Low temperature rheology of calcite and dolomite: Experiment and nature

Carbonates are common in shear zones in the upper and middle crust and influence deformation processes during collision and orogenesis. Experimental and field-based studies of carbonate rheology have found that dolomite aggregates are generally stronger than calcite aggregates. However, a recent experimental characterization of dolomite rheology has found that fine-grained shear zones nucleate readily due to intragranular fracturing and dynamic recrystallization that causes a switch to diffusion creep. Extrapolating these results to nature suggests that fracturing and dynamic recrystallization during dislocation creep would cause fine-grained shear zones to form in dolomites at temperatures as low as 250 °C. Below 250 °C, rheology of fine-grained dolomites is not grain-size sensitive and is unlikely to lead to strain weakening or localization, whereas calcite rheology is grain-size sensitive at these conditions. An exposure of the Copper Creek (CC) thrust fault, southern Appalachians, TN provides an opportunity to test the extrapolation of these experimental results to natural conditions. The CC fault contains massive, fractured dolomite in the hanging wall and interbedded limestone and shale with fractures and abundant calcite veins in the footwall, allowing a comparison of the deformation behavior of these two common carbonates under natural conditions. The CC fault accommodated 15-20 km displacement at a depth of 4-6 km (100-180 °C), and deformation of both minerals is dominated by grain fracture. The fault zone that separates the dolomite hanging wall from the shale footwall is 0.5-4 cm thick. The fault zone contains dolomite (50-1000 μm) and shale (10-300 μm) clasts separated by finer clasts of dolomite and shale (10-75 μm). Dolomite and shale are cross-cut by randomly oriented calcite veins. Twins in dolomite are rare. The fault zone also contains a 0.5 cm thick layer of fault parallel calcite veins. Within the fault parallel veins, calcite grain sizes range from 0.1-8.2 μm (mean 0.5 μm). Coarse calcite grains within these veins contain twins with voids at twin-twin intersections. The ultrafine-grained calcite contains interpenetrating grain boundaries, voids at grain boundaries, four-grain junctions and fractures. These microstructures suggest calcite deformed primarily by plasticity-induced fracturing associated with twinning, resulting in ultrafine-grains, while dolomite deformed by fracturing. The ultrafine-grained calcite deformed by diffusion creep and strain was localized in these fault-parallel veins. Both calcite and dolomite experienced grain size reduction to very fine grain sizes. Dolomite grain size reduction to ~10 μm took place by grain fracturing; calcite grain size reduction to ~0.5 μm took place by plasticity-induced fracturing. The ultrafine-grained calcite deformed by diffusion creep and strain localized within the fault parallel calcite veins, while strain did not localize in the fine-grained dolomite. This observation may be explained, in part, by the finer-grain size developed in the calcite. However, these results also support experimental data that suggest diffusion creep in calcite occurs more readily than in dolomite under low temperature conditions.