The Fluid and Tectonic Evolution of Convergent Margins as Preserved in the Chemistry of Authigenic Carbonates

James C. Sample

Geochemical tracers from pore waters in accretionary wedges are useful indicators of fluid sources. Constraining fluid sources is important in understanding the present-day tectonics of active convergent margins. But geochemical tracers preserved in authigenic carbonate deposits in accreted sediments may yield more information about some aspects of the fluid and tectonic evolution of convergent margins over longer times. For example, in accretionary wedge ODP Sites of the Nankai, Barbados, Peru, and Cascadia convergent margins, only the carbonate veins found in Nankai Site 808 are in oxygen isotopic equilibrium with observed geochemical and temperature conditions of the pore fluids at the same depth. At the other Sites, oxygen isotopes in many of the authigenic c rbonates are lighter than expected compared with oxygen isotopes measured in the pore fluids. This is particularly pronounced at the Cascadia margin, where ^dgr¹⁸O_PDB of Site 891 carbonates ranges from to -16.5^pmil to -6.2^pmil, while pore fluids from the same Site range from ^dgr¹⁸O_SMOW = -0.1^pmil to -1.3^pmil. The inferred temperature at the bottom of Site 891 (^approx25°C) is not great enough to cause the fractionation. Possible explanations for low oxygen isotope values include presence of detrital carbonate, low-temperature alteration of volcanic matter, or influx of meteoric fluids, but both are unlikely given other geologic constraints. Rapid influx of warm fluids migrating from depth along faults and rapid precipitation of carbon te cements and veins during pressure release of the fluids may lead to precipitation of carbonates with low oxygen isotope values. These migration events might be related to seismic events.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California