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The greater Paris area has some 12 million inhabitants and 48 heating network production units that exploit the heat capacity of a 1.5 km deep aquifer, the Bathonian limestone. This is one of the most productive aquifers in the world for district heating, with an annual output of about 1.7 TWh of energy. The current challenge for Paris is to triple the number of heating networks using geothermal energy so as to reduce dependence on fossil fuels (40% in 2020 in France). As no detailed geothermal reservoir model is available, drilling involves geological risk. A recent well, drilled at Grigny (20 km south of Paris), turned out to be a failure in transmissibility because the bed of permeable limestones it tapped into was very thin (meter-scale). The main aim of this study is to create a digital database and a 3D geological model of this aquifer to minimize geological risks and optimize the location of future geothermal operations around Paris. By compiling data from 168 wells, a high-resolution 3D geological model of 360 km3 size is constructed (about 40 km x 50 km x 0.2 km), made up of 12.2 million cells and displaying sedimentary facies, sequence stratigraphy, porosity (Φ) and permeability (k). About 20% of the oolitic and bioclastic facies are of good reservoir quality (Φ > 13% and k > 350 mD), especially in two targeted, high-quality reservoir sequences. These facies of interest probably correspond to giant dunes and a shoal/barrier prograding from east to west. In these facies, permeable zones are generally 4 m thick and form patches of 1600 m x 1100 m, on average, elongated perpendicular to the depositional slope. 2D and 3D maps of temperature, salinity, porosity, transmissivity, and permeability allow us to understand the areas of interest for geothermal exploration, as demonstrated around Grigny. This model helps us to apprehend better the heterogeneous character of the reservoir for geothermal prospection and to reduce the risk of future doublets during well implantation. Detailed local models may be extracted to anticipate better the implantation of new doublets in areas with already densely spaced existing wells.

We have developed a multiscale approach for solving 2D and 3D nonlinear inverse problems of gravity data in estimating the basement topography. The inversion is carried out in two stages in which the long-wavelength features of the basement are first estimated from smoothed gravity data via a stochastic optimization algorithm. The solution of this stage is used as the starting model for a deterministic optimization algorithm to reconstruct the short-wavelength features from the full-spectrum gravity data. The forward problem is capable of handling lateral and vertical variations in the density of sediments. Two cases are considered regarding prior knowledge about the density: (1) The density contrast between sediments at the surface and the underlying basement and its vertical variations are a priori known, and (2) only the density contrast at the surface is known with its vertical gradient to be recovered in the inversion. In the former case, the unknowns of the problem are the depths, whereas in the latter case, they are the depths and density gradients defined individually for each prism. Therefore, the inverse problem is ill-posed and has many local minima. The stochastic optimization algorithm uses a random initial model and estimates a coarse model of the basement topography. By repeating the stochastic inversion, an ensemble of solutions is formed defining an equivalent domain in the model space supposed to be within the neighborhood of the global minimum of which several starting solutions are extracted for the secondary deterministic inversion. The presented methodology has been tested successfully in converging to the global minima in 2D and 3D cases with 50 and 2352 total number of prisms, respectively. Finally, the inversion algorithm is used to calculate the thickness of the sediments in the South Caspian Basin using the EIGEN-6c4 global gravity model.

Distributed temperature sensing is known to provide sharp signals which are very efficient for mapping hydraulically active fractures in wellbores. High-resolution temperature sensing has specifically demonstrated its capacity to characterize very low flows in wellbores. But as sharp as they can be, temperature profiles are often difficult to decipher. The aim of the present work is to provide and to test the “Borehole Heat Budget Calculator” (BHB Calculator), which is implemented as a fast and easy to use tool for the quantitative analysis of depth-temperature profiles. The Calculator is suitable for most pumping and draining configurations, as the heat budget is generalized for modelling multidirectional flow systems within the same wellbore. The formatted worksheet allows the quick exploitation of temperature logs, and is applicable for the characterization of distributed fractures in long screened wellbores. Objectives of the heat modelling are to enhance the readability of complex depth-temperature data, as well as to quantify distribution of inflow intensities and temperatures with depth. The use of heat budget helps to clearly visualize how heat conduction and heat advection contributions are distributed along wellbores profiles. Calculations of inflow temperatures and their evolution through pumping duration is a prerequisite to infer about the nature of aquifer properties (i.e. conduits, distributed or discrete fractures, porous media), as well as to give insight information about the mapping of effective flow paths draining the aquifer. The efficiency and limitations of the BHB Calculator are being tested through high-resolution temperature logging, along with complementary flowmetering and televiewing logging in fractured aquifers located in the St-Lawrence Lowlands, Quebec, Canada.

Due to uncertainty of single-method interpretation we applied 2-D integrated lithospheric modelling along three CELEBRATION 2000 profiles CEL01, CEL04 and CEL05. Modelling of the lithospheric thermal structure is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. The models for each profile were constrained by seismic modelling results of the large-scale international project CELEBRATION 2000. The results indicate large variations of the lithosphere thickness from the old and cold East European Craton (~200 km) and the Trans European suture zone via the Western Carpathian orogeny to the young and hot Pannonian Basin (~90 km). Important differences in the lithospheric thickness were also found along-strike of the Western Carpathian orogeny and the Trans-European Suture Zone. The western part of the Western Carpathians is characterized by weak thickening of the lithosphere (only about 145 km), while their eastern segment presents strong lithospheric thickening (~190 km). The Malopolska unit in southern Poland has a lithospheric thickness of about 130 km. Thickest lithosphere (220 km) is observed around the junction of the Carpathian Foredeep and the East-European Craton. The crustal thickness follows generally the course of the lithosphere-asthenosphere boundary. The results suggest different geodynamic evolution of the collision of the ALCAPA microplate with the European platform on the one hand and the East-European Craton on the other hand. It is suggested that the tectonic evolution of this very complex area consisting of different tectonic units has changed in time and space.

In France, heating networks are largely dependent on fossil fuels (42%), and deep geothermal energy represents less than 5% of the energy mix of heating networks. Deployment of geothermal energy in large cities is limited by a geological risk, difficult to predict. This risk constitutes an obstacle to the future development of geothermal energy in the Ile-de-France region. The aim of this work is to develop a predictive 3D reservoir model in terms of stratigraphic geometries, facies, porosity, permeability and temperature at a given location in Ile-de-France. We focus on the main geothermal reservoirs in the area: the Middle Jurassic limestones. In order to create this 3D model, 80 wells (630 logs), drilled over the last 60 years, were studied over an area of 800 km2. The first phase of this study consisted in digitizing the old well data, particularly log data on 80 wells (GR, Sonic, resistivity) and adding all recent wells (with neutron porosity and NMR logs). We also compiled from the drilling reports 694 porosity (phi) – permeability (k) values previously measured on cores from plugs, and we imported them into the geomodeller Petrel®. Two reference wells with cores of the reservoir were studied in detail from a sedimentological and stratigraphic point of view in order to link sedimentary facies, logs and phi-k in a well-defined sedimentological framework. We also digitized temperature in 40 wells. The sequence stratigraphy framework allows to define 11 3rd order stratigraphic sequences from the Bajocian (jason Zone) to the Middle Callovian (zigzag Zone). Twelve surfaces from Bj5 to Ca3 corresponding to 3rd order Maximum Regressive Surfaces (MRS) allow to correlate all wells and to define stratigraphic geometries. A total of 10 facies are grouped into 4 facies associations (1) marls of lower offshore (facies association FA1), (2) marl-limestone alternations of upper offshore (FA2), (3) oolitic grainstones of the shoreface (FA3) and (4) lagoon micritic limestones (FA4). These facies associations were coded in all wells according to the log depths. The best reservoir is mainly located in the oolitic and bioclastic grainstones (FA3) with average porosity of 12% and permeability of 130 mD. The lagoon micritic facies also presents interesting properties with average porosity of 8.2% and permeability of 46 mD. The model has been meshed into 6.5 million of cells split on 64 vertical cell layers of 150 m x150 m x about 5 m (length ×width ×height) each bearing specific property information (facies, porosity, permeability). The final model shows a high variability of the facies distribution over the 11 depositional sequences. The maximum thickness of the oolitic reservoir is about 50 m in the western part of the study area between surface Bt2 and Bt4. By combining the isopach map of oolitic facies between surface Bt2 and Bt4 with porosity above 10%, permeability of more than 100 mD and temperature larger than 60°C, we locate areas of interest for geothermal development in the Paris Basin.

International audience; This study presents laboratory experiments using surface Ground Penetrating Radar measurements to exhibit and monitor hysteresis of the water retention function of Fontainebleau sand. A commercial impulsionnal GPR system monitored the volumetric water content changes in a large sand column subject to different hydraulic heads applied at its bottom during drainage/wetting cycles. Coupled hydrodynamic and electromagnetic modeling was used to simulate radargrams whereas hydrodynamic modeling coupled with 1 D optical ray tracing was used to estimate the hydrodynamic parameters of the sand from GPR reflection two-way travel times. Statistical and uncertainty analysis were performed on numerical and experimental data. The range of optimized parameters obtained from experimental data were compared to those obtained with classical laboratory methods such as the hanging water column and the constant head permeameter. The range of parameters retrieved using GPR monitoring was consistent for the hydraulic conductivity at saturation and the van Genuchten parameters alpha(d), alpha(w). The difference between the GPR method and classical methods for the retrieved lambda and n van Genuchten's parameters are believed to be caused by the dynamic character of the proposed method.

International audience; Dated paleosea level markers and eustatic sea level changes are necessary but not sufficient information to calculate vertical motion rates on oceanic islands. Therefore, we use a procedure in which we work progressively back in time to incorporate the more recent vertical motion rates implied by the youngest paleoshorelines into the vertical motion history of all older shorelines. Specifically, we calculate the time‐averaged vertical motion rates required to explain the present‐day elevations of the dated sequence of paleoshorelines on Santiago volcanic island (Cape Verde). We thus obtain a vertical motion history consisting of time‐averaged vertical motion rates spanning the five intervening periods between paleoshoreline formation and the present day: (1) 5.06 to 3.29 Ma—seamount growth or island subsidence because all the rocks in this period are submarine; (2) fast uplift (approximately 0.96 mm/a) from 3.29 to 2.87 Ma, mostly responsible for putting submarine lavas currently close to 410 m altitude; (3) relatively fast subsidence (approximately −0.11 mm/a) between 2.87 and 2.18 Ma; (4) stagnation from 2.18 to 0.811 Ma; and (5) relatively fast uplift (approximately 0.14 mm/a) between 0.811 and 0 Ma. We numerically tested top‐down (volcanic loading) and bottom‐up (lithosphere thinning, underplating, and mantle plume) mechanisms to explain the inferred vertical movements, and we conclude that volcanic loading and crustal underplating are capable of producing the observed subsidence and uplift, respectively.

International audience; The Hradište border fault zone has played an important role in the development of the tectonic contact of the Cenozoic sediments of the Turiec Basin with the Malá Fatra Mountains crystalline basement. Seismic, geoelectric, radiometric, gravimetric, magnetometric and ground penetrating radar measurements were used to study the physical properties and determine the exact position and inclination of this fault zone down to a depth of up to 40 m. The Hradište border fault zone represents an almost vertical physical boundary characterized by decreasing seismic velocity (from 3.0 km.s−1 to 2.2 km.s−1) and decreasing electrical resistivity (500 to 150 Ω.m) when passing from the basement west of the fault to sediments to its east. It corresponds also to a compact segment of the lowest volume activity of radon 222Rn values in soil air (8 kBq.m−3 on average) and maximum horizontal gravity gradient (−0.0076 mGal.m−1). The discovery of this anomalous zone also helps us to distinguish two different anomalous blocks. The block west of the fault represents the orthogneisses of the Tatric crystalline complex belonging to the Malá Fatra Mountains. The eastern block is built-up by the Bystrička Member Pliocene sediments of the Turiec Basin. Our study serves as a case study for geophysical research on faults in different tectonic units of the Western Carpathians and other similar orogens.

Potential field data such as geoid and gravity anomalies are globally available and offer valuable information about the Earth's lithosphere especially in areas where seismic data coverage is sparse. For instance, non-linear inversion of Bouguer anomalies could be used to estimate the crustal structures including variations of the crustal density and of the depth of the crust-mantle boundary, i.e. Moho. However, due to non-linearity of this inverse problem, classical inversion methods would fail whenever there is no reliable initial model. Swarm intelligence algorithms, such as Particle Swarm Optimization (PSO), are a promising alternative to classical inversion methods because the quality of their solutions does not depend on the initial model, they do not use the derivatives of the objective function, hence allowing the use of L1 norm, and finally they are global search methods, meaning the problem could be non-convex. In this paper, Quantum-behaved Particle Swarm (QPSO), a probabilistic swarm intelligence-like algorithm, is used to solve the non-linear gravity inverse problem. The method is first successfully tested on a realistic synthetic crustal model with a linear vertical density gradient and lateral density and depth variations at the base of crust in the presence of white Gaussian noise. Then, it is applied to the EIGEN 6c4, a combined global gravity model, to estimate the depth to the base of the crust and the mean density contrast between the crust and the upper-mantle lithosphere in the Eurasia-Arabia continental collision zone along a 400 km profile crossing the Zagros Mountains (Iran). The results agree well with previously published works including both seismic and potential field studies. This article is protected by copyright. All rights reserved

International audience; We study electrical anisotropy using azimuthal electrical resistivity tomography (A-ERT) for identifying geological layers and determining the preferential aquifer flow direction. The work presented in this paper aims at calculating the anisotropy coefficient from the inverted resistivity measurements, relating the geophysical results to the site geology and water flow direction, and comparing the results with those obtained from prior studies using hydrogeological approaches. The study area is located in the Eastern Mitidja basin, about 15 km east of Algiers. The work carried out includes three measurement points, totalling 24 ERT profiles. For each point, eight A-ERT profiles, using Wenner-Schlumberger array configuration, were performed every 22.5 degrees around a fixed central point. The data processing includes two-dimensional inversion of each profile, representation of the inverted resistivities at the central points as a function of the azimuth in polar diagrams, and implementation of an inversion program to determine the best fitting azimuthal anisotropy parameters. The origin of electrical anisotropy is probably due to grain alignment, which is often observed in fine-grained sediments such as clays and some sands. This alignment creates a higher electrical resistivity perpendicular to the alignment than parallel to it, which is an indicator for higher hydraulic permeability in the minimum resistivity direction, which corresponds to the preferential groundwater flow direction.

International audience; In this study, we developed a method to invert jointly Rayleigh wave group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface wave group velocities are sensitive mainly to shear wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18–33 km) and Makran fore-arc (33–37 km) to the volcanic-arc (44–46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.

International audience; The Pico-Faial ridge is a steep WNW-ESE volcanic ridge that has developed within the Nubia-Eurasia diffuse plate boundary, close to the Azores Triple Junction. The ridge comprises two islands, Pico and Faial, separated by a shallow (< 100 m depth) and narrow (< 8 km) channel. Despite some similarities, the two islands show contrasting features still deserving explanation: (1) meaning of GPS data; (2) meaning of palaeomagnetic data; (3) the island-scale Faial Graben does not have a counterpart in Pico; (4) both islands comprise a main central volcano each, but the one in Faial is symmetrical and the one in Pico is asymmetrical; (5) Pico shows evidence of at least two large-scale flank collapses, but none has been recognized so far in Faial. The new data reported here lead to the following probable answers: (1) the GPS data show much larger vertical and horizontal ground motions in Faial, which could be the result of recent volcanism and tectonics; (2) the cyclicity inferred from the paleomagnetic data in Pico could be related to inflation/deflation cycles that could have triggered the inferred flank collapses; (3) the Faial Graben ends abruptly at the eastern edge of the island, which we interpret as inhibited propagation to the east because of the load and stresses imposed by the nearby large Pico Volcano; (4) we attribute the asymmetry of the Pico Volcano to partial flank collapse recognizable in new offshore seismic profiles; (5) either the Faial island is actually gravitationally more stable, or there have been flank collapses not yet recognized. All these processes have concurred to shape the two islands and the ridge to their current morphology, but the current morphologies of Pico and Faial result mainly from three of these processes: volcanism, tectonics and large-scale landslides.

Magnetic anomaly analysis is an important method to study the structure of deep crust. With the assumption of random magnetization sources, the structure of a magnetic layer can be inverted via spectral analysis. Curie point depth (CPD), the depth at which rocks lose their ferromagnetic properties, is the bottom of a magnetic layer. In this study, we estimate from the magnetic anomaly data of EMAG2 dataset the CPD and the top of the magnetic layer in North China. With a moving window of 180 km × 180 km, we calculate the average top and centroid depth of the magnetic layer in each window and determine the regional CPD distribution across North China. The CPD of North China varies from 18 to 32 km. In addition, the CPD in the western part of the North China Craton is deeper than that in the eastern part. The shallowest CPD is located near the Bohai Sea. When compared to available heat flow data, the estimated CPD values are consistent with thermal conductivity of 1.8–3.2 Wm−1 K−1 and on heat production value of 0.4–1.3 µWm−3.

The lithospheric structure of ancient cratons provides important constraints on models relating to tectonic evolution and mantle dynamics. Here we present the 3D lithospheric structure of the North China Craton (NCC) from a joint inversion of gravity, geoid and topography data. The NCC records a prolonged history of Archean and Paleoproterozoic accretion of crustal blocks through subduction and collision building the cratonic architecture, which was subsequently differentially destroyed during Mesozoic through extensive magmatism. The thermal structure obtained in our study is considered to define the lithosphere-asthenosphere boundary (LAB) of the NCC, and reflects the density variations within the mantle lithosphere. Employing the Moho depths from deep seismic sounding profiles for the inversion, and based on repeated computations using different parameters, we estimate the Moho depth, LAB depth and average crustal density of the craton. The Moho depth varies from 28 to 50 km and the LAB depth varies from 105 to 205 km. The LAB and Moho show concordant thinning from West to East of the NCC. The average crustal density is 2870 kg m − 3 in the western part of the NCC, higher than that in the eastern part (2750 kg m − 3 ). The results of joint inversion in our study yielded LAB depth and lithospheric thinning features similar to those estimated from thermal and seismic studies, although our results show different depth and variations in the thickness. The lithosphere gently thins from 145 to 105 km in the eastern NCC, where as the thinning is much less pronounced in the western NCC with average depth of about 175 km. The joint inversion results in this study provide another perspective on the lithospheric structure from the density properties and corresponding geophysical responses in an ancient craton.

This study showcases the value of drone photogrammetry in creating a meter-scale geological model of complex carbonate geobodies. Although drone photogrammetry is now commonly used for modeling the sedimentary facies and architecture of sandstone outcrops, its use is not widespread in creating geomodels of carbonate geobodies. Drone photogrammetry can generate accurate line-drawing correlation and detailed architecture analysis along inaccessible vertical faces of outcrops and permits observations of unreachable places. This work models the Bathonian limestones of Massangis quarry (Burgundy) as an example. The quarry covers an area of 0.4 km2 and is considered as an analogue for the Oolithe Blanche geothermal reservoir in the center of the Paris Basin. The Massangis quarry model represents a good analogue for reservoir microporosity and secondary porosity associated with dedolomitization. Ten facies are described and grouped into three facies associations (1) clinoforms, (2) tidal to subtidal facies, and (3) lagoonal facies. The clinoforms are sets of very large marine dunes 15–20 m high that prograded N70° across the platform as part of a regressive systems tract. Moldic rhombohedral pore spaces associated with dedolomitization are well-expressed within clinoforms and in the bioturbated levels of lagoonal facies. Drone photogrammetry combined with the “Truncated Gaussian with Trends” algorithm implemented in Petrel® software is used to create a geological model that faithfully reproduces the facies architecture observed in the quarry cliffs. Drone photogrammetry can be combined with field work to describe and locate facies and so constrain the spatial distribution of petrophysical properties. It also helps constraining the shapes of geobodies in the model grid for more realistic geological static models and helps providing 3D petrophysical models from an outcropping analogue for geothermal and petroleum reservoirs.

International audience; We present a comprehensive lithospheric model of the Black Sea comprised of thermal structure, sediment thickness, and depths to Moho and lithosphere-asthenosphere boundary (LAB). The crustal structure (depths to Moho boundary and basement) is first estimated via a simultaneous joint 3D nonlinear inversion of satellite gravity data and the Gzz component of gravity gradiometry tensors. The results are then used for detailed 2D iterative forward modeling of potential field data based on the thermal structure of the lithosphere. In addition to potential field data, heat flow measurements are used for the iterative forward modeling. Eight 2D profiles across the Black Sea were selected to achieve a full coverage of the main geological structures. The average depth to basement is estimated as 10-11 km in the East Black Sea (EBS) basin and 11-12 km in the West Black Sea (WBS) basin. The average depth to basement beneath the ridges is estimated between 2 and 5 km. The modeling results show a mean thickness of 20 km and 21 km for the crust and 150 and 100 km for the lithosphere in the central parts of EBS and WBS, respectively. While the lithosphere is thicker in the EBS than in the WBS, a LAB uplift in the central basins is obvious in the reconstructed model. Also, the calculated surface heat flow is highest at the ridges while the lowest values are found over thick sediments in the central basins. After correcting the calculated heat flow data for the sedimentation effect, the resulting values match the published measurements indicating the reliability of the obtained thermal structure for the Black Sea.

International audience; The inversion of Rayleigh wave group velocity dispersion curves is challenging, because it is non-linear and multimodal. In this study, we develop and test a new Rayleigh wave dispersion curve inversion scheme using the Shuffled Complex Evolution (SCE) algorithm. Incorporating this optimization algorithm into the inverse procedure not only can effectively locate the promising areas in the solution space for a global minimum but also avoids its wandering near the global minimum in the final stage of search. In addition, our approach differs from others in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for thickness, velocities and densities and their vertical gradients of four layers, sediments, upper-crust, lower-crust and upper mantle. The proposed inverse procedure is applied to non-linear inversion of fundamental mode Rayleigh wave group dispersion curves for shear and compressional wave velocities. At first, to determine the efficiency and stability of the SCE method, two noise-free and two noisy synthetic data sets are inverted. Then real data for Makran region in SE Iran are inverted to examine the usage and robustness of the proposed approach on real surface wave data. In a second step, we applied 3D Gravity Modeling based on surface wave analysis results to obtain the density structure and thickness of each layer. The reason for using both types of data sets, is that gravity anomaly has a bad vertical resolution and surface wave group velocities are good for placing layer limits at depth, but they are not very sensitive to densities. Therefore, using gravity data increases the overall resolution of density distribution. In a final step, we used again the SCE method to invert the fundamental mode Rayleigh wave group dispersion curves based on the gravity results. Gravity results like thicknesses and sediment densities have been used to constrain the limit of search space in the SCE method. Results show a high shear and compressional velocity under the Gulf of Oman which reduce to the North of the Makran region. The Moho depth of the Oman Gulf is about 18–28 km and it increases to 46–48 km under the Taftan-Bazman volcanic-arc. The density image shows an average crustal density with maximum values under the Gulf of Oman decreasing northward to the Makran.

International audience; The interpretation of five 2D PSTM seismic reflection sections (14 s TWT) covering the northern Levant Basin revealed a total of 10 horizons, among which, one is interpreted as an interface that may represent the Moho. The interpretation of seismic packages and their bounding surfaces as well as the seismic facies analysis were constrained by published 2D seismic interpretations of the northern Lebanese offshore. A total of nine seismic packages are identified in the basin with ages varying from the Mid Jurassic to the Quaternary. The filling of the basin is made up of thick Cenozoic and Mesozoic strata deposited above rifted Triassic – Early Jurassic interval. The sediments are deposited in deep water mixed-settings resulting from high-stand systems (various types of carbonate platforms) and low-stand systems (siliciclastic and carbonate deep-water turbidite complexes). Carbonate and siliciclastic systems are sealed by 1–1.5 km of evaporites, and underlie Plio-Quaternary hemipelagic and pelagic sediments intercalated by turbiditic sheets.The time horizons were converted into depth using two methods; the first one is based on stacking velocities and the second one on velocities resulting from refraction data. 2D crustal modeling was achieved by integrating free-air gravity anomaly, geoid heights and topography data on the five interpreted PSTM seismic lines. The models representing five sections across the northern Levant Basin, show a progressively attenuated crystalline crust in an EW direction (away from the basin's eastern margin). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower continental crust. The Moho appears to be situated between 20 and 23 km in the central and southern Lebanese offshore. Estimated surface heatflow in the basin is around 40 mW/m2, which is lower than reported values for the onshore and the margin. These differences in heatflow values between the offshore, the margin and the onshore have an important impact on hydrocarbon maturation and assessment of potential petroleum systems.

International audience; This work aims to provide an overview of the thermal processes that shape wellbore temperature profiles under static and dynamicconditions. Understanding of the respective influences of advection and conduction heat fluxes is improved through the useof a new heat budget at the borehole scale. Keeping in mind the thermal processes involved, a qualitative interpretation of thetemperature profiles allows the occurrence, the position, and the origin of groundwater flowing into wellbores from hydraulicallyactive fractures to be constrained.With the use of a heat budget developed at the borehole scale, temperature logging efficiency hasbeen quantitatively enhanced and allows inflow temperatures to be calculated through the simultaneous use of a flowmeter. Undercertain hydraulic or pumping conditions, both inflow intensities and associated temperatures can also be directly modelled fromtemperature data and the use of the heat budget. Theoretical and applied examples of the heat budget application are provided.Applied examples are shown using high-resolution temperature logging, spinner flow metering, and televiewing for three wellsinstalled in fractured bedrock aquifers in the St-Lawrence Lowlands, Quebec, Canada. Through relatively rapid manipulations,thermal measurements in such cases can be used to detect the intervals or discrete positions of hydraulically active fractures inwellbores, as well as the existence of ambient flows with a high degree of sensitivity, even at very low flows. Heat budget calculationsat the borehole scale during pumping indicate that heat advection fluxes rapidly dominate over heat conduction fluxes with theborehole wall. The full characterization of inflow intensities provides information about the distribution of hydraulic propertieswith depth. The full knowledge of inflow temperatures indicates horizons that are drained from within the aquifer, providingadvantageous information on the depth from which groundwater originates during pumping.1. Introduction

ACT; International audience; Surface Ground-Penetrating Radar (GPR) data have been acquired along the floor as well as along the vertical walls of a tunnel inside a karstic limestone reservoir in Rustrel. Geological study previously demonstrated the existence of stratification planes with an average dip of 25° to the south and numerous subvertical fault planes. The mono-offset GPR profile analysis acquired along the vertical wall of the tunnel demonstrates the presence of dipping reflectors that can be followed as deep as 16 m from the acquisition surface with 250 MHz nominal antennas. The position of these reflectors coincides with observations of faults recorded in a report written during the tunnel excavations. © 2017 IEEE.

We present four new 2D lithosphere-scale transects crossing central Europe from the West European Platform in the North to the Aegean Sea in the South and from the Adriatic Sea in the West to the East European Platform in the East. Modelling is based on the joint interpretation of gravity, geoid, topography and surface heat flow data with temperature-dependent density. Wherever possible, crustal structure is constrained by seismic data. The thickness of the lithosphere decreases from the older and colder platforms to the younger and hotter Pannonian Basin with a maximum thickness under the Eastern and Southern Carpathians. Similar to earlier studies, the thickness of the Carpathian arc lithosphere varies between 150 km in the North (the Western Carpathians) and about 300 km in the Vrancea zone (the Eastern and Southern Carpathian junction). In the Platform areas it is between 120 and 150 km and in the Pannonian Basin it is about 70 km. The lithosphere thickens strongly underneath the Transylvanian Basin reaching locally values of nearly 200 km which is thicker than in earlier studies. New results are given for the Southern Carpathians and the Moesian Platform. The models show that the Moesian Platform is overthrust from the North by the Southern Carpathians and from the South by the Balkanides and characterised by bending of this platform. In all transects, the thickest crust is found underneath the Carpathian Mountains or, as in the case of the Vrancea area, under their immediate foreland. The thickest crust outside the orogens is modelled for the Moesian Platform with Moho depths of up to 45 km. The thinnest crust is located under the Pannonian Basin with about 26–27 km.

The arch-and-basin geometry that characterizes North Africa was achieved at the end of Paleozoic times. It has been subsequently reactivated during the Mesozoic-Cenozoic with, in particular, the development of large topographic anomalies. Among these, the Tuareg Shield forms a topographic high in which the Pan-African basement reaches 2400 m above sea level (Hoggar core). While Cretaceous sedimentary remnants suggest a possible stage of subsidence during the Mesozoic, currently the area forms a swell, emphasized by Cenozoic volcanic episodes since 35 Ma. In this context, we present the first apatite (U-Th)/He thermochronological data acquired across this swell, with mean ages ranging from 78 ± 22 Ma to 13 ± 3 Ma. These results demonstrate the existence of a widespread Eocene exhumation of the shield before volcanic activity began, which reflects large-scale vertical processes. In the northeastern part of the swell, Cretaceous continental sedimentary remnants unconformably lying on the basement close to our samples evidence that they were near the surface at that time. This study shows that basement rocks have undergone subsequent heating at ∼60–80 °C, suggesting a burial of more than 1 km after the Early Cretaceous. This conclusion can be possibly extended over the whole Tuareg Shield.

We present a 3D algorithm to obtain the density structure of the lithosphere from joint inversion of free air gravity, geoid and topography data based on a Bayesian approach with Gaussian probability density functions. The algorithm delivers the crustal and lithospheric thicknesses and the average crustal density. Stabilization of the inversion process may be obtained through parameter damping and smoothing as well as use of a priori information like crustal thicknesses from seismic profiles. The algorithm is applied to synthetic models in order to demonstrate its usefulness. A real data application is presented for the area of northern Iran (with the Alborz Mountains as main target) and the South Caspian Basin. The resulting model shows an important crustal root (up to 55 km) under the Alborz Mountains and a thin crust (ca. 30 km) under the southernmost South Caspian Basin thickening northward to the Apsheron–Balkan Sill to 45 km. Central and NW Iran is underlain by a thin lithosphere (ca. 90–100 km). The lithosphere thickens under the South Caspian Basin until the Apsheron–Balkan Sill where it reaches more than 240 km. Under the stable Turan platform, we find a lithospheric thickness of 160–180 km.

International audience; We present a 2D profile of density and temperature distribution in the lithosphere across Iran along a more than 1600 km long profile extending from the Oman Gulf in the South to the Kopeh-Dagh and the Turan platform in the North. Gravity, geoid, topography and surface heat flow data were used for modeling, assuming local isostatic equilibrium. As much as possible, crustal structure has been constrained by seismic data. Crustal thickening is found under the Taftan-Bazman volcanic-arc (up to 47 km), under the Binalud foreland (∼54 km) and beneath the Kopeh-Dagh mountains (up to 50 km) whereas thin crust has been obtained under the Oman Gulf (20 km). Moho depth under the Lut block and the Turan platform is about 40 km. The lithospheric thickness is ∼90 km under the Oman Gulf and increases slightly until the Jazmourian depression. Then the lithospheric-asthenospheric boundary (LAB) bends significantly and sinks to ∼260 km under the Taftan-Bazman volcanic-arc. The LAB depth is about 190 km beneath the Lut block. A slight increase of LAB depth under the Binalud foreland towards the North may indicate a suture zone. Under the Turan platform, the LAB depth reaches ∼210 km. We also modeled two possible positions of the deep suture zone in NE Iran (along the main Kopeh-Dagh fault or along the Atrak River) and concluded that, when the suture zone is along the Atrak River, we obtained the better fit between calculated and measured data.

This paper discusses the deep structure of the lithosphere underlying the easternmost Mediterranean region, in particular the Levant Basin and its margins, where the nature of the crust, continental versus oceanic, remains debated. Crustal thickness and the depth of the lithosphere-asthenosphere boundary (LAB) as well as the crustal density distribution were calculated by integrating surface heat flow data, free-air gravity anomaly, geoid and topography. Accordingly, two-dimensional, lithospheric models of the study area are discussed, demonstrating the presence of a progressively attenuated crystalline crust from E to W (average thickness from 35 to 8 km). The crystalline crust is best interpreted as a strongly thinned continental crust under the Levant Basin, represented by two distinct components, an upper and a lower crust. Further to the west, the Herodotus Basin is believed to be underlain by an oceanic crust, with a thickness between 6 and 10 km. The Moho under the Arabian Plate is 35–40 km deep and becomes shallower towards the Mediterranean coast. It appears to be situated at depths ranging between 20 and 23 km below the Levant Basin and 26 km beneath the Herodotus Basin, based on our proposed models. At the Levantine margin, the thinning of the crust in the transitional domain between the onshore and the offshore is gradual, indicating successive extensional regimes that did not reach the beak up stage. In addition, the depth to LAB is around 120 km under the Arabian and the Eurasian Plates, 150 km under the Levant Basin, and it plunges to 180 km under the Herodotus Basin. This study shows that detailed 2D lithosphere modeling using integrated geophysical data can help understand the mechanisms responsible for the modelled lithospheric architecture when constrained with geological findings.

The Harrat Ash Shaam volcanic province (HASV) is the largest volcanic field in the Arabian plate. It developed during the Cenozoic close to the southern part of Dead Sea fault system and has been linked to the tectonic evolution of the Red Sea rifting since the early Oligocene. The HASV is an ideal environment to study volcanism adjacent to a strike–slip fault (the Dead Sea fault system) and constrain the development of regional deformation along such lithospheric structures. We here present a morpho-structural analysis based on digital elevation data coupled with remote sensing observations and new K–Ar ages on fresh separated groundmass which allow us to propose a new volcano-tectonic model of the HASV. From Landsat7 Enhanced Thematic Mapper plus (ETM +) and SRTM data, we recognize more than 350 monogenetic volcanic cones grouped in three main clusters. Topographical variations between these clusters are interpreted as reflecting different volcanic phases. The new ages measured here range between 7.13 ± 0.10 Ma and 0.056 ± 0.009 Ma. Together with previous geochronological data, they reveal two main periods of volcanic activity. The first period lasted from late Oligocene up to early-middle Miocene (26–16 Ma), and the second period has been active since 13 Ma, indicating a gap of volcanic activity between ~ 16 and ~ 13 Ma. The volcano-tectonic evolution model suggests two different extensional styles, “en-echelon” rifting during the first period, and counterclockwise rotation during the second one. Alternative to the Afar plume hypothesis, a thinned lithosphere underneath the HASV as supported by geophysical modeling, can explain magma genesis in the northern part of the Arabian plate.

Subbasalt imaging can be improved by carefully applying pre-stack depth migration. Pre-stack depth migration requires a detailed velocity model and an accurate traveltime calculation. Ray tracing methods are fast but, often fail in calculating traveltimes in complex models, specially, when they feature high velocity contrasts. Finitte difference solutions of the eikonal are more stable and can produce a traveltime field for the whole model avoiding shadow zones. A synthetic test was carried out to check the performance of a new pre-stack depth migration algorithm in a model that features a high velocity layer surrounded by lower velocities. The results reasonably reproduce the original model. The same scheme was used to process long-offset reflection data from the Faroe Shelf where conventional techniques (stack) were insufficient to assess the structure under a basalt layer. Pre-stack depth migration produced an improved image which recovered the main features in the stacked section and allowed to identify some subbasalt coherent events.

International audience; Three-dimensional joined inversion of topography, gravity and geoid data was carried out in the collision zone between the Eurasian and Arabian plates, comprising Central and Eastern Anatolia, the Black Sea Basin and the Caucasus region. The 3D algorithm, first introduced by Motavalli-Anbaran et al. (2013), is based on a Bayesian approach with Gaussian density functions in which the targeted area is divided into vertical columns, each comprised of four layers namely water (of known thickness i.e. bathymetry) if present, crust, lithospheric mantle, and asthenosphere. The inversion results are Moho depth, average crustal density and depth to the lithosphere–asthenosphere boundary, defined here as the 1350 °C isotherm. Existing seismic Moho depths were used as a priori information in order to constrain crustal thicknesses. Thickest crust (up to 50 km) was found underneath the Great Caucasus Mountains, thinnest crust (22–25 km) underneath the Black Sea Basin. The East-Anatolian plateau is underlain by relatively thick crust (up to 45 km), thinning to less than 40 km towards the Central Anatolian Plateau. Eastern Anatolia and the eastern part of Central Anatolia are underlain by strongly thinned lithosphere (up to 100 km). Thickest lithosphere (up to 200 km) underlies the Caspian Basin and the Russian Platform.

SUMMARY A new 3-D inversion algorithm for the interpretation of magnetic total field anomalies is presented. Using Bayes' theorem the algorithm allows the inclusion of a priori information of model parameters and to treat this information as well as the measured data in a probabilistic manner. In this way a rather fast and stable method is obtained. The models consist of a system of vertical rectangular prisms with its top, base, susceptibility and remanent magnetization intensity and direction as variable parameters. Several synthetic examples show the possibilities that the algorithm offers. Finally the interpretation of an anomaly measured in NE Spain is presented, which shows a strong remanent magnetization component oblique to the local geomagnetic field.

The parameters used for the acquisition of the IBERSEIS deep seismic reflection profile in the SW Iberian Peninsula provide seismic images of the deep crust as well as a high resolution section of the shallow subsurface. A very dense array of sources and receivers allowed high resolution tomographic studies in zones of special interest (granitic plutons). The three dimensional tomographic inversion produced velocity models along a 500 m wide and 1000 m deep strip along the IBERSEIS transect in the areas of La Bazana, La Dehesilla, Feria and Villafranca. In these high resolution velocity models (sampled by 50 × 50 × 50 m cells), high velocity anomalies indicate the geology and extension of the granitic plutons at depth. This directly correlates with the surface outcrops. Moreover, tomographic models provided valuable information for the geometry and characterization of fractured and fresh domains in a rock volume. Furthermore, a piggy back seismic acquisition experiment using additional seismic instrumentation from the University of Paris Sud (40-channel DMT) provided perpendicular, offline recordings of the Vibroseis sources. This additional recording system was deployed perpendicularly to the main IBERSEIS seismic reflection line and provided additional 3D control.

A new methodology for automatic modelling of crustal and lithospheric thickness or density variations that integrates both elevation and geoid anomalies is presented. We used this algorithm to image the main lithospheric heterogeneities within the Atlantic–Mediterranean transition zone. Main assumptions in the modelling are local isostasy and a two-layered model comprising crust and lithospheric mantle with constant densities, plus sea water and asthenosphere. The results show a wide zone of thickened lithosphere, oriented NE–SW and located beneath the western Betics and Rif, the Gulf of Cadiz and the northwestern Atlantic Moroccan margin. In addition, a zone of thinned lithosphere with the same orientation is shown beneath the Atlas Mountains and the eastern Alboran Basin. To cite this article: J. Fullea Urchulutegui et al., C. R. Geoscience 338 (2006).

We discuss the implications of a lithospheric model of the Moroccan Atlas Mountains based on topography, heat flow, gravity and geoid anomalies, taking into account the regional geology. The NW African cratonic lithosphere, some 160‐ 180 km thick, thins to c. 80 km beneath the Atlas fold-thrust belts, in contrast with the shortening regime prevailing there since the early Cenozoic. This fact explains several geological and geophysical features as high topography with modest tectonic shortening, the occurrence of alkaline magmatism contemporaneous to compression, the absence of large crustal roots to support elevation, the scarce development of foreland basins, and a marked geoid high. The modelled lithosphere thinning is related to a thermal upwelling constrained between the Iberia‐Africa convergent plate boundary and the Saharan craton.

Atmospheric-derived noble gases He, Ne, Ar, Kr and Xe dissolved in ground waters record the temperature at the moment of their recharge. Palaeotemperature reconstructions should include all the parameters affecting the soil air–ground water thermodynamic equilibrium. Particularly, the occurrence of excess air, that is ANG in excess compared to the equilibrium concentrations and often mass-fractionated compared to the atmospheric pattern, is likely produced by air entrapment during water infiltration in the nonsaturated zone (NSZ). Here, we show that water persisting in the NSZ has peculiar capillary properties controlled by the internal water pressure, which decreases (even down to negative pressures) with decreasing soil–air humidity. Decreasing water pressure induces mass-dependent increasing solubility of ANG from He to Xe. In addition, in nonsaturated soils where water and air coexist, air entrapment is favored allowing nonfractionated excess air. This capillary approach has been coded in an inverse-method program called Thermo_Inver, and tested on two well-characterized noble gas datasets used to constrain palaeotemperatures in Midwestern United States and Brazil during the last glacial maximum [Quat. Res. 43 (1995) 209; Science 269 (1995) 379]. Results of these simulations highlight that under suitable climatic conditions and in fine-porous media, the capillary pressure of water may be a prominent parameter controlling the addition of ANG to ground waters. Under these conditions, the recharge zone for ANG is the NZS and this water–air coexistence zone easily produces excess air.

Metastable systems are created when the interface between the atmosphere (in which Patm = 1 bar) and water forms a spherical meniscus either concave toward the air (water filling capillaries, wherein Pwater Patm). Soil water, undergoing negative pressure (“capillary potential”) remains bound to the solid matrix (instead of flowing downward) by the capillary meniscus, concave toward the undersaturated dry atmosphere. The positive counterpart of tensile water in soils is the pressurized water contained in fine droplets suspended in oversaturated humid air, as in clouds. All these systems are anisobaric domains the phases of which have different pressures. Geochemical consequences of such characteristics are assessed here by calculating the consequences of the positive or negative water potential on the equilibrium constants of reactions taking place in stretched or pressurized aqueous solutions. Thermodynamic properties of aqueous species are obtained by using the TH model, used explicitly for positive pressures but extrapolated to negative ones for soil solutions. It appears that soil water dissolves gases, offering an alternative explanation of the observed enrichment of atmospheric noble gases in groundwater and of carbonic gas in the unsaturated zone below the root zone. Water droplets obviously show the opposite behavior, that is, a decreasing dissolutive capability with decreasing droplet size (water pressure increases), inducing some climatic consequences. An application of this approach to the solid-solution equilibria is performed by comparing experimental solubility of amorphous silica in unsaturated media on the one hand, to theoretical calculations taking account of the negative water pressure on the other hand. This comparison outlines the potential complexity of anisobaric situations in nature and the necessity to develop a suitable approach for solid pressure.

International audience; We analyzed satellite gravity and geoid anomaly and topography data to determine the 3D lithospheric density structure of the Singhbhum Protocontinent. Our density model shows that distinct vertical density heterogeneities exist throughout the lithosphere beneath the Singhbhum Protocontinent. The crustal structure identified includes a lateral average crustal density variation from 2800 to 2890 kg/m3 as well as a relatively flat Moho at 35–40 km depth in Singhbhum Protocontinent and Bastar Craton. A similar Moho depth range is found for the Mahanadi, Damodar, and Bengal basins. In the northern part of the area, Moho undulates between more than 40 km under the confluence of Mahanadi–Damodar Gondwana basins and the Ganga foreland basin, and 36–32 km under the Eastern Ghats Mobile belt and finally reaches 24 km in the Bay of Bengal. The lithosphere–asthenosphere boundary (LAB) across the Singhbhum Protocontinent is at a depth of about 130–140 km. In the regions of Bastar Craton and Bengal Basin, the LAB dips to about 155 ± 5 km depth. The confluence of Mahanadi and Damodar Gondwana basins toward the north-west and the foreland Ganga Basin toward the north are characterized by a deeper LAB lying at a depth of over 170 and 200 km, respectively. In the Bay of Bengal, the LAB is at a shallower depth of about 100–130 km except over the 85 0E ridge (150 km), and off the Kolkata coast (155 km). Significant density variation as well as an almost flat crust–mantle boundary indicates the effect of significant crustal reworking. The thin (135–140 km) lithosphere provides compelling evidence of lithospheric modification in the Singhbhum Protocontinent. Similarities between the lithospheric structures of the Singhbhum Craton, Chhotanagpur Gneiss Complex, and Northern Singhbhum Mobile Belt confirm that the repeated thermal perturbation controlled continental lithospheric modification in the Singhbhum Protocontinent.

Various interpretations of seismic, magnetotelluric and geothermal data have resulted in distinctly different models of the structure and thickness of the lithosphere in the Western Carpathian–Pannonian Basin region. The model inconsistencies, which are at least partly due to the use of non-unique single-method data inversion, led us to the application of integrated lithospheric multi-method modelling. Our approach combines the interpretation of surface heat flow, gravity and topography (local isostasy) data in order to constrain the continental lithospheric thermal structure along five transects crossing the Western Carpathians from the Polish platform and the Bohemian Massif to the Pannonian Basin. The results indicate clear variations of the lithosphere thickness along the Western Carpathian orogen not only in NS direction but also in WE direction. The lithospheric thickness is found to be 90–120 km underneath the Bohemian Massif, 115–140 km beneath the Polish platform and 75–100 km beneath the Pannonian Basin. A thick lithosphere (up to 130–150 km) is indicated underneath the central and eastern segments of the Western Carpathians with a tendency of thickening eastwards. This structure is interpreted as a remnant of a subducted slab of the European plate. In contrast, no lithospheric root shows up in our models of the westernmost segment of the Western Carpathians. We suggest that this difference indicates that in the transition zone from the Eastern Alps to the Carpathians, strike-slip movements predominated during the entire convergence period and no clear slab was produced in this area.

Crustal roots formed beneath mountain belts are gravitationally unstable structures, which rebound when the lateral forces that created them cease or decrease significantly relative to gravity. Crustal roots do not rebound as a rigid body, but undergo intensive internal deformation during their rebound and cause intensive deformation within the ductile lower crust. 2-D numerical models are used to investigate the style and intensity of this deformation and the role that the viscosities of the upper crust and mantle lithosphere play in the process of root rebound. Numerical models of root rebound show three main features which may be of general application: first, with a low-viscosity lower crust, the rheology of the mantle lithosphere governs the rate of root rebound; second, the amount of dynamic uplift caused by root rebound depends strongly on the rheologies of both the upper crust and mantle lithosphere; and third, redistribution of the rebounding root mass causes pure and simple shear within the lower crust and produces subhorizontal planar fabrics which may give the lower crust its reflective character on many seismic images.

International audience; We present the 3D crustal and lithospheric structure and crustal average density distribution of southern Indian shield (south of 18 degrees N), Sri Lanka and adjoining oceans. The model is based on the assumption of local isostatic equilibrium and is derived from joint inversion of free air gravity and geoid anomalies and topography data. The derived crustal thickness of 10-25 km in the oceanic region increases to 34-35 km along the coast. A crustal thickness of 34-38 km is obtained beneath the Eastern Dharwar Craton and 36-45 km beneath the Western Dharwar Craton and the Southern Granulite Terrain. Sri Lanka has a thinner crust of 30-35 km. The lithosphere-asthenosphere boundary is located at depths of 70-120 km under oceanic regions and similar to 150-180 km below the Dharwar Craton and the Northern block of Southern Granulite Terrain. A notably thinned lithosphere of similar to 130 km near Bangalore in the Eastern Dharwar Craton, similar to 140 km beneath the Southern block of Southern Granulite Terrain and similar to 130 km in Sri Lanka is observed. The thickness of the lithosphere (similar to 130 km) near Bangalore is inferred as the frozen in signature of a small fossil mantle plume and/or tectono-compositional effect of a rifted margin and a suture. Considerable stretching and/or convective removal of pristine lithosphere in the Southern block of Southern Granulite Terrain and adjoining Sri Lanka, before disappearing completely in the Archaean Northern block of Southern Granulite Terrain and Dharwar Craton, is suggested. (C) 2014 Elsevier Ltd. All rights reserved.

New refraction-seismic data have been obtained in the northern French Massif Central. Seven shots were recorded simultaneously on two parallel, NNW-SSE (i.e., perpendicular to the surface structures) oriented profiles allowing for inline and fan interpretations. The results of a 2D-interpretation reveal a large-scale updoming of the Moho of about 2 km, compared to average western Europe, corresponding to the Cenozoic uplift of the Massif Central. Local crustal thinning of up to 20% is restricted to the Cenozoic grabens as seen at the surface. Beneath the volcanic area of Cantal, one of the largest Cenozoic European shield volcanoes, which was active in Miocene to recent times, significant crustal thickening of 2–3 km is observed. Velocities in the upper mantle are generally in the range of 7.9–8.1 km/s beneath the Limagne graben and surrounding undisturbed areas. Underneath the volcanic area of Cezallier, however, velocity is reduced to about 7.7 km/s, corresponding to the area of crustal thickening. Therefore, the low velocity in this zone is explained by the remainders of cooling magma chambers and heating by ascending hot plume material. The Cenozoic tectonic evolution of the Massif Central has affected crust and upper mantle in different ways. Oligocene rifting corresponds well to local crustal thinning, but no correlation is detected to velocities of the upper mantle. In contrast, the volanic event occurring since Miocene times is related to upper mantle velocity reduction and crustal thickening due to underplating.

International audience; Seismic anisotropy of a fractured karstic limestone massif in sub-parallel underground galleries is studied. As the fractures are mostly vertically oriented, the seismic properties of the massif are approximated by horizontal transverse isotropy (HTI). Several data inversion methods were applied to a seismic dataset of arrival-times of P and S-waves. The applied methods include: isotropic tomography, simple cosine function fit, homogeneous Monte-Carlo anisotropic inversion for the parameters of horizontal transverse isotropy and anisotropic tomography for tilted transversely isotropic bodies. All methods lead to the conclusion that there is indeed an anisotropy present in the rock massif and confirm the direction of maximum velocity parallel to the direction of fracturing. Strong anisotropy of about 15% is found in the studied area. Repeated measurements show variations of the P-wave parameters, but not of the S-wave parameters, which is reflecting a change in water saturation.

A new technique for interpretation of 3-D seismic tomographic models in terms of temperature, degree of partial melt and rock composition is presented and tested. We consider both anharmonic and anelastic temperature effects on seismic velocities as well as the effects of mineral reactions, composition and partial melt. It is shown that composition effect is small (less than 1% of velocity) if there are no strongly depleted, Mg-rich harzburgites. We calculate anharmonic temperature derivatives of seismic velocities from compositions of mantle xenoliths. The parameters of a non-linear frequency and temperature-dependent model of attenuation have been taken from published laboratory experiments and calibrated using global Q observations in the upper mantle. For every block of the tomographic model we calculate the absolute temperature and melt fraction required to fit the observed V p perturbation, the average temperature of the tomographic layer being constrained by the observed surface heat flow. With these temperatures we calculate attenuation, density, V p and V s from petrophysical modelling, using the average for 80 mantle xenoliths samples from the French Massif Central. The technique is applied to a recently published 3-D teleseismic P wave tomographic model of the upper mantle beneath the French Massif Central. The observed velocity perturbations are probably caused there by variations in temperature. Temperature does not reach the dry solidus temperature (except for a few tomographic blocks), although it comes close to it at the depth of 60–100 km below volcanic areas. At high subsolidus temperatures the contribution of anelasticity to velocity perturbations is at least as important as the combined effect of anharmonicity and mineral reactions. Our model is consistent with the Pn velocities from refraction seismic studies, Q S estimations from surface waves, observed gravity, geoid, topography and surface heat flow, as well as with the composition and temperatures derived from mantle xenoliths. We suggest that the lithosphere-asthenosphere boundary is uplifted to 50–60 km depth beneath the main volcanic fields. The central and southern part of the Massif Central is underlain by the hot mantle body (plume?) with a potential temperature that is 100–200°C higher than the average potential temperature of the upper mantle.

International audience; For the present 2-D lithospheric density modelling, we selected three geotransects of more than 1000 km in length each crossing the southern Indian shield, south of 16 degrees N, in N-S and E-W directions. The model is based on the assumption of local isostatic equilibrium and is constrained by the topography, gravity and geoid anomalies, by geothermal data, and where available by seismic data. Our integrated modelling approach reveals a crustal configuration with the Moho depth varying from similar to 40 km beneath the Dharwar Craton, and similar to 39 km beneath the Southern Granulite Terrane to about 15-20 km beneath the adjoining oceans. The lithospheric thickness varies significantly along the three profiles from similar to 70-100 km under the adjoining oceans to similar to 130-135 km under the southern block of Southern Granulite Terrane including Sri Lanka and increasing gradually to similar to 165-180 km beneath the northern block of Southern Granulite Terrane and the Dharwar Craton. This step-like lithosphere-asthenosphere boundary (LAB) structure indicates a normal lithospheric thickness beneath the adjoining oceans, the northern block of Southern Granulite Terrane and the Dharwar Craton. The thin lithosphere below the southern block of Southern Granulite Terrane including Sri Lanka is, however, atypical considering its age. Our results suggest that the southern Indian shield as a whole cannot be supported isostatically only by thickened crust; a thin and hot lithosphere beneath the southern block of Southern Granulite Terrane including Sri Lanka is required to explain the high topography, gravity, geoid and crustal temperatures. The widespread thermal perturbation during Pan-African (550 Ma) metamorphism and the breakup of Gondwana during late Cretaceous are proposed as twin cause mechanism for the stretching and/or convective removal of the lower part of lithospheric mantle and its replacement by hotter and lighter asthenosphere in the southern block of Southern Granulite Terrane including Sri Lanka. Unusually thinned LAB beneath the region near Bangalore apparently indicates the preserved tectonocompositional effect of late Proterozoic rifted margin of Dharwar Craton.

Two-dimensional kinematic modelling of extension based on the numerical solution of the heat transport equation is used to investigate the lateral evolution of lithospheric yield strength during rifting and during postrift relaxation. Two yield strength minima are shown to exist, one beneath the rift centre and the other beneath the undeformed area adjacent to the rift. These are separated by a relative maximum in the transition zone between the rift and the outer area. Initially cold (thick) lithosphere and fast extension give rise to an absolute strength minimum beneath the rift, leading to a narrow rift. On the other hand, initially hot (thin) lithosphere and slow extension lead to an absolute strength minimum beneath the rift sides, which could cause outward migration of the area of principal strain and formation of a wide rift. In contrast to some existing one-dimensional analyses, no lithospheric hardening is necessary for the formation of a wide rift and the conditions for its formation are consequently less restricted. The maximum of strength in the transition zone suggests a third mode of extension characterized by the activation of extension parallel to the rift, with undeformed areas in between. During postrift evolution, the strength of the thinned area progressively increases until the zone adjacent to the rift becomes the weakest area, inducing a widening of the rift in subsequent rifting episodes. The minimum time required for migration of deformation in successive rifting stages is termed the critical relaxation time (CRT) and is shorter for an initially hot lithosphere and low strain rate. CRT values range from few million years to more than 130 Ma for a stretching factor of 1.65. This study highlights major differences with respect to previous one-dimensional analyses when considering lateral heat transport and a progressive crustal and lithospheric thinning from the rift flanks.

A two-dimensional algorithm to determine the steady state thermal structure of the lithosphere that integrates thermal, gravity, and local isostasy analyses is presented. Gravity analyses together with seismic data are used to constrain spatial variations in density and crustal structure, while absolute elevation is used to determine the lithospheric mantle thickness. The calculation is performed using a finite element technique that links the different physical equations. The program optionally calculates the temperature at any material boundary and, with given rheological parameters, the strength distribution and the total lithospheric strength in selected columns. We apply the algorithm to the Northeastern Spanish Geotransect which extends from the Pyrenees to the Balearic Promontory and along which a strong variation in crustal and lithospheric thickness is evident. We assess the use of two different inferred density models for the lithospheric mantle: The first assumes a linear decrease in density with increasing temperature using the asthenospheric density as a reference; the second model assumes a constant density for the whole lithospheric mantle. Although conceptually the two hypotheses differ substantially, the results obtained do not show significant differences. Lithospheric thicknesses of 120–130 km below the Pyrenees, 60–65 km in the Valencia Trough, and 65–75 km below the Balearic Promontory are deduced. In all cases the mean lithospheric mantle density has to be 40–60 kg m−3 higher than the asthenospheric density. The algorithm is shown to be a powerful tool in lithospheric thermal modeling especially in areas where surface heat flow is poorly constrained because of the temperature-density-elevation relationship.