Analysis of Oblique Wedges Using Analog and Numerical Models

Oblique plate motion is understood to be a primary factor in determining the style and location of deformation at many convergent margins. These margins are frequently characterized by a dominant strike-slip fault parallel to the margin, which accommodates margin-parallel motion and shear and is adjacent to partitioned and near margin-normal thrusting. We have performed a series of analog experiment in which we have simulated oblique wedges with frictional and layered, friction over viscous, rheologies. Using the detailed analysis of topography and strain from these analog models we have compared them to geometrically similar 2D and 3D numerical models. While our pure frictional analog wedges are characterized by numerous discrete thrust faults in the pro-wedge and a zone of shear between the pro-wedge and the retro-wedges, our layered wedges have a dominate shear zone that is long-lived. In all models the highest rate of contractional deformation is at the thrust front, while the highest rate of shear is isolated in a relatively narrow zone at the back of the pro-wedge. Because the layered analog wedge is better able isolate shear behind the pro-wedge it can better partition strain into dip-slip thrusting normal to the margin. Our numerical simulations support the assertion that a relatively small amount of extensional stress is needed to play a significant role in the structural evolution of convergent systems. However, the manner in which this stress is localized on discrete structures, and in particular, how the style of strain (extension or contraction) will evolve, is a strong function of rheology and its strength at depth for a given initial geometry.