Running Head: Rock uplift west of the Fairweather fault, Alaska
R.C. Witter1, H.M. Kelsey2, R.O.
Lease1, A.M. Bender1, K.M.
Scharer3, and P.J. Haeussler1, and
D.S. Brothers4
1U.S. Geological Survey Alaska Science Center,
Anchorage, AK, USA
2Department of Geology, Cal Poly Humboldt, Arcata, CA,
USA
3U.S. Geological Survey Earthquake Science Center,
Pasadena, CA, USA
4U.S. Geological Survey, Pacific Coastal and Marine
Science Center, Santa Cruz, CA, USA
Abstract [286/300 words]
Contraction along the Yakutat-(Pacific)-North America plate boundary
drives extreme rock uplift along Earth’s fastest slipping (≥49 mm/yr)
ocean-continent transform fault, the Fairweather fault. Between Icy
Point and Lituya Bay, the near-vertical Fairweather fault focuses rock
uplift and rapid right-lateral slip by accommodating both vertical and
fault-parallel strain during ruptures with a substantial vertical-slip
component and separate, predominantly strike-slip events. We use 1.0 m
resolution digital elevation models and offshore seismic reflection
profiles to map active faults, uplifted marine and fluvial terraces, and
document past reverse fault earthquakes with maximum 3–5 m of coseismic
uplift per event. Radiocarbon and luminescence dating provide timing to
estimate 4.6–9.0 mm/yr Holocene rock uplift rates, which match 5–10
km/Myr Quaternary exhumation rates estimated from thermochronometry.
These unusually high uplift rates result from plate-boundary strain that
is partitioned onto reverse faults that form, together with the steeply
dipping Fairweather fault, a 10-km-wide, asymmetric, positive flower
structure along a 20°, ~30-km-long restraining double
bend in the Fairweather fault. The principal reverse fault in the flower
structure is the offshore, blind Icy Point-Lituya Bay fault, which
ruptures no more than every 460–1040 years evidenced by uplifted
Holocene marine shorelines. Evaluated over a range of dips, the uplift
on this reverse fault implies maximum 3.1–10 m dip slip per event and
estimated earthquake magnitudes of Mw 7.0–7.5. Our
model implies oblique slip on the Fairweather fault at seismogenic
depths with and without co-rupture on the reverse fault. Oblique slip on
the Fairweather fault is evident where it vertically offsets fluvial and
marine terraces by >25 m, strikes >20° west of
plate boundary motion, juxtaposes near-surface rocks of different
strength, and where the Yakutat block collides obliquely into North
America.