6 Conclusions
The tectonic mechanism driving high rates (>5 km/my) of rock uplift along Earth’s fastest slipping ocean-continent transform plate boundary is a one-sided, positive flower structure along a prominent restraining double bend in the Fairweather fault. Uplifted shorelines at Icy Point record repeated earthquakes coincident with west-side-up displacement on the Fairweather fault and contraction on interconnected reverse faults offshore to the west—a mode of deformation that did not occur during the 1958 Mw7.8 Fairweather earthquake.
In a region of rapid, present-day uplift caused by post-LIA glacial rebound, tectonic contributions to land-level change can be obscured by isostatic processes. We circumvent this problem by comparing the contrasting RSL histories on either side of the Fairweather fault. Marine terrace elevations and luminescence ages of terrace deposits support estimates of -42.3 ±2.1 m of RSL change (fall) at Icy Point, west of the Fairweather fault, since sometime between 5.5 and 9.4 ka. East of the fault in Icy Strait, Mann and Streveler (2008) estimated +4.1 +1/-0.75 m of RSL change (rise) over the past 6.9–7.2 ka. The difference between the two opposing RSL histories juxtaposed by the fault, amounts to -46.4 ±2.4 m of RSL change over the past ca. 7 ka driven by tectonic uplift occurring west of the Fairweather fault.
Active fault traces between Lituya Bay and Icy Point delineate a 30-km-long, ~20° restraining double bend at the southern end of the Fairweather fault expressed by contractional deformation in the landscape (Witter et al., 2021). Offshore, the Icy Point-Lituya Bay thrust fault (Plafker, 1971; Carlson et al., 1988), imaged in seismic reflection profiles (Balster-Gee et al., 2022a; 2022b), is an east-side-up, buried reverse fault (Figure 2). This offshore reverse fault, and other faults mapped onshore that displace Holocene marine terraces, like the Finger Glacier fault (Witter et al., 2021), reveal a complex fault system that accommodates oblique contractional deformation within the Fairweather fault restraining double bend. We interpret this fault system as an asymmetric, positive flower structure (Figure 13; Woodcock and Fischer, 1986) that decouples deformation of weaker Yakutat sedimentary rocks from stronger oceanic plateau rocks in the lower crust (Lease et al., 2021). Rock uplift rates of 4.6 to 9.0 mm/yr are implied by the uplifted Holocene terraces at Icy Point, and these estimates agree with Quaternary rock exhumation rates of 5–10 km/m.y. (Lease et al., 2021) and late Pleistocene terrace uplift rates of 6–8 m/k.y. (Mann, 1986) occurring within the restraining bend.
Our structural model explains uplift rates observed at Icy Point by invoking variable modes of slip on the Fairweather fault—including strike-slip, vertical-, or oblique-slip during separate earthquakes within the restraining double bend. We suggest that in addition to ruptures like the 1958 Fairweather earthquake, which involved only strike-slip displacement at Icy Point, other earthquakes result from joint rupture of the Fairweather and Icy Point-Lituya Bay faults that collectively uplift coastal terraces and shorten the eastern edge of the Yakutat block that impinges on the North America plate. If the Holocene shorelines on Terrace B at Icy Point record coseismic uplift during past ruptures of the Icy Point-Lituya Bay thrust fault, then our results suggest such earthquakes are relatively rare and occur no more than every 460–1040 years, or roughly one for every 5–10 strike-slip ruptures on the Fairweather fault. Fault rupture scenarios that accommodate convergence require a maximum of 3.1–10 m dip-slip-per-event on an offshore thrust fault and at most a 3–5 m vertical component of slip along the Fairweather fault. Ruptures of the offshore Icy Point-Lituya Bay thrust fault may extend 40–70 km in length along strike and produce Mw7–7.5 earthquakes accompanied by coseismic uplift of Icy Point. Complex earthquakes that involve simultaneous rupture of both the Icy Point-Lituya Bay thrust fault offshore and the Fairweather fault may reach Mw7.9.