INTUSION OF THE ST. KEVIN BATHOLITH INTO THE HOMESTAKE SHEAR ZONE, NORTHERN SAWATCH RANGE, CENTRAL COLORADO; EFFECTS ON THE MECHANICS OF AN ACTIVELY DEFORMING BRITTLE-PLASTIC SHEAR ZONE

The Homestake shear zone (HSZ) is one of the principle structures of the Colorado Mineral Belt (CMB), and is located in the northern Sawatch Range of central Colorado. First studied by Ogden Tweto in the 1960’s, the geology of the HSZ has been well characterized. Originally described as a braided system of shear zones overprinted by mylonites and psuedotachylites, the HSZ is now understood to have gone through two distinct phases of deformation; a 1.8-1.6 Ga high-temperature event that generated the gneissic fabrics, and a second 1.45-1.35 Ga brittle-plastic phase that resulted in the extensive, through-going mylonite/ultramylonite and psuedotachylite shear zones. Research on the later has yielded vital information about the nature of rocks and the mechanics of deformation at the brittle-plastic transition, as well as the nature of the coupling between these two deformation regimes. Now, research on the interactions between the HSZ and St. Kevin batholith (1.4 Ga) has revealed new information about how these two dynamic systems interact with each other.

Intrusion of the St. Kevin into the actively deforming HSZ had a profound effect on the mechanics of the shear zone. Magmatic, sub-magmatic, and solid-state deformational features within the margins of the pluton and rarity of the mylonite/ultramylonite shear zones, so characteristic of the HSZ elsewhere, suggest the accommodation of strain by the St. Kevin during the time of its emplacement and/or the increased thermal gradient from the St. Kevin altered the rheology of the host rock, making the strain more diffuse and preventing the formation of the more through-going mylonite/ultramylonite shear zones. The presence of extensive psuedothachylite shear zones indicate strain was still high, or temperatures still low enough, to allow brittle failure of the host rocks.