DUCTILE-BRITTLE RELATIONSHIPS ON THE WESTERN FLANK OF THE RALEIGH METAMORPHIC BELT, NORTH CAROLINA

NCGS STATEMAP and USGS EDMAP 1:24,000-scale mapping on the western flank of the Raleigh metamorphic belt documents a close spatial association between strike-slip and normal, ductile to brittle faults in the Raleigh and Henderson 100K sheets. Alleghanian dynamo-thermal metamorphism overprints Late Proterozoic to Cambrian volcanogenic rocks in the Carolina, Falls Lake, Crabtree, Raleigh, and Spring Hope terranes. Subvertical to steeply west-dipping, ductile strike-slip faulting produced northeast-trending, subhorizontal stretch lineation and dextral kinematic indicators along anastomosing strands of the eastern Piedmont fault system. Mineral assemblages in mylonite and phyllonite indicate that these intra- and inter-terrane, high strain zones formed under amphibolite to greenschist conditions.

Ductile and brittle normal faults form discrete to interconnected zones. They define intra-terrane faults having similar trends to Alleghanian faults, and overprinting inter-terrane faults that lie parallel to or crosscut the dextral faults. Greenschist to sub-greenschist composite fabrics, boudinage, and slickenlines indicate oblique northwest-trending to down-dip normal displacements. Locally, fault breccia and fracture sets, both silicified, and hydrothermal alteration ornament the faults. They are identified within the crystalline western flank adjacent and up to 10 km east of the Mesozoic Deep River rift basin.

Based upon the proximity of dextral and normal faults, an antecedent relationship between late Paleozoic structures and early Mesozoic rift basin development exists. Similar relationships occur northward along the Hylas fault zone (Bobyarchick and Glover, 1979). Preliminary ⁴⁰Ar/³⁹Ar study results yield an age of 255 ± 2 Ma for muscovite within one of the ductile-brittle normal faults. This ductile-brittle transition may be a progressive event reflecting a plate motion change from transpressional collision of Laurentia, the Carolina zone, and Gondwana to a Pangean transtensional breakup, or it may represent a distinct, very late Paleozoic to early Mesozoic extensional overprint upon a Paleozoic transpressional, structural culmination which was exhumed due to crustal thinning and uplift during rift basin development.