Abstract

Deformation of dolomite along a minor thrust fault within the Mountain City window, Tennessee, took place by twinning, fracturing, pressure solution, and recrystallization processes. Deformation otherwise unobservable can be seen using cathodoluminescence and doubly polished ultrathin (∼5 μm) sections.

A transect across the hanging wall, starting at 14 m above and approaching the hanging-wall/footwall (hw/fw) contact, and crosscutting relationships suggest a temporal sequence of deformation: (1) Twinning occurred in the coarser fraction of grains, and material was removed along bedding parallel stylolites. (2) Dolomite recrystallized at the edges of veins. Fine-grained deformation zones composed of dynamically recrystallized dolomite formed. Transgranular hairline fractures formed, visible only under cathodoluminescence. (3) Grain size decreased. Recrystallization occurred at grain boundaries, along veins, and within fine-grained deformation zones. The proportion of twinned grains decreased with twinning only within the coarser fraction of grains. The proportion of veins and fractures increased. (4) Bedding-parallel stylolites offset deformation zones and veins, indicating reactivation. Intersection relationships indicate that stylolites, fractures, and deformation zones all acted concurrently. (5) At the hw/fw contact, the rock is finer grained and homogeneous. Very few veins or stylolites remain.

The regional geology (Boyer and Elliott, 1982) suggests that deformation took place at a depth of 9 km (270 °C at a 30 °C/km geothermal gradient). Experimental data (for example, Turner and others, 1954) suggest that dolomite would not deform by dynamic recrystallization under these conditions. However, fluids may reduce the strength and increase the ductility of calcite rocks (Rutter, 1972). Similarly, fluids may have reduced the temperature at which dynamic recrystallization could take place in these dolomitic rocks.

The volume of mesoscopic fractures, microscopic luminescing fractures, and the mean grain size decrease as the hw/fw contact is approached. Grain-size reduction in conjunction with the influx of fluids may have resulted in a change in the dominant deformation mechanism from fracturing to dynamic recrystallization.

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