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.