DUCTILE-BRITTLE EXTENSIONAL DEFORMATION ALONG THE NORTHERN DEEP RIVER RIFT BASIN TERMINATION IN THE NORTHEASTERN NORTH CAROLINA PIEDMONT

The Deep River Mesozoic basin, a northeast-trending, east-side-down half graben, separates the North Carolina central and eastern Piedmont. The Durham basin with its clastic sedimentary infill is the northern extent of this rift-related structure. The northeast-trending, west-dipping, high-angle ductile-brittle Jonesboro normal fault (JBF) separates much of the Durham basin from greenschist and amphibolite facies metaigneous rocks of the Cary and Falls Lake terranes to the east. Recent NCGS STATEMAP mapping indicates that where the Durham basin trends abruptly north near Wilton, the informally named and west dipping ductile-brittle Fishing Creek normal fault (FCF) separates clastic rocks from Carolina terrane greenschist facies rocks, while the JBF maintains a northeast trend beyond Wilton.

Along the JBF, silicified cataclasite and mylonite ridges separate the Wilton Granite and amphibolite facies rocks of the Falls Lake, Crabtree, and Raleigh terranes to the east from Carolina terrane greenschist facies rocks and Ruin Creek Gneiss to the west, forming a high grade-low grade horst-graben structure. The main steep JBF tip line and shallow-dipping mylonitic to cataclastic splay zones merge into, and overprint the dextral Alleghanian Nutbush Creek fault zone at least 25 km northeast of Wilton.

Between the FCF and JBF, anastomosing zones of phyllonite and mylonite have steep west-dipping composite planar and dip-parallel linear stretch fabrics, and localized disharmonic folds that overprint mafic to felsic plutons. Brittle fabrics that transition with and overprint ductile fabrics are concentrated along the basin contact, and include tops-down vein fiber surfaces, cataclastic to brecciated phyllonite, and closely spaced fractures with slickenlines. Domains of silicification, seritization, saussuritization, and chloritization contribute to or overprint earlier fabrics. Both the JBF and FCF may have originated as ductile Alleghanian transpressional faults that progressed to ductile-brittle, Mesozoic transtensional or extensional ductile faults. The southern trace of the FCF appears to truncate against the JBF, suggesting it is an older graben-horst rift structure. Its northern tip lies 6 km north of the last outcrops of basin rocks, which may have extended farther northeast of the present basin.