Role of structural inheritance on deformation pattern and paleofluid evolution in folded platform carbonates: the case of the Parmelan anticline, Bornes Massif, external Western Alps

Abstract

In convergent plate margins, foreland basin systems develop at orogenic wedge toes and typically include extensional deformation domains in forebulges, formed in response to foreland flexure. Consequently, synorogenic extensional longitudinal and transversal deformation structures provide a quite common structural inheritance playing a role in controlling the development of fold-and-thrust geometries and related deformation patterns. Similarly, fluids responsible for cementation during the long-lasting tectonic evolution of convergent margins can start evolving their physical conditions and chemical compositions by migrating along primary and secondary porosity, well before the onset of contraction. Therefore, the pre-contractional geological evolution of convergent plate margins intended as an interplay of tectonic, stratigraphic and climatic factors, impacts on the structural, thermal, and chemical evolution of geofluids and rocks that hence represent two components of a unique system. In this PhD thesis, we investigated on the role of structural (and geological) inheritance in controlling deformation patterns and paleofluid evolutions in regional-scale anticlines that involve carbonate-dominated successions in the external sectors of foreland thrust-fold belts. We selected the Parmelan anticline, in the Bornes Massif (northern Subalpine Chain, France), for the amazing exposures of Lower Cretaceous platform carbonates (Urgonian Limestones). Through a detailed structural study of the deformation pattern exposed in different fold sectors, we intended to track the stress and strain evolution during the long-lasting tectonic journey of the convergent margin from the alpine foreland into the alpine wedge toe. We investigated on how complex stress histories and inherited structural fabrics, from the plurikilometric to the outcrop scales, can lead to the development of strain patterns that significantly differs from “classical textbook templates”. Through a detailed study of syn-tectonic calcite cements filling deformation features, we traced fluid sources and their migration pathways. For these purposes, we used a multidisciplinary approach coupling cement optical and cold cathodoluminescence petrography, with stable (C and O) and Sr isotope analysis, major and trace element analysis, clumped isotope thermometry, and fluid inclusions microthermometry. Such a multidisciplinary approach allowed us to constrain the evolution of fluid temperature and chemistry during the long-lasting deformation evolution from extension in the alpine foreland to post-folding exhumation. We outlined important conclusions on the fluid-rock system opening in fixed-hinge folds involving platform carbonates (potential reservoirs) and shale units (potential seals) and we discussed the role of syn-folding deformation structrues in controlling vertical fluid migration in the folded stratigraphy. The dataset presented in the thesis shows that the opening of the system to external (meteoric) fluids occurred only during fold tightening and not during incipient folding and that the regional stratigraphic seals underlying the Urgonian Limistones prevent the upward migration of heated basement-derived fluids during the entire deformation history of this anticline. The results of this thesis bear important implications for a better understanding of fluid-rock systems in folds developed at the toe of thrust wedges and, consequently, of the migration and accumulation of fluids in the subsurface.