Geodynamics of Terrane Accretion within Southern Alaska

The subduction and accretion of an exotic terrane at the southern margin of Alaska is driving uplift of the St. Elias and Alaska Ranges, and is responsible for some of the largest strain releases in history. Here are presented results from numerical models conditioned by geological observations that reproduce the tectonic landscape, deformation, and strain patterns at macro- (1000-km) and meso- (\u3c100 \u3ekm) scales. These models utilize completely coupled thermal and mechanical solutions that account for the development of heterogeneities to both the thermal and rheological structure of the lithosphere. Perturbation to the thermal structure related to flattening of the buoyant down-going slab offsets the hot mantle wedge flow, cooling the fore-arc region of the orogen developing a thin sliver of material that behaves frictionally. This frictional sliver provides a primary control on the transfer of strain to the over-riding crust and influences the observed deformation patterns. Strengthening of the fore-arc causes a large-scale discontinuous jump in the deformation front. Initial deformation consists of the development of the Alaska Range orogenic wedge and dextral Denali Fault system. The deformation pattern reorganizes most of the strain captured by the St. Elias orogenic wedge forming above the down-dip limit of the frictional sliver. These model results are consistent with the observed slip on the Denali Fault indicating the partitioning of northwestward translation of the accreting terrane into the fold-thrust belt of the Alaska Range, relatively fast uplift within the St. Elias Range, and the temporal shift in deformation patterns observed within the thermochronological and stratigraphic records. The mesoscale model strain patterns, including the effects of evolving topography and erosion, are consistent with the geological observations; the St. Elias Range thin-skinned fold-thrust belt develops with uplift reaching a maximum within the kinematic tectonic corner. The basic strain pattern is controlled by the tectonic geometry, with the surface conditions providing a secondary influence on the rates and magnitudes of deformation. The results of this study indicate that the non-linear feedback between rheology, temperature, and geometry provide a primary control on strain patterns during orogenesis