Seismic and Potential Field Constraints on Upper Crustal Architecture of Inner Bering Shelf, Offshore Southwestern Alaska

Abstract

Southwestern Alaska encompasses a group of fault-bounded tectonostratigraphic terranes that were accreted to North America during the Mesozoic and Paleogene. To characterize the offshore extension of these terranes and several significant faults identified onshore, we reprocessed three intersecting multichannel deep seismic reflection profiles totaling ~750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, the migrated reflection images are supplemented with high-resolution P wave velocity models derived by traveltime tomography of the recorded first-arrivals to depths of up to 2000 m. Additionally, other geophysical datasets such as well logs, ship-board gravity, ship-board magnetics, satellite-altimetry gravity and air-borne magnetics are also incorporated into an integrated regional interpretation. We delineate the offshore extension of the major mapped geological elements, including the Togiak-Tikchik fault, East Kulukak fault, Chilchitna fault, Lake Clarke fault, Togiak terrane, Goodnews terrane, Peninsular terrane, Northern and Southern Kahiltna flysch deposits, and the Regional Suture Zone. We interpret the offshore Togiak-Tikichik fault to be a terrane bounding fault separating the Togiak terrane and Goodnews terrane. We also locate the offshore boundaries of the Regional Suture Zone using satellite gravity anomaly and air-borne magnetic data. Furthermore, we suggest that the sedimentary fill in the graben-like features offshore, as identified by seismic tomographic velocity models, is constituted by the deposits of Northern and Southern Kahiltna flysch.