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Toulouse, France

Lagabrielle Y.,French National Center for Scientific Research | Labaume P.,French National Center for Scientific Research | De Saint Blanquat M.,LMTG
Tectonics | Year: 2010

The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Barn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.Copyright 2010 by the American Geophysical Union.

Venchiarutti C.,Alfred Wegener Institute for Polar and Marine Research | Venchiarutti C.,French National Center for Scientific Research | Roy-Barman M.,CEA Saclay Nuclear Research Center | Freydier R.,LMTG | And 4 more authors.
Biogeosciences | Year: 2011

Dissolved and particulate excess 230Th and 231Pa concentrations (noted 230Th xs and 231Paxs respectively) and 231Pa xs/ 230Th xs activity ratios were investigated on and out of the Kerguelen plateau (Southern Ocean) in the framework of the Kerguelen Ocean and Plateau compared Study project in order to better understand the influence of particle flux and particle chemistry and advection on the scavenging of 231Pa. In the wake of Kerguelen, particulate 231Paxs is relatively abundant compared to its content in the dissolved phase. This, together with the low fractionation observed between 230Th and 231Pa (F Th/Pa ranging from 0.06 ± 0.01 to 1.6 ± 0.2) reflects the domination of the biogenic silica in the particle pool. Along the eastern escarpment of the Kerguelen plateau, the strong 231Paxs horizontal gradient in the deep waters highlights the intense removal of 231Pa at depth, as already observed for 230Th xs. This local boundary scavenging was attributed to re-suspension of opal-rich particles by nepheloid layers, resulting in fractionation factors F Th/Pa ≤ 1 along the Kerguelen plateau slope. Therefore, both the composition (biogenic opal) and the flux (intense along the margin) of particles control the scavenging of the two radionuclides in the Kerguelen wake. The modelling of 231Pa distribution with an advection-scavenging model demonstrates that lateral advection of open ocean water on the Kerguelen plateau could supply most of the 231Pa, which is then efficiently scavenged on the highly productive plateau, as previously proposed for 230Th xs. It stresses that lateral advection can play a significant role in the overall budget of particle reactive trace elements in a coastal-open ocean system. © 2011 Author(s).

Credoz A.,French Atomic Energy Commission | Credoz A.,Amphos XXI Consulting S.L | Bildstein O.,French Atomic Energy Commission | Jullien M.,French Atomic Energy Commission | And 3 more authors.
Applied Clay Science | Year: 2011

The reactivity of mixed-layer illite-smectite was investigated in batch and mixed-flow reactors in the presence of K-bearing mineral impurities (K-feldspar) in order to determine the reaction pathways and the kinetic rate of the reactions in a range of pH conditions pertinent to the context of CO2 geological storage.The mixed-layered illite-smectite (I-S) mineral was extracted from a natural clayey caprock and was studied at 80 °C, with a solution salinity of 0.1. M NaCl and over a pH range from 3 to 8.5. The experimental results show that the most important process taking place is illitization of the initial I-S. In the most acidic conditions, an intense illitization is observed, which was interpreted as a new proton-promoted illitization process. At neutral pH, illitization is less intense and is attributed to a classical thermal effect.In terms of the overall reaction rate (I/S and K-feldspar dissolution in a mixed flow experiment), the values calculated from the evolution of SiO2 concentrations fall in the range of the experimental values reported in the literature for smectites and illites.Implications of illitization processes on CO2 storage and clayey caprock integrity are also discussed. © 2011 Elsevier B.V.

Laurent G.,French National Center for Scientific Research | Caumon G.,French National Center for Scientific Research | Jessell M.,LMTG | Royer J.J.,French National Center for Scientific Research
ECMOR 2012 - 13th European Conference on the Mathematics of Oil Recovery | Year: 2012

Most current approaches for building structural reservoir models focus on geometrical aspects and consistency with seismic and well data. Few approaches account for the validity of 3D geological models regarding structural compatibility. It may be done using restoration to check the kinematics or mechanics. This is generally performed a posteriori, which also provides critical insights on the basin/reservoir history, but requires significant modeling efforts. This paper presents an approach introducing a first-order kinematic and mechanical consistency at the early stages of the structural modeling. Because the full deformation path is generally poorly constrained, we suggest using simplified approaches to generate plausible structures and assess first-order deformations, making efficiency and robustness more important than physical accuracy. A mechanical deformable model based on rigid elements linked by a non-linear energy has been adapted to geological problems. The optimal deformation is obtained by minimizing the total energy with appropriate boundary conditions. Last, the displacement field is transferred to the geological objects embedded into the rigid elements. With this approach, 3D structural models can be obtained by successively modeling the tectonic events. The underlying tectonic history of resulting models is explicitly controlled by the interpreter and can be used to study structural uncertainties.

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