Structure of normal faults in the western U. S

Fault zone morphology is partly controlled by secondary faults that grow laterally and link together. Deformation eventually focuses into a narrow, active fault zone surrounded by an elongated mosaic of mostly inactive faults. These secondary faults reflect deformation processes in the brittle crust, and are characterized by power-law distribution in trace length (L) for 100 m < L < 10 km. Larger scale, 30 km to 60 km long segmentation may reflect fundamental spatial variations in strain-rate and rheology in the lower crust and mantle. Stream drainage patterns and topography of mountain ranges and basins reflect variations in uplift, subsidence and erosion related to isostatic forces on the large scale, and fault structure on smaller scale. Structural fault segments are bounded by jogs, curved and cuspate fault traces, cross faults, bedrock spurs and ridges that extend across basins in the surface and subsurface. These structures form where faults link up, or where faults intersect a rheological boundary in the crust. Fault ends of tip lines are embedded in a mosaic of secondary faults with a crudely radial fault pattern. Seismicity and fluid flow are partly controlled by secondary faulting. Belts of seismicity and hydrothermal springs are localized along spurs andmore » near cross-basin ridges because of secondary faulting and enhanced fracture permeability in these regions. Large antithetic faults with discontinuous Quaternary scarps intersect the main faults at depths of 10 to 20 km. The depth at which antithetic faults intersect the main fault zone may reflect either changes in rheology (stick-slip to stable sliding transition) or geometry of the main fault zone.« less