MEADOW FORK AND RECTOR BRANCH MYLONITE ZONES SOUTHWEST OF THE HOT SPRINGS WINDOW, WESTERN NC

We present recent NCGS mapping in the Lemon Gap and Spring Creek 7.5' quadrangles and detailed structural observations along two traverses crossing the Meadow Fork and Rector Branch mylonite zones SW of the Hot Springs window in western NC. Deformed rocks associated with the Hot Springs window vary from cataclasites to low grade mylonites. On the S side of the window, Precambrian Spring Creek gneiss overlies Cambrian metasediments along the Rector Branch mylonite zone. Mylonitic foliation strikes NE-SW to ENE-WSW with SE-plunging lineations. The mylonite zone truncates lithological contacts and other mylonitic zones within the Spring Creek gneiss at high angles. The Meadow Fork mylonite zone defines the southwestern corner of the window, where Spring Creek gneiss and Max Patch granite overly the metasediments. Mylonitic foliation strikes NNE-SW. Locally, minor folds plunge gently NNW and lineations plunge SE to E.

The two mylonite zones overlap SW of the window, with Max Patch and Spring Creek units structurally below and above, respectively. The merged zone includes slices of mylonitized Max Patch granite, Spring Creek gneiss, metasediments, and a highly seritized mylonite unit. Mylonitization increases in intensity across strike over 500 meters approaching the mylonite zone. By ~1 km SW of the window, distributed mylonitic foliation in the Max Patch granite and Spring Creek gneiss, and foliation in the merged mylonite zone, all strike NE-SW and dip moderately to the SE. Microstructural shear sense indicators include S-C fabric defined by mica minerals and asymmetric porphyroclasts. Work to date suggests that, overall, mineral lineations and stretching lineations plunge SE and shear sense is SE-side-up. Locally intensive seritization, cataclasis of feldspar grains, and evidence for dynamic recrystallization of quartz are consistent with greenschist facies conditions for deformation.

Some observations indicate that deformation varied from SE-side-up, dip-slip shear in the merged mylonite zone, at least locally. Examples include (1) lineations in higher strain metasedimentary lenses locally pitching 10-20° to the SW, indicating a component of oblique motion; and (2) conjugate extensional microfaults in feldspar, indicating a component of shortening perpendicular to the foliation.