A fluid inclusion study of fluid pressure and salinity variations in the footwall of the rector branch thrust, North Carolina, U.S.A.

Abstract

Last melting and homogenization temperatures of fluid inclusions from plastically deformed bedding-parallel quartz veins in the footwall of the Rector Branch thrust, North Carolina, were studied as a function of distance from the thrust. Fluid inclusions and microstructures in mylonitic rocks within the thrust zone were also examined. Fluid inclusions in quartz veins which display evidence for intracrystalline plasticity (e.g. subgrain polygonization) occur along subgrain boundaries and have higher homogenization temperatures ( T n) and a wider range (120–320°C) compared to less deformed samples. Maximum T h values, which approach the temperature of deformation (300 ± 20° C), apparently reflect leakage of inclusions along subgrain boundaries. Minimum T h values (120–160°C), on the other hand, record near lithostatic conditions (2.6 kb) at 300°C. Maximum last melting temperatures ( T m) increase from −20 to −4°C with decreasing distance to the thrust, corresponding to a decrease in salinity of the fluid from 23 to 3 wt% (NaCl equivalent). The decrease in salinity towards the fault is interpreted as due to infiltration of the fault at depth (to approximately 10 km) by surface derived waters during periods of fault zone dilatancy. Inclusions along healed microcracks in quartz from the fault zone display higher salinity (17–26 wt% NaCl equiv.) and are interpreted to reflect enhanced fluid-rock interaction in the fault zone due to hydration reactions. The fluid pressure and salinity variations are consistent with a combined dilatancy-hydraulic fracturing model for the Rector Branch thrust. Previously documented bulk rock volume losses for this fault zone are inferred to have been produced by the fluxing of the fault zone with undersaturated surface derived fluids.