13299-33 Site D 0.11 5.106 0.15 294.86 1.0 0.92 5.136 0.10 8 of 8 100.0 0.87 5.076 0.13KS-01?18 Site A 0.09 4.156 0.26 295.96 2.0 0.50 4.166 0.22 8 of 8 100.0 0.44 4.206 0.19aAges calculated relative to 28.201 Ma for the Fish Canyon sanidine [Kuiper et al., 2008] using decay constant of Min et al. [2000]. Uncertain-ties reflect 2 analytical uncertainties. Preferred age in bold.Figure 8. Histograms of age dates obtained from SKS andthe adjacent edifices (ages in 0.25 Myr bins on the vertical axisand number of samples on the horizontal), superimposed withprecise ages (symbols, horizontal position is arbitrary) and 2errors. Black dots mark K-Ar dates for the SKS samples. Eachhistogram is positioned horizontally according to distancesbetween the centers of each feature and the summit of Kilauea,projected along a trajectory of the current Pacific Plate motion[Gripp and Gordon, 2001]. Dates of Kaua?i?s post-shield arecircled. Data sources: Kaua?i [McDougall, 1964, 1979; Clagueand Dalrymple, 1988; Garcia et al., 2010]; W. Ka?ena[Greene et al., 2010]; Ni?ihau [Sherrod et al., 2007].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592339for Nu?uanu and Wailau landslides combined (3.8x 103 km3), and is comparable to our estimate forthe Wai?anae Slump (5.8 x 103 km3, Table 3).Method 2 yields a volume for SKS of 146 3.4 x103 km3. This volume is comparable those ofKa?ena Ridge (14.86 2.0 x 103 km3) and the smallshield volcano, Mahukona (13.06 1.0 x 103 km3,Table 3). The two estimates for SKS are 14%(method 1 for landslides) and 32% (method 2 forshields) of the total volume of Kaua?i without SKS(43.56 7.9 x 103km3, area shown in Figure 1).8. Origin of South Kaua?i Swell[27] Secondary volcanism, landslides, and shieldvolcanism are the three main processes that createthe submarine features of the Hawaiian chain. Allthree processes have also played a role in the evo-lution of the SKS. Each is discussed and evaluatedin terms of its relative importance in the formationof the SKS beginning with the most recent process.8.1. Secondary Volcanism[28] Lavas from secondary volcanism contrastwith shield lavas in generally being more silicaundersaturated, containing higher abundances ofincompatible elements, originating from moredepleted sources [e.g., Fekiacova et al., 2007] andhaving younger ages (by 0.6?2.0 Myr) than theassociated shield lavas [Ozawa et al., 2005]. Onland, secondary volcanism is often referred to as??rejuvenated volcanism?? because it overliesshield volcanism; it is usually separated from theshield or postshield phase by a thick soil and/orsedimentary sequence [e.g., Macdonald et al.,1983]. In the submarine environment, secondaryvolcanism is generally associated with high-acoustic backscatter, indicating relatively thinsediments on these younger lavas [Lipman et al.,1989; Clague et al., 1990; Dixon et al., 2008;Greene et al., 2010].[29] The South Kaua?i Swell displays characteris-tics that differ from those of secondary volcanism.The relatively thick sediments overlying the vol-canic rocks as detected by the low-acoustic back-scatter and seen in JASON imagery contrast withthe observations of known submarine secondaryvolcanism. In addition, samples collected byJASON from the SKS have variable but com-monly thick Mn rinds (up to 14 mm) indicatingresidence on the ocean floor for several millionyears [e.g., Moore and Clague, 2004]. The excep-tions are the relatively rare alkalic SKS lavas (col-lected at two sites), which have thin or no Mnoxide coatings. These alkalic lavas have geochem-ical (Figures 6 and 7) and age (0.08?1.86 Ma, Fig-ure 8) characteristics of secondary volcanism, thusrevealing that a few of the seamounts on SKSare probably secondary volcanic. However, theTable 3. Estimated Volumes in Units of 103 km3 Ordered Smallest to LargestEdifice (outlines shown in Figure 1)Volume estimatesappropriate for landslidesaVolume estimatesappropriate for shield volcanoesbEstimates of Robinsonand Eakin?s [2006]Wailau Slide 1.1 (8.56 2.0) 1.6S. O?ahu Volcanic Field (2.1) 9.66 3.7Nu?uanu Slide 2.7 (13.46 4.9) 2.4Mahukona Volcano (4.7) 13.06 1.0c 13.5Wai?anae Slump 5.8 (15.96 2.0)South Kaua?i Swell 6.0 14.06 3.4Ka?ena Ridge 14.86 2.0Ko?olau Volcano without Nu?uanu Slide 34.36 2.9 31.7Wai?anae Volcano with Wai?anae Slumpand without Ka?ena Ridge36.86 3.9Kaua?i without S. Kaua?i Swell 43.56 7.9Ko?olau with Nu?uanu Slide 47.76 7.6Wai?anae with Ka?ena Ridge andWai?anae Slump51.66 5.8 52.9Kaua?i with S. Kaua?i Swell 57.56 11.0 57.6aVolume above a flat abyssal seafloor at a depth of 4.6 km.bVolume between the seafloor and the pre-existing oceanic basement, which flexes downward beneath the islands in the shape of elastic plate(see text). Uncertainty is based on an uncertainty of 60.5 km of the depth of basement. Adding ?0.5 km to the depth puts the basement near thatimaged seismically by Watts and ten Brink [1989] near O?ahu. The shown volume estimates are based on the assumption that 0.5 km of pre-existing pelagic sediment lie between the pre-existing basement and each edifice following Robinson and Eakin?s [2006].cMethod 2 assumes the base of Mahukona is the surface of the pre-existing seafloor, flexed downward beneath the island chain. Here the startingpoint of this surface is the seafloor outside the flexural moat of the Island of Hawaii at a depth of 4.6 km. Garcia et al. [2012] assumed the samestarting depth but used a point on the southern margin of Mahukona, which is well within the flexural moat, thus producing a volume estimate of 6x 103 km3.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592340dominance of tholeiitic lavas from the othersampled seamounts and their older ages indicatethat most of the SKS is not related to secondaryvolcanism. The bulk of the SKS was derived fromshield volcanism.8.2. Landslide Origin[30] Giant landslides are common on and aroundoceanic volcanoes worldwide (e.g., Canary Islands[Watts and Masson, 1995; Masson et al., 2002],Cape Verde [Ancochea et al., 2010], La Reunion[Oehler et al., 2008], Aleutians [Coombs et al.,2007], Stromboli [Romagnoli et al., 2009]); andsuch was the original interpretation of the SKS[Holcomb et al., 1988; Moore et al., 1989, 1994].Submarine landslides are usually associated withoversteepened flanks of volcanoes. Steep headscarps and bounding walls commonly formamphitheater-shaped scars, which are the sourceregion of the landslides. Landslides can occur atany time during the evolution of Hawaiian volca-noes: the preshield stage (Lo?ihi seamount, [Malah-off, 1987; Fornari et al., 1988; Moore et al.,1989]), the shield stage (South Kona slide; [Mooreet al., 1994]), and after most of the volcano hasformed (East Moloka?i [Moore et al., 1994]).[31] Two types of massive landslides are commonon oceanic islands: debris avalanches and slumps[Moore et al., 1994]. Debris avalanches are consid-ered to be catastrophic events creating debris fieldsof giant (tens of kilometers wide) blocks or smallerblocks (tens to hundreds of meters wide) and hum-mocky topography [Moore et al., 1994]. As seenaround the Canary Islands, La Reunion and Strom-boli, hummocky debris avalanches often display aconcave-up topographic profile that is steepest nearthe head wall and flattens with distance away fromthe volcano, eventually merging asymptoticallywith the abyssal seafloor [Watts and Masson, 1995;Urgeles et al., 1999; Coombs et al., 2007; Romag-noli et al., 2009]. Hawaiian debris avalanches rep-resent some of the largest debris avalanches in theworld [Hampton and Lee, 1996]. The Nu?uanuslide, for example, extends 150 km away fromO?ahu and is composed of intact blocks up to 35km long by 18 km across and 1.5 km tall [Garciaet al., 2006]. In contrast, the smaller South Kona,?Alika 1 & 2, and Clark slides are examples ofmore hummocky deposits made up of smaller (hun-dreds of meters or less across) and more uniformlysized fragments [Moore et al. 1994]. The ?Alika 1& 2 slides display well-defined chutes, bounded bylevees [Moore et al., 1994]. In contrast to debrisfields, slumps (e.g., Wai?anae and Hilina slumps,Figure 1) are characterized by deeply rooted,mostly intact blocks of flank material that slide epi-sodically over geologic time [Moore et al., 1989,1994; Hampton and Lee, 1996].[32] The SKS displays some characteristics ofboth debris field and slumps, but fails to conformfully to either model. In support of a landslide ori-gin, the rounded southern border of the SKS inmap view is not unlike the distal outline of hum-mocky debris avalanches among other islandchains (e.g., Canaries [Watts and Masson, 1995;Masson et al., 2002], La Reunion [Oehler et al.,2008], Aleutians [Coombs et al., 2007]). In addi-tion, the SKS has relatively smooth, long-wavelength topography that is populated withnumerous small seamounts producing a hum-mocky surface (Figures 1?3), superficially resem-bling the deposits of an ?Alika 2-type debrisavalanche [Moore et al., 1989]. The most compel-ling evidence for a landslide are the lack of clearlyinsitu pillow lavas where the tholeiitic sampleswere obtained, the highly vesicular lava samples,as well as the diversity of ages of tholeiites foundin close proximity to each other, sometimes on thesame seamount. These findings indicate thatextensive erosion and material transport was im-portant to the evolution of the SKS.[33] A number of characteristics of SKS, however,are contradictory with those of other landslides. Interms of its geomorphology, the SKS has a convexsurface and meets the abyssal seafloor with a dis-tinct break in slope (Figure 4), which contrast withthe form of most debris avalanches near otherocean islands as discussed above. While the notedhummocky surface resembles that of the ?Alika 2avalanche, the SKS differs significantly in itsmuch larger scale (6700 km2 versus 1700 km2 inarea [Moore et al. 1989]) and having larger sea-mounts (median width of 700 m) than the debrisof the Alika 2 avalanche (again, widths typically102 m or less). Volumetrically, if the SKS were adebris avalanche, it would represent an extremeend-member: the estimated volume above theabyssal seafloor depth of 6.0 x 103 km3 (Table 3)is larger than that of the Nu?uanu slide (2.7 ? 5 x103 km3), and three to six times the volumes of thelargest debris avalanches of the Canary Islands[Masson et al., 2002]. Only two debris avalancheson Earth have been estimated to be comparable orgreater in volume: the Storegga slide offshoreNorway at 5.6 x 103 km3 [Bugge et al., 1988] andthe Agulhas slide off South Africa at 20 x 103 km3[Dingle, 1977].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592341[34] Relative to typical slump deposits, the 2?2.5km thickness of SKS over a broad area comparesfavorably with the thickness and extent of, forexample, the Wai?anae slump. However, the SKScontrasts with Hawaiian slumps (or the Nu?uanu orWailau avalanches for that matter) by its lack oflarge angular blocks (Figures 1 and 4), the absenceof a prominent head scarp, and the presence of arounded, rather than an irregular margin with thesurrounding seafloor. If the SKS originally con-sisted of one or more largely intact blocks, theseblocks would have had to be extensively erodedand the area between them filled in order to erasetheir original blocky morphology, leaving the rela-tively smooth, convex-up morphology and roundedmargin of the SKS. The central axis and side lobesthat produce the SKS?s subtle pinnate morphologyis especially troublesome to explain with either adebris avalanche or a slump origin.[35] The possible source regions for a SKS landslideare Ni?ihau?s east flank and Kaua?i?s south flank[Moore et al., 1989]. Both areas show evidence ofcollapse based on gaps in the original shorelines ofeach island?s shield volcano (Figures 2 and 9). Typi-cally, the margins of the submarine platforms sur-rounding the Hawaiian Islands have slope-breaks,marking the most distal shorelines that formed dur-ing the shield-building volcanic phase [Mark andMoore, 1987]. Gaps in these slope breaks aroundNi?ihau and Kaua?i were mapped by Flinders et al.[2010] (Figures 2 and 9a). Around Ni?ihau, the vol-cano?s missing eastern paleo-shoreline is the proba-ble location where a large collapse occurred[Stearns and Macdonald, 1947]. Most of Ni?ihau?ssoutheast shoreline, however, is probably intact andcan be traced to a point just below the most westernextent of Kaua?i southern shoreline (Figure 2).Thus, in order for material from eastern Ni?ihau tobe part of SKS, a collapse would have had to occurearly enough in Ni?ihau?s shield stage for the south-east shoreline to rebuild (Figure 10a). About half ofthe dated samples from SKS are old enough (4.3?5.4 Ma) to have come from the Ni?ihau shield (4.3?6.3 Ma; [Sherrod et al., 2007]), however, the otherhalf of the dated SKS tholeiites are probably tooyoung (3.9?4.2 Ma).[36] A generous estimate for the volume of miss-ing material from east Ni?ihau is 103 km3 (dimen-sions of 35 km N-S along the paleo-shorelineseast of Ni?ihau, 35 km between Ni?ihau andKaua?i, and 1 km in average thickness). Thisestimate is only 17% of the volume of SKS abovethe in-filled moat sediments (Table 3). This dis-crepancy is aggravated by the fact that it is impos-sible for all of east Ni?ihau to have collapsed tothe SSE (Figure 10a): the southeast flank mayhave, but not the northeast flank. Hence, onlyabout half of Ni?ihau?s missing flank potentiallycould have contributed to the SKS. Furthermore, theprominent, eastward-dipping scarp on Ni?ihau?snortheast flank suggests that the landslide from thisarea traveled to the east (and now underlies or ispart of Kaua?i [Flinders et al., 2010], or to the north-east, forming the debris field presently located northof Kauai [Moore et al., 1989] (Figures 10a and10b). Therefore, it is unlikely that east Ni?ihau con-tributed significantly to the volume of SKS.[37] This leaves Kaua?i as the main landslidesource. Indeed, a 30 km wide gap in Kaua?i?ssouthern paleo-shoreline (Figures 2 and 9a) indi-cates that a portion of south Kaua?i has experi-enced mass wasting. To evaluate whether themissing volume of Kaua?i?s south flank matchesthat of the inferred debris deposits, we recon-structed the area of the SKS prior to the hypothe-sized landslide. This was done following themethods of Satake et al. [2002]. First, the bathym-etry points within the SKS border were deletedfrom the bathymetry grid. Second, several controlcontours were placed across the gap in data points,connecting with the real contours on either side ofthe SKS. The location of the pre-SKS shorelinewas estimated by visually interpolating a smootharc through the gap in the paleo-shoreline. Third,from the data surrounding the gap and the controlpoints within the gap, a smooth surface was inter-polated to fill the gap. The interpolation was doneusing the ??surface?? routine of the GMT softwarepackage [Wessel and Smith, 1995], which com-putes a continuous curvature spline in tension. Thetension parameter was varied in numerous runsuntil a geomorphologically reasonable preswellsurface was attained (Figure 9b).[38] Subtracting the bathymetry of the reconstructedseafloor from the present-day bathymetry yields avolume of landslide debris of 2.9 x 103 km3 (i.e.,the volume SKS as landslide above the infillingmoat sediments if their surface shoaled towardKaua?i, rather than remained flat at 4.6 km asassumed for method 1). The missing volume ofKaua?i?s flank is computed based on the differencein topography between the reconstructed andpresent-day flank (between Kaua?i and the northernboundary of the SKS, mark in red in Figures 9a and9b), and is 84 km3. This missing volume fromsouth Kaua?i represents only 3% of the inferred vol-ume of the debris.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592342[39] To adequately account for the volume of theSKS by a landslide, the prefailure shoreline alongKauai?s south shore would have to display a far dif-ferent and geologically problematic morphology(Figure 9c). The prefailure shoreline would need toprotrude 30?40 km south of Kaua?i?s current shore-line. While such a protrusion could have representeda broad rift zone extending from the Kaua?i shieldvolcano, land and marine gravity surveys on southKaua?i [Flinders et al., 2010] show no evidence forthe remnants of dense cumulate material within thecore of the rift zone as is detected beneath other Ha-waiian rift zones [Kauahikaua et al., 2000]. Instead,the observed gravity anomaly decreases continu-ously from a peak value near the center of Kaua?i,southward through Kaua?i?s southern shore, andreaching a minimum in the area of the hypotheticalprotrusion in this geologic reconstruction (Figure 5).The crust below the area of the hypothesized protru-sion has a low, rather than a high density. In addi-tion, generating SKS from such a protrusionrequires most of the debris to have travelled southand southeast, roughly parallel to the long axis ofthe protrusion. This behavior is counter that of mostknown flank failures, whereby the run-out tends tobe perpendicular to established rift zones [Swansonet al., 1976; Moore et al., 1989; Smith et al., 1999].[40] Another possibility that would not require sucha large protuberance invokes one or more landslidesthat incised deeper into the interior of the islandand removed a wider portion of Kaua?i?s southflank (Figure 10b). Subsequent to these events, thesame flank would have been reconstructed byFigure 9. Perspective views of (a) existing bathymetry (b) reconstructed bathymetry in the absence of SKS,with Kaua?i?s paleo-shoreline interpolated between the identified paleo-shorelines (Figure 2), and (c) recon-structed bathymetry with Kaua?i?s paleo-shoreline protruding southward so that it would account for the fullvolume of the SKS above the shown abyssal seafloor. Contour interval is 0.5 km and vertical exaggeration is3:1 (a1) Close-up and (a2) profile (location marked by A-A? of inset map) of Kaua?i?s existing southern flank.(b1) Close-up and (b2) profile of Kaua?i?s southern flank reconstructed in (b). No vertical exaggeration in (a2)and (b2).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592343volcanism, a new paleo-shoreline would haveformed, and then one or more small landslideswould have created the present-day gap in thepaleo-shoreline. Temporally, the main landslide(s)must have occurred late enough during shield-building to construct a sufficient volume, but earlyenough for subsequent volcanism to rebuild Kauai?ssouth flank and create the island?s roughly circularplanform that exists today. A difficulty with thispremise is that the overlap in ages between the SKStholeiites (3.9?5.4 Ma) and the Kaua?i shield stage(3.6?5.1 Ma) allows for little time for the hypothe-sized reconstruction. Such a massive reconstructionof the south flank of Kauai has also not been recog-nized in the geology of Kaua?i [Garcia et al.,2010], although a structural trough did form on thesouthwest flank of Kaua?i near the end of shieldvolcanism (4.0 Ma [Macdonald et al., 1960]).[41] Ultimately, the most fundamental problemfor a landslide origin of the SKS is the lack of asuitable source region. It would be extremely in-congruous for one of Earth?s largest submarinelandslides to have its amphitheater scar almostcompletely filled by subsequent volcanism. Insummary, the lack of a source, the large size ofthe SKS, and the deviations in morphology fromother submarine debris avalanches and slumpslead us to conclude that the main volume ofSKS is probably not a landslide deposit.8.3. The SKS as a Low-Relief ShieldVolcano[42] An alternative origin for the SKS is a low-relief shield volcano. Perhaps most contradictory toFigure 10. Illustrations of hypothesized origins of the SKSoverlain on existing bathymetry (contoured at 500 m). Smallred symbols mark sample locations as in Figure 2. Large redovals mark gravity highs over the inferred magmatic centers ofKaua?i and Ni?ihau [Flinders et al., 2010]. Small yellow sym-bols and short dashed curves show mapped slope breaks, inter-preted as paleo-shorelines of Ni?ihau (red) and Kaua?i (blue)[Flinders et al., 2010] (see also Figure 2). Long dashed curvesare hypothetical paleo-shorelines that have been destroyed bylandslides on the south flank of Kaua?i. (a) SKS is composed ofapproximately half of the collapsed mass of east Ni?ihau (lightblue), overlain by debris from Kaua?i?s missing southern paleo-shoreline (dark blue). The other half of Ni?ihau?s east flank col-lapsed northeast (light green) (b) SKS is composed of depositsfrom a massive sector of the south flank of Kaua?i (light blue).Kaua?i?s southern flank was then rebuilt by volcanism, andlater experienced a small collapse on to the SKS (dark blue).Figures 10a and 10b are unlikely as discussed in the text. (c)Most likely, the SKS is an elongate, low-relief shield volcanothat never reached sea level (light brown). It was later partlycovered with the small volume of debris from Kaua?i?s missingsouthern paleo-shoreline (dark blue).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592344a shield origin, is the evidence provided by the ge-ology, vesicularity, and irregular age distributionfor widespread mass wasting. A massive landslideorigin is a simple explanation, but an alternative ex-planation is one or more relatively small landslidesfrom Kaua?i?s missing southern shoreline, whichveneered a large area of a SKS shield volcano. Am-biguous evidence for a shield origin includes thegeochemistry (Figures 6 and 7) and the range ofages (Figure 8) obtained from the SKS. These dataare consistent with either a massive landslide fromKaua?i, or a separate shield of similar compositionand age to Kaua?i, perhaps partially overlain with asmall amount of material from Kaua?i.[43] Weakly supportive of a low-relief shield isthe residual gravity anomaly. On one hand, thelack of a strong positive residual gravity anomalyover the SKS (Figure 5) could be consistent with alandslide origin. While on the other hand, thisfinding is also consistent with a low-relief shieldvolcano, in which magmatism was not stronglyfocused to a central accumulation zone but wasinstead more distributed. A more distributed mag-matic plumbing system, in fact, may be expectedfor such a broad, low-relief volcano. Mahukona isa taller, but still low-relief feature that also doesnot display a large gravity high over its summit[Garcia et al., 2012] (the gravity high over thesouthern part of Mahukona (Figure 5) is largelyattributed to a submarine extension of Hualalai?srift zone). The negative gravity anomaly separat-ing the high over Kaua?i from the low-amplitudehigh over the SKS is, again, difficult toexplain with a landslide origin. Instead, this obser-vation is consistent with Kaua?i?s south flankbeing composed of low-density debris fromKaua?i, and most of the volume of SKS compris-ing higher-density, intact lava due to shieldvolcanism.[44] The most supportive evidence of a shield ori-gin is the overall morphology and size. This evi-dence includes the smooth long-wavelengthtopography, convex surface, and large continuousheight of the SKS, which resembles parts ofKa?ena Ridge and Mahukona (Figures 1 and 4).The 14 x 103 km3 volume of the hypothesizedSKS shield volcano as measured from the top ofthe pre-existing seafloor is comparable to somesmall Hawaiian volcanoes (e.g., Mahukona, 13 x103 km3 and Ka?ena Ridge, 15 x 103 km3 fromTable 3; West Maui, 9 x 103 km3 and Hualalai 15x 103 km3 from Robinson and Eakins [2006]). Theproposed SKS volcano is lower in relief thanthe other Hawaiian volcanoes but higher than theSouth West O?ahu Volcanic Field (SWOVF).[45] The existence of a SKS shield would alsoreduce the large distance between adjacent shieldvolcanoes represented by Wai?anae and Kaua?i.Without a SKS shield, the spacing between thecenters of Wai?anae and Kaua?i is 140 km ornearly twice the typical spacing of 72637 kmbetween adjacent Hawaiian shield volcanoes [tenBrink, 1991]. With the SKS shield, the averagespacing between the three shields is closer to thetypical spacing (average of 90 km with 130km between the SKS and Wai?anae and 50 kmbetween SKS and Kaua?i). If Ka?ena Ridge is alsoa separate volcano, the average spacing betweenthe four volcanoes also fits with the typical spac-ing (average of 63 km based on distances of 50km, 100 km, 50 km for Wai?anae-Ka?ena,Ka?ena-SKS, SKS-Kaua?i, respectively).[46] In summary, although there is no evidencerequiring a shield origin, this explanation has theleast profound contradictions with observationsand employs the most straightforward geologicprocesses as presently understood. We thereforesuggest that a low-relief shield is the most likelyorigin for the major (>90%) volume of the SKS.[47] Figure 10c illustrates the model of most of theSKS forming as low-relief shield volcano. Thismodel has the construction of the SKS shield (3.9?5.4 Ma, Tables 1 and 2) overlapping with the mid-to-late shield phase of Ni?ihau (4.3?6.3 Ma; Sher-rod et al. [2007]) and the shield phase of Kaua?i(4.0?5.1 Ma; McDougall [1979]; Garcia et al.[2010]). The SKS shield was later partially over-lain by a small volume of debris from Kaua?i, asindicated by the gap in Kauai?s southern paleo-shoreline. These debris contributed to some of thetopography between the SKS and Kaua?i and tothe negative residual gravity in this area. Finally, afew monogenetic, alkalic seamounts formedbetween 1.9 and 0.2 Ma during a secondary vol-canic phase.[48] As a shield volcano, the SKS would be veryunusual on Earth given the combination of itsappreciable area (6700 km2), low relief (2?2.5km) and thus low slope (< 1.5, Figure 4). Bycomparison, Mauna Loa on the Island of Hawaiihas slopes of 5?10 on land and slopes up to 18offshore. Iceland is well known for having a num-ber of small, low-slope volcanoes (29 documentedby [Rossi, 1996] with a median slope of 2.7).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592345These Icelandic volcanoes, however, are muchsmaller (diameters of 0.5?11 km, heights of 12?520 m) and monogenetic, thus probably represent-ing a different type of volcano than a SKS shield,the latter of which presumably would be composedof many eruptive events. The feature most similarto the SKS that we know of on Earth is the SWO?ahu Volcanic Field (slope 0.5), whereas theareally extensive lava flows north of O?ahu andsoutheast of the Island of Hawai?i [Lipman et al.,1989; Clague et al., 1990] may represent the mostextreme examples of volcanism over a broad areawith little, or in this case, no relief.[49] While unusual for Earth, large shield volca-noes with such low slope, in contrast, are morecommon on other planets. The largest shield-likeedifices on Venus, for example, have lateraldimensions (hundreds of km) a few times greaterthan the SKS, and slopes (< 1?3) [Stofan et al.,2001] comparable to the SKS. On Mars, the tallestvolcanoes are much steeper than the SKS, but atleast twenty Martian volcanoes have areas of thesame order (103 km2), and slopes much lower thanthe SKS [Baratoux et al., 2009]. A swarm of about30 volcanoes on Syria Planum, identified byBaptista et al. [2008], have smaller areas(80?1400 km2) than, but comparable slopes (0.2?1) to the SKS. This comparison leads us to ques-tion whether there are some general physical con-ditions promoting the formation of broad, low-slope shields that occurred on Mars and Venus,were active at the Hawaiian hotspot during the cre-ation of the SKS, but otherwise rarely occur onEarth. That said, few terrestrial oceanic volcanochains are as well surveyed as the HawaiianIslands, so there may be other edifices like SKSthat are as yet not recognized. A shield origin forthe SKS would imply that shield volcanism inHawaii, and on Earth in general, spans a range ofsizes and shapes from the large island-buildingshields, to smaller edifices protruding from theislands such as Mahukona and Ka?ena Ridge, andfinally, to even lower-relief volcanoes representedby the SKS and possibly the SW O?ahu VolcanicField.9. Conclusions[50] The SKS is a 110 km x 80 km ovoid bathy-metric feature with a convex surface, punctuatedwith numerous small (<1 km wide) seamounts.Most of the SKS has a low acoustic backscatterindicating relatively thick sediment cover as con-firmed by JASON dive images. The residual grav-ity over SKS is negative on the very northern partof the SKS and neutral or slightly positive over thecentral portion. Lavas from two of the seamountssampled are alkali basalts with ages of 0.2?1.9Ma, and thus represent a phase of secondary volca-nism. The majority of the SKS samples are tho-leiitic and have ages of 3.9?5.4 Ma, which arecoeval with Ni?ihau?s mid-to-late shield phase andKaua?i?s shield phase. The SKS tholeiites have87Sr/86Sr and 206Pb/204Pb compositions similar tothose of Kaua?i, West Ka?ena, and Wai?anae.[51] A landslide origin, as originally proposed, isproblematic. Morphologically, the SKS is unlikeany other landslide of comparable size. The mostprofound discrepancy is that the estimated volumeof SKS above the surrounding seafloor (6 x 103km3) is greater than almost all other estimates forlandslides on Earth, however, there is no obvioussource region that could have housed this enormousvolume. A landslide origin of SKS requires subse-quent shield volcanism to nearly completely fill thescar of a massive sector collapse and to constructKaua?i?s circular planform?a requirement that isin conflict with the overlap in ages of Kaua?i andthe SKS tholeiites as well as the lack of geologicevidence on Kaua?i for a major sector collapse.[52] Among the three hypotheses that were eval-uated for the origin of the SKS, the low-reliefshield volcano model most readily explains thegeomorphologic and geophysical evidence. Theshield was later mantled by mass wasting eventsfrom Kaua?i, which created the gap in the southernpaleo-shoreline. Subsequently, a few isolated sec-ondary volcanic seamounts formed on the SKS,further complicating its geologic history. The largearea and low slope of the SKS make it a rather un-usual terrestrial shield volcano, although notunlike many volcanoes on Venus and Mars.Acknowledgments[53] The efforts of the crews of the R/V Kilo Moana andWHOI JASON ROV helped make the field campaign trulysuccessful. University of Hawaii undergraduates, LindsaySpencer and Kyle Taniguchi, are thanked for help with sam-ple preparation for geochronology and geochemistry. Com-ments by John Sinton and Peter Mouginis-Marks led to morecareful presentation in various parts of the manuscript. Wegratefully acknowledge the thorough and constructive reviewsby an anonymous reviewer, Michelle Coombs, and EditorJames Tyburczy, which led to substantial improvements. Thisresearch was supported by NSF grants EAR-0510482, EAR-1219955, and OCE-1155098. This is SOEST contribution#8933.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592346ReferencesAncochea, E., M. J. Huertas, F. Hernan, and J. L. 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Prof. Pap. 963, 39 pp., U.S. Government PrintingOffice, Washington, D. C.Swinnard, L. F. (2008), Geochemical variations of Kaua?iIsland and South Kaua?i Swell volcanics, 97 pp., M.S. The-sis, Univ. of Hawaii, Manoa.Takahashi, E., et al. (2001), A newly recognized shield vol-cano southwest of Oahu Island, Hawaii, Eos Trans. AGU,82(47), Fall Meet. Suppl., Abstract V12B-0981.ten Brink, U. S. (1991), Volcano spacing and plate rigidity,Geology, 19, 397?400.Urgeles, R., D. G. Masson, A. B. Watts, and T. Le Bas (1999),Recurrent large-scale landsliding on the west flank of LaPalma, Canary Islands, J. Geophys. Res., 104(B11), 25,331?25,348.Watts, A. B., and D. G. Masson (1995), A giant landslide onthe north flank of Tenerife, Canary Islands, J. Geophys. Res.,100(B12), 24,487?24,498.Watts, A. B., and U. S. ten Brink (1989), Crustal structure,flex-ure, and subsidence history of the Hawaiian islands, J. Geo-phys. Res., 94, 10,473?10,500.Watts, A. B., U. S. t. Brink, P. Buhl, and T. M. Brocher (1985),A multichannel seismic study of lithospheric flexure acrossthe Hawaiian-Emperor seamount chain, Nature, 315, 105?111.Wessel, P., and W. H. F. Smith (1995), New version of theGeneric Mapping Tools released, Eos Trans. AGU, 76, 329.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592348 A low-relief shield volcano origin for the SouthKaua?i SwellGarrett ItoDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USA (gito@hawaii.edu)Michael O. GarciaDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAJohn R. SmithHawaii Undersea Research Laboratory, SOEST, University of Hawai?i, Honolulu, Hawai?i, USABrian TaylorDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAAshton FlindersGraduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USABrian JichaDepartment of Geoscience, University of Wisconsin, Madison, Wisconsin, USASeiko YamasakiTono Geoscience Center, Japan Atomic Energy Agency, Gifu, JapanDominique WeisPacific Centre for Isotopic and Geochemical Research, Department of Earth Ocean Sciences,University of British Columbia, Vancouver, British Columbia, CanadaLisa SwinnardDepartment of Geology and Geophysics, SOEST, University of Hawai?i, HonoluluHawai?i, USAChuck BlayTEOK, Kaua?i, Hawai?i, USA[1] The South Kaua?i Swell (SKS) is a 110 km x 80 km ovoid bathymetric feature that stands >2 kmhigh and abuts the southern flank of the island of Kaua?i. The origin of the SKS was investigated usingmultibeam bathymetry and acoustic backscatter, gravity data, radiometric ages, and geochemistry of rocksamples. Most of the SKS rock samples are tholeiitic in composition with ages of 3.9?5.4 Ma indicatingthey were derived from shield volcanism. The ages and compositions of the SKS rocks partially overlapwith those of the nearby Ni?ihau, Kaua?i and West Ka?ena volcano complexes. The SKS was originallydescribed as a landslide; however, this interpretation is problematic given the ovoid shape of SKS, itsrelatively smooth, flat-to-convex surface, and the lack of an obvious source region that couldaccommodate what would be one of Earth?s most voluminous (6 x 103 km3) landslides. The morphology,? 2013. American Geophysical Union. All Rights Reserved. 2328ArticleVolume 14, Number 729 July 2013doi: 10.1002/ggge.20159ISSN: 1525-2027size, and the surrounding gravity anomaly are more consistent with the SKS being a low-relief shieldvolcano, which was partially covered with a small volume of landside debris from south Kaua?i and laterwith some secondary volcanic seamounts. A shield origin would imply that Hawaiian and possibly otherhotspot shield volcanoes can take on a wider variety of forms than is commonly thought, ranging fromtall island-building shields, to smaller edifices such as Ka?ena Ridge and Mahukona, to even lower-reliefvolcanoes represented by the SKS and possibly the South West O?ahu Volcanic Field.Components: 13,156 words, 10 figures, 3 tables.Keywords: South Kauai Swell ; submarine landslides; hotspot volcanism; shield volcanism; secondary volcanism.Index Terms: 3037 Oceanic hotspots and intraplate volcanism: Marine Geology and Geophysics; 3070 Submarine land-slides: Marine Geology and Geophysics; 3075 Submarine tectonics and volcanism: Marine Geology and Geophysics; 8137Hotspots, large igneous provinces, and flood basalt volcanism: Tectonophysics;8415 Intra-plate processes: Volcanology;1033 Intra-plate processes: Geochemistry; 3615 Intra-plate processes: Mineralogy and Petrology.Received 30 March 2013; Revised 24 April 2013; Accepted 25 April 2013; Published 29 July 2013.Ito, G., M. Garcia, J. Smith, B. Taylor, A. Flinders, B. Jicha, S. Yamasaki, D. Weis, L. Swinnard, and C. Blay (2013), A low-relief shield volcano origin for the South Kaua?i Swell, Geochem. Geophys. Geosyst., 14, 2328?2348, doi:10.1002/ggge.20159.1. Introduction[2] The Hawaiian-Emperor Chain is one of the mostextensively studied in the world and has greatlyinfluenced our knowledge of how oceanic islandsform and evolve (Figure 1). The Hawaiian islandsare perched on broader subsided platforms (pinkareas in Figure 1) from which emanate elongateridges that are, or once were volcanic rift zones [For-nari, 1987]. The platform flanks are mantled withthe products of mass wasting, which range in sizeand character from localized turbidite channels tomassive landslides [Moore et al., 1989; Smith et al.,2002; Morgan et al., 2007]. Some of the latter formsare the remnants of huge island sector collapses rep-resenting the most dramatic landslides on Earth[Moore et al., 1989, 1994; Hampton and Lee, 1996].[3] Other submarine features along the Hawaiian-Emperor Chain include products of secondary vol-canism, forming extensive lava fields on the flex-ural arches north of O?ahu (200 km wide) andsouth of Hawai?i (50 km wide) [Lipman et al.,1989; Clague et al., 1990], as well as on the north-east flank of Ni?ihau (100 km wide) [Clagueet al., 2000; Dixon et al., 2008] and around Ka?ulaIsland [Garcia et al., 2008]. Another lava field sitson a seafloor bulge southwest of O?ahu (100 kmwide) [Moore et al., 1989]. One interpretation ofthe SW O?ahu Volcanic Field (SWOVF) is that itis a veneer of secondary volcanism overlying thebulging deposits of a landslide from O?ahu[Coombs et al., 2004]. Alternatively, the lava andseafloor bulge together, could be part of a verylow-relief shield volcano [Takahashi et al., 2001;Noguchi and Nakagawa, 2003]. The causes of theabove forms of volcanism far from the hotspotcenter are poorly known.[4] Another class of submarine construct that ispoorly understood is represented by the large edificesprotruding northwest from the Big Island of Hawai?i(Mahukona) and northwest from O?ahu (Ka?enaRidge). Mahukona is interpreted as a separate low-relief shield volcano as evidenced by geophysical,geochronological, and geochemical data [Garciaet al., 1990; Clague and Moore, 1991; Garcia et al.,2012]. Ka?ena Ridge, however, is not as well studied.It has been proposed as being the submerged exten-sion of the Wai?anae Rift Zone or an entirely sepa-rate, volcanic system [Smith, 2002; Coombs et al.,2004], and has been compared to the large submarineHana Ridge east of Maui. Thus, Hawaiian construc-tional volcanism is manifested in a variety of formsfrom small, but extensive lava fields, to more volu-minous ridges, to massive shield volcanoes.[5] A prominent feature that is particularly enig-matic is the large bathymetric swell south of theIsland of Kaua?i (Figures 1 and 2). The SouthKaua?i Swell (SKS) was originally described as asubmarine landslide based on the U.S. GeologicalSurvey?s, GLORIA side-scan surveys, whichlacked swath bathymetry data [Moore et al.,ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.2015923291994; Groome et al., 1997; Holcomb and Robin-son, 2004]. Subsequent reconnaissance bathymet-ric mapping showed that the gross morphology isquite unlike other large Hawaiian landslides. Forexample, many of the features that were consid-ered as debris blocks are actually cone-shapedseamounts, some with acoustically reflective lavaflows (Figure 2). These morphological featuresled us to consider other hypotheses for the originof the SKS: (1) widespread secondary volcanismsuch as the vast lava field northwest of Ni?ihau,and (2) a low-relief shield volcano, intermediate inrelief between Mahukona and the SW O?ahu Vol-canic Field. To test these and the landslide hypoth-eses, we undertook a marine expedition of the SKSin 2007 on the R/V Kilo Moana, operated by theUniversity of Hawaii, during which we performedgeophysical surveys and sampled rocks usingWoods Hole Oceanographic Institution?s JASONROV. The results from this expedition are com-bined with those from three dredge hauls madefrom the R/V Kilo Moana in 2005, and samplestaken during a JASON test dive in 2006. Here wereport on the findings and interpretations of themultibeam bathymetry and acoustic imagery, thegravity surveys combined with an extensive localgravity dataset, and the lava geochemistry and geo-chronology analyses.2. Bathymetric Data and AcousticImagery[6] The South Kaua?i Swell (SKS) is a SSE-trending, ovoid feature (110 km x 80 km) thatspans an area of 6.7 x 103 km2 adjacent to thesouth flank of Kaua?i (Figures 1?3). It stands a max-imum height of 2?2.5 km above the deep abyssalseafloor and merges with the steep-sided southernmargin of Kaua?i at a depth of 2 km. The overallsurface of SKS is slightly convex, having minimalslope near a depth of 2.5 km and sloping slightlysteeper at greater depths toward the surroundingabyssal seafloor (Figures 3 and 4). The swell has acentral axis extending SSE, flanked by slopes dip-ping to the SSW and E. A few low-relief (10?30 kmwide) lobes, or spur-like features, splay SSW andFigure 1. Multibeam bathymetry map of Hawaiian Islands, illuminated from the NW (grid available athttp://www. soest.hawaii.edu/HMRG/cms). Areas discussed in the text are outlined (black dashed). Whitelines mark locations of profiles labeled as shown in Figure 4.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592330Figure 2. ContinuedITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592331SE away from the central axis, producing a subtle,large-scale pinnate structure. Numerous seamounts(>400 with height> 60 m and ellipticity< 3, me-dian and standard deviation of the major axis lengthis 6726 562 m) are present; a few are circular andsmooth-topped (e.g., at 15935?W, 2144N),although most are more irregular in shape.[7] The acoustic backscatter of SKS is relativelylow in amplitude over large areas (light gray towhite; Figure 2b), which indicates that much ofthe SKS is relatively thickly sedimented as wasconfirmed by inspection during JASON dives.Higher acoustic backscatter is found along alluvialchannels extending south from Kaua?i and onmany of the seamounts (Figure 2b). Three stronglyreflective areas just east of SKS (Figure 2b) wereconfirmed to be lava flows [Greene et al., 2010].Video from the JASON dives confirmed that themore reflective escarpments and seamounts, whereFigure 2. (a) Multibeam bathymetry data around Kaua?i and Ni?ihau from our survey (KM0718) and from the Hawai?i Multi-beam Synthesis (http://www.soest.Hawai?i.edu/HMRG/cms), with 111 m grid spacing, colored and illuminated from the NW.Contours are every 0.5 km from depths of 4.5?2 km. JASON dive numbers are labeled; sample locations are marked with redcircles; dredge numbers (KS1?3) are labeled with locations marked by triangles. SKS is outlined with dashed line and its centralaxis is marked by large arrows. Gravity anomaly highs ( ?80 mGal) overly the inferred magmatic centers of the two shield vol-canoes are marked by translucent yellow patches [Flinders et al., 2010]. White symbols mark locations of identified slope breaks,interpreted as the paleo-shorelines of Kaua?i (triangles) and Ni?ihau (circles) [Flinders et al., 2010]. Dotted lines indicate the sec-tions where the slope break continues between the white symbols. Small black arrow marks where a slope-break of Kaua?i?s ter-race lies above a slope-break of Ni?ihau?s terrace. (b) Mosaic of acoustic backscatter from KM0718 and the Hawai?i MultibeamSynthesis. Dark is high return, light is low return. Contours are the same as in Figure 2a. Small red arrows point to acousticallyreflective areas confirmed to be young (< 0.4 Ma) alkalic lava flows as reported in Greene et al. [2010].Figure 3. Perspective views of South Kaua?i Swell bathymetry (color scale in km) from (top) azimuth of210, illuminated from 300, and (bottom) azimuth of 110, illuminated from 30. Color bar indicates depthin km. Vertical exaggeration is about 5:1.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592332we focused our sampling efforts, are generallymore sparsely sedimented. The areas sampled con-tain loose to partially lithified sediments, fracturedbasement rock, talus, breccia, and few insitu pillowflows. The samples recovered from the pillowbasalts were found to be young (<2 Ma) andalkalic, not tholeiitic (discussed below). The overalllow and sporadic backscatter of SKS clearly con-trast with the more uniformly high backscattersurrounding Ni?ihau and Ka?ula (small islandWSW of Ni?ihau), which denotes extensive areasof flows and volcanic cones produced by secondaryvolcanism [Dixon et al., 2008; Garcia et al., 2008].3. Gravity Data[8] Hawaiian volcanoes typically have gravityhighs over their magmatic centers [e.g., Krivoyand Easton, 1961; Kinoshita et al., 1963]; thus,gravity may be used to test the shield volcano hy-pothesis for the SKS. A regional compilation ofgravity data [Flinders et al., in press] was madefor the northern Hawaiian Islands from our andother R/V Kilo Moana cruises, the National Geo-physical Data Center (www.ngdc.noaa.gov), aswell as onshore gravity [Flinders et al., 2010].Details of the gravity data processing and reduc-tion are given in Flinders et al. [2010]; only theessential points are summarized here. Free-airgravity data from individual ship survey lineswere hand-edited for noisy sections (typicallyassociated with course and speed changes), brokeninto straight line segments, and then corrected fordiscrepancies between line crossings [Prince andForsyth, 1984]. The standard deviation of thereduced crossing errors was 2 mGal over theFigure 4. Profiles of various offshore features along the lines shown and labeled in Figure 1. Vertical exag-geration is 5:1. Labeled horizontal axes bound each profile triplet (for SKS) or pair of the labeled features.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592333area of free-air gravity shown in Figure 5. As iscommon, the free-air gravity anomaly shows astrong correlation with topography.[9] We produced a complete Bouguer anomaly(CBA) by subtracting from the free-air gravityanomaly the gravitational attraction of the submar-ine and subaerial topography (Figure 5b). The den-sity of water, the crust above, and the crust belowsea level were taken to be 1000, 2400, and 2700kg/m3, respectively [Flinders et al., 2010]. Varia-tions in the CBA reflect density structure that dif-fers from the above reference densities. Forexample, short wavelength (50 km wide) highsprobably mark dense cumulate-rich crust at thecenters of the Ni?ihau, Kaua?i [Flinders et al.,2010], Wai?anae, and Ko?olau shields (Figure 5b).From the southwest to northeast across the islandchain, the CBA is relatively high (> 300 mGal),decreases near the center of the chain (170 mGaljust west of Maui), and then increases again to thenortheast. This long wavelength variation iscaused by the crust-mantle interface taking theshape of an elastic lithospheric plate, which hasbeen flexed downward by the weight of the vol-cano chain [Watts and ten Brink, 1989].[10] To isolate the shorter-wavelength signals dueto local density variations beneath the SKS and theother volcanic features and landslides, weremoved wavelengths >150 km from the completeBouguer anomaly (using the Gaussian tapered fil-ter of ??grdfilter?? in the GMT software package[Wessel and Smith, 1995]). This filtering producesthe residual gravity anomaly (Figure 5c). Over thecentral portion of SKS, the residual anomaly flat-tens to neutral or low-amplitude positive values(5 mGal, yellow to orange in Figure 5c), similarto that over the SW O?ahu Volcanic Field(SWOVF). This low-amplitude high is interruptedto the south by a linear residual gravity low (5 to10 mGal, light green) that trends ENE toward theisland of Moloka?i and overlies the northern branchof the Moloka?i Fracture Zone (the southern branchis evident as a similar linear low projecting towardMaui). The slightly high-residual gravity over theSKS is bounded to the north by an area of verylow-residual gravity. This low-gravity anomalyresembles the low gravity on the other flanks sur-rounding most of Kaua?i and is most negative onKaua?i south shelf area (20 mGal). This promi-nent low clearly separates the strong positive grav-ity signature on the Island of Kaua?i from theneutral-to-small positive gravity over the SKS.[11] The gravity signatures over the SKS andSWOVF contrast with the larger-amplitude gravityhighs over the Wai?anae rift zone (10?40 mGal)and the southern part of Mahukona (10?20 mGal)[Garcia et al., 2012], as well as the negative resid-ual gravity low over the Wai?anae Slump (westflank of O?ahu). The Nu?uanu (NE of O?ahu) andFigure 5. (a) Free-air gravity anomaly is colored along shiptracks. (b) Complete Bouguer anomaly is free-air anomalyminus the effects of submarine and subareal topography. (c)The residual gravity anomaly is the complete Bouguer anom-aly with wavelengths >150 km removed. Depth contours areevery 1 km and islands are outlined in black. Relevant fea-tures are outlined with bold dashed lines.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592334Wailau (N of Moloka?i) landslide deposits have nosignificant residual gravity anomalies relative tothe areas immediately adjacent to them. Thus,although the gravity signatures of the SKS andSWOVF can distinguish them from the Wai?anaeSlump, their gravity anomalies alone are not dis-tinctive with respect to the Nu?uanu and Wailaulandslide deposits.4. Lava Samples[12] Samples from the South Kaua?i Swell were col-lected during four JASON dives and three dredge-hauls (Figure 1) to determine rock type,vesicularity, and chemical and isotopic composi-tions. The dives traversed many kilometers of sea-floor and thus enabled numerous samples frommultiple seamounts or morphologically distinctlocations (see Appendix for detailed maps of divesand sample locations; supporting information).1 Atotal of 110 samples were obtained from 19 sea-mounts at depths 2970?4170 m below sea level.Most of the 74 lava samples are breccias, consistingof lithologically identical (monomict), angularclasts. The other SKS samples include mudstones,sandstones, pebbly conglomerates with rounded,matrix-supported clasts of diverse lithologies (poly-mict) suggesting the clasts have been transported.For comparison, monomict as well as polymict vol-canoclastic rocks were found interbedded in the sub-marine portion of Mauna Kea?s deep drill core(3100 m) (HSDP2; [Garcia et al., 2007]). TheHSDP2 monomict fragmental deposits were inter-preted to be of local origin, formed as lavas eruptedon steep submarine slopes, whereas the polymictbreccias were thought to have formed by mass wast-ing on the unstable slopes of the active volcano[Garcia et al., 2007]. Mn coatings on the SKS sam-ples vary from <1 to 14 mm in thickness. Glass wasfound on only a few samples and was mostlyaltered. The interiors of the SKS rocks show varyingdegrees of alteration from unaltered with pristine ol-ivine, to moderate levels of alteration with partialreplacement of olivine by iddingsite and, in rarecases, clay and or zeolite in the vesicles.[13] Vesicularity shows no systematic variationwith sample location or rock composition but dis-plays a somewhat bimodal distribution with mostsamples having <5 vol. % or >20 vol. % vesicles.By comparison, subaerial HSDP2 lavas have meanvesicularities of 9, 11 and 18 vol.% for MaunaLoa, Mauna Kea and Kilauea volcanoes respec-tively, and a larger total variation (<1 to >30vol.%) for each volcano. The HSDP2 submarinelavas tend to have lower vesicularity, averaging<3 vol.%, but ranging widely (0.1 to 19 vol.%).Offshore Hawai?i, highly vesicular submarinelavas are found near submarine vents (e.g., up to33 vol.% in a tholeiitic lava from Loihi seamount)and tend to decrease with eruption depth [Moore,1965; Garcia et al., 1995]. Thus, the highly vesic-ular SKS lavas (>20 vol.%) probably erupted inshallow water depths (< 1000 m) and some per-haps subaerially. Because all of the SKS sampleswere extracted from water depths of >2500 m, thehighly vesicular lavas were probably transportedfrom shallower depths. These lavas were collectedfrom the western side (dive 297, water depths3330?3770 m) and central part (dives 252 and 299water depths 3260?3322 m) of SKS, but not fromthe site (298) furthest southeast from Kaua?i. Thissoutheastern-most dive (298) recovered rocks withvesicularities all <0.1 %. Thus, the highly vesicu-lar lavas from SKS could be landslide debris.5. Petrology and Geochemistry[14] The primary objective of the petrological andgeochemical components of the investigation wasto identify the type of lava (tholeiitic versusalkalic) and to determine whether the SKS lavasare compositionally similar to or distinct fromKaua?i lavas. Fifty SKS samples were analyzed byXRF for whole-rock major and trace element com-positions. Nineteen of these samples were also an-alyzed by ICP-MS for trace elements, and byTIMS and MC-ICP-MS for Pb, Sr, Nd and Hf iso-topes. Only an overview of the geochemicalresults is presented here to provide a basic charac-terization and address the origin of the swell ; thegeochemistry data are reported in Swinnard[2008], and the detailed examination of these datawill be presented in a separate study [Garciaet al., in preparation].[15] The SKS rocks range in composition fromalkalic basalt and basanite to tholeiitic basalts andpicrites (Figure 6). Only seven alkali rocks werecollected, and they were from the western flank ofthe SKS (dive 297 and dredge KS2, Figure 2) inareas of locally high-acoustic backscatter near andon relatively smooth-topped and broad seamounts.Those from dive 297 came from an outcrop of pil-low basalts. These rocks are similar in major and1Additional supporting information may be found in the onlineversion of this article.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592335trace element composition to the rejuvenated lavason Kauai?i [Garcia et al., 2010](Figure 6).[16] Tholeiites are the most abundant rock typesampled and range widely in composition (Figure6, e.g., 6?25 wt.% MgO, 0.16?0.62 K2O wt.%,and 0.73?0.91 CaO/Al2O3) in the least alteredsamples [Swinnard, 2008]. The rocks show widecompositional variations at the same MgO content,which cannot be attributed to olivine addition oralteration; they must reflect distinct parental mag-mas. Some seamounts show a small range in com-position (e.g., Zr/Nb, which is resistant toalteration, ranges 11.5?12.5 among six samplesfrom one seamount), whereas others have a largerrange (e.g., Zr/Nb ranges 10.1?13.3 among threesamples from another seamount). Compositiondoes not appear to vary systematically with posi-tion within individual dives sites or among theseven sample locations. The SKS tholeiitic com-positions generally overlap with those for Kaua?ishield lavas, although some have higherNa2O?K2O contents and Zr/Nb ratios (Figure 6),as well as somewhat lower CaO/Al2O3 at a givenMgO (not shown) content than Kaua?i lavas, sug-gesting slightly distinct sources.[17] The isotopic data for SKS lavas form twogroups that correspond with the rock types: thealkalic lavas have distinctly lower 87Sr/86Sr thanthe tholeiites and plot close to the compositionalfield of the Koloa (rejuvenated) lavas on Kaua?i(Figure 7). The tholeiites show overlap in87Sr/86Sr and 206Pb/204Pb with tholeiitic lavasfrom Kaua?i and Wai?anae, as well as some fromWest Ka?ena.6. Geochronology[18] A two-pronged approach was used in datingthe SKS lavas. Initially, 12 samples spanning awide range in rock composition were analyzed bythe unspiked K-Ar method at Kyoto University(see Table 1 and Appendix for details on methodsused; supporting information). Two samples wereanalyzed also at Japan Atomic Energy Agency(JAEA) by similar method. These K-Ar methodshave proven useful for dating young Hawaiianbasalts [e.g., Ozawa et al., 2005; Garcia et al.,2012]. In addition, sixteen tholeiitic samples wereanalyzed by 40Ar/39Ar methods at the Universityof Wisconsin-Madison (Tables 2, A1, and Appen-dix; supporting information). The plateau ages ofthese samples represent a high percentage of the39Ar (all greater than 93% and 100% for 14 of 15analyses), have low MSWD values (all <1.3 andmost <0.75), and are used here as the preferredages.[19] Among the three samples analyzed with bothK-Ar and Ar-Ar methods, two yielded remarkablyconsistent results: sample 297-09 gave ages4.246 0.46 Ma by K-Ar vs. 4.146 0.14 Ma and4.226 0.12 Ma by Ar-Ar (all errors are 2) ; sam-ple 299-29 gave ages of 4.036 0.12 Ma by K-Arand 4.026 0.13 Ma by Ar-Ar (Tables 1 and 2).For K-Ar dating of sample 299-29, the weightedmean uncorrected age is used because 38Ar/36Arratio was not well determined and no technicalcause was identified for this problem. In anotherstudy on rocks collected from seamounts east ofKaua?i [Greene et al., 2010], 40Ar/39Ar andunspiked K-Ar ages produced consistent resultsfor young alkalic lavas (0.37 vs. 0.24 Ma) as wellas for older tholeiitic lavas (3.6?4.9 vs. 4.3?4.7Ma). Thus, we think both geochronology methodsprovide useful constraints for the eruption ages ofthe least altered SKS lavas.Figure 6. Major element compositions of SKS samplescompared with compositions of the shield and rejuvenated(Koloa) stages on Kaua?i and tholeiitic lavas from westKa?ena [Garcia et al., 2010; Greene et al., 2010]. Red sym-bols denote tholeiites; blue denotes alkalic lavas.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592336[20] The 21 new ages for tholeiitic lavas rangefrom 3.9 to 5.4 Ma (Figure 8). We see no clear ge-ographic pattern to the distribution of ages (seeAppendix for detailed maps of sample locationsand ages; supporting information). For examplefrom Dive 252, we obtained two samples from ad-jacent seamounts separated by 2 km that yieldedidentical ages (both 4.29 Ma), and from dive 297,two samples from adjacent seamounts yielded dis-tinct but similar ages (4.03?4.34 Ma). Whereasfrom the two other dives, samples from the sameseamount yielded appreciably different ages (4.00and 4.29 Ma from Dive 298, and 4.02, 4.44, 5.11Ma from Dive 299).[21] The frequency distributions of the ages of thetholeiites show a large cluster (16 samples) withages 3.9?4.5 Ma, and a smaller cluster (3 samples)spans ages of 4.75?5.4 Ma (Figure 8). The oldestages (>5 Ma) were obtained for samples fromdives 297 and 299 on the central and western partof SKS (Figure 2). These areas are south anddownslope of the gap in Kaua?i?s ancient shoreline(Figure 2), and thus could well contain debrisfrom Kaua?i. The oldest rock (5.40 Ma, dive 297,sample 26) was sampled from a loosely stratified,alluvial deposit near the end of a submarine chan-nel that originated at the outlet of the WaimeaRiver. This sample is therefore our most-likelycandidate for being from Kaua?i. The SKS tholei-ite ages overlap with the ages for the Kaua?i shield(3.6?5.1 Ma [McDougall, 1979; Garcia et al.,2010]), Ni?ihau (4.3?6.3Ma) and W. Ka?ena (2.9?4.9 Ma), but extend to younger ages than Ni?ihauand older ages than W. Ka?ena.[22] The SKS alkalic lavas are much younger(0.08?1.9 Ma, Figure 8), indicating a 1.8 Myr hi-atus in volcanism after the tholeiites. This age gapand the alkalic compositions, together, are consist-ent with these rocks representing a secondary vol-canic phase.7. South Kaua?i Swell Volume[23] Size is a critical characteristic of the SouthKaua?i Swell. The volume of SKS was computedbased on the border of the swell shown in Figure 2using two methods. Method 1 is most appropriatefor landslides and method 2 for shield volcanoes.We also estimated the volumes of large Hawaiianlandslides and near-by shield volcanoes (bordersshown in Figure 1) for comparison.[24] Method 1 is appropriate for landslides thatformed on top of sediments infilling the flexuralmoat [Watts and ten Brink, 1989] surrounding theoriginal shield volcanoes. We estimated the vol-ume between the seafloor and a flat base at a depthof 4.6 km, which corresponds to the depth of theabyssal seafloor just outside of the flexural moatsof Kaua?i. Method 2 is more appropriate for ashield volcano that grew directly on top of the pre-existing seafloor. Following Robinson and Eakins[2006], we approximated the surface of the pre-existing seafloor as that of an elastic lithosphericplate with an effective thickness of 35 km beingflexed downward by the weight of the island chain.We used a submarine crustal density of 2700 kg/m3 and a subaerial crustal density of 2400 kg/m3(same as those used for computing the completeBouguer anomaly). This model predicts a flexedsurface that closely matches the shape of the base-ment of the pre-existing oceanic crust on the flanksof the island of O?ahu as was modeled and seismi-cally imaged by Watts and ten Brink [1989] andused by Robinson and Eakins [2006]. Then, weremoved 0.5 km [Robinson and Eakins, 2006]above the modeled pre-existing basement toaccount for the pre-existing pelagic sediments thatare not part of the volcanoes.[25] To verify our methods, we compared our vol-ume estimates of other edifices in the area withFigure 7. Isotope compositions of lavas from SKS (trian-gles) compared to of tholeittic basalts from near-by volcanicprovinces. Data sources: SKS [Swinnard, 2008]; West ofKa?ena [Greene et al., 2010]; Wai?anae [Coombs et al.,2004]; Kaua?i [Garcia et al., 2010]. The portion of Koloafield that lies within the plot is outlined (black); the wholefield spans 18.0895-18.6136 and 0.702959-0.703248 in206Pb/204Pb and 87Sr/86Sr, respectively [Garcia et al., 2010].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592337Table1.K-ArDatingResultsSamplenameSamplesiteRocktypeAlterationaK2O/P 2O5LOIK2O(wt.%)LabSamplewt.(g)40Ar/36Ar38Ar/36Ar40Ar/36ArInit.b40Arrad.(108cm3STP/g)40Arair(%)Uncorrectedagec(Ma)62Correctedage(Ma)62297-9coneBtholeiiteB21.880.160.50Kyoto0.703359.761.00.186160.0022293.067.06.960.881.44.0860.164.2460.46297-18coneCtholeiiteB21.520.070.32Kyoto0.615332.160.80.185660.0017291.565.74.660.787.83.9160.164.3460.64297-20coneDbasaniteA22.710.610.74Kyoto0.606304.963.50.189360.0106302.7634.80.162.099.20.2360.080.0560.84JAEA0.597300.660.70.186260.0011293.463.70.560.397.60.1660.020.2360.12weightedmeancorrectedagefromtwoexperiments:0.2360.12297-21coneDbasaniteA22.430.600.73Kyoto0.602299.860.80.186660.0016294.565.30.660.698.20.2060.040.2560.25Kyoto0.627298.960.70.187360.0017296.865.70.360.899.30.2160.040.1360.36Kyoto0.705298.860.70.187160.0017296.065.40.460.899.10.1960.040.1660.32weightedmeancorrectedagefromthreeexperiments:0.1960.18297-24coneDbasaniteA21.411.200.50Kyoto0.614297.661.00.187860.0030298.269.80.1262.4100.20.3260.140.08615.4JAEA0.128297.760.80.187460.0016297.065.40.161.099.80.2760.100.0860.70297-26AreaEtholeiiteB21.590.120.63Kyoto0.202332.361.00.186860.0022295.267.411.162.288.85.3460.225.4061.10299-29SiteCtholeiiteB21.790.170.50Kyoto0.550334.261.8295.55.960.388.43.6560.20Kyoto0.455333.060.6295.56.960.286.74.2860.16weightedmeanuncorrectedagefromtwoexperiments:4.0360.12KS-2dredgebasaniteA13.551.631.67Kyoto0.731384.561.60.186960.0016295.665.010.160.676.91.8660.061.8660.12KS-3-1dredgetholeiiteB11.810.500.48Kyoto0.740410.161.60.188760.0018301.065.66.160.473.44.0560.123.8660.24A1A2B1B2FreshnessofolivinephenocrystfreshfreshminordminordSecondarymineralsinvesiclesnopresentnopresentFreshnessofgroundmassolivinefreshfreshfreshfreshPreferredageinbolda Alterationcriteriabythinsectionobservation.bInitial40Ar/36Arcalculatedfrom38Ar/36Arassumingmassfractionation.c Massfractionationuncorrected.dMinoroxidationatthemargin(<10%ofthediameter).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592338previously published estimates. Method 1 pro-duces a volume for the Wailau debris avalanche of1.1 x 103 km3 (Table 3), which is consistent withestimates by Satake et al. [2002] (1.56 0.5 x 103km3), Robinson and Eakins [2006] (1.6 x 103km3), and Moore et al. [1989] (1 x 103 km3). Forthe Nu?uanu debris avalanche, our estimate of 2.7x 103 km3 compares favorably with estimates bySatake et al. [2002] (3.56 0.5 x 103 km3) andRobinson and Eakins [2006] (2.4 x 103 km3), butis less than that by Moore et al. [1989] (5 x 103km3). An important difference is that Moore et al.[1989] attributed a 0.9 km thick layer within thesediments that infill O?ahu?s flexural moat as partof the Nu?uanu debris avalanche (based on seismicevidence [Watts et al., 1985; Brocher and tenBrink, 1987]), whereas Satake et al. [2002] and wedo not. For the Mahukona, Ko?olau, Wai?anae,and Kaua?i shield volcanoes, method 2 producesvolumes that match those of Robinson andEakins?s [2006] (Table 3) within 8%. The aboveconsistencies lend confidence to our methods ofestimating volumes of landslides (method 1) andshield volcanoes (method 2).[26] For the SKS, method 1 yields a volume of 6 x103 km3. This volume is greater than our estimatesTable 2. Summary of 40Ar/39Ar Dating ResultsaSample no. Location K/Ca totalTotal fusion IsochronNPlateauAge (Ma)6 2 40Ar/36Ari6 2 MSWD Age (Ma)6 239Ar % MSWD Age (Ma)6 2252-04 Cone A 0.13 4.286 0.14 296.46 2.1 0.65 4.236 0.14 8 of 8 100.0 0.59 4.296 0.12252-10 Cone B 0.18 4.336 0.16 297.66 2.9 0.64 4.246 0.10 8 of 8 100.0 0.62 4.296 0.12297-04 Cone B 0.18 4.056 0.24 297.06 4.0 0.13 3.786 0.66 8 of 8 100.0 0.14 4.036 0.22297-09 Cone B 0.15 4.066 0.18 287.06 6.5 0.85 4.596 0.34 8 of 8 100.0 1.00 4.146 0.140.18 4.126 0.14 292.36 4.4 0.42 4.406 0.25 7 of 8 97.2 0.43 4.226 0.12Weighted mean plateau age from two experiments: 4.196 0.09298-12 Cone D 0.20 4.016 0.15 297.16 3.7 0.83 3.796 0.46 8 of 8 100.0 0.74 4.006 0.13298-20 Cone D 0.13 4.606 0.18 295.66 1.3 1.28 4.746 0.10 8 of 9 96.7 1.10 4.756 0.110.11 4.836 0.16 296.76 0.9 0.94 4.756 0.09 10 of 10 100.0 1.04 4.846 0.13Weighted mean plateau age from two experiments: 4.796 0.08299-04 Site A 0.08 4.536 0.32 295.06 0.9 0.61 4.666 0.17 10 of 10 100.0 0.58 4.606 0.260.08 4.396 0.30 295.76 1.0 1.42 4.446 0.16 9 of 9 100.0 1.25 4.466 0.22Weighted mean plateau age from two experiments: 4.516 0.16299-08 Site A 0.07 4.346 0.35 295.16 2.9 0.95 4.496 0.50 10 of 10 100.0 0.85 4.426 0.280.13 4.376 0.29 296.76 1.9 0.72 4.216 0.35 9 of 9 100.0 0.69 4.426 0.25Weighted mean plateau age from two experiments: 4.426 0.18299-15 Site A 0.10 4.116 0.20 294.16 5.8 1.29 4.286 0.43 10 of 10 100.0 1.16 4.176 0.16299-20 Site B 0.13 5.266 0.15 296.26 2.0 0.35 5.086 0.11 6 of 8 93.7 0.30 5.126 0.120.18 5.196 0.17 295.96 1.8 0.16 5.096 0.12 6 of 8 96.8 0.14 5.116 0.14Weighted mean plateau age from two experiments: 5.116 0.09299-21 Site B 0.09 4.486 0.17 296.36 0.4 0.79 4.406 0.07 10 of 10 100.0 1.04 4.446 0.13299-23 Site B 0.13 4.026 0.16 296.36 3.3 0.28 3.996 0.13 8 of 8 100.0 0.25 4.026 0.12299-29 Site C 0.17 4.036 0.15 295.06 2.3 0.22 4.086 0.27 10 of 10 100.0 0.20 4.026 0.13299-33 Site D 0.11 5.106 0.15 294.86 1.0 0.92 5.136 0.10 8 of 8 100.0 0.87 5.076 0.13KS-01?18 Site A 0.09 4.156 0.26 295.96 2.0 0.50 4.166 0.22 8 of 8 100.0 0.44 4.206 0.19aAges calculated relative to 28.201 Ma for the Fish Canyon sanidine [Kuiper et al., 2008] using decay constant of Min et al. [2000]. Uncertain-ties reflect 2 analytical uncertainties. Preferred age in bold.Figure 8. Histograms of age dates obtained from SKS andthe adjacent edifices (ages in 0.25 Myr bins on the vertical axisand number of samples on the horizontal), superimposed withprecise ages (symbols, horizontal position is arbitrary) and 2errors. Black dots mark K-Ar dates for the SKS samples. Eachhistogram is positioned horizontally according to distancesbetween the centers of each feature and the summit of Kilauea,projected along a trajectory of the current Pacific Plate motion[Gripp and Gordon, 2001]. Dates of Kaua?i?s post-shield arecircled. Data sources: Kaua?i [McDougall, 1964, 1979; Clagueand Dalrymple, 1988; Garcia et al., 2010]; W. Ka?ena[Greene et al., 2010]; Ni?ihau [Sherrod et al., 2007].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592339for Nu?uanu and Wailau landslides combined (3.8x 103 km3), and is comparable to our estimate forthe Wai?anae Slump (5.8 x 103 km3, Table 3).Method 2 yields a volume for SKS of 146 3.4 x103 km3. This volume is comparable those ofKa?ena Ridge (14.86 2.0 x 103 km3) and the smallshield volcano, Mahukona (13.06 1.0 x 103 km3,Table 3). The two estimates for SKS are 14%(method 1 for landslides) and 32% (method 2 forshields) of the total volume of Kaua?i without SKS(43.56 7.9 x 103km3, area shown in Figure 1).8. Origin of South Kaua?i Swell[27] Secondary volcanism, landslides, and shieldvolcanism are the three main processes that createthe submarine features of the Hawaiian chain. Allthree processes have also played a role in the evo-lution of the SKS. Each is discussed and evaluatedin terms of its relative importance in the formationof the SKS beginning with the most recent process.8.1. Secondary Volcanism[28] Lavas from secondary volcanism contrastwith shield lavas in generally being more silicaundersaturated, containing higher abundances ofincompatible elements, originating from moredepleted sources [e.g., Fekiacova et al., 2007] andhaving younger ages (by 0.6?2.0 Myr) than theassociated shield lavas [Ozawa et al., 2005]. Onland, secondary volcanism is often referred to as??rejuvenated volcanism?? because it overliesshield volcanism; it is usually separated from theshield or postshield phase by a thick soil and/orsedimentary sequence [e.g., Macdonald et al.,1983]. In the submarine environment, secondaryvolcanism is generally associated with high-acoustic backscatter, indicating relatively thinsediments on these younger lavas [Lipman et al.,1989; Clague et al., 1990; Dixon et al., 2008;Greene et al., 2010].[29] The South Kaua?i Swell displays characteris-tics that differ from those of secondary volcanism.The relatively thick sediments overlying the vol-canic rocks as detected by the low-acoustic back-scatter and seen in JASON imagery contrast withthe observations of known submarine secondaryvolcanism. In addition, samples collected byJASON from the SKS have variable but com-monly thick Mn rinds (up to 14 mm) indicatingresidence on the ocean floor for several millionyears [e.g., Moore and Clague, 2004]. The excep-tions are the relatively rare alkalic SKS lavas (col-lected at two sites), which have thin or no Mnoxide coatings. These alkalic lavas have geochem-ical (Figures 6 and 7) and age (0.08?1.86 Ma, Fig-ure 8) characteristics of secondary volcanism, thusrevealing that a few of the seamounts on SKSare probably secondary volcanic. However, theTable 3. Estimated Volumes in Units of 103 km3 Ordered Smallest to LargestEdifice (outlines shown in Figure 1)Volume estimatesappropriate for landslidesaVolume estimatesappropriate for shield volcanoesbEstimates of Robinsonand Eakin?s [2006]Wailau Slide 1.1 (8.56 2.0) 1.6S. O?ahu Volcanic Field (2.1) 9.66 3.7Nu?uanu Slide 2.7 (13.46 4.9) 2.4Mahukona Volcano (4.7) 13.06 1.0c 13.5Wai?anae Slump 5.8 (15.96 2.0)South Kaua?i Swell 6.0 14.06 3.4Ka?ena Ridge 14.86 2.0Ko?olau Volcano without Nu?uanu Slide 34.36 2.9 31.7Wai?anae Volcano with Wai?anae Slumpand without Ka?ena Ridge36.86 3.9Kaua?i without S. Kaua?i Swell 43.56 7.9Ko?olau with Nu?uanu Slide 47.76 7.6Wai?anae with Ka?ena Ridge andWai?anae Slump51.66 5.8 52.9Kaua?i with S. Kaua?i Swell 57.56 11.0 57.6aVolume above a flat abyssal seafloor at a depth of 4.6 km.bVolume between the seafloor and the pre-existing oceanic basement, which flexes downward beneath the islands in the shape of elastic plate(see text). Uncertainty is based on an uncertainty of 60.5 km of the depth of basement. Adding ?0.5 km to the depth puts the basement near thatimaged seismically by Watts and ten Brink [1989] near O?ahu. The shown volume estimates are based on the assumption that 0.5 km of pre-existing pelagic sediment lie between the pre-existing basement and each edifice following Robinson and Eakin?s [2006].cMethod 2 assumes the base of Mahukona is the surface of the pre-existing seafloor, flexed downward beneath the island chain. Here the startingpoint of this surface is the seafloor outside the flexural moat of the Island of Hawaii at a depth of 4.6 km. Garcia et al. [2012] assumed the samestarting depth but used a point on the southern margin of Mahukona, which is well within the flexural moat, thus producing a volume estimate of 6x 103 km3.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592340dominance of tholeiitic lavas from the othersampled seamounts and their older ages indicatethat most of the SKS is not related to secondaryvolcanism. The bulk of the SKS was derived fromshield volcanism.8.2. Landslide Origin[30] Giant landslides are common on and aroundoceanic volcanoes worldwide (e.g., Canary Islands[Watts and Masson, 1995; Masson et al., 2002],Cape Verde [Ancochea et al., 2010], La Reunion[Oehler et al., 2008], Aleutians [Coombs et al.,2007], Stromboli [Romagnoli et al., 2009]); andsuch was the original interpretation of the SKS[Holcomb et al., 1988; Moore et al., 1989, 1994].Submarine landslides are usually associated withoversteepened flanks of volcanoes. Steep headscarps and bounding walls commonly formamphitheater-shaped scars, which are the sourceregion of the landslides. Landslides can occur atany time during the evolution of Hawaiian volca-noes: the preshield stage (Lo?ihi seamount, [Malah-off, 1987; Fornari et al., 1988; Moore et al.,1989]), the shield stage (South Kona slide; [Mooreet al., 1994]), and after most of the volcano hasformed (East Moloka?i [Moore et al., 1994]).[31] Two types of massive landslides are commonon oceanic islands: debris avalanches and slumps[Moore et al., 1994]. Debris avalanches are consid-ered to be catastrophic events creating debris fieldsof giant (tens of kilometers wide) blocks or smallerblocks (tens to hundreds of meters wide) and hum-mocky topography [Moore et al., 1994]. As seenaround the Canary Islands, La Reunion and Strom-boli, hummocky debris avalanches often display aconcave-up topographic profile that is steepest nearthe head wall and flattens with distance away fromthe volcano, eventually merging asymptoticallywith the abyssal seafloor [Watts and Masson, 1995;Urgeles et al., 1999; Coombs et al., 2007; Romag-noli et al., 2009]. Hawaiian debris avalanches rep-resent some of the largest debris avalanches in theworld [Hampton and Lee, 1996]. The Nu?uanuslide, for example, extends 150 km away fromO?ahu and is composed of intact blocks up to 35km long by 18 km across and 1.5 km tall [Garciaet al., 2006]. In contrast, the smaller South Kona,?Alika 1 & 2, and Clark slides are examples ofmore hummocky deposits made up of smaller (hun-dreds of meters or less across) and more uniformlysized fragments [Moore et al. 1994]. The ?Alika 1& 2 slides display well-defined chutes, bounded bylevees [Moore et al., 1994]. In contrast to debrisfields, slumps (e.g., Wai?anae and Hilina slumps,Figure 1) are characterized by deeply rooted,mostly intact blocks of flank material that slide epi-sodically over geologic time [Moore et al., 1989,1994; Hampton and Lee, 1996].[32] The SKS displays some characteristics ofboth debris field and slumps, but fails to conformfully to either model. In support of a landslide ori-gin, the rounded southern border of the SKS inmap view is not unlike the distal outline of hum-mocky debris avalanches among other islandchains (e.g., Canaries [Watts and Masson, 1995;Masson et al., 2002], La Reunion [Oehler et al.,2008], Aleutians [Coombs et al., 2007]). In addi-tion, the SKS has relatively smooth, long-wavelength topography that is populated withnumerous small seamounts producing a hum-mocky surface (Figures 1?3), superficially resem-bling the deposits of an ?Alika 2-type debrisavalanche [Moore et al., 1989]. The most compel-ling evidence for a landslide are the lack of clearlyinsitu pillow lavas where the tholeiitic sampleswere obtained, the highly vesicular lava samples,as well as the diversity of ages of tholeiites foundin close proximity to each other, sometimes on thesame seamount. These findings indicate thatextensive erosion and material transport was im-portant to the evolution of the SKS.[33] A number of characteristics of SKS, however,are contradictory with those of other landslides. Interms of its geomorphology, the SKS has a convexsurface and meets the abyssal seafloor with a dis-tinct break in slope (Figure 4), which contrast withthe form of most debris avalanches near otherocean islands as discussed above. While the notedhummocky surface resembles that of the ?Alika 2avalanche, the SKS differs significantly in itsmuch larger scale (6700 km2 versus 1700 km2 inarea [Moore et al. 1989]) and having larger sea-mounts (median width of 700 m) than the debrisof the Alika 2 avalanche (again, widths typically102 m or less). Volumetrically, if the SKS were adebris avalanche, it would represent an extremeend-member: the estimated volume above theabyssal seafloor depth of 6.0 x 103 km3 (Table 3)is larger than that of the Nu?uanu slide (2.7 ? 5 x103 km3), and three to six times the volumes of thelargest debris avalanches of the Canary Islands[Masson et al., 2002]. Only two debris avalancheson Earth have been estimated to be comparable orgreater in volume: the Storegga slide offshoreNorway at 5.6 x 103 km3 [Bugge et al., 1988] andthe Agulhas slide off South Africa at 20 x 103 km3[Dingle, 1977].ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592341[34] Relative to typical slump deposits, the 2?2.5km thickness of SKS over a broad area comparesfavorably with the thickness and extent of, forexample, the Wai?anae slump. However, the SKScontrasts with Hawaiian slumps (or the Nu?uanu orWailau avalanches for that matter) by its lack oflarge angular blocks (Figures 1 and 4), the absenceof a prominent head scarp, and the presence of arounded, rather than an irregular margin with thesurrounding seafloor. If the SKS originally con-sisted of one or more largely intact blocks, theseblocks would have had to be extensively erodedand the area between them filled in order to erasetheir original blocky morphology, leaving the rela-tively smooth, convex-up morphology and roundedmargin of the SKS. The central axis and side lobesthat produce the SKS?s subtle pinnate morphologyis especially troublesome to explain with either adebris avalanche or a slump origin.[35] The possible source regions for a SKS landslideare Ni?ihau?s east flank and Kaua?i?s south flank[Moore et al., 1989]. Both areas show evidence ofcollapse based on gaps in the original shorelines ofeach island?s shield volcano (Figures 2 and 9). Typi-cally, the margins of the submarine platforms sur-rounding the Hawaiian Islands have slope-breaks,marking the most distal shorelines that formed dur-ing the shield-building volcanic phase [Mark andMoore, 1987]. Gaps in these slope breaks aroundNi?ihau and Kaua?i were mapped by Flinders et al.[2010] (Figures 2 and 9a). Around Ni?ihau, the vol-cano?s missing eastern paleo-shoreline is the proba-ble location where a large collapse occurred[Stearns and Macdonald, 1947]. Most of Ni?ihau?ssoutheast shoreline, however, is probably intact andcan be traced to a point just below the most westernextent of Kaua?i southern shoreline (Figure 2).Thus, in order for material from eastern Ni?ihau tobe part of SKS, a collapse would have had to occurearly enough in Ni?ihau?s shield stage for the south-east shoreline to rebuild (Figure 10a). About half ofthe dated samples from SKS are old enough (4.3?5.4 Ma) to have come from the Ni?ihau shield (4.3?6.3 Ma; [Sherrod et al., 2007]), however, the otherhalf of the dated SKS tholeiites are probably tooyoung (3.9?4.2 Ma).[36] A generous estimate for the volume of miss-ing material from east Ni?ihau is 103 km3 (dimen-sions of 35 km N-S along the paleo-shorelineseast of Ni?ihau, 35 km between Ni?ihau andKaua?i, and 1 km in average thickness). Thisestimate is only 17% of the volume of SKS abovethe in-filled moat sediments (Table 3). This dis-crepancy is aggravated by the fact that it is impos-sible for all of east Ni?ihau to have collapsed tothe SSE (Figure 10a): the southeast flank mayhave, but not the northeast flank. Hence, onlyabout half of Ni?ihau?s missing flank potentiallycould have contributed to the SKS. Furthermore, theprominent, eastward-dipping scarp on Ni?ihau?snortheast flank suggests that the landslide from thisarea traveled to the east (and now underlies or ispart of Kaua?i [Flinders et al., 2010], or to the north-east, forming the debris field presently located northof Kauai [Moore et al., 1989] (Figures 10a and10b). Therefore, it is unlikely that east Ni?ihau con-tributed significantly to the volume of SKS.[37] This leaves Kaua?i as the main landslidesource. Indeed, a 30 km wide gap in Kaua?i?ssouthern paleo-shoreline (Figures 2 and 9a) indi-cates that a portion of south Kaua?i has experi-enced mass wasting. To evaluate whether themissing volume of Kaua?i?s south flank matchesthat of the inferred debris deposits, we recon-structed the area of the SKS prior to the hypothe-sized landslide. This was done following themethods of Satake et al. [2002]. First, the bathym-etry points within the SKS border were deletedfrom the bathymetry grid. Second, several controlcontours were placed across the gap in data points,connecting with the real contours on either side ofthe SKS. The location of the pre-SKS shorelinewas estimated by visually interpolating a smootharc through the gap in the paleo-shoreline. Third,from the data surrounding the gap and the controlpoints within the gap, a smooth surface was inter-polated to fill the gap. The interpolation was doneusing the ??surface?? routine of the GMT softwarepackage [Wessel and Smith, 1995], which com-putes a continuous curvature spline in tension. Thetension parameter was varied in numerous runsuntil a geomorphologically reasonable preswellsurface was attained (Figure 9b).[38] Subtracting the bathymetry of the reconstructedseafloor from the present-day bathymetry yields avolume of landslide debris of 2.9 x 103 km3 (i.e.,the volume SKS as landslide above the infillingmoat sediments if their surface shoaled towardKaua?i, rather than remained flat at 4.6 km asassumed for method 1). The missing volume ofKaua?i?s flank is computed based on the differencein topography between the reconstructed andpresent-day flank (between Kaua?i and the northernboundary of the SKS, mark in red in Figures 9a and9b), and is 84 km3. This missing volume fromsouth Kaua?i represents only 3% of the inferred vol-ume of the debris.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592342[39] To adequately account for the volume of theSKS by a landslide, the prefailure shoreline alongKauai?s south shore would have to display a far dif-ferent and geologically problematic morphology(Figure 9c). The prefailure shoreline would need toprotrude 30?40 km south of Kaua?i?s current shore-line. While such a protrusion could have representeda broad rift zone extending from the Kaua?i shieldvolcano, land and marine gravity surveys on southKaua?i [Flinders et al., 2010] show no evidence forthe remnants of dense cumulate material within thecore of the rift zone as is detected beneath other Ha-waiian rift zones [Kauahikaua et al., 2000]. Instead,the observed gravity anomaly decreases continu-ously from a peak value near the center of Kaua?i,southward through Kaua?i?s southern shore, andreaching a minimum in the area of the hypotheticalprotrusion in this geologic reconstruction (Figure 5).The crust below the area of the hypothesized protru-sion has a low, rather than a high density. In addi-tion, generating SKS from such a protrusionrequires most of the debris to have travelled southand southeast, roughly parallel to the long axis ofthe protrusion. This behavior is counter that of mostknown flank failures, whereby the run-out tends tobe perpendicular to established rift zones [Swansonet al., 1976; Moore et al., 1989; Smith et al., 1999].[40] Another possibility that would not require sucha large protuberance invokes one or more landslidesthat incised deeper into the interior of the islandand removed a wider portion of Kaua?i?s southflank (Figure 10b). Subsequent to these events, thesame flank would have been reconstructed byFigure 9. Perspective views of (a) existing bathymetry (b) reconstructed bathymetry in the absence of SKS,with Kaua?i?s paleo-shoreline interpolated between the identified paleo-shorelines (Figure 2), and (c) recon-structed bathymetry with Kaua?i?s paleo-shoreline protruding southward so that it would account for the fullvolume of the SKS above the shown abyssal seafloor. Contour interval is 0.5 km and vertical exaggeration is3:1 (a1) Close-up and (a2) profile (location marked by A-A? of inset map) of Kaua?i?s existing southern flank.(b1) Close-up and (b2) profile of Kaua?i?s southern flank reconstructed in (b). No vertical exaggeration in (a2)and (b2).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592343volcanism, a new paleo-shoreline would haveformed, and then one or more small landslideswould have created the present-day gap in thepaleo-shoreline. Temporally, the main landslide(s)must have occurred late enough during shield-building to construct a sufficient volume, but earlyenough for subsequent volcanism to rebuild Kauai?ssouth flank and create the island?s roughly circularplanform that exists today. A difficulty with thispremise is that the overlap in ages between the SKStholeiites (3.9?5.4 Ma) and the Kaua?i shield stage(3.6?5.1 Ma) allows for little time for the hypothe-sized reconstruction. Such a massive reconstructionof the south flank of Kauai has also not been recog-nized in the geology of Kaua?i [Garcia et al.,2010], although a structural trough did form on thesouthwest flank of Kaua?i near the end of shieldvolcanism (4.0 Ma [Macdonald et al., 1960]).[41] Ultimately, the most fundamental problemfor a landslide origin of the SKS is the lack of asuitable source region. It would be extremely in-congruous for one of Earth?s largest submarinelandslides to have its amphitheater scar almostcompletely filled by subsequent volcanism. Insummary, the lack of a source, the large size ofthe SKS, and the deviations in morphology fromother submarine debris avalanches and slumpslead us to conclude that the main volume ofSKS is probably not a landslide deposit.8.3. The SKS as a Low-Relief ShieldVolcano[42] An alternative origin for the SKS is a low-relief shield volcano. Perhaps most contradictory toFigure 10. Illustrations of hypothesized origins of the SKSoverlain on existing bathymetry (contoured at 500 m). Smallred symbols mark sample locations as in Figure 2. Large redovals mark gravity highs over the inferred magmatic centers ofKaua?i and Ni?ihau [Flinders et al., 2010]. Small yellow sym-bols and short dashed curves show mapped slope breaks, inter-preted as paleo-shorelines of Ni?ihau (red) and Kaua?i (blue)[Flinders et al., 2010] (see also Figure 2). Long dashed curvesare hypothetical paleo-shorelines that have been destroyed bylandslides on the south flank of Kaua?i. (a) SKS is composed ofapproximately half of the collapsed mass of east Ni?ihau (lightblue), overlain by debris from Kaua?i?s missing southern paleo-shoreline (dark blue). The other half of Ni?ihau?s east flank col-lapsed northeast (light green) (b) SKS is composed of depositsfrom a massive sector of the south flank of Kaua?i (light blue).Kaua?i?s southern flank was then rebuilt by volcanism, andlater experienced a small collapse on to the SKS (dark blue).Figures 10a and 10b are unlikely as discussed in the text. (c)Most likely, the SKS is an elongate, low-relief shield volcanothat never reached sea level (light brown). It was later partlycovered with the small volume of debris from Kaua?i?s missingsouthern paleo-shoreline (dark blue).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592344a shield origin, is the evidence provided by the ge-ology, vesicularity, and irregular age distributionfor widespread mass wasting. A massive landslideorigin is a simple explanation, but an alternative ex-planation is one or more relatively small landslidesfrom Kaua?i?s missing southern shoreline, whichveneered a large area of a SKS shield volcano. Am-biguous evidence for a shield origin includes thegeochemistry (Figures 6 and 7) and the range ofages (Figure 8) obtained from the SKS. These dataare consistent with either a massive landslide fromKaua?i, or a separate shield of similar compositionand age to Kaua?i, perhaps partially overlain with asmall amount of material from Kaua?i.[43] Weakly supportive of a low-relief shield isthe residual gravity anomaly. On one hand, thelack of a strong positive residual gravity anomalyover the SKS (Figure 5) could be consistent with alandslide origin. While on the other hand, thisfinding is also consistent with a low-relief shieldvolcano, in which magmatism was not stronglyfocused to a central accumulation zone but wasinstead more distributed. A more distributed mag-matic plumbing system, in fact, may be expectedfor such a broad, low-relief volcano. Mahukona isa taller, but still low-relief feature that also doesnot display a large gravity high over its summit[Garcia et al., 2012] (the gravity high over thesouthern part of Mahukona (Figure 5) is largelyattributed to a submarine extension of Hualalai?srift zone). The negative gravity anomaly separat-ing the high over Kaua?i from the low-amplitudehigh over the SKS is, again, difficult toexplain with a landslide origin. Instead, this obser-vation is consistent with Kaua?i?s south flankbeing composed of low-density debris fromKaua?i, and most of the volume of SKS compris-ing higher-density, intact lava due to shieldvolcanism.[44] The most supportive evidence of a shield ori-gin is the overall morphology and size. This evi-dence includes the smooth long-wavelengthtopography, convex surface, and large continuousheight of the SKS, which resembles parts ofKa?ena Ridge and Mahukona (Figures 1 and 4).The 14 x 103 km3 volume of the hypothesizedSKS shield volcano as measured from the top ofthe pre-existing seafloor is comparable to somesmall Hawaiian volcanoes (e.g., Mahukona, 13 x103 km3 and Ka?ena Ridge, 15 x 103 km3 fromTable 3; West Maui, 9 x 103 km3 and Hualalai 15x 103 km3 from Robinson and Eakins [2006]). Theproposed SKS volcano is lower in relief thanthe other Hawaiian volcanoes but higher than theSouth West O?ahu Volcanic Field (SWOVF).[45] The existence of a SKS shield would alsoreduce the large distance between adjacent shieldvolcanoes represented by Wai?anae and Kaua?i.Without a SKS shield, the spacing between thecenters of Wai?anae and Kaua?i is 140 km ornearly twice the typical spacing of 72637 kmbetween adjacent Hawaiian shield volcanoes [tenBrink, 1991]. With the SKS shield, the averagespacing between the three shields is closer to thetypical spacing (average of 90 km with 130km between the SKS and Wai?anae and 50 kmbetween SKS and Kaua?i). If Ka?ena Ridge is alsoa separate volcano, the average spacing betweenthe four volcanoes also fits with the typical spac-ing (average of 63 km based on distances of 50km, 100 km, 50 km for Wai?anae-Ka?ena,Ka?ena-SKS, SKS-Kaua?i, respectively).[46] In summary, although there is no evidencerequiring a shield origin, this explanation has theleast profound contradictions with observationsand employs the most straightforward geologicprocesses as presently understood. We thereforesuggest that a low-relief shield is the most likelyorigin for the major (>90%) volume of the SKS.[47] Figure 10c illustrates the model of most of theSKS forming as low-relief shield volcano. Thismodel has the construction of the SKS shield (3.9?5.4 Ma, Tables 1 and 2) overlapping with the mid-to-late shield phase of Ni?ihau (4.3?6.3 Ma; Sher-rod et al. [2007]) and the shield phase of Kaua?i(4.0?5.1 Ma; McDougall [1979]; Garcia et al.[2010]). The SKS shield was later partially over-lain by a small volume of debris from Kaua?i, asindicated by the gap in Kauai?s southern paleo-shoreline. These debris contributed to some of thetopography between the SKS and Kaua?i and tothe negative residual gravity in this area. Finally, afew monogenetic, alkalic seamounts formedbetween 1.9 and 0.2 Ma during a secondary vol-canic phase.[48] As a shield volcano, the SKS would be veryunusual on Earth given the combination of itsappreciable area (6700 km2), low relief (2?2.5km) and thus low slope (< 1.5, Figure 4). Bycomparison, Mauna Loa on the Island of Hawaiihas slopes of 5?10 on land and slopes up to 18offshore. Iceland is well known for having a num-ber of small, low-slope volcanoes (29 documentedby [Rossi, 1996] with a median slope of 2.7).ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592345These Icelandic volcanoes, however, are muchsmaller (diameters of 0.5?11 km, heights of 12?520 m) and monogenetic, thus probably represent-ing a different type of volcano than a SKS shield,the latter of which presumably would be composedof many eruptive events. The feature most similarto the SKS that we know of on Earth is the SWO?ahu Volcanic Field (slope 0.5), whereas theareally extensive lava flows north of O?ahu andsoutheast of the Island of Hawai?i [Lipman et al.,1989; Clague et al., 1990] may represent the mostextreme examples of volcanism over a broad areawith little, or in this case, no relief.[49] While unusual for Earth, large shield volca-noes with such low slope, in contrast, are morecommon on other planets. The largest shield-likeedifices on Venus, for example, have lateraldimensions (hundreds of km) a few times greaterthan the SKS, and slopes (< 1?3) [Stofan et al.,2001] comparable to the SKS. On Mars, the tallestvolcanoes are much steeper than the SKS, but atleast twenty Martian volcanoes have areas of thesame order (103 km2), and slopes much lower thanthe SKS [Baratoux et al., 2009]. A swarm of about30 volcanoes on Syria Planum, identified byBaptista et al. [2008], have smaller areas(80?1400 km2) than, but comparable slopes (0.2?1) to the SKS. This comparison leads us to ques-tion whether there are some general physical con-ditions promoting the formation of broad, low-slope shields that occurred on Mars and Venus,were active at the Hawaiian hotspot during the cre-ation of the SKS, but otherwise rarely occur onEarth. That said, few terrestrial oceanic volcanochains are as well surveyed as the HawaiianIslands, so there may be other edifices like SKSthat are as yet not recognized. A shield origin forthe SKS would imply that shield volcanism inHawaii, and on Earth in general, spans a range ofsizes and shapes from the large island-buildingshields, to smaller edifices protruding from theislands such as Mahukona and Ka?ena Ridge, andfinally, to even lower-relief volcanoes representedby the SKS and possibly the SW O?ahu VolcanicField.9. Conclusions[50] The SKS is a 110 km x 80 km ovoid bathy-metric feature with a convex surface, punctuatedwith numerous small (<1 km wide) seamounts.Most of the SKS has a low acoustic backscatterindicating relatively thick sediment cover as con-firmed by JASON dive images. The residual grav-ity over SKS is negative on the very northern partof the SKS and neutral or slightly positive over thecentral portion. Lavas from two of the seamountssampled are alkali basalts with ages of 0.2?1.9Ma, and thus represent a phase of secondary volca-nism. The majority of the SKS samples are tho-leiitic and have ages of 3.9?5.4 Ma, which arecoeval with Ni?ihau?s mid-to-late shield phase andKaua?i?s shield phase. The SKS tholeiites have87Sr/86Sr and 206Pb/204Pb compositions similar tothose of Kaua?i, West Ka?ena, and Wai?anae.[51] A landslide origin, as originally proposed, isproblematic. Morphologically, the SKS is unlikeany other landslide of comparable size. The mostprofound discrepancy is that the estimated volumeof SKS above the surrounding seafloor (6 x 103km3) is greater than almost all other estimates forlandslides on Earth, however, there is no obvioussource region that could have housed this enormousvolume. A landslide origin of SKS requires subse-quent shield volcanism to nearly completely fill thescar of a massive sector collapse and to constructKaua?i?s circular planform?a requirement that isin conflict with the overlap in ages of Kaua?i andthe SKS tholeiites as well as the lack of geologicevidence on Kaua?i for a major sector collapse.[52] Among the three hypotheses that were eval-uated for the origin of the SKS, the low-reliefshield volcano model most readily explains thegeomorphologic and geophysical evidence. Theshield was later mantled by mass wasting eventsfrom Kaua?i, which created the gap in the southernpaleo-shoreline. Subsequently, a few isolated sec-ondary volcanic seamounts formed on the SKS,further complicating its geologic history. The largearea and low slope of the SKS make it a rather un-usual terrestrial shield volcano, although notunlike many volcanoes on Venus and Mars.Acknowledgments[53] The efforts of the crews of the R/V Kilo Moana andWHOI JASON ROV helped make the field campaign trulysuccessful. University of Hawaii undergraduates, LindsaySpencer and Kyle Taniguchi, are thanked for help with sam-ple preparation for geochronology and geochemistry. Com-ments by John Sinton and Peter Mouginis-Marks led to morecareful presentation in various parts of the manuscript. Wegratefully acknowledge the thorough and constructive reviewsby an anonymous reviewer, Michelle Coombs, and EditorJames Tyburczy, which led to substantial improvements. Thisresearch was supported by NSF grants EAR-0510482, EAR-1219955, and OCE-1155098. This is SOEST contribution#8933.ITO ET AL.: ORIGIN OF THE SOUTH KAUA?I SWELL 10.1002/ggge.201592346ReferencesAncochea, E., M. J. Huertas, F. Hernan, and J. L. 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At Kyoto University Argon isotope ratios were measured using a VG Isotech? VG3600 mass spectrometer operated in the static mode, connected to extraction and purification lines. Sensitivity of the mass spectrometer was determined by analyzing a known amount of the air standard, which was generally ~1.6 ? 107 V/cm3STP. Blank levels were less than 4.6 ? 10?9 cm3 STP for mass 40. No peak drift was observed during analyses. A VG Isotech? VG 5400 mass spectrometer was used at JAEA, and the sensitivity was generally ~4.7 ? 107 V/cm3STP and Blank levels were less than ~3.0 ? 10?9 cm3. Mass discrimination in the mass spectrometer was corrected assuming 40Ar/36Ar and 38Ar/36Ar of the air standard to be 295.5 and 0.1869, respectively [Matsumoto and Kobayashi, 1995]. The initial 40Ar/36Ar is calculated from measured 38Ar/36Ar assuming mass-dependent isotopic fractionation during rock formation. The air standard was analyzed each day and a hot blank was measured every five to ten samples. SORI93 biotite was used for calibration of the air standard. Errors for 40Ar, 40Ar/36Ar and 38Ar/36Ar were estimated from multiple analyses of the air standard, and were 2.0%, 0.2%?0.4% and 0.4%?0.8%, respectively. For measurement of potassium content, an aliquot of each sample was ground further and analyzed using a flame emission spectrometer, Asahi Rika FP-33D, operated in a peak integration mode with a lithium internal standard. Analytical error for potassium measurement is ~1%, estimated from standard deviation of multiple analyses of standard JB-3 and JA-2. See [Yamasaki et al., 2011] for additional information on the unspiked K-Ar dating method.  Results are given in Table A1. 2. University of Wisconsin 40Ar/39Ar Dating Methods 40Ar/39Ar incremental heating experiments were made on 16 South Kaua?i Swell lavas. Holocrystalline groundmass separates were prepared from porphyritic lava samples by crushing,  Appendix:  Ito et al. South Kaua?i Swell, revised for G-cubed 2013 2   sieving to 250-350 ?m, magnetic sorting, density separation using methylene iodide, and ultimately hand picking under a binocular microscope. Phenocrysts, xenoliths and any grains with secondary alteration were carefully removed to minimize the presence of extraneous argon and altered material. Purified groundmass separates were weighed and then wrapped in 99.99% copper foil packets placed into aluminum disks with the 28.201 Ma Fish Canyon sanidine [Kuiper et al., 2008]. The aluminum disks were irradiated for three hours at the Oregon State University TRIGA reactor in the Cadmium-Lined In-Core Irradiation Tube (CLICIT) facility.  At the University of Wisconsin Rare Gas Geochronology Laboratory, ~20-25 mg of groundmass was placed in a 3 mm x 20 mm copper trough and incrementally heated using a 25 Watt CO2 laser.  Prior to each incremental heating experiment, samples were degassed at 2% power to potentially remove large amounts of water and atmospheric argon.  Fully automated experiments consisted of 8-10 steps; each step included a scan across the trough at 150 ?m/sec at a given laser power, followed by an additional 15 minutes for gas cleanup.  The gas was cleaned during and after the heating period with two SAES C50 getters.  Argon isotope analyses were done using a MAP 215-50, and the isotopic data was reduced using ArArCalc software version 2.5 [Koppers, 2002].  All ages were calculated using the decay constants of Min et al. [2000]. The age uncertainty determined for each sample is the 2? analytical error, unless otherwise noted.  Replicate experiments were performed on several samples to check for accuracy and improve precision.  Isochron regressions agree with plateau ages and do not reveal evidence that excess argon is present in any of the lavas, therefore, we consider the plateau ages to give the best estimate of the time elapsed since eruption (Table A2).  See Jicha et al. [2012] for additional information on the 40Ar/39Ar dating method at the University of Wisconsin-Madison.     Appendix:  Ito et al. South Kaua?i Swell, revised for G-cubed 2013 3   Appendix References Jicha, B. R., J. M. Rhodes, B. S. Singer, and M. O. Garcia (2012), 40Ar/39Ar geochronology of submarine Mauna Loa volcano, J. Geophys. Res., 117(B09204), doi:10.1029/2012JB009373. Koppers, A. A. P. (2002), ArArCALC?software for 40Ar/39Ar age calculations, Comp. Geosc., 28, 605-619. Kuiper, K. F., A. Deino, F. J. Hilgen, W. Krijgsmann, P. R. Renne, and J. R. Wijbrans (2008), Synchronizing rock clocks of Earth history, Science, 320, 500-504. Matsumoto, A., and T. Kobayashi (1995), K-Ar age determination of late Quaternary volcanic rocks using the ?mass fractionation correction procedure?: application to the younger Ontake volcano, central Japan, Chem. Geol., 125, 123-135. Matsumoto, A., K. Uto, and K. Shibata (1989), K-Ar dating by peak comparison method --New technique applicable to rocks younger than 0.5 Ma, Bull. Geol. Surv. Jpn., 40, 565-579. Min, K., R. Mundil, P. R. Renne, and K. R. Ludwig (2000), A test for systematic errors in 40Ar/39Ar geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite, Geochim. Cosmochim. Acta, 64, 73-98. Yamasaki, S., R. Sawada, A. Ozawa, T. Tagami, Y. Watanabe, and E. Takahashi (2011), Unspiked K-Ar dating of Koolau lavas, Hawaii: Evaluation of the influence of weathering/alteration of age determinations, Chem. Geol., 287, 41-53.   Table A1.  Complete 40Ar/39Ar ResultsSample:  J2-298-12  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.5 % 6.610E-15 1.120024 0.001588 0.029434 0.000203 0.000997 0.000014 0.018551 0.002011 0.003530 0.000020 6.99 2.66088 0.20718 4.33 ? 0.67 0.6822 3.3 % 1.001E-14 1.695214 0.001911 0.059550 0.000213 0.001725 0.000026 0.082579 0.002321 0.005263 0.000018 8.63 2.45817 0.09452 4.00 ? 0.31 0.3103 4.3 % 1.103E-14 1.868037 0.002045 0.072152 0.000204 0.002048 0.000026 0.202156 0.003506 0.005802 0.000020 9.07 2.35193 0.08838 3.82 ? 0.29 0.1534 5.5 % 6.885E-15 1.166597 0.001716 0.048813 0.000223 0.001335 0.000018 0.259511 0.004207 0.003610 0.000010 10.30 2.47102 0.07380 4.02 ? 0.24 0.0815 6.5 % 3.163E-15 0.535926 0.001575 0.024156 0.000198 0.000641 0.000014 0.194991 0.003455 0.001662 0.000008 11.21 2.50057 0.11954 4.07 ? 0.39 0.0536 7.7 % 1.983E-15 0.336013 0.001620 0.015358 0.000200 0.000399 0.000016 0.184973 0.003335 0.001059 0.000009 11.18 2.46640 0.20624 4.01 ? 0.67 0.0357 10.0 % 1.813E-15 0.307242 0.000194 0.013531 0.000156 0.000367 0.000015 0.280953 0.007812 0.001010 0.000008 10.00 2.30334 0.18610 3.75 ? 0.60 0.0208 25.0 % 2.043E-15 0.346221 0.000244 0.015167 0.000158 0.000420 0.000014 0.595273 0.010831 0.001193 0.000009 11.60 2.72014 0.18918 4.42 ? 0.61 0.011Weighted Mean Age (8 of 8): 4.00 ? 0.13Sample:  J2-299-29  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 2.648E-15 0.448659 0.000814 0.012267 0.000036 0.000500 0.000016 0.011216 0.000921 0.001405 0.000012 7.67 2.80750 0.29105 3.93 ? 0.81 0.4702 3.0 % 6.535E-15 1.107209 0.001373 0.034966 0.000068 0.001190 0.000017 0.050343 0.001067 0.003414 0.000017 9.23 2.92445 0.14676 4.10 ? 0.41 0.2983 3.6 % 7.063E-15 1.196758 0.001413 0.043428 0.000096 0.001244 0.000032 0.089147 0.001003 0.003664 0.000012 10.12 2.79213 0.08990 3.91 ? 0.25 0.2094 4.2 % 6.167E-15 1.044918 0.001473 0.041513 0.000099 0.001096 0.000032 0.129412 0.001492 0.003167 0.000011 11.40 2.87473 0.08938 4.03 ? 0.25 0.1385 4.9 % 3.711E-15 0.628790 0.001066 0.027937 0.000080 0.000692 0.000028 0.135049 0.001539 0.001884 0.000010 13.12 2.96349 0.11435 4.15 ? 0.32 0.0896 5.7 % 5.094E-15 0.863154 0.001144 0.018164 0.000106 0.000772 0.000028 0.138359 0.001527 0.002781 0.000013 6.06 2.89413 0.21892 4.06 ? 0.61 0.0567 7.0 % 1.249E-15 0.211589 0.001065 0.012981 0.000081 0.000301 0.000028 0.151634 0.001711 0.000627 0.000011 18.01 2.95848 0.26226 4.15 ? 0.73 0.0378 9.0 % 7.346E-16 0.124468 0.001061 0.008427 0.000077 0.000209 0.000025 0.192510 0.002154 0.000393 0.000008 18.78 2.81717 0.30087 3.95 ? 0.84 0.0199 12.0 % 3.402E-16 0.057634 0.001073 0.004084 0.000074 0.000094 0.000027 0.187147 0.002032 0.000207 0.000010 18.96 2.76126 0.77393 3.87 ? 2.17 0.00910 25.0 % 2.510E-16 0.042523 0.001067 0.002701 0.000073 0.000059 0.000026 0.215330 0.002395 0.000174 0.000009 18.32 3.04758 1.11357 4.27 ? 3.12 0.005Weighted Mean Age (10 of 10): 4.02 ? 0.13Sample:  J2-299-23  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.3 % 1.341E-15 0.227274 0.000302 0.012532 0.000059 0.000311 0.000017 0.014019 0.000454 0.000666 0.000008 13.95 2.53113 0.19902 4.12 ? 0.65 0.3842 2.8 % 1.584E-15 0.268345 0.000380 0.028704 0.000080 0.000537 0.000013 0.052768 0.000843 0.000681 0.000008 26.48 2.47899 0.08544 4.03 ? 0.28 0.2343 3.3 % 1.381E-15 0.233923 0.000453 0.032676 0.000082 0.000529 0.000011 0.081815 0.001255 0.000553 0.000011 32.80 2.35238 0.10368 3.83 ? 0.34 0.1714 3.8 % 1.190E-15 0.201697 0.000413 0.031190 0.000054 0.000489 0.000016 0.108882 0.001651 0.000447 0.000008 38.67 2.50629 0.07443 4.08 ? 0.24 0.1235 4.4 % 8.769E-16 0.148569 0.000236 0.023465 0.000078 0.000337 0.000013 0.106849 0.001606 0.000333 0.000010 39.33 2.49773 0.12127 4.06 ? 0.39 0.0946 5.6 % 1.380E-15 0.233771 0.000256 0.029446 0.000047 0.000473 0.000013 0.158705 0.002389 0.000588 0.000008 30.98 2.46861 0.07865 4.01 ? 0.26 0.0797 8.0 % 2.601E-15 0.440722 0.000612 0.028249 0.000073 0.000591 0.000018 0.313803 0.004687 0.001335 0.000017 16.02 2.51801 0.18106 4.09 ? 0.59 0.0388 25.0 % 2.798E-15 0.474148 0.000696 0.018315 0.000078 0.000533 0.000014 0.460618 0.006952 0.001576 0.000020 9.36 2.46451 0.33056 4.01 ? 1.07 0.017Weighted Mean Age (8 of 8): 4.02 ? 0.12Sample:  J2-297-04  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.5 % 8.712E-15 1.476052 0.001954 0.034528 0.000156 0.001265 0.000022 0.033897 0.006762 0.004699 0.000025 6.11 2.61384 0.22350 4.25 ? 0.73 0.4382 3.3 % 1.756E-14 2.975773 0.001811 0.083028 0.000158 0.002732 0.000026 0.139172 0.007050 0.009424 0.000036 6.78 2.43184 0.13103 3.95 ? 0.43 0.2563 4.3 % 1.843E-14 3.122305 0.001679 0.098352 0.000207 0.003157 0.000027 0.293342 0.007959 0.009803 0.000051 7.95 2.52920 0.15404 4.11 ? 0.50 0.1444 5.5 % 9.765E-15 1.654447 0.002140 0.059346 0.000173 0.001759 0.000021 0.309596 0.008087 0.005184 0.000025 8.87 2.48221 0.12910 4.04 ? 0.42 0.0825 6.5 % 3.475E-15 0.588742 0.001240 0.022589 0.000410 0.000641 0.000032 0.170060 0.003606 0.001858 0.000016 8.98 2.35222 0.22096 3.83 ? 0.72 0.0576 7.7 % 1.724E-15 0.292142 0.001020 0.011821 0.000408 0.000344 0.000029 0.139418 0.003267 0.000926 0.000017 10.09 2.51354 0.44499 4.09 ? 1.45 0.0367 10.0 % 9.365E-16 0.158672 0.001020 0.006641 0.000404 0.000223 0.000027 0.142882 0.003299 0.000518 0.000010 10.49 2.54295 0.51572 4.14 ? 1.68 0.0208 25.0 % 9.537E-16 0.161584 0.001013 0.006360 0.000405 0.000215 0.000028 0.202780 0.003890 0.000547 0.000012 9.72 2.52302 0.63519 4.10 ? 2.06 0.013Weighted Mean Age (8 of 8): 4.03 ? 0.22Sample:  J2-299-15  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 2.363E-15 0.400455 0.000995 0.012193 0.000155 0.000504 0.000012 0.018886 0.001321 0.001234 0.000015 9.28 3.05042 0.36380 4.27 ? 1.02 0.2772 3.0 % 3.931E-15 0.666057 0.001036 0.028196 0.000052 0.000916 0.000016 0.079977 0.000951 0.001999 0.000015 12.27 2.90447 0.16135 4.07 ? 0.45 0.1513 3.6 % 2.982E-15 0.505300 0.000785 0.027312 0.000056 0.000773 0.000019 0.112597 0.001348 0.001456 0.000007 16.59 3.07722 0.08623 4.31 ? 0.24 0.1044 4.2 % 2.303E-15 0.390207 0.000776 0.023255 0.000091 0.000703 0.000017 0.140535 0.001603 0.001123 0.000007 17.78 2.99508 0.09921 4.20 ? 0.28 0.0715 4.9 % 1.690E-15 0.286300 0.000759 0.015262 0.000046 0.000483 0.000015 0.125705 0.001360 0.000845 0.000011 16.21 3.05857 0.22394 4.29 ? 0.63 0.0526 5.7 % 1.148E-15 0.194468 0.000732 0.011195 0.000073 0.000361 0.000010 0.119410 0.001405 0.000593 0.000008 14.66 2.56427 0.21223 3.59 ? 0.59 0.0407 7.0 % 1.037E-15 0.175740 0.000732 0.009405 0.000062 0.000255 0.000012 0.149420 0.001675 0.000547 0.000010 14.71 2.77864 0.33852 3.89 ? 0.95 0.0278 9.0 % 9.194E-16 0.155772 0.000728 0.006852 0.000053 0.000215 0.000012 0.194479 0.002101 0.000525 0.000008 10.06 2.33080 0.36007 3.27 ? 1.01 0.0159 12.0 % 6.624E-16 0.112233 0.000726 0.004451 0.000048 0.000164 0.000018 0.173719 0.001906 0.000386 0.000009 10.38 2.68864 0.63749 3.77 ? 1.78 0.01110 25.0 % 5.601E-16 0.094894 0.000731 0.003438 0.000045 0.000111 0.000016 0.174856 0.002005 0.000325 0.000008 13.32 3.80815 0.78381 5.33 ? 2.19 0.008Weighted Mean Age (10 of 10): 4.17 ? 0.16Sample:  J2-297-09  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.5 % 3.961E-15 0.671141 0.003809 0.042801 0.000414 0.000865 0.000020 0.053698 0.001262 0.001908 0.000017 16.60 2.60503 0.14885 4.24 ? 0.48 0.3422 3.3 % 6.313E-15 1.069672 0.004060 0.087103 0.000401 0.001567 0.000021 0.196482 0.003002 0.002903 0.000026 21.22 2.61010 0.10015 4.24 ? 0.33 0.1903 4.3 % 5.332E-15 0.903504 0.003993 0.079884 0.000440 0.001457 0.000018 0.312585 0.004658 0.002446 0.000007 22.68 2.57177 0.05884 4.18 ? 0.19 0.1104 5.5 % 2.794E-15 0.473475 0.003739 0.041582 0.000406 0.000759 0.000020 0.269488 0.004034 0.001329 0.000013 21.47 2.45540 0.13142 3.99 ? 0.43 0.0665 6.5 % 1.278E-15 0.216464 0.003739 0.017705 0.000396 0.000365 0.000016 0.158171 0.002502 0.000640 0.000007 18.32 2.25360 0.25125 3.67 ? 0.82 0.0486 7.7 % 8.111E-16 0.137422 0.003734 0.010098 0.000393 0.000235 0.000022 0.137925 0.002242 0.000422 0.000012 17.07 2.34468 0.53220 3.81 ? 1.73 0.0317 10.0 % 7.860E-16 0.133180 0.003737 0.008455 0.000056 0.000194 0.000010 0.184525 0.002682 0.000448 0.000009 11.42 1.82576 0.54463 2.97 ? 1.77 0.0198 25.0 % 1.370E-15 0.232153 0.003747 0.006535 0.000046 0.000270 0.000012 0.389960 0.005645 0.000860 0.000010 3.60 1.33374 0.76038 2.17 ? 2.47 0.007Weighted Mean Age (8 of 8): 4.14 ? 0.14Sample:  J2-297-09  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.3 % 2.089E-15 0.353950 0.000772 0.009409 0.000073 0.000346 0.000013 0.006013 0.000281 0.001179 0.000020 1.67 0.62784 0.62441 1.02 ? 2.03 0.6722 2.8 % 2.607E-15 0.441648 0.000666 0.024172 0.000068 0.000563 0.000017 0.024820 0.000435 0.001285 0.000012 14.45 2.64237 0.14927 4.30 ? 0.48 0.4183 3.3 % 3.560E-15 0.603167 0.000954 0.042765 0.000115 0.000831 0.000019 0.062438 0.001027 0.001677 0.000021 18.67 2.63536 0.14423 4.29 ? 0.47 0.2944 3.8 % 4.040E-15 0.684433 0.000890 0.054447 0.000102 0.001010 0.000015 0.108608 0.001752 0.001856 0.000016 21.10 2.65655 0.08777 4.32 ? 0.29 0.2155 4.4 % 3.425E-15 0.580306 0.000705 0.049568 0.000092 0.000913 0.000019 0.131374 0.002074 0.001573 0.000020 21.68 2.54306 0.11981 4.14 ? 0.39 0.1626 5.6 % 4.548E-15 0.770593 0.000712 0.066173 0.000111 0.001222 0.000019 0.268158 0.004225 0.002098 0.000014 22.24 2.59735 0.06357 4.22 ? 0.21 0.1067 8.0 % 3.719E-15 0.630147 0.000693 0.055339 0.000062 0.001016 0.000023 0.386823 0.006059 0.001749 0.000013 22.76 2.60423 0.07138 4.23 ? 0.23 0.0618 25.0 % 3.385E-15 0.573473 0.000691 0.029321 0.000064 0.000756 0.000018 0.576162 0.009051 0.001859 0.000015 12.06 2.39021 0.15534 3.89 ? 0.50 0.022Weighted Mean Age (7 of 8; step 1 excluded): 4.22 ? 0.12Sample:  KS1-18  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.3 % 5.557E-15 0.941462 0.001245 0.012383 0.000074 0.000735 0.000012 0.015661 0.001315 0.003090 0.000032 3.15 2.39545 0.77160 3.90 ? 2.51 0.3402 2.8 % 5.296E-15 0.897320 0.001470 0.020035 0.000080 0.000779 0.000019 0.039253 0.001420 0.002873 0.000021 5.71 2.56301 0.31943 4.17 ? 1.04 0.2193 3.3 % 3.419E-15 0.579352 0.000666 0.018501 0.000095 0.000573 0.000021 0.062955 0.001601 0.001812 0.000008 8.43 2.64543 0.14006 4.30 ? 0.45 0.1264 3.8 % 3.923E-15 0.664742 0.000733 0.025267 0.000096 0.000710 0.000020 0.103371 0.002030 0.002047 0.000015 10.21 2.69224 0.18045 4.38 ? 0.59 0.1055 4.4 % 2.868E-15 0.485887 0.000497 0.023297 0.000094 0.000581 0.000018 0.133990 0.002417 0.001488 0.000013 11.63 2.43535 0.17217 3.96 ? 0.56 0.0746 5.6 % 2.565E-15 0.434677 0.000440 0.025135 0.000127 0.000602 0.000012 0.240576 0.003950 0.001323 0.000012 14.39 2.50462 0.14847 4.07 ? 0.48 0.0457 8.0 % 3.360E-15 0.569354 0.000695 0.039290 0.000074 0.000913 0.000015 0.711518 0.010860 0.001765 0.000013 18.13 2.66035 0.10320 4.33 ? 0.34 0.0238 25.0 % 2.509E-15 0.425133 0.000989 0.030817 0.000079 0.000629 0.000020 1.325849 0.019961 0.001544 0.000018 16.99 2.41379 0.18415 3.93 ? 0.60 0.010Weighted Mean Age (8 of 8): 4.20 ? 0.19Sample:  J2-252-10  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.5 % 4.418E-15 0.748497 0.001146 0.022576 0.000385 0.000723 0.000019 0.012578 0.008816 0.002316 0.000022 8.70 2.88642 0.29545 4.69 ? 0.96 0.7722 3.3 % 3.335E-15 0.565123 0.000675 0.043817 0.000402 0.000845 0.000019 0.061226 0.008868 0.001531 0.000020 20.77 2.68135 0.13772 4.36 ? 0.45 0.3073 4.3 % 1.875E-15 0.317675 0.000622 0.047817 0.000407 0.000752 0.000020 0.129981 0.009027 0.000695 0.000010 38.51 2.56329 0.06808 4.17 ? 0.22 0.1584 5.5 % 1.169E-15 0.198140 0.000556 0.036177 0.000400 0.000552 0.000020 0.176918 0.009190 0.000386 0.000007 49.37 2.71273 0.07247 4.41 ? 0.24 0.0885 6.5 % 8.589E-16 0.145524 0.000577 0.024639 0.000395 0.000404 0.000012 0.168286 0.009169 0.000323 0.000008 43.34 2.57136 0.11350 4.18 ? 0.37 0.0636 7.7 % 7.777E-16 0.131765 0.000561 0.021807 0.000389 0.000358 0.000018 0.213081 0.009360 0.000301 0.000009 45.03 2.73860 0.13479 4.45 ? 0.44 0.0447 10.0 % 9.676E-16 0.163946 0.000517 0.025464 0.000392 0.000449 0.000017 0.393633 0.010484 0.000431 0.000014 40.97 2.66544 0.17137 4.33 ? 0.56 0.0288 25.0 % 1.037E-15 0.175662 0.000509 0.026728 0.000393 0.000477 0.000015 0.650833 0.012835 0.000538 0.000010 38.36 2.56277 0.12940 4.17 ? 0.42 0.017Weighted Mean Age (8 of 8): 4.29 ? 0.12Sample:  J2-252-04  J = 0.0008685 ? 0.0000006 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.5 % 4.808E-15 0.814605 0.001266 0.014505 0.000071 0.000705 0.000016 0.010505 0.000747 0.002611 0.000018 5.39 3.02637 0.38296 4.74 ? 1.20 0.5932 3.3 % 4.021E-15 0.681352 0.000673 0.036917 0.000097 0.000834 0.000016 0.054822 0.000814 0.001975 0.000010 14.98 2.76820 0.08538 4.33 ? 0.27 0.2893 4.3 % 3.801E-15 0.643974 0.001306 0.057143 0.000101 0.001001 0.000019 0.134127 0.001964 0.001670 0.000013 24.99 2.82093 0.07302 4.41 ? 0.23 0.1834 5.5 % 2.776E-15 0.470387 0.001095 0.048511 0.000171 0.000830 0.000009 0.182530 0.002599 0.001195 0.000017 27.97 2.71860 0.10796 4.25 ? 0.34 0.1145 6.5 % 2.157E-15 0.365488 0.000334 0.031003 0.000061 0.000592 0.000011 0.176279 0.002510 0.001012 0.000012 21.91 2.59305 0.11238 4.06 ? 0.35 0.0756 7.7 % 2.652E-15 0.449353 0.000316 0.027814 0.000058 0.000624 0.000015 0.218079 0.003116 0.001324 0.000014 16.73 2.71675 0.14837 4.25 ? 0.46 0.0557 10.0 % 4.520E-15 0.765786 0.001233 0.042254 0.000091 0.001013 0.000021 0.479057 0.006803 0.002341 0.000019 14.53 2.65388 0.13450 4.15 ? 0.42 0.0388 25.0 % 5.946E-15 1.007382 0.000707 0.053710 0.000122 0.001339 0.000021 1.080320 0.015268 0.003206 0.000021 14.33 2.72424 0.11812 4.26 ? 0.37 0.021Weighted Mean Age (8 of 8): 4.29 ? 0.12Sample:  J2-299-08  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 3.183E-15 0.539343 0.003422 0.006079 0.000039 0.000410 0.000015 0.012769 0.000290 0.001764 0.000010 3.56 3.16644 0.74497 4.44 ? 2.08 0.2042 3.0 % 5.281E-15 0.894738 0.004872 0.014336 0.000064 0.000687 0.000009 0.045503 0.000551 0.002908 0.000012 4.37 2.73141 0.42294 3.83 ? 1.18 0.1353 3.6 % 4.075E-15 0.690457 0.001722 0.014803 0.000050 0.000633 0.000015 0.067303 0.000767 0.002182 0.000009 7.38 3.45103 0.21055 4.83 ? 0.59 0.0944 4.2 % 3.163E-15 0.535942 0.001506 0.013993 0.000047 0.000519 0.000011 0.096050 0.001074 0.001704 0.000008 7.47 2.87434 0.19586 4.03 ? 0.55 0.0625 4.9 % 2.077E-15 0.351973 0.001092 0.010320 0.000069 0.000356 0.000016 0.099963 0.001098 0.001105 0.000010 9.43 3.23813 0.30387 4.54 ? 0.85 0.0446 5.7 % 1.610E-15 0.272828 0.000798 0.008770 0.000040 0.000291 0.000014 0.108838 0.001195 0.000855 0.000007 10.48 3.28716 0.26266 4.61 ? 0.74 0.0347 7.0 % 1.548E-15 0.262363 0.000882 0.008254 0.000052 0.000283 0.000015 0.139508 0.001453 0.000836 0.000012 9.98 3.20780 0.44019 4.49 ? 1.23 0.0258 9.0 % 1.488E-15 0.252075 0.000866 0.007281 0.000041 0.000257 0.000011 0.184718 0.001967 0.000822 0.000009 9.37 3.29896 0.37321 4.62 ? 1.04 0.0179 12.0 % 9.742E-16 0.165062 0.000753 0.004246 0.000029 0.000148 0.000012 0.156618 0.001747 0.000570 0.000010 5.30 2.11472 0.74818 2.96 ? 2.10 0.01110 25.0 % 1.114E-15 0.188711 0.000821 0.003174 0.000037 0.000155 0.000009 0.206013 0.002291 0.000657 0.000008 5.64 3.50706 0.86828 4.91 ? 2.43 0.006Weighted Mean Age (10 of 10): 4.42 ? 0.28Sample:  J2-299-08  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 2.897E-15 0.490921 0.001697 0.004868 0.000048 0.000388 0.000015 0.009837 0.000250 0.001610 0.000006 3.23 3.26627 0.49168 4.58 ? 1.38 0.2122 3.0 % 5.827E-15 0.987358 0.001635 0.014301 0.000048 0.000797 0.000016 0.037091 0.000549 0.003190 0.000018 4.82 3.33699 0.39768 4.67 ? 1.11 0.1663 3.6 % 5.081E-15 0.860954 0.001705 0.018306 0.000064 0.000761 0.000013 0.067564 0.000864 0.002739 0.000014 6.59 3.10738 0.25035 4.35 ? 0.70 0.1164 4.2 % 4.227E-15 0.716213 0.001649 0.018710 0.000079 0.000722 0.000012 0.105123 0.001343 0.002236 0.000009 8.89 3.41788 0.17164 4.79 ? 0.48 0.0765 5.0 % 3.041E-15 0.515189 0.001620 0.015268 0.000065 0.000502 0.000020 0.129851 0.001644 0.001618 0.000006 9.17 3.11171 0.16190 4.36 ? 0.45 0.0506 6.0 % 2.383E-15 0.403749 0.001631 0.012783 0.000078 0.000401 0.000013 0.153239 0.002075 0.001278 0.000010 9.43 3.00201 0.26255 4.21 ? 0.73 0.0367 7.0 % 1.650E-15 0.279555 0.001615 0.009119 0.000051 0.000301 0.000012 0.147619 0.001832 0.000901 0.000013 8.91 2.76274 0.45306 3.87 ? 1.27 0.0268 10.0 % 2.240E-15 0.379584 0.001600 0.011152 0.000061 0.000413 0.000011 0.297332 0.003674 0.001261 0.000010 7.91 2.74273 0.31866 3.84 ? 0.89 0.0169 25.0 % 1.697E-15 0.287498 0.001607 0.005063 0.000058 0.000270 0.000013 0.367123 0.004640 0.001020 0.000008 5.16 3.07914 0.62435 4.31 ? 1.75 0.006Weighted Mean Age (9 of 9): 4.42 ? 0.25Sample:  J2-299-21  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 7.558E-15 1.280578 0.001359 0.002652 0.000031 0.000889 0.000022 0.006996 0.000249 0.004294 0.000008 0.95 4.60561 1.00194 6.36 ? 2.76 0.1632 3.0 % 9.229E-15 1.563784 0.002452 0.006736 0.000046 0.001100 0.000022 0.027650 0.000382 0.005209 0.000009 1.71 3.99171 0.54199 5.51 ? 1.49 0.1043 3.6 % 6.104E-15 1.034287 0.001918 0.009647 0.000034 0.000735 0.000020 0.047291 0.000568 0.003418 0.000018 2.70 2.90039 0.58737 4.01 ? 1.62 0.0874 4.2 % 4.067E-15 0.689167 0.001609 0.015269 0.000048 0.000591 0.000014 0.072529 0.000817 0.002194 0.000010 6.76 3.06230 0.21499 4.23 ? 0.59 0.0905 4.9 % 1.924E-15 0.326043 0.001239 0.018595 0.000079 0.000388 0.000014 0.074691 0.000851 0.000905 0.000008 19.75 3.47296 0.14166 4.79 ? 0.39 0.1076 5.7 % 1.339E-15 0.226892 0.001231 0.021851 0.000071 0.000364 0.000020 0.074065 0.000841 0.000551 0.000007 30.84 3.20953 0.11352 4.43 ? 0.31 0.1277 7.0 % 1.179E-15 0.199773 0.001234 0.029078 0.000048 0.000418 0.000012 0.096651 0.001074 0.000382 0.000011 47.20 3.25030 0.11541 4.49 ? 0.32 0.1298 9.0 % 1.068E-15 0.180974 0.001239 0.027698 0.000079 0.000436 0.000014 0.159248 0.001676 0.000362 0.000008 47.77 3.13330 0.09988 4.33 ? 0.28 0.0759 12.0 % 6.922E-16 0.117287 0.000681 0.020246 0.000052 0.000332 0.000014 0.182767 0.002172 0.000227 0.000008 54.91 3.20042 0.12935 4.42 ? 0.36 0.04710 25.0 % 7.859E-16 0.133158 0.001227 0.019027 0.000064 0.000338 0.000010 0.335923 0.006003 0.000341 0.000008 43.90 3.10932 0.14411 4.29 ? 0.40 0.024Weighted Mean Age (10 of 10): 4.44 ? 0.13Sample:  J2-299-04  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 9.545E-15 1.617234 0.001816 0.007519 0.000061 0.001137 0.000016 0.013414 0.000820 0.005389 0.000017 1.60 3.45215 0.69795 4.84 ? 1.95 0.2412 3.0 % 9.252E-15 1.567545 0.002619 0.017039 0.000081 0.001283 0.000020 0.049956 0.000888 0.005156 0.000020 3.04 2.80534 0.37556 3.93 ? 1.05 0.1463 3.6 % 4.552E-15 0.771264 0.001520 0.018352 0.000103 0.000837 0.000028 0.082730 0.001109 0.002425 0.000019 7.94 3.34618 0.31173 4.69 ? 0.87 0.0954 4.2 % 2.522E-15 0.427316 0.001272 0.017513 0.000084 0.000524 0.000011 0.122224 0.001463 0.001294 0.000012 12.73 3.11982 0.21694 4.37 ? 0.61 0.0615 4.9 % 1.414E-15 0.239652 0.000877 0.013482 0.000091 0.000296 0.000010 0.132132 0.001656 0.000684 0.000009 19.93 3.56550 0.21597 4.99 ? 0.60 0.0446 5.7 % 9.803E-16 0.166101 0.000880 0.010375 0.000103 0.000233 0.000009 0.141787 0.001661 0.000490 0.000007 19.42 3.13771 0.23006 4.40 ? 0.64 0.0317 7.0 % 9.452E-16 0.160144 0.000889 0.009712 0.000078 0.000240 0.000013 0.192954 0.002226 0.000484 0.000007 20.06 3.35274 0.23945 4.70 ? 0.67 0.0218 9.0 % 9.239E-16 0.156543 0.000859 0.008484 0.000092 0.000214 0.000019 0.298002 0.003233 0.000511 0.000008 18.36 3.46890 0.31044 4.86 ? 0.87 0.0129 12.0 % 5.448E-16 0.092314 0.000849 0.004752 0.000082 0.000114 0.000013 0.223704 0.002614 0.000321 0.000011 16.22 3.25435 0.74076 4.56 ? 2.07 0.00910 25.0 % 4.228E-16 0.071631 0.000888 0.003460 0.000079 0.000092 0.000009 0.256231 0.002754 0.000278 0.000007 13.02 2.83590 0.71546 3.97 ? 2.00 0.006Weighted Mean Age (10 of 10): 4.60 ? 0.26Sample:  J2-299-04  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0034 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 9.857E-15 1.670125 0.003293 0.006291 0.000037 0.001157 0.000015 0.008307 0.000850 0.005573 0.000019 1.43 3.80618 1.03743 5.33 ? 2.90 0.3252 3.0 % 1.484E-14 2.514607 0.003300 0.018184 0.000093 0.001792 0.000015 0.045057 0.001032 0.008364 0.000033 1.85 2.55946 0.57066 3.59 ? 1.60 0.1733 3.6 % 7.544E-15 1.278174 0.001061 0.022320 0.000053 0.001089 0.000022 0.085211 0.001328 0.004102 0.000016 5.69 3.26728 0.21496 4.58 ? 0.60 0.1124 4.2 % 3.873E-15 0.656246 0.000591 0.022458 0.000083 0.000696 0.000012 0.128712 0.001833 0.001999 0.000012 11.51 3.37499 0.15907 4.73 ? 0.45 0.0755 5.0 % 2.239E-15 0.379354 0.000561 0.019172 0.000079 0.000476 0.000011 0.159170 0.002122 0.001114 0.000008 16.49 3.28113 0.12268 4.60 ? 0.34 0.0526 6.0 % 1.540E-15 0.260971 0.000483 0.016141 0.000073 0.000373 0.000017 0.194691 0.002503 0.000769 0.000009 18.70 3.04789 0.17731 4.27 ? 0.50 0.0357 7.0 % 1.096E-15 0.185753 0.000457 0.011432 0.000045 0.000258 0.000014 0.188895 0.002696 0.000545 0.000013 21.27 3.49443 0.35002 4.90 ? 0.98 0.0268 10.0 % 1.586E-15 0.268696 0.000506 0.015161 0.000060 0.000394 0.000011 0.466608 0.005963 0.000894 0.000010 15.28 2.76454 0.20941 3.87 ? 0.59 0.0149 25.0 % 1.088E-15 0.184312 0.000488 0.010352 0.000036 0.000236 0.000010 0.615736 0.007651 0.000687 0.000008 15.88 2.94489 0.26291 4.13 ? 0.74 0.007Weighted Mean Age (9 of 9): 4.46 ? 0.22Sample:  J2-298-20  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0036 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 4.941E-15 0.837123 0.000371 0.008291 0.000100 0.000642 0.000022 0.002098 0.003300 0.002783 0.000018 1.78 1.79727 0.63922 2.52 ? 1.79 1.6992 3.0 % 1.002E-14 1.697736 0.001454 0.022678 0.000105 0.001320 0.000022 0.022439 0.003318 0.005523 0.000034 3.98 2.98104 0.45363 4.18 ? 1.27 0.4343 3.6 % 7.539E-15 1.277392 0.001345 0.024908 0.000047 0.001082 0.000023 0.045687 0.000594 0.004052 0.000019 6.56 3.36598 0.23651 4.72 ? 0.66 0.2344 4.2 % 5.343E-15 0.905204 0.001535 0.024833 0.000102 0.000830 0.000024 0.077680 0.000995 0.002791 0.000016 9.57 3.49557 0.19841 4.90 ? 0.56 0.1375 5.0 % 4.062E-15 0.688190 0.001223 0.027145 0.000091 0.000687 0.000016 0.113820 0.001507 0.002044 0.000014 13.51 3.43430 0.16480 4.81 ? 0.46 0.1026 6.0 % 2.981E-15 0.505072 0.001217 0.034417 0.000067 0.000649 0.000016 0.152828 0.002019 0.001343 0.000006 23.81 3.50510 0.06456 4.91 ? 0.18 0.0977 7.0 % 1.933E-15 0.327548 0.001120 0.032050 0.000066 0.000531 0.000023 0.141868 0.001828 0.000784 0.000007 32.65 3.34722 0.07796 4.69 ? 0.22 0.0978 10.0 % 2.711E-15 0.459393 0.001145 0.041311 0.000082 0.000728 0.000020 0.325533 0.004109 0.001185 0.000013 29.27 3.27278 0.09570 4.59 ? 0.27 0.0549 25.0 % 3.090E-15 0.523499 0.000879 0.035042 0.000067 0.000694 0.000015 0.539388 0.006751 0.001534 0.000014 21.47 3.24154 0.12004 4.54 ? 0.34 0.028Weighted Mean Age (8 of 9; step 1 excluded): 4.75 ? 0.11Sample:  J2-298-20  J = 0.0007663 ? 0.0000005 (1?) D/amu: 1.0036 ? 0.0001 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 4.476E-15 0.758408 0.003262 0.005738 0.000239 0.000571 0.000014 0.006656 0.000558 0.002521 0.000015 1.83 2.41661 0.97960 3.39 ? 2.74 0.3702 3.0 % 9.438E-15 1.599169 0.001697 0.019340 0.000072 0.001230 0.000018 0.027689 0.000781 0.005184 0.000020 4.35 3.59813 0.32033 5.04 ? 0.90 0.3003 3.6 % 6.907E-15 1.170262 0.001675 0.021807 0.000083 0.000958 0.000019 0.048821 0.000934 0.003700 0.000014 6.89 3.70210 0.20757 5.19 ? 0.58 0.1924 4.2 % 5.186E-15 0.878679 0.001427 0.023985 0.000055 0.000818 0.000022 0.080866 0.001277 0.002702 0.000009 9.85 3.61650 0.12198 5.07 ? 0.34 0.1275 4.9 % 3.433E-15 0.581604 0.001408 0.024810 0.000129 0.000594 0.000017 0.101790 0.001513 0.001707 0.000016 14.64 3.44034 0.20567 4.82 ? 0.58 0.1056 5.7 % 2.429E-15 0.411490 0.001399 0.026205 0.000076 0.000509 0.000012 0.116828 0.001676 0.001114 0.000014 22.24 3.50301 0.16616 4.91 ? 0.46 0.0967 7.0 % 2.461E-15 0.416905 0.001460 0.033635 0.000067 0.000612 0.000021 0.153042 0.002094 0.001055 0.000010 28.11 3.49470 0.09900 4.90 ? 0.28 0.0948 10.0 % 2.680E-15 0.454163 0.001397 0.037952 0.000098 0.000692 0.000014 0.322593 0.004225 0.001193 0.000009 27.89 3.35657 0.08186 4.70 ? 0.23 0.0509 25.0 % 2.628E-15 0.445198 0.001419 0.031303 0.000065 0.000623 0.000016 0.433555 0.005685 0.001259 0.000010 24.01 3.44754 0.10946 4.83 ? 0.31 0.03110 35.0 % 1.267E-15 0.214615 0.001397 0.013826 0.000081 0.000288 0.000012 0.218759 0.003009 0.000643 0.000009 19.41 3.04541 0.22908 4.27 ? 0.64 0.027Weighted Mean Age (10 of 10): 4.84 ? 0.13Sample: J2-299-33  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.3 % 5.987E-15 1.014435 0.001168 0.015526 0.000063 0.000832 0.000020 0.032433 0.000848 0.003294 0.000020 4.31 2.81737 0.38650 4.58 ? 1.26 0.2062 2.8 % 6.692E-15 1.133805 0.000949 0.028516 0.000071 0.001042 0.000023 0.073617 0.001329 0.003564 0.000009 7.63 3.03884 0.10111 4.94 ? 0.33 0.1663 3.3 % 4.184E-15 0.708838 0.000946 0.030378 0.000057 0.000787 0.000021 0.077998 0.001386 0.002095 0.000019 13.51 3.15895 0.18851 5.14 ? 0.61 0.1674 3.8 % 4.474E-15 0.757985 0.001029 0.043455 0.000096 0.000854 0.000022 0.124555 0.002015 0.002100 0.000025 19.41 3.39260 0.17128 5.51 ? 0.56 0.1505 4.4 % 3.560E-15 0.603253 0.000693 0.045190 0.000114 0.000819 0.000031 0.162150 0.002530 0.001595 0.000012 23.98 3.20902 0.08202 5.22 ? 0.27 0.1206 5.6 % 3.392E-15 0.574757 0.000844 0.058037 0.000098 0.000995 0.000020 0.246994 0.003776 0.001415 0.000015 30.61 3.03997 0.07804 4.94 ? 0.25 0.1017 8.0 % 2.467E-15 0.417977 0.000721 0.044464 0.000144 0.000800 0.000021 0.374201 0.005640 0.001046 0.000015 33.00 3.11945 0.10412 5.07 ? 0.34 0.0518 25.0 % 2.319E-15 0.392848 0.000677 0.031502 0.000073 0.000656 0.000020 0.585954 0.008875 0.001157 0.000010 24.59 3.10605 0.09688 5.05 ? 0.31 0.023Weighted Mean Age (8 of 8): 5.07 ? 0.13Sample:  J2-298-20  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.3 % 3.308E-15 0.560411 0.000983 0.003695 0.000179 0.000386 0.000022 0.000000 0.000000 0.001835 0.000009 3.25 4.93158 0.79931 8.01 ? 2.59 0.0002 2.8 % 6.500E-15 1.101323 0.001275 0.014125 0.000181 0.000829 0.000012 0.018951 0.002704 0.003517 0.000012 5.78 4.51048 0.27229 7.33 ? 0.88 0.3203 3.3 % 5.541E-15 0.938906 0.000724 0.024970 0.000181 0.000822 0.000016 0.039670 0.002756 0.002906 0.000018 8.86 3.33527 0.21713 5.42 ? 0.70 0.2704 3.8 % 4.311E-15 0.730347 0.000691 0.033102 0.000204 0.000768 0.000018 0.060548 0.002835 0.002140 0.000020 14.07 3.10766 0.17964 5.05 ? 0.58 0.2355 4.4 % 2.886E-15 0.488990 0.000680 0.031524 0.000175 0.000566 0.000016 0.076551 0.002935 0.001343 0.000012 20.03 3.11227 0.11939 5.06 ? 0.39 0.1776 5.6 % 3.475E-15 0.588785 0.001015 0.046732 0.000179 0.000831 0.000018 0.145315 0.003476 0.001538 0.000011 24.73 3.12275 0.07360 5.08 ? 0.24 0.1387 8.0 % 3.341E-15 0.566153 0.000508 0.065353 0.000214 0.001021 0.000027 0.278264 0.005006 0.001289 0.000011 36.54 3.17454 0.05308 5.16 ? 0.17 0.1018 25.0 % 3.853E-15 0.652830 0.001833 0.062524 0.000189 0.001104 0.000019 0.706109 0.010793 0.001746 0.000022 29.38 3.09157 0.10949 5.03 ? 0.36 0.038Weighted Mean Age (6 of 8, steps 1,2 excluded): 5.12 ? 0.12Sample:  J2-299-20  J = 0.0009024 ? 0.0000009 (1?) D/amu: 1.0020 ? 0.0002 (1?) Groundmass: 25 mgN Percent40Ar 40Ar ? ??? 39Ar ? ??? 38Ar ? ??? 37Ar ? ??? 36Ar ? ??? %40Ar* 40Ar*/39ArK ? 1? Age ? 2? K/CaPower (moles) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Volts) (Ma) (Ma)1 2.4 % 2.910E-15 0.493085 0.000730 0.003273 0.000050 0.000344 0.000024 0.001838 0.002047 0.001602 0.000011 4.04 6.08091 1.04233 9.87 ? 3.38 0.7652 3.0 % 5.588E-15 0.946866 0.001104 0.008671 0.000042 0.000699 0.000019 0.011982 0.002063 0.003084 0.000020 3.84 4.19667 0.69011 6.82 ? 2.24 0.3113 3.6 % 7.132E-15 1.208375 0.001099 0.020959 0.000057 0.000953 0.000022 0.026763 0.002096 0.003884 0.000033 5.19 2.99644 0.46582 4.87 ? 1.51 0.3364 4.5 % 8.225E-15 1.393512 0.000669 0.050351 0.000142 0.001390 0.000021 0.067832 0.002322 0.004182 0.000024 11.70 3.24158 0.14411 5.27 ? 0.47 0.3195 6.0 % 6.367E-15 1.078782 0.000917 0.074309 0.000112 0.001380 0.000023 0.130751 0.002905 0.002903 0.000021 21.42 3.11280 0.08453 5.06 ? 0.27 0.2446 8.0 % 4.998E-15 0.846850 0.001158 0.082667 0.000113 0.001362 0.000019 0.235432 0.004241 0.002050 0.000028 30.63 3.14404 0.10060 5.11 ? 0.33 0.1517 11.0 % 3.424E-15 0.580078 0.002785 0.075519 0.000555 0.001131 0.000017 0.314827 0.005299 0.001245 0.000014 40.79 3.14211 0.07075 5.11 ? 0.23 0.1038 25.0 % 4.282E-15 0.725566 0.002875 0.061164 0.000531 0.001114 0.000020 0.608065 0.009728 0.001969 0.000026 26.32 3.14376 0.13867 5.11 ? 0.45 0.043Weighted Mean Age (6 of 8, steps 1,2 excluded): 5.11 ? 0.14Constants used SourceAtmospheric argon ratios 40Ar/36Ar 295.5 ? 0.5 Steiger R. H. and J?ger E. (1977) Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology,Earth Planet. Sci. Lett. 36 , 359?362. 38Ar/36Ar 0.1880 ? 0.0003 Nier, A.O., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Phys. Rev. 77,  789?793.Interfering isotope production ratios(40Ar/39Ar)K (5.4 ? 1.4) x 10-4 Jicha, B.R. and Brown, F.H. (2013) An age for the Korath Range, Ethiopia and the viability of 40Ar/39Ar dating of kaersutite in Late Pleistocene volcanics, Quat. Geochron., in press.(38Ar/39Ar)K (1.210 ? 0.002) x 10-2 Ibid.(39Ar/37Ar)Ca (6.95 ? 0.09) x 10-4 Renne, P.R., et al. 2013. Time scales of critical events around the Cretaceous-Paleogene boundary. Science 339, 684-687.(38Ar/37Ar)Ca (1.96 ? 0.08) x 10-5 Ibid.(36Ar/37Ar)Ca (2.65 ? 0.022) x 10-4 Ibid.Decay constants 40K??? (0.580 ? 0.014) x 10-10 a-1 Min, K., R. Mundil, P. R. Renne, and K. R. Ludwig (2000), A test for systematic errors in  40Ar/39Ar  geochronology through comparison with U/Pb analysis of a 1.1-Ga rhyolite, Geochim. Cosmochim. Acta, 64,  73-9840K??? (4.884 ? 0.099) x 10-10 a-1 Ibid.39Ar (2.58 ? 0.03) x 10-3 a-1 Stoenner, R.W., Schaeffer, O.A., Katoff, S. (1965) Half-Lives of Argon-37, Argon-39, and Argon-42, Science 148, 1325-1327.37Ar (5.4300 ? 0.0063) x 10-2 a-1 Renne, P.R., Norman, E.B., 2001. Determination of the half-life of 37Ar by mass spectrometry. Physical Review C 63 047302, 3 pp.36Cl ?? (2.35 ? 0.02) x 10-6 a-1 Endt, P.A., (1998) Supplement to energy levels of A = 21-44 nuclei (VII), Nucl. Phys. A, 633, 1-220 280028503100335033503400345035003500Depth (km)159?24'W159?24'W159?22'W159?22'W159?20'W159?20'W21?18'N 21?18'N21?19'N 21?19'N21?20'N 21?20'N21?21'N 21?21'N21?22'N 21?22'N4000          3800          3600            3400         3200            3000          2800          2600 Cone A#01-064.29 MaCone B#08-114.29 MaDive 252Figure A1. Detailed bathymetry (colored) of the area of dive 252.  Contour interval is 25 m.  Circles show sample locations. Labels show the sample numbers and ages obtained from within the adjacent cluster of circles.34503500355036003650370037503800Depth (km)159?56'W159?56'W159?54'W159?54'W159?52'W159?52'W159?50'W159?50'W159?48'W159?48'W21?29'N 21?29'N21?30'N 21?30'N21?31'N 21?31'N21?32'N 21?32'N21?33'N 21?33'N21?34'N 21?34'N21?35'N 21?35'N4200          4000          3800          3600            3400         3200            3000          2800 Dive 297Area A#01-03Cone D#19-240.19, 0.23 MaCone B#04-104.03, 4.19, 4.24 MaCone C#11-184.34 MaArea E#25-275.40 MaFigure A2. Same as Figure A1 but for dive 297. 385039003950400040504100Depth (km)159?24'W159?24'W159?22'W159?22'W159?20'W159?20'W159?18'W159?18'W21?03'N 21?03'N21?04'N 21?04'N21?05'N 21?05'N21?06'N 21?06'N21?07'N 21?07'N4400          4200          4000          3800          3600            3400         3200            3000Cone A#01-05Dive 298Site B#06-07Cone DCone C#09-11#124.00 Ma#204.79 MaFigure A3. Same as Figure A1 but for dive 298. 2950300032003250330033003350340034503450350035503600Depth (km)159?34'W159?34'W159?32'W159?32'W159?30'W159?30'W21?20'N 21?20'N21?21'N 21?21'N21?22'N 21?22'N21?23'N 21?23'N21?24'N 21?24'N21?25'N 21?25'N21?26'N 21?26'N21?27'N 21?27'N21?28'N 21?28'N3800          3600            3400         3200            3000          2800          2600           2400 Dive 299Site C#26-314.02, 4.03 MaSite A#01-094.42, 4.51 Ma#19-234.02, 4.44, 5.11 Ma#10-194.17Ma#24-25#33-345.07 MaSite B#32#35-38Site DKS-14.02MaFigure A4. Same as Figure A1 but for dive 299. 

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