Crustal Structure across the Coast Mountains in SE Alaska and British Columbia from Wide-Angle P- and S-wave Seismic Data
Morozov, I B (1), Smithson, S B (1), Hollister, L S (2)
(1)Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071-3006 United States; (2) Department of Geosciences, Princeton University, Princeton, NJ 08544 United States
Abstract:
Crustal and uppermost mantle structure across the Coast Mountains, SE Alaska and British Columbia is analyzed using P- and S-wave wide-angle seismic data from the 1994 ACCRETE experiment. High-quality S-wave records present one of the most unique features of this experiment. A combination of travel-time analysis methods, pre-stack migration and correlation with a coincident vertical-incidence multichannel seismic section results in a detailed P- and S- wave velocity structure. The study reveals moderately high crustal velocities, low velocity gradient in the middle crust, and decreasing average crustal velocity and a deepening Moho north of the Coast Shear Zone (CSZ). Termination of crustal reflectivity across a vertical zone indicates that the CSZ is most likely a strike-slip fault. A mid-Cretaceous thrust system mapped on the surface to the west of the CSZ is imaged by several groups of mid- to lower crustal reflectors extending close to the Moho indicating it was a thick-skinned thrust system. NE dipping fabric imaged within the Coast Mountains batholith (CMB) is associated with a ductile deformation during early Eocene crustal extension. The crustal section under the CMB, which has an average P-wave velocity of 6.55 km/s and shallower than average crustal thickness of 31 km, can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high-velocity tonalite to gabbro sills.
