The continuously distributed, well-preserved fracture-filling carbonates were collected from the cores retrieved from the Taiwan Chelungpu-fault Drilling Program between 400 and 2000 m depth for carbon and oxygen isotope analyses to constrain the fluid compositions and processes associated with brittle deformation within the accretionary wedge. The isotope analyses of fracture-filling carbonates yielded δ18O values steadily increasing with depth. The δ13C values exhibited a pattern similar to that of δ18O values at shallower depths, but an opposite trend at greater depths. Such variations in the isotopic compositions could be best explained by a mixing of shallow, meteoric water and deeply rooted fluid carrying isotopic signatures of various microbial processes. The notable excursion of δ18O values might infer a conduit for the lateral fluid transport along the high vein density zone. Overall, the isotopic patterns preclude the substantial influence of early diagenesis and suggest pervasive fluid percolation through the fracture network.
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