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:
- 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.
- 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.
- 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.
- 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.