CONTRASTING HYDROGEOLOGIC REGIMES ALONG STRIKE-SLIP AND THRUST FAULTSIN THE OREGON CONVERGENT MARGIN - EVIDENCE FROM THE CHEMISTRY OF SYNTECTONIC CARBONATE CEMENTS AND VEINS

Samples of carbonate-cemented sedimentary rocks were collected during 11 Alvin dives in two regions along the Cascadia margin, a northern st rike-slip fault zone and a southern thrust-fault zone (''second ridge' '). We characterized 35 samples petrographically and chemically. North ern-area samples are dominantly sandstones; second-ridge samples are a ll mudstones, Vugs and high percentages of carbonate cement in the mud stones suggest that these rocks were never deeply buried, The abundanc e of veins and brecciation in samples from both regions attests to the ir proximity to faults, Northern-area cements are dominantly calcite o r magnesian calcite, whereas second-ridge cements generally are more d olomitic, Oxygen and Sr isotopic values of carbonate cements indicate the involvement of two very different fluid reservoirs during cementat ion, One reservoir generally has low delta(18)O, very low delta(13)C, and high Sr-87/Sr-86 ratios, but the other has high delta(18)O, modera tely negative delta(13)C, and Sr-87/Sr-86 ratios lower than modern sea water values, In the northern area, diagenetic carbonates are subdivid ed into two populations of coupled oxygen and carbon isotopic, values, Population I has delta(18)O(PDB) values of +3 parts per thousand to 5 parts per thousand and delta(13)C(PDB) values of -55 parts per thous and to -45 parts per thousand. Population II has delta(18)O values of -13 parts per thousand to 4 parts per thousand and delta(13)C values o f -25 parts per thousand to -1 parts per thousand. Such common low del ta(18)O values in carbonate cements hale not been observed at any othe r accretionary wedge. Sr-87/Sr-86 ratios of northern-area samples rang e from 0.7128 to 0.7088, Oxygen and Sr isotopic, values show a correla tion between increasing Sr-87/Sr-86 ratios and decreasing delta(18)O v alues, Oxygen and carbon isotopic values of most diagenetic carbonates from the second ridge range from +3 parts per thousand to +10 parts p er thousand and -55 parts per thousand to -38 parts per thousand, resp ectively. Enrichments of O-18 probably result from a combination of do lomitic mineralogy, cold bottom-water temperatures. and the presence o f decomposing gas hydrate in the subsurface, Sr-87/Sr-86 ratios of car bonates from the second ridge range from 0.7086 to 0.7091, The geochem ical signatures of diagenetic cements from the northern area suggest t hat fluids were derived from greater than 2 km depth, perhaps from the decollement. Prominent gullies throughout the northern area are proba bly underlain by strike-slip faults that provide conduits for upward f luid migration, Cements at the second ridge precipitated from shallowl y derived fluids, The difference in fluid source depth is related to p roximity to the fluid conduits and different orien- tations of minimum principal stress and resultant hydrofractures in the two fault regime s.