ACCRETIONARY TECTONICS OF SOUTHERN ALASKA CONSTRAINED BY GPS

Wrangell-St. Elias and Glacier Bay National Parks in southcentral and southeast Alaska are home to some of the most spectacular mountain landscapes in the world. The tectonics behind the topography is currently dominated by the on-going collision of the Yakutat block, an allochthonous terrane that moved north along the Fairweather – Queen Charlotte transform system, with the southern Alaska margin. Previous studies established high rates of motion for the Yakutat block, but insufficient data prevented the development of a detailed, comprehensive regional tectonic model. We present a GPS dataset that spans the region and use that data to characterize deformation styles and constrain relative block motions and fault slip rates.

In southeast Alaska, extremely large, northwest-directed GPS velocities are observed along the coast west of the Fairweather fault. Inboard of the Fairweather fault, GPS velocities have smaller magnitudes and display a clockwise rotation. The Yakutat block moves to the northwest at a rate of about 50 mm/yr relative to North America, resulting in nearly 45 mm/yr of NW-SE-directed convergence between the block and southern Alaska.

The highest strain rates in the region occur across Icy Bay and the western edge of the Malaspina Glacier. Rates approach -1 microstrain/yr, a value higher than that observed in parts of the Himalaya. These strain rates suggest that the current deformation front between the Yaktuat block and southern Alaska is located within the Icy Bay area. Our preliminary modeling indicates that multiple NW- and N- directed thrust faults in Icy Bay, along the western edge of the Malaspina Glacier, and between Icy Bay and Mount St. Elias are required to explain the GPS observations. Over 50% of the relative convergence may be accommodated by slip on these faults.

The second major focus of compressive strain in the region is centered over the Yakataga fold-and-thrust belt. Strain rates there are in the range of -0.40 to -0.50 microstrain/yr. Little significant strain is observed across the Bagley ice valley or to the north of this feature. This suggests that most of the convergence between the Yaktuat block and southern Alaska occurs in a fairly narrow band across the Icy Bay, upper Malaspina/Mount St. Elias, and Yakataga fold-and-thrust belt areas.