EVIDENCE FOR SEISMIC AND ASEISMIC SLIP ALONG A FORELAND THRUST FAULT, SOUTHERN APPALACHIANS
The Copper Creek thrust (CC), TN accommodated 15-20 km displacement, at depths of 4-6 km, as estimated from balanced cross-sections. At the Diggs Gap exposure of the CC, an ~ 2 cm thick, vein-like shear zone separates shale layers in the hanging wall and footwall. The shear zone is composed of anastomosing layers of ultrafine-grained calcite and/or shale as well as aggregate clasts of ultrafine-grained calcite or shale. The shear zone and the footwall are separated by a 350 µm-thick layer of ultrafine-grained calcite. Fault parallel and perpendicular calcite veins are common in the footwall and increase in density towards the shear zone. Microstructures within the shear zone that are similar to those observed in experimental studies of unstable slip include: ultrafine-grained calcite (~0.34 µm), nano-aggregate clasts (100-300 nm), injection structures, and vein-wrapped and matrix-wrapped clasts.
Not all structures within the shear zone or ultrafine-grained calcite layer suggest seismic slip. Coarse-grained calcite contains pores at twin-twin intersections suggesting plasticity-induced fracturing as the main mechanism for grain size reduction. Interpenetrating grain boundaries in ultrafine-grained calcite and a lack of a lattice preferred orientation suggest ultrafine-grained calcite deformed by diffusion creep accommodated grain boundary sliding.
Microstructures suggest both seismic and aseismic slip along this ancient fault zone. During periods of aseismic slip, deformation is accommodated by plasticity-induced fracturing and diffusion creep. Calcite veins suggest an increase in pore-fluid pressure, contributing to fluidized flow and seismic slip, but also providing the calcite that deformed by diffusion creep during aseismic creep.