Footwall Refrigeration vs. Footwall Hydration: Reassessing Strong Thermal Gradients Below Detachment Faults with in situ SIMS Oxygen Isotope Quartz-Epidote Analysis.
Abstract
Current understanding suggests that the flux of meteoric fluids into shallow areas of footwall rocks can drive rapid cooling along extensional detachment faults. The Oligo-Miocene Whipple detachment fault (WDF) in Southeastern California is an important orogen-scale structure associated with fault-controlled hydrothermal activity and fluid-assisted deformation processes. To better understand the extent of rock-fluid interaction at the grain-scale, and how it influences cooling and deformation along the WDF, we present in situ oxygen isotope analyses in quartz and epidote by secondary ion mass spectrometry (SIMS) from three quartzofeldspathic mylonite samples. Samples were collected from different structural depths below the detachment fault and show different degrees of brittle and ductile deformation microstructures. Petrography shows different generations of epidote with distinct core-rim zoning in many grains, and SIMS data confirm significant intergrain O-isotope variations within samples. In the sample nearest to the fault (<1 m below), quartz O-isotope values range from +7 to +9.5 and epidote O-isotope values range from +6 to -1.5. At 5 m below the fault, quartz O-isotope values are +8 to +9.5 and epidote O-isotope values are +6 to +3.5. In the structurally deeper sample 40 m below the fault, quartz and epidote O-isotope values are tightly clustered near 9.5 and 6, respectively, and we interpret this sample to preserve O-isotope equilibrium at ~450 C. Low O-isotope epidote values and the decreasing quartz and epidote O-isotope trends require exchange with a low O-isotope meteoric fluid at high temperature (300-450C) and moderate water-rock ratio. Mean quartz-epidote O-isotope fractionations are highest near the fault and lower farther from the fault, but our data suggest that this trend reflects mainly increasing degree of O-isotope exchange with meteoric fluid toward the detachment and does not record equilibrium between quartz and epidote. Instead, our data suggest that fluid-rock interaction associated with the WDF occurred over a range of temperatures within the same rocks, such that mineral O-isotope records temporal rather than spatial variation in crustal thermal structure around the detachment fault.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.T14B..01R