MICROSTRUCTURAL OBSERVATIONS OF SHEAR ZONES IN THE ROSS OROGENIC BELT, SHACKLETON GLACIER AREA, CENTRAL TRANSANTARCTIC MOUNTAINS, ANTARCTICA

The Ross Orogenic belt represents a major Neoproterozoic-Early Paleozoic mobile belt that developed during the assembly of the supercontinent Gondwana. Structural, stratigraphic, and magmatic assemblages within the Ross Orogenic belt are typically viewed as the remnants of a continental marginal arc that formed in response to oceanic lithosphere subduction along the margin of the East Antarctic craton. Previous authors have explored kinematic models for Ross orogenesis that differs from orogen normal contraction, transpression, and translation based on stratigraphic, isotopic, and structural data. One key reason why previously proposed kinematic models differ so markedly is that virtually no data exists on the kinematics of deformation along the majority of the belt. We conducted a micro-structural analysis to determine the movement directions and deformation conditions of several major shear zones within the Central and Southern Transantarctic Mountains. Oriented thin sections from a shear zone cutting granitic rock in the O'brien Peak area are characterized by S/C fabrics and mica fish, which collectively indicate that movement involved a component of left-lateral parallel to the grain of the orogen. Feldspars in thin section exhibit subgrain boundaries, and patchy undulatory extinction, while quartz exhibits sutured re-crystallized boundaries and undulatory extinction. Collectively, these observations indicate that deformation occurred around temperatures of 300-350°C. Oriented thin sections from two shear zones located on opposite sides of Shackleton Glacier contain steeply plunging stretching lineations and are characterized by strain caps and shadows that flank larger porphyroclasts. Quartz exhibits deformation banding, undulatory extinction, and sutured re-crystallized boundaries. By contrast, feldspar porphyroclasts show rigid behavior in the form of microfaults and fracturing. These differences indicate deformation occurred at temperatures around 250-300°C.