Isotopic constraints on the age of fluid migration and faulting in the deep Nankai accretionary prism at IODP Site C0002
Abstract
We examined carbonate veins from Site C0002 in the Nankai convergent margin for information about fluid-rock interactions and deformation at this seismically active plate boundary. Drill cutting samples were collected at intervals in the accretionary prism from 875.5 to 3058.5 meters below seafloor (mbsf). A cored interval from 2163-2219 mbsf, including a fault with apparent normal shear sense, was also sampled. A total of 74 samples were analyzed for carbon and oxygen isotopes. We analyzed a subset for carbonate-clumped isotopes, and attempted U-series age dating on 10 samples. Generally minimum oxygen isotope values decrease and maximum clumped-carbonate temperatures increase downhole. Veined samples dominate the population with the largest 18O depletions; matrix cements dominate the less depleted population, in many cases reflecting seawater values. Carbonates in the cored fault are similar to deeper samples, but fault veins have slightly depressed δ18O values and slightly elevated temperature values in the context of the downhole gradients. One cored-fault vein has a higher temperature (63.8±10.6°C) and calculated fluid composition (δ18OVSMOW = +0.3±1.7‰) relative to the other fault veins (43.9±6.9°C to 51.6±4.2°C and -2.8±1.2‰ to -1.9±0.7‰), suggesting a different, deeper source. Two of the analyzed calcite veins from one vein generation yielded uncorrected 230Th ages of 106±1 and 161±2 ka. As the analyzed samples have high 232Th concentrations (19 and 56 ppb, respectively) and any initial 230Th correction would result in younger U-Th ages, we argue that these dates likely represent maximum ages. The stable isotopic results support veins dominated by locally derived fluids migrating from the sedimentary matrix into the fault. Some migration of deeper fluids may have periodically occurred along the cored fault. Vein temperatures are most consistent with geothermal gradient estimates of Harris et al. (2011). The U-Th ages imply: 1) vein formation and by implication fault activity postdate accretion of Nankai Trough sediments and deposition of most of the Kumano forearc basin; 2) vein formation could still be occurring in the cored fault; and 3) the cored fault is most likely a recent normal fault rather than an older, rotated thrust fault. Harris, R. N., et al., 2011. Geochem. Geophys. Geosys. 12: Q0AD16.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.T31D2929S
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRYDE: 7240 Subduction zones;
- SEISMOLOGYDE: 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8413 Subduction zone processes;
- VOLCANOLOGY