Faulting Patterns in North-Central Nevada and Strength of the Crust

NNE normal fault trends characterize much of the northern Basin and Range province. These faults make sharp bends to NNW and ENE trends in north- central Nevada in the vicinity of a mid-Miocene rift characterized by a zone of diabase dike swarms, graben-filling flows, and a coinciding aeromagnetic anomaly. Despite a roughly 45 ° change in the least principal stress direction since mid-Miocene time, pre-existing NNW- and ENE-trending faults in the vicinity of the rift accommodated the extension whereas regionally, major crustal blocks were faulted along a NNE trend, approximately perpendicular to the modern least principal stress direction. An assumed uniform regional stress field (derived from geologic and geophysical indicators of the modem principal stress field) and the observed oblique slip on the pre-existing faults were combined in an analysis utilizing an empirically derived frictional sliding law and the Coulomb failure criterion. This analysis constrained the ratio of the least principal stress to the greatest principal stress(S3/S1 as well as the inherent shear strength of intact crustal rocks, τc. While both parameters S3/S1 and ,τc, are functions of unknowns including pore pressure and the cohesion (frictional strength) of the pre-existing faults, reasonable assumptions about these parameters lead to ,τc estimates that agree well with values obtained from laboratory experiments simulating crustal conditions. At a depth of 10 km, the analysis indicates that the minimum inherent shear strength of intact crustal rocks must range between 150--450 bars for zero pore pressure and 150-350 bars for hydrostatic pore pressure, whereas the corresponding maximum shear stresses at 10-km depth are 970-1200 bars for zero pore pressure and 640-770 bars for hydrostatic pore pressure