BRITTLE AND PLASTIC DEFORMATION IN THE HOMESTAKE SHEAR ZONE, COLORADO: IMPLICATIONS FOR DEFORMATION PROCESSES IN THE MIDDLE CRUST AND THE EVOLUTION OF THE COLORADO MINERAL BELT

The Homestake shear zone (HSZ) provides an outstanding case study of the evolution of a persistent tectonic zone in the continental lithosphere and an opportunity to investigate mid-crustal processes in deeply exhumed rocks from near the base of the seismogenic zone. The HSZ is a ~10 km wide zone of sub-vertical, NE-trending, anastomosing mylonite, pseudotachylyte, and cataclasite zones and is one of the major shear zones of the Colorado mineral belt (CMB) shear zone system. At different times in its history the CMB has been the locus of intracontinental deformation, plutonism, and mineralization and coincides with major mid-crustal gravity and upper mantle velocity anomalies. The currently exposed level of the HSZ evolved as a brittle-plastic shear system in the middle crust. Mylonite, ultramylonite, and pseudotachylyte zones in the HSZ record a complex interplay of slow plastic creep and seismogenic faulting that overlap in time and space. The shear zone forms part of the northern margin of the ~1.40 Ga St. Kevin batholith and the complex contact zone suggests synkinematic emplacement. In-situ electron microprobe dates on monazite within mylonite and ultramylonite confirm a minimum age for ductile shearing of ~1.40 Ga. At least one major >25 km-long zone of en-echelon pseudotachylyte zones parallels the ductile shear zones and other zones of uncertain extent have also been identified. Microfaults including pseudotachylyte and microbreccia seams are closely associated with ultramylonite. Field and microstructural relationships indicate that at least some of the pseudotachylyte and microbreccia is broadly coeval with the ultramylonite and suggest that deformation may have cycled between plastic deformation and seismogenic faulting. Sixteen ⁴⁰Ar/³⁹Ar dates and evidence for incipient plasticity in feldspar indicate that temperatures in the country rock were probably ~500-550° C during deformation at ~1.4 Ga and cooled through 300° C by ~1.38 Ga. Melt temperatures are estimated to be 1050-1200° C in parts of the main pseudotachylyte zone. A preliminary kinematic model interprets the HSZ and associated structures as a complex transpressive system in the middle crust where plutonism, shearing, and faulting interact at seismogenic to orogenic time scales.