Stable isotope evidence for contrasting paleofluid circulation in thrust faults and normal faults of the central Apennines, Italy

We have investigated the structures and stable isotope geochemistry of faul t rocks within thrust faults and normal faults of the central Apennines in order to understand the fluid circulation during late Miocene-early Pliocen e contraction and late Pliocene-Pleistocene extension of the thrust belt. S table isotope data were obtained for 70 carbonate host rocks and 80 fault r ocks. Cataclasites, veins, and slickenfibers of the thrust faults have delt a C-13 values ranging from 0 to 3 parts per thousand, which are similar to the host rocks' values, and delta O-18 values from 25 to 34 parts per thous and, which are slightly lower than the 28 to 34 parts per thousand delta O- 18 values of the host rocks. Cemented cataclasites, veins, and slickenfiber s of the normal faults have delta C-13 values ranging from -5 to 3 parts pe r thousand and delta O-18 values from 19 to 30 parts per thousand, which ar e significantly lower than the host rocks' values. These data are consisten t with the contraction-related structures having formed in a semiclosed hyd rologic system. In contrast, the extension-related structures formed in sem i-open to open system conditions. During shortening, there was little to no advection of large quantities of isotopically distinct fluids from the sil iciclastic wedge on top of the subducting Adriatic plate and/or from the ov erlying groundwater aquifers. Only during exhumation and extension of the o rogen were fluids able to penetrate downward into large normal faults that at present are seismically active.