Nicolas Carry

Strain partitioning along the anatectic front in the Variscan Montagne Noire massif (southern French Massif Central)

We decipher late-orogenic crustal flow characterized by feedback relations between partial melting and deformation in the Variscan Montagne Noire gneiss dome. The dome shape and finite strain pattern of the Montagne Noire Axial Zone (MNAZ) result from the superimposition of three deformations (D1, D2 and D3). The early flat-lying S1 foliation is folded by D2 upright ENE-WSW folds and transposed in the central and southern part of the MNAZ into steep D2 high-strain zones consistent with D2 NW-SE horizontal shortening, in bulk contractional coaxial deformation regime that progressively evolved to noncoaxial dextral transpression. The D2 event occurred under metamorphic conditions that culminated at 0.65 ± 0.05 GPa and 720 ± 20°C. Along the anatectic front S1 and S2 foliations are transposed into a flat-lying S3 foliation with top-to-NE and top-to-SW shearing in the NE and SW dome terminations, respectively. These structures define a D3 transition zone related to vertical shortening during coaxial thinning with a preferential NE-SW to E-W directed stretching. Depending on structural level, the metamorphic conditions associated with D3 deformation range from partial melting conditions in the dome core to subsolidus conditions above the D3 transition zone. We suggest that D2 and D3 deformation events were active at the same time and resulted from strain partitioning on both sides of the anatectic front that may correspond to a major rheological boundary within the crust.

P/T ratio in high-pressure rocks as a function of dip and velocity of continental subduction

High-pressure–low-temperature (HP-LT) metamorphic rocks that belong to the same orogen commonly show alignment of their peak pressure and related temperature within a pressure-temperature (PT) diagram, defining a pressure/temperature (P/T) ratio. In the Aegean region, for example, two metamorphic belts of different ages, the Eocene Cycladic blueschists and the Miocene Cretan blueschists, show contrasting P/T ratios that are characterized by the slope and the temperature at 10 kbar (ΔP/ΔT and T@10 kb). Peak PT data for the Cyclades yield a ΔP/ΔT of 0.056 and T@10 kb of 375 °C, while the Cretan blueschists give a much larger ΔP/ΔT with a lower T@10 kb. One-dimensional (1D) modeling of the thermal evolution of a subducted continental margin shows that subduction velocity controls both the slope ΔP/ΔT and T@10 kb of the P/T ratio, and the subduction dip only controls T@10 kb. On these bases, the variations of P/T ratios in the Aegean region reflect variations through time of subduction dip and velocity. Eocene subduction for Cycladic blueschists burial occurred at a rate of 1.5 cm a −1 , while subduction velocity during Cretan blueschists formation is found to be 2.75 cm a −1 . Because the convergence rate between Africa and Eurasia is constant and ~1–1.5 cm a −1 at these times, the active southward rollback of the Aegean slab during the Oligo-Miocene likely explains the larger subduction velocity for the Cretan HP-LT rocks. These results exemplify the use of this new modeling approach as a proxy to quantify dip and velocity of continental subduction from the P/T ratio of high-pressure rocks.

Hydrochemical response of a fractured carbonate aquifer to stress variations: application to leakage detection of the Vouglans arch dam lake (Jura, France)

Leakage detection and the prediction of the behavior of fractured rocks subjected to variations in hydrostatic pressure are important issues in hydropower engineering. Some large water reservoirs are constructed in karstic carbonate areas. In order to understand underground circulation in the limestone/dolomite foundation of the Vouglans dam (Jura, France), groundwater chemistry analysis, according to geological conditions, is used. Statistical analyses (PCA and DFA) are carried out: (1) to accurately characterize the contrast in chemical composition resulting from the interaction between surface and groundwater in a poorly contrasted environment, due to the shared carbonated context, and (2) to reassign individuals into homogeneous groups with respect to the variables studied. PHREEQC is used to determine the potential for the precipitation of secondary minerals from the water chemistry mainly in order to determine the geochemical control of clogging. Three types of hydrodynamic behavior were identified in the sector. The origin and transfer time of inflows vary with respect to seasons. The area of influence of the rapid transit of the lake water varies predominantly in response to changes in the lake water level. The hydrostatic pressure of the water column influences the opening and closing of cracks at the bottom of the dam. Drains intercepting an upstream fissure are highlighted by hydrochemical measurements. Inflows are oversaturated with respect to calcite and are thus likely to precipitate this mineral. Zones with a strong clogging capacity correspond to the zones with slow transit groundwater.

Groundwater—Surface waters interactions at slope and catchment scales: implications for landsliding in clay-rich slopes

Understanding water infiltration and transfer in soft-clay shales slopes is an important scientific issue, especially for landsliding. Geochemical investigations are carried out at the Super-Sauze and Draix-Laval landslides, both developed in the Callovo-Oxfordian black marls, with the objective to define the origin of the groundwater. In situ investigations, soil leaching experiments and geochemical modeling are combined to identify the boundaries of the hydrological systems. At Super-Sauze, the observations indicate that an external water flow occurs in the upper part of the landslide at the contact between the weathered black marls and the overlying formations, or at the landslide basement through a fault network. Such external origin of water is not observed at the local scale of the Draix-Laval landslide but is detected at the catchment scale with the influence of deep waters in the streamwater quality of low river flows. Hydrogeological conceptual models are proposed emphasizing the role of the interactions between local (slope) and regional (catchment) flow systems. The observations suggest that this situation is a common case in the Alpine area. Expected consequences of the regional flows on slope stability are discussed in term of rise of pore water pressures and physicochemical weathering of the clay shales.

Comparison of Flow Processes in Drains and Low Permeability Volumes of a Karst System in the French Jura Mountains: High-Resolution Hydrochemical Characterization During a Flood Event

The Fertans karst system is a field site of the hydrogeological observatory “Jurassic Karst” in the French Jura Mountains. The site is located within karstified upper Oxfordian limestones. Two monitoring stations were installed for simultaneous monitoring of a spring, draining a fracture, and a borehole within the microfractured limestone giving access to the low permeability volume (LPV). A flood event was followed in June 2012, with high-frequency physicochemical (pressure, T°C, E.C) and hydrochemical measurements (major and trace elements, total organic carbon) at both stations. Infiltration (TOC, Fe, Al) and long-time residence markers (Mg) were studied in order (1) to analyze the response of the drain (spring SA) and the LPV (borehole C3) to the rainfall event, (2) to identify the origin of the flood waters, and (3) to characterize water exchange between the 2 compartments. Based on statistical analysis (PCA, DFA), it was possible to characterize different mixing end-members and to identify exchanges between drains and LPV. The hydrochemical signal of the flood was observed rapidly at the spring (3 h); in contrast, the borehole showed a much higher inertia and an arrival of infiltration water in two steps: a first peak attributed to drain contribution, and a second peak attributed to diffusively infiltrated rain water, spreading out slowly through the LPV. The infiltration water signature was typically enriched in organic carbon, Fe, and Al derived from interaction with the clay–humus complex of the surface soil. Nitrates which are often used as indicator for direct surface infiltration seemed to have accumulated over longer time periods prior to the flood in the LPV, which behaved like a diffusive nitrate source.