SHEAR ZONE ROOTS IN THE MIDDLE CRUST: THE TRANSITION FROM PARTITIONED DEFORMATION TO PENETRATIVE DUCTILE FLOW IN THE NORTHERN WET MOUNTAINS, CO

Proterozoic gneisses of the northern Wet Mountains, Colorado, contrast in metamorphic grade and structural style from those in adjacent ranges. The Wet Mountains form part of a ca. 1.4 Ga thermal culmination and mark a transition from deformation partitioned upon large discrete shear zones, characteristic in the Front Range (Shaw et al., 2001; in press), to distributed deformation in a partial melt / magma- rich horizon. In the northern Wet Mountains, non-migmatitic shear zones exposed in the Arkansas River Canyon contrast with high strain rocks in Copper Gulch, 6 km to the south, that are flooded with leucosome and heterogeneous granite intrusions. Fabric geometries, crosscutting relationships and small scale structures at these two sites were measured and documented in detail in order to understand the transition from localized deformation upon steep high strain zones to partial melt dominated flow.

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.