Abstract

The intersecting Totschunda and Denali strike-slip faults of southern Alaska have been active since at least the Cretaceous. As long-lived structures, the history of deformation along these lithospheric-scale faults can be used to investigate slip distribution between pre-existing fault strands and the transfer of stress inland from plate boundaries due to relative plate motion changes. We apply geochronology (zircon U-Pb) and thermochronology (apatite fission track, zircon and apatite (U-Th)/He) along the southern Totschunda fault and integrate our new data with published regional low temperature thermochronology. U-Pb ages from bedrock samples document Cretaceous gabbroic magmatism and Oligocene Wrangell arc volcanism. New thermochronology data from along the Totschunda fault indicate an increase in rapid cooling, interpreted as exhumation in the Late Miocene that continues until the present. We link this increase in vertical tectonics to Totschunda fault strike-slip motion increasing from ~2 mm/yr in the Oligocene to the modern rate of ~14 mm/yr at ca. 6 Ma based on palinspastic reconstructions. The Pacific plate-North America plate convergence vector rotated ~18° clockwise at ca. 6 Ma resulting in an incoming vector that is preferentially aligned with the pre-existing Totschunda fault. This relative plate motion change and the subsequent alignment of the Totschunda fault with the Fairweather transform, led to slip-rejuvenation along the Totschunda fault and diminution of strike-slip motion on the northern Eastern Denali Fault. We further suggest in other locations around the Pacific margin the Pacific plate motion change in the Late Miocene also triggered fault system reorganization towards mechanical efficiency.