A Quantitative Microstructural Analysis of Quartz Slickenfibers from the Crestone Thrust Fault in the Sangre de Cristo Mountains, Southern Colorado

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

Presenting Author:

Authors:

Abstract:

We investigate fault rock from a sample associated with the WSW-dipping Crestone thrust in the Sangre de Cristo Mountains in Colorado, which preserves shortening developed during Laramide and/or Ancestral Rocky Mountain orogenesis. We combine optical microscopy with quantitative microstructural data collected using electron backscatter diffraction (EBSD) to document the structural evolution of quartz slickenfibers preserved along a minor fault in the footwall of the Crestone thrust. Fluid and solid inclusion trails oriented at high angles to long dimension of the slickenfibers and spaced about 10-15 microns apart are interpreted to record growth by the crack-seal mechanism. Quartz within the fibers is generally oriented with the crystallographic c-axis oriented parallel or subparallel to the inferred direction of slip. The slickenfibers show evidence for plastic overprinting that varies along the length of the fiber. Younger, unrecrystallized fiber grades into elongated, sheared grains that themselves grade into material fully recrystallized by subgrain rotation recrystallization. Quartz c-axis distributions in the recrystallized material are dispersed around the initial fiber orientations, suggesting that grain boundary sliding worked to scatter crystallographic orientations. Misorientation axis distributions from partially recrystallized grains indicate plastic deformation occurred primarily by basal <a> slip, with possible minor contribution of rhomb <a>. Application of an EBSD-based quartz piezometer gives a mean recrystallized grain size of ~7.5 microns, which corresponds to a differential stress of about ~140 MPa. Given the progressive but incomplete nature of plastic overprinting of the slickenfibers, we infer that deformation must have proceeded at conditions near the brittle-plastic transition. The alternating brittle and plastic deformation may relate to variable strain-rates, with crystal-plastic processes dominating when strain-rates are low, and brittle deformation corresponding to seismic slip.