ROLE OF TECTONICS AND CLIMATE IN THE TOPOGRAPHY, GLACIATION, DENUDATION, AND FLOW OF ROCKS IN THE CHUGACH/ST. ELIAS RANGE (SOUTHERN ALASKA) ACTIVE COLLISIONAL OROGEN

Glacial erosion has accompanied development of the Chugach/St. Elias Range orogenic belt in southern Alaska over the last ~6 My. partial subduction of the Yakutat terrane (YT) across the Chugach- St. Elias thrust fault (CSE), which marks the southern edge of the North American plate (NA) drives crustal deformation. Restoration of an area-balanced cross-section indicates ~55 km of internal shortening in the YT near the eastern edge of the collision. Assuming this internal deformation accumulated after 6 Ma, ~25% of the YT tectonic influx has accreted at ~10 mm/yr onto the NA upper plate. Low-T cooling ages imply a progressive increase in exhumation (from <1 mm/yr to ~2 mm/yr) northward across strike from the deformation front on the south to the CSE. Progressive exposure of deeper structural levels constrains erosional denudation from <5 km to > 7 km from south-to-north, respectively. Plate interface dip, detachment stratigraphic locations and shortening distribution in the YT, and plate convergence obliquity control rock uplift variation and orogenic width. Glaciers and glacial erosion likely control the amplitude of the topography. Mean elevation corresponds closely with the modern equilibrium line altitude of glaciers (ELA). Orographic control on climate is revealed by the glacier distribution on the windward, southern and leeward, northern flanks of the range (south and north of the CSE, respectively), during Holocene glacial cycles. Glaciers moved 10’s of km up- and down-valley over 10^3 yr cycles on the windward side of the range. Limited redistribution of glaciers marks the leeward side (a few km) on the same timescales. Climate thus influences erosion rate by controlling the proportion of landscape covered by glaciers, which in turn modulates base level for hillslopes, rivers in unglaciated portions of valleys, and sediment production, routing, storage, and delivery out of the orogen. The greater magnitude and frequency of glacier distribution may explain the larger degree of exhumation of the windward relative to the leeward flank of the range. Whereas the relatively low mean elevation of the range (2500 – 1100 m) reflects the 60° latitude and maritime setting of active deformation, the long-term exhumation is paced by the rock uplift associated with the tectonic influx represented by accretion of the YT to NA.