THE ROLE OF CHLORITE IN THE TRANSITION FROM BRITTLE TO DUCTILE FAULTING: AN EXAMPLE FROM AN ALLEGHANIAN THRUST IN THE NORTH CAROLINA BLUE RIDGE

The Crabtree-Penland fault of western North Carolina preserves a record of the transition from brittle microfracturing to macroscopically ductile flow during deformation at greenschist facies conditions. The evolution from protolith to highly deformed fault rock within the Crabtree-Penland fault is accompanied by the growth of chlorite in connected, anastamosing shear zones, which grew primarily by neocrystallization and to a lesser extent by retrogression of biotite and garnet. The abundant chlorite creates weak zones in the rock because slip occurs easily along basal slip planes of chlorite grains and within chlorite aggregates. Additionally, the macroscopic ductility within the Crabtree-Penland fault was aided by (1) a lack of abundant quartz in the protolith and (2) the tendency of chlorite grains and aggregates to coat quartz grain boundaries and thus limit the amount of quartz surface area available for the growth of new quartz. Chlorite within the Crabtree-Penland fault increases significantly within transitional and highly sheared zones of the fault (up to 33%). This is important, especially during the initial stages of fault development, as chlorite precipitated in connected open fractures would coat exisiting quartz grains, thereby limiting quartz nucleation sites and cement growth.

Microstructures, mineralogy, and analysis of fluid inclusions contained within synkinematic quartz are consistent with deformation temperatures of 275-300 °C. Assuming a geothermal gradient of 25 C°/km, depth at the time of faulting is estimated to be 11-12 km. The combination of northwest-directed thrust faulting and low-temperature, sub-greenschist facies conditions suggest that faulting within the Crabtree-Penland fault resulted from Alleghanian deformation. However, the low-temperature character of the fault zone and its shallow depth of formation suggest the Crabtree-Penland fault likely represents a late-stage Alleghanian deformational event (<300 Ma), post-dating the faulting responsible for creating the ubiquitous Alleghanian ductile mylonites.