Subduction of an Oceanic Plateau Across Southcentral Alaska: Scattered-Wave Imaging

An oceanic plateau, the Yakutat terrane, has entered the subduction system across southcentral Alaska. Its down-dip fate and relationship to overlying volcanism is still debated. Broadband seismometers from the Wrangell Volcanism and Lithospheric Fate (WVLF) temporary experiment were deployed with P and S coda of teleseismic P waves for imaging discontinuity structure. This procedure is applied to WVLF and other dense seismic arrays across southcentral Alaska in a manner that accounts for near-surface wavespeed variations. Two imaging techniques are employed: two-dimensional migration and three-dimensional common-conversion-point (CCP) stacking. Migrating the scattered phases along WVLF stations shows the similar to 18 +/- 4 km thick Yakutat crust subducting beneath the Wrangell Volcanic field to the NNE. It is offset from the Alaska-Aleutian seismic zone laterally by 250 km to the southeast at 100 km depth, and dips more steeply (45 degrees). At depths <45 km, CCP stacking reveals that the Yakutat crust is continuous for over 450 km along strike. This shallow continuity and deeper offset suggest a tear in the subducting Yakutat slab at depths >45 km, around 146 degrees W. CCP stacking also reveals a continuous thin low-velocity layer atop the underthrust Yakutat crust for >450 km along strike, at all depths <35 km. The uniform low-velocity thrust zone indicates consistent properties through multiple rupture-zone segments, showing that low-velocity channels generally correspond with subduction megathrusts.