THREE DIMENSIONAL DEFORMATION IN THE EASTERN ST. ELIAS SYNTAXIS

We synthesize results for earthquake focal mechanism, finite source models, seismic tomography, 3D imaging with P and S receiver functions, GPS deformation modeling, and surface geology in a region of southeast Alaska near Mt. St. Elias using modern 3d visualization techniques. Reanalysis of finite source models for the 1979 St Elias earthquake suggest that event occurred on two low angle surfaces: a western, low-angle, NW dipping source downdip from the Malaspina fault and an eastern, NE dipping source beneath the high St. Elias Range. The NE dipping segment is consistent with recent finite fault models of the 1899 Yakatat Bay earthquakes described by Plafker and Thatcher (2008). Collectively these surfaces are interpreted as segments of the megathrust underlying the fully convergent Yakataga thrust belt to the west vs the oblique transpressional margin to the east. In the spirit of a midwestern US meeting we propose a middlebuster plow model as a conceptual model for the 3D geometry of this system. Surface faults suggest a significant change in kinematics along surface faults on either side of the Seward Glacier outlet in direct association with this deep, subsurface structure. Geologic relationships in the Icy Bay region add a time dimension to this process through a systematic change in fold-thrust trends with structural level with Paleogene and early Neogene rocks below an angular unconformity recording NW trending fold-thrust structures, variably overprinted by younger structures whereas syn-orogenic strata of the Yakataga Formation show successive rotations in kinematic axes through the stack of thrust sheets. These observations suggest the NW trending structures were formed in the oblique transpressional orogen, and transported into the syntaxis where they experienced rapid kinematic changes as they were carried into the fully convergent core of the orogen.