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.

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