GEOMETRY AND KINEMATICS OF SEISMIC RUPTURE IN THE MIDDLE CRUST: THE HOMESTAKE SHEAR ZONE IN THE WESTERN SAWATCH RANGE, COLORADO

The Proterozoic Homestake shear zone (HSZ) presents a unique case study of a mid-crustal seismogenic fault system across an exposure area spanning more than 25 km along strike. The HSZ is well known in the NE Sawatch Range from extensive geologic mapping, but the extent of the HSZ is not well established in the western Sawatch Range. The HSZ generally bounds part of the NW margin of the Colorado mineral belt, a diffuse zone of Laramide (~85-40 Ma) plutons and mineral deposits. In the NE Sawatch Range, the HSZ consists of a subvertical, dip-slip mylonite and ultramylonite zone that progressively developed at ~1.4 Ga in a mid-crustal setting in supracrustal gneisses. This >20-m-wide zone strikes 055˚ and can be traced southwestward for 17 km along strike from the Eagle River canyon near Hornsilver Mountain on the NE, to Fancy Pass in the north-central Sawatch Range. SE of this zone, dextral strike- and oblique-slip pseudotachylyte zones parallel the mylonite across a width of 2–4 km. Field relations, including the presence of recrystallized and mylonitic pseudotachylyte in and near the ultramylonite suggest these tectonites are at least in part coeval.

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.