ARCHITECTURE, KINEMATICS, AND EXHUMATION OF A GLACIATED OROGENIC WEDGE: THE CENTRAL ST. ELIAS OROGEN, ALASKA

The kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate, but the long term effects of glacial erosion on orogenesis have yet to be fully revealed by case studies. We use thermochronometry to develop a new architectural model of the St. Elias orogen in southern Alaska, an ideal example of a glaciated orogenic wedge. Apatite and zircon (U-Th)/He dating illustrate that this orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding, thin-skinned fold and thrust belt on the windward flank. These distinct deformational and thermal domains are separated by a structure beneath the Bagley ice field, which we propose is a backthrust that makes the orogen doubly-vergent. Within the orogenic wedge, denudation and deformation are strongly influenced by glacial erosion. Exhumation, at rates of up to 5 mm/yr, is concentrated in a narrow band along the windward flank, where the mean Quaternary glacial equilibrium line altitude intersects the landscape. This zone of focused denudation cuts across the structural trend and is more closely correlated with equilibrium line than with orographic precipitation. This corroborates model predictions that glaciers and climate control deformation and removal of crust from orogens. The effect of climate is also evident in a temporal and spatial shift in exhumation associated with late Cenozoic climate change. The onset of enhanced glaciation following the Mid-Pliocene Warm Interval coincided with a ten-fold increase in cooling rates within the wedge and a major shift in deformation away from the North American-Yakutat terrane suture (Chugach St. Elias fault). We propose that accelerated denudation redistributed strain along forethrusts and the backthrust, which in turn truncates the southward-vergent thrusts. A high degree of coupling between climate and tectonics is thus apparent in this glacially-dominated orogen.