Glacial Erosion as a Primary Control on Landscape Evolution of the Active Chugach/St. Elias Range of Southern Alaska

Studies of landscape evolution have recognized the significance that erosion plays on the development of active orogens. The influence of fluvial erosion on active mountain belts has been documented, but the role played by glacial erosion has yet to be fully recognized. The growth of the glacially-dominated Chugach/St. Elias Range of southern Alaska over the last 5 m.y. provides a unique setting to study the effects of glaciation on the denudation patterns of active orogens. The range has been built from rapid collision (5 cm/yr) of the Yakutat microplate with North America. Its close proximity to the coast of the Gulf of Alaska and the orographic effect of topography result in extensive glaciation (low equilibrium line altitudes) on the windward flank and sporadic glaciation (high equilibrium line altitudes) on the leeward flank. If glacial erosion is a maximum at equilibrium line altitude, we would expect erosion to be maximized there and topography to be limited by a negative feedback loop. As a result, exhumation rates should increase southwards toward the coast. However, tectonic convergence in the range varies along strike and is spread across a fold and thrust belt, of which the coastal region lies predominantly in the footwall. If active structures control uplift and erosion, we would instead expect a correlation between exhumation pattern and the distribution of faults and folds. To determine the impact of glacial erosion on exhumation in the Chugach/St. Elias Range, we have measured radiogenic helium ages of apatite (AHE) from numerous bedrock samples collected along and across the range. The low-temperature sensitivity of the AHE technique allows determination of exhumation rates for rocks from shallow crustal depths. Results indicate that glaciation contributes significantly to the exhumation of the range. AHE ages from coastal samples are much younger (~1-5 Ma) than sample ages from further inland (~10-20 Ma). The youngest AHE sample age found in the eastern part of the range reflects rapid denudation from extensive glaciers that surround Mt. St. Elias and converge at sea level. The oldest AHE sample ages found in the western portion of the range reflect a decrease in exhumation due to the inland location of the rocks on the hanging wall of the Aleutian Megathrust where convergence is minimal. Although only a fraction of recently-collected samples have been analyzed thus far, the pattern of exhumation favors glacial erosion as a primary control on landscape evolution.