The Eastern Syntaxis of the St. Elias Fold and Thrust System, Alaska

The transition from oblique convergence in the St. Elias fold and thrust system to transform motion along the Fairweather fault system forms the Eastern syntaxis of the St. Elias orogen. Structural studies suggest that the Dangerous River Zone is holding the position of the syntaxis fixed and separates slip-partitioned, basement-involved, transpressional structures to the east from the foreland fold and thrust belt to the west. An oblique component of slip was previously (>3 Ma?) partitioned within the fold and thrust belt, but is currently being accommodated by reactivated thrusts and younger oblique and en echelon structures including the Malaspina fault. In addition, recent thermochronology data suggests that there is large differential exhumation across several faults within the syntaxis area, including a splay fault in the upper the Seward throat, a narrow and dramatic glacial channel separating the Seward and Malaspina glaciers. This relationship may be suggestive of a linkage between localized deformation, glacial erosion, and the generation of high topography. Numerical modeling based on the unique geometry of the Eastern syntaxis predicts the formation high topography in concert with erosion and strain localization. Fault activity in the syntaxis area is likely associated with the leading deformation front and represents a significant crustal boundary (at a moderate to high angle to the orogenic margin) that continues to enable the accretion of the Yakutat microplate to North America, analogous to the Bering Glacier fault and the Ragged Mountain fault to the west.