Microstructures of cataclasites in a limestone-on-shale thrust fault: implications for low-temperature recrystallization of calcite

Abstract

The Hunter Valley thrust (HVT), a low-temperature foreland thrust fault in the Valley and Ridge province (Southern Appalachians), produced two distinct cataclasites: a limestone cataclasite, derived predominantly from the hanging wall limestone, and a shale cataclasite, derived predominantly from the footwall shale. The limestone cataclasite consists of fragments of calcite, limestone, pre-existing limestone cataclasite, quartz, and quartz aggregates in a fine-grained (<3 μm) matrix. TEM observations show that the dominant microstructures preserved in the fine-grained calcite matrix is that of high dislocation densities, bulging grain and twin boundaries, some polygonal grains, and euhdedral precipitates of calcite. In contrast, grains greater than approximately 5 μm in diameter are twinned, contain variable dislocation densities and microfractures, but show evidence for only limited migration of boundaries. Grain size measurements support a mechanism for grain size reduction by microfracturing down to approximately 7 μm. Based on TEM observations and the grain size data we suggest that grain size reduction in calcite grains greater than 7 μm in diameter occurred predominantly by microcracking and that at grain sizes less than 3 μm in diameter, grain size reduction occurred predominantly by twin and grain boundary migration and subsequent recrystallization, in conjunction with calcite precipitation; between 3 and 7 μm microcracking, boundary migration and solution transfer processes operated concurrently. Evidence for the presence of fluids during deformation is abundant in the hanging wall and we suggest that the combination of small grain sizes and fluid–grain interactions led to increased mobility of dislocations and point defects in the calcite structure. The importance of dynamic recrystallization versus solution transfer processes and cataclasis in accommodating displacement is, however, difficult to quantify. Development of a footwall cataclasite indicates that the footwall shales accommodated significant strain during deformation. TEM microstructures suggest that deformation within the shale cataclasite occurred by frictional sliding of clay packets and diffusive mass transfer.