SHEAR ZONE ROOTS IN THE MIDDLE CRUST: THE TRANSITION FROM PARTITIONED DEFORMATION TO PENETRATIVE DUCTILE FLOW IN THE NORTHERN WET MOUNTAINS, CO
In the Arkansas River canyon, NNW- to N-S-striking shear zones in Five Points Gulch and Sheep Basin exhibit steep to moderately steep high strain fabrics and share a NE- to NNE-plunging mineral lineation, defined in the Five Points shear zone by prismatic sillimanite and symmetrical strain shadows upon garnet. The shear zones, developed in monotonous gray quartz gneisses, cut across WNW-ESE to NW-SE polyphase fabrics and map-scale, ESE- to SE-plunging folds outside the zones. Asymmetrical folds and rotated boudins record reverse oblique motion on the Sheep Basin shear zone. A late-tectonic pegmatite dike involved in the zone yields a U-Pb zircon age of 1430±5 Ma, constraining the time of shear zone activity. Work in progress in Copper Gulch to the south, at a deeper structural level, finds comparable rocks invaded by granite sheets and distinct leucosomes. Dominant fabrics remain steep but are variable in orientation, host gneisses and granites are closely interfolded, and fold trends seem variable; suggesting gradual onset of melt-dominated behavior as framework gneisses weakened. Reverse movement on discrete zones, as at Sheep Basin, potentially drew incompetent, melt rich materials in to the shear zones, acting to progressively broaden the deformation zones and give way to distributed deformation.
Shaw, C.A.et al., 2004,in K.E. and Keller, G.R., eds., AGU Monograph, in press.