Microstructures and rheology of a calcite-shale thrust fault

Abstract

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.

Publisher

Elsevier

Publication Date

8-1-2014

Publication Title

Journal of Structural Geology

Department

Geology

Document Type

Article

DOI

https://dx.doi.org/10.1016/j.jsg.2014.04.002

Keywords

Calcite, Shale, Low-temperature deformation, Plasticity-induced fracturing, Grain boundary sliding, Diffusion creep

Language

English

Format

text

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