Date of Award

6-2011

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Geology

First Advisor

John Garver

Language

English

Keywords

exhumation, Paleocene, geology, orca, Valdez Group, fault

Abstract

The Prince William terrane is a major component of the Mesozoic-Tertiary accretionary complex of the North American Cordillera that is well exposed for ~2200 km in southern Alaska and is inferred to be one of the thickest accretionary complexes in the world. Detrital zircons from Prince William terrane record the thermal evolution and exhumation history of the accretionary wedge. Samples of the Paleocene-Eocene Orca Group of the Prince William terrane were analyzed using detrital zircon fission track techniques to understand the thermochronology of the region near Cordova. Six sandstones from the Orca Group and one sample of the Sheep Bay Granite were collected, dated, and track lengths were analyzed. In the Orca Group, the six samples collected along a transect from Sheep Bay to Cordova have a common cooling age of ~49 Ma, as well as older populations at ~79 Ma and older. Samples collected to the east of the NE-SW-trending Rude River fault have a young cooling age population at ~31 Ma that is essentially absent to the west of the fault. The ~52 Ma Sheep Bay granite has a majority ZFT cooling age population that is overdispersed with grains at ~40 Ma and at ~28 Ma. Ages and track-length distributions vary on either side of the Rude River fault. To the east of the Rude River fault, in the Orca Group, track lengths are long and unimodal, suggesting full resetting of grains. To the west of the Rude River fault, track lengths are bimodal with an abundance of shortened tracks, indicating a significant degree of partial annealing. A similar and contemporaneous study found that the Valdez Group of the Chugach terrane to the north has a majority cooling age population at ~38 Ma, as well as a minority population at ~51 Ma that is evident in half of the samples studied. The Orca Group and the Valdez Group share a memory of cooling at c. 50 Ma that is likely related to an increase in the geothermal gradient of the region related to granitic intrusions of the Sanak-Baranof belt driven by passage of the slab window and appears to lag that event by only several Myr. The young cooling ages record a landward age progession for the exhumation of the orogenic wedge in the Late Eocene (~38 to 32 Ma).

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