Orogenic Response to Intense Quaternary Glaciation: The St. Elias Orogen, Alaska

Numerical modeling of subaerial orogenic systems suggests that spatial and temporal variations in climate can have a profound influence on orogen kinematics, architecture, topography, and thermal evolution. However, due to the co-evolution of climate and tectonically constructed topography, the role of atmospheric processes in natural systems continues to be elusive. The tractable size and high latitude of the St. Elias orogen provides an ideal setting to address the real world orogenic response to focused denudation and climate change. Independent of any known change in the rate of convergence, the St. Elias orogen was subjected to the most severe transition in climate during Cenozoic time, the Late Neogene and Pleistocene onset and advance of glaciers. Low-temperature thermochronometry, offshore seismic reflection, borehole, and earthquake data from the STEEP project demonstrate an association between glacial denudation and orogenic evolution. Coeval with the onset of enhanced glacier coverage in mid-Pleistocene time, onshore denudation and offshore sedimentation accelerated ~ten-fold, with the highest rates of augmented exhumation (4 km/Myr (±25%)) located around a narrow zone where the mean Quaternary glacial equilibrium line altitude (ELA) intersects mean topography on the windward flank of the orogen. This climatically driven mass redistribution coincided with accelerated backthrust motion and a shift in deformation away the seaward deformation front. In a cause and effect response, the expansion of glaciers thus appears to have resulted in an orogen scale structural reorganization and a narrowing of the orogenic wedge to maintain critical Coulomb taper. Many of these results have previously not been observed in a natural system and have far-reaching implications for the classic "chicken and egg" debate of Late Cenozoic climate change, the glacial buzz-saw hypothesis, as well as the processes governing orogenesis.