APPLYING LOW-TEMPERATURE THERMOCHRONOLOGY TO CONSTRAIN THE TECTONIC HISTORY OF THE STRIKE-SLIP TOTSCHUNDA FAULT SYSTEM, SOUTHEASTERN ALASKA

The Totschunda fault (TF) is a prominent northwest-striking strike-slip fault located in southeastern Alaska. The TF branches from the Denali fault system and likely connects to the Fairweather fault to the south via the proposed “Connector fault.” The 2002 M7.9 earthquake that originated in the eastern Alaska Range propagated east along the Denali fault and then southeast along the TF. Apatite fission track thermochronology (AFT) and apatite (U-Th)/He dating is being applied to rocks along and across the TF in order to better constrain the history of this fault and its role in regional tectonics of southern Alaska. Samples were collected west of Cooper Pass near the Nabesna River. There the TF resembles a right-handed step-over structure with two overlapping strands. Sampling strategy included sampling either side of the western strand, a horizontal transect away from the western strand, a vertical profile on this western side, and one detrital sample from Notch Creek, a tributary of the nearby Chisana River. Most samples were collected from the Cretaceous Nabesna and Devils Mountain plutons or nearby Jurassic igneous stocks. Extensive volcanism in the proximal Wrangell volcanic belt eruptive centers date from ~26 Ma to active. Whole rock ⁴⁰Ar/³⁹Ar ages from Wrangell volcanic belt hypabyssal igneous rocks collected as part of this study west of the TF gave ages of ~23 Ma. Preliminary AFT results immediately adjacent to the western side of the western strand of the TF step-over yield AFT ages of ~25 Ma. Along the TF, on its western side AFT ages increase to the northwest and southeast outside the central part of the step-over, suggesting less exhumation. A K-feldspar ⁴⁰Ar/³⁹Ar age from a feldspar-rich vein within meters of these ~25 Ma fault-adjacent AFT ages gave an unreset plateau age of ~112 Ma. West of the western TF strand, AFT ages increase (up to ~75 Ma) with distance from the fault, and also increase with elevation. Samples within the possible TF step-over (to the west of Coopers Pass) are much older, up to ~190 Ma, suggesting minimal Cenozoic exhumation there as compared to the western side of the western strand. The preliminary ~25 Ma AFT age data and ~23 Ma hypabyssal igneous rocks are coincident with initial highly coupled flat slab subduction (~25 Ma) of the Yakutat Microplate.