RHEOLOGIC EVOLUTION DURING THRUSTING WITHIN CARBONATE FAULT ROCKS

Determining the changing deformation processes in fault rocks reveals temporal variations in fault zone rheology. Foliated fault rocks from the Hunter Valley (HV) and Copper Creek (CC) thrusts in the southern Appalachians record the transition from cataclasis to diffusive mass transfer (DMT) and grain boundary sliding (GBS). Previous work documented (using optical microscopy) early grain fracture followed by DMT and (using TEM) intracrystalline plasticity. The current research uses SEM to map microstructures at a scale intermediate to the previous work in order to focus on the spatial and temporal evolution of microstructures.

Both HV and CC fault rocks contain coarse fragments of wall rock and earlier fault rocks in a fine-grained matrix. In HV fault rocks, fragments of coarse or very-fine grained carbonate or quartz (0.1-3 mm) contain fractures and/or calcite- or quartz-filled veins. Undeformed, calcite-filled veins (perpendicular to foliation) cut deformed, ankeritic dolomite-filled veins generally parallel to banding but often disrupted, folded and offset along with foliated matrix. Matrix grains are ~10 µm. In CC fault rocks, mainly carbonate fragments are 0.1-1 mm; matrix grains are 0.25 µm. Heavily twinned carbonate veins perpendicular to foliation are offset along twinned carbonate-filled veins parallel to foliation. Stylolites are common, best observed within veins. Flow structures are present in both fault zones, evident as matrix-filled veins (HV) and foliation wrapping around clasts (HV and CC).

The fine grain size, stylolites and matrix flow structures suggest that DMT and GBS predominated once the proportion of fine-grained matrix increased due to continued cataclasis. Veins normal to foliation at HV are disrupted by flowing matrix; the formation of veins alternated with flow of matrix. DMT/GBS of submicron diameter matrix grains in CC fault rocks would occur at low differential stresses, yet alternated with a rheology sufficiently competent to sustain brittle fracture and the formation of veins. Variation in pore pressure could cause alternating fracture and flow in these rocks. These cross-cutting microstructural relationships thus suggest that deformation alternated between brittle and ductile deformation, influenced by changes in grain size, pore pressure, and the influx of fluids.