Microstructures and rheology of a calcite-shale thrust fault
A thin (similar to 2 cm) layer of extensively sheared fault rock decorates the similar to 15 km displacement Copper Creek thrust at an exposure near Knoxville, TN (USA). In these ultrafine-grained (<0.3 mu m) fault rocks, interpenetrating calcite grains form an interconnected network around shale clasts. One cm below the fault rock layer, sedimentary laminations in non-penetratively deformed footwall shale are cut by calcite veins, small faults, and stylolites. A 350 mu m thick calcite vein separates the fault rocks and footwall shale. The vein is composed of layers of (1) coarse calcite grains (>5 mu m) that exhibit a lattice preferred orientation (LPO) with pores at twin twin and twin-grain boundary intersections, and (2) ultrafinegrained (0.3 mu m) calcite that exhibits interpenetrating grain boundaries, four-grain junctions and lacks a LPO. Coarse calcite layers crosscut ultrafine-grained layers indicating intermittent vein formation during shearing. Calcite in the fault rock layer is derived from vein calcite and grain-size reduction of calcite took place by plasticity-induced fracture. The ultrafine-grained calcite deformed primarily by diffusion-accommodated grain boundary sliding and formed an interconnected network around shale clasts within the shear zone. The interconnected network of ultrafine-grained calcite indicates that calcite, not shale, was the weak phase in this fault zone. (C) 2014 Elsevier Ltd. All rights reserved.
Wells, Rachel K., Julie Newman, and Steven Wojtal. 2014. "Microstructures and rheology of a calcite-shale thrust fault." Journal of Structural Geology 65: 69-81.
Journal of Structural Geology
Calcite, Shale, Low-temperature deformation, Plasticity-induced fracturing, Grain boundary sliding, Diffusion creep