Three-Dimensional Crust-Upper Mantle Velocity Structure of Southern Alaska

Jack Pelton, Boise State University

Abstract

The USGS Southern Alaska seismograph network spans an area in which a significant tectonic transition occurs from northwestward underthrusting of the Pacific Plate beneath the Kenai Peninsula – Cook Inlet to right-lateral transform motion southeast of Yakutat Bay. In order to study the deep structure of this region, we measured travel-times of P-waves from 53 teleseismic sources recorded by the USGS network. A total of 1570 residuals were calculated and used in an inversion for 3-D velocity structure.

In the crustal layer (0-40km) large negative velocity (slow) anomalies coincide with the Cook Inlet, and the Yakutat block along coastal Alaska. Positive (fast) anomalies mark the volcanic chain which borders the NW side of Cook Inlet, the SE half of Kenai Peninsula, and Prince William Sound. These velocity variations appear to reflect changes in crustal composition.

In the upper mantle (3 layers from 40-440km), west of about 147 <degree>W longitude, velocity anomalies define linear features that persist to the bottom of the model. The velocity anomalies have a NE-SW trend that changes farther north to a N-S grain and can be related to the subduction of the Pacific Plate. East of this line, the upper mantle is relatively homogeneous indicating that if the Pacific Plate is subducting beneath this area, as some have proposed, then the plate does not penetrate deeply into the upper mantle; however, the presence of low-velocity pockets (40-140 km deep) beneath the Wrangell Mountains and the Yakutat Bay area might be indicative of some sort of convective instability related to past or present subduction.