MESOSCOPIC STRUCTURES IN PSEUDOTACHYLYTE FROM THE HOMESTAKE SHEAR ZONE, COLORADO

Pseudotachylyte in the Homestake shear zone crops out as dark, aphanitic veins that have a distinct linear map- and outcrop-scale geometry that suggests they are a product of seismogenic faulting. Individual pseudotachylyte fault veins are commonly as much as several cm thick and are exposed within eight discrete, NE-striking fault zones that discontinuously crop out for more than 7.3 km along strike. Each individual fault zone is typically less than 5 to 28 m wide and consists of one or more parallel to sub-parallel fault veins, which may be solitary, paired, or complexly interconnected in a network. The fault zones systematically diverge and thin in width to the northeast; cumulative thickness of pseudotachylyte veins across each fault zone also decreases northeastward from a maximum of 57.2 cm to < 2.8 cm. If the majority of the system formed in response to a single episode of slip, the multi-km-scale map relations permit: (1) interpretation of rupture directivity, and (2) quantitative assessment of dynamic processes associated with rupture, including seismic moment, energy release, and 1-D heat flow. In addition to the systematic regional distribution of pseudotachylyte-bearing fault zones, some meso-scale structures are regularly distributed along strike. For example, some fault veins contain light-gray bands within the otherwise dark matrix. In thin section, the bands are differentiated from adjacent pseudotachylyte by the presence of a higher density of magnetite octahedra within the matrix. These bands are locally exposed as wall-parallel isoclinal folds that commonly exhibit hinges that are thicker than limbs. These mesoscopic folds are mostly observed in veins thicker than ca. 1 cm. Most are located near the transition between thin and thick (> 1 cm) fault-vein segments. Interestingly, at least seven well-exposed folds along two fault zones verge in the same direction (southwestward), suggesting a possible common flow-related origin that will be explored. Other meso-scale structures include pseudotachylyte “breccia”, which appears to be preferentially developed where fault veins cross-cut host-rock foliation.