GEODYNAMIC MODELS OF FAULT-CONTROLLED EXTENSION

Results from geodymamic models for lithospheric extension that include s 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 co mposed of thermoelastic-plastic material with viscous creep that is su bject to extension, buoyant supporting forces and sedimentary loads, a nd a coincident, but thicker, thermal lithosphere that includes the ef fect of sediment blanketing and radiogenic heating. We have chosen for comparison purposes a ''standard'' reference model which minimizes cr eep in the crust; consequently, our results depend strongly on plastic deformation. Our models show that large faults can control the positi on and growth history of the mantle instabilities that can lead to rup ture. We observe that these instabilities have secondary, ''normal mod e'' like character. Models with two faults show that the normal mode b ehavior can interact and create an enhancement to the necking process, provided the faults are an ideal distance apart. We compare model wit h 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 refer ence model with models using ''wet'' and ''dry'' rheologies and observ e that flow in the crust attenuate the propagation effect created by t he faults.