6. Conclusions

By integrating our new geochronology and thermochronology data from the southern part of the Totschunda fault with a compilation of geologic data from across southern Alaska we increased our understanding of the Totschunda-Denali fault system and southern Alaska tectonics. The key findings are:
  1. Inverse thermal models (HeFTy) of apatite Fission track, (U-Th)/He, and U-Pb indicate a rapid exhumation event (~13°C/Ma) initiated on the southern segment of the Totschunda fault at ca. 6 Ma.
  2. We suggest slip rates of ~2 mm/yr on the Totschunda fault from ca. 25 Ma to 6 Ma to accommodate ~40 km of dextral displacement.
  3. Starting in the Late Miocene (~6 Ma) slip rates on the Totschunda fault accelerated from ~2mm/yr to ~14 mm/yr as strain from the southern Alaska margin was accommodated by the Totschunda fault while the northern Eastern Denali fault was effectively abandoned. This change is a result of a relative plate motion change between the Pacific-Yakutat plates and North America.
  4. We suggest that circum-Pacific Plate fault reorganizations were compelled by the ca. 6 Ma Pacific plate motion change with the nature for slip along fault systems to be redistribution towards mechanical efficiency (Cooke and Madden, 2014; McBeck et al., 2017).
Ancillary findings from the study area include 1) Cretaceous rapid cooling from ~95 Ma to ~85 Ma (~10°/Ma) is documented on a small Cretaceous gabbro unit, 2) U-Pb zircon ages document Oligocene Wrangell Arc volcanism along the Totschunda fault and refine a previously mapped “Triassic gabbro” as Cretaceous in age and 3) we suggest many basins formed along strike-slip faults are likely to be poor targets to apply cobble thermochronology in order to constrain the timing of basin inversion because of their relatively short-lived nature and small size.