105 Tectonics (Posters)

Monday, 1 November 2010
8:00 AM - 6:00 PM
Hall D

USING DETRITAL ZIRCON FISSION-TRACK AGES FROM MODERN GLACIAL SEDIMENTS TO EVALUATE EXHUMATION OF THE WESTERN CHUGACH MOUNTAIN RANGE, SOUTH CENTRAL ALASKA

Paper number: 105-2
Program: Technical Sessions
Time: 8:00 AM – 6:00 PM

Authors

Abstract

The western Chugach Mountains reside ~20-30 km above the highly-coupled megathrust between the relatively buoyant Yakutat microplate and the overlying North American plate. Shallow subduction of the Yakutat terrane began about 25 Ma forming a syntaxial region marked by the curvature of major faults and mountain belts in the area; these features trend northeast in the west to northwest in the east. The Chugach Mountains and Prince William Sound area may be a locus of rapid uplift and exhumation caused by Yakutat subduction. Direct sampling of the Chugach Mountain bedrock is difficult due to steep slopes, high elevation, and heavy glaciation. However, glaciers erode and carry sediment throughout their expanse. Outwash samples were collected at the termini of Harvard and Barry glaciers, which are tidewater glaciers that drain the south (windward) side of the western Chugach Mountains for detrital zircon fission-track (ZFT) analysis. Binomial peak-fitting analysis was used to assess ZFT age distributions and peaks.

Harvard glacier yields ZFT ages that range from 16.1 to 50.1 Ma with peak ages of 19.1, 27.8, 33.3, and 44.1 Ma (n=100). Barry glacier yields similar age peaks of 23.0, 29.6, and 45.4 Ma, with ages that range from 14.0 to 53.9 Ma (n=100). All of these ages are younger than the depositional age of the Late Cretaceous Valdez Group from which the samples were primarily derived and are therefore reset ages. The younger peaks may be the result of rapid exhumation in the southwest Chugach Mountains related to Yakutat subduction. However, some of these peaks are similar to ages determined for various dikes and sills throughout the Valdez Group, suggesting at least some resetting is related to intrusive heating events. Peak ages from the Prince William Sound side of the Chugach Mountains are generally younger than glacial outwash ZFT age peaks from glaciers that drain the north side of the range. Two sampled glaciers on the north side yield peak ages that range from 36 to 86 Ma, with many unreset ages. The much younger peak ages from the Prince William Sound area may indicate that more rapid, higher magnitude exhumation occurs on the south (windward) side of the Chugach Mountains.