The thermochronological record of tectonic and surface process interaction at the Yakutat-North American collision zone in southeast Alaska

Abstract

We investigate the material fluxes in space and time as a result of exhumation and erosion processes at the ongoing Yakutat-North American collision in southeast Alaska. Many thermochronologic studies using a variety of sampling strategies are challenged by the widespread ice cover that limit field observations and accessibility. This paper reviews new and published low-temperature thermochronological data from southeast Alaska to give a comprehensive interpretation of the exhumation patterns through time and how they are influenced by surface processes and climate change. We find that the southeastern margin of Alaska was exhumed and eroded long before the late Miocene-Pliocene Yakutat collision, but since the beginning of the subduction of the Yakutat lithosphere in the Oligocene/early Miocene. Today there is a distinct pattern of exhumation in southeast Alaska with a localized very rapid and deep-seated exhumation at the Yakutat plate corner (St. Elias syntaxis), where strike slip motion changes to convergence. Exhumation is also rapid, but less deep along the dextral Fairweather fault, and in the evolving fold and thrust belt. We present a re-interpretation of the exhumation pattern in the fold and thrust belt and suggest that mass transport by exhumation is parallel to the observed active thrust faults and oblique to the suture zone and orogenic strike. The locus of most rapid exhumation migrated from northwest to southeast with Recent exhumation occurring near the St. Elias syntaxis. Exhumation of the Chugach terrane rocks is still active, however to a lesser degree than on the south side of the orogen where precipitation rates are much higher. The Wrangellia terrane to the north has experienced little exhumation and has essentially formed the backstop for terrane accretion in southeast Alaska since the Early Cretaceous. Apatite U-Th/He ages give the first evidence that rocks of the Wrangell Range have only been recently uplifted and eroded as a consequence of the continuing Yakutat collision. In general the thermochronology in southeast Alaska reveals that climate variations across the region as well as changes through time have a limited influence on the pattern of erosion and that the location of deep exhumation is primarily influenced by tectonic processes.