EVALUATING THE LINKS BETWEEN MOUNTAIN BUILDING, CLIMATE AND FLAT SLAB SUBDUCTION ALONG THE SOUTHERN ALASKA CONVERGENT MARGIN: AN EXPERIMENTAL APPROACH

The Yakutat microplate is a 15-30 km thick block of oceanic crust that is actively subducting beneath the heavily glaciated St. Elias Range along the southern Alaskan convergent margin. We use sandbox analog models to understand the linkages between orogenic wedge development and glacial erosion and sedimentation during the subduction of thickened oceanic crust. Our various models simulate the growth and development of an accreting wedge whose structure is governed primarily by frictional deformation mechanisms and factors which perturb the models’ otherwise steady state deformation. These models are then compared against a baseline model to understand how such factors as flat-slab subduction and glacial sedimentation and erosion influence the deformational history of such systems. We use image correlation techniques (particle image velocimetry) to calculate detailed kinematics of the developing models. Each model recorded structural development characterized by accretion at the deformation front leading to periodic activation of a backthrust and the development of out-of-sequence fore thrusts. The location and amount of fault slip was significantly influenced by the location of both erosion and deposition and was manifested in both the taper and width of the wedge. Whereas faults steepened progressively with age in the wedge for all models, those produced in our model which simulated glacial erosion were significantly shallower due to longer sustained slip on the frontal thrust and occasional uplift on the backthrust of the wedge. The geometry of the accretionary prism varied in each experiment, producing a narrower zone of deformation during glacial erosion and a wider orogenic wedge during glacial sedimentation relative to the baseline model. In addition, our model simulating glacial sedimentation produced fewer faults with greater slip accommodated by growth structures analogous to those described in the St. Elias Range. Two key results of our modeling demonstrate that subduction of a thicker oceanic plate will cause a regional rapid uplift of the entire accretionary prism and that exhumation is most significant immediately after the orogenic wedge is adjusting to an event of localized erosion.