The Topographically Asymmetrical Alaska Range: Multiple Tectonic Drivers Through Space And Time

Abstract

The topographically segmented, ~700 km long Alaska Range evolved over the last ~50 Ma in response to both far-field driving mechanisms and near-field boundary conditions. The eastern Alaska Range follows the curve of the Denali Fault strike-slip system, forming a large arc of high topography across southern Alaska. The majority of the topography in the eastern Alaska Range lies north of the Fault. A region of low topography separates the eastern Alaska Range from the central Alaska Range, where most of the high topography lies south of the Denali Fault. To the west, there is a restraining bend in the Fault. Southwest of the bend, the north-south trending western Alaska Range takes an abrupt 90 degree turn away from the Denali Fault. I applied 40Ar/39Ar thermochronology to over forty granitic samples to constrain the thermal history of the western and eastern Alaska Range. I combine the 40Ar/39Ar analyses with available apatite fission track and apatite (U-Th)/He dating. I then inferred the Alaska Range's exhumation history from the region's rates and patterns of rock cooling. Periods of mountain building within the Alaska Range are related to Paleocene-Eocene ridge subduction and an associated slab window (~50 Ma to ~35 Ma), Neogene flat-slab subduction of the Yakutat microplate (~24 Ma to present), Yakutat microplate latitudinal variation in thickness (~6 Ma to present), block rotation/migration, and fault reorganization along the Denali Fault. However, it is clear from basin, petrological and thermochronological constraints that not all of the far-field driving mechanisms affected every segment of the Alaska Range to the same degree or at the same time. Alaska Range tectonic reconstruction is also complicated by near-field structural controls on both the timing and extent of deformation. Fault geometry affects both the amount of exhumation (e.g., ~14 km in the Susitna Glacier region of the eastern Alaska Range) and location of topographic development (e.g., north or south of the Denali Fault). The topographic signature we see today is also in part the result of a pre-existing landscape modified by Plio-Quaternary (~3 Ma to present) surface processes.