Active Tectonics of An Onshore-Offshore Fold-Thrust Belt Undergoing Intense Mass Redistribution by Erosion and Deposition: Initial Results from the ST. Elias Erosion and Tectonics Project (STEEP)

Southern Alaska represents an oblique collisional orogen where high tectonic rates and very high glacial erosion rates interact to form a system uniquely suited to the study of erosion and tectonic interactions. STEEP is examining this interaction through a comprehensive study of the orogen architecture and history. The orogen is comprised of three segments: a dextral transpressive system on the east, a central fold-thrust belt, and a western syntaxis where the collision transfers into the Aleutian trench. STEEP speakers in this session will focus on specific results from studies in the central and western segments, and marine seismic work in 04 and 08 as well as ongoing geodetic and passive seismology studies will provide further insights. Results summarized here include: 1) The finite structure of the fold-thrust belt resembles other fold-thrust belts, yet it has been fundamentally reshaped by Pleistocene glacial erosion. Deformation has shifted in response to this interaction--in particular, glacial ice streams play a fundamental role in mass transfer within the orogen transporting mass from the orogen core to offshore basins which have buried older fold-thrust structures, shifting deformation primarily to onland structures; 2) major changes in deformation patterns appear to be closely tied in time to the onset of Pleistocene glacial cycles, underscoring the role of glacial systems in the reshaping of the orogen; and 3) the western syntaxis represents one of the most actively deforming segments, but contains a complex deformational history from long-term indentation during the collision. The syntaxis is an immature analog of Himalayan syntaxes and like the Himalayan syntaxes, active structures are closely tied to sites of focused erosion, but rock uplift rates are presumably lower because of a thinner crustal section and glaciers, rather than rivers, do the erosive work in the St. Elias syntaxis producing rapid feedbacks.