Figure 1. Active faults (red lines, barbs point down dip) along the Pacific-Yakutat-North American plate boundary where the Queen Charlotte fault steps onshore and becomes the Fairweather fault (bold red line). The 1958 Mw7.8 Fairweather fault rupture extended >260 km (bold red line); the earthquake epicenter (star) is located near Cross Sound (Doser and Lomas, 2000; Doser, 2010). Yakutat block geodetic velocity from Elliott and Freymueller (2020). CS, Cross Sound; FF, Fairweather Foothills; GB, Glacier Bay; IS, Icy Strait; YB, Yakutat Bay; YF, Yakutat Foothills. Source of basemap: General Bathymetric Chart of the Oceans, https://www.gebco.net/; NOAA National Centers for Environmental Information, https://www.ncei.noaa.gov/maps/bathymetry/.
the restraining double bend. However, Icy Point, a peninsula beveled by late Pleistocene and Holocene marine and fluvial terraces at the southern end of this complex restraining double bend, conspicuously lacked evidence for vertical displacement during the 1958 earthquake (Tocher, 1960).
The tectonic geomorphology at Icy Point (Witter et al., 2021), an area of unusually high (5–10 mm/yr) Quaternary rock uplift rates (Mann, 1986; Lease et al., 2021), presents a unique setting to investigate high-rate deformation along a major transpressional plate boundary fault system. The principal strand of the southern Fairweather fault accommodates >90% of plate boundary strike-slip motion (Elliott and Freymueller, 2020) and structurally controls vertical uplift along the eastern flank of the Icy Point peninsula (Witter et al., 2021). Previous studies implicate a west-vergent reverse fault offshore and west of Icy Point as the mechanism driving marine terrace uplift (Plafker, 1967, 1971; Carlson et al., 1988). However, geomorphic evidence for vertical slip on the Fairweather fault raises several motivating questions, including: What fault rupture scenarios sustain such high uplift rates at Icy Point along one of the fastest strike-slip faults on Earth? What kinematic model of the Fairweather fault can account for both Holocene vertical displacement and the documented right-lateral surface fault rupture in 1958 that lacked coseismic uplift of Icy Point?
Here, we present the results of investigations at Icy Point to assess strain partitioning and the Holocene earthquake history along the southern Fairweather fault through the restraining double bend south of Lituya Bay (Figure 2). Using offshore and onshore observations, we assess how slip is partitioned on strike-slip and reverse faults in the complex corner of a restraining bend between the Queen Charlotte and Fairweather faults. We quantify rates of oblique contraction in the restraining bend that provide insights about how fault slip may vary on a principal, strike-slip fault and we propose a range of fault rupture scenarios occurring over multiple earthquake cycles that explain our observations. Our findings contribute a conceptual understanding of oblique contraction along the Fairweather fault that can both improve kinematic fault models and provide better tectonic context for paleoseismic data in southeastern Alaska that underpin seismic hazard assessments.