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.