STRUCTURE OF THE ACTIVELY DEFORMING FOLD-THRUST BELT OF THE ST. ELIAS OROGEN WITH IMPLICATIONS FOR GLACIAL EXHUMATION AND 3D TECTONIC PROCESSES

The fold and thrust belt of the St. Elias orogen in southern Alaska is becoming an archetype for erosion-tectonic interactions in a collisional orogen exhumed by glacial processes, yet to date most studies have considered the problem with little reference to geologic structure. As part of the St. Elias Erosion and tectonics Project (STEEP) we conducted geologic mapping along two corridors across the f&t belt: Duktoth River and Icy Bay. Mapping together with balanced cross section construction reveals the orogen is the site of both 3D processes from dextral-oblique motion and structural complications at depth that appear to be related to focused erosion. 3D complications include: 1) young, probably active, NE trending, out-of-sequence thrusts that take early ~EW trending folds and warp them around NW trending fold systems within the Duktoth transect; and 2) kinematic changes over time recognized in the Icy Bay transect demonstrated by variations in fold-axis orientation across angular unconformities, and exhumation of deep structural levels in the Mt. St. Elias massif. We infer different processes for each of these 3D complexities: younger cross-cutting thrust systems in the Duktoth transect vs kinematic affects of rocks entering the "tectonic corner" in the Icy Bay region. Although the structure is 3D, it is sufficiently 2D that cross-sectional restoration is possible for both two transects. In both sections we projected the top of basement recognized in STEEP seismic studies beneath the onshore sections and assumed a subsurface structural geometry of stacked duplex systems based on the observed structural style onland as well as analog model studies of Malavielle (2009, GSA Today). These sections can account for 150-200km of shortening within the fold thrust system; well short of likely convergence of ~600km in the last 10m.y. based on the subsurface geometry of the subducted yakutat terrane basement, but 2-4x larger than previous estimates from balanced cross sections in the area. A simple prediction from section restorations is that uplift and warping above a young duplex can account for the observed elongate bullseye pattern of low-T cooling ages in the thrust belt and is interpreted as a direct effect of focused erosion and offshore deposition.