Date of Award
Bachelor of Science
tectonics, fossil, schist, geology, plates
During the Eocene in the North Pacific, the Kula, Farallon, and Pacific plates met in a trench-ridge-trench triple junction, bordered to the east by a continental margin along the edge of the North American plate. The flysch of the Chugach-Prince William terrane, a deformed accretionary complex, accreted onto this margin in the late Cretaceous to Paleocene. This terrane is framed to the north by the Border Ranges fault, a large strike-slip fault system that has accommodated northward movement of the Chugach-Prince William since the Eocene. One of the easternmost units of the Chugach-Prince William is the Baranof Schist on Baranof Island, a metasandstone unit metamorphosed to garnet-biotite to andalusite grade near the Crawfish Inlet pluton ca. 47-52 Ma. The Crawfish Inlet pluton is the youngest of a series of plutons of the Sanak-Baranof plutonic belt, which decreases in age from west to east, beginning with 61 Ma on Sanak Island. The Baranof Schist experienced post-intrusive cooling after the metamorphic event and this cooling was determined through zircon fission track dating. Twelve samples from a transect of Whale Bay on Baranof Island were collected for analysis to determine the thermal and tectonic history of the schist unit. Zircon fission track cooling ages indicate that the Baranof Schist cooled uniformly but relatively slowly, as young fission track ages are between 27-39 Ma. Six of the samples are overdispersed, failing χ2, and have two age populations potentially reflecting different thermal events at ~42 and ~35 Ma. The younger of the two populations of cooling ages are consistent with cooling ages from those samples that passed χ2 and reflect cooling of the Baranof Schist after intrusion of the Crawfish Inlet pluton. Combining these fission track data with cooling ages from other studies produces the cooling curve of the Baranof Schist, which shows that cooling of the unit lasted from 42 Ma until at least 27 Ma. This cooling curve can be compared to the cooling of the Leech River Schist and the Chugach metamorphic complex, two metamorphosed units believed to have been contiguous with the Baranof Schist at 50 Ma. The Chugach metamorphic complex to the north of the Baranof Schist began cooling 52-55 Ma and the Leech River Schist on Vancouver Island began cooling 49-50 Ma. It is hypothesized that these three units were accreted as a belt and were then metamorphosed successively by the passing of a slab window between the Kula and Farallon plates. After metamorphism, the Baranof Schist and Chugach metamorphic complex were translated northward as indicated by paleomagnetic data, while the Leech River Schist remained in place.
Kaminski, Kate, "Exhumation of the Baranof Schist in Whale Bay Alaska determined through zircon fission track dating" (2014). Honors Theses and Student Projects. 538.