Figure 7: Schematic palinspastic reconstructions of strike-slip
translation and basin formation in eastern Alaska and the Yukon from 25
Ma to present. Pacific- Yakutat microplate vectors through time
(Doubrovine and Tarduno, 2008; Elliot et al., 2010;Elliott and Freymuller
2020) and strike of Eastern Denali fault and Totschunda fault shown in
upper right corner of figures when applicable. Major faults and geologic
units are labeled in each figure. Blue pin is arbitrary and fixed
relative to the red and black pins. Blue-red pins demonstrate 130 km of
right-lateral slip on the Eastern Denali fault from 25 Ma to 6 Ma using
long term slip rates of ~7mm/yr. (Waldien et al., 2018)
for the given time period. Black-red pins demonstrate
~40 km of allowable right-lateral slip on the Totschunda
fault from 25 Ma to 6 Ma (~2 mm/yr) and an additional 85
km of right-lateral slip on the Totschunda fault from 6 Ma to present
(14mm/yr) using long term slip rates and total separation between the
correlative Nutzotin basin sediments (Red Pin) and Clearwater sediments
(Black Pin) (Waldien et al., 2018; Berkelhammer et al., 2019; Allen et
al., 2022). Strike-slip motion is minimal if at all along the northern
Eastern Denali fault after ca. 6 Ma (e.g., Marechal et al., 2018; Choi
et al., 2021). WR - Wrangellia; ARSZ - Alaska Range Suture Zone;
CG-Chugach Terrane; INT-Intermontane Terrane / North America; Ol
Sds-Oligocene Sediments; Mio Sds-Miocene Sediments; Pleis
Sds-Pleistocene Sediments.
Our new thermochronology data documents an increase in cooling, inferred
exhumation, and slip rates at 6 Ma along the Totschunda fault. We
conclude the ca. 6 Ma Pacific-Yakutat plate vector change (Engebretson
et al., 1985) was responsible for transferring slip away from the
northern Eastern Denali fault and onto the Totschunda fault at this
time. The ca. 1 Ma collision of the ~30 km thick Yakutat
segment and subsequent jamming of the Wrangell arc trench (Richter et
al., 1990; Christenson et al., 2012; Gulick et al., 2013; Reece et al.,
2013; Trop et al., 2022; Brueseke et al., 2023) may have further
accelerated stress transfer from the plate boundary along the
Fairweather transform to the Totschunda fault via the inferred Connector
fault (e.g., Richter et al., 1971; Lahr and Plafker, 1980; Spotila and
Berger 2010; Doser, 2014).
The Late Miocene (ca. 6 Ma) southern Alaska tectonic event documented by
numerous previous thermochronology studies across and along the Denali
fault system (e.g., Fitzgerald et al., 1993, 1995; Waldien et al., 2018;
Benowitz et al., 2022a, b) has been linked to the Pacific-Yakutat plate
vector change at ca. 6 Ma. This event led to a profound change in slip
distribution on faults across southern Alaska. Rocks along other fault
systems such as the Fairweather fault corridor (McAleer et al., 2009),
Queen Charlotte fault zone (Cromwell, 2021), St. Elias Range (Enkelmann
et al., 2017); western Chugach mountains (Arkle et al., 2013); as well
as the Tordrillo Mountains (Haeussler et al., 2008) all experienced a
pronounced increase in exhumation at ca. 6 Ma. Additional studies
document increases in the progression of magmatism in the Wrangell
volcanic field ca. 6 Ma (Richter et al., 1990; Trop et al., 2022),
initiation of the magmatism on the Seward Peninsula (Mukasa et al.,
2007), rejuvenation of the Aleutian island arc (Jicha et al., 2006),
extension in the Bering Sea (Marincovich et al., 1999), formation of the
Mount McKinley restraining bend (Benowitz et al., 2022b) and coarse
clastic sedimentation rates along the Denali fault and Cook Inlet
increasing in the Late Miocene (Wahraftig, 1975; Ridgway et al., 2007;
Finzel et al., 2015; Allen et al., 2022) (Figure 8).
Furthermore, the increase in deformation along the west side of the
Fairweather fault related to the ca. 6 Ma Pacific-Yakutat plate vector
change (McAleer et al., 2009) resulted in a “double-trouble” tectonic
scenario along the St. Elias syntax. The continued flat slab-subduction
of the Yakutat oceanic plateau combined with convergence of the crustal
welt along the western Fairweather fault led to increased deformation
along the St. Elias coastal mountains (Enkelmann et al., 2010). However,
there is no evidence the indention of the Fairweather Fault sliver led
to increased slip rates on the Totschunda fault since 6 Ma.