Science, Faculty of
Earth, Ocean and Atmospheric Sciences, Department of
DSpace
UBCV
Russell, Megan B.
Master of Science - MSc
University of British Columbia
In this thesis, an investigation of volcanism associated with Aramaiti Corona, Venus, is presented. Two features were identified in images and altimetry data collected during the NASA Magellan mission, which orbited Venus 1990–1994, and are studied here using higher resolution topography later derived from stereo SAR images. The first is Narina Tholi, a steep-sided volcanic dome located on the western fracture annulus of Aramaiti Corona. Around this dome is a topographic moat characteristic of a lithospheric flexural signature. By modelling lithospheric flexure on terrestrial planets and icy moons, subsurface heat flow, which is an important metric in investigating a thermal history, can be estimated. The topographic signature is compared with models for the flexure of a thin elastic shell to estimate the lithospheric thickness and the associated heat flow. The results for Narina Tholi indicate a thinner lithospheric thickness and correspondingly higher heat flow than previously estimated regionally from flexure around Aramaiti Corona. This implies a locally higher heat flow, consistent with the emplacement of Narina Tholi late in the corona’s evolution. The second feature is associated with the north portion of Aramaiti Corona and is inferred to be the result of volcanism associated with the corona fracture annulus. This suggests a different style of volcanism at Aramaiti Corona than has been previously observed, one that is possibly late-stage.
Heat Flow and Late-Stage Volcanism at Aramaiti Corona, Venus by Megan B. Russell B.Sc. (Hons), York University, 2009 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES (Geophysics) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) March 2020 © Megan B. Russell, 2020 ii The following individuals certify that they have read, and recommend to the Faculty of Graduate and Postdoctoral Studies for acceptance, a thesis/dissertation entitled: Heat Flow and Late-Stage Volcanism at Aramaiti Corona, Venus, submitted by Megan B. Russell in partial fulfillment of the requirements for the degree of Master of Science in Geophysics. Examining Committee: Catherine L. Johnson, Professor, Department of Earth, Ocean and Atmospheric Sciences, UBC Supervisor A. Mark Jellinek, Professor, Department of Earth, Ocean and Atmospheric Sciences Supervisory Committee Member Lori Kennedy, Associate Professor, Department of Earth, Ocean and Atmospheric Sciences UBCUBC Additional Examiner Additional Supervisory Committee Members: Valentina Radić, Associate Professor, Department of Earth, Ocean and Atmospheric Sciences Supervisory Committee Member iii Abstract In this thesis, an investigation of volcanism associated with Aramaiti Corona on Venus is presented. Two features were identified in images and altimetry data collected during the NASA Magellan mission, which orbited Venus 1990–1994, and are studied here using higher resolution topography later derived from stereo SAR images. The first is Narina Tholi, a steep-sided volcanic dome located on the western fracture annulus of Aramaiti Corona. Around this dome is a topographic moat characteristic of a lithospheric flexural signature. By modelling lithospheric flexure on terrestrial planets and icy moons, subsurface heat flow, which is an important metric in investigating a thermal history, can be estimated. The topographic signature is compared with models for the flexure of a thin elastic shell to estimate the lithospheric thickness and the associated heat flow. The results for Narina Tholi indicate a thinner lithospheric thickness and correspondingly higher heat flow than previously estimated regionally from flexure around Aramaiti Corona. This implies a locally higher heat flow, consistent with the emplacement of Narina Tholi late in the corona’s evolution. The second feature is associated with the north portion of Aramaiti Corona and is inferred to be the result of volcanism associated with the corona fracture annulus. This suggests a different style of volcanism at Aramaiti Corona than has been previously observed, one that is possibly late-stage. iv Lay Summary This thesis presents an investigation of two aspects of volcanic activity at Aramaiti Corona, a class of large volcanic-tectonic features on Venus. This is carried out using data collected during the NASA Magellan mission, which orbited Venus 1990 – 1994. The main feature of Aramaiti Corona is a 350 km diameter, 100 km wide annulus of concentric fractures. A steep-sided volcano located on the fracture annulus, and hints of additional volcanic activity associated with its northern rim, is characterized using images and elevations. Surrounding the volcano is a circular moat, possibly from the ground flexing under the weight of the volcano. Surface heat flow at the time the volcano was created is estimated by comparing the moat to the predictions of models for flexure of an elastic shell. Our results show a signature of locally high heat flow at the volcano, and further activity volcanic associated with the north rim. v Preface This thesis is original work completed by Megan Russell in collaboration with MSc supervisor Catherine Johnson, who provided an endless amount of support and guidance. During my time as a graduate student at the University of British Columbia, I investigated numerous volcanic features on Venus, specifically associated with Aramaiti Corona, a large tectono-volcanic feature in the southern hemisphere plains. The thesis comprises two distinct self-contained studies. Chapter 2, the main content, is a study of flexure and heat flow around a volcano on Aramaiti Corona. I am responsible for identifying the project area, the idea for modelling the flexure was Dr. Johnson’s and I did the modelling using code adapted from Johnson and Sandwell (1994). The discussion and implications were my work, guided by Dr. Johnson. This chapter is a manuscript that will be submitted to a journal, first authored by me and co-authored by Dr. Johnson. The identification and analysis of the features of interest in Chapter 3 were mine and the idea for comparison with terrestrial features was suggested by Dr. Jellinek. The writing of the thesis and manuscript was done by me, with input and feedback from Dr. Johnson. vi Table of Contents Abstract ......................................................................................................................................... iii Lay Summary ............................................................................................................................... iv Preface .............................................................................................................................................v Table of Contents ......................................................................................................................... vi List of Tables .............................................................................................................................. viii List of Figures ............................................................................................................................... ix List of Symbols .............................................................................................................................xv List of Abbreviations ................................................................................................................. xvi Acknowledgements ................................................................................................................... xvii Chapter 1: Introduction ................................................................................................................1 1.1 Background: Exploration and Some Basic Properties of Venus ........................ 1 1.2 Magellan Mission Data Sets ............................................................................... 3 1.3 Surface Topography and Geological Features .................................................... 6 1.4 The Resurfacing Puzzle .................................................................................... 11 1.5 Overview of the Next Two Chapters ................................................................ 13 Chapter 2: Investigation of Lithospheric Flexure and Heat Flow at a Volcanic Construct on Venus .............................................................................................................................................17 2.1 Introduction ....................................................................................................... 17 2.2 Aramaiti Corona and Narina Tholi: Geological Setting ................................... 20 2.3 Axisymmetric Flexural Modelling .................................................................... 23 2.3.1 Elastic Flexure Model ................................................................................... 23 2.3.2 Modelling Approach ..................................................................................... 26 vii 2.4 Results ............................................................................................................... 28 2.4.1 Predicted Elastic Lithospheric Thickness ..................................................... 28 2.4.2 Predicted Elastic Flexural Stresses ............................................................... 30 2.5 Discussion ......................................................................................................... 31 2.5.1 Young’s Modulus.......................................................................................... 33 2.5.2 Estimating the Volume of Narina Tholi........................................................ 37 2.5.3 Mechanical Thickness and Heat Flow at Narina Tholi ................................. 37 2.6 Conclusions ....................................................................................................... 41 Chapter 3: Further Evidence for Volcanism Associated with the Fracture Annulus of Aramaiti Corona ..........................................................................................................................43 3.1 Topography Associated with the North Rim .................................................... 46 3.2 Blocky Deposit.................................................................................................. 50 3.2.1 Observations ................................................................................................. 50 3.2.2 Discussion ..................................................................................................... 55 3.3 Conclusions ....................................................................................................... 60 Chapter 4: Conclusion .................................................................................................................62 Bibliography .................................................................................................................................63 viii List of Tables Table 1.1: Basic Properties of Venus. ............................................................................................ 2 Table 2.1: Parameters and their values used in flexural modelling. ............................................ 25 Table 2.2: Summary of elastic thicknesses, root-mean-square (RMS) misfits, peak stress magnitude and location for profiles 2–10 for different values of E. The location of the peak radial stress on each profile is the same for different values of E. ......................................................... 36 Table 2.3: Elastic thicknesses, bending moments calculated from (A9) in Solomon and Head (1979), corresponding temperature gradients, mechanical thicknesses and heat flow for each profile. The thermal gradient is calculated from a yield strength envelope incorporating dry Maryland diabase crust and dry olivine mantle rheologies, and a strain rate of 10-16 s-1. ............ 39 ix List of Figures Figure 1.1: Global topographic map of the Global Topography Data Record (GTDR) derived from Magellan altimetry elevations. Horizontal resolution is ~10–20 km (Ford and Pettengill, 1992). Yellow labels are broad geographic regions as named by the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature. The locations of chasmata (filled purple circles), major planitia i.e., low-lying plains (filled lime green circles) and craters (smaller aquamarine circles) are shown. Colour-bar taken from the Planetary GIS web server from the US Geological Survey (USGS). ....................................................................................... 4 Figure 1.2: Global map in pseudocylindrical projection showing left-look Synthetic Aperture Radar (SAR) Full Resolution Map (FMAP). Areas with no data are white. Shading in this and several other radar images in this thesis is inverted such that darker shading represents regions of high radar reflectivity (i.e. radar bright). ........................................................................................ 5 Figure 1.3: Global map in pseudocylindrical projection showing left-look SAR FMAP images in greyscale, with stereo-topography (Herrick et al., 2012) coverage in aquamarine. Regions with stereo topography coverage were produced by two left-look SAR images at different look angles.......................................................................................................................................................... 6 Figure 1.4: Left-look synthetic aperture radar images overlain on Global Topography Data Record (GTDR) for two types of coronae. (a) Fatua Corona, a type 1 corona near Alpha Regio. Map is in Mercator projection. (b) Three type 2 coronae, Cassatt (south-east), Muzamuza (south-west), Nzingha (north) located just west of Metis Regio. Map is in Albers (equal-area conic projection) projection. The bright line down the centre of the image is an area of no SAR data. (c–e) Histograms of Type 1 coronae that have annulus width measurements (~300 out of 515) for corona radius, annulus width and annulus width as a fraction of radius. These measurements x were compiled by me, to update and extend the original reported histograms in Stofan et al., 1992............................................................................................................................................... 10 Figure 1.5: Synthetic aperture radar (SAR) image overlain on the Global Topography Data Record (GTDR) of Aramaiti Corona and surrounding region. Ovda Region, an intensely deformed plateau is in the north, as is a small outcrop of deformed tesserae in the east. Illumination from the right............................................................................................................ 14 Figure 2.1: (a) Magellan right-look SAR image of Aramaiti Corona. Narina Tholi (center left) appears as two adjacent domes that are superposed on the west outer fracture annulus of the corona. Annulus fractures are seen as concentric radar-bright lineations. Narina Tholi lies on a large radar-bright flow (Oilule Fluctus) that hosts Ahsabkab Vallis (AV), a channel about 5 km wide and over 700 km long. Also seen in the vicinity of the corona are various radar dark and bright deposits, a southwest-northeast trending fracture belt (NEFB), and small shield volcanoes. Scale bar at bottom of image. Here, radar-bright terrains are light grey to white, radar-dark terrains are dark grey to black. (b) Magellan right-look SAR image (illumination from right) overlain on altimetry-derived topography over Aramaiti (Gridded Topographic Data Record, GTDR, of Ford and Pettengill, 1992). “FA” indicates the fracture annulus in the north and south. Total elevation range is -800 m (dark blue) to +1600 m (dark red). ............................................ 21 Figure 2.2: Narina Tholi and Aramaiti Corona fracture annulus. Magellan right-look SAR image is superimposed on color elevation derived from (a) the gridded altimetry (GTDR) and (b) stereo-SAR. In the images, blue indicates low and red indicates high elevations. Total elevation range is ~1.8 km (altimetry) and ~1.5 km (stereo-SAR). In (b), lines indicate the locations of topographic profiles examined for modelling, and gray filled circles on profiles 1–10 examined xi show the radial extent of the profile included in the modelling. Profiles 11–15 were too short to allow reliable modelling. Grayscale regions lack stereo-SAR coverage. ..................................... 23 Figure 2.3: (a) Topographic profile 5 (location shown in Fig. 2.2b), showing elevation as a function of distance from the tholi center (solid black curve). The predicted profile from the best-fit elastic flexure model is shown as a red dashed line, and circles along the profile show the locations of the parameters rLOAD (black), r1 (blue), and r2 (orange). (Inset) he and corresponding root-mean-square (RMS) misfit curve. (b) RMS vs he for different combinations of r1 and r2 listed as r1; r2 in the legend. r1 was increased from 21–41 km in 5 km increments and r2 from 40–60 km in 10 km increments. The black dashed line shows the RMS misfit curve for the choices of r1 and r2 used for profile 5. ....................................................................................................... 27 Figure 2.4: (a) Topographic profiles vs distance from tholi center (black) and best fit flexural model (red dashed) for profiles 1 (bottom) to 10 (top) (see Fig. 2.2b for track locations). Best fit he is indicated at the end of each profile. The best-fit elastic thickness values corresponding to E = 65 GPa are located at the end of the profiles, with those corresponding to E = 5 GPa in parentheses. (b) Root-mean-square (RMS) misfit vs he (km) for each profile. ............................ 29 Figure 2.5: Topographic profiles with the location of the maximum tensile radial stress associated with lithospheric flexure (triangles) for E = 65 GPa. Colour of the triangle indicates stress magnitude with values shown in the legend. (Inset) Zoom on a single concentric fracture collated with the flexure, and the peak stress locations. Arrows show location of fracture. ........ 31 Figure 2.6: Surface radial stress vs distance from tholi center for three values of Young’s modulus. Profile 3 (top) and Profile 4 (bottom) both cross the fracture concentric to the load, which is marked on the profiles as a circle (see Fig. 2.5 inset). The stress at the location of the fracture is ~20 MPa for E = 5 GPa and ~120 MPa for E = 65 GPa. Compressional and tensional xii stresses are represented by negative and positive values respectively. The profiles begin at the radius of the load. The radial location of the fracture is plotted on the profiles as a circle. The gray box is showing the range of tensile strengths of basalt for different RMR values, from heavily fractured to intact lab sample quality (0.1–25 MPa). ....................................................... 36 Figure 3.1: Magellan right-look SAR image (illumination from right) overlain on (a) altimetry-derived topography (Gridded Topographic Data Record, GTDR of Ford and Pettengill, 1992) showing coverage over Aramaiti Corona (centre) and Narina Tholi (centre-left). Narina Tholi is superposed on the western fracture annulus (FA) of Aramaiti Corona. Arrows indicate the inner FA in the north and the radially-continuous FA in the south. Blue indicates low and red indicates high elevations relative to the mean planetary radius. Total elevation range is -800 m (dark blue) to +1600 m (dark red). All SAR images in this chapter are shaded such that radar-bright materials are darker greys, and radar-dark (smooth) materials are light. (b) Stereo coverage over Aramaiti Corona (Herrick et al., 2012). Stereo-topography is not continuous over the region because two SAR images with appropriate look-geometry were not always available. Note that the absolute scale is different from that in Fig. 3.1a because stereo topography gives the relief relative to a baseline of 6040.0 km (Herrick et al., 2012), whereas altimetry is relative to the mean planetary radius of 6051.8 km. Colours represent extent of available topographic coverage, which ranges from -1700 m (blue) to + 300 m (red). ................................................................... 44 Figure 3.2: Zoom of the north rim of Aramaiti Corona. Magellan right-look (illumination from the right) SAR image superposed on colour elevations derived from (a) gridded altimetry (GTDR) and (b) stereo-SAR. Line segments on (b) indicate the locations of the along-strike (A1) profiles taken from the GTDR and stereo, as well as the cross-strike (C1, C2, C3) profiles, all shown in Fig. 3.4. Blue indicates low and red high elevations relative to mean planetary radius. xiii Total elevation range is -100 to 750 m (altimetry) and -1000 to -100 m (stereo-SAR). Dark shading in the SAR image represents brighter radar-return. ......................................................... 45 Figure 3.3: (a) Along-strike profiles using the GTDR and stereo-SAR elevations from west (left) to east (right). The location for both profiles is shown in Fig. 3.2b. Horizontal distance and vertical height is in metres. As noted in Fig. 3.1 the absolute scale for the stereo is offset from that of the altimetry. (b) Across-strike profiles across three topographic highs, from north (left) to south (right). .................................................................................................................................. 47 Figure 3.4: (a) Google Earth view of Tarawera Volcanic Dome Complex, containing Ruawahia Dome (RD) in the centre, and two other adjacent domes. Location of topographic profile in Fig. 3.4b is shown in white. (b) Along-strike profile of the domes. Horizontal length is 5 km, and vertical scale is ~260 m. Vertical exaggeration ~5x. Vertical and horizontal scale bars are given on the figure, because of the small font in the Google Earth generated profile. .......................... 49 Figure 3.5: Right-look SAR image of Aramaiti northern rim showing the location of blocky deposit relative to the FA. In the west the deposit is blocky, with clear distinct blocks above the resolution of the radar, and toward the east it comprises finer material. North is pointing up in this image. The higher portion of the corona rim is on the lower half of the image and the blocky deposit sits outboard of this at the base of the topography. The deposit (dark) is superposed on the smooth plains unit (light). ....................................................................................................... 50 Figure 3.6: Deposit with boundaries of distinct units mapped (see text). Main blocky boundary is labelled ‘B1’, boundaries of smoother materials, ‘S’ (S1, S1), and wispy boundaries with ‘W’ (W1, W2). “o” and “i” are discussed in the text. .......................................................................... 52 Figure 3.7: Right-look SAR image of north rim, with two zooms of the rubble boundary. Red boundary corresponds to the outermost boundaries (consisting of B1 plus W1). Isolated dark xiv features interpreted as blocks in the far field are outlined with blue circles. These are automatically detected via their radar contrast with the underlying unit. Individual blocks contained within the main deposit are visible in the two zooms. The radar-facing side shows up as a coherent radar-bright (shaded) feature. .................................................................................. 53 Figure 3.8: (a) Zoom on the area just west of that in Fig. 3.2. Magellan right-look SAR image superposed on colour elevations derived from stereo-SAR. Line segment indicates the location of the profile shown in (b). On the profile (b), the extent of the rubble pile is indicated with circles along the profile around 50 km. The profile starts outboard of the fracture annulus (0 km) with distance increasing towards corona interior. ................................................................................. 54 Figure 3.9: Synthetic aperture radar (SAR) images of two debris avalanches associated with volcanic domes. Illumination is from the left, and the images are shaded such that radar-bright materials are darker purple, and radar-dark (smooth) materials are beige. North is up in both images. (a) Albers (equal-area conic) projection of blocky deposit near Metis Regio. Note the fan-like planform of the deposit, with individual blocks up to ~2.5 km visible within the deposit. (b) Custom UTM projection of blocky deposit associated with volcanic dome between Atla and Phoebe Regiones. Some individual blocks up to 4 km in length are visible. ............................... 58 xv List of Symbols Symbol Definition Unit α Flexural wavelength m D Rigidity Pa m3 dT/dz Thermal gradient through the lithosphere K m-1 E Young’s Modulus Pa g Gravitational acceleration m s-2 he Elastic thickness m hm Mechanical thickness m k Thermal conductivity Wm-1 K-1 κ Curvature m-1 κo Curvature at first zero crossing m-1 ρ Lithospheric density kg m-3 Tm Mantle temperature K Ts Surface temperature K σ Lithospheric strength Pa   Time since lithospheric overturn event years qs Heat flow Wm-2 ν Poisson’s ratio dimensionless xvi List of Abbreviations FA Fracture annulus GIS Geographic Information System GTDR Global Topographic Data Record NASA National Aeronautics and Space Administration SAR Synthetic aperture radar YSE Yield strength envelope xvii Acknowledgements I am grateful not only for the amazing research that surrounds me every day, but also for the people behind that research. To my friends from the lab who extend across 5 different groups, we only seem to be divided by the planetary bodies we study as we have made so many great memories: road trips to national monuments, battling cows to visit volcanoes, getting rescued at the Grand Canyon, new board games, old family recipes at Christmas, a crutch to lean while grocery shopping, manuscript editing, thoughtful feedback during presentations, thoughtful complete and total reworking of said presentations, hikes and snacks, lunches in the sun and Friday socials. Gabriela, thanks for your generosity and thoughtfulness, whether it be in a happy celebration or a “keep looking up” kind of occasion. My gratitude goes to my committee, Catherine, Mark and Valentina, for being great at what you do, and supporting me inside and outside research. The breadth of scientific study you collectively cover is admirable. To Catherine Johnson and Mark Jellinek. Thank you for answering that first email before welcoming me into the group, sharing your love of science and your ability to be so successful at it. Through thoughtful investigations of physical phenomena, and insight into the meteoric amount of work and passion that goes into successful space missions, I now have a deep understanding of science. Thank you for the opportunity to travel to conferences and rocket launches, eat tacos near air force bases, the temporary relocation to a warmer climate and a lifelong appreciation of Scripps and Southern California in general. You’ve put together an awesome group and we are all lucky to be part of it. To my three non-nerds from across Canada, Cecilia, Marie and Rachel. Thanks for your support through undergrad and continued friendship, and congrats Cess, can’t wait for a xviii Pasadena visit. Thanks to other friends for providing excitement outside of academia, cheers to 61 punk rock shows per month, the Kits Girls Club, and road trips to weddings in Europe and Squamish, and also Area 51 in a Camaro. Thank you Lisa for being super supportive, I will always appreciate your late night texts and endless work snacks. To my mom – thank you for your enthusiastic support when I wanted to switch gears while living in Vancouver to pursue something I love; I will never be able to thank you enough. You knew it was time to switch gears and I would not have been able to do it without you. 1 Chapter 1: Introduction 1.1 Background: Exploration and Some Basic Properties of Venus Venus, the second planet from the Sun, is often referred to as Earth’s sister planet because of its similar mass, size and orbital distance (Table 1.1). However, Earth-based observations are limited by its thick atmosphere. The first mission to Venus, Venera 4 in 1967, was primarily designed to return atmospheric data during entry. Data from this and the subsequent Soviet Venera mission probes and landers have been foundational to understanding the surface geology and interior geophysics, elucidated by later missions. The atmosphere is dominated by carbon dioxide and the lower atmosphere contains minimal hydrogen and oxygen (Johnson and Oliveira, 2019). Furthermore, the deuterium to hydrogen ratio (~0.024) is a factor of 150 higher than that of Earth’s oceans (Basilevsky and Head, 2003; Johnson and Oliveira, 2019), indicating that the Venus atmosphere once had significantly more water than today (Donahue, 1999). Venera 4 returned the first measurements of surface temperature, TS, and pressure, PS (Table 1.1). During the 1960s–1980s several more Venus missions were carried out by the USA and USSR space programs in the form of fly-by, orbiter, and lander missions. Collectively these missions showed a surface that is predominantly shaped by volcanic and tectonic features, as well as some impact features. The Venera landers, and two landers from the Vega mission, measured the abundances of the major heat-producing elements uranium (U), thorium (Th) and potassium (K) at five landing sites (see Table 1 of Surkov et al., 1987), and petrogenic constituents at three sites (see Table 2 Surkov et al., 1997). In general, these values are similar to those of terrestrial basalts, suggesting similar present-day heat production in the two planets. At one site, Venera 8, somewhat higher Th and U concentrations were detected, 2 suggestive of more evolved (i.e., more silicic) surface rocks (Shellnut, 2019; Surkov et al., 1984; Vinogradov et al., 1973). In the late 1970s, the U.S. Pioneer Venus project comprised a probe and orbiter component; the probe providing atmospheric information and the orbiter mapping surface elevations with radar. About 10 years later, the NASA Magellan spacecraft orbited the planet from 1990 until 1994, providing the most complete global data sets of the surface that exist to date. After a long hiatus in Venus exploration following the Magellan mission (see below), the European Space Agency’s (ESA) Venus Express, an atmospheric and interplanetary environment mission, was inserted into orbit around Venus in 2005. Mean orbital distance from Sun rorbit 0.72 AU Mean planetary radius RP 6051.8 km Gravitational acceleration g 8.87 ms-2 Average surface temperature TS 728 K / 455° C Average surface pressure PS 93 bar (9.3 x 106 Pa) Atmospheric constituents CO2 (96.5%); N2 (2.5%); trace SO2, Ar Table 1.1: Basic Properties of Venus. This thesis is based on data collected during the Magellan mission. Below I briefly describe the data sets available from this mission (Section 1.2) and review the global physiography of Venus (Section 1.3). One of the outstanding questions resulting from the Magellan mission is how Venus loses heat – the driving observation for this is the impact crater record seen in radar images of the surface (Section 1.4). This question motivated the main focus of my thesis, a study of lithospheric thickness and heat flow at a Venusian volcano (summarized in Section 1.5, detailed in Chapter 2). During the course of that investigation, I noticed some 3 interesting nearby features and these became the focus of a second, shorter study (summarized in Section 1.5 and detailed in Chapter 3). 1.2 Magellan Mission Data Sets The key scientific objectives of the Magellan mission included global characterization of volcanic landforms, tectonism and impact features, as well as identification of landforms resulting from erosional, depositional and chemical processes and providing the collection of data to enable modelling of the interior density distribution. The thick atmosphere, optically opaque to visible wavelengths, requires the use of radar to achieve these goals from orbit. Magellan carried a single radar instrument, used in different modes to collect synthetic aperture radar (SAR) images (98% geographical coverage), and make altimetric measurements of surface elevations. Two-way ranging (Earth-to-spacecraft-to-Earth) was used to infer the planet’s gravity field (95% coverage). These measurements were achieved over six mapping cycles, where one mapping cycle refers to a complete rotation of Venus beneath the spacecraft orbit, i.e., 243 days (Stofan et al., 1993). The SAR images were compiled into a mosaic with a pixel resolution of 75 m (actual resolution ~150 m) and the altimetry profiles into a grid with a pixel resolution of ~4.6 km (Figs. 1.1 and 1.2). The actual horizontal resolution of the gridded altimetry data, known as the Global Topographic Data Record or GTDR, is typically 10–20 km. The gravity field resolution is limited by spacecraft altitude which varied because of the spacecraft’s elliptical orbit. As a result, the best spatial resolution in the measured gravity field is ~250 km, but in some regions of the planet is closer to ~1000 km (Johnson, 1994; Konopliv et al., 1999; Smrekar et al., 2018). 4 Figure 1.1: Global topographic map of the Global Topography Data Record (GTDR) derived from Magellan altimetry elevations. Horizontal resolution is ~10–20 km (Ford and Pettengill, 1992). Yellow labels are broad geographic regions as named by the International Astronomical Union (IAU) Working Group for Planetary System Nomenclature. The locations of chasmata (filled purple circles), major planitia i.e., low-lying plains (filled lime green circles) and craters (smaller aquamarine circles) are shown. Colour-bar taken from the Planetary GIS web server from the US Geological Survey (USGS). 5 Figure 1.2: Global map in pseudocylindrical projection showing left-look Synthetic Aperture Radar (SAR) Full Resolution Map (FMAP). Areas with no data are white. Shading in this and several other radar images in this thesis is inverted such that darker shading represents regions of high radar reflectivity (i.e. radar bright). SAR is an “active” remote sensing technique, whereby a pulse of radar power is sent toward the planet’s surface and the reflected return power is measured as a function of time. The flight-path of an airborne or orbiting SAR instrument is used to create a synthetically larger antenna, greatly increasing the imaging resolution. This reduces the physical size requirements of the antenna. These signals are then processed into planform images resembling optical greyscale images. At imaging incidence angles (the angle of the downgoing radar beam with the local normal to the surface) θ < 20°, the radar return is most sensitive to topographic slope at the scale of the imaging resolution. For imaging incidence angles, θ > 20° and θ < 60°, it is most sensitive to surface roughness on the scale of the wavelength of the radar (~12.6 cm). Dielectric properties 6 of the imaged material also affect the return signal (Stofan et al., 1993). Some, but not all, locations on Venus were imaged with the SAR more than once, but from different viewing geometries, allowing stereo topographic products to be generated. The most comprehensive of these (Herrick at al., 2012) has a spatial resolution of ~1 km, an order of magnitude improvement in spatial resolution compared to the GTDR, and covers about 20% of the planet (Fig. 1.3). Figure 1.3: Global map in pseudocylindrical projection showing left-look SAR FMAP images in greyscale, with stereo-topography (Herrick et al., 2012) coverage in aquamarine. Regions with stereo topography coverage were produced by two left-look SAR images at different look angles. 1.3 Surface Topography and Geological Features Magellan revealed global topography that has a unimodal distribution of elevations relative to the mean planetary radius, RP = 6051.8 km (Fig. 1.1). In contrast, Earth has a bimodal distribution of elevations, reflecting the relatively deeper ocean basins and higher continental regions. The surface of Venus is covered by volcanic features, varied tectonism and surprisingly 7 few craters (Figs 1.1 and 1.2). However, unlike Earth, Venus shows no evidence for a global system of plate tectonics (Grimm and Solomon, 1989; Solomon et al., 1991, 1992). On Earth subduction of cold plates accounts for as much as 70% of the planet’s present-day heat loss (Sclater et al., 1980; Solomon and Head, 1982), and as I will discuss in Section 1.4, one of the enduring questions left unanswered by the Magellan mission to Venus (Saunders et al., 1992) is how Earth’s sister planet loses heat. Basilevsky and Head (2003) classified the surface of Venus into three major features: volcanic plains (planitiae), upland plateaus hosting intensely deformed terrain (tesserae), and long-wavelength topographic rises. Regional plains, interpreted to be the result of intensive volcanic flooding, cover about 80% of the surface (Basilevsky and Head, 2003). The majority of the plains are smooth, hosting low relief “wrinkle ridges”, interpreted to result from moderate compressional deformation. The large-scale topographic lowlands associated with the regional plains, together with the strong correlation of gravity with topography, led to the interpretation that these regional plains are dynamically supported and overlie large-scale mantle downwellings (Simons et al., 1997). The highly deformed tessera terrain is typically associated with elevated plateaus that rise 1–2 km above mean planetary radius. Examples include Ovda, Thetis, Themis and Phoebe Regiones in Fig. 1.1. The modest gravity signatures over these plateaus, again correlated with topography, point to isostatic compensation by a thickened crust (Simons, 1994; James et al., 2013). Broad topographic rises (horizontal wavelengths >1000 km) e.g., Beta and Atla Regiones (Fig. 1.1) are associated with extensive volcanism, rifts, and large positive gravity anomalies indicating dynamic support, possibly related to large-scale mantle upwelling (McKenzie, 1994; Phillips, 1994; Smrekar, 1994; Simons et al., 1997). There are also linear, 8 arcuate troughs (Hoogenboom et al., 2005), known as chasmata, interpreted to be extensional rift zones, that extend for up to 40,000 km (Basilevsky and Head, 2003; Solomon et al., 1992). Volcanic products are ubiquitous on Venus and extrusive volcanic features include extensive flow fields (more than 200 with individual areas larger than 50,000 km2), shield volcanoes (as small as 1 km and up to 100–1000 km), volcanic domes (average diameter ~20 km), channels up to 7,000 km in length and pits interpreted to be collapse features (Basilevsky and Head, 2003; Crumpler et al., 1993; Fink et al., 1993; Magee and Head, 1993; Pavri et al., 1992). Stratigraphically younger flows are often superposed on the larger plains units. These younger flows are often less deformed and sometimes show lobate margins. Individual flows are identified in the SAR images as large-scale homogenous features with smooth to lobate margins that contrast in radar brightness with the underlying surface. Depending on the imaging angle, the flow margins may also appear radar bright if they possess sufficient positive relief relative to the underlying surface (e.g., a levee). Importantly, and a major challenge for understanding the evolution of the Venusian surface, the margins of plains units may be visible in radar images only if there is a topographic boundary or a difference in surface properties between adjacent units. Some volcanic features seen in the SAR images are interpreted as intrusive. For example, large fracture systems have been proposed to be magmatic dike systems (Ernst et al., 1995) and some domes have been proposed to be analogous to laccoliths and cryptodomes on Earth (Head et al., 1991). Magellan data also shed light on a class of features unique to Venus known as coronae, which combine both volcanism and tectonism. Coronae are large quasi-circular features, ranging from ~100 km to over 1000 km in diameter, and were first identified in Venera 15 orbiter radar images of the surface (Barsukov et al., 1986) as concentric-elliptical fractures following 9 topographic ridges. They are typically defined by a partial annulus of concentric fractures (Stofan et al., 1992) that have widths of ~10 km to 150 km. A second category of coronae was later defined via the presence of quasi-circular to circular features with topographic rims but with only “partial” or no fracture annuli i.e., fracture annuli that are contiguous in azimuth around less than half of the corona. These features otherwise have similar characteristics to coronae (Stofan et al., 2001), and were denoted as Type 2 coronae versus the earlier Type 1 coronae that have complete or almost-complete fracture annuli. The ~500 catalogued coronae (Stofan et al., 1992, 2001) often have extensive associated volcanism such as flows, domes and cones. Coronae have been grouped into nine topographic classes (Smrekar and Stofan, 1997), for example those with interior topographic highs versus interior lows. Many are too small to be resolved in gravity data (Johnson and Richards, 2003). Coronae are concentrated in and around the major topographic rises (Stofan et al, 1992; Glaze et al., 2002). On the basis of their quasi-circular planform, associated volcanism, and observed tectonism they are interpreted to be surface manifestations of transient mantle upwellings (Johnson and Richards, 2003; Schubert et al., 1989). Therefore, these features may provide insights into current and past heat loss on Venus, as well as convective flow regimes within the mantle (Johnson and Richards 2003; Robin and Jellinek, 2007; Smrekar and Stofan, 1997; Stofan et al., 1991), both important for understanding the planet’s resurfacing history. 10 Figure 1.4: Left-look synthetic aperture radar images overlain on Global Topography Data Record (GTDR) for two types of coronae. (a) Fatua Corona, a type 1 corona near Alpha Regio. Map is in Mercator projection. (b) Three type 2 coronae, Cassatt (south-east), Muzamuza (south-west), Nzingha (north) located just west of Metis Regio. Map is in Albers (equal-area conic projection) projection. The bright line down the centre of the image is an area of no SAR data. (c–e) Histograms of Type 1 coronae that have annulus width measurements (~300 out of 515) for corona radius, annulus width and annulus width as a fraction of radius. These measurements were compiled by me, to update and extend the original reported histograms in Stofan et al., 1992. 11 1.4 The Resurfacing Puzzle A major and surprising result from the Magellan mission was the presence of less than 1000 impact craters on the planet’s surface, with a spatial distribution indistinguishable from a uniform distribution (Schaber et al., 1992, see also Fig. 1.1). Furthermore, very few of the craters appeared to be volcanically or tectonically modified, suggesting limited geological activity since their emplacement on the surface (Izenberg et al., 1994; Schaber et al., 1992; Strom et al., 1994). The areal density of craters over a region of a planet’s surface is an indicator of the time since that surface has been exposed to incoming impactors, i.e. the time since resurfacing. The Moon, Mars and Mercury all exhibit very heavily cratered regions interpreted to be ~4 Gyr old, as well as less heavily cratered, smooth volcanic flows interpreted to be much younger. Converting a crater density to an absolute age requires models for the impactor flux as a function of time, for the crater diameter expected for a given size, density and energy of impactor, and in the case of Venus for the effect of the atmosphere on smaller impactors (Herrick and Phillips, 1994; Phillips et al., 1991; Schaber et al., 1992). On Venus, the areal crater density is so low that it is not possible to obtain statistically reliable relative ages for distinct regions (Phillips et al., 1992; Hauck et al., 1998; Campbell, 1999), and only an average global surface age can be inferred. The average surface age, 𝜏, has been suggested to range from 300 Ma to ~1 Ga, depending on the impactor flux assumed (Schaber et al., 1992; Phillips et al., 1992; McKinnon et al., 1997). Here we refer to this average surface age as either simply, 𝜏, and where we use absolute ages, these are referenced to 𝜏 = 750 Myr (McKinnon et al., 1997). Interpretation of the Venusian crater distribution in terms of resurfacing models has been a controversial topic since the earliest results from the Magellan mission. Two end-member resurfacing models have been proposed, colloquially referred to as the ‘catastrophic’ and 12 ‘equilibrium’ models. In the ‘catastrophic’ model, Venus was volcanically resurfaced globally over a very short interval at time 𝜏 ago, with subsequent volcanism and tectonism confined to the major rises and rift zones. This model predicts very little recent volcanic or tectonic activity and a directed global sequence of geological events (Basilevsky and Head, 1998; Ivanov and Head, 2011). It further predicts that the lithosphere, the mechanically stronger outer layer of the planet, has cooled conductively over the time 𝜏, resulting in a present-day mechanical thickness, hm, that is too large for the planet to be in thermal equilibrium (hm = 190–350 km for 𝜏 = 300 Myr–1 Gyr). The surface heat flow, qs = -k dT/dz = -k (Tm - Ts) / hm, where the base of the mechanical lithosphere is defined by a temperature isothere, Tm (~1013 K, see Chapter 2), and k is the thermal conductivity (~4 Wm-1K-1). Thus qs = 12–6 mW m-2, much less than that predicted (~70 mW m-2) on the basis of the observed abundance of heat producing elements, scaled to Earth’s value (Solomon and Head, 1982). As a consequence, in this model, the interior of Venus is heating up, so another ‘catastrophic’ resurfacing is predicted. The predicted repeated occurrences of such resurfacing have led to this scenario also being referred to as ‘episodic’ resurfacing. In contrast, in the ‘equilibrium’ model, volcanic or tectonic resurfacing occurs continuously and randomly in space over patches smaller than the average intercrater spacing (Guest and Stofan, 1999; Hansen and Young, 2007). This model permits present-day volcanic and tectonic activity, and spatially variable heat flow and lithospheric thicknesses that on average are in equilibrium with heat production. Recent re-investigations of crater modification states also suggest that some Venusian craters are volcanically flooded in their interiors (Herrick and Rumpf, 2011), also suggesting that ongoing volcanism may have occurred over the time . Observational tests of the two hypotheses could include (i) detailed geological mapping requiring higher resolution images than those available from Magellan, (ii) identification of 13 current volcanism and (iii) assessment of temporal and spatial variations in heat flow via estimates of lithospheric thickness. To date there has been no definitive identification of active volcanism, although there have been tantalizing suggestions of locally enhanced surface or brightness temperatures (Bondarenko et al., 2010; Shalygin et al., 2015) and high near-infrared emissivity suggestive of geologically recent (less than 0.25 Myr old) flows (Filiberto et al., 2020; Smrekar et al., 2010). Lithospheric thickness estimates from gravity and topography (Anderson and Smrekar, 2006; Hoogenboom et al., 2004; Moore and Schubert, 1995; Phillips, 1994), as well as from topographic flexural signatures (Brown and Grimm, 1996; Johnson and Sandwell, 1994; Sandwell and Schubert, 1992), have been used to argue for both end-member hypotheses. As described in more detail in Chapter 2, Section 2.1, topographic flexural features (e.g., flexure at ocean islands and subduction zones on Earth) provide an estimate of lithospheric thickness and hence heat flow at the time the load causing the flexure was emplaced. On Venus, topographic flexure is observed at several coronae and chasmata; however, distinguishing between spatial and temporal variations in heat flow is not possible because of the challenges regarding surface ages discussed above. Furthermore, resolution of both the gravity field and the altimetry-derived topographic grid have, until recently, precluded lithospheric thickness investigations at smaller features. 1.5 Overview of the Next Two Chapters This thesis focusses on two distinct studies of volcanic features associated with Aramaiti Corona, which is situated in Aino Planitia, a unit of volcanic plains (Fig. 1.5). The extensive volcanism that occurs at about 40% of coronae may hold clues about coronae formation and the 14 local and regional properties of the lithosphere on which the coronae form (Stofan et al., 1992). Figure 1.5: Synthetic aperture radar (SAR) image overlain on the Global Topography Data Record (GTDR) of Aramaiti Corona and surrounding region. Ovda Region, an intensely deformed plateau is in the north, as is a small outcrop of deformed tesserae in the east. Illumination from the right. 15 Stratigraphic mapping suggests that in general, volcanism associated with coronae typically occur prior to, but also synchronous with, the formation of the corona annulus (Roberts and Head, 1993). Volcanic flows can fill the coronae interiors and/or extend radially for 150–300 km from the source (Roberts and Head, 1993). Fields of individual shield volcanoes less than 1 km in diameter are found on the interior plateaus of coronae and exterior to the annulus, and may form as eruptive products late in the stage of coronae evolution. Both styles of volcanism point to low viscosity basaltic lavas. Less ubiquitous associated with coronae is the presence of steep-sided domes. Where observed, these are also inferred to form later in the evolution of an individual corona, as extrusive volcanism waned (Lang and Lopez, 2015; Roberts and Head, 1993). Steep-sided domes may reflect distinct composition, rheology or emplacement conditions (Fink and Bridges, 1993; Fink and Griffiths, 1998; Pavri et al., 1992). There have been studies of the morphologies of steep-sided Venusian domes, but not necessarily in association with the rims of coronae (Gleason et al., 2010). The next two chapters contain two studies of late stage volcanism associated with the concentric fracture annulus of Aramaiti Corona. The first study (Chapter 2) focuses on a steep-sided volcanic dome, Narina Tholi, which is located on the western section of the fracture annulus of Aramaiti Corona. Using stereo-topography (Herrick et al., 2012) a flexural signature associated with the volcanic load is investigated following the approach of Solomon and Head (1979), Johnson and Sandwell (1994) and O’Rourke and Smrekar (2018). Flexural modelling is used to estimate the subsurface heat flow at time of emplacement of this late-stage volcanic construct. Stereo-topography has previously been used investigate flexure signal around Aramaiti Corona and infer a regional heat flow (O’Rourke and Smrekar, 2018). The local heat flow value at Narina Tholi is compared to the regional value estimated at the host corona at time of formation, and to published results 16 from global heat flow studies. Chapter 2 forms the bulk of my thesis work and is about to be submitted to a journal for publication. I have shortened Section 2.1 to avoid repetition of some of the material in this broader thesis introduction, but other than this Chapter 2 is a self-contained manuscript. In Chapter 3, the area of interest is the north section of the fracture annulus of Aramaiti Corona. Volcanism specifically associated with the fracture annuli of coronae has received little attention until now; however, I suggest that the northern rim of Aramaiti shows evidence for volcanism that is different from the low viscosity lavas in the interior of the corona, but also different from Narina Tholi. Stereo-derived topography and SAR images are used to investigate the topography of the corona annulus and identify volcanic constructs with an appearance different from those previously found at coronae. Specifically, the corona annulus shows irregular along-strike topographic variations and excessive relief. The shape and short wavelength of relief points to a high strength material. There is also an extensive blocky deposit at the base of the topography interpreted to be sourced from the anomalous topography. I suggest that the pervasive fracturing may provide pathways for such volcanism, and furthermore that this volcanism may be late-stage in the evolution of the corona. 17 Chapter 2: Investigation of Lithospheric Flexure and Heat Flow at a Volcanic Construct on Venus 2.1 Introduction As motivated in Chapter 1, a major puzzle for Venus in the post-Magellan era is how the planet loses heat. In situ heat flow measurements have so far not been carried out on any extra-terrestrial surface, other than the Moon (Langseth et al., 1976) and, perhaps soon, Mars (Spohn et al., 2018). Instead, heat flow on terrestrial planets and icy moons is typically inferred indirectly via thermal emission remote sensing observations (e.g., Spencer et al., 2013) or lithospheric thickness estimates determined from flexural signals in topography and/or gravity data (e.g., Turcotte and Schubert, 1982; Watts et al., 2001). Topography and gravity data have been used to infer present-day Venusian lithospheric thickness (e.g., Nimmo and McKenzie, 1998; Simons et al., 1994), but the spatial resolution of the Magellan gravity data set ranges from 380 km to 950 km (Konopliv et al., 1999; Smrekar et al., 2018), restricting such studies to larger physiographic features. Alternatively, topography data alone can be used to investigate signatures of lithospheric flexural deformation, in particular at features too small to be resolved in Magellan gravity data. Such flexural signals record the mechanical lithospheric thickness, hm, at the time of deformation. The general procedure for estimating heat flow from topographic flexure is as follows. Elastic plate models are used to model the flexural signature in the topography and estimate an effective elastic thickness, he. This is converted to a physically meaningful quantity, the mechanical thickness, hm, assuming a yield strength envelope (YSE) for the crust and/or mantle and matching the moment sustained by the elastic plate model and the mechanical thickness 18 (McNutt et al., 1982). The base of the mechanical lithosphere corresponds to an isotherm, Tm, where the yield strength of the geologic material, most often dry olivine for mantle compositions, drops below 50 MPa and ductile deformation begins (typically taken at T = 700°C–800°C). Assuming a thermal conductivity, k and knowing the surface temperature, Ts, allows the thermal gradient dT/dz = (Tm - Ts) / hm and the corresponding heat flow, qs = k dT/dz to be computed. As noted in Chapter 1, establishing the chronology, even in a relative sense, of such estimates of hm, or equivalently qs, is a major challenge in Venusian studies. Previous studies of lithospheric flexure on Venus have focused on topographic signatures at chasmata and coronae (Sandwell and Schubert 1992; Johnson and Sandwell 1994). Coronae appear to be unique to Venus and may contribute as much as 25% to the present-day heat flux (Smrekar and Stofan, 1997). A small subset of coronae shows topographic flexural signatures, typically extending outwards from the fracture annulus (Sandwell and Schubert, 1992; Johnson and Sandwell, 1994; O’Rourke and Smrekar, 2018). Thin elastic plate models have been used to estimate he at these features, with most studies using Magellan altimetry profiles (Sandwell and Schubert, 1992; Johnson and Sandwell, 1994), that have an along-track resolution of ~10–20 km and a vertical precision of ~50–100 m (Ford and Pettengill, 1992). This restricted previous investigations to flexural features with a planform roughly perpendicular to the satellite tracks, and with relatively large (> ~50–100 km) flexural wavelengths. Flexure modelled at ~15 coronae and chasmata led to heat flow estimates in the range 27–70 mW m-2, consistent with predictions of 10–76 mW m-2 from stagnant lid models, chondritic thermal evolution models and terrestrial scaling (Ghail, 2015; Phillips and Malin, 1983; Solomon and Head, 1990). More recently, higher resolution stereo topography derived from Magellan Synthetic Aperture Radar (SAR) images (Herrick et al., 2012), has allowed a systematic study of flexural 19 features at smaller coronae (O'Rourke and Smrekar, 2018). The stereo topography has an order of magnitude improvement in spatial resolution compared with the gridded altimetry data (Ford and Pettengill, 1992). Results from this study indicate a much wider range of heat flow estimates at 18 features ranging from ~20 mW m-2 to ~150 mW m-2, with one estimate of ~290 mW m-2 (Fatua Corona). Because of the challenges in establishing ages, the relative contributions of spatial and temporal variations to these heat flow estimates is unknown. Here we investigate heat flow at a specific volcanic construct, Narina Tholi, spatially associated with the outer fracture annulus of Aramaiti Corona using high resolution topography of a flexural signature. The feature is of interest because it has been mapped as forming late in the sequence of tectonic and volcanic processes associated with the evolution of the corona (Lang and Lopez, 2015). Furthermore, a regional-scale heat flow estimate of ~100 mW m-2 was obtained from flexural signatures in the topography north and south of the corona (O’Rourke and Smrekar, 2018). Narina Tholi therefore provides the opportunity to establish a local estimate of heat flow in the context of the regional study and geological constraints on relative timing of events. First, we review previous studies of the geologic setting of Narina Tholi and Aramaiti Corona inferred from SAR imagery and topography (Section 2.2). The axisymmetric elastic flexure formulation used to model the topography (Brotchie, 1971) and to calculate surface stresses is described (Section 2.3). The resulting elastic thickness and stress estimates are summarized (Section 2.4). The magnitude of the predicted stresses for different values of Young’s modulus are compared with the expected strength of the material surrounding Narina Tholi, and are also compared with any evidence for circumferential fracturing around the volcanic construct (Section 2.5.1). Finally, the implications of the results for mechanical thickness and heat flow are discussed (Section 2.5.2). 20 2.2 Aramaiti Corona and Narina Tholi: Geological Setting Aramaiti Corona (Fig. 2.1) is located in Aino Planitia, within a unit of regional fractured plains material comprising multiple volcanic flows and hosting numerous volcanic and tectonic features such as shield volcanoes, fractures and wrinkle ridges (Head et al., 1992; Stofan and Guest, 2003). Aramaiti Corona is ~350 km in diameter and is classified as topographic class 3b (Smrekar and Stofan, 1997), i.e., a rim surrounding lower elevations, that in turn surround a central high. The topographic rim reaches elevations up to 600 m above the surrounding plains. Interior to the rim is a trough with minimum elevations about 1400 m lower than those of the rim. The center of the corona exhibits a dome at approximately the height of the surrounding plains. Broad topographic flexural signatures, represented by a trench and outer rise located just to the north and south of the corona rim, were previously modelled to return estimates of the regional effective elastic thickness (O’Rourke and Smrekar, 2018). Regional fracturing and deformation are observed around Aramaiti Corona, which lies on a NE trending fracture belt that extends for thousands of kilometers. An outcrop interpreted to be “basement” material, e.g., stratigraphically older fractured and ridged plains units, formed from regional compression (Basilevsky and Head 1997, Lang and Lopez, 2015), is visible to the east. The compression does not appear to have deformed the corona, and is thus interpreted to predate the formation of Aramaiti. Aramaiti’s tectonic annulus is about 100 km wide (Fig. 2.1) and lies on the topographic rim, extending inboard into the trough. The concentric fractures appear to be mainly extensional faults (Squyres, 1992) that deform the host local plains unit (Stofan and Guest, 2003). In the north, a concentric outer trough (Basilevsky and Head, 1998) separates the outermost concentric fractures from the main part of the fracture annulus. In and around the corona, including within 21 the troughs, localized radar-dark volcanic flow units as well as individual volcanic constructs are found to generally superpose the regional plains unit (Stofan and Guest, 2003). Figure 2.1: (a) Magellan right-look SAR image of Aramaiti Corona. Narina Tholi (center left) appears as two adjacent domes that are superposed on the west outer fracture annulus of the corona. Annulus fractures are seen as concentric radar-bright lineations. Narina Tholi lies on a large radar-bright flow (Oilule Fluctus) that hosts Ahsabkab Vallis (AV), a channel about 5 km wide and over 700 km long. Also seen in the vicinity of the corona are various radar dark and bright deposits, a southwest-northeast trending fracture belt (NEFB), and small shield volcanoes. Scale bar at bottom of image. Here, radar-bright terrains are light grey to white, radar-dark terrains are dark grey to black. (b) Magellan right-look SAR image (illumination from right) overlain on altimetry-derived topography over Aramaiti (Gridded Topographic Data Record, GTDR, of Ford and Pettengill, 1992). “FA” indicates the fracture annulus in the north and south. Total elevation range is -800 m (dark blue) to +1600 m (dark red). Narina Tholi is a steep-sided dome with a flat top (Gleason et al., 2010; Head et al, 1992; Sigurdsson, 2000), spatially associated with the western outer fracture annulus and interpreted to 22 be volcanic in origin (Fig. 2.1). Superposition relationships suggest that the tholi postdates the local fracturing in the annulus (Lang and Lopez, 2015). Furthermore, the tholi does not exhibit appreciable evidence of later tectonic modification. Radial and polygonal fracturing on the tops of steep-sided domes have been noted to occur at 28% and 34%, respectively, of domes due to cooling or expansion during endogenous growth after an outer carapace has formed (Fink and Griffiths, 1998; Stofan et al., 2000). Parallel northeast-southwest trending fractures are visible on the top of the tholi, however their strike is different than the regional trending fractures and they do not appear thoroughgoing. They are therefore not interpreted to be related to post emplacement deformation from surrounding tectonism. Venusian steep-sided domes are on average ~20 km in diameter, much larger than their terrestrial counterparts, which are typically 0.1–1 km across (Fink et al., 1993; Osmond and Griffiths, 2001; Pavri et al., 1992). Thus, Narina Tholi is large compared with most Venusian domes: it has a diameter of ~40 km and rises ~1 km above the surrounding terrain (Gleason et al., 2010). The positive relief is immediately surrounded by a topographic low (Fig. 2.2). Peak-to-trough relief (from the top of Narina Tholi to the base of the surrounding low) is over 1 km. The dome is generally visible in the gridded altimetric data (Ford and Pettengill, 1992), although the limited spatial resolution prohibits detailed correlation of elevation with the dome morphology seen in the SAR images (Fig. 2.2a), and cross-track resampling of the data distorts the planform shape of the dome. In contrast, the stereo-SAR-derived topography (Herrick et al., 2012) clearly shows the axisymmetric shape of the dome and a surrounding moat (Fig. 2.2b). An extensive mapped flow, Oilule Fluctus, continues northwest of the tholi, and is about 400 km long with a variable width of 50–100 km. The flow hosts Ahsabkab Vallis, a constructional lava channel (Gregg and Greeley, 1993) of about 5 km width (Lang and Lopez, 2015). 23 Figure 2.2: Narina Tholi and Aramaiti Corona fracture annulus. Magellan right-look SAR image is superimposed on color elevation derived from (a) the gridded altimetry (GTDR) and (b) stereo-SAR. In the images, blue indicates low and red indicates high elevations. Total elevation range is ~1.8 km (altimetry) and ~1.5 km (stereo-SAR). In (b), lines indicate the locations of topographic profiles examined for modelling, and gray filled circles on profiles 1–10 examined show the radial extent of the profile included in the modelling. Profiles 11–15 were too short to allow reliable modelling. Grayscale regions lack stereo-SAR coverage. 2.3 Axisymmetric Flexural Modelling 2.3.1 Elastic Flexure Model We investigated the flexural signature surrounding Narina Tholi using SAR-derived stereo topography (Herrick et al., 2012) and thin elastic plate flexure models that assume an axisymmetric geometry. Previous studies have applied cartesian and/or axisymmetric models to larger features such as coronae and chasmata (e.g., Brown and Grimm, 1995; Johnson and Sandwell, 1994, O’Rourke and Smrekar, 2018; Sandwell and Schubert, 1992). Motivated by the 24 cross-sectional shape of the tholi revealed in the stereo topography (Fig. 2.2b) we assumed a disk load on a thin elastic shell, following the approach of Solomon and Head (1979). The small size, correspondingly high planform curvature and symmetry of the load (e.g., the tholi) motivate an axisymmetric model. The corresponding solution to the differential equation for the flexural deflection, w(r), as a function of radial distance, r, from the load center is given by 𝑤(𝑟) = 𝐶0 [𝑑1𝐾𝑒𝑟 (𝑟𝛼) + 𝑑2𝐾𝑒𝑖 (𝑟𝛼)] + 𝐶1r + 𝐶2 for 𝑟 > 𝑟𝐿𝑂𝐴𝐷 (1), where 𝐶0 = 𝑞𝐸ℎe 𝑅2⁄ + 𝜌𝑔 𝑑1 = 𝑑 𝐵𝑒𝑟′(𝑑) 𝑑2 = −𝑑 𝐵𝑒𝑖′(𝑑). 𝑑 = 𝑟𝐿𝑂𝐴𝐷𝛼 𝐶1 and 𝐶2 are the background topographic gradient (𝐶1) and mean elevation (𝐶2), C0 is the loading amplitude, he is the thickness of the elastic shell, R is the radius from the center of the planet to the mid-plane of the shell (i.e., R = 𝑅𝑝 −ℎ𝑒2), rLOAD is the radius of the modelled disk load, and E, ρ, g are Young’s modulus, density and gravitational acceleration respectively (values given in Table 1). Ber, Ker, Bei and Kei are the real and imaginary parts of the Bessel-Kelvin functions of order 0, and ′ denotes the first derivative. The flexural wavelength α is given by 𝛼 = √𝐷𝐸ℎe 𝑅2⁄ +𝜌𝑔4 (2)and is dependent on the flexural rigidity, D 25 𝐷 = 𝐸ℎe312(1 − 𝜈2) (3), where ν is Poisson’s ratio (Table 2.1). Symbol Parameter Value E Young's Modulus 5–65 GPa ρ Density contrast across the lithosphere 3300 kgm-3 g Gravitational acceleration 8.87 ms-2 𝜈 Poisson's ratio 0.25 Rp Radius of Venus 6051.8 km Table 2.1: Parameters and their values used in flexural modelling. The net horizontal elastic stresses at the surface of the shell are 𝜎𝑟 = 𝑁𝑟ℎ𝑒+ 12𝑀𝑟ℎ𝑒3 (𝑟𝑝 − 𝑅) (4) 𝜎𝜙 = 𝑁𝜙ℎ𝑒+ 12𝑀𝜙ℎ𝑒3 (𝑟𝑝 − 𝑅). (5) R is the radius to the midplane of the shell (𝑅𝑝 −ℎ𝑒2), 𝜎𝑟 and 𝜎𝜙 are the radial and azimuthal stresses respectively (Solomon and Head, 1979). Outside the load the radial and azimuthal stress resultants (𝑁𝑟 and 𝑁𝜙) and bending moments (𝑀𝑟 and 𝑀𝜙) are given by 𝑁𝑟 =𝐶0𝐸ℎ𝑒𝑅[12(𝑑𝛼𝑟)2+ 𝛼𝑟(𝑑1𝐾𝑒𝑖′ (𝑟𝛼) − 𝑑2𝐾𝑒𝑟′ (𝑟𝛼)) ] (6) 𝑁𝜙 = −𝑁𝑟 +𝐸ℎ𝑒𝑤𝑅(7) 𝑀𝑟 = −𝑞𝛼2 [𝑑1 (𝐾𝑒𝑖 (𝑟𝛼) +𝛼(1 − 𝜈)𝑟𝐾𝑒𝑟′ (𝑟𝛼)) − 𝑑2 (𝐾𝑒𝑟 (𝑟𝛼) − 𝛼(1 − 𝜈)𝑟𝐾𝑒𝑖′ (𝑟𝛼))] (8)26 𝑀𝜙 = −𝑞𝛼2 [𝑑1 (𝜈𝐾𝑒𝑖 (𝑟𝛼) − 𝛼(1 − 𝜈)𝑟𝐾𝑒𝑟′ (𝑟𝛼)) − 𝑑2 (𝜈𝐾𝑒𝑟 (𝑟𝛼) +𝛼(1 − 𝜈)𝑟 𝐾𝑒𝑖′ (𝑟𝛼))] (9).2.3.2 Modelling Approach Sixteen radial elevation profiles originating at the center of Narina Tholi and spaced 22.5° apart in azimuth were extracted from the stereo-SAR gridded topography data. Most profiles are ~100 km long, but some are shorter because of gaps in the stereo topography coverage (Fig. 2.2b). Profiles 0, 14 and 15 cross a second smaller emplacement northwest of the main tholus and do not show a flexural moat, and profiles 11–13 have truncated stereo coverage so we discarded these. The remaining ten profiles of elevation versus distance from the tholi center were modelled. The flexural modelling begins at r = r1. Regional-scale signals unrelated to flexure local to the tholi were estimated and subtracted from the data as follows. The background gradient and mean elevation were solved for using a portion of the profiles outboard of the flexural signature (represented by the parameter r2 in Fig. 2.3a), and subtracted from the profile. The elastic flexure model (first two terms in equation 1) was then fit to the detrended profile at radial distances r > r1 (see Fig. 2.3a for example choice for r1). The load magnitude and flexural wavelength can be solved for simultaneously using a non-linear least squares inversion. Alternatively, a linear least squares estimate of the load magnitude, C0, can be made for a specified value of α (or equivalently he). We chose to take the latter approach, obtaining the best-fit solutions for C0 for a range of he. For each value of he, the RMS misfit of the best-fit profile to the data was computed. The minimum RMS misfit as a function of he yields the effective elastic thickness that best fits the data for each profile and the RMS misfit curve provides information on how well-constrained 27 that best-fit value is. The vertical deflection, w(r), and the horizontal radial surface stress were computed for the best-fit models for each profile. Figure 2.3: (a) Topographic profile 5 (location shown in Fig. 2.2b), showing elevation as a function of distance from the tholi center (solid black curve). The predicted profile from the best-fit elastic flexure model is shown as a red dashed line, and circles along the profile show the locations of the parameters rLOAD (black), r1 (blue), and r2 (orange). (Inset) he and corresponding 28 root-mean-square (RMS) misfit curve. (b) RMS vs he for different combinations of r1 and r2 listed as r1; r2 in the legend. r1 was increased from 21–41 km in 5 km increments and r2 from 40–60 km in 10 km increments. The black dashed line shows the RMS misfit curve for the choices of r1 and r2 used for profile 5. 2.4 Results 2.4.1 Predicted Elastic Lithospheric Thickness The parameters r1 and r2 were picked manually and the sensitivity of the solution to the choices of r1 and r2 was investigated for profiles 2–10. As an example, we show the results of this process for profile 5 (Fig. 2.3). We initially used the canonical value for E = 65 GPa. The corresponding best-fit model for topographic profile 5 results in an elastic thickness of he = 3.5 km, with r1 = 21 km and r2 = 50 km (Fig. 2.3a). The best-fit value for he is insensitive to the choice of r1 or r2 provided that the profile is sufficiently long to include most of the flexural bulge (i.e. r1 is close to the edge of the load). The solution is better determined for longer profiles as reflected by the depth of the well in the misfit curves (Fig. 2.3b). Furthermore, we find that the choice of r2 is unimportant to the best-fit value for he (Fig. 2.3b) because most profiles have background slopes that are small. The best-fit elastic thickness values for E = 65 GPa are 2.0–6.0 km (Fig. 2.4). The misfit curves show that the results for individual profiles are generally well-determined. The exception is profile 1 and, as most of this profile outboard of the flexure extends along the fracture annulus, we exclude it from further discussion. The average elastic thickness for profiles 2–10 is 3.9 ± 1.0 km. 29 Figure 2.4: (a) Topographic profiles vs distance from tholi center (black) and best fit flexural model (red dashed) for profiles 1 (bottom) to 10 (top) (see Fig. 2.2b for track locations). Best fit he is indicated at the end of each profile. The best-fit elastic thickness values corresponding to E 30 = 65 GPa are located at the end of the profiles, with those corresponding to E = 5 GPa in parentheses. (b) Root-mean-square (RMS) misfit vs he (km) for each profile. 2.4.2 Predicted Elastic Flexural Stresses We calculated the radial horizontal stress for our best-fit elastic thickness for profiles 2–10 for E = 65 GPa. The peak values and their locations are shown for each track (Fig. 2.5) and the predicted stress along two profiles are shown in Fig. 2.6. The average peak horizontal radial stress is ~190 MPa (for E = 65 GPa) and is tensile. Although this is high, the SAR images show little evidence for fracturing associated with the flexure at Narina Tholi. The exception is a single fracture directly east of Narina Tholi that parallels the corona fracture annulus, but further north changes direction to continue concentric to the tholi. This fracture that crosses profiles 2, 3, and 4, inboard of the predicted peak stress (Fig. 2.5 inset). 31 Figure 2.5: Topographic profiles with the location of the maximum tensile radial stress associated with lithospheric flexure (triangles) for E = 65 GPa. Colour of the triangle indicates stress magnitude with values shown in the legend. (Inset) Zoom on a single concentric fracture collated with the flexure, and the peak stress locations. Arrows show location of fracture. 2.5 Discussion Our results suggest a thinned elastic lithosphere in the vicinity of Narina Tholi. A thinner lithosphere results in increased modelled stresses, and here we find peak tensile stresses that are much larger than the expected tensile strength of intact basalt (5–25 MPa), determined from experiments (e.g., Schultz, 1995). However, only a single candidate concentric fracture is 32 observed crossing profiles 2–4. One possibility is that concentric fractures have been buried by subsequent lava flows from Narina Tholi. According to previous geologic mapping, there is little evidence for post-emplacement flow material on the east margin of Narina Tholi extending to a sufficient radial distance to erase concentric fractures associated with flexure (Basilevsky and Head, 1997; Lang and Lopez, 2015; Stofan and Guest, 2003). A second consideration is that the flexural response is governed by the flexural rigidity, D, which in turn is proportional to Ehe3. Given a smaller value for E, an observed flexural signature will correspond to a larger value for he and reduced horizontal radial stress magnitudes. Third, the regional stress field likely plays a role in any tectonic deformation local to Narina Tholi. In modelling lunar rilles spatially associated with large mare units, Solomon and Head (1979) propose that the interplay of flexural stresses from lunar mare flood events and regional stresses are important in the formation and final appearance of the rilles. This influence is dependent on the direction of regional stresses with respect to the lithospheric flexural stresses induced by mare loading emplacement events, and if they are extensional or compressional. Narina Tholi sits on the fracture annulus of Aramaiti Corona, and these fractures have been interpreted to be extensional graben (Lang and Lopez, 2015), presumably formed from tensile radial stress associated with the formation of Aramaiti Corona. The southwest-northeast trending fracture belt (NEFB) on which both the corona and tholi sit are also interpreted to be extensional graben. Below we investigate values of Young’s modulus that might be more appropriate to the lithosphere local to Narina Tholi, and the consequences for elastic thickness and predicted stresses. We compare the predicted stresses with constraints on the tensile strength of the material and discuss the role that regional stresses might play in affecting the presence or 33 absence of concentric fractures around Narina Tholi. Finally, we examine the implications of our elastic thickness results for heat flow. 2.5.1 Young’s Modulus Young’s modulus describes the relationship between stress and resulting strain, and values have largely been derived from laboratory experiments on intact samples (Schultz, 1993). Canonical values used in engineering and tectonic modelling range from 65–100 GPa (e.g., Johnson and Sandwell 1994; O’Rourke and Smrekar, 2018; Turcotte and Schubert, 1982). At larger scales, the concept of a rock mass is important. This takes into account features such as joints, bedding, and faults, which reduce the strength of the rock compared with that of intact test samples (Bieniawski, 1989; Schultz, 1993). Thus, to more accurately model deformation, Young’s modulus is replaced by a ‘deformation modulus’ (Hoek and Brown 1980; Klimczak, 2015; Klimczak et al., 2015; Schultz, 1993). In terrestrial applications, values for deformation moduli are found either by in-situ testing or indirect estimates using a classification system (Palmström and Singh, 2001), such as the Rock Mass Rating (RMR) system. Images from the surface of Venus captured at the Venera 9, 10, 13, and 14 landing sites located in plains regions reveal pervasive bedrock fractures, fines and fragments up to ~1 m in size. In the absence of environmental weathering, it is proposed that the fragments could be the result of physical degradation of flows (Garvin et al., 1984). Although images at the resolution of the Venera landers are not available for our study area, it is clear from the Magellan SAR images that the environment around Narina Tholi is at least moderately fractured at larger scales. Both corona and tholi are situated in what has been mapped as wrinkle-ridge plains and plains having previously undergone localized extension and compression (Basilevsky and Head, 1997; Stofan and Guest, 2003). Furthermore, locally Narina Tholi is superposed on and surrounded by the 34 heavily fractured annulus of Aramaiti Corona. Schultz (1993) proposes an upper limit of E = 50 GPa for moderately altered environment, and 5 GPa for a more fractured, weaker rock mass environment. This range of 5–50 GPa is considerably less than the canonical values of 65–100 GPa. We calculate the predicted stresses using E = 5, 30 GPa and compare these with our results for E = 65 GPa for profiles 3 and 4 (Fig. 2.6). The stress at the location of the fracture is ~120 MPa for E = 65 GPa, ~65 MPa for E = 30 GPa, and ~20 MPa for E = 5 GPa. For E = 5 GPa the best-fit elastic thickness for each profile is more than twice that for E = 65 GPa (Table 2.2), as expected (i.e. he,5 GPa ~ (65/5)1/3 he,65 GPa). The average best-fit elastic thickness for profiles 2–10 is 9.1 ± 1.9 km with E = 5 GPa. Even taking into account the effect of a reduced Young’s modulus, the average elastic thickness beneath and around Narina Tholi is less than the value of he = 13 ± 3 km found from flexure to the north and south of Aramaiti Corona, outboard of the fracture annulus (O’Rourke and Smrekar, 2018). We discuss this further in Section 2.5.2. Our results indicate substantially reduced stresses for reduced values of Young’s modulus. This brings the peak stress values down to the high end of experimental tensile strength of intact basalt (5–25 MPa) and closer to the expected tensile strength of fractured basalt (0.1–2.5 MPa) (Schultz, 1995). Other factors contributing to the strength of the geologic material taking part in flexure include the confining pressure of the Venusian atmosphere, which would increase the rock strength. This, however, is only a modest effect. More important is the dehydrated state of Venusian surface rocks, which terrestrial experiments have shown increases strength (Mackwell et al., 1998). Thus at the lowest Young’s modulus investigated, the maximum modelled stresses would not predict a zone of concentric fracturing around Narina Tholi, more in line with observations. 35 The presence of a single, partial fracture suggests that the lower values of Young’s modulus are more appropriate and/or that the flexural stresses from Narina are modified substantially by the regional stresses. At the scale and location of Narina Tholi, the stresses resulting in the corona fracture annulus are orientated east-west. The one fracture that we posit to be associated with flexural deformation from Narina Tholi follows the N-S strike of the fracture annulus locally, before turning northwest to form a partial concentric fracture, suggesting that indeed the pre-existing stress field influences the formation of flexurally-induced fractures around Narina. 36 Figure 2.6: Surface radial stress vs distance from tholi center for three values of Young’s modulus. Profile 3 (top) and Profile 4 (bottom) both cross the fracture concentric to the load, which is marked on the profiles as a circle (see Fig. 2.5 inset). The stress at the location of the fracture is ~20 MPa for E = 5 GPa and ~120 MPa for E = 65 GPa. Compressional and tensional stresses are represented by negative and positive values respectively. The profiles begin at the radius of the load. The radial location of the fracture is plotted on the profiles as a circle. The gray box is showing the range of tensile strengths of basalt for different RMR values, from heavily fractured to intact lab sample quality (0.1–25 MPa). E = 65 GPa E = 30 GPa E = 5 GPa Profile RMS Misfit (m) he (km) Peak σφ (MPa) he (km) Peak σφ (MPa) he (km) Peak σφ (MPa) Position (km) 2 17 6 117 7.5 734 14 21 35 3 9 4 144 5.5 78 9.5 25 29 4 29 3 141 4 80 7 26 27 5 25 3.5 165 4 119 7.5 34 27 6 20 3.5 171 5 86 8.5 29 27 7 29 4 230 5.5 124 9.5 41 28 8 23 4 309 5.5 165 9.5 55 28 9 26 3.5 286 5 145 8.5 49 26 10 16 3.5 190 4.5 114 8 37 26 Mean 3.9 ± 0.9 5.2 ± 1.1 9.1 ± 2.0 Table 2.2: Summary of elastic thicknesses, root-mean-square (RMS) misfits, peak stress magnitude and location for profiles 2–10 for different values of E. The location of the peak radial stress on each profile is the same for different values of E. 37 2.5.2 Estimating the Volume of Narina Tholi A volume estimate of Narina Tholi can be derived by combining the above-ground topography with the modelled deflection under the load by using equation A3 in Solomon and Head (1979). After averaging all topographic profiles, the modelled elastic thickness is he = 3.5 km. The true load height, estimated by the difference between the above-ground elevation and the below-ground minimum of the modelled deflection is about 1.6 km, with about half that above-ground. The resulting volume is calculated by estimating the load as series of rings with heights equal to the elevations along the topographic profile. The resulting volume estimate is about 1300 km3. This is compared to various terrestrial volcanic products such as the voluminous, silicic Badlands lava flow in southwestern Idaho (15 km3), the Chao Dacite Flow, Chile (26 km3) and Lōʻihi, the youngest basaltic volcano in the Hawaiian Ridge (1700 km3) (Fink and Griffiths, 1998; Robinson and Eakins, 2006). 2.5.3 Mechanical Thickness and Heat Flow at Narina Tholi The maximum stress supported by a flexing plate as a function of depth is the minimum of the elastic stress and either the pressure-dependent brittle strength at shallow depths, or the temperature-dependent ductile strength at greater depths. These regions make up the mechanical lithosphere, hm, the base of which is defined by a temperature isotherm (~1013 K), at which the ductile strength is less than a specified cut off, typically taken as 50 – 100 MPa (McNutt, 1984). For curvatures, κ, less than ~10-7 m-1, most of the plate behaves elastically and hm is approximated by he (McNutt, 1984). For κ less than ~5 x 10-7 m-1, O’Rourke and Smrekar (2018) found hm ≈ 1.1 he to 1.5 he as expected from McNutt (1984). A more precise conversion is attained by assuming a yield strength envelope (YSE) for the crust and/or mantle and matching the moment sustained by the elastic plate model and the mechanical thickness. The brittle-38 elastic-ductile YSE models brittle strength by frictional sliding with Byerlee’s Law and ductile strength via steady state creep. The bending moment of a plate that sustains the YSE stress profile is matched to the bending moment calculated from equation (A9) from Solomon and Head (1979). This gives Mr, the horizontal radial bending moment for a purely elastic model and an axisymmetric geometry as a function of radial distance from load centre and is typically calculated at the first zero crossing of the deflection profile (McNutt, 1984). The YSE incorporates a crust and mantle with different rheologies. Based on in situ geochemical measurements combined with high surface/subsurface temperatures, dry diabase and dry olivine rheologies are assumed for the crust and mantle materials, respectively (Grimm and Hess, 1997; Mackwell, 1998; Surkov et al., 1984). The modelled thickness of the Venusian crust is 8–25 km (Anderson and Smrekar, 2006; James et al., 2013; Simons et al., 1994). Incorporating two layers in the YSE may introduce mechanical stratification and a de-coupled layer, where the strong upper crust overlies a weak lower crust above a strong mantle. This effectively introduces a mechanically weak middle layer. Alternatively, for example in the case of a higher temperature gradient, the strong upper crust overlies a weak lower crust above a similarly weak mantle, with no mechanical de-coupling between these two lower layers. These are often referred to as the jelly sandwich and crème brulee models respectively (Burov and Watts, 2006; see also Mackwell et al., 1998). Parameters for an 8-km-thick dry Maryland diabase crust overlying a dry olivine mantle with a strain rate of 10-16 s-1 are used for the yield strength envelopes (Mackwell et al., 1998; McNutt, 1984; Watts and Burov, 2003). We note that the strain rate for the Venusian mantle is poorly constrained. The base of the mechanical lithosphere is then defined as the depth along the 39 ductile flow curve for the matching thermal gradient at which the ductile strength, σD, drops below 50 MPa. The heat flow out of the surface is qs = k(Tm - Ts) /hm, where k = 4 Wm-1K-1 is thermal conductivity. Ts ~728 K and Tm ~1013 K. The resulting average mechanical thickness for profiles 2–10 is hm = 4.4 ± 0.8 km. As predicted by McNutt (1984), the ratio hm/he ranges from 1–1.25, with calculated curvatures at the first zero crossing, k0 = 2–9 x10-7 m-1. Heat flow values correspond to qs = 299 mW m-2. The results are summarized in Table 2.3. Profile he (km) Mr from A9 S&H (1014 N) dT/dz (K m-1) hm (km) qs (mW m-2 ) 2 6 2.60 50.5 6.3 202 3 4 1.52 73 4.3 292 4 3 0.86 96.5 3.3 386 5 3.5 1.37 81 3.9 324 6 3.5 1.37 82 3.9 328 7 4 2.43 68 4.7 272 8 4 3.23 65 4.8 260 9 3.5 2.31 76 4.2 304 10 3.5 1.56 80 4 320 3.9 4.4 299 Table 2.3: Elastic thicknesses, bending moments calculated from (A9) in Solomon and Head (1979), corresponding temperature gradients, mechanical thicknesses and heat flow for each profile. The thermal gradient is calculated from a yield strength envelope incorporating dry Maryland diabase crust and dry olivine mantle rheologies, and a strain rate of 10-16 s-1. The mechanical thicknesses are smaller than the minimum modelled crustal thickness in the vicinity of Aramaiti Corona (Anderson and Smrekar, 2006; James et al., 2013), and our results therefore indicate a mechanical lithosphere residing entirely in the crust. There is an absence of a mechanically detached lower crustal layer whereby the mantle does not contribute 40 to the strength of the lithosphere, consistent with the ‘crème brulee’ lithospheric model (Burov and Watts, 2006). The surface heat flow is much larger, an order of magnitude so in some cases, compared with stagnant lid, chondritic thermal evolution or terrestrial scaling models (q = 10–76 mW m-2) or that from previous estimates of heat flow from flexural signatures including at Aramaiti Corona (103 mW m-2) (Brown and Grimm, 1996; Johnson and Sandwell, 1994; O’Rourke & Smrekar, 2018; Sandwell and Schubert 1992). In previous studies, Narina Tholi was interpreted as a stratigraphically young feature (Basilevsky and Head, 1997; Lang and Lopez, 2015), appearing to be a late stage volcanic product superposed on both Oilule Fluctus and the annulus of Aramaiti Corona. Lang and Lopez (2015) extend this to two additional coronae in their study, interpreting late-stage volcanic domes as reflecting a change in emplacement conditions or magma rheology during the later in the generalized coronae evolution sequence (Stofan et al., 1991; Stofan et al., 1992). Although Stofan et al. (2000) note that 53% of domes exhibit ‘throughgoing fractures’ that postdate emplacement, there appears to be little to no tectonic deformation postdating the emplacement of Narina Tholi, further strengthening the argument of a later-stage volcanic product. Modelling results and observations therefore indicate this elevated heat flow is associated with a relatively young volcanic construct. The small, contemporaneous resurfacing events associated with the equilibrium resurfacing model are hypothesized to occur above patches of thinned lithosphere, with the spatial extent of the resurfacing less than the average intercrater distance (Schaber, 1992). Evidence for this is given by Anderson and Smrekar (2006), who estimate that 47% of the elastic lithosphere is less than 20 km with large variation thicknesses, up to 70 km. This points to a complex history of geological processes operating in the relatively 41 recent past, at least on timescales less than the mean surface age. This contrasts with catastrophic resurfacing models, in which the global lithosphere thickens more or less in a spatially homogenous fashion over time, until a global event is triggered, resetting both lithospheric thickness and surface age, albeit with some small regional variations. Narina Tholi, with an associated heat flow estimate similar to that of very young terrestrial oceanic crust, for example the <20 Ma portion near the Juan de Fuca ridge, could therefore be a prime example of localized thermal rejuvenation revealed by regional variations in he (Jaupart and Mareschal, 2007; Sclater, 1980). 2.6 Conclusions In this study we identify a flexural signature at a volcanic construct, Narina Tholi, and estimate lithospheric thickness using flexural modelling. The model assumes a thin elastic shell, canonical mechanical parameters, and, motivated by the planform shape of the tholi, an axisymmetric load. By comparing the modelled profiles to topographic data, we determine a mean best-fit elastic thickness he,65 GPa = 3.9 ± 1.0 km from nine profiles taken around the volcanic construct. We use a yield strength envelope to convert this value to mechanical thickness, and determine a corresponding heat flow of qs = 299 mW m-2 at the time the flexural signature was emplaced. This heat flow is higher than those obtained from previous studies of flexure at larger features such as coronae and chasmata (e.g., Brown and Grimm, 1995; Johnson and Sandwell, 1994, Sandwell and Schubert, 1992) that estimated average global heat flow to be 27–70 mW m-2. The heat flow is also elevated or comparable to heat flow estimates at coronae from the study of O’Rourke and Smrekar (2018). Narina Tholi sits on the western fracture annulus of Aramaiti Corona, and the heat flow inferred at the tholi location is more than twice that estimated from flexural signatures associated outboard of the north and south rim of the 42 corona (O’Rourke and Smrekar, 2018). The stratigraphy of the region suggests Narina Tholi is a relatively young volcanic construct, and thus these results suggest locally enhanced heat flow, late in the evolution of the corona. The predicted elastic horizontal radial stresses associated with flexure are large, on average 190 MPa. However, unlike around many coronae hosting flexural signatures, SAR images show minimal fracturing associated with the flexure associated with Narina Tholi. Motivated by terrestrial engineering experiments and the effect of fracturing on the strength of geological materials, we calculated the predicted stresses for a reduced value of Young’s modulus, E. For E = 5 GPa, possibly appropriate to the fracture annulus on which Narina Tholi sits, the peak stresses are reduced to ~20 MPa, more consistent with SAR-image observations and the expected material strength. Regional stresses likely also affect the presence or absence of fracturing associated with Narina Tholi. The investigation of Narina Tholi, a relatively small construct compared to those featured in previous investigations, was made feasible due to high-resolution stereo SAR-derived topography (Herrick et al., 2012). The horizontal resolution, vertical precision and noise floor of the altimetry-based elevation data previously limited investigations to larger features with long flexural wavelengths or high topographic amplitudes. Globally, this high-resolution stereo topographic coverage is limited to about 20% of the planet, and is not generally contiguous (e.g., Fig. 2.2b). More complete coverage of high-resolution elevation data would permit the investigation of additional small features and aid in understanding the spatial and temporal variations in heat loss on Venus. 43 Chapter 3: Further Evidence for Volcanism Associated with the Fracture Annulus of Aramaiti Corona In this chapter, evidence for volcanism associated with the fracture annulus of Aramaiti Corona is presented. The study area for this chapter is the north portion of the inner fracture annulus. Two aspects are highlighted: (1) irregular topography along the strike of the fracture annulus and its qualitative similarity to terrestrial domes/volcanic constructs sometimes seen in association with calderas and fissures, and (2) an extensive blocky deposit observed immediately outboard of the fracture annulus. Both of these observations are unusual in that they have not previously been reported in association with coronae. Observations that suggest this volcanism at the northern fracture annulus may also be late stage are discussed. Narina Tholi, which sits on the western fracture annulus of Aramaiti Corona, was investigated in the previous chapter. A view of the entire corona (Fig. 3.1) indicates that the 100 km wide fracture annulus (FA) is almost spiral in planform, as noted by Basilevsky and Head (1997). The area of interest for this chapter is the northern portion of the FA, which is divided by an arcuate, radar-dark (smooth) trough, unlike the uninterrupted southern FA (Fig. 3.1). The altimetry-derived elevations (Fig. 3.1a) and higher-resolution stereo-derived elevations (Fig. 3.2b) show high topography of both the northern and southern segments of the FA. High resolution stereo-derived topography is not contiguous over Aramaiti Corona (Fig. 3.2b) but some coverage exists over the northern FA. In the altimetry, the along-strike topography of the high relief north rim is smooth, however it has an undulating, irregular appearance in the stereo (Fig. 3.3). We first examine the topography of this portion of the FA, then discuss an extensive blocky deposit situated outboard of the FA. The deposit lies at the base 44 of high relief and extends concentrically westward. We suggest possible connections between high relief along the corona FA and the blocky deposit. Figure 3.1: Magellan right-look SAR image (illumination from right) overlain on (a) altimetry-derived topography (Gridded Topographic Data Record, GTDR of Ford and Pettengill, 1992) showing coverage over Aramaiti Corona (centre) and Narina Tholi (centre-left). Narina Tholi is superposed on the western fracture annulus (FA) of Aramaiti Corona. Arrows indicate the inner FA in the north and the radially-continuous FA in the south. Blue indicates low and red indicates high elevations relative to the mean planetary radius. Total elevation range is -800 m (dark blue) to +1600 m (dark red). All SAR images in this chapter are shaded such that radar-bright materials are darker greys, and radar-dark (smooth) materials are light. (b) Stereo coverage over Aramaiti Corona (Herrick et al., 2012). Stereo-topography is not continuous over the region because two SAR images with appropriate look-geometry were not always available. Note that the absolute scale is different from that in Fig. 3.1a because stereo topography gives the relief relative to a baseline of 6040.0 km (Herrick et al., 2012), whereas altimetry is relative to the 45 mean planetary radius of 6051.8 km. Colours represent extent of available topographic coverage, which ranges from -1700 m (blue) to + 300 m (red). Figure 3.2: Zoom of the north rim of Aramaiti Corona. Magellan right-look (illumination from the right) SAR image superposed on colour elevations derived from (a) gridded altimetry (GTDR) and (b) stereo-SAR. Line segments on (b) indicate the locations of the along-strike (A1) profiles taken from the GTDR and stereo, as well as the cross-strike (C1, C2, C3) profiles, all shown in Fig. 3.4. Blue indicates low and red high elevations relative to mean planetary radius. Total elevation range is -100 to 750 m (altimetry) and -1000 to -100 m (stereo-SAR). Dark shading in the SAR image represents brighter radar-return. 46 3.1 Topography Associated with the North Rim Motivated by the along-strike appearance of the topography (Fig. 3.2) we extracted profiles from both the altimetry derived Global Topographic Data Record (GTDR) and stereo imagery (Fig. 3.3). Two or three distinct topographic highs are visible in the along-strike profile (Fig. 3.3a). The far west high rises steadily to about 250 m above the existing topographic rim along a distance of ~4 km before descending ~200 m again over ~4 km. The second rises ~200 m over 7 km (Fig. 3.3a). The undulations are significant with respect to the vertical precision of the topographic data, which is on the order of tens of meters (Herrick et al., 2012). This short-wavelength topography is superposed on a rim that already has high relief. The stereo topography in the radial, or across-strike, direction shows the short wavelength, high relief of the feature. On the profiles C1–C3, the topography rises 300–550 m above the smooth plains immediately outboard of FA over a horizontal distance of 7 km before dropping into the inboard trough. This across-strike relief of the rim accounts for over 50% of the vertical relief of the entire corona over only 5% of the corona diameter. 47 Figure 3.3: (a) Along-strike profiles using the GTDR and stereo-SAR elevations from west (left) to east (right). The location for both profiles is shown in Fig. 3.2b. Horizontal distance and vertical height is in metres. As noted in Fig. 3.1 the absolute scale for the stereo is offset from 48 that of the altimetry. (b) Across-strike profiles across three topographic highs, from north (left) to south (right). Slope maps have been used to delineate the edges of steep-sided domes, and look for multiple emplacement events (Gleason et al., 2010). The built-in ArcGIS slope map function was applied to the north rim stereo topography. The edges of the irregular planform topography are well delineated as abrupt increases in slope from ~0° to ~9° across the horizontal distance of one pixel (~75 m) of the stereo topography. The signatures of other coronae in topographic classes with rims were visually inspected in the high-resolution stereo data but the along-strike irregularity appears rare. Unfortunately, as seen for Aramaiti in general, the altimetry outside of the limited stereo coverage does not have sufficient resolution to investigate similar features. The occurrence of multiple, irregular, short-wavelength, topographic highs is qualitatively similar to that seen at some terrestrial volcanic domes. On Earth, such domes can occur in association with large calderas and/or rift systems. Here, I simply offer a qualitative comparison with a terrestrial example, and in Section 3.3 comment on the possible implications. One particular example is Ruawahia Dome, part of the Tarawera dome complex (Fig. 3.4), which in turn is part of a larger volcanic complex on the north island of New Zealand that follows the strike of a large rift system. Ruawahia Dome is about 1.5 km diameter (Ashwell, 2018), and rises ~110 m over a horizontal distance of 300 m. It is built on top of previous volcanic constructs, a series of older domes, and is one of three more recent (~0.7 ka) main domes of similar size and planform shape, in association with their associated pyroclastics. Gilbert Dome in California, part of the Long Valley Caldera, is also similar in general planform 49 shape at a smaller scale, rising 200 m over a distance of 850 m. It is weathered and has an oblong shape, about 2 km long and 500 m wide. Figure 3.4: (a) Google Earth view of Tarawera Volcanic Dome Complex, containing Ruawahia Dome (RD) in the centre, and two other adjacent domes. Location of topographic profile in Fig. 3.4b is shown in white. (b) Along-strike profile of the domes. Horizontal length is 5 km, and vertical scale is ~260 m. Vertical exaggeration ~5x. Vertical and horizontal scale bars are given on the figure, because of the small font in the Google Earth generated profile. 50 3.2 Blocky Deposit 3.2.1 Observations Visible in the SAR image is an extensive blocky deposit outboard of and concentric to the fracture annulus. Its outer edge is crenulated and this, together with its texture, creates a good radar backscatter contrast with the underlying surface (Fig. 3.5). The underlying surface has been mapped by Basilevsky and Head (1997) as radar-dark wrinkle-ridge plains. Figure 3.5: Right-look SAR image of Aramaiti northern rim showing the location of blocky deposit relative to the FA. In the west the deposit is blocky, with clear distinct blocks above the resolution of the radar, and toward the east it comprises finer material. North is pointing up in this image. The higher portion of the corona rim is on the lower half of the image and the blocky deposit sits outboard of this at the base of the topography. The deposit (dark) is superposed on the smooth plains unit (light). Distinct trails of the deposit emanate radially from the edge of the deposit and comprise both blocky and finer material. To the west, the deposit appears to be made up of coherent, angular blocks with radar-facing sides showing up as radar bright (dark gray in the inverted images) linear features several hundred meters in size. Toward the east along the exterior of the 51 rim, the radar signature of the deposit transitions from a blocky appearance to a more homogenous radar-bright signal that composed of features below the resolution limit of the radar. In this region the deposit does not show a specific texture, but is radar-bright (dark grey in the images), and thus must be rough at the wavelength of the radar (12.6 cm). The outer boundary is wavy and crenulated, but takes on a ‘wispy’ appearance reminiscent of those seen in the leeward side of some Venusian volcanic cones or impact crater parabolic ejecta deposits (Cohen et al., 2016). The deposit appears to be made up of distinct units. The boundaries of these units were automatically mapped based on a contour extraction tool in ArcGIS applied to the right-look SAR image. The tool contours pixels of similar radar brightness values in the raster, analogous to generating contours from a gridded elevation data set. Because brightness in radar images is dependent on surface properties, primarily roughness, then under optimal conditions, the boundaries can denote units with different properties. An example of an optimal condition is a radar-bright deposit (rough) on a flat radar-dark (smooth) surface. Another example is an isolated small feature creating a single bright radar return, such as a boulder with a side facing the radar, situated on top of a radar-dark surface. This approach led to identification of about five distinct boundaries within the extensive deposit (Fig. 3.6), visually corresponding to more blocky, or more fine, deposits. One inner edge (S1 in Fig. 3.6) was mapped manually as there is insufficient contrast for it to be identified automatically. The portion of the deposit containing large blocks in the west is about 70 km long, along the strike of the corona FA. The deposit then graduates into finer material in the east, and continues for another 90 km along the base of the northern rim, for a total length of about 160 km. The radial extent of the deposit is 6–7 km from the base of the corona rim or outermost 52 extent of the FA. Along the blockiest portion of the deposit, the long-wavelength structure of outer edge (labeled “o” in Fig. 3.6) is quasi-parallel to the inner edge (labeled “i” on Fig. 3.6). The approximate spatial scale of the planform undulations in the outer and inner edges is 10–20 km. The blocky deposit appears to have a similar spatial density of blocks as a function of azimuth around the fracture annulus with little to no size-sorting with azimuth. However, in the radial direction, the larger blocks are generally proximal to the FA, with relatively smaller blocks and finer material more distal. Isolated blocks are seen in the radial far-field, extending beyond the outer edge of main deposit (Fig. 3.7). These can be clearly seen and were automatically detected due to their clear contrast with the underlying smooth unit (Fig. 3.7). This contrast allowed the block sizes to be assessed, and most are one to four pixels across, i.e., 85–340 m. Figure 3.6: Deposit with boundaries of distinct units mapped (see text). Main blocky boundary is labelled ‘B1’, boundaries of smoother materials, ‘S’ (S1, S1), and wispy boundaries with ‘W’ (W1, W2). “o” and “i” are discussed in the text. 53 Figure 3.7: Right-look SAR image of north rim, with two zooms of the rubble boundary. Red boundary corresponds to the outermost boundaries (consisting of B1 plus W1). Isolated dark features interpreted as blocks in the far field are outlined with blue circles. These are automatically detected via their radar contrast with the underlying unit. Individual blocks contained within the main deposit are visible in the two zooms. The radar-facing side shows up as a coherent radar-bright (shaded) feature. Stereo topography (Figs. 3.1, 3.2) clearly shows that the eastern part of the deposit (comprising fines) lies at the base of the relief associated with the FA of the north rim of Aramaiti Corona. Unfortunately, stereo coverage ends in the west and only covered a portion of the blocky deposit (Fig. 3.2b) in the vicinity of the blocky deposit, however the altimetry-based topography provides some information. The altimetry shows that beyond the west portion of 54 stereo coverage, there is little relief associated with the FA. in contrast to the relief in the east (Section 3.1). Figure 3.8: (a) Zoom on the area just west of that in Fig. 3.2. Magellan right-look SAR image superposed on colour elevations derived from stereo-SAR. Line segment indicates the location of the profile shown in (b). On the profile (b), the extent of the rubble pile is indicated with circles 55 along the profile around 50 km. The profile starts outboard of the fracture annulus (0 km) with distance increasing towards corona interior. At the western edge of the deposit, the limited stereo topography coverage shows that outboard of the rubble pile, the topography gradually slopes down towards the corona interior, and levels off at the rubble pile (Fig. 3.8), creating a shelf. The rubble pile is sitting on this horizontal shelf, and here the stereo topography ends, but the GTDR shows that the slope continues downhill towards the corona interior, and there is no further high topography. The volume contained within the deposit is difficult to constrain; however, an order of magnitude estimate can be provided by assuming an average thickness. The median size of the distal blocks and those contained within the deposit can be used to estimate an average thickness of 0.5–1 SAR pixels. This gives an approximate volume for the western blocky portion of 70 km x 5 km x (0.375–1 km) = 131–350 km3. The absence of vertical relief associated with the fines to the east suggests this is a thin deposit up to a few meters in thickness. 3.2.2 Discussion The hummocky texture of the aerially extensive deposit with distinct blocks contained within it is noted to occur on Earth associated with volcanic debris avalanches and on Venus at other volcanic domes (Bulmer and Guest, 1996; Lopez, 2011; Siebert, 1984). However, unlike the deposit around the northern FA of Aramaiti, other Venusian deposits form fan-like debris aprons. These are proposed to be sourced from over-steepened portions of the volcanic edifice after most of the volcanic activity ceases. The strength of the cooled material is high, but the over-steepened slopes can collapse catastrophically into large blocks and flows. Additionally, fan-like deposits inferred to be mass movements not necessarily associated with recent volcanic activity were initially identified in the Magellan SAR images by Malin 56 (1992). These were interpreted to be flows and debris avalanches (e.g., Fig. 3.9), with some occurring at domes and others in association with high relief e.g., near tectonic troughs. Interestingly, radar-dark/smooth surfaces were also noted to surround or be emplaced on top of some rock slide avalanches (Malin, 1992). Within some individual slides there is a progression of continuous surface to isolated blocks. To our knowledge there has been no previous identification of blocky deposits directly associated with coronae. The surfaces and edges of blocks that are orientated with their normal towards the radar instrument will create a relatively brighter return than those not facing the radar (Stofan et al., 1993). That the edges of individual blocks within the hummocky deposit are visible in the SAR images indicates that they have sufficient vertical expression to create a radar-bright return from the radar-facing side. The image shows a preferential north-south orientation of the edges, likely resulting from the east-west SAR imaging geometry. Often adjacent to inferred block edges is a small radar shadow region with reduced return (Fig. 3.7) (Ford et al., 1993). These are interpreted to be the top edges of the blocks that are facing away from the radar and are thus in radar shadow. Previous regional mapping led to interpretation of the blocky and fine deposits at the base of Aramaiti Corona as “downslope talus embayed by Pwr-d (dark wrinkle-ridge) plains” (Basilevsky and Head, 1997). However, closer examination of both blocky and finer deposits indicates that they and the block trails are superposed on the plains unit outboard of the FA, and thus stratigraphically younger. The blocks, with their angular corners and distinct sides, appear relatively pristine. The deposit outboard of the corona and at the base of the steep topography is qualitatively similar to the debris aprons associated with a specific class of terrestrial volcanic domes. A prominent characteristic of these terrestrial volcanic domes is talus slopes, which 57 contributes strongly to the dome’s morphology during emplacement and at the end (Blake, 1990). Growth and destruction cycles create a “distinctive apron of crumble breccia” (Blake, 1990) and blocks (Fink and Griffiths, 1998). 58 Figure 3.9: Synthetic aperture radar (SAR) images of two debris avalanches associated with volcanic domes. Illumination is from the left, and the images are shaded such that radar-bright materials are darker purple, and radar-dark (smooth) materials are beige. North is up in both images. (a) Albers (equal-area conic) projection of blocky deposit near Metis Regio. Note the fan-like planform of the deposit, with individual blocks up to ~2.5 km visible within the deposit. (b) Custom UTM projection of blocky deposit associated with volcanic dome between Atla and Phoebe Regiones. Some individual blocks up to 4 km in length are visible. 59 We considered possible scenarios that could explain the presence of this enigmatic deposit. - Lava flows: Sticky, strong flows, emanating from one or more vents located on the corona rim. These might be similar to Hawaiian a’a or blocky lava, but must be rubbly with relief on scales much greater than the radar wavelength and thus substantially larger than their terrestrial a’a counterparts. This would be a very unusual feature on Venus, making it difficult to explain or provide terrestrial analogues. Additionally, isolated material, presumably sourced from the extensive blocky deposit, is found more than 5 km from the base of the topography. If this was sourced from a flow, a smaller spatial extent might be expected. - Collapse of the outer slope of relief associated with the FA: This appears to be a plausible hypothesis for the eastern part of the deposit, however the absence of positive FA relief to the west is problematic, i.e. the blocky deposit in the west does not lie at the base of positive FA relief. We considered two possible explanations. First, that the blocky deposit to the west was in fact sourced from the annulus relief to the east, and migrated around the FA to the west, possibly down a regional slope. Examination of the GTDR indicates there is no such regional slope, and so does not support this unless the regional slope around the FA has changed since the deposit was emplaced. Furthermore, the size sorting and morphology of the blocky part of the deposit suggests any large-scale movement of the deposit was radially outward away from the FA, rather than concentric around the FA. A second possibility is that the blocky deposit originated from high FA relief along the west portion of the rim that has since subsequently fully collapsed or deflated, explaining why we see no current 60 signature of topography here. One mechanism could be deflation of previous volcanic constructs consistent with the interpretation of the high relief to the east as volcanic domes (see Section 3.1). The waviness of the rubble deposit boundary has similar spatial scale (10–20 km) to the spatial scale of the along-strike undulating relief described in Section 3.1. If deflation of a portion of the domes is indeed the explanation, the observations of similar wavelength would point to prior domes similar in spatial scale to those in the east. The mechanism causing the gradation of the deposit from hummocky/coherent blocks, to a homogenous fine deposit remains unresolved. The fines could either be a very thin mantle of material, simply blocks below the resolution of the SAR, or older material that has degraded under Venusian surface conditions. 3.3 Conclusions We have identified signatures of substantial topographic relief associated with the north rim in the stereo topography, that are qualitatively similar to terrestrial domes found at caldera/fissure systems. To our knowledge, such along-strike variations in topography at the fracture annuli of coronae have not been discussed, possibly because their recognition requires the high resolution stereo data which is not available everywhere. This suggests further evidence for volcanic activity associated with the fracture annulus of a corona. This volcanism is clearly different from the extensive fluid flows interior and exterior to the corona, and the fields of small shield volcanoes therein, that are typically interpreted as basaltic flood volcanism and shield volcanoes. However, although the morphology at the north rim suggests more steep-sided domes, it is also very different from typical Venusian steep-sided domes, such as that of Narina Tholi on the western fracture annulus of Aramaiti Corona. 61 We have also identified an extensive blocky deposit outboard of the north rim that appears to stratigraphically overlie a previous flow unit, and that has moved radially outward during emplacement. Contributing to the fresh-looking appearance of the deposit is the angular nature of the blocks, the clear outline of the deposit and the diffuse, crenulated nature of the homogenous fine portion of the deposit. Although there is evidence for blocky deposits associated with Venusian steep-sided domes, the combination of the planform of the deposit (a narrow annulus concentric to the topography rather than fan-like), the hummocky texture of the blocks and homogeneous texture of the fine portion of the deposit associated with a corona rim is rare (to our knowledge). Also unlike many flank failure and mass movements that are associated with steep-sided domes and are embayed by later flows (Lopez, 2011), this deposit has not been embayed. The absence of later embayment and fresh-looking morphology suggest that the deposit has occurred late in the evolution of Aramaiti Corona. 62 Chapter 4: Conclusion The results presented in Chapters 2 and 3 provide speculation for spatially restricted late-stage volcanism during coronae formation, specifically in association with corona annuli. 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Thesis/Dissertation
eng
Geophysics
Vancouver : University of British Columbia Library
Attribution-NonCommercial-NoDerivatives 4.0 International
Graduate
Heat flow and late-stage volcanism at Aramaiti Corona, Venus
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