MODELING THE EVOLUTION OF CORDILLERAN PLATEAUS

Numerical and analytical models of plateau development emphasize viscous behavior of the crust as a necessity for plateau growth, whereas geologic studies of high elevation plateaus stress the importance of brittle deformation. The role of brittle deformation in plateau formation is particularly apparent for the Andean Plateau in Bolivia where balanced cross-sections through the Andean fold-thrust belt indicate upper crustal shortening of 300-330 km through predominantly eastward propagating brittle fault systems. Although associated mantle deformation is not fully understood, only half of the original 880 km length of mantle lithosphere is preserved in the orogen. Much of the preserved mantle lithosphere is interpreted to be Brazilian shield. We model the formation of the Andean plateau through numerical 2-D experiments. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions. The brittle parts of the lithosphere are modeled as a frictional and cohesional material. The ductile lithosphere is modeled as a non-Newtonian Maxwell visco-elastic material. Faults in the brittle parts of the model are formed by locally decreasing the friction and cohesion as a function of plastic strain. The initial conditions for the model include a strong cold lithosphere adjacent to a younger, warmer lithosphere that corresponds to the 400-200 Ma passive margin/ extensional basin on the western edge of the South American continent. Subduction of the Nazca oceanic plate is not included in the model. It is however included as the driving mechanism applied as compressional velocity boundary conditions of the model. Initial deformation in the model is focused in the center of the passive margin basin, on a fault that breaches the lithosphere. Intracontinental subduction of mantle lithosphere pulls down the overlying crust and facilitates the eastward propagation of brittle deformation. With time heat associated with a radiogenic, thickened crust decreases the viscosity of the lower crust and facilitates plateau growth through viscous processes. The height of the plateau is controlled by the strength of the brittle crust. The transition from brittle- to viscous-dominated deformation is controlled by crustal thickness and thermal age of the lithosphere.