Three-dimensional numerical modeling of upper crustal extensional systems

We focus on understanding the evolution and structural style of crustal extension in three dimensions using state of the art forward dynamic models. To date, few 3-D models exist that follow the evolution of tectonic processes into large deformation modes with sufficient resolution to resolve individual faults and shear zones. We use an arbitrary Lagrangian-Eulerian fully parallel finite element code that solves for viscoplastic flows in three dimensions. Plastic materials weaken with accumulating strain. To localize deformation, a weak seed region is introduced at the base of a plastic model extended by velocity boundary conditions. Controls on the geometry and spacing of three-dimensional plastic shear zones are investigated. The sensitivity of varying the offset between weak seeds and the sensitivity of strain weakening parameters on the linkage between offset rift zones and on the efficiency of rift propagation are tested. The model results indicate the primary controls of strain-dependent plastic rheology and rift offset on the efficiency of rift propagation and the style of rift segment interaction. The amount and onset of strain weakening also play a large role in the degree to which the primary segments link and propagate toward each other, resulting in a trade-off effect between the amount of offset between the initial grabens and the strain weakening ratio. The three-dimensional models indicate three main rift modes for linkage between two upper crustal rift segments in three dimensions: (1) small-offset grabens with a single relay zone, (2) intermediate-offset grabens with one or more secondary step-over graben segments, and (3) large-offset grabens with limited to no significant segment interaction.