CRUSTAL STRUCTURE OF THE YAKUTAT BLOCK: CONSTRAINTS FROM STEEP WIDE-ANGLE SEISMIC DATA

The collision of the Yakutat block with the North American plate has resulted in flat-slab subduction and the formation of the St. Elias orogen. Existing models for the Yakutat block disagree as to whether it is 1) a continental fragment attached to normal oceanic crust, or 2) an oceanic plateau. In Fall 2008 we acquired two marine wide-angle seismic profiles constraining Yakutat block structure as part of the St. Elias Erosion/tectonics Project (STEEP). STEEP01 is a northwest-southeast oriented profile on the continental shelf. The Yakutat basement on STEEP01 is close to the seafloor to the east near the Dangerous River Zone, and at ~15 km depth in the west beneath the Pamplona Zone; Moho depth is at 30-35 km. STEEP02 is a northeast-southwest oriented profile extending from the Pacific ocean basin across the Transition fault and into Yakutat Bay. Crustal thickness changes dramatically across the Transition fault, with Moho depth increasing from 11 km for Pacific ocean crust to 30-35 km for Yakutat crust. Within Yakutat Bay there is no evidence for the Dangerous River Zone, and sediment thickness remains as much as 4 km. We interpret the Yakutat block, based on the crustal velocity structure, as a piece of an oceanic plateau emplaced along the Transition strike-slip fault. Buoyancy due to the thick crust of the block provides a causal explanation for the flat slab subduction that has produced North America’s highest mountain range.