Development of simple strike-slip fault zones, Mount Abbot quadrangle, Sierra Nevada, California
Simple strike-slip fault zones mark the third stage of faulting in granitic plutons in the Mount Abbot quadrangle of the Sierra Nevada of California. Deformation began with the opening of nearly vertical subparallel joints. These joints were filled mostly with epidote and chlorite, are up to a few tens of meters long, and typically are less than 1 cm wide. Next, some of these joints slipped left-laterally and became small faults. Small faults accommodated up to ∼2 m of displacement and are characterized by mylonitic fabrics and ductilely deformed quartz. Oblique fractures commonly developed near the ends of small faults and in many cases linked faults end-to-end. Simple fault zones developed as abundant oblique fractures linked small faults side-to-side. These fractures opened and were filled with chlorite, epidote, and quartz. Such fractures are scarce outside the two faults that mark the boundaries of a zone. Simple fault zones typically are 0.5-3 m wide, hundreds of meters long, and laterally displace dikes up to ∼10 m. Displacement is concentrated along the boundary faults, which are characterized by cataclastic textures and brittlely deformed quartz. The fault zones consist of noncoplanar segments a few tens of meters long that join at steps or bends. The segmentation reflects the initial joint pattern and indicates that fault zones grew in length as noncoplanar faults linked end-to-end. Away from bends, the most prominent internal fractures have straight traces and strike 20°-60° counterclockwise from the boundaries, whereas near bends they have gentle S-shaped traces and are nearly perpendicular to the boundaries. We suggest that as some faults linked to form longer structures, a "shear stress shadow" was cast over adjacent smaller faults, causing slip on them essentially to cease. In this manner, displacement progressively became localized on the longer faults and fault zones. If the regional shear strain rate remained constant during this process, then the shear strain rate across the still active faults must have increased. This may have caused cataclastic textures to develop in the boundary faults.