Abstract
In situ secondary ionization mass spectrometry (SIMS) analyses of oxygen isotopes in authigenic calcite veins were obtained from an active thrust fault system drilled at Ocean Drilling Program (ODP) Site 892 (44°40.4′N, 125°07.1′W) along the Cascadia subduction margin. The average δ18OPDB value of all samples is −9.9‰ and the values are the lowest of any measured in active accretionary prisms. Ranges in individual veins can be as much as 19.6‰. There is an isotopic stratigraphy related to the structural stratigraphy. Mean isotope values in the hanging wall, thrust, and footwall are −14.4‰, −9.5‰, and −5.2‰, respectively. Several veins and crosscutting vein sequences show a general trend from lower to higher δ18O values over time. Isotopic and textural data indicate several veins formed by a crack-seal mechanism and growth into open fractures. The best explanation for the strong 18O depletions is periodic rapid flow from 2–3km deeper in the prism. Relatively narrow isotopic ranges for most veins suggest that fluids were derived from a similar source depth for each episode of fluid pulse and calcite crystallization. Structural and mass balance considerations are consistent with a record preserved in the veins of ten to hundreds of thousands of years. The fluid pulses may relate to periodic large earthquake events such as those recognized in the paleoseismicity records from the Cascadia margin.
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