Geological evidence for post–40 m.y. B.P. large-scale northwestward displacement of part of southeastern Alaska

Abstract

The Leech River complex on southern Vancouver Island and schistose rocks on southern Baranof Island have very similar geologic histories. Both were derived from probable Upper Jurassic–Cretaceous terrigenous clastic rocks and minor associated volcanogenic rocks and chert. Each underwent (1) at least two penetrative deformations locally involving folding, disruption, and transposition of thinly layered sequences, and (2) synkinematic, low-pressure, Buchan-type metamorphism. Progressive metamorphic zones ranging from greenschist to amphibolite facies are related to granitic intrusive bodies. Deformation, metamorphism, and intrusion culminated about 40 to 42 m.y. ago. Apparently, these units, now separated by more than 1,100 km, were once contiguous and lay at least as far south as the latitude of southern Vancouver Island. Shortly after 40 m.y. B.P., the allochthonous Leech River complex was emplaced at the southern edge of Vancouver Island, and the schists of Baranof Island and perhaps the entire Chugach terrane began their long northwestward journey along a system of dextral transcurrent faults.