Figure 2. (a) Topographic and bathymetric digital elevation models along
the Gulf of Alaska coast in the area surrounding Lituya Bay and Icy
Point. The bathymetric compilation includes NOAA data
(https://www.ncei.noaa.gov/maps/bathymetry/) in the Gulf of Alaska
(100-m resolution) and Lituya Bay (15-m resolution). Ten-meter
resolution data in Palma Bay, the area offshore Icy Point and along the
Queen Charlotte fault comes from recent multibeam surveys in 2015
(Dartnell et al., 2022). Topographic data come from the 5-m resolution
Alaska IfSAR digital elevation model (https://elevation.alaska.gov/).
(b) South of Palma Bay, bathymetric data show the linear, submarine
trace of the Queen Charlotte fault, which projects northwest toward Icy
Point. The linear Queen Charlotte fault trace terminates near a series
of folds expressed on the sea floor 5-10 km south of Icy Point. Onshore,
the Fairweather fault borders the eastern edge of the uplifted Icy Point
peninsula and strikes ~20° more westerly than the Queen
Charlotte fault offshore. West of the Fairweather fault, the Finger
Glacier fault (FGF) and the inferred Icy Point-Lituya Bay fault
(Plafker, 1971; Carlson et al., 1988) are the northwestern extensions of
faults imaged in seismic reflection profiles. Coastal geomorphology
preserves remnants of six marine terraces identified by Mann (1986).
Cyan symbols locate key exposures of marine terraces described by Mann
(1986) and dated by Don Miller (Rubin and Alexander, 1958).
High topography along the coast north of Icy Point, named informally
here as the ‘Fairweather Foothills,’ reflects contraction west of the
Fairweather fault (Figure 1). The Fairweather Foothills continue
northward for 200 km, locally exceeding elevations of 1300 m, and merge
with the Yakutat Foothills, which are bound by an active reverse fault
along their southwest flank (Bruhn et al., 2004). Convergence occurs at
the northern end of the Fairweather fault on mapped reverse faults in
the Yakutat Foothills (Bruhn et al., 2004). The primary reverse fault is
variously called the Yakutat fault (Schartman et al., 2019; Walton et
al., 2022), the Yakutat Bay thrust fault (Bruhn et al., 2004; Plafker
and Thatcher, 2008), and the Foothills fault (Elliott et al., 2010); in
all cases, the fault delineates the western margin of the Yakutat
Foothills (Figure 1). Bruhn et al. (2004) described the Yakutat fault as
part of a system of faults that form an asymmetric flower structure
(terminology after Sylvester and Smith (1976)), which accommodates
contractional deformation southwest of the structural syntaxis at the
northern end of the Fairweather fault (Bruhn et al., 2012). In a block
model by Elliott and Freymueller (2020), the fault accommodates 5.4±0.7
mm/yr of contraction and connects to the offshore Icy Point-Lituya Bay
fault, which is discussed in the next section.
Lease et al. (2021) provide geologic context for the convergence modeled
by Elliott et al (2010) and Elliott and Freymueller (2020) by estimating
exhumation rates along the restraining bend and further north along the
plate margin. Lease et al. (2021) conclude that the flight of emergent
marine terraces at Icy Point imply >6–8 km/Myr rock uplift
rates along the Fairweather fault restraining double bend. Northwest of
the restraining bend, rock uplift in the Fairweather Foothills reflects
transient, rapid exhumation of rock advected through the Fairweather
fault restraining bend where the Icy Point-Lituya Bay fault branches
from the Fairweather-Queen Charlotte fault. The rocks that underlie high
topography of the Yakutat and Fairweather foothills suggest a long-term
strike-slip rate of ~54 km/Myr since ~3
Ma (Lease et al., 2021).
2.2 Geology of the Icy Point peninsula and the offshore Icy Point-Lituya
Bay thrust fault
At Icy Point, the Fairweather fault juxtaposes rocks of considerably
different ages, rock types, and strengths (Figure 3). East of the fault,
Paleogene amphibolite-facies and layered gabbro metamorphic rocks,
>9-km-thick, support the high topography of the Fairweather
Range; west of the fault, Cretaceous-Cenozoic sedimentary rocks,
~12 km thick, overlie the eastern Yakutat block
(Plafker, 1994). The Tertiary rocks at Icy Point include the Topsy
Formation and the disconformably overlying Yakataga Formation. The Topsy
Formation consists of marine concretionary siltstone and greenish-gray
argillaceous and carbonaceous sandstone deposited during the Miocene
(Miller, 1961; Plafker, 1967; 1971; 1987). The Yakataga Formation is as
young as Pleistocene (Marincovich, 1980; Rau et al, 1983), and includes
interbedded siltstone and sandstone that grade into overlying beds
containing minor conglomerate and mudstone that incorporates
ice-transported clasts of diverse lithologies (Plafker, 1967; 1971).