Wide-angle seismic imaging across accreted terranes, southeastern Alaska and western British Columbia

Abstract

This study addresses the question of crustal, Moho, and uppermost mantle structure across an accreted terrane, continental arc, and fold and thrust belt in southeastern Alaska and western British Columbia. The 186-km-long Portland Canal line of the ACCRETE wide-angle seismic dataset across the Coast Mountains is analyzed using a combination of travel-time analysis techniques, including delay-time tomography, turning ray tomography, reciprocal time analysis, intercept-time inversion, and forward ray tracing. Additional constraints on the model are obtained from a coincident vertical-incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section. The study reveals moderately high crustal velocities, low-velocity gradient in the middle crust, and decreasing average crustal velocity and a north-deepening Moho. Termination of crustal reflectivity across a vertical zone (the Coast Shear Zone, CSZ) indicates that the CSZ is most likely a strike-slip fault associated with a transpressive tectonic regime. 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 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.