INFLUENCE OF GLACIATION ON OROGENESIS IN THE ST. ELIAS RANGE, ALASKA AND IMPLICATIONS FOR OTHER ACCRETING TERRANES

The St. Elias orogen is an appropriate location to study the effects of glacial loading and unloading on orogenesis because it maintains the highest coastal mountains on Earth, has an extensive temperate glacier network, and the region is actively deforming with convergent rates greater than 40 mm/yr. Today the coast of southeast Alaska is defined by a subduction boundary to the north, a strike-slip fault (Fairweather fault) on the east, and an enigmatic, Transition fault to the southwest that strikes northwest. Using three-dimensional modeling of the rheology, kinematics, and geometry of coastal Alaska and dynamic ice sheet modeling of the Last Glacial Maximum (LGM) extent, we test the sensitivity of the Transition fault to rheological structure and increased normal stress (LGM glacial load). The Transition fault follows a well defined boundary between the Pacific plate and the Yakutat plate. The rheology of the area can be constrained by the fact that the strength of oceanic crust is well known and there is a large step in crustal thickness between the Pacific and Yakutat crust. Based on hypocenter data we suggest that the lower 5-10 km of the Yakutat crust is weaker relative to its surroundings. This weakness, when coupled with additional perturbations (e.g. glacial load), allows for deformation or strain to be taken up across the Transition fault. Model results show that when there is no weakening of the lower crust, all of the strain is accommodated along the strike-slip and subduction boundaries. With the addition of a LGM sized load some strain is partitioned into the Yakutat plate but does not reach the Transition fault. For simulations with a weaker lower Yakutat crust and with an applied glacial load this pattern changes and strain is distinctly accommodated on the Transition Fault and there is no longer a continuous boundary between the strike-slip and subduction boundaries. Results from this study suggest that the spatial and temporal distribution of glaciers influences the tectonics, seismicity, and orogenic evolution of accreting terranes and can be used to constrain the rheology of a region.