QUARTZ CRYSTALLOGRAPHIC FABRICS FROM THE NORTHERN MYLONITE ZONE OF THE CHEYENNE BELT, SE WYOMING

The Cheyenne belt, exposed in SE Wyoming, is a system of high-strain zones interpreted as the suture zone between the <1.8-Ga Colorado province and the >2.5-Ga Wyoming province. These high-strain zones exhibit steeply dipping foliations and dominantly down-dip mineral lineations or apparent flattening strain (S-tectonites). Numerical simulations suggest that this strain geometry may record transpressional deformation with a large strike-slip component. To test this transpression hypothesis, we measured quartz CPO in quartz-rich samples from the northern mylonite zone on the west side of the Medicine Bow Mountains to look for shallowly plunging a-axis maxima indicative of large components of horizontal simple shear during transpression.

In the study area, the northern mylonite zone appears as a 250–300-m-wide, NE-trending domain of intensely foliated and lineated quartzite and amphibolite. Average orientations of foliations and lineations in this area are 049, 72-NW and 002, 66. Foliation and lineation intensity diminishes abruptly along the northern margin of the high-strain zone, but is gradational along the southern margin. The map pattern suggests isoclinal folding of quartzite and amphibolite within the zone, but no outcrop-scale folds were observed in the area.

Quartz CPOs were measured in thirteen samples using SEM-EBSD. In most samples from the high-strain zone, quartz c-axis fabrics exhibit well-developed single girdles and quartz a-axis fabrics exhibit strong steeply plunging maxima that are distinctly asymmetrical about the foliations. A sample collected 190 m outside of the high-strain zone also yields a strong CPO despite the absence of a strong foliation and lineation in hand sample. CPOs from six samples display a triclinic symmetry in the foliation-perpendicular, lineation-parallel reference frame. The central c-axis girdles and the a-axis maxima in these samples are offset 7–20° from the centers and primitives of the plots respectively. Nevertheless, the consistent strong asymmetry of the quartz CPOs combined with the steeply plunging a-axis maxima argue against the transpression hypothesis, and instead indicate that this part of the northern mylonite zone records SE-side-up, oblique-slip motion with a large dip-slip component consistent with existing interpretations.