Mapping Sub-Glacier Geomorphology and Strucutre in a Collisional Orogen; AN Example from the Agassiz and Malaspina Glaciers, AK

Structural boundaries in the Saint Elias Mountains of southern Alaska and Yukon, Canada are covered in ice, concealing the underlying geology and making a tectonic interpretation difficult. The Malaspina Glacier covers the active deformation front formed by the subduction of the Yakutat micro-plate beneath southern Alaska. The Malaspina fault is an active structure located beneath the Agassiz lobe of the Malaspina glacier and is the primary focus of this study which aims to constrain the orientation of the fault in the sub surface.We analyzed ice surface topography, ice velocity fields, and folds and crevasses to infer bedrock topographic features with the anticipation of deriving the geologic structures that will clarify the tectonic transition from transform to subduction in the micro-plate. Calculation of the ice velocity field is done by cross-correlating temporal sequences of Landsat images. The topography and structures of the glacier are mapped using Digital Elevation Models, ICESat profiles, and optical imagery. Perturbations in the ice-velocity field, topography, and glacial structures reflect the orientation of ridges and troughs that form from by glacial erosion of geologic structures. To reinforce the observations made on the Malaspina glacier analog models were constructed in the lab to physically model glacial ice flow as it moves over basins and ridges that may reflect faulting and folding at depth. Results indicate that the Agassiz lobe flows eastward across the gentle west dipping back limb of the Malaspina fault propagation fold, implying that the Malaspina fault dips westward but crops out beneath the ice to the east, where the steep forelimb of the fold lies buried. The east facing topographic escarpment through which the Agassiz lobe flows is therefore an erosional feature of the fold, and not a direct expression of the Malaspina fault. Steeply dipping beds of the fold's forelimb are buried where the Agassiz lobe merges with the main body of the Malaspina glacier. Further work is being done to refine the location of the fold limbs, and project the position of the Malaspina fault tip in that area