Synthesis of the Seismic Structure of the Greater Alaska Region: Subducting Slab Geometry

We utilized a normalized vote method to define common features in teleseismic, body-wave tomography models produced from the Alaska USArray data. The metric is used with three-dimensional visualizations to define subducting slab geometry. We observe the following features. (1) The eastern end of the slab downstream from the Yakutat microplate has a flap-like structure extending to 300–350 km depth that is bounded on the east by an edge consistent with rigid plate kinematics and on the west by a tear located downstream from the western limit of Yakutat rocks. (2) The geometry matches Slab2.0. (3) Below Slab2.0, the dip steepens to form a vertical structure that extends to the top of the mantle transition zone. Within the transition zone, the geometry changes from north to south dipping to form a lithospheric scale, recumbent fold connected to a lower-mantle structure located 1,000 km south of the trench. (4) A previously unknown mantle structure extends eastward from the flat slab to an edge predicted by rigid plate kinematics suggesting it is an older section of the Pacific Plate emplaced before the Yakutat tear initiated. We propose a new kinematic model of subduction history in Alaska since 40 Ma consistent with the new data.