The Coast Mountains orogen is thought to have formed as a result of accretion of the Alexander and Wrangellia terranes against the western margin of the Stikine and Yukon–Tanana terranes, but the nature and age of accretion remain controversial. The Chatham Sound area, which is located along the west flank of the Coast Mountains near the Alaska–British Columbia border, displays a wide variety of relations that bear on the nature and age of the boundary between inboard and outboard terranes. Geologic and U–Pb geochronologic studies in this area reveal a coherent but deformed and metamorphosed sequence of rocks belonging to the Yukon–Tanana terrane, including pre-mid-Paleozoic marble, schist, and quartzite, mid-Paleozoic orthogneiss and metavolcanic rocks, and upper Paleozoic metaconglomerate and metavolcanic rocks. These rocks are overlain by Middle Jurassic volcanic rocks (Moffat volcanics) and Upper Jurassic – Lower Cretaceous strata of the Gravina basin, both of which also overlie Triassic and older rocks of the Alexander terrane. This overlap relationship demonstrates that the Alexander and Wrangellia terranes were initially accreted to the margin of inboard terranes during or prior to mid-Jurassic time. Accretion was apparently followed by Late Jurassic–Early Cretaceous extension–transtension to form the Gravina basin, left-slip along the inboard margin of Alexander–Wrangellia, mid-Cretaceous collapse of the Gravina basin and final structural accretion of the outboard terranes, and early Tertiary dip-slip motion on the Coast shear zone.

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