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The evolution of deformation structures and associated past fluid flow is a key to better appraise both the reservoir properties of a rock and the regional evolution of an area. Indeed, the meso-scale fracture network often develops over a long period of time, granting access to a long term evolution of a past fluid system (i.e. the temperature, origin and migration pathways of the fluids) on the one hand. On the other hand, major thrusts are believed to enable episodic fluid migrations on larger spatial scales, in relation to their activity calendar, whether this activity is over or ongoing. It is seldom however to reconstruct both the past fluid flow and the present-day fluid flow on the same structure, yet it can be very enlightening about the evolution of the thrust connectivity at depth since before a fold developed over it.In this study we reconstructed the deformation pattern and associated past fluid system related to the development of the Sierra de Orba Anticline (Spain), and we compare it to the current-day fluids resurging in the vicinity of the fault. The Sierra de Orba anticline is part of a fault-propagation fold system that developed in the northern part of the Jaca Basin, Southern Pyrenean foreland, Spain. This fold affects the sedimentary succession including the Triassic decollement level, the Upper Cretaceous to Paleocene carbonate strata and the Middle Eocene marls. The N110 striking major thrust, where the Upper Cretaceous Marboré Fm. lies unconformably onto the Eocene marls, was sampled, along with the fracture network in the hangingwall and the footwall of the thrust. The fracture network includes a sequence of prefolding sets of joints and veins, striking E-W and N060 and flexural-slip related reverse faults. Then, the network encompasses the orogeny history since likely the forebulge development until the strata tilting during folding. Syn-kinematic calcite cements were characterized petrographically in both joints and faults, then studied by means of oxygen and carbon isotopic measurements coupled with fluid inclusion microthermometry. Results highlight that the fluid system recorded an alternation between meteoric fluids (δ18O signature of the fluid: -5‰ SMOW) heated up at 70-80°C, and evolved seawater (δ18O signature of the fluid: +5 to +10‰SMOW) heated up at 70-100°C. That past fluid system around the main thrust did not record any deep-sourced fluids, unlike similar structures in the southern Pyrenean foreland. The major gas content of current day fluids was analysed from a resurgence in the footwall of the thrust. This gas is especially nitrogen-rich (>86%), and is characterized by an heavy δ13C signature of the methane content (4‰PDB). Both these features could relate to deep processes, such as an oxidation of a potential abiotic carbon that can be related to a serpentinization process. This hypothesis needs to be further evaluated by means of a study of noble gas content. Beyond regional implications, this case study illustrate how a combination of geochemical proxies can help to unravel the evolution of a fluid system at a fold-thrust scale, from its onset to today.

Helium-rich gases (up to 6%) upwell along a fault that bounds the western edge of the granitic Morvan Massif (northeastern part of the French Massif Central) and the sedimentary rocks of the southeastern edge of the Paris Basin. The sampled gas is mainly composed of nitrogen (90 %.). The radiogenic 3He/4He (0.02 Ra, where Ra is the atmospheric ratio) and nucleogenic 21Ne/22Ne (0.031) imply that the gas is basement-derived, in good agreement with the presence of granite in the area. The high radiogenic He concentration can be explained by a rock/ water system isolated over geological time scales and might be linked with the presence of a close-by fluorite ore deposit, dated 130 Ma, located above the fault and at the basement/sediment unconformity in Pierre-Perthuis, formed by leaching of granite (Gigoux et al., 2015).The possible link between the fluorite ore and the He-rich gases should imply that groundwater in which He released from U and Th in granite was trapped for several millions of years in the granite or in the permeable basement/sediment unconformity reservoir, below the sedimentary cover probably reaching 1.5 km in the area at the end of the Cretaceous. Deep groundwaters trapped in the reservoir accumulated helium before tectonic exhumation of the Morvan Massif and remobilization of fluids to the surface through the Bazois fault, whose onset began 40 My ago (Lenoir et al., 2021). Mixing between old deep crystalline-water with superficial water is evidenced by the presence of high amount of atmosphere-derived nitrogen.The high chloride content of the waters may originate from the marine water trapped in the Early Jurassic clays, and released by faulting, or by water-rock interaction and hydrolysis of minerals, reinforcing the link between chlorine, fluorine, and water from granite.This geological example shows that in quiescent area, inactive fault can act as a fluid pathway over long time scale.

The Eastern Alps hold an abundance of landscapes with noticeably low topographic gradients at higher elevations above much steeper slopes. Many of these elevated low-relief landscapes (ELRL) are organized in distinct surface levels. Sub-horizontal cave systems can often be found at similar elevations. Utilizing spatial statistics of these ELRL and over 15000 caves, we show that the formation of both the surface and sub-surface landscapes is connected and can help deciphering the landscape evolution of the Eastern Alps from the Late Neogene until today. New cosmogenic nuclide data (10Be, 21Ne, 26Al) of allogenic quartzous sediments from caves and surfaces of distinct elevation levels in the Eastern Alps are used to quantify the incision and ultimately surface uplift history. Burial ages of cave sediments scatter between 0.5 and over 5 Ma. The preliminary data indicate a mean surface uplift of some 0.15 – 0.25 mm/year for much of the Pliocene. We also show that most ELRL in the Eastern Alps can be interpreted in terms of pre-Pleistocene relict landscapes, especially in the only minorly glaciated eastern part. However, the data also show some impact of the Pleistocene glacial cycles on the ELRL and the mobilization of sediments associated with them.

Basaltic rocks generated by upwelling mantle plumes display a range of trace element and isotope compositions indicative of strong heterogeneity in deep material brought to Earth surface. Helium isotopes are an unrivalled tracer of the deep mantle in plume-derived basalts. It is frequently difficult to identify the composition of the deep mantle component as He isotopes rarely correlate with incompatible trace element and radiogenic isotope tracers. It is supposed that this is due to the high He concentration of the deep mantle compared to degassed/enriched mantle reservoirs dominating the He in mixtures, although this is far from widely accepted. The modern Afar plume is natural laboratory for testing the prevailing paradigm.The 3He/4He of basalt glasses from 26°N to 11°N along the Red Sea spreading axis increases systematically from 7.9 to 15 Ra. Strong along-rift relationships between 3He/4He and incompatible trace element ratios are consistent with a binary mixture between moderately enriched shallow asthenospheric mantle in the north and plume mantle evident in basalts from the Gulf of Tadjoura, Djibouti (the Ramad enriched component of Barrat et al. 1990). The high-3He/4He basalts have trace element-isotopic compositions that are similar, but not identical, to the high 3He/4He (22 Ra) high Ti (HT2) flood basalts erupted during the initial phase of the Afar plume volcanism (Rogers et al. in press). This suggests that the deep mantle component in the modern Afar plume has a HIMU-like composition. From the hyperbolic 3He/4He-K/Th-Rb/La mixing relationships we determine that the upwelling deep mantle has 3-5 times higher He concentration than the asthenosphere mantle beneath the northern Red Sea.Barrat et al. 1990. Earth and Planetary Science Letters 101, 233-247.