Nascent Development of Focused Rock Uplift and Erosion in the St. Elias Orogen, Alaska

The St. Elias orogen in Alaska, the world's highest coastal mountain range, is heavily covered by ice and aggressively eroded by temperate glaciers. Formed through the ongoing oblique collision of the Yakutat terrane with North America, the St. Elias orogen hosts a transition from subduction to strike-slip tectonics, and thus provides an excellent setting for studying coupling between deformation, erosion, and climate. Previous dating studies involving low-temperature thermochronometers on bedrock samples report the youngest cooling ages where precipitation rates are highest, suggesting a strong coupling between erosion and modern climate, and relatively homogenous magnitudes and rates of erosion along orogenic strike. We employed a complementary sampling approach involving fission-track dating of detrital zircons in order to obtain information about the substantial portions of the orogen that are ice-covered. In addition, because of the relatively high closure temperature (>300°C) for fission-tracks in young pristine zircon, such ages provide a better test for geodynamically significant exhumation of 5 to 10 km that exceeds local relief and this requires rock uplift to be a contributing driver in the system. We find that in general fission-track ages of zircons in glacial sediments are only moderately young along strike (> 10 Ma). However, the Seward-Malaspina Glacier transports a substantial fraction of young zircon: 25-41 % of grains are less than 3 Ma, and a number of grains give ages as young as 0.4 Ma. This glacial system must tap a zone of focused rock uplift and erosion that is colocated with the orogen's area of highest relief, and high seismicity, in the transition from subduction to transform tectonics. U/Pb dating on zircons that yield the youngest fission track ages verifies that these young ages are indeed cooling ages due to exhumation, and supports the inference that the source of the fast exhuming rocks is located underneath the Seward Glacier. These new results are suggestive of glacially mediated coupling of rock uplift and erosion, and may indicate incipient development of a 'tectonic aneurysm' involving thermomechanically mediated strain localization by erosion.