Geochemical and stable isotope resetting in shear zones from Täschalp: constraints on fluid flow during exhumation in the Western Alps

Abstract

Fluid flow at greenschist facies conditions during exhumation of the western Alps occurred in several penecontemporaneous systems, including shear zones at lithological contacts, deformed contacts between serpentinite bodies and metabasalts, albite veins within metabasalts, and calcite + quartz veins within calcareous schists. Fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the Piemonte Unit reset O and H isotope ratios. δ18O values are buffered by the wall rocks; however, calculated fluid δ2H values are similar within all the shear zones suggesting that they formed an interconnected network. The similarity of δ2H values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from, or had equilibrated with, the schists that envelop the ophiolite rocks. Time‐integrated fluid fluxes at the sheared contacts estimated from changes in Si in metabasalts were up to 105 m3 m−2, with the fluid flowing up temperature driven either by topography or seismic pumping. Individual shear zones were active for c. 2–3 Myr, implying average fluid fluxes of up to 10−9 m3 m−2 s−1. Rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes, implying that fluid volumes decreased into the ophiolite unit, consistent with the source of fluids being the metasediments. Fluids were generated by dehydration reactions that were intersected during exhumation and, while many rocks show the affects of fluid–rock interaction, large‐scale fluid flow between major units was not common.

Subduction zone metamorphic pathway for deep carbon cycling: I. Evidence from HP/UHP metasedimentary rocks, Italian Alps

“…Thus, we suggest that it is possible that the O isotope evolution of the more carbonate-rich rocks reflects exchange of these rocks with H 2 O-rich fluids generated within the Schistes Lustrés thrust slices (cf. Henry et al, 1996;Cartwright and Buick, 2000;Cartwright and Barnicoat, 2003). As an additional possibility, infiltration of these subducting sediments by fluids generated by devolatilization in underlying mafic mineralogies (δ 18 O = +6 to +20‰; Alt et al, 1992) could have caused this shift (see Angiboust et al, 2014, for a discussion of fluid sources for the Monviso locality).…”

Stable isotope and Ar/Ar evidence of prolonged multiscale fluid flow during exhumation of orogenic crust: Example from the Mont Blanc and Aar Massifs (NW Alps)

“…However, the size, origin, and extent of fluid migration are still a matter of debate. In shear systems, there are indeed evidences of outcrop‐scale up to crustal‐scale fluid redistribution [e.g., Kerrick et al ., , ; Glazner and Bartley , ; McCaig et al ., ; Dipple and Ferry , ; Marquer et al ., ; Oliver and Bons , ; Richards et al ., ; Cartwright and Barnicoat , , and references therein; Raimondo et al ., ]. In the External Alps (Figure ), several studies have examined stable isotopes and fluid inclusions [e.g., Poty et al ., ; Marquer and Bukhard , 1992; Mullis et al ., ; Sharp et al ., ] in order to constrain fluid‐rock interactions associated with the formation of shear zones at midcrustal depths in the External Crystalline Massifs (ECM) and at upper crustal depth in the Helvetic nappes (sedimentary cover sequence of the ECM, subjected to nappes stacking during the Alpine orogeny).…”

Mass transfer in the lower crust: Evidence for incipient melt assisted flow along grain boundaries in the deep arc granulites of Fiordland, New Zealand

“…The two main mechanisms advocated for significant mass transfer through the crust occur during deformation, either within the brittle regime, through flow in dykes [e.g., Sleep , ; Spence et al ., ; Weinberg , ] or the ductile regime, through flow in shear zones [e.g., Beach and Fyfe , ; Carter and Dworkin , ; Cartwright and Barnicoat , ; Streit and Cox , ]. Conceptually, there is also the possibility of a third mechanism, diffuse porous flow, which occurs without deformation and is commonly invoked in the mantle.…”