Figure 1: Simplified terrane map of Alaska and the Northern Cordillera with major faults. Most major faults in Alaska are terrane boundary structures (e.g., Coney et al., 1984; Nokleberg et al., 1985). Convergence rates for the Pacific Plate and Yakutat microplate with respect to a fixed north America are from Elliot et al. (2010). Approximate extent of Figure 2 is shown by the white box. Extent of imaged subducted Yakutat microplate is based chiefly on tomography and seismicity data (adapted from Eberhart-Phillips et al., 2006; Bauer et al., 2014; and Wech, 2016; Pavlis et al., 2019; Mann et al., 2022). Geologic units modified from Wilson et al. (2015). MZ sediments represent the Alaska Range Suture Zone. Offset Maclaren - Kluane schist shown with blue 1*. Potentially offset Tetelna formation and Station Creek formation shown with yellow 2*. Offset Dezeadesh-Nutzotin-Clearwater basins shown with green 3*. Offset Ruby Range - Susitna Batholith pink 4*.
We present thermochronologic evidence from near-fault and fault-zone bedrock samples along the southern Totschunda fault demonstrating the initiation of rapid exhumation at ca. 6 Ma. We also present geochronology and thermochronology data from cobbles within a nearby basin of Late Miocene-Pleistocene age lying to the east of the Totschunda fault and cut by splays of that fault. Rapid exhumation beginning in the Late Miocene along the Totschunda fault is interpreted as a response to the ca. 6 Ma Pacific-Yakutat plate motion change. This plate motion change led to a more favorable convergence orientation such that slip (with associated exhumation) was primarily accommodated on the Totschunda fault, effectively abandoning strike-slip on the northern Eastern Denali fault. Our results build upon our understanding of how plate boundary forces are transferred and distributed inboard onto strike-slip fault systems like the San Jacinto-San Andreas faults in California (Spotila et al. 2001; Blythe et al., 2002). Furthermore, this data provides insight into long-term slip distribution along strike slip fault systems which can improve seismic hazard forecasts in Alaska and other regions with long lived strike-slip faults and evolving plate boundary conditions (e.g., Alpine fault, Dead Sea, San Andreas fault).