Pressure, temperature, and composition history of syntectonic fluids in a low-grade metamorphic terrane

Abstract

Analysis of stable isotopes and fluid inclusions in centimeter-scale fibrous quartz-calcite strain fringes from the Taconic thrust belt in Vermont and New York allows reconstruc- tion of fluid conditions and sources over the course of strain-fringe growth. Oxygen isotope ratios in quartz show regular variations in d 18 O, ranging between 19.2‰ and 20.0‰ in different parts of strain fringes. Fluid inclusions have low salinities (2.4% NaCl equivalent) in all parts of strain fringes, and we propose that fluids were derived from clay dehydra- tion. Variations in fluid temperature and pressure derived from stable isotope and fluid- inclusion analysis mimic changes in the orientations of fibers, indicating a strong corre- lation between tectonics and fluids. Earliest fluids had temperatures of ;240 8C; temperatures declined over time to ;200 8C and rose again to ;260 8C. Fluid pressures began at ;1.6 kbar (160 MPa) and fell to ;0.8 kbar (80 MPa) before rising to ;2 kbar (200 MPa). We interpret these results in the context of thrust faulting and the dynamics of thrust wedges. Our model begins with thrusting along the Bird Mountain fault, placing rocks from depth on shallower ones and raising the temperature of the footwall. Increasing temperatures would have stimulated clay dehydration, raising fluid pressures and weak- ening the rocks within the orogen. Reduced strength of the Taconic accretionary wedge would have required lower critical taper angles, stopping motion on the Bird Mountain fault and stimulating horizontal elongation within the Taconic thrust wedge. As fluid pressures and temperatures declined, increasing strength of the rocks would have de- manded higher critical taper angles, stimulating renewed Bird Mountain thrusting and increases in temperature and fluid pressure. We cannot uniquely determine the depth of burial during strain-fringe growth, but we can constrain the depth for different geother- mal gradients and evaluate how close fluid pressures were to lithostatic. The highest fluid pressures were ;70% of lithostatic, and could have been higher. Similar mechanisms tying fluid conditions to tectonics in a feedback loop may be active in accretionary prisms and thrust belts worldwide.