CNRS ENS Geology Laboratory

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Gabuchian V.,California Institute of Technology | Rosakis A.J.,California Institute of Technology | Bhat H.S.,CNRS ENS Geology Laboratory | Madariaga R.,CNRS ENS Geology Laboratory | Kanamori H.,California Institute of Technology
Nature | Year: 2017

Many of Earth's great earthquakes occur on thrust faults. These earthquakes predominantly occur within subduction zones, such as the 2011 moment magnitude 9.0 eathquake in Tohoku-Oki, Japan, or along large collision zones, such as the 1999 moment magnitude 7.7 earthquake in Chi-Chi, Taiwan. Notably, these two earthquakes had a maximum slip that was very close to the surface. This contributed to the destructive tsunami that occurred during the Tohoku-Oki event and to the large amount of structural damage caused by the Chi-Chi event. The mechanism that results in such large slip near the surface is poorly understood as shallow parts of thrust faults are considered to be frictionally stable. Here we use earthquake rupture experiments to reveal the existence of a torquing mechanism of thrust fault ruptures near the free surface that causes them to unclamp and slip large distances. Complementary numerical modelling of the experiments confirms that the hanging-wall wedge undergoes pronounced rotation in one direction as the earthquake rupture approaches the free surface, this torque is released as soon as the rupture breaks the free surface, resulting in the unclamping and violent flapping of the hanging-wall wedge. Our results imply that the shallow extent of the seismogenic zone of a subducting interface is not fixed and can extend up to the trench during great earthquakes through a torquing mechanism. © 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

Pawlyta M.,Silesian University of Technology | Rouzaud J.-N.,CNRS ENS Geology Laboratory | Duber S.,University of Silesia
Carbon | Year: 2015

The aim of this paper is to describe carbonisation and partial graphitization of carbon blacks (CB). Raman spectrometry is used to investigate a series of five CB grades during heat treatment (up to 2600 °C). Obtained results are discussed by comparing Raman data with X-ray diffraction and high-resolution transmission electron microscopy (HRTEM) observations. For Raman spectra interpretation, the usual curve fitting method proposed by Sadezky et al. for soot and disordered carbonaceous material is applied. As the same procedure can be applied over all the heat-treatment temperature range, the determination of band parameters from five band decompositions appears to be the most convenient to follow the CB's structural improvement. We demonstrate that only a partial graphitization takes place and the graphitizability is limited by the diameter of the primary particles. Our observations generalize the results obtained for cokes: graphitization degree of carbonaceous materials after the heat-treatment is limited by the diameter of the volumes within polyaromatic layers are oriented in parallel. © 2014 Elsevier Ltd. All rights reserved.

Buffetaut E.,CNRS ENS Geology Laboratory | Angst D.,University Claude Bernard Lyon 1
Earth-Science Reviews | Year: 2014

The stratigraphic distribution of the three main groups of large flightless birds known from the Palaeogene of Europe, Gastornithidae, Phorusrhacidae and Ratitae, is reviewed. The huge, herbivorous gastornithids, represented by the single genus Gastornis, are known from the Selandian (Middle Palaeocene) to the late Lutetian (Middle Eocene), being recorded from reference levels MP5 to MP13. The carnivorous phorusrhacids are represented by a single species, Eleutherornis cotei, from the late Lutetian (MP14, late Middle Eocene). The ratites have a patchy distribution, being represented by two species of moderate size, Remiornis heberti from the Thanetian (MP6, Late Palaeocene) and Palaeotis weigelti from the Lutetian (MP11 to MP13, Middle Eocene). The stratigraphic distributions of large eggs referred to gastornithids in the Late Palaeocene and Early Eocene of southern Europe and the occurrence of enigmatic large avian footprints in the Late Eocene of France are discussed. Whereas gastornithids and ratites co-existed in both the Palaeocene and the Middle Eocene, phorusrhacids seem to have been the only large ground birds in Europe at the end of the Middle Eocene. The palaeobiogeographical and evolutionary implications of the stratigraphic distributions of those groups of large birds in Europe are discussed. As Gastornis first appears in North America and in Asia in the Early Eocene, it is likely that gastornithids originated in Europe and later spread to other land masses during a dispersal event close to the Palaeocene-Eocene boundary. Prior to that, gastornithids evolved on the European "island continent", where they were the largest terrestrial tetrapods during the Palaeocene. Gastornithids do not seem to have been significantly affected by the PETM. Ratitae have a more patchy record and relationships between Remiornis and Palaeotis remain unclear. Nevertheless, those European forms are among the earliest known ratites and this should not be overlooked in discussions of ratite evolution and palaeobiogeography. Phorusrhacids appear to have been present in Europe for only a short time and are interpreted as the result of dispersal from Africa followed by local extinction. © 2014 Elsevier B.V.

Olcott Marshall A.,University of Kansas | Jehlicka J.,Charles University | Rouzaud J.-N.,CNRS ENS Geology Laboratory | Marshall C.P.,University of Kansas
Gondwana Research | Year: 2014

The Pilbara Craton in Western Australia contains the best-preserved and most complete record of Archean rocks in the world. As such, they are some of the most studied rocks in the world; paleontologists, isotopic geochemists, geologists and geobiologists have all investigated these rocks for clues about the early biosphere and atmosphere. Here we show using high-resolution transmission electron microscopy that the carbonaceous material found in the Apex chert, and potentially in other associated units, was formed by multiple processes such as abiotic catalytic synthesis and/or biological synthesis. We use these data as well as the geological history of the craton to demonstrate that when the rocks of the Pilbara Craton experienced a high degree of post-depositional hydrothermal alteration, carbonaceous material could have been remobilized and redeposited. As the carbonaceous material within the Apex chert samples was formed over nearly a billion years, bulk chemistry, even at the micron level, will be unable to unambiguously delineate the presence of life in these ancient rocks, although nanoscale observations may provide a way forward in the search for ancient life. © 2013 International Association for Gondwana Research.

Aubourg C.,Cergy-Pontoise University | Aubourg C.,French National Center for Scientific Research | Pozzi J.-P.,CNRS ENS Geology Laboratory
Earth and Planetary Science Letters | Year: 2010

We investigate the effects of burial and moderate experimental heating on claystones from three regions with different degrees of maturation: immature (burial temperature ~40°C) of Bure Callovo-Oxfordian claystones in the Basin of Paris (France); early mature (burial temperature ~85°C) of Opalinus Lower Dogger claystones from the Mont Terri anticline in front of the Jura fold belt (Switzerland); and mature to overmature (burial temperature <170°C) of Chartreuse Callovian-Oxfordian claystones from Chartreuse Sub-Alpine chains. To have information about the nature of the magnetic assemblage, we perform low-temperature (10K-300K) investigation of an isothermal remanent magnetization. In a first set of laboratory heating experiments, we aim to impart a chemical remanent magnetization (CRM) at 95°C for several weeks in Bure and Opalinus claystones. Thermal demagnetization of the CRM reveals that magnetite is formed by heating the Opalinus claystones while an assemblage of magnetite and iron sulphide is formed in Bure claystones. Further, we document the appearance of a magnetic transition at ~35K in Bure claystones after heating. We name this transition the P-transition and we propose that it is related to the formation of fine-grained pyrrhotite (Fe7S8). The P-transition is also detected in early mature to mature Opalinus and Chartreuse claystones. We conduct additional experimental heating of natural Opalinus claystones. One set of experiments is referred to as short-term heating (1h) from 100°C to 200°C. It is dedicated to an investigation of the effect of short-lived heating processes in geology. A second set of heating experiments is designed to approach burial conditions using a gold capsule. In burial-like experiments, we heated Opalinus claystones from 150°C to 250°C for several weeks under a pressure of 100MPa. In both experiments, we observe a correlative diminution of the pyrrhotite signature at 35K with increasing temperature. We interpret this trend as the appearance of magnetite. We derive a parameter PM from the warming curve of a saturated isothermal remanent magnetization acquired at 10K (ZFC). We report on a consistent evolution of PM with temperature in the range of 40°C to 250°C, including natural samples, heated samples at 95°C, and burial-like heated samples. PM first increases between ~40°C up to ~85°C, implying that pyrrhotite gradually dominates the magnetic assemblage at low temperature. For temperatures above 85°C, PM decreases up to 250°C, implying that the formation of magnetite gradually overshadows the magnetic input of pyrrhotite. PM values obtained from mature to overmature claystones from the Chartreuse are lower than the PM values obtained from the burial-like heated Opalinus claystones, suggesting that the formation of magnetite is driven by kinetics. The continuous trend of the PM parameter suggests that the magnetic properties of pyrrhotite-magnetite claystones can be used to infer paleo-temperatures and we propose to name this geothermometer MagEval. © 2010 Elsevier B.V.

Lambeck K.,Australian National University | Lambeck K.,CNRS ENS Geology Laboratory | Rouby H.,Australian National University | Rouby H.,CNRS ENS Geology Laboratory | And 3 more authors.
Proceedings of the National Academy of Sciences of the United States of America | Year: 2014

The major cause of sea-level change during ice ages is the exchange of water between ice and ocean and the planet's dynamic response to the changing surface load. Inversion of ∼1,000 observations for the past 35,000 y from localities far from former ice margins has provided new constraints on the fluctuation of ice volume in this interval. Key results are: (i) a rapid final fall in global sea level of ∼40 m in <2,000 y at the onset of the glacial maximum ∼30,000 y before present (30 ka BP); (ii) a slow fall to -134 m from 29 to 21 ka BP with a maximum grounded ice volume of ∼52 × 106 km3 greater than today; (iii) after an initial short duration rapid rise and a short interval of near-constant sea level, the main phase of deglaciation occurred from ∼16.5 ka BP to ∼8.2 ka BP at an average rate of rise of 12 m·ka-1 punctuated by periods of greater, particularly at 14.5-14.0 ka BP at ≥40 mm·y-1 (MWP-1A), and lesser, from 12.5 to 11.5 ka BP (Younger Dryas), rates; (iv) no evidence for a global MWP-1B event at ∼11.3 ka BP; and (v) a progressive decrease in the rate of rise from 8.2 ka to ∼2.5 ka BP, after which ocean volumes remained nearly constant until the renewed sea-level rise at 100-150 y ago, with no evidence of oscillations exceeding ∼15-20 cm in time intervals ≥200 y from 6 to 0.15 ka BP.

Buffetaut E.,CNRS ENS Geology Laboratory
Annales de Paleontologie | Year: 2011

Samrukia nessovi was described as a giant bird on the basis of a pair of mandibular rami from the Late Cretaceous of Kazakhstan. Anatomical comparison shows that the specimen bears no distinctive avian characters, and that its purported autapomorphies, as well as all its other characters, are in fact well-known pterosaurian features. The published phylogenetic analysis placing Samrukia within Aves is flawed because it did not include pterosaurs. Samrukia nessovi is clearly a large pterosaur, not a giant bird. © 2011 Elsevier Masson SAS.

Brantut N.,CNRS ENS Geology Laboratory | Schubnel A.,CNRS ENS Geology Laboratory | Gueguen Y.,CNRS ENS Geology Laboratory
Journal of Geophysical Research: Solid Earth | Year: 2011

Triaxial tests on gypsum polycrystal samples are performed at confining pressure (Pc) ranging from 2 to 95 MPa and temperatures up to 70C. During the tests, stress, strain, elastic wave velocities, and acoustic emissions are recorded. At Pc 10 MPa, the macroscopic behavior is brittle, and above 20 MPa the macroscopic behavior becomes ductile. Ductile deformation is cataclastic, as shown by the continuous decrease of elastic wave velocities interpreted in terms of microcrack accumulation. Surprisingly, ductile deformation and strain hardening are also accompanied by small stress drops from 0.5 to 6 MPa in amplitude. Microstructural observations of the deformed samples suggest that each stress drop corresponds to the generation of a single shear band, formed by microcracks and kinked grains. At room temperature, the stress drops are not correlated to acoustic emssions (AEs). At 70C, the stress drops are larger and systematically associated with a low-frequency AE (LFAE). Rupture velocities can be inferred from the LFAE high-frequency content and range from 50 to 200 m s-1. The LFAE amplitude also increases with increasing rupture speed and is not correlated with the amplitude of the macroscopic stress drops. LFAEs are thus attributed to dynamic propagation of shear bands. In Volterra gypsum, the result of the competition between microcracking and plasticity is counterintuitive: Dynamic instalibilities at 70C may arise from the thermal activation of mineral kinking. © 2011 by the American Geophysical Union.

Buffetaut E.,CNRS ENS Geology Laboratory
Geological Magazine | Year: 2010

A fragmentary bone from the Dinosaur Park Formation (Campanian) of Dinosaur Provincial Park (Alberta, Canada), originally described as a pterosaur tibiotarsus, is reinterpreted as the distal end of the tibiotarsus of a basal bird, probably an enantiornithine, on the basis of several distinctive characters. It is the first report of such a bird from the Dinosaur Park Formation and shows that this group was present, together with various more derived ornithurines, in the relatively high-latitude environments of Late Cretaceous western Canada. © Cambridge University Press 2010.

Gueguen Y.,CNRS ENS Geology Laboratory | Fortin J.,CNRS ENS Geology Laboratory
Geophysical Research Letters | Year: 2013

In this paper we focus on the case of sandstones for which many experimental data are available. We present a simple 2D model derived from granular media mechanics. This model assumes that the granular microstructure is a key point to understand the mechanical behavior. We consider a periodic grain network and focus on the firstorder neighbors of a given grain. These approximations are sufficient to explain the overall mechanical behavior in the Q versus P stress space. In the low pressure range, the controlling micromechanism is assumed to be tensile failure at grain contacts. The "dilatant" envelope is found to be a straight line in the stress space. In the high pressure range, the controlling micromechanism is assumed to be grain fragmentation. The "compactant" envelope is found to be a straight line in the stress space. We observed that this 2D model slightly overestimates Q versus P slopes determined experimentally (2.3 instead of 1.5), which can be explained by the approximations made. © 2013. American Geophysical Union. All Rights Reserved.

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