A unified three-dimensional model of the lithospheric structure at the subduction corner in southeast Alaska: Summary results from STEEP

Abstract

Abstract We merge structural results from the ST. Elias Erosion/tectonics Project (STEEP), other studies, and seismicity data to build a comprehensive, three-dimensional model of the lithosphere of the subduction corner in southern Alaska. The model is defined by three surfaces: (1) a top of the subducting lithosphere surface, (2) Moho surfaces, and (3) a base of subducting lithosphere surface. We model the eastern edge of the subducting lithosphere using the southern tip of the Yakutat microplate as an anchor. Kinematic reconstructions using that anchor suggest the modern Fairweather fault is likely inherited from motion of the margin in the 6–10 Ma period. We constructed a 4D kinematic model of crustal deformation in the vicinity of Mount St. Elias. We call this model the middlebuster model because the geometry is similar to a two-sided plow with that name. The west side of the plow is the eastern limit of the Aleutian megathrust constructed from the union of constraints from STEEP seismic results and slip models of the 1979 St. Elias earthquake. The east side is inferred from geologic mapping and slip models of the 1899 Yakutat Bay earthquake sequence. The top of the plow is near the Seward Glacier, where previous studies showed near world-record exhumation rates. GPS velocity vectors show a large rotation across the syntaxis at Mount St. Elias. West of the syntaxis, faults inferred from inversion of the GPS data are above the megathrust inferred from seismic imaging. That and other evidence suggest the presence of a wedge of ductile crust that partially decouples the subducting mantle lithosphere from the upper crust in the area near the suture with the Yakutat microplate.