Imaging the Lithosphere-Asthenosphere Boundary of the Northern Cordillera Using SP Receiver Functions

The complex collage of accreted terranes in Alaska and the Yukon and Northern British Columbia provinces provide an excellent setting for investigation of lateral variations in lithospheric and crustal thickness. Ongoing convergence throughout the Phanerozoic has juxtaposed Proterozoic to Triassic pericratonic continental margin sedimentary provinces along interior accreted belts, and Paleozoic to Mesozoic magmatic, sedimentary, and metamorphic assemblages further outboard along the margin. This complex geometry has been achieved through variable subduction rates and obliquities, along-strike terrane translations, and large-scale oroclinal bending. We apply the S receiver function (SRF) method to utilize S-to-p conversions at the lithosphere-asthenosphere boundary (LAB) and Moho. The wavelength of the teleseismic S phase is appropriate to image the gradational nature of the LAB, and contamination of from multiple energy pathways, which is an issue with P receiver functions, is avoided because the Sp conversion arrives as a precursor to the direct S phase. All data available from previous and existing seismic networks (250 stations) are used to analyze 700 teleseismic direct S events and 500 SKS events for SRF. Preliminary results from the CaNOE array in northern British Columbia are consistent with steps in lithospheric thickness across the eastern limit of Cordilleran deformation and at the boundary of allocthounous terranes along the western leg of the array, revealing structure related to Paleozoic growth and modification of the Canadian Craton. Extension of this analysis into the south-central region of Alaska as well as sparsely located station coverage in the broader AK seismic network will search for similar signals and is presented here. Contrasts between the structure of these more recently accreted terranes (late Mesozoic and Cenozoic) and those of the more interior terranes in the Yukon and British Columbia provinces will provide comparisons of the time progression of continental growth through time. The well-instrumented region encompassing the Alaska Range, Kenai Peninsula and Wrangell Mountains will be suitable for Common Conversion Point stacking, allowing for a robust determination of the three dimensional geometry of the LAB. Comparison of these results of lithospheric scale imaging of the northern Cordillera with shear wave splitting analyses provide insight into the variation of lithospheric-scale fabric and potential differences in the ages or nature of the accreted terranes.