DELINEATION OF A KILOMETER-SCALE COSEISMIC RUPTURE AT HYPOCENTRAL DEPTHS USING PSEUDOTACHYLYTE VEIN GEOMETRY: AN EXAMPLE FROM THE NORTHERN SAWATCH RANGE, COLORADO

Pseudotachylyte fault veins record frictional melting during coseismic slip and offer unique insight into the types and scales of lithologic heterogeneities that control earthquake ruptures. To date, most field studies of pseudotachylyte veins have focused on meter- to decameter-scale field relations. However, detailed field mapping at 1:4900 scale within the Homestake shear zone in central Colorado defines a remarkable system of pseudotachylyte fault veins that exceed 25 km in cumulative length. The system consists of eight discrete northeast-striking faults that are exposed for 7.3 km along strike and are hosted within an early Proterozoic biotite gneiss. The faults splay to the northeast from a narrow 170 m wide zone to a maximum known cross-strike width of 2.3 km. The width of individual pseudotachylyte fault veins systematically decreases northeastward from as much as 39 cm to less than 1.5 cm. The faults exhibit dextral displacement of 0.4-1.2 m. These relationships suggest that the rupture was a strike-slip fault that nucleated on the southwestern end of the fault system and propagated to the northeast. An absence of brittle cross-cutting relationships within and among individual fault strands suggests that most, if not all, of the entire system was produced as a result of a single seismic event. The geometry of the rupture is strongly controlled by the structure of the host rock. The faults clearly have propagated parallel to the primary vertical foliation within the gneiss. Further, map-scale, rootless isoclinal folds with vertical axes define a second, but more substantial control on the overall geometry of the fault system. Most faults have reactivated the attenuated limbs of the folds which are interpreted to be parasitic to a larger regional fold. Careful regional mapping of pseudotachylyte may thus serve as a unique paleoseismic tool that provides a direct glimpse at processes that occur near the earthquake source.