Contribution of the RSCM Geothermometry to Detect the Thermal Anomalies and Peak Temperatures Induced by Fluid Circulation in Metasediments

International audienceThe occurrence of deposits hosted by carbonaceous materials-rich metasediments is widespread. For this reason, we propose in this study to investigate the potential of the Raman Spectroscopy of Carbonaceous Material (RSCM) geothermometry to detect thermal anomalies in hydrothermal ore deposits environment. The chosen geological context is the Lucia subterrane in the Franciscan Complex (California, USA), which includes gold-bearing quartz veins (Underwood et al., 1995). Estimated Raman temperatures 1) confirmed the increase in the metamorphic grade towards the north already shown by Underwood et al. (1995), using classical methods like mineralogy and vitrinite reflectance and 2) exhibit anomalous values. These anomalies are probably due to the later hydrothermal event. This result suggests that RSCM could be used as a reliable tool to determine thermal anomalies caused by hot fluid-flow

Exhumation kinematics of the Cycladic Blueschists unit and back-arc extension, insight from the Southern Cyclades (Sikinos and Folegandros Islands, Greece)

International audienceCurrent models for the Oligo-Miocene post-orogenic back-arc extension of the Aegean domain suggest that stretching is accommodated by two bivergent detachment systems of opposing shear sense. The co-existence in the Eocene of a top-to-the-S thrust at the base of the Cycladic Blueschists unit and top-to-the-N extensional shear zones at the roof raises the problem of differentiating synorogenic and post-orogenic deformations with similar directions and shear senses. Based on structural field data, this study shows that the post-orogenic deformation recorded in the Southern Cyclades is extremely asymmetric as the Cycladic Blueschists unit is pervasively affected by top-to-the-N shearing deformation distributed on four main shear zones. All activated in greenschist-facies conditions, some of these shear zones operated in the brittle regime during the final part of the exhumation. The Cycladic Blueschists/Cycladic Basement contact displays clear polyphased deformation with the preservation of top-to-the-S thrust kinematics. Thermal structure of the Cycladic Blueschists unit with regards to position of ductile shear zones was retrieved using the Raman Spectroscopy of Carbonaceous Material peak-metamorphic temperatures. This study shows a series of major metamorphic gaps accommodating an upward and stepwise decrease of more than 200 °C within the Cycladic Blueschists unit. Pressure-temperature estimates show that only lower parts of the Cycladic Blueschists unit recorded ca. 18–20 kbar for 530 °C peak-conditions. While flanking the West Cycladic Detachment System, which shows a top-to-the-S shear sense, the Southern Cyclades are dominated by a top-to-the-N non-coaxial shearing. Deformation is therefore genuinely asymmetric in the center of the Aegean domain

The Ikaria high-temperature Metamorphic Core Complex (Cyclades, Greece): Geometry, kinematics and thermal structure

International audienceThis work attempted at clarifying the structure of Ikaria using primarily intensive geological mapping combined with structural analysis and a geothermometry approach of Raman spectrometry of carbonaceous material. Foliation over the whole island defines a structural dome cored by high-grade to partially-molten rocks. Its exhumation was completed by two top-to-the-N ductile extensional shear zones, operating in the ductile and then the brittle fields, through a single extensional event coeval with progressive strain localization. The thermal structure of the dome with regard to position of ductile shear zones was retrieved using the Raman spectroscopy of carbonaceous material. Peak-metamorphic temperatures range from 390 °C in the upper parts of the structure down to 625 °C in the core of the dome in the vicinity of migmatites and S-type granite. Pioneer in situ U-Th-Pb analyses on monazite performed on the leucosome parts of these rock yielded a 15.7 ± 0.2 Ma age. Ikaria Island thus completes the series of Miocene migmatite-cored Metamorphic Core Complex in the central part of the Aegean domain where a genuine high-temperature zone can be defined as the central Aegean HT zone. There, the extreme stretching of the continental crust is associated with dominantly top-to-the-N kinematics

Thermal imprint of rift-related processes in orogens as recorded in the Pyrenees

International audience19 The extent to which heat recorded in orogens reflects thermal conditions inherited from 20 previous rift-related processes is still debated and poorly documented. As a case study, we 21 examine the Mauléon basin in the north-western Pyrenees that experienced both extreme 22 crustal thinning and tectonic inversion within a period of ~30 Myrs. To constrain the time-23 temperature history of the basin in such a scenario, we provide new detrital zircon fission-24 track and (U-Th-Sm)/He thermochronology data. The role of rift-related processes in 25 subsequent collision is captured by inverse modeling of our thermochronological data, using 26 relationships between zircon (U-Th-Sm)/He ages and uranium content, combined with 27 thermo-kinematic models of a rift-orogen cycle. We show that the basin recorded significant 28 heating at about 100 Ma characterized by high geothermal gradients (~80°C/km). Our 29 thermo-kinematic modeling and geological constraints support the view that subcontinental 30 lithospheric mantle was exhumed at that time below the Mauléon basin. Such a high 31 geothermal gradient lasted 30 Myr after onset of convergence at ~83 Ma and was relaxed 32 during the collision phase from ~50 Ma. This study suggests that heat needed for ductile 33 shortening during convergence, is primarily inherited from extension rather than being only 34 related to tectonic and/or sedimentary burial. This should have strong implications on tectonic 35 reconstructions in many collision belts that resulted from inversion of hyper-extended rift 36 basins

Basement – Cover decoupling and progressive exhumation of metamorphic sediments at hot rifted margin. Insights from the Northeastern Pyrenean analog

International audienceWe compile field data collected along the eastern part of the North Pyrenean Zone (NPZ) to point to a tectonic evolution under peculiar thermal conditions applying to the basin sediments in relation with the opening of the Cretaceous Pyrenean rift. Based on this compilation, we show that when thinning of the continental crust increased , isotherms moved closer to the surface with the result that the brittle-ductile transition propagated upward and reached sediments deposited at the early stage of the basin opening. During the continental breakup, the pre-rift Mesozoic cover was efficiently decoupled from the Paleozoic basement along the Triassic evaporite level and underwent drastic ductile thinning and boudinage. We suggest that the upper Albian and upper Creta-ceous flysches acted as a blanket allowing temperature increase in the mobile pre-rift cover. Finally, we show that continuous spreading of the basin floor triggered the exhumation of the metamorphic, ductily sheared pre-rift cover, thus contributing to the progressive thinning of the sedimentary pile. In a second step, we investigate the detailed geological records of such a hot regime evolution along a reference-section of the eastern NPZ. We propose a balanced restoration from the Mouthoumet basement massif (north) to the Boucheville Albian basin (south). This section shows a north to south increase in the HT Pyrenean imprint from almost no metamorphic recrystallization to more than 600 °C in the pre-and syn-rift sediments. From this reconstruction, we propose a scenario of tectonic thinning involving the exhumation of the pre-rift cover by the activation of various detachment surfaces at different levels in the sedimentary pile. In a third step, examination of the architecture of current distal passive margin domains provides confident comparison between the Pyrenean case and modern analogs. Finally, we propose a general evolutionary model for the pre-rift sequence of the Northeastern Pyrenean rifted margin

Tectono-thermal history of an exhumed thrust-sheet-top basin : an example from the south Pyrenean thrust belt

This paper presents a new balanced structural cross-section of the Jaca thrust-sheet-top basin of the southern Pyrenees combined with paleo-thermometry and apatite fission track (AFT) thermochronology data. The cross-section, based on field data and interpretation of industrial seismic reflection profiles, allows refinement of previous interpretations of the south-directed thrust system, involving the identification of new thrust faults, and of the kinematic relationships between basement and cover thrusts from the middle Eocene to the early Miocene. AFT analysis shows a southward decrease in the level of fission track resetting, from totally reset Paleozoic rocks and lower Eocene turbidites (indicative of heating to Tmax>~120°C), to partially reset middle Eocene turbidites and no/very weak resetting in the upper Eocene-lower Oligocene molasse (Tmax<~60°C). AFT results indicate a late Oligocene-early Miocene cooling event throughout the Axial Zone and Jaca Basin. Paleo-maximum temperatures determined by vitrinite reflectance measurements and Raman spectroscopy of carbonaceous material reach up to ~240°C at the base of the turbidite succession. Inverse modelling of AFT and vitrinite reflectance data with the QTQt software for key samples show compatibility between vitrinite-derived Tmax and the AFT reset level for most of the samples. However, they also suggest that the highest temperatures determined in the lowermost turbidites correspond to a thermal anomaly rather than burial heating, possibly due to fluid circulation during thrust activity. From these results, we propose a new sequential restoration of the south Pyrenean thrust system propagation and related basin evolution

The Rocroi inlier, highlight of grey areas in its geological story using modern tools (U-Pb on Zircon with LA-ICP-MS and Raman Spectroscopy of Carbonaceous Material)

The Rocroi inlier, part of the Ardenne Allochton, is composed of alternation of quartzite and metapelite, Cambrian to Ordovician in age. We update its story by performing 1) U-Pb dating on zircon with LA-ICP-MS; 2) Raman Spectroscopy on Carbonaceous Materiel (RSCM) on metasedimentary rocks. With these results we can propose an eo-Variscan scenario: 1) Back-arc basin opening (Pridoli), associated with a magmatic event; 2) At the paroxysm of subsidence (Mid. Devonian) the thickness of the crust allow the heating of the sediments which led to the metamorphic pick of temperature recorded by rocks; 3) Beginning of the Variscan orogeny at ca. 330Ma

Frictional Heating Processes and Energy Budget During Laboratory Earthquakes

International audienceDuring an earthquake, part of the released elastic strain energy is dissipated within the slip zone by frictional and fracturing processes, the rest being radiated away via elastic waves. While frictional heating plays a key role in the energy budget of earthquakes, it could not be resolved by seismological data up to now. Here we investigate the dynamics of laboratory earthquakes by measuring frictional heat dissipated during the propagation of shear instabilities at stress conditions typical of seismogenic depths. We estimate the complete energy budget of earthquake rupture and demonstrate that the radiation efficiency increases with thermal-frictional weakening. Using carbon properties and Raman spectroscopy, we map spatial heat heterogeneities on the fault surface. We show that an increase in fault strength corresponds to a transition from a weak fault with multiple strong asperities and little overall radiation, to a highly radiative fault behaving as a single strong asperity. Plain Language Summary In nature, earthquakes occur when the stress accumulated in a medium is released by frictional sliding on faults. The stress released is dissipated into fracture and heat energy or radiated through seismic waves. The seismic efficiency of an earthquake is a measure of the fraction of the energy that is radiated away into the host medium. Because faults are at inaccessible depths, we reproduce earthquakes in the laboratory under natural in situ conditions to understand the physical processes leading to dynamic rupture. We estimate the first complete energy budget of an earthquake and show that increasing heat dissipation on the fault increases the radiation efficiency. We develop a novel method to illuminate areas of the fault that get excessively heated up. We finally introduce the concept of spontaneously developing heat asperities, playing a major role in the radiation of seismic waves during an earthquake

Evaluation du degré de maturité de la matière organique par spectroscopie Raman: extension du géothermomètre RSCM vers la gamme 100-200°C.

National audienceLa connaissance de températures de formation des roches est un paramètre important pour la reconstitution et la modélisation de l'histoire de domaines et de processus géologiques. Parmi les nombreuses possibilités d'estimation des températures, le géothermomètre RSCM (Raman Spectroscopy of Carbonaceous Materials) apporte un critère relativement précis pour reconstituer l'évolution thermique d'un domaine géologique (histoire température-temps) à partir de l'étude des composés carbonés ubiquistes (e.g. Beyssac et al., 2002, Aoya et al., 2010, Lahfid et al., 2010). Cet outil fournit ainsi des contraintes pertinentes sur l'histoire thermique des formations géologiques. Cependant, il n'a été calibré que pour la gamme 200-640°C avec une précision absolue de +/- 50°C et une reproductibilité interne bien meilleure de l'ordre de 10-15°C. La présente étude vise à étendre l'applicabilité du géothermomètre RSCM vers des températures plus basses que celles investiguées précédemment. Il s'agit de la gamme 100-200°C. Une série d'échantillons a été sélectionné. Parmi les zones d'échantillonnage figurent des bassins de la chaîne pyrénéenne. Les pics de températures de ces échantillons ont été estimés par des méthodes conventionnelles parmi lesquelles figurent le pouvoir réflecteur de la vitrinite, les inclusions fluides et la cristallinité de l'illite. Un nouveau protocole de mesure, par microspectroscopie Raman, pour caractériser la structure des matériaux carbonés très désordonnés a été établi. Lors de l'analyse, plusieurs paramètres ont été ajustés tels que la puissance du laser, le nombre d'analyses par échantillon, le temps d'acquisition... Une série de spectres de référence ont été alors acquis. Ces spectres ont permis de réaliser dans un premier temps une calibration qualitative de la méthode RSCM pour la gamme 100-200°C. Afin de proposer une calibration quantitative, nous sommes entrain de définir une procédure fiable d'ajustement des spectres Raman afin de déterminer des paramètres Raman pertinents pour la quantification thermique des matériaux carbonés très désordonnés dont les pics thermiques correspondent à la gamme 100-200°C. Cette calibration pourra être appliquée pour reconstituer l'histoire thermique des bassins de marge passive profonde (cible actuelle pour l'industrie pétrolière)

Raman Spectroscopy of Carbonaceous Materials geothermometry: a reliable method to investigate thermal history of foreland basins.

To investigate the thermal evolu-tion of foreland basins, many classical methods includ-ing clay mineralogy, vitrinite reflectance, fluid inclu-sions and illite cristalinity are used. These methods are probably not perfectly reliable taken individually, but provides a robust estimate when they give a coherent results. Raman Spectroscopy of Carbonaceous Materials (RSCM) could be an alternative method to constrain paleotempeartures of rocks. This method has been calibrated firstly in the range 330-640°C [1] then in the range 200-350° [2]. The aim of this study is to extend the applicability of this method towards palotemperatures lower than 200°C. For this purpose, we discuss a new fitting pro-cedure which would allow us to define a new Raman parameter R3. This parameter is different of RA1 [1] and R2 [2] used previously to characterize the thermal evolution of the Carbonaceous Materials. Raman spectra measured and treated are obtained by analysing samples from turbiditic deposits of differ-ent basins located in the western part of the South Pyrenean Zone. The thermal history of these samples, that underwent diagenesis, is well known using a vari-ous techniques. These methods are vitrinite reflec-tance, fluid inclusion thermometry, low-T thermochro-nology, index mineralogy and illite cristallinity. Although our results are preliminary, they suggest that the RSCM geothermometer could be a reliable method to constrain the thermal history of foreland basins. We are currently working on the acquisition of more data to determine temperatures of the basin that underwent diagenesis conditions during the Pyrenean activity. Referenc