FAULT GEOMETRIES AND GLACIAL CONDUITS IN ICY BAY, ALASKA

The St. Elias orogen in Southeast Alaska continues to be formed by the Yakutat Terrane colliding with and subducting beneath North America. Convergence of 50 mm/yr between the Yakutat Terrane and the North America Plate is accommodated by the onshore Yakataga fold and thrust belt and the offshore Pamplona fault zone. These structures are thought to merge somewhere near Icy Bay, but the exact fault geometry in this region is unknown.

We acquired 450km of high-resolution seismic reflection data during a 2016 joint USGS-NSF cruise in Icy bay and Taan Fiord. We combine new data in Icy Bay with legacy seismic data to provide an updated map of the Pamplona fault zone and its relationship to the Yakataga fold and thrust belt on land. Seismic images do not provide direct evidence of faulting within Icy Bay due to the evacuation of ice from the bay in the last ~120 years, but glacial basins and moraines in the bay align with onshore-offshore structural trends. We infer these glacial features are structurally controlled, and also infer the location of the offshore extension of the Malaspina thrust fault as crossing Icy Bay near its western side based on the location of these glacial troughs and moraines. Studies of nearby glaciers have found that ice preferentially flows and erodes parallel to faults, and creates topographic steps when flowing over faults. Aside from the Malaspina system, we characterize a second fault to the east of the bay, the Foreland thrust, using offshore seismic data and previous studies of ice flow within the Malaspina glacial system.

The southeast Alaskan coast has not seen great earthquakes since the September 4^th M_W 8.1 and 10^th M_W 8.2 1899 earthquakes. This update to the characterization of the faults near Icy Bay will help to assess earthquake and tsunami hazard in the area. Additionally, Icy Bay provides a nice example of tectonically modified glacial conduits.