High-resolution receiver function imaging of the subduction zone across Southcentral Alaska

Abstract:

The Yakutat Terrane (YT) subducts in the Denali segment of the Alaska-Aleutian subduction zone. At least 400 km has subducted, and it generates intermediate-depth earthquakes (IDEs) in its western part. The IDEs end abruptly east of 147W, and it is unclear if the terrane accretes farther east or subducts beneath the Wrangell Volcanic Field (WVF). We deployed the Wrangell Volcanism and Lithospheric Fate (WVLF) experiment of 36 broadband seismometers with 15 km spacing across the eastern half of Southcentral Alaska (SA) for two years to study this transitional region. We use data from WVLF, the EarthScope Transportable Array, and two other dense deployments (BEAAR and MOOS) to image major seismic discontinuity structure beneath SA. We employ two receiver function imaging techniques: 2D scattered-wave migration (SWM) and 3D common-conversion point (CCP) stacking. SWM along a N15E transect just west of the WVF shows the upper-plate Moho beneath the Yukon-Tanana terrane at 32 km depth, the Wrangellia Composite terrane at 40 +/- 5 km depth, and the WVF at 48 +/- 5 km depth. South of the WVF, a continuous low-velocity layer enters the subduction zone at a shallow dip between 24 km and 50 km depth at the coastline, striking approximately N75W, and steepens beneath the WVF. This 20 +/- 5 km thick low-velocity layer, dipping 10 degrees at <40 km depth, extends from the coast 140 km inland and reaches depths of ~100 km beneath the WVF, placing the top of the layer 80 +/- 5 km depth beneath Mount Drum, the westernmost WVF volcano. Earthquakes project along strike within this layer down to 100 km, similar to the subducting YT crust imaged beneath the Denali segment. The 3D CCP stacking images the upper plate Moho north of the subducting slab. The subducting YT Moho is imaged in map view across the array where it is subhorizontal (depths < 65 km), showing that the Alaska-Aleutian subduction zone is continuous at shallow depths with the YT collision farther east, at least to the Canadian border. The shallow continuity of the YT contrasts with clear seismicity and migration images of the dipping Denali and WVF segment slabs, which show an offset of several hundred km at 80-100 km depth. Most likely there is a tear in the slab at the eastern edge of Alaska-Aleutian seismicity at depths greater than 40-50 km.