TECTONICS AND STRUCTURAL EVOLUTION OF THE EASTERN ST. ELIAS SYNTAXIS, ALASKA

The transition from transform motion to oblique convergence as the Yakutat terrane collides with North America forms the eastern syntaxis of the Saint Elias orogen in southern Alaska. It is unclear whether the syntaxis is a static feature of the orogenic system or if deformation accumulates as material migrates through the region into the fold and thrust belt to the west. The syntaxis contains the highest topography (> 5 km peaks) and deeply exhumed high-grade metamorphic rocks suggesting prolonged development. However, the orogen also contains synorogenic deposits and is < 40 km wide in the syntaxis area (compared to > 100 km elsewhere along strike), suggestive of either recent activity or sustained localized deformation. The syntaxis is the nexus between the transfer of strike slip motion from the Fairweather fault into the orogenic interior, likely retrothrust motion in the backstop, an active thrust front in the foreland, and the Dangerous River Zone where basement changes from continental to oceanic affinities and pre-tectonic sedimentary cover thickens abruptly to the west. In addition, the Seward and Malaspina glacial system (one of the largest and most dynamic in North America) occupies the syntaxis. Preliminary geometric modeling of interactions between presumably active structures appears to closely correlate with the glacial system, possibly indicative of a linkage between concentrated glacial erosion and localized deformation. Recent field studies also constrain the current tectonic configuration and provide insight into the structural evolution of the region. In particular, there may be a component of previously unrecognized strike-slip motion accommodated within the eastern syntaxis as transitional Yakutat terrane basement is thrust beneath the backstop assemblage.