GEODYNAMIC MODELS OF FAULT-CONTROLLED EXTENSION

Results from geodymamic models for lithospheric extension that includes one or two large-scale upper crustal faults are presented. The study does not address the origin of these faults, but the consequences if they exist. Model components include a solid mechanical lithosphere composed of thermoelastic-plastic material with viscous creep that is subject to extension, buoyant supporting forces and sedimentary loads, and a coincident, but thicker, thermal lithosphere that includes the effect of sediment blanketing and radiogenic heating. We have chosen for comparison purposes a ''standard'' reference model which minimizes creep in the crust; consequently, our results depend strongly on plastic deformation. Our models show that large faults can control the position and growth history of the mantle instabilities that can lead to rupture. We observe that these instabilities have secondary, ''normal mode'' like character. Models with two faults show that the normal mode behavior can interact and create an enhancement to the necking process, provided the faults are an ideal distance apart. We compare model with two rates of extension, 1.2 cm yr-1 (''fast'') and 0.038 cm yr-1 (''slow''), which show remarkably little difference. We compare our reference model with models using ''wet'' and ''dry'' rheologies and observe that flow in the crust attenuate the propagation effect created by the faults.