ISOTOPIC EVIDENCE FROM AUTHIGENIC CARBONATES FOR RAPID UPWARD FLUID-FLOW IN ACCRETIONARY WEDGES

A comparison of pore fluids and authigenic carbonates sampled by the O cean Drilling Program from the Nankai, Peru, Barbados, and Cascadia ac cretionary wedges illustrates significant disequilibria in oxygen isot opes, which in some cases may be related to rapid incursions of fluids along fault conduits in the past, The Peru, Barbados, and Cascadia ca rbonates have lower than expected delta(18)O(PDB) (Peedee belemnite) v alues for the measured downhole temperatures and isotope compositions of the pore fluids. In the Fern wedge the lower values may have result ed from past incursion of meteoric water, but in the Barbados and Casc adia wedges, the low delta(18)O(PDB) values were caused in part by pas t incursion of fluids with higher temperatures, probably along fault c onduits, The temperature of fluids in the Cascadia wedge may have exce eded 100 degrees C, which is 90 degrees C higher than measured current downhole temperatures, The required high fluid-flow rates suggest a m echanism of seismically induced fluid migration to bring fluids rapidl y from depths of greater than 2 km to the surface.