MICROSTRUCTURES OF CATACLASITES IN A LIMESTONE-ON-SHALE THRUST-FAULT - IMPLICATIONS FOR LOW-TEMPERATURE RECRYSTALLIZATION OF CALCITE

The Hunter Valley thrust (HVT), a low-temperature foreland thrust faul t in the Valley and Ridge province (Southern Appalachians), produced t wo distinct cataclasites: a limestone cataclasite, derived predominant ly from the hanging wall limestone, and a shale cataclasite, derived p redominantly from the footwall shale. The limestone cataclasite consis ts of fragments of calcite, limestone, pre-existing limestone cataclas ite, quartz, and quartz aggregates in a fine-grained (<3 mu m) 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 euhdedr al precipitates of calcite. In contrast, grains greater than approxima tely 5 mu m in diameter are twinned, contain variable dislocation dens ities 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 mu m. Based on TEM observations and the grain size data we suggest that grain size reduction in calcite grains greater than 7 mu m in diameter occurred predominantly by microcracking and that at grain sizes less than 3 mu m in diameter, grain size reduction occurred predominantly by twin and grain boundary migration and subsequent recrystallization, in conjunc tion with calcite precipitation; between 3 and 7 mu m microcracking, b oundary migration and solution transfer processes operated concurrentl y Evidence for the presence of fluids during deformation is abundant i n the hanging wall and we suggest that the combination of small grain sizes and fluid-grain interactions led to increased mobility of disloc ations and point defects in the calcite structure. The importance of d ynamic recrystallization versus solution transfer processes and catacl asis in accommodating displacement is, however, difficult to quantify. Development of a footwall cataclasite indicates that the footwall sha les accommodated significant strain during deformation. TEM microstruc tures suggest that deformation within the shale cataclasite occurred b y frictional sliding of clay packets and diffusive mass transfer. (C) 1998 Elsevier Science B.V. All rights reserved.