Figure 8: a)Compilation of the location tectonic and magmatic events across southern Alaska that occurred in response to the ca. 6 Ma Pacific plate motion change. Yakutat slab position at ca. 6 Ma is inferred based on restored modern ~50 mm/yr slab slip rates and does not include ~1000 km of likely eclogitized slab by that time (Rossi et al., 2006). References: fault modification:1Benowitz et al. (2022a); 2Benowitz et al. (2022b); 3Waldien et al. (2018);4this study, 5Choi et al. (2021);6McAleer et al. (2009); 7Hyndman (2015). Orogenesis increase: 8Fitzgerald et al. (1993, 1995); 2Benowitz et al. (2022b);9Haeussler et al. (2008); 10Arkle et al. (2013); 11Enkelmann et al. (2017). Basin evolution: 12Marincovich and Gladenkov (1999);13Finzel et al. (2015); 14Ridgway et al. (2007); 15Allen et al. (2022). Magmatism change:16Mukasa et al. (2007); 17Richter et al. (1990). Also see text for additional references for these ca. 6 Ma southern Alaska tectono-magmatic events. Base layer from Lim et al. (2011). b) and c) Schematic of pre and post ca. 6 Ma slip distribution and block motion along the Totschunda and eastern Denali faults. Before ca. 6 Ma the Totschunda and Duke River faults were translated along the curve of the Denali fault leading to variable orientations in relation to relative Pacific plate convergence direction. Modified from Marechal et al. (2018).
Lastly, the Totschunda fault strikes 326°, the northern Eastern Denali fault has a general strike of 308°, while the incoming Yakutat microplate is converging at 337° relative to southeastern Alaska (Elliott et al., 2010). The strike of these faults and when they are active is correlative with the relative convergence direction of the subducting plate. The change in relative plate motion and ~37% increase in net convergence rate in the Late Miocene (ca. 6 Ma) to a more northerly orientation is better aligned with the strike of the Totschunda fault versus the strike of the Eastern Denali fault. This Late Miocene event correlates with the inferred change in slip distribution from 7 mm/yr of horizontal motion on the Eastern Denali fault and <2 mm/yr on the Totschunda fault to <1mm/yr on the Eastern Denali fault and 14 mm/yr on the Totschunda fault (Figure 7). We suggest that the strike of the Totschunda fault was more favorably aligned than the Eastern Denali Fault to accommodate slip following the Late Miocene relative plate motion change since the Totschunda fault is nearly parallel to the plate vector of the Pacific-Yakutat plate. Conversely, prior to the Late Miocene the Eastern Denali fault was nearly parallel to the plate motion vector of the then slower moving Pacific-Yakutat plate. Thus, the fault system has changed from the Central Denali fault receiving slip from the Eastern Denali fault to the Central Denali fault receiving slip from the Totschunda fault (Figure 7).
In summary the newly presented thermochronologic constraints from the Totschunda fault in conjunction with: 1) The general absence of younger-than-Late Miocene rapid exhumation on the northern Eastern Denali fault (e.g., McDermott et al., 2019) (Figure 2a); 2) The Eastern Denali fault acting as a backstop with little to no dextral slip component since the Late Miocene (e.g., Enkelmann et al., 2022), 3) Limited to no Holocene strike-slip motion on the modern Eastern Denali fault (Marechal et al., 2018), and 4) Seismic analysis indicating the Eastern Denali fault is not presently an active strike-slip fault due to unfavorable perpendicular principal stresses (Choi et al., 2021) supports that the ca. 6 Ma Pacific-Yakutat plate vector change redistributed almost all horizontal slip from the Eastern Denali fault onto the Totschunda fault. Hence, we concur with numerous other studies that suggest a less active (“sleepy”) northern Eastern Denali fault after ca. 6 Ma (Waldien et al., 2018; Choi et al., 2021; Trop et al., 2022; Allen et al., 2022). An increase in exhumation rates in the central Alaska Range (Fitzgerald et al., 1993, 1995), the development of the Mount McKinley restraining bend (Benowitz et al., 2022b), expansion of the Alaska Range Northern foothills fold and thrust belt and Hayes Range deformation front (Bemis et al., 2015, Benowitz et al., 2022a); and exhumation in the Tordrillo Mountains (Haeussler et al., 2008) are all in part a deformational response to this Late Miocene plate boundary modification.

5.7 Global Context

Around the Pacific plate, convergence was either redistributed along preferentially aligned pre-existing faults or fault systems were modified to accommodate a change in incoming strain direction following the ca. 6 Ma Pacific plate motion change. This is observed on the Denali fault: (Fitzgerald et al., 1993, 1995; Benowitz et al., 2022a, b); the Totschunda fault (this study), the Queen Charlotte fault (Hyndman, 2015); the San Andreas fault (Kellogg and Minor, 2005; Townsend et al., 2021), the La Cruz fault (Bennett et al., 2016), and the Alpine fault system of New Zealand (Walcott 1998; Batt et al., 2004; Collett et al., 2019; Duvall et al., 2020). This change in relative plate motion in the Late Miocene (ca. 6 Ma) resulted in geodynamic responses that were near geologically instantaneous. Hence, these slip redistribution events support the theorem that deformation seeks the path of least resistance or work minimalization, such that the “Earth is lazy” (Cooke and Madden, 2014) and fault systems evolve towards mechanical efficiency both through time and space to accommodate new stress regimes. As fault intersections are common along strike-slip fault systems (e.g., San Jacinto-San Andreas faults), fault strike obliquity relative to incoming plate vectors can be used, with caution for seismic hazard prediction (Schwartz et al., 2012; Lozos, 2016) and to evaluate which fault is the strand of principal slip (Christie-Blick and Biddle,1985; Passchier and Platt, 2017).