Preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks

Abstract

Grain-size sensitive flow of fine-grained rocks, such as are often found in mylonitic shear zones, is commonly inferred to be accomplished by grain-boundary sliding, so that no preferred crystallographic orientation is expected to develop, pre-existing fabrics are supposed to weaken and grain shapes to remain equant. However, many fine-grained mylonites commonly exhibit marked preferred crystallographic orientation, whilst displaying a stable but weak grain-shape fabric. We have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic, ultrafine-grained (<10 μm), hot-pressed calcite rocks, Solnhofen limestone and Carrara marble. Samples were deformed experimentally both in extension and compression, under conditions encompassing the transition from grain-size insensitive crystal-plastic flow to grain-size sensitive superplastic flow (300– >700°C). The strong crystal-plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow. This is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain-boundary sliding. From experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working, it was shown that although the microstructure could be totally annealed, the early formed fabric survived even subsequent high-temperature crystal-plastic or superplastic flow. This may explain the frequent occurrence of strong, low-temperature fabrics in calcite ultramylonites.