MARINE RECORDS OF FLAT SLAB SUBDUCTION INFLUENCED BY TEMPERATE GLACIATION IN THE ST. ELIAS OROGEN, GULF OF ALASKA

The St. Elias orogen formed ~10 Ma due to the collision of the Yakutat Block with North America in the Gulf of Alaska. Approximately 500 km of Yakutat Block crust has underthrust North America at less than a 3 degree dip, a flat slab subduction event that causes a gap in the Aleutian volcanic arc and may be a modern day analog for the Laramide orogeny. The nature of the Yakutat Block has been long debated but new crustal scale seismic reflection and refraction data collected during the St. Elias Erosion/tectonics Project (STEEP) suggests a potential large-igneous plateau origin. The modern orogenic wedge, and highest topography, lies between the Bagley Ice Field onshore and the Pamplona Zone offshore with the deformation front cutting diagonally from the Khitrov slope across the Bering shelf to Icy Bay. A décollement is present at depths starting at 10 km from this front to the west. During three intervals since 6 Ma, temperate glaciers have occupied and eroded the coastal mountain belt. The record of the tectonic collision and glacial denudation is contained in outcrops of glacimarine sequences of the Yakataga Formation onshore, which reach up to 2 km thick, in the shelf deposits of Yakataga offshore, which reach up to 6 km thick, and in strata farther offshore contained within the deepsea Surveyor Fan which are over 3 km thick proximal to the shelf. The STEEP data, in concert with onshore thermochronology, suggest that deformation patterns within the orogen have been fundamentally modified by the patterns of erosion and deposition in at least the most recent and most intense phase of glaciation, which has been ongoing since the Mid-Pleistocene climate transition, and potentially earlier. STEEP data, combined with previous geophysical imaging, depict changes in the relative balance of deposition on the shelf versus within the Surveyor Fan and an evolution of sediment delivery processes with changes in glacial intensity. Unraveling the interplay of tectonics and climate within the orogen ultimately requires the sequences within these Yakataga deposits to be correlated regionally and placed in a chronologic and climatic framework, goals requiring an ocean drilling expedition.