MICROSTRUCTURAL ANALYSIS AND KINEMATICS OF QUARTZOFELDSPATHIC MIGMATITIC GNEISSES WITHIN THE WILDHORSE DETACHMENT, PIONEER MOUNTAINS, IDAHO

The Wildhorse detachment within the Pioneer metamorphic core complex in south-central Idaho preserves Eocene extensional strain and exhumation of ductile mid-crustal rocks. Quartzofeldspathic migmatitic gneisses of the Wildhorse Gneiss Complex experienced partial melting and high temperature metamorphism in the Early Eocene followed by late Eocene mylonitic deformation. To investigate changes in strain across the detachment from gneiss to mylonite, we collected samples from the top of the detachment footwall along a 300 m vertical transect perpendicular to the detachment. The migmatitic gneisses range from protomylonitic near the base of the detachment to localized mylonitic zones to strongly mylonitic near the top of the detachment.

Mylonitic fabrics preserve WNW-trending lineations defined by stretched quartz. Microstructures including asymmetric plagioclase porphyroclasts, S-C’ structures, and oblique foliations of recrystallized quartz record normal sense shear. Quartz grains occur as stretched ribbons containing subgrains and small, recrystallized grains at their margins. Ribbons of quartz wrap around feldspar porphyroclasts. Plagioclase grains display fracturing, kinking, and deformation lamellae. These microstructures indicate subgrain rotation recrystallization at greenschist to lower amphibolite facies conditions, at or near the brittle-ductile transition for feldspar. EBSD analysis of the mylonitic gneisses show strong quartz crystallographic preferred orientation (CPO) dominated by prism <a> slip, with minor rhomb <a> slip in some samples. In the protomylonitic rocks at the base of the detachment, the lowermost sample displays a cross-girdle pattern, whereas the next sample along the transect records prism <c> slip. Differential stresses calculated using the quartz recrystallized grain size piezometer range from 40–50 MPa toward the base of the transect and 90–100 MPa near the top of the transect. In comparison, a second Wildhorse detachment transect three km to the west within structurally higher quartzites and marbles records predominantly rhomb <a> slip. These results indicate the preservation of high temperature fabrics in the lowermost gneisses and an increase in differential stress and mylonitic deformation toward the top of the Wildhorse detachment.