STRESS-FIELD ROTATION AND ROOTED DETACHMENT FAULTS - A COULOMB FAILURE ANALYSIS

Several well-known mechanical models have shown that unusual boundary or loading conditions can alter principal-stress orientations into con figurations consistent with low-angle normal faulting. Such models, ho wever, have not demonstrated whether magnitudes of reoriented stresses are sufficient to initiate and promote slip on low-angle surfaces. We present the results of simple Coulomb failure analyses to determine w hether, and where, such models predict frictional slip, assuming geolo gically plausible boundary stresses, pore pressures, and rock strength s. Models that invoke a sizable shear traction at the base of the uppe r crust or spatially varying loads on the upper crust reorient princip al stresses and failure planes but do not produce frictional failure o n crustal-scale detachments either in the absence of pore fluids or at hydrostatic pore fluid pressures. Models that reorient stresses by mi dcrustal dike intrusion produce slip on low-angle surfaces at relative ly deep crustal levels but only in the area of the dike tip; the low-a ngle failure surfaces curve into a high-angle orientation a short dist ance from the dike. All of these models also imply unsustainably high absolute tensile stresses in the upper 5 km of the crust and suggest t hat, in any system in which stresses are allowed to evolve over time, failure and stress release will occur on high-angle faults before low- angle ones have developed. These assertions are true even when near-li thostatic pore pressures are assumed, unless there is an inhomogeneous , extraordinarily fortuitous distribution of pore pressures and rock s trengths at the time of initiation of a new detachment fault. One mode l we tested, for example, required pore pressures exceeding 0.96 times lithostatic in the area of the hypothesized low-angle normal fault, w ith lower pore pressures both above and below the detachment to preven t slip and stress release on high-angle normal faults in the upper par t of the modeled region and on low-angle thrust faults in the lower pa rt.