Fault produced fabrics adjacent to the Newberry Mountains detachment fault, Clark County, Southern Nevada

Thesis, (M.S.) Geological Sciences.Brittle deformation dominates the detachment fault zone exposed in the Newberry Mountains in southern Nevada, indicating that mid-Tertiary extension faulting occurred at shallow crustal levels. Below the microbreccia ledge marking the fault, the rocks grade from cataclasites to fine crush breccias that extend tens of meters into the lower plate. The cataclasites are deformed by grain-size reduction. Similar brittle behavior of minerals of contrasting rheology indicates brittle response throughout the deformation. The primary deformation processes in the fine crush breccias are linked, oriented shears and fractures, fold rotations, and pressure solution of porphyroclasts. The shears and fractures are highly asymmetric both in orientation and occurrence, and may have origins in the progressive rotation of a preexisting conjugate pair during subsequent extensional faulting. Superimposed rotational strain related to extensional faulting was accommodated on the favorably oriented synthetic conjugate, offsetting and pulling apart the antithetic conjugate. The shears cut both feldspar and quartz, often breaking perpendicular to cleavage and transecting grain boundaries, indicating deformation in the brittle field. Clorite, epidote, and sericite in pressure shadows and on shear surfaces indicates lower greenschist-grade conditions during faulting. Penetrative anastomosing fracture patterns are developed on all scales throughout the zone. Shears, striae, fibers, and folded porphyroclasts indicate northeast directed, higher over lower, movement of lower-plate rocks. Preexisting deformation may have localized strain, reduced the cohesion and friction values, and facilitated the extensional faulting at its characteristic low angle. This preexisting deformation however, was itself in the brittle field and may be a shallow crustal level expression of the Mesozoic mylonitic horizons utilized by detachment faults elsewhere in the Cordillera