MICROSTRUCTURE AND KINEMATIC VORTICITY OF KINNIKINIK QUARTZITE, PIONEER METAMORPHIC CORE COMPLEX, IDAHO

The Pioneer metamorphic core complex is a domal structure that developed during Eocene extensional deformation. In the northern part of the complex, a N-dipping, low-angle detachment fault (Wildhorse detachment), separates amphibolite facies metamorphic rocks in the footwall from unmetamorphosed rocks in the hanging wall. At Kane Creek, within the Wildhorse detachment, quartzite, marble, and granodioritic rocks display mylonitic structures including S-C fabrics and asymmetric porphyroclasts. Within the Kinnikinic Quartzite unit in Kane Creek the mylonitic foliation strikes NE-SW and moderately dips to the NW, and the stretching lineation trends NW. We performed microstructure and EBSD analyses on these quartzites in order to decipher the nature of detachment activation during exhumation of the Pioneer MCC. Ten oriented samples of quartzite were collected along a transect across an ~100 m thick shear zone within the Wildhorse detachment of Kane Creek. The samples typically contain ~85% quartz, ~10% K-feldspar porphyroclasts, and ~5% mica fish. EBSD data of quartz show a pattern that indicates simple shear deformation with top-to-the-NW shear sense. Analyses using rigid grain technique (Wallis et al., 1993) and rigid grain net (Jessup et al., 2007) indicate a kinematic vorticity number (Wk) of ~0.75–0.8 which corresponds to ~40–45% shortening in the Wildhorse detachment during extensional exhumation. In addition, aspect ratio vs. angle-to-foliation plots indicate a concentration of data points in a linear pattern that reflects the preferred orientation of synthetically rotated feldspar porphyroclasts. This line trends from ~+30° for clasts with a low aspect ratio to ~10° for clasts with an aspect ratio of 3. This linear pattern persists across the section, suggesting that the type of flow in this detachment mylonite favored this stable orientation.