Date of Award

6-2012

Document Type

Union College Only

Degree Name

Bachelor of Science

Department

Geology

First Advisor

John Garver

Language

English

Keywords

zircon, thermal dynamics, Alaska, terrane

Abstract

The Chugach-Prince William (CPW) composite terrane is a Mesozoic-Tertiary accretionary complex 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 zircon in the flysch of the CPW record the thermal evolution and exhumation history of the accretionary wedge. Samples of the Campanian-Maastrichtian Valdez Group of the Chugach terrane and the Paleocene-Eocene Orca Group of the Prince William terrane, were analyzed using detrital zircon fission track techniques to understand the thermochronology of the rocks. One sandstone from the Valdez Group and nine sandstones from the Orca Group in addition to one sample from the Hidden Bay pluton were collected and dated along a NW-SE trending transect from near Anchorage to Montague Island in Prince William Sound (PWS). A sharp thermal discontinuity has been recognized at the Montague Strait fault (based on FT and VR). Samples inboard of the Montague Strait fault all have reset ages younger than deposition, and a heterogeneous cooling history that gets progressively younger closer to the fault. These rocks record three distinct cooling ages at ~49-53 Ma, ~37-40 Ma and 25-30 Ma, which are likely related to regional-scale heating or exhumation. The ~50 Ma cooling age is likely related to regional heating driven by plutonism of the Sanak-Baranof belt that passed through PWS at ~54 Ma. The second major thermal event at ~38 Ma is probably related to plutons of the Eshamy Suite, which have recently been dated to between 37 and 40 Ma. Inboard samples collected closest to the Montague Strait fault record a 25-30 Ma cooling age that may be related to erosion and exhumation caused by the initiation of the Yakutat microplate collision to the east. Outboard of the Montague Strait fault on Montague Island, the rocks are inferred to be Upper Eocene (< 35 Ma), and they are not thermally reset and thus retain original provenance ages. The youngest coherent grain-age population is ~52 Ma, similar to the timing of widespread uplift and exhumation in the British Columbia Coast ranges. Double dating shows that there are two primary fractions of grain ages: 1) 150 to 200 Ma with 100 to 120 Ma cooling ages; and 2) 90 to 60 Ma with 40 to 60 Ma cooling ages. DZFT cooling age population (~40-60 Ma) lags behind the young crystallization age population (~60-90 Ma) by approximately 20 Myr. We propose that the farthest outboard Orca Group on Montague Island may have been translated by one either the Pacific plate or one of the numerous dextral strike-slip faults in PWS and accreted onto the main CPW terrane after 35 Ma (youngest U/Pb ages). Crystallization ages are closely compatible with those from the northern Coast Plutonic Complex along the western Canadian margin, making it a potential source terrane.

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