Figure 3: Geologic map of Lime Creek field area with sample
locations (Map is adapted from MacKevett (1978) where detailed unit
descriptions are recorded). For sample 19SOLO06, the dashed green line
leading away from the sample location denotes inferred sample path for a
rock that clearly originated from the outcrop denoted with a green X.
2.3 Yakutat Flat Slab Subduction, The Wrangell Arc, and the
Pacific
Plate
The Yakutat microplate is an 11-30 km thick oceanic plateau, with a dip
and crustal thickness contrast between the eastern and western segments
and thinning slab thickness towards the north (Ferris et al., 2003;
Rossi et al., 2006; Worthington et al., 2012; Mann et al., 2022). The
Yakutat microplates convergence angle reflects Pacific plate vector
changes through time (e.g., McAleer et al., 2009). The thick
(>11 km) Yakutat slab extends for ~250 km
northwestward beneath Alaska at a subduction angle of
~20° before the dip angle increases to
~60° beneath the Alaska Range. Seismic images by Ferris
et al. (2003) showed that thickened crust of the Yakutat terrane carries
through to the steeper slab section to depths of at least 150 km beneath
the Alaska range (Mann et al., 2022).
The Wrangell volcanic arc, which is related to subduction of the Yakutat
slab, initiated by ca. 30 Ma (Brueseke et al., 2019; Berkelhammer et
al., 2019; Trop et al., 2022), coeval with the cessation of magmatism in
the Alaska Range arc (Trop et al., 2019; Regan et al., 2021; Benowitz et
al., 2022a). This change in location of arc magmatism, from the west to
the east has been linked to the commencement of Yakutat flat-slab
subduction under southern Alaska (Richter et al., 1990; Trop et al.,
2019; Jones et al., 2021). Other upper plate proxies such as exhumation
in the Alaska Range (Benowitz et al., 2011, 2012, 2019; Riccio et al.,
2014; Lease et al., 2016), deformation of the Susitna Basin (Shaw et
al., 2020) and modification of Alaska’s southern margin forearc (Finzel
et al., 2015; Betka et al., 2017; Rosenthal et al., 2018; Pavlis et al.,
2019) have also been linked with the initiation of Yakutat flat-slab
subduction.
A significant Alaska plate boundary change occurred at ca. 25 Ma, when
the Pacific-Yakutat plate vector relative to North America underwent 8°
- 15° of counter- clockwise rotation (Jicha et al., 2018). Inboard of
the plate margin, this relative plate margin change resulted in
significant oblique convergence across the Denali fault system as
southern Alaska moved to the northwest (e.g., Benowitz et al., 2014;
Trop et al., 2019). A 18° clockwise rotation in convergence angle and a
37 % increase in convergence rate between the Pacific-Yakutat plate
relative to North America at ca. 6 Ma (Engebretson et al., 1985;
Doubrovine and Tarduno, 2008; Austerman et al., 2011) further increased
the northwestward-directed motion of southern Alaska. Fitzgerald et al.
(1993, 1995) correlated this plate motion change with a significant
pulse of Late Miocene uplift and exhumation in the central Alaska Range,
that in combination with the coeval formation of the Mount McKinley
restraining bend (Benowitz et al., 2022b) formed the present-day
high-standing mountains of the central Alaska Range (Denali; 6190 m).
This Pacific-Yakutat plate motion change is also assumed to result in
the transfer of slip onto the Totschunda fault from the Denali fault
(Waldien et al., 2018; Choi et al., 2021; Allen et al., 2022).