Abstract

Southern Alaska is a collage of fault-bounded accreted terranes. The deformation history of these crustal blocks and geometric history of the bounding faults reflect both inherited features and subsequent convergent margin events. Multiple dense (<20-km spacing) arrays of broadband seismometers across southern Alaska has previously allowed for imaging of crustal structure across the region using various seismic imaging methods. Here, we employ S-toP receiver functions to investigate the crustal structure of southern Alaska for signals of dynamic tectonic activity. The subduction zone plate interface and subducting slab Moho are imaged dipping at shallow (<60-km) depths across the southernmost part of the subduction zone. Along two different transects, an inboard-dipping (~15°) boundary is imaged intersecting the trace of the Border Ranges Fault at the surface that we infer represents an unrotated inboard-dipping paleosubduction (Mesozoic) interface. This observation is combined with previous seismic imaging along both the Border Ranges Fault and the next seaward terrane-bounding fault-the Contact Fault-to buttress a known history of convergent tectonics that varies along the margin. Along with large (>10-km) crustal thickness offsets imaged across both the Denali Fault system and the Manuscript in press at upcoming AGU Monograph Tectonics and Seismicity of Alaska and Western Canada: Earthscope and Beyond 2 Eureka Creek Fault, this feature supports a Mesozoic-to-Present inboard-dipping (east and northward) subduction polarity in the region. Additionally, the Sp CCP volume reveals a 100-km x 50-km sized positive velocity gradient with depth (PVG) at ~25-km depth beneath the Copper River Basin, which we interpret as the top of a region of active underplating and/or intrusion of basaltic magmatism into the lower crust. This feature may be related to the generation of a new Wrangell Volcanic Field volcano, resulting from the underlying tear in the subducting slab.