Fluid evolution history of brittle-ductile shear zones on the hanging wall of Yellow Spring thrust, Valley and Ridge Province, Pennsylvania, U.S.A.

This work presents the results of structural and fluid inclusion analyses of brittle-ductile shear zones (bdsz) cutting Lower Paleozoic carbonates in the Appalachian fold and thrust belt. Strike parallel extension of the beds riding up the ramp of Yellow Spring thrust resulted in development of conjugate bdsz by progressively non-coaxial deformation. During the shearing, syntectonic extension veins were formed by successive episodes of mode-I cracking and sealing by the precipitation of quartz, saddle dolomite and calcite in paragenetic order. Geobarometry on co-existing methane-rich and water-rich fluid inclusions reveals the trapping of fluids at large range of fluid pressures. The values of maximum and minimum fluid pressures correspond to the events of crack propagation and complete sealing respectively. Contrary to ductile shear zones in high-grade metamorphic rocks, this study in unmetamorphosed rocks shows that bdsz evolve by influx of locally derived methane bearing brines of connate or evolved formation water origin at pressures of 80-40 MPa, temperatures of 155-165°C and a depth of 6 km. It is shown that the salinity and composition of the mineralizing fluids change from one crack-seal episode to another, in spite of a broad consistency in the pressure-temperature conditions.