Alleghanian tectono-thermal evolution of the dextral transcurrent Hylas zone, Virginia Piedmont, U.S.A.

The Hylas zone is the northernmost segment of the Eastern Piedmont fault system and lies in the Grenville Goochland terrane, Virginia. Like the other major segments, the Nutbush Creek zone, the Hollister zone and the Modoc (Irmo) zone, the Hylas zone experienced late Paleozoic dextral transcurrent shearing. Early deformation in the zone was ductile and produced type I and type II S-C mylonites under amphibolite-grade metamorphic conditions. Later deformation was at the brittle-ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons. The feldspar exhibits a strain-dependent sequence of microstructures, including kink-bands, antithetic extensional microfaults (pull-apart), bends in the microfaults by development of transverse fractures and microboudinage. Correlation of mineral mechanical response with a thermal-decay curve based on isotopic mineral ages yields a temporally constrained deformation history. Ductile dextral shearing occurred subsequent to the intrusion of the 330 Ma Petersburg granite and passed into the brittle-ductile transition by approximately 260 Ma. Dextral faulting terminated before 240 Ma. The Appalachian dextral transcurrent faulting event therefore continued through Permian in some areas.