Numerical Investigations of Lithospheric Deformation and 3D Mantle Flow in the Pacific-North America Plate Boundary Corner in Southern Alaska

Continental deformation in convergent settings has traditionally been interpreted in terms of two end-member models: continuous, viscous, thin-sheet models and discontinuous, fault-bounded block models. However, these end-member models fail to account for isolated, broad regions of deformation that occur far from plate boundaries, such as the central Alaska Range, a young (5--6 Ma) localized region of uplift, located approximately 500 km inland from the Pacific-North America plate boundary in southern Alaska. Several complexities in crust, lithosphere and mantle structure may play an important role in transferring deformation far inboard from the plate boundary. First, southern Alaska is characterized by a history of accretion of terranes with varying lithology, and perhaps variable crust-lithosphere strength. Second, the central Alaska Range straddles a bend in the Denali Fault System, which may be a lithosphere-scale weak zone. Local regions of variable crust-lithosphere strength may act to localize deformation. Third, the spatial correlation of the central Alaska Range with a change in the geometry of the subducted slab, from moderately dipping in the west to a shallow or flat dip near the plate boundary corner, implies that features in the subsurface, such as slab shape and/or mantle flow patterns around the edge of the slab, may be genetically linked to the uplift in this region. We present initial results of 3D viscous flow models that are from the first phase of our investigations into the causes for the uplift of the Alaska Range. These models investigate the influence of the slab geometry on viscous flow patterns in the mantle around slab edges and how they may inturn influence deformation patterns in the overriding lithosphere. These models are instantaneous and use either a Newtonian viscosity or an effective viscosity that includes Newtonian and non-Newtonian rheology and a yield stress criterion for cold regions. The slab is represented by a simplified density (and thermal) anomaly. In order to isolate the effects of the mantle structure on surface deformation, a uniform lithosphere structure is used throughout the model, except for a narrow low viscosity region, which follows the 3D plate boundary surface to a depth of 100 km. The results of these 3D numerical models have general applicability to understanding the dynamics of mantle flow and slab behavior in plate boundary corners.