Geology of the western flank of the Coast Mountains between Cape Fanshaw and Taku Inlet, southeastern Alaska
The western flank of the Coast Mountains batholith between Cape Fanshaw and Taku Inlet is underlain primarily by Jura-Cretaceous strata of the Gravina belt; pre-Permian(?), Permian, and Triassic strata of the Taku terrane; and mid-Proterozoic(?) to upper Paleozoic rocks of continental margin affinity. The continental margin rocks include mid-Proterozoic(?) to lower Paleozoic(?) quartzite and marble of the Tracy Arm assemblage; mid-Paleozoic metavolcanic and subordinate metasedimentary rocks of the Endicott Arm assemblage; and upper Paleozoic quartz-rich metaturbidites and metaconglomerate of the Port Houghton assemblage. We suggest that these three assemblages are correlative with components of the Yukon-Tanana terrane, which underlies a large region of Yukon and eastern Alaska. Rocks of the Gravina belt, Taku terrane, and Yukon-Tanana terrane are juxtaposed along west-vergent thrust faults of mid-Cretaceous age and are internally deformed and disrupted along latest Cretaceous to early Eocene dip-slip and possibly strike-slip shear zones. These rocks and structures, together with mid-Cretaceous to Eocene plutons of the Coast Mountains batholith, separate rocks of the Alexander-Wrangellia terrane to the west from rocks of the Stikine terrane to the east. Mid-Cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the Alexander-Wrangellia terrane from central Alaska to northwestern Washington. The continental margin rocks in the northern Coast Mountains may be part of the in situ North American margin that has been overthrust by the Stikine and adjacent terranes. Alternatively, these rocks may have (1) rifted from and then returned to the North American margin, (2) moved >800 km along left-lateral or right-lateral faults from elsewhere along the North American margin, or (3) originated near a continent other than North America.