PLASTIC OVERPRINTING OF QUARTZ SLICKENFIBERS ALONG THRUST FAULTS IN THE SANGRE DE CRISTO MOUNTAINS IN SOUTHERN COLORADO

Slickenfibers often develop along active fault surfaces and therefore are a hallmark of brittle deformation. Here we describe slickenfibers associated with minor thrust faults in Pennsylvanian sandstone from the Sangre de Cristo Mountains in southern Colorado. These faults are located adjacent to the WSW-dipping Crestone thrust, which records basement-involved shortening during the Laramide and/or Ancestral Rocky Mountains orogeny. The slickenfibers record numerous episodes of crack-seal, and notably many of them exhibit evidence for plastic overprinting, such as dynamic recrystallization. The degree of recrystallization is variable throughout our samples, with some domains that are completely recrystallized with development of a polygonized texture. Grain size in the fully recrystallized samples is ~10–20 µm, implying stresses significantly below what would be expected at the brittle plastic transition in a thrust regime. We use electron backscatter diffraction (EBSD) analysis to characterize the crystallographic orientations and microstructures associated with this plastic deformation. Preliminary results in at least one sample show a degrading of the quartz crystallographic preferred orientation during dynamic recrystallization, potentially indicating a transition to a grain-size sensitive diffusion creep mechanism. Plastic strain and the degree of dynamic recrystallization in the slickenfiber increases towards older crack-seal domains, and recrystallized domains directly above the sandstone host rock are associated with fine-grained mica fabrics with an oblique preferred orientation consistent with thrust shear. Our analysis provides insight to the brittle-plastic deformation conditions, fluid rock conditions, and rheology of these shear zones.