Figure 13. (a) Oblique view looking to the northwest where the offshore Queen Charlotte fault steps 3 km east to the Fairweather fault. Data sources for onshore and offshore elevation models described in Figure 2 caption. The location of the southern termination of the offshore Icy Point-Lituya Bay reverse fault is inferred from seismic profiles shown in Figure 5. (b) Conceptual block model depicting transpression, or oblique contraction, in a right-lateral transform plate boundary. Arrows indicate relative fault slip rates (after Wesnousky, 2005). (c) Our tectonic model for the restraining double bend along the southern Fairweather fault consists of a one-sided, positive flower structure that includes the steeply dipping Fairweather fault to the east of Icy Point and two reverse faults, the Icy Point-Lituya Bay and Finger Glacier faults. The Icy Point-Lituya Bay fault splays off the Fairweather fault at seismogenic depths; the Finger Glacier thrust fault is a shallow secondary structure. The Icy Point-Lituya Bay fault and the Fairweather fault accommodate oblique contraction where the Yakutat block obliquely collides into North America.
Uplift and contraction at Icy Point are accommodated by faults with reverse slip, strike slip, and oblique-slip behaviors. Because of its near-vertical dip (Tocher, 1960; Doser and Lomas, 2010), the Fairweather fault does not accommodate contraction. Shortening along the restraining double bend is accommodated by reverse faults, most notably the offshore Icy Point-Lituya Bay fault. Because the Icy Point peninsula is uniformly uplifted (Figure 9b and c), there must be vertical displacement on the Fairweather fault during some earthquakes. Kinematically, this requires that some earthquakes on the Fairweather fault involve oblique slip. Alternatively (or, in addition), the Fairweather fault on the east side of the Icy Point peninsula may slip in a bimodal fashion during independent strike-slip and vertical-slip fault ruptures (e.g., Barnhart et al., 2015).
Several observations guide our conceptual model of fault-driven deformation west of the restraining bend at Icy Point. First, the obliquity between the strike of the Fairweather fault and Yakutat-North America plate motion (22° ± 8°; Brothers et al., 2020), provides the setting for the restraining double bend and accounts for the topographic relief of the contractional foothills (Fairweather and Yakutat foothills) to the west of the Fairweather fault. Second, at Icy Point, uplifted Holocene shorelines on Terrace B imply an earthquake cadence that averages 3.4 m of coseismic uplift every 460–1040 yrs. Third, the Fairweather fault on the east margin of Icy Point has accommodated at least 25 m of vertical, west-side-up displacement in the past 10 ky based on uplifted periglacial and outwash deposits at the Kaknau Cliff section. Finally, because the 1958 Mw7.8 Fairweather earthquake was predominantly strike slip and caused no detectable vertical displacement at Icy Point, there must be a set of earthquakes with different rupture modes than the 1958-type earthquake because, every 460–1040 years, at least one plate boundary earthquake causes measurable, ~3.4 m, coseismic uplift of the Icy Point peninsula. By comparison, the average recurrence interval for predominantly strike-slip earthquakes on the Fairweather fault like the 1958 event (3.5 m per event) is <100 years (Witter et al., 2021).
5.4 A one-sided, positive flower structure fault model
A conceptual fault model of deformation at the restraining double bend that accommodates a fast-slipping transform plate boundary with extraordinary rock uplift rates at Icy Point is one in which the principal strand of the Fairweather fault forms the eastern margin of a “one-sided,” positive flower structure (Figure 13) (Woodcock and Fischer, 1986; Bruhn et al., 2004; Pavlis et al., 2004). The structure initiates 1–2 km south of Icy Point, where the offshore Queen Charlotte fault steps 3 km east to the onshore Fairweather fault, undergoes a ~20° bend, and ends in the north near Lituya Bay (Witter et al., 2021). The Queen Charlotte fault may merge with the east-dipping Finger Glacier fault as implied by offshore seismic profiles (Figure 5). Transpression imposed by the >20° obliquity between the strike of the Fairweather fault and Yakutat-North America plate motion (Brothers et al.., 2020) drives the asymmetrical flower structure at Icy Point; contraction is accommodated primarily on the Icy Point-Lituya Bay reverse fault that splays off the Fairweather fault at seismogenic depths (<20 km depth, Table 5). The simple fault model depicted in Figure 13 does not explain the complex dynamics required by the continual lateral advection of crust through the corner of a stationary restraining bend.
The asymmetrical flower structure model accommodates shortening between crustal slivers along the northeastern edge of the Yakutat block (Bruhn et al., 2004; Pavlis et al., 2004). The model we propose is consistent with geodetic block models that place convergence on reverse faults west of the Fairweather fault (Elliott et al., 2010; Elliott and Freymueller, 2020) and differs from models of slip partitioning that infer substantial convergence on unidentified structures east of the fault under the Fairweather Range (McAleer et al., 2009). We present an alternative model that infers that the strain-weakened edge of the Yakutat block abuts the strong crystalline core of the Fairweather Range and that high rates of horizontal and vertical deformation are localized west of the Fairweather fault. Our contention that the near vertical Fairweather fault accommodates substantial vertical slip is similar to models of vertical extrusion attributed to oblique convergence along the sub-vertical Denali fault (Benowitz et al., 2022).
5.5 Fault rupture scenarios accommodating oblique contraction
Several fault rupture scenarios could account for the uplift rates recorded at Icy Point in the Holocene. A ‘joint rupture’ scenario involves the simultaneous ruptures of the Fairweather fault and the offshore Icy Point-Lituya Bay fault. In this scenario, coseismic slip on the Fairweather fault at depth propagates upward and intersects the Icy Point-Lituya Bay fault where a component of slip splays off the primary strand of the Fairweather fault. Slip is partitioned into reverse slip on the blind Icy Point-Lituya Bay fault and vertical- or oblique-slip on the Fairweather fault. This scenario consists of joint rupture of both faults and causes uplift of the Icy Point peninsula. There may be seaward (westward) tilting during uplift because a blind reverse fault like the Icy Point-Lituya Bay fault can exhibit diminishing slip towards the tip, promoting more uplift above the location where the reverse fault splays off the Fairweather fault. However, the measured westward slopes of terraces B and C (Figure 9) do not exceed seaward gradients typically cut by shore platform formation along the coast.
An alternative to joint rupture of both faults is a scenario that invokes independent ruptures of the Fairweather fault and the Icy Point-Lituya Bay fault. This ‘independent rupture’ scenario entails vertical- or oblique-slip on the Fairweather fault that relieves a substantial component of vertical strain, and subsequent reverse slip on the offshore, blind Icy Point-Lituya Bay fault that relieves strain oriented perpendicular to the plate boundary. The oblique-slip events on the Fairweather fault must be balanced by events with sufficient horizontal slip that eventually sum to the long-term fault slip rate. The oblique-convergent ruptures that caused the 2010 and 2021 Haiti earthquakes offer comparative analogs of complex, serial ruptures along an oblique contractional fault system involving strike-slip and reverse faults (Hayes et al., 2010; Okuwaki and Fan, 2022).
Both earthquake scenarios require that the Fairweather fault is an oblique-slip fault at seismogenic depths and accommodates both vertical and horizontal slip. The vertical component of slip is recorded in the northern two seismic profiles (panels A and B) in Figure 5, which mark the commencement of growth of the “Fairweather foothills.” The Fairweather foothills, and the Yakutat foothills to the north, record contraction of crustal slivers along the northeastern edge of the Yakutat block (Bruhn et al., 2004; Pavlis et al., 2004; Elliott and Freymueller, 2020). Horizontal shortening occurs through slip on the Icy Point-Lituya Bay and Yakutat faults; rock uplift is the result of the vertical component of slip on the Fairweather fault and reverse faults that define the crustal slivers to the west.
Multiple earthquake scenarios are supported by observations of coseismic slip in other transpressional fault systems that change the Coulomb stress on adjacent strike-slip and reverse faults and either promote or inhibit failure (Lin and Stein, 2004). Along the Fairweather fault, rupture of adjacent reverse faults can promote failure along strike-slip faults. For example, the 1899 Mw8.1 Yakutat Bay earthquake promoted failure on the Fairweather fault at the northwestern section of the 1958 rupture (Rollins et al., 2020) and blind thrust faults promote failure over broad areas of the overlying crust (Lin and Stein, 2004). Rupture of the Fairweather fault also likely promoted failure on reverse faults northwest of Yakutat Bay (Rollins et al., 2020). For comparison, the Mw 7.9 San Andreas fault earthquake of 1857 promoted failure on nearby thrust systems including the Coalinga and White Wolf reverse faults (Lin and Stein, 2004). The Mw 7.9, 2002 Denali fault earthquake and its foreshock, the Mw 6.7 Nenana Mountain earthquake, provide an example of stress transfer from the Nenana Mountain strike-slip foreshock to the hypocentral area of the Denali earthquake mainshock (Anderson and Ji, 2003). Stress transfer from the Nenana Mountain earthquake promoted complex reverse-oblique and strike-slip ruptures on the Susitna Glacier thrust and Denali faults, respectively (Eberhart-Phillips et al., 2003; and Aagaard et al., 2004). In this context, Coulomb stress changes resulting from complex oblique-slip, reverse, and strike-slip fault ruptures along the restraining double bend north of Icy Point may periodically promote vertical slip on the Fairweather fault.
5.6 Earthquake source parameters for the offshore Icy Point-Lituya Bay thrust fault
Slip on the offshore Icy Point-Lituya Bay thrust fault represents a source of earthquakes and tsunamis along the Gulf of Alaska coast in addition to strike-slip ruptures along the Fairweather fault. The 1958 Mw7.8 Fairweather earthquake primarily relieved shear strain parallel to the plate boundary. Published slip rates for the Fairweather fault (46–58 mm/yr) imply average recurrence intervals of 60–140 years for Mw>7 strike-slip earthquakes (Plafker et al., 1978; Witter et al., 2021). This investigation derives fault source parameters for repeated Holocene ruptures on the Icy Point-Lituya Bay thrust fault that relieved fault-normal strain, caused coseismic uplift at Icy Point, and included substantial vertical displacement on the Fairweather fault. Moreover, rupture of offshore thrust faults may explain large tsunamis in Lituya Bay in 1853–1854, ca. 1874, and ca. 1899 (Miller 1960).
Reverse slip on the Icy Point-Lituya Bay thrust fault evidenced by uplifted Holocene terraces at Icy Point occur no more than every 460–1040 years. Assuming a simple fault geometry for a thrust or reverse fault striking subparallel to the plate boundary and dipping between 45°–75° and coseismic uplift of 3–5 m per earthquake based on terrace riser heights, we estimate that the slip during past events on the Icy Point-Lituya Bay fault ranged between 3.1 and 10 m (Table 5). Because the reverse fault is blind, its dip is unknown. Shallower fault dips (30°–45°) are most consistent with geodetic estimates of fault-normal rates of motion (5–14 mm/yr of shortening) perpendicular to the plate boundary (Table 5) (Elliott and Freymueller, 2020); steeper dips (60°–75°) are required if the reverse fault splays off the Fairweather fault at seismogenic depths (10–16 km). Hypothetical ruptures of the Icy Point-Lituya Bay fault with rupture lengths equal to or exceeding the distance between Icy Point and Lituya Bay (40–70 km) could potentially generate Mw7–7.5 earthquakes (Wesnousky, 2008; Stirling et al., 2013); larger events are suggested by the 3.4 m average vertical separation of shorelines on Terrace B (Moss et al., 2022). Complex events that include simultaneous rupture of the Icy Point-Lituya Bay fault (e.g., Mw7–7.5) and the Fairweather fault (e.g., Mw7.8) are implied by our results, and could potentially generate Mw7.9 earthquakes (derived by summing the moments of multiple events (Kanamori, 1983)).