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.