Nucleation and growth of retrograde shear zones: an example from the Needle Mountains, Colorado, U.S.A.

Abstract

Brittle and ductile deformation features are heterogeneously developed in late Proterozoic retrograde, normal-slip shear zones that cross-cut amphibolites and quartzo-feldspathic gneisses in the Needle Mountains. Replacement of amphibolite-grade minerals by greenschist facies assemblages is spatially related to transgranular fractures that cross-cut the older gneissosity. Steeply dipping, chlorite-lined slip surfaces are associated with chlorite + quartz ± muscovite-filled dilational jogs and narrow breccia zones. Arrays of slip surfaces are transitional into similarly oriented phyllonite zones, within which the foliation locally overprints early cataclastic zones. The nucleation of these shear zones under greenschist facies conditions involved fluid infiltration along early transgranular fractures and hydration of Fe-Mg-bearing minerals. Initial displacement occurred by slip along these fractures and local cataclasite development. Fluids were episodically drawn into dilatant sites along these slip surfaces. A transition to plastic deformation resulted directly from the metamorphic breakdown of feldspars and amphiboles to phyllosilicates, which allowed strain localization into narrow phyllonite zones. Thus, dilatancy produced by early brittle deformation appears to be the main process by which fluids infiltrate into developing greenschist-grade, retrograde shear zones. Reaction-softening via hydration reactions causes a brittle-to-plastic transition and results in strain localization.