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).