Accretion and subduction tectonics in the Chugach Mountains and Copper River Basin, Alaska: Initial results of the Trans-Alaska Crustal Transect

Abstract

Geologic, seismic, gravity, and magnetic data from the northern Chugach Mountains and southern Copper River Basin, Alaska, indicate that the Chugach terrane (CGT) and the composite Peninsular/Wrangellia terrane (PET/WRT) are thin (< 10 km), rootless sheets bounded on the south by north-dipping thrust faults that sole into a shallow, horizontal, low-velocity zone. The CGT has been thrust at least 40 km beneath the PET/WRT along the Border Ranges fault system (BRFS). Adjacent to the BRFS, uplift and erosion of 30-40 km since Jurassic time have exposed blueschist-facies rocks in the CGT and mafic and ultramafic cumulate rocks in the PET/WRT. Four paired north-dipping layers of low and high seismic velocities extend beneath the northern CGT and southern PET/WRT and may be slices of subducted oceanic crust and upper mantle; the upper two pairs may now be joined to the continental plate. 15 references, 5 figures.