The geological-event reference system, a step towards geological data harmonization

The temporal dimension is an inherent component of geology. In this regard, traditional geological maps can represent a few geological events, yet they hardly account for the entire complex rock history whether sedimentary, crystalline or volcanic. Here, using the RGF research program (French Geological Reference platform) we propose a new methodology based on digital technology and the French historical collection of 1:50 000-scale geological maps. This innovative approach consists of describing, organizing and hierarchizing a series of geological events within a reference framework and linking it to GIS map geometries (polygons, faults, points). In this way, the complete history of geological features can be compiled and stored in digital maps, combining distinct geological events and properties. For a single event, all associated transformations can be represented on maps, facilitating the production of real “palaeo-geological” maps that consider not only traditional sedimentary environments but also possible synchronous weathering, metamorphism, and volcanism. We discuss here an example of French orogenic history. The approach demonstrated here on geological maps can be used with other geological data media (boreholes, seismic reflection profiles, etc.) and thus facilitate a 3D-to-4D scale, with a significant ability to address not only academic community needs, but also themes or issues related to applications required by politics, civil engineering, and society itself, to confront challenges such as natural and anthropic risk reduction and subsurface uses

Tectonic evolution and paleogeography of the Kırşehir Block and the Central Anatolian Ophiolites, Turkey

In Central and Western Anatolia two continent-derived massifs simultaneously underthrusted an oceanic lithosphere in the Cretaceous and ended up with very contrasting metamorphic grades: high pressure, low temperature in the Tavsanli zone and the low pressure, high temperature in the Kirsehir Block. To assess why, we reconstruct the Cretaceous paleogeography and plate configuration of Central Anatolia using structural, metamorphic, and geochronological constraints and Africa-Europe plate reconstructions. We review and provide new Ar-40/Ar-39 and U/Pb ages from Central Anatolian metamorphic and magmatic rocks and ophiolites and show new paleomagnetic data on the paleo-ridge orientation in a Central Anatolian Ophiolite. Intraoceanic subduction that formed within the Neotethys around 100-90 Ma along connected N-S and E-W striking segments was followed by overriding oceanic plate extension. Already during suprasubduction zone ocean spreading, continental subduction started. We show that the complex geology of central and southern Turkey can at first order be explained by a foreland-propagating thrusting of upper crustal nappes derived from a downgoing, dominantly continental lithosphere: the Kirsehir Block and Tavsanli zone accreted around 85 Ma, the Afyon zone around 65 Ma, and Taurides accretion continued until after the middle Eocene. We find no argument for Late Cretaceous subduction initiation within a conceptual "Inner Tauride Ocean" between the Kirsehir Block and the Afyon zone as widely inferred. We propose that the major contrast in metamorphic grade between the Kirsehir Block and the Tavsanli zone primarily results from a major contrast in subduction obliquity and the associated burial rates, higher temperature being reached upon higher subduction obliquity.European Research Council ; Netherlands Organization for Scientific Research (NWO

Controls on Trace Element Distribution in Oxides and Silicates

International audienceUnderstanding and quantifying the partitioning of elements at low concentrations is important for petrology, as minor and trace elements are used as tracers of many geological processes. The lattice strain model has consequently been found to be very useful, as it links mineral/melt partition coefficients to the elastic properties of the mineral and to the difference in ionic radius between the trace element and the cation it replaces. However, this model has limitations, particularly in terms of describing crystal strain, due to the form of the equation and from the choice of a hard-sphere model. After a review of the thermodynamics of trace element incorporation into crystal sites and equilibrium between mineral phases, we present classical atomistic modelling using transferable empirical potentials. Following incorporation of one (or more) mismatching element(s), crystal strain appears strongly related to the environment of the site of exchange, with anisotropic, vacancy-rich minerals deforming more than densely-packed minerals, due to anisotropic deformation, rotation and tilting of the surrounding polyhedra. As shown by the computed cation–oxygen length in strained crystal sites, bond strain is smaller than the difference in cation radii, and varies between structures, with densely-packed minerals being less strained. Consequently, computed relaxation energies are smaller in less compressible minerals. This has implications for modelling the partitioning of trace elements, and highlights the limits of using continuum mechanics below crystal cell scale. Neither oxides nor silicates have isotropic elastic properties, and at nanoscale they do not deform like continuous material. Predictions of partitioning between mineral and melt (or fluid) remain hampered by several factors, amongst which are (i) knowledge of the thermodynamics properties of the dissolved species (in melts and fluids); (ii) the precision of estimated defect and strain energies; and (iii) the presence of solid solution in natural systems, even when limited to a few percent. The latter may have orders-of-magnitude effects on the calculated partition coefficients due to interactions between strain fields around minor cations, leading to a chemical mixing regime at low concentration different from solid solutions where phase components are found in high proportions. Partitioning between mineral phases in an assemblage is described with similar equations as for mineral/liquid equilibria. In cases where terms linked to fluid species disappear, such as incorporation with similar substitutions in two minerals, the assumption that crystal strain energy is the preponderant term during cation exchange becomes unnecessary and it is preferable to use defect energies rather than strain energies. Application and discussion of these concepts are presented for mineral/melt partitioning and for partitioning between minerals, using garnet/clinopyroxene equilibria. The advantage of atomistic modelling is that it does not rely on fitting. Agreement with experimental data shows predictive accuracy within an order of magnitude, which is poorer than what may be achieved by fitting the lattice strain model to experimental data, but validating the theoretical approach and sufficient for application to some petrology problems. Improvements of the modelling for better application to minerals like augite involve systematic work on the effect of solid solutions on the strain field around defects and imply solving ordering problems

Relationship between microstructures and resistance in mafic assemblages that deform and transform

Syn-kinematic mineral reactions play an important role for the mechanical properties of polymineralic rocks. Mineral reactions (i.e., nucleation of new phases) may lead to grain size reduction, producing fine-grained polymineralic mixtures, which have a strongly reduced viscosity because of the activation of grain-size-sensitive deformation processes. In order to study the effect of deformation–reaction feedback(s) on sample strength, we performed rock deformation experiments on “wet” assemblages of mafic compositions in a Griggs-type solid-medium deformation apparatus. Shear strain was applied at constant strain rate (10−5 s−1) and constant confining pressure (1 GPa) with temperatures ranging from 800 to 900 ∘C. At low shear strain, the assemblages that react faster are significantly weaker than the ones that react more slowly, demonstrating that reaction progress has a first-order control on rock strength. With increasing strain, we document two contrasting microstructural scenarios: (1) the development of a single throughgoing high-strain zone of well-mixed, fine-grained aggregates, associated with a significant weakening after peak stress, and (2) the development of partially connected, nearly monomineralic shear bands without major weakening. The lack of weakening is caused by the absence of interconnected well-mixed aggregates of fine-grained reaction products. The nature of the reaction products, and hence the intensity of the mechanical weakening, is controlled by the microstructures of the reaction products to a large extent, e.g., the amount of amphibole and the phase distribution of reaction products. The samples with the largest amount of amphibole exhibit a larger grain size and show less weakening. In addition to their implications for the deformation of natural shear zones, our findings demonstrate that the feedback between deformation and mineral reactions can lead to large differences in mechanical strength, even at relatively small initial differences in mineral composition

Tectonic-metamorphic evolution of the Jebilet massif (Morocco) in the context of the Variscan orogeny

International audienceThe Jebilet massif belongs to the western Moroccan Meseta characterized by nearly complete Paleozoic sedimentary sequence folded and metamorphosed under low-grade greenschist-to amphibolite-facies during the Variscan orogeny, and intruded by widespread syn-to late-orogenic Carboniferous granitoids (Michard et al., 2008, 2010 and references therein). It has been previously proposed that Jebilet massif first underwent a regional, greenschist facies metamorphic event (D1 phase), followed by a high-T regional and contact metamorphism that reached the hornfels/amphibolite facies conditions (D2 and D2/D3 phases). However our recent observations of staurolite likely belonging to D1 assemblage obscure the previously proposed metamorphic conditions. In order to refine the metamorphic model we propose to revise the entire metamorphic conditions of the Jebilet massif. To address this issue, detailed structural, mineralogical, thermobarometric and Raman Spectroscopy on Carbonaceous Material (RSCM) methods have been employed to study the tectono-metamorphic evolution of the Jebilet. The results obtained for the metapelitic rocks that underwent D2/D3 higher metamorphism grades (hornfels/amphibolite facies), show four dominant mineral assemblages: (1) Chlorite– Biotite, (2) Cordierite–Biotite, (3) Andalusite–Garnet–Biotite, and (4) Andalusite– Cordierite–Biotite. The corresponding RSCM temperatures vary between 474 ± 50 °C and 628 ± 50 °C. The computed pseudo-sections for samples from the hornfels/amphibolite facies confirm the peak temperatures measured by the RSCM method. Our structural and mineralogical results support the occurrence of the Garnet– Staurolite assemblage during the D1 regional metamorphism event. These results bring new constrains on the evolution of the Jebilet massif during the Variscan orogeny of the western Moroccan Meseta and strongly suggest that the D1 phase reached higher conditions than those initially proposed, refining the metamorphic history of the Jebilet massif. As the Jebilet massif contains many ore deposit, this work will contribute to determine the main thermal event responsible of concentration the mineralization

Geophysical signature of the alpine slab: Field analogues and direct models

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Lithospheric-scale dynamics during continental subduction: Evidence from a frozen-in plate interface

International audienceContinental subduction and collision are not merely follow-ups of oceanic subduction but mark the transition from lithospheric-scale deformation localized along the subduction interface to crustal-scale deformation distributed across the orogen. In order to unravel the processes typifying the dynamic changes from oceanic subduction to collision, we have characterized the pressure-temperature (P-T) and spatio-temporal evolution of rocks on each side of the tectonic contact (Briançonnais–Liguro-Piemont [Br-LP] contact) separating the subducted oceanic remnants from the subducted continental fragments along the Western Alps. Results indicate that the maximum temperature and pressure difference on each side of the contact is generally <30 °C and <0.3 GPa, evidencing that no significant metamorphic gap exists. The preservation of similar P-T conditions on both sides of the Br-LP contact is interpreted as resulting from offscraping of the Liguro-Piemont and later Briançonnais units at similar depths, as supported by the ~10 m.y. gap between peak burial ages of both zones. The similar depth range reached by the various units reflects systematic variations of slicing and mechanical coupling along the plate interface suggesting that (1) similar slicing mechanisms and strain localization prevailed during both oceanic and continental subduction and (2) the Br-LP contact represents a frozen-in subduction interface. The end of high-pressure and low-temperature metamorphism and continental subduction at ca. 33 Ma would mark the stalling of subduction interface dynamics and the onset of strain distribution across the plate interface and into the lower plate

The effect of obliquity on temperature in subduction zones: insights from 3-D numerical modeling

The geotherm in subduction zones is thought to vary as a function of the subduction rate and the age of the subducting lithosphere. Along a single subduction zone the rate of subduction may strongly vary due to changes in the angle between the trench and the plate convergence vector, i.e., the subduction obliquity, due to trench curvature. We currently observe such curvature in, e.g., the Marianas, Chile and Aleutian trenches. Recently, strong along-strike variations in subduction obliquity were proposed to have caused a major temperature contrast between Cretaceous geological records of western and central Turkey. We test here whether first-order temperature variation in a subduction zone may be caused by variation in the trench geometry using simple thermo-kinematic finite-element 3-D numerical models. We prescribe the trench geometry by means of a simple mathematical function and compute the mantle flow in the mantle wedge by solving the equation of mass and momentum conservation. We then solve the energy conservation equation until steady state is reached. We analyze the results (i) in terms of mantle wedge flow with emphasis on the trench-parallel component and (ii) in terms of temperature along the plate interface by means of maps and the depth-temperature path at the interface. In our experiments, the effect of the trench curvature on the geotherm is substantial. A small obliquity yields a small but not negligible trench-parallel mantle flow, leading to differences of 30 °C along-strike of the model. Advected heat causes such temperature variations (linked to the magnitude of the trench-parallel component of velocity). With increasing obliquity, the trench-parallel component of the velocity consequently increases and the temperature variation reaches 200°C along-strike. Finally, we discuss the implication of our simulations for the ubiquitous oblique systems that are observed on Earth and the limitations of our modeling approach. Lateral variations in plate sinking rate associated with curvature will further enhance this temperature contrast. We conclude that the synchronous metamorphic temperature contrast between central and western Turkey may well have resulted from reconstructed major variations in subduction obliquity

Petrogeodynamics of HP-LT rocks : state of the art and application to processes along the subduction

International audienceBased on examples from the Betics, Oman and the W. Alps, this contribution attempts to (1) critically evaluate the precision with which metamorphic P-T-t histories are retrieved at present and (2) discuss the implications on our understanding of orogenic processes, focusing on those processes taking place along the subduction channel. Thanks to improved interconsistent thermodynamic databases, multiple thermodynamic softwares and analytical tools, numerous quantitative constraints on metamorphic histories are now accessible. We herein assess the merits and shortcomings of our present knowledge. In particular, we stress that, despite impressive improvements, parts of the P-T-time history still remain systematically ill-constrained. Metamorphic histories do not provide a quite continuous record either, and whether mineral reequilibrations are discrete or continuous remains fondamentally ambiguous. We then focus on the early stages of orogenic build-up, namely on processes related with oceanic and continental subduction. After briefly reviewing our current knowledge of metamorphic histories worlwide, we focus on the example of the W. Alps. We present the results of the comparison between the metamorphic evolution of the Schistes Lustrés paleoaccretionary complex and the evolution of some of the major ophiolitic bodies of the W. Alps exhumed from the Alpine subduction (Zermatt-Saas, Avic, Rocciavre, Monviso). We show how this allows placing constraints on the nature and characteristics of the plate interface (eg, the so-called 'subduction channel') and reconstructing the geodynamic processes at work in a subduction zone. We particularly discuss the dimensions of the bodies sliced up and stacked in the subduction channel, the depths and possible mechanisms at which this happens, and the role of fluid transfer. Along strike variations of the exhumation processes within the same subduction zone are also emphasized. In the light of these findings, we finally address other related issues: insights from thermomechanical models combined with petrological techniques, timing of mineral reequilibration with respect to deformation (and implications for rheology), fluid behaviour and kinetic processes

Pegmatite emplacement: insights from two-phase flow numerical modelling.

International audienceAmongst the fluids present in lithospheric processes, silicate rich melts with their range of composition are one of the prime sources of ore on Earth. Understanding how such fluids propagate and interact with the porosity of the lithosphere is therefore of prime interest both for the long-term lithospheric deformation in relation with the weakening properties of fluids or to predict ore concentration linked or not to the chemical evolution of the melts. Pegmatites are igneous rocks, generally with a granitic composition and characterised by a crystal growth dominated texture. They are often enriched in rare elements such as Lithium, Tantalum or Beryllium (amongst others) offering valuable ore deposit. In nature they are observed as pegmatites fields showing an internal organisation, with respect to the host rock (either magmatic or metamorphic) and the tectonic structures. Here, we focus on the geometry of pegmatite fields using a numerical modelling approach. The numerical model allows us to resolve the formation and propagation of pipe structures in a viscous porous media also called porosity waves. We use the finite difference approach to solve the two-phase flow problem that couples a Stokes solver to predict the deformation of the porous matrix to a non-linear Darcy flow representing the pore fluid displacement (Räss et al., in prep). We present here the first results of our study and discuss the effects heterogeneities in the host porous media and compare the statistical distribution of the modelled pipes with natural examples for the Montagne Noire and the Massif Central. Räss, L., Duretz, T., and Y. Podladchikov. Hydro-mechanical coupled flow in deforming porous media