CHARACTERIZING THE 3-D INFLUENCE OF LOWER CRUSTAL LOCALIZATION ON THE SURFICIAL KINEMATICS OF AN OROGEN

The geomorphic structure of Earth's surface in actively deforming areas is a result of the interplay of surficial and tectonic processes. This study uses numerical modeling to explore the sensitivity of surface kinematics and the resulting topographic field to rheologic variation in the lower crust. Models will test whether lower crustal shear zones which are on the order of five times weaker than the surrounding crust, with lengths on the 10’s of km scale and widths on the 100’s of meter scale have a mechanical influence on the kinematics of an orogen. Shear zone geometry and position are changed in both two and three dimensions to observe the circumstances under which their influence on surface kinematics is visible. The sensitivity of orogenic topographic development to shear zone location is evaluated using a three-dimensional numerical model of an orthogonal collisional setting. The orogen is modeled using a two-layered crust overlying an elasto-plastic mantle slab. Shear zones are implemented in the lower crust by defining zones of weakness which are in contrast to the surrounding, elasto-plastic lower crust with material properties defined by experimental flow laws. Initial runs are completed using a shear-zone free version as the reference model. Subsequent runs introduce a 10 - 20 km long, 50 – 200 m wide dipping shear zone five to ten times weaker than the surrounding crust. Along with testing for shear zone geometry, the influence of position is tested by moving the shear zone laterally through the lower crust. Findings suggest that lateral changes in shear zone positions have a significant effect on the shape and elevation of the orogenic belt and that there may be a critical position in the model at which the orogen transitions between topographic styles. The development of a more or less prominent inlet orogen varies with shear zone position, allowing the effect of shear zone placement and geometry to be viewed at the surface.