Revisiting the Baranof–Leech River hypothesis for early Tertiary coastwise transport of the Chugach–Prince William terrane

Abstract

According to the Baranof–Leech River hypothesis originally proposed in 1982, (1) schists on southern Baranof Island in southeastern Alaska were contiguous with the Leech River schist on southern Vancouver Island until 40 Ma, and (2) both rock units were part of the 2200 km long Chugach–Prince William terrane, which was displaced northward about 1100 km after 40 Ma. Isotopic data obtained since 1982 show that the syn-magmatic metamorphism that produced the Baranof and Leech River schists occurred at 50 Ma, not at 40 Ma. Large-magnitude coastwise slip of the terrane is therefore post-50 Ma. Igneous rocks in the Baranof and Leech River units are part of the Sanak–Baranof magmatic belt of forearc magmatism, which has been ascribed to the early Tertiary subduction of an oceanic ridge. The slab window also gave rise to early Eocene, near-trench plutonic and volcanic rocks on North American basement in the North Cascades of Washington State, and probably to coeval igneous rocks on the western coast of Vancouver Island. These igneous suites in the forearc fix the location of the intersection of the ridge with the continental margin 50 Myr ago at latitude ca. 48–49°N (present-day coordinates). Paleomagnetic data obtained since 1982 imply that before 50 Ma, the parts of the Chugach–Prince William terrane that were to become the Baranof and Leech River schists were south of 48–49°N. From 61 to 50 Ma, the northward movement of the terrane relative to North America can be reconciled with the southward migration of forearc magmatism in the Chugach–Prince William terrane if the ridge–trench intersection was fixed at 48°N (present-day coordinates). The Border Ranges fault system is the on-land structure that most likely accommodated hundreds of kilometers of post-early Eocene displacement.