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UBC Theses and Dissertations

Lithospheric structure across the Canadian cordillera of Northeastern British Columbia from seismic refraction and potential field data Welford, Joanna Kim


The region of northeastern British Columbia includes the transition from cratonic North America to the deformed Canadian Cordillera. While geological mapping has progressed for over 120 years in the northern Cordillera, the subsurface has remained terra incognita as few geophysical surveys have been conducted. The SNoRE '97 seismic refraction/wideangle reflection (R/WAR) survey was carried out to determine the velocity structure of the lithosphere in northwestern Canada. It sampled four separate seismic profiles along the three corridors of the LITHOPROBE SNorCLE transect. This thesis focuses on the results for Line 21. From east to west, the line begins in the Proterozoic Fort Simpson magmatic arc terrane, crosses the accreted Nahanni terrane, transects the Foreland Belt and samples the edge of the Omineca Belt just west of the Tintina Fault. The refraction data set shows good signal-to-noise ratios; primary phases (Pg, PmP and Pn) are clearly observed for most shots. A velocity structural model is achieved first through τ — p analysis, then forward ray trace modeling and 2-d inversion of traveltimes and forward amplitude modeling using RAYINVR, a widely applied, ray-theory-based algorithm developed for R/WAR studies. The resolved velocity structural model demonstrates a westward thickening package of low upper crustal velocities (6.2 km/s and less) extending to almost 20 km depth along the eastern portion of the profile below the WCSB. Beneath the westward thickening low velocities, a west-facing ramp of higher velocities, resolved in the middle and lower crust, is interpreted as the rifted margin of cratonic North America, which was overlain by low velocity, passive margin sediments during the mid-to-late Proterozoic and early Paleozoic. The eastern part of the Foreland Belt shows high velocities (6.4 km/s) in the upper five kilometers, indicating rocks upthrust from deeper in the crust. At the western end of the line, a narrow trench of low velocities at the surface of the model is inferred to represent sediments and brecciated surface rocks filling in the scar of the crustal-scale Tintina Fault. The location of the fault is resolved ~ 20 km to the west of its current inferred location. The depth to Moho is ~ 34 km in the Omineca Belt, deepens to ~ 38 km below the eastern Foreland Belt, and shallows to 34 km at the Cordilleran deformation front. The upper mantle is laterally heterogeneous with anomalously high velocities (up to 8.3 km/s) beneath the Foreland Belt flanked by regions of low velocities (7.7-7.8 km/s). Results from 3-D Bouguer gravity inversion, 2.5-D Bouguer gravity forward modeling and aeromagnetic comparisons performed in this thesis are combined with results from heat flow analyses and previous geophysical studies to check the consistency of the resolved velocity model and to constrain the interpretation of the tectonic evolution of northeastern British Columbia.

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