COOPERATION AMONG TECTONIC AND SURFACE PROCESSES IN THE ST. ELIAS RANGE; EARTH’S HIGHEST COASTAL MOUNTAINS

Investigations of tectonic and surface processes have shown a clear relationship between climate-influenced erosion and long-term exhumation of rocks. Evaluation of the driver for exhumation and the understanding of key feedback mechanisms are complicated partly because climate has changed significantly in the late Cenozoic. The effects of climate change are particularly pronounced at high latitude where modern climate change is largest and where the growth and decay of enormous ice masses during the last ~2.5 Myr have profoundly affected the landscape. Numerical models suggest most orogens are in a transient state, but observational evidence of a spatial shift in mountain building processes due to tectonic-climate interaction are missing.

A synthesis of new and published thermochronology data in the St. Elias Mountains reveals large spatial and temporal differences in rates and amounts of exhumation along the strike of the Yakutat – North American collision zone. The region of the indenting Yakutat plate corner, where dextral transform motion transitions to convergence, is the focal point for deformation, erosion, and exhumation. This region is currently characterized by the highest topography in the region and large ice fields that fuel fast moving glaciers (the Seward and Hubbard). The data reveal that the region was undergoing extremely rapid exhumation between ~4–2 Ma; it slowed down sometime after 2 Ma. Moreover, the most rapid exhumation shifted towards the south during the Pleistocene and currently occurs at the southern flanks of the orogenic corner in the region between Icy Bay and Yakutat Bay. This is the region where geodynamic models predict very high strain rates and rapidly changing kinematics as thrust systems from the transpressional Fairweather transform converge into the central thrust belt producing rapid but shallow uplift and exhumation in a band that parallels the active thrust front. This thrust front coincides with largest GPS-constrained shortening rates, concentrated seismicity and the greatest erosive potential. We infer that the high sedimentation caused rheological modification and the emergence of the southern St. Elias Range, intercepting orographic precipitation and shifting focused erosion and exhumation to the south.