From Core Complex to Wide Rift Mode: Modeling the Evolution of the Basin and Range.

From Triassic to late Cretaceous the Basin and Range province underwent uplift, thrusting and magmatism. In the Cenozoic two phases of extensional tectonics occurred; the first phase began in the early Neogene and continued until mid-Miocene time and was associated with large amount of localized extension on single normal faults as it is observed in the core complex belt. The second extensional phase lasted from the mid-Miocene to the end of the Miocene and was associated with broadly distributed extension on multiple normal faults which produced the Basin and Range morphology. Before extension began in the Neogene the previous phases of convergent tectonics and uplift determined the initial geometry and rheology of the basin. Thickened crust and features such as major preexisting thrust faults, possible rheological contrast between the lower and upper plate of the orogeny, as well as the presence of weak lower crust or lower density magmatic bodies are likely to have influenced the style of extensional tectonics observed in the Basin and Range Province. Here, I present 2D numerical models of the extension of a thickened visco-elasto plastic lithosphere in which the effect of preexisting structures on the evolution of the basin is analyzed. 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 cohesion as a function of plastic strain. The rheological structure of the model is controlled by the initial temperature distribution and the temperature boundary conditions. Model results show that the presence of a preexisting major thrust boundary, the possible rheological contrast between the upper and lower plate of the orogeny and the presence of a weaker, lighter (molten) lower crust could help localize the deformation on a single graben as observed in the core complex extensional phase. The presence of a preexisting major thrust boundary also enables the asymmetric thinning of the crust by viscous flow in the lower crust. After this first phase of very localized extension thinning of the crust generates enough buoyancy to delocalize the extension. This leads to a second phase of broadly distributed extension and to the well known Basin and Range morphology.