GEOMETRY AND KINEMATICS OF SEISMIC RUPTURE IN THE MIDDLE CRUST: THE HOMESTAKE SHEAR ZONE IN THE WESTERN SAWATCH RANGE, COLORADO
In the western Sawatch Range, existing mapping (Tweto et. al., 1978; Tweto, 1979) shows the HSZ to continue southwestward at ~050˚ for another 30 km across the entire range to Hunter Creek at the eastern margin of the Aspen 7.5’ quadrangle. This area is dominated by a ~1.4 Ga quartz monzonite in which we have not observed tectonites typical of the NE Sawatch Range. Instead, our recent mapping shows the HSZ to abruptly bend by ~25˚ to an 080˚ strike S-SE of Fancy Pass in the north-central Sawatch Range. At present, our work shows the pseudotachylyte to extend westward for at least another 8 km, and the ultramylonite zone to significantly narrow (<2 m) and extend westward for 3 km. In the area of the bend, the pseudotachylyte is present in top-to-south, dextral reverse faults and dextral strike- and oblique-slip faults. The reverse faults are concordant with a N-dipping foliation (50–60˚), whereas the strike-slip faults are in subvertical rocks. The HSZ therefore preserves a broad, complex record of partitioned displacement, including: (1) a creeping subvertical, dip-slip ultramylonite zone, and (2) more than a dozen seismogenic rupture zones partitioned into reverse and strike-slip faults.