Modeling the formation of a half graben using realistic upper crustal rheology

The evolution of a half graben in a strong upper crust with a Byerlee strength envelope, underlain by an inviscid substratum, has been modeled using elasto-viscoplastic finite element analysis. The nonlithostatic stresses, regions of failure, and flexure profiles have been computed as the layer is extended by increments. The weaker upper part of the layer fails on stretching, and more extensive regions of tensional failure extend progressively downward beneath the downbending half graben and upward from the bottom of the layer beneath the footwall uplift. Compressional failure occurs locally due to bending or unbending. Plastic yielding progressively reduces the widths of the half graben and footwall uplift as extension proceeds, as the outer hinge line of the half graben migrates toward the fault. The plastic deformation reduces the effective elastic thickness of the layer by up to 67%, helping to explain the anomalously low observed values associated with normal faulting. Throughgoing failure in tension eventually occurs beneath the footwall uplift near the fault, when subsidence slows down and ceases, The strength of the upper crust thus limits the amount of subsidence that can occur. For an 8 km thick layer and sediment infill 400 kg/m(3) less dense than the basement, the half graben width decreases from over 80 km at the onset of extension to 42 km when the subsidence has reached its limit of 2.3 km; the equivalent elastic layer thickness is then reduced to 3.3 km. Thicker layers have correspondingly greater widths and limiting depths. Less dense sediments result in a narrower half graben and smaller limiting depth.