Story posted October 25, 2010
Summer 2010 brought together faculty and students from Bowdoin College and Colby College to study quartz fabrics from the Cheyenne Belt, an ancient tectonic suture zone in Wyoming. In Bowdoin's SEM lab, Alex Roberts-Pierel, Bowdoin '12, and Emma Beck, Colby '12, ran Electron Backscatter Diffraction (EBSD) analyses on 13 samples to evaluate the crystallographic preferred orientations (CPO) of quartz. The research was advised by Prof. Rachel Beane, Bowdoin, and Prof. Bill Sullivan, Colby, and supported by a Colby-Bates-Bowdoin Collaborative Faculty Enhancement Grant, funded by the Mellon Foundation. Alex presented the research results November 1 at the 2010 Geological Society of America Annual Meeting in Denver (abstract below).
QUARTZ CRYSTALLOGRAPHIC FABRICS FROM THE NORTHERN MYLONITE ZONE OF THE CHEYENNE BELT, SE WYOMING
Sullivan, W., Roberts-Pierel, A. (presenter), Beck, E. and Beane, R.
Geological Society of America Abstracts with Programs, v. 42, no. 5, p. 261.
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.