Reconnaissance geology and U-Pb geochronology of the western flank of the Coast Mountains between Juneau and Skagway, southeastern Alaska

Reconnaissance-scale mapping and related structural, stratigraphic, and U-Pb geochronologic studies suggest that the western flank of the Coast Mountains between Juneau and Skagway, Alaska, preserves structural and stratigraphic relations that have not been recognized along other segments of the Coast Mountains orogen. In much of the area southeast of Juneau, the western Coast Mountains area is underlain by moderate- to high-grade metamorphic rocks that are imbricated along mid-Cretaceous thrust faults and intruded by deep-level mid-Cretaceous plutons. Near and just north of Juneau, the metamorphic rocks and associated thrusts are truncated by the latest Cretaceous-early Tertiary Coast shear zone and associated tonalite sills, and mid-Cretaceous plutons have not been recognized. North of this zone of truncation, the western flank is underlain by relatively low grade and only moderately deformed Triassic strata of the Wrangellia terrane and overlying Jurassic-Cretaceous strata of the Gravina belt. Stratigraphic relations preserved in these rocks allow for the recognition of a new unit within the Gravina belt, the Treadwell Formation, and provide evidence that Triassic strata of the Taku terrane are lateral facies equivalents of Triassic basalts of the Wrangellia terrane. The northern Coast Mountains also preserve important relations between the Coast shear zone and the Denali and Chatham Strait faults. All of these structures are known or suspected to have moved during early Tertiary time, with ∼370 km of dextral offset on the Denali fault, ∼150 km of dextral motion on the Chatham Strait fault, and predominantly dip-slip displacement on the Coast shear zone. Where the Denali fault swings abruptly westward into the northeast-striking Chatham Strait fault, the nearby segment of the Coast shear zone records oblique dextral and/or reverse motion. The apparent linkage between these structures supports Lanphere's (1978) suggestion that the difference in offset of the Denali and Chatham Strait faults (∼220 km) is accommodated by dextral slip on the Coast shear zone and related structures along the west flank of the Coast Mountains. This provides an explanation for dextral offset within and adjacent to the Coast shear zone that does not involve large-scale (1500-2000 km) transform motion.