Ductile Shear Zones

Abstract

Mylonite with a big delta mantled porphyroclast of K-feldspar in a matrix of quartz, mica and feldspar. Mylonite is derived from a porphyritic granite characterized by large K-feldspar phenocrysts (up to 20 cm long). Medium-grade conditions can be inferred from the recrystallization of K-feldspar along the rims of porphyroclast. Shear sense is dextral as indicated by the stair stepping to the right across the delta structure. Section subparallel to the lineation and normal to the foliation. Width of view 154 mm. Location: Sierra de Velasco, La Rioja Province, NW Argentina; GPS point (WGS84): S 28°36′26.2″ - W 67°10′38.5″; Rock type: Mylonite; Formation name: Ortogneiss Antinaco (TIPA Shear Zone); Age (relative): Post-Lower Ordovician / Pre-Carboniferous; Facies/grade: medium grade.