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.