Using Low-Temperature Thermochronology to Constrain the Role of the Totschunda Fault in Southeastern Alaskan Tectonics

• Main Author: Milde, Edward Richard
• Format: Text
• Language: unknown
• Published: SURFACE at Syracuse University 2014
• Subjects: Alaska; Tectonics; Totschunda; Devils Mountain; Fairweather; Yakutat
• Online Access: https://surface.syr.edu/thesis/29; https://surface.syr.edu/cgi/viewcontent.cgi?article=1030&context=thesis

The Totschunda fault is a prominent northwest-striking strike-slip fault located in southeastern Alaska. The Totschunda fault branches southeast off of the Denali fault system and likely connects to the Fairweather fault to the south via the proposed "Connector fault". The significance of the Totschunda fault became apparent after the 2002 M 7.9 Denali fault earthquake propagated 227 km east along the Denali fault and onto the Totschunda fault. Deformation and movement along the Totschunda fault is most likely a result of tectonic processes along the active southern margin of Alaska. Apatite fission track thermochronology (AFT) was applied to rocks along and across the Totschunda fault in order to better constrain its history and role in regional tectonics of southern Alaska. Samples were collected west of the Cooper Pass region near the Nabesna River in southeastern Alaska. There the Totschunda fault 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 Permian and Pennsylvanian igneous stocks. Extensive volcanism in the nearby Wrangell volcanic belt eruptive centers date from ~26 Ma to active. Whole rock 40Ar/39Ar ages from Wrangell volcanic belt hypabyssal rocks collected as part of this study yielded ages of ~23 Ma. AFT ages ranged between 24 and 190 Ma. The samples closest to the western strand of the Totschunda fault yielded the youngest AFT ages. Ages grew older with distance from the Totschunda fault and with elevation. Samples located within the stepover feature were significantly older than any of the other samples in the study area. HeFTy inverse thermal models combined with AFT results indicate episodic cooling events since at least ~90 Ma. These events are interpreted as due to exhumation associated with local faulting. The near-fault samples recorded episodes of rapid cooling as recent as the Late Miocene. Other inverse models from this study confirmed ~25 Ma and ~80 Ma events coinciding with the accretion of the Yakutat microplate and the change in structural dynamics of southern Alaska respectively. Another ca. ~55 Ma event recorded in the data most likely correlates to changes in plate boundary processes associated with the "Resurrection", Kula and Farallon plates. Accommodation of strain along the Totschunda fault can be related to plate boundary processes along the southern margin of Alaska. It is significant that the initial collision of the Yakutat microplate began in the Oligocene (ca. 25 Ma), and it is very likely that this event is what is driving most of the young cooling events analyzed in this study.