THE BARANOF-LEECH RIVER HYPOTHESIS REVISITED

In 1982, I proposed that (1) schists on southern Baranof Island in SE Alaska were contiguous with the Leech River schist on southern Vancouver Island until 40 Ma, and (2) the Baranof schists and the adjoining Chugach terrane had been displaced northward about 1100 km by post-40 Ma coastwise dextral slip. Diverse evidence obtained by others since 1982 is compatible with the Baranof-Leech River hypothesis, which I modify in one respect. Isotopic data (Brew et al., 1991; Groome et al., 2000) show that the syn-magmatic metamorphism that produced the Baranof and Leech River schists occurred at 50 Ma, not 40 Ma. Therefore, large-magnitude coastwise slip, accommodated partly on the Border Ranges fault system, is post-50 Ma.

Following Bradley et al. (1993) and others, I ascribe the Baranof-Leech River, near-trench magmatism to the subduction of an ocean ridge. The intersection of the ridge with the contintental margin 50 m.y. ago is fixed at latitude ca. 48 degrees N (present-day coordinates). The slab window gave rise not only to plutonic rocks in the Baranof and Leech River units but also to coeval peraluminous plutonic and volcanic rocks on North American basement in the northwestern Cascades, and probably to the coeval Catface-Flores suite on the southwestern coast of Vancouver Island. Paleomagnetic data from Kodiak Island (Plumley et al., 1983) and the Resurrection Peninsula (Bol et al. 1992) imply that before 50 Ma, the parts of the Chugach terrane that were to become the Baranof and Leech River schists were south of 48 degrees N.