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UBC Theses and Dissertations

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UBC Theses and Dissertations

Structure of the western margin of the Queen Charlotte Basin, British Columbia Young, Ian Fairley

Abstract

The thesis investigates the geology and structural development of the western margin of the Neogene Queen Charlotte Basin, British Columbia. They study area, situated along the Sandspit Fault trend, includes both the onshore portion of the forearc basin on Graham Island (Queen Charlotte Islands) and offshore portion in southwestern Hecate Strait and northwestern Queen Charlotte Sound. Field investigations comprised marine high resolution seismic and magnetic profiling and sea bottom sampling in vicinity of the presumed southern extension of the Sandspit Fault and gravity measurements across the fault zone on Queen Charlotte Islands. Samples were collected for geochronometry and analysis of magnetic properties. Additional unpublished data, including aeromagnetic survey maps and seismic profiles, have also been incorporated. A synthesis of available information on the geologic and tectonic history of the Queen Charlotte Islands and the Queen Charlotte Basin and plate motions in the northeastern Pacific Ocean serve as a basis for interpretation of the field data. Newly reported radiometric dates have confirmed or revised the ages of Mesozoic-Cenozoic volcanic and plutonic rocks of the Queen Charlotte Islands. Masset volcanic episodes occured in ? Paleocene, Upper Eocene and Lower Miocene time. Coeval "post-tectonic" plutons were emplaced in Upper Eocene-Lower Oligocene, Lower-Middle Miocene and later and appear to postdate Masset volcanic episodes. The timing of wrench faulting on the Sandspit and Rennel-Louscoone systems is constrained by the new age data. Magnetic susceptibilities and remnant intensities of Queen Charlotte Islands volcanic and plutonic rocks were measured to aid interpretation of airborne and marine magnetic data. High magnetizations and corresponding high amplitude magnetic anomalies are associated with the Masset volcanic and syn- and post-tectonic plutons. High Q ratios and peak demagnetizing fields indicates Masset NRM is stable and comprises the larger percentage of the magnetic field. Interpretation of marine magnetics has outlined probable seacrop patterns on an inner shelf between Queen Charlotte Islands and the basin edge. Anomaly patterns over major basin elements, including Hecate and Charlotte sub-basins and Moresby and Cape St. James ridges, reflect depth to source and nature of pre-Neogene subcrop. Offshore extensions of the Sandspit, Rennel, and Louscoone faults and associated wrench structures are suggested by anomaly trends. Seismic profiles illustrate structural development in shallow Neogene sediments of the western Queen Charlotte Basin. In Skidegate Inlet, the seismic data shows possible evidence of late Pleistocene faulting. In Hecate sub-basin, continuous parallel to en-echelon folds, that evidently experienced some growth in Neogene time, are draped over Mesozoic wrench folds in a deformation zone colinear with the onshore Sandspit Fault. Lack of a throughgoing fault zone in the sedimentary cover indicates that post-Lower Miocene wrench fault development is in an incipient stage of development. In the northern Charlotte sub-basin folds are poorly developed and deformation has been less severe. In the southern portion of the subbasin faults that parallel the Sandspit trend have beenmapped. Marine arkosic sands dredged from seacrop in the Charlotte sub-basin contain an Upper Miocene-early Pliocene microfloral assemblage. The sands are probably equivalent to the upper marine member of the Skonun Formation on Graham Island. The nature and timing of Cenozoic wrench faulting and associated volcanism and plutonism on Queen Charlotte Islands and in the western Queen Charlotte Basin may be related to changes in oceanic plate motions and possible plate edge effects at the continental margin.

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