Collaborative Research: Seismic and Geodetic Imaging of Subducting Terranes Under North America (S-TUNA)

The collision of thickened crust with subduction zones significantly modifies subduction. These accretion events lead to net growth of continents and drive much of the subduction-related tectonism. Terrane collision may also have a profound effect on the size, coupling, and rupture characteristics of large intraplate earthquakes. The present accretion of exotic terranes with the Alaska subduction system represents one of the few examples of this process currently active. In this region, the collision of a region of thickened crust, the Yakutat terrane, occurs at the largest rupture asperity known, part of the 1964 Mw9.2 Alaska earthquake. The collision produces mountains along the Alaska coast and perhaps far inland, and may drive westward extrusion of distal parts of Alaska. Recently, an unusual layer, perhaps thickened crust, has been imaged at the top of the subducting plate beneath central Alaska from 70 to 150 km depth, using receiver functions from the BEAAR (Broadband Experiment Across the Alaska Range) IRIS-PASSCAL experiment. If continuous with the shallow structure, this would represent the largest deeply-subducted fragment of thickened crust yet observed. Subduction of such thick crust may help explain the size of the 1964 asperity. However, the lack of continuity between deep and shallow structures makes it difficult to tell; have these signals imaged the largest piece of thick subducted crust on the planet, or something else? In any case, what is the effect of subducting terranes on mechanics of the thrust zone?