MULTI FLUID-FLOW EVENTS DURING MODE I OPENING: A MICROSTRUCTURAL AND SIMS STUDY

Syntectonic veins have been employed to assess the physical-chemical conditions and the source of fluids involved within fault zones during their movement. Because they form in response to tectonic stress, veins may also inform about the stress conditions during deformation/mineralization. On the basis of bulk O and C stable isotopes analyses and fluid-inclusion studies at macroscopic scale, many studies have revealed that syntectonic veins represent a snapshot of fluid and stress conditions during the fault history. This consensus is largely accepted for mode I extension veins. However, an important and unanswered question is how much of the fault history information can veins record at micrometer-scale. New detailed in-situ micron-scale analyses of δ¹⁸O values by SIMS and fluid inclusion microthermometry are presented here to document the fluid and temperature conditions during tensile vein formation related to the Cotiella thrust activity (Pyrenees, Spain). In-situ δ¹⁸O analyses were coupled to cathodoluminescence and fluid-inclusion microthermometry. We identified 3 generations of calcite fractures, successively opened at 140, 90 and <70°C under the same stress field, but with distinct δ¹⁸O values. This supports that veins can record a complex fluid and temperature history throughout fault activity. This study hence demonstrates that micrometer-scale analyses offer new perspectives on the behavior and history of faults.