Pre- to syn-folding closed paleofluid system conditions and their opening in late- to post-folding structural evolution: the case of Urgonian platform carbonates folded in the Parmelan anticline, Bornes Massif, external Western Alps

Abstract

<p>Fluid systems in fold-thrust belts typically evolve from hydrologically closed to open, as a consequence of the incremental development of brittle deformation. The spatial distribution of fold-related fractures depends, among other factors, on the kinematics of folding and on the presence of inherited pre-folding structures. An improved understanding of the impact of the incremental evolution of deformation patterns on fluid migration and accumulation is crucial for industrial purposes. Here, we discuss the evolution of the fluid-rock system of the Parmelan anticline, in the Dauphinois units of the northern Subalpine Chains (Bornes Massif). We combined a detailed structural analysis in the Lower Cretaceous units (e.g. Urgonian Limestones) with the study of syn-tectonic calcite cements, by coupling stable and clumped isotope analysis with trace and major element geochemistry, radiogenic Sr isotopic data, and fluid inclusion microthermometry. The older calcite cements associated with the pre-folding structural assemblages precipitated from an <sup>18</sup>O-enriched fluid at temperatures between 90 and 115 °C. This first fluid type was thermally equilibrated with the host rock under maximum burial conditions in the Alpine Foreland and its isotopic composition has been interpreted to reflect a high degree of fluid-rock interaction in a closed system. Trace and major elements and Sr isotopes support a mixed meteoric-marine origin of this fluid, possibly trapped during subaerial platform exposure in the forebulge and then mixed with Eocene seawater. Closed system and rock-buffered conditions persisted during incipient folding whereas, during late folding, longitudinal (i.e. axial parallel) deformation structures allowed fluid circulation in an open system. Open system conditions initially occurred only in crest-limb transitional domains characterized by an higher deformation intensity. By contrast, during post-folding transpression,  the formation of a persistent vein set oblique to fold axis allowed external fluids to migrate in the anticline crest. Younger calcite cements precipitated from moderately warm (55-66 °C) <sup>18</sup>O-depleted meteoric fluids during the late- to post-folding stages. Our compositional and Sr isotopic data exclude any contribution from basement-derived ascending fluids and rule out a possible downward circulation of these meteoric fluids at basement depths. Our results indicate that, in regional anticlines of shallow crustal sectors in foreland fold-thrust belts, a significant amount of secondary porosity can be produced in the pre-folding stages when the hydromechanical stratigraphy likely preserves closed conditions and regional stratigraphic seals can prevent upward fluid migration during the entire tectonic evolution.</p>