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).