PALEOFLUIDS IN BRITTLE FAULTS FROM THE WESTERN BORDER FAULT ZONE OF THE HARTFORD BASIN: EVIDENCE FROM FLUID INCLUSIONS

Sinsitral oblique- to strike-slip brittle faults cut metamorphic rocks to the west of the main western border fault of the Hartford basin in central Connecticut. Fault strikes range from 190° - 230°, and dip 50°-65° with slip lineations raking 0°-15° to the north and south. Displacements are indeterminate, most likely decimeters to meters. The fault surfaces are coated with stepped quartz crystal fibers, massive quartz, and/or small euhedral quartz crystals within rhombochasms. The orientation, slip data and brittle nature of these faults suggests that they are associated with the shifting phase of Hartford Basin evolution. Fluid inclusion microthermometry was used to investigate syn-deformational fluids along these faults. The fluid inclusions found in the quartz fault zone mineralization are two-phase brine inclusions. The inclusions occur in three assemblages: 1) primary and pseudosecondary NaCl-rich inclusions with T_h values ranging from 235° to 282°C and with 7.5 to 8.5 wt. % NaCl equiv. salinity, 2) primary and pseudosecondary NaCl-rich inclusions with T_h values ranging from 225° to 293°C and 16.5 to 17.6 wt. % NaCl equiv. salinity, and 3) pseudosecondary NaCl-CaCl₂-rich inclusions with Th values ranging from 121° to 147°C and 5.5 to 12.8 wt. % NaCl equiv. salinity. The two high temperature fluids appear to be coeval, and the low temperature fluids later. It cannot be determined if the high temperature inclusions represent ambient conditions during faulting or high temperature fluids that were migrating along the faults from a deeper reservoir. If ambient, pressure-corrected trapping temperatures are >280° to 410°C with pressures of >120 to 160 MPa (burial depths of >12 to 16 km). On the other hand, the lower temperature fluids give pressure-corrected trapping temperatures of >130° to 180°C and pressures of >50 to 70 MPa, (burial depths of at >5 to 7 km). One interpretation is that the faults have hade two different episodes of motion, one when deeply buried, the other when more shallow. An alternative interpretation is that the faults were active only when relatively shallow and had episodic fluid influx from multiple reservoirs due to seismic pumping. Interestingly, the low temperature fluid inclusions are similar to those found in quartz from the Bristol copper mines along the western border fault.