MACRO- TO MICRO-SCALE PARTITIONING OF STRAIN AND KINEMATICS WITHIN THE CHUNKY GAL MOUNTAIN FAULT, BLUE RIDGE PROVINCE, SOUTHERN APPALACHIAN MOUNTAINS

The type exposure of the terrane-bounding Chunky Gal Mountain Fault (CGMF) preserves dynamic shear zone fabrics in the Southern Appalachian Blue Ridge Province. In this roadcut, the CGMF separates porphyroclastic biotite gneiss from amphibolite gneiss and is defined by several NE-trending, steeply SE-dipping, ~1m-wide, mylonitic, shear zones with evidence of outcrop- to micron-scales partitioning of strain and kinematics.

In outcrop view, foliation deflected across each shear zone indicates apparent normal (SE-side-down) offset. We observe both steep and shallow elongation lineations; locally both appear to be preserved. Here we focus on samples from 2 shear zones, referenced ‘L’ and ‘J’. Shear zone J lies at the lithologic boundary. All shear zone L samples preserve shallow lineations; shear zone J samples preserve both steep and shallow lineations. In top-down view, asymmetric fabrics in samples with shallow lineations indicate sinistral kinematics in some thin sections and dextral in others. Kinematic indicators associated with steep lineations show SE-side-down movement. Dynamically recrystallized quartz textures suggest regime 2/3 dislocation creep, with mostly regime 3 in higher strain domains, possibly indicating higher strain conditions at higher temperatures.

Quartz CPO’s from several domains within each sample, collected using Electron Backscatter Diffraction (EBSD), suggest partitioning of pure vs. simple shear within the zones. Although we observe two lineations, quartz CPO patterns appear to be associated with only the shallow lineation. Possible explanations: a) shallow movement followed steep movement and overprinted earlier CPO fabrics; b) CPO was formed by earlier shallow movement, but little quartz recrystallization was associated with later steep movement.

For most samples, EBSD kinematics are consistent with textural kinematics; however, in two samples with shallow lineations they disagree. Analysis of individual domains in a sample with sinistral fabrics may suggest differing CPO kinematics for domains with c-axis clusters typical of prism <a> vs. rhomb <a> slip. Detailed CPO mapping may help resolve the discrepancy in kinematics and clarify possible partitioning of strain, kinematics and deformation conditions that are suggested by this data.