COASTAL DEFORMATION IN THE NORTHEASTERN GULF OF ALASKA SAINT ELIAS OROGEN: GREAT EARTHQUAKES, ACTIVE STRUCTURES, AND GLACIAL FLUCTUATIONS

Holocene coastal stratigraphy and geomorphology in the Saint Elias orogen provide evidence of large to great magnitude earthquakes, tsunami potential, and the linkage between active onshore and offshore structures in the Yakutat microplate. Transient and permanent patterns of uplift and subsidence correlate with earthquake history and zones of structural deformation dominated by concealed faults. Co-seismic uplift created by M 8 & 9 earthquakes dominates the coast from Prince William Sound 400 km eastward to the Malaspina Glacier. However, long-term deformation is spatially variable, with zones of uplift separated by tracts of null to finite interseismic subsidence that may shift laterally with time. This tectonic deformation is superimposed upon widespread glacio-isostatic deformation. Net tectonic uplift around the Katalla Valley at about 1 mm/yr since 6 ka, subsidence farther east near the Bering River, and long-term uplift at Cape Suckling reflect deformation related to the Kayak Island Zone and Bering Glacier thrust. Uplifted coastal terraces between Cape Yakataga and Icy Bay record paleoearthquakes, thrusting and folding above a ramp in the Malaspina thrust, where tectonic plus glacio-isostatic uplift is about 10 mm/yr over 5 ka. The high-relief mountain welt above the Malaspina fault traps precipitation in large alpine glaciers that flow east into Icy Bay, and spill westward as ice falls. Although structurally linked to the Aleutian megathrust, the Malaspina fault system ruptures independently as demonstrated by an M 8+ earthquake in 1899. However, earthquakes about 900 and 1500 years ago may have ruptured eastward on the megathrust to the Malaspina – Pamplona deformation front creating an earthquake and tsunami larger than the great Alaskan M 9.2 earthquake of 1964. A similar history is revealed around the Katalla Valley, where uplifted beach berms record several M 9 megathrust events, but cored marsh deposits record other more localized earthquake ruptures.