DUCTILE-BRITTLE FABRIC DEVELOPMENT IN CAROLINA TERRANE ROCKS ALONG THE NORTHWESTERN BOUNDARY OF THE DURHAM SUB-BASIN, DEEP RIVER TRIASSIC RIFT BASIN, NORTH CAROLINA

In the northeastern NC Piedmont, the greenschist facies Hyco, Aaron, and Virgilina Formations define the Late Proterozoic Virgilina sequence in the Carolina terrane, the largest subduction-related lithotectonic element of the peri-Gondwanan Carolina Zone superterrane. Recent NCGS STATEMAP efforts have focused upon the Stem, Lake Michie and Rougemount 7.5-minute Quadrangles straddling the boundary between Virgilina sequence rocks to the west and sedimentary rocks of the Durham sub-basin of the Deep River Triassic rift basin to the east. A suite of 613 Ma and younger diorite-tonalite and granodiorite plutons have been mapped whose western wallrocks are crystal lithic dacite tuffs, lavas, and domes, and lesser amounts of microdiorite-andesite and volcanogenic sedimentary rocks of the 615 Ma upper Hyco Formation. The 578-551 Ma Virgilina deformation overprints the Virgilina sequence and is characterized by gently plunging, overturned, SE-verging folds, N-trending, SE-dipping axial surface cleavage, and ductile-brittle faults due to intra-arc basin closure. Plutons do not develop the folds and penetrative cleavage.

Unlike its normal faulted eastern boundary, the northwestern boundary of the Durham sub-basin is a noncomformity as arkosic sandstone overlies the Hyco Fm. and associated plutons. However, within these rocks, overprinting high strain and brittle fracture zones are oriented parallel to the NE trend of the basin. These domains display NE-SW trending, steeply dipping phyllonite foliation and transitional-tensile and shear fracture planes containing a down-dip mineral lineation. They are also associated with sulfide-rich mineralization, silicification, epidotization, and cataclasis, and texturally differ and overprint Virgilina deformation fabric. While these fabric element orientations mimic the Virgilina deformation, ductile-brittle zones having similar fabric development and orientation also occur along the eastern boundary of the Durham sub-basin and are linked with the Fishing Creek and Jonesboro normal faults. These zones may be manifestations of Permo-Triassic rift deformation on the western side of the sub-basin, and may be using Virgilina foliation anisotropy to guide their development and contribute to brittle failure that produced the Deep River Triassic basin.