CONSTRAINING THE TRANSITION FROM SHALLOW TO STEEP SUBDUCTION IN THE WESTERN CHUGACH AND KENAI MOUNTAINS (ALASKA) USING LOW-TEMPERATURE THERMOCHRONOLOGY

The western Chugach Mountains are located in a syntaxial bend above the shallow subducting Yakutat microplate. The region is characterized by arcuate fault systems and steep mountainous topography. Distributions of apatite fission-track and (U-Th)/He (AHe) ages are interpreted to represent focused exhumation in the syntaxial core region. In this study, five new samples collected along a roughly north-to-south transect in the western Chugach and northern Kenai Mountains were analyzed using AHe dating. The average AHe ages for the samples, based on 3-4 single-grain replicates each, range from 36.0 to 8.7 Ma. The AHe ages display a pattern of exhumation in the region and support previous work by Arkle et al. (2013-GSA Bull.), which show a bulls eye pattern of young (4-5 Ma) ages in the vicinity of the Harriman Fiord and Mt. Marcus Baker that increase outward towards the west and south into our study area. South of Turnagain Arm, ages increase abruptly and are generally ~12 Ma. This abrupt age change may be the surface expression of exhumation above the transition from shallow Yakutat subduction to steeper subduction of the Pacific plate south of Turnagain Arm. These data and interpretations are also supported by geomorphic features in the region and provide constraints on the rock uplift effects of the transition from shallow to steep subduction beneath south-central Alaska.