Abdeltif Lahfid

In foreland fold‐thrust belts, tectonically deformed aquifers remain underexplored, largely due to the complex architecture of their hydrogeological reservoirs. This limited understanding prevents the development of sustainable management policies to face the current decline in groundwater availability. In the southwestern Subalpine Chains, we characterised the structure of the folded Senonian karst aquifer of the Dévoluy Massif, which is transected by the Median Dévoluy Thrust. Using surface structural data, we performed an original 3D structural model of the massif. Quantitative analyses of this model indicate a total Senonian package volume of 114 km ³ , of which 25 km ³ lie below the elevation of Gillardes Spring, the main karst discharge. A structural restoration of the base of the Senonian indicates Alpine shortening of ~4.4 km in the southern part of the massif and ~1.2 km in the northern part. The deformation of the karst reservoir into two N‐trending synclines, separated by the Median Dévoluy Thrust, is identified as a key control on the northward groundwater drainage pattern, culminating at Gillardes Spring. This 3D structural modelling approach also allowed for identification of suitable areas for the exploration of new deeper resources. Finally, this study highlights the potential of a hydrostructural approach and 3D structural modelling for assessing the hydrogeology in tectonically deformed karst reservoirs.

Evaporites have a strong impact on the structural and sedimentary evolution of sedimentary basins and fold-and-thrust belts. They also have a thermal conductivity that can be more important than other sedimentary rocks and are thus able to modify the thermal history of these sedimentary basins and fold-and-thrust belts. Even though this property is known and has been of interest for the oil and gas industry, no field examples have been studied trying to decipher how salt rock impacts temperature distribution in fold-and-thrust belts. In this paper, we use the Raman Spectroscopy on Carbonaceous Material (RSCM) to track the record of the peak thermal event around three salt structures from the southern sub-Alpine fold-and-thrust belt in SE France. These three salt structures are (1) the Astoin allochthonous salt sheet and the associated overturned megaflap, (2) the Rocher de Hongrie and (3) the Daluis diapir. Our results show that the resulting record of peak temperatures around the structures is different depending on the type of salt structure and its kinematic. The Astoin structure shows that salt tectonics during the Jurassic-Cretaceous has impacted the temperature distribution around the allochthonous salt sheet while at Daluis and the Rocher de Hongrie, the temperatures have overprinted an already existing salt-related structure. The impact of the salt structure on temperature distribution is always local but the interpretation of the RSCM temperatures may systematically be difficult without considering early salt tectonics in the structural evolution of the area.

Whereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (HT/LP) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (R_o) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures (T_max) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously publishedT_maxdata, we propose a new map depicting the spatial distribution of the HT/LPmetamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with R_oandT_maxdata to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of R_ovalues measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the HT/LPmetamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no HT/LPmetamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.

We describe a singular structure in Upper Cretaceous rocks of the Eaux‐Chaudes massif of the western Pyrenees, consisting of a kilometre‐scale fold nappe with a sheared overturned limb. High ductile strain attests a deformation style rarely reported for the alpine Pyrenees, and peak temperature in Upper Cretaceous carbonates is estimated by Raman spectroscopy of carbonaceous material palaeothermometry in the lower greenschist facies (>300°C). The normal fold limb retains the original sedimentary textures, while the overturned limb shows calcite crystal‐plastic deformation and dynamic recrystallization, with crystallographic preferred orientation. The observed ductility and metamorphic temperature bear similarities with the lower Helvetic nappes of the Alps, suggesting deep burial and/or possibly high geothermal gradient in this part of the Pyrenees.

The fossil rift in the North Pyrenean Zone, which underwent high temperature‐low pressure metamorphism and alkaline magmatism during Early Cretaceous hyperextension, was studied to explore the geothermal regime at the time of rifting. In this work, we combined Raman lab analysis and thermal numerical modelling to shed light on the distribution of geothermal gradients across the inverted hyperextended Mauléon rift basin during Albian and Cenomanian time, its period of active extension. Data were acquired from a set of 155 samples from densely spaced outcrops and boreholes, analyzed using Raman spectroscopy of carbonaceous material. The estimated paleogeothermal gradient is strongly related to the structural position along the Albian‐Cenomanian rift, increasing along a proximal‐distal margin transect from ~34°C/km in the European proximal margin to ~37–47°C/km in the two necking zones and 57–60°C/km in the hyperextended domain. This pattern of the paleogeothermal gradient induced a complex interaction between brittle and ductile deformation during crustal extension. A numerical model reproducing the thermal evolution of the North Pyrenees since 120 Ma suggests that mantle heat flow values may have reached 100 mW/m² during the rifting event. This model reveals that above the thermal pulse, the temperature gradient varied within a small range of 55 to 62°C/km, as inferred from RSCM peak temperatures. We demonstrate that the style of reactivation during subsequent convergence influenced the thermal structure of the inverted rift system.

Taiwan is an archetypal example of continental accretionary wedges. Yet the generally poor knowledge of three‐dimensional strain distribution over time and role of architecture of the rifted margin shed doubt on the cylindrical two‐dimensional kinematic models of Taiwan collision. Here we provide new field‐based constraints on strain distribution, new Raman Spectroscopy on Carbonaceous Materials temperatures and apply mica‐chlorite multiequilibrium approach to determine pressure‐temperature in the Central Range of Taiwan. We identify three distinct structural domains that define zones of orthogonal shortening in the western Backbone Range and left‐lateral ductile shearing overprinted by left‐lateral transtensional brittle deformation in eastern Central Range. Field surveys show the lack of nappe stacking in the Backbone Range. Combining new temperature estimates with existing thermochronological constraints we emphasize that western Taiwan mostly inherited preorogenic thermal history. We show that metamorphic peak conditions of 5–6 kbar and 330–400 °C in the eastern Backbone Range and HP rocks of the Yuli Belt exhumed along the P‐T paths related to transcurrent deformation. We propose a three‐dimensional kinematic model of Taiwan accounting for the oblique motion of the Philippine Sea Plate relative to the plate boundary and the reactivation of a NS striking transform fault in the South China Sea rifted margin. Recent and ongoing strain partitioning in the Taiwan accretionary wedge is reflected by the coexistence of brittle left‐lateral shear, oblique extension, and contraction. Our results have impact on orogen‐based plate kinematic reconstructions that consider two‐dimensional kinematic evolution of orogens.

The architecture of the Pyrenean‐Cantabrian belt results from the inversion of a series of former Cretaceous rift basins. A HT‐LP metamorphic event dated at 105 to 85 Ma ago is commonly associated with an Albo‐Cenomanian episode of hyperextension of the continental crust. This metamorphism is well known in the eastern Basque‐Cantabrian Basin within the Nappe des Marbres Unit (NMU) that is preserved from intense compressional deformation during the Pyrenean orogeny. Based on a structural study at the scale of the eastern Basque‐Cantabrian Basin and on a dense sampling for T_RSCM estimates with the Raman spectroscopy of carbonaceous material (RSCM) method (Raman Spectrometry on Carbonaceous Material), we show following results: (1) the NMU has recorded two major phases of deformation, related to a localized extensional ductile foliation (S₁) during the Cretaceous rifting and later by the regional N‐S shortening, recorded by a regional cleavage (S₂) observed only in shale sediments. (2) The NMU is affected by early salt tectonics related to several diapirs and salt walls fed by the Upper Triassic evaporitic layer. (3) The NMU recorded maximum temperatures exceeding 550°C, which represent some of the highest temperatures along the Pyrenees. These new data demonstrate that there is no E‐W lateral metamorphic gradient across the belt and that hyperextension rifting occurred consistently within a high‐thermal regime; (4) metamorphic isograds are oblique in respect of the main structures, suggesting that the metamorphic event postdates most of the early salt‐related deformations prior to or coeval with early stages of rifting. As it represents one of the best preserved example of preorogenic hyperextended basin, the eastern Basque‐Cantabrian Basin record may be used regionally to better understand the rift to orogen evolution of the Pyrenean Internal Metamorphic Zone but also more generally as an analog of salt‐bearing hyperextended rift record.