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).