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Field rheology and structural evolution of the Homestake shear zone, Colorado

¹ Department of Earth Sciences, Montana State University, Bozeman, MT 59717, U.S.A.
² Department of Geology and Physical Sciences, Concord University, Athens, WV 24712, U.S.A.

^* Correspondence should be addressed to: colin.shaw1{at}montana.edu

Proterozoic tectonites within the Homestake shear zone (HSZ) in the northern Sawatch Range, Colorado, record two major cycles of progressive deformation under contrasting P–T conditions. Field relations and microstructures in these tectonites can be used to constrain the geometry, kinematics, and rheology of rocks in the HSZ during these events. The first deformation cycle occurred 1.7 Ga at temperatures near the granite solidus and produced structures indicative of pervasive viscous flow at low differential stress levels. The second deformation cycle at 1.4 Ga resulted in localized plastic and brittle failure concentrated in relatively narrow shear zones and seismogenic faults. This second deformation occurred at temperatures of 350–450°C and produced mylonite, ultramylonite, and pseudotachylyte. Overprinting relationships between brittle-frictional faults and plastic shear zones suggest that both modes of deformation were active at about the same time. Thus, the latter deformation cycle probably occurred near the brittle-plastic transition. Simple calculations using standard rheologic relationships show that tectonic loads of 300–400 MPa—sufficient to drive brittle failure—could have been transmitted by ductile ultramylonite zones deforming at relatively high strain rates (on the order of 10^–9–10^–11). Thus, plastic shear zones and brittle-frictional faults probably acted together as a dynamically coupled deformation system.

Key Words: Homestake shear zone • Colorado mineral belt • brittle-plastic transition • structural geology • rheology • faults • mylonite • ultramylonite • pseudotachylyte