Geodesy Papers | Ice Papers | Vegetation Papers

Ice-Lidar Related Publications

Abstract. Southern Alaska is a continent-scale region of ongoing crustal deformation within the Pacific-North American plate boundary zone. Glaciers and glacial erosion have dictated patterns of denudation in the orogen over the last ~5 Myr. The orogen comprises three discrete topographic domains from south to north, respectively: (1) the Chugach/St. Elias Range; (2) the Wrangell Mountains; and (3) the eastern Alaska Range. Although present deformation is distributed across the orogen, much of the shortening and uplift are concentrated in the Chugach/St. Elias Range. A systematic increase in topographic wavelength of the range from east to west reflects east-to-west increases in the width of a shallowly dipping segment of the plate interface, separation of major upper plate structures, and a decrease in the obliquity of plate motion relative to the plate boundary. Mean elevation decays exponentially from ~2500 to ~1100 m from east to west, respectively. Topographic control on the present and past distribution of glaciers is indicated by close correspondence along the range between mean elevation and the modern equilibrium line altitude of glaciers (ELA) and differences in the modern ELA, mean annual precipitation and temperature across the range between the windward, southern and leeward, northern flanks. Net, range-scale erosion is the sum of (1) primary bedrock erosion by glaciers and (2) erosion in areas of the landscape that are ice-marginal and are deglaciated at glacial minima. Oscillations between glacial and interglacial climates controls ice height and distribution, which, in turn, modulates the locus and mode of erosion in the landscape. Mean topography and the mean position of the ELA are coupled because of the competition between rock uplift, which tends to raise the ELA, and enhanced orographic precipitation accompanying mountain building, which tends to lower the ELA. Mean topography is controlled both by the 60° latitude and maritime setting of active deformation and by the feedback between shortening and uplift, glacial erosion, and orographic effects on climate accompanying mountain building.

Abstract. The changes of the solid Earth in south central Alaska in response to two major glacial fluctuations on different temporal and spatial scales have been estimated and we evaluated their influence on the stress state and ongoing tectonic deformation of the region. During the recent (1993-1995) Bering Glacier surge, a large transfer of ice from the Bagely Ice Field to the Bering Glacier terminus region occurred. We estimated the elastic displacment of the solid Earth due to ice mass redistribution from Global Positioning System (GPS) measurements at sites near the surging glacier. We can account for these displacments by transfer of an ice volume of about 14 km³ from the surge reservoir area to the terminus region. We examined the background seismicity (M_L ≥ 2.5) before, during, and after the surge. We found that the occurrence of small earthquakes (M_L ≤ 4.0) in the surge reservoir region increased during the surge time interval possibly in response to a decrease in ice mass. This suggests that a small decrease in the vertical stress, σ₃, could be enough to modulate the occurrence of small, shallow earthquakes in this dominantly thrust fault setting. During this century the southern Alaska coastal glaciers have been undergoing an overall decrease in volume. Based on our compilation of changes in the extent and thickness of the coastal glaciers between the Malaspina and Bering, we calculated surface displacements due to the Earth's viscoelastic response to annual thinning and to the cumulative retreat over the last 100 years. The uplift of the region due to an average annual thinning rate of 1-6 m/yr in the ablation region is 1-12 mm/yr. For our reference model with a viscosity of 5x10¹⁹ Pas for depths between ≈ 40 and 200 km the toatal viscoelastic response due to the retreat over the last century may be as much as a couple of meters within the coastal ablation zone near Icy Bay. The maximum decrease in σ_V between 0 and 10 km was ≈ 1.0 MPa, which is significant in relation to the stress drops in recent earthquakes (≈ 2 to 10 MPa) but small in relation to the estimated tectonic stress magnitude. Therefore the occurrence of an earthquake such as St. Elias (1979, M_S = 7.2) may have been advanced in time; however most of the ongoing stress accumulation would be primarily due to tectonic forces.

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