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Lecomte E.,Heriot - Watt University | Le Pourhiet L.,CNRS Paris Institute of Earth Sciences | Lacombe O.,CNRS Paris Institute of Earth Sciences
Geophysical Research Letters | Year: 2012

The existence of active low-angle normal faults is much debated because (1) the classical theory of fault mechanics implies that normal faults are locked when the dip is less than 30° and (2) shallow-dipping extensional fault planes do not produce large earthquakes (M > 5.5). However, a number of field observations suggest that brittle deformation occurs on low-angle normal faults at very shallow dip. To reconcile observations and theory, we use an alternative model of fault reactivation including a thick elasto-plastic frictional fault gouge, and test it at large strain by the mean of 2D mechanical modeling. We show that plastic compaction allows reducing the effective friction of faults sufficiently for low-angle normal faults to be active at dip of 20°. As the model predicts that these faults must be active in a slip-hardening regime, it prevents the occurrence of large earthquakes. However, we also evidence the neoformation of Riedel-type shear bands within thick fault zone, which, we believe, may be responsible for repeated small earthquakes and we apply the model to the Gulf of Corinth (Greece). Copyright 2012 by the American Geophysical Union.

Tranos M.D.,Aristotle University of Thessaloniki | Lacombe O.,CNRS Paris Institute of Earth Sciences
Journal of Geodynamics | Year: 2014

We investigate the geometry and kinematics of the faults exposed in basement rocks along the Strouma River in SW Bulgaria as well as the sequence of faulting events in order to place constraints on the Cenozoic kinematic evolution of this structurally complex domain. In order to decipher the successive stress fields that prevailed during the tectonic history, we additionally carried out an analysis of mesoscale striated faults in terms of paleostress with a novel approach. This approach is based on the P-T axes distribution of the fault-slip data, and separates the fault-slip data into different groups which are characterized by kinematic compatibility, i.e., their P and T axes have similar orientations. From these fault groups, stress tensors are resolved and in case these stress tensors define similar stress regimes (i.e., the orientations of the stress axes and the stress shape ratios are similar) then the fault groups are further unified. The merged fault groups after being filled out with those fault-slip data that have not been incorporated into the above described grouping, but which present similar geometric and kinematic features are used for defining the final stress regimes. In addition, the sequence of faulting events was constrained by available tectonostratigraphic data.Five faulting events named D1, D2, D3, D4 and D5 are distinguished since the Late Oligocene. D1 is a pure compression stress regime with σ1 stress axis trending NNE-SSW that mainly activated the WNW-ESE to ENE-WSW faults as reverse to oblique reverse and the NNW-SSE striking as right-lateral oblique contractional faults during the Latest Oligocene-Earliest Miocene. D2 is a strike-slip - transpression stress regime with σ1 stress axis trending NNE-SSW that mainly activated the NNW-SSE to N-S striking as right-lateral strike-slip faults and the ENE-WSW striking faults as left-lateral strike-slip ones during the Early-Middle Miocene. D3 extensional event is associated with a NW-SE to WNW-ESE extension causing the activation of mainly low-angle normal faults of NE-SW strike and NNE-SSW to NNW-SSE striking high-angle normal faults. D4 is an extensional event dated from Late Miocene to Late Pliocene. It activated NNW-SSE to NW-SE faults as normal faults and E-W to WNW-ESE faults as right-lateral oblique extensional faults. The latest D5 event is an N-S extensional stress regime that dominates the wider area of SW Bulgaria in Quaternary times. It mainly activated faults that generally strike E-W (ENE-WSW and WNW-ESE) normal faults, along which fault-bounded basins developed. The D1 and D2 events are interpreted as two progressive stages of transpressional tectonics related to the late stages of collision between Apulia and Eurasia plates. These processes gave rise to the lateral extrusion of the Rhodope and Balkan regions toward the SE along the Strouma Lineament. The D3 event is attributed to the latest stage of this collision, and represents the relaxation of the overthickened crust along the direction of the lateral extrusion. The D4 and D5 events are interpreted as post-orogenic extensional events related to the retreat of the Hellenic subduction zone since the Late Miocene and to the widespread back-arc Aegean extension still prevailing today. © 2013 Elsevier Ltd.

Angiboust S.,CNRS Paris Institute of Earth Sciences | Agard P.,CNRS Paris Institute of Earth Sciences | Yamato P.,University Rennes1 | Raimbourg H.,University of Orléans
Geology | Year: 2012

Understanding processes acting along the subduction interface is crucial to assess lithospheric-scale coupling between tectonic plates and mechanisms causing intermediate-depth seismicity. Despite a wealth of geophysical studies aimed at better characterizing the subduction interface, we still lack critical data constraining processes responsible for seismicity within oceanic subduction zones. We herein report the finding of eclogite breccias, formed at ∼80 km depth during subduction, in an almost intact 10-km-scale fragment of exhumed oceanic lithosphere (Monviso ophiolite, Western Alps). These eclogite breccias correspond to meter-sized blocks made of 1-10 cm fragments of eclogite mylonite cemented by interclast omphacite, lawsonite, and garnet, and were later embedded in serpentinite in a 30-150-m-wide eclogite facies shear zone. At the mineral scale, omphacite crack-seal veins and garnet zoning patterns also show evidence for polyphased fracturing-healing events. Our observations suggest that a possible seismic brecciation occurred in the middle part of the oceanic crust, accompanied by the input of externally derived fluids. We also conclude that these eclogite breccias likely mark the locus of an ancient fault zone associated with intraslab, intermediate-depth earthquakes at ∼80 km depth. © 2012 Geological Society of America.

Rosenberg C.L.,University Pierre and Marie Curie | Rosenberg C.L.,CNRS Paris Institute of Earth Sciences | Kissling E.,ETH Zurich
Geology | Year: 2013

Accommodation of collisional shortening in the Central Alps varies dramatically along strike, and this change is inferred to result from along-strike changes of rheology. In the western Central Alps, 90% of shortening is accommodated in the thickened lower plate. In the eastern Central Alps, 90% of shortening is accommodated in the upper plate. In the central Central Alps, shortening is almost equally partitioned between the two plates. The lower crust of the Adriatic plate forms a wedge that reaches a maximum north-south extension of almost 70 km in the Engadine section, progressively decreasing westward and disappearing along the Simplon section. This difference indicates an along-strike increase of intra-plate decoupling, limiting shortening of the Adriatic plate to the middle and upper parts of the crust. Whereas the upper plate indents into the thickened accreted lower plate in the Simplon section, it is the lower plate that indents an intensely deforming upper plate in the Engadine section. In the west, the Ivrea mantle body increases the strength of the Adriatic upper plate, and Barrovian metamorphism weakens the lower plate. Therefore, along-strike transfer of shortening from one plate to the other appears to be a manifestation of along-strike changes in rheology deep in the crust. © 2013 Geological Society of America.

Saleeby J.,California Institute of Technology | Saleeby Z.,California Institute of Technology | Le Pourhiet L.,CNRS Paris Institute of Earth Sciences
Geosphere | Year: 2013

We investigate the putative Pliocene-Quaternary removal of mantle lithosphere from beneath the southern Sierra Nevada region using a synthesis of subsidence data from the Great Valley, and geomorphic relations across the Sierra Nevada. These findings are used to test the results and predictions of thermomechanical modeling of the lithosphere removal process that is specific to the Sierra Nevada, as presented in an accompanying paper referenced here as Part I. Our most successful thermomechanical model and the observational data that it explains are further bundled into an integrated physiographic evolution-geodynamic model for the threedimensional epeirogenic deformation field that has affected mainly the southern Sierra Nevada-San Joaquin Basin region as a result of underlying mantle lithosphere removal. The coupled Sierra Nevada mountain range and Great Valley basin are recognized as a relatively rigid block (Sierra Nevada microplate) moving within the San Andreas- Walker Lane dextral plate juncture system. Our analysis recognizes that the Sierra Nevada possessed kilometer-scale local and regional paleotopographic relief, and that the Great Valley forearc basin possessed compara ble structural relief on its principal stratigraphic horizons, both dating back to the end of Cretaceous time. Such ancient paleorelief must be accounted for in considering late Cenozoic components of uplift and subsidence across the microplate. We further recognize that Cenozoic rock and surface uplift must be considered from the perspectives of both local epeirogeny driven by mantle lithosphere removal, and regional far-field-forced epeirogeny driven by plate tectonics and regional upper-mantle buoyancy structure. Stratigraphic relations of Upper Cretaceous and lower Cenozoic marinestrata lying on northern and southern Sierra Nevada basement provide evidence for near kilometerscale rock uplift in the Cenozoic. Such uplift is likely to have possessed positive, and then superposed negative (subsidence) stages of reliefgeneration, rendering net regional rock and surface uplift. Accounting for ancient paleorelief and far-field-driven regional uplift leaves a residual pattern whereby ̃1200 m of southeastern Sierra crest rock and similar surface uplift, and ̃700 m of spatially and temporally linked tectonic subsidence in the southern Great Valley were required in the late Cenozoic by mantle lithosphere removal. These values are close to the predictions of our modeling, but application of the model results to the observed geology is complicated by spatial and temporal variations in the regional tectonics that probably instigated mantle lithosphere removal, as well as spatial and temporal variations in the observed uplift and subsidence patterns. Considerable focus is given to these spatial-temporal variation patterns, which are interpreted to reflect a complex three-dimensional pattern resulting from the progressive removal of mantle lithosphere from beneath the region, as well as its epeirogenic expressions. The most significant factor is strong evidence that mantle lithosphere removal was fi rst driven by an east-towest pattern of delamination in late Miocene- Pliocene time, and then rapidly transitioned to a south-to-north pattern of delamination in the Quaternary. © 2013 Geological Society of America.

Calcite twins have been used for a long time as indicators of stress/strain orientations and magnitudes. Recent developments during the last 15 years point toward significant improvements of existing techniques as well as new applications of calcite twin analysis in thrust belts and forelands. This paper summarizes the principles of the most common techniques in this tectonic field and illustrates some aspects of the use of calcite twins to constrain not only stress/strain orientations and magnitudes, but also to some extent paleotemperature or paleoburial in orogenic forelands. This review is based in a large part on the studies that I conducted in various geological settings such as the forelands of Taiwan, Pyrenees, Zagros, Rockies and Albanides orogens. The contribution of calcite twin analysis to the understanding of the intraplate stress transmission away from plate boundaries is also emphasized. © 2010, IFP Energies nouvelles.

Chorowicz J.,CNRS Paris Institute of Earth Sciences
Comptes Rendus - Geoscience | Year: 2016

The Pieniny Klippen Belt (PKB) is a narrow, discontinuous zone rich in olistostromes and olistoliths (Klippen) in the western Carpathians. This paper, based on prior works including tectonic and stratigraphic evidences, suggests that the PKB rocks were deposited from the Triassic to the Early Paleogene along the eastern footwall of a major Split-Karlovac-Initial PKB-Crustal-Zone (SKICZ) paleotransform fault zone. This transform fault was then separating the continental crust of the Austro-Alpine zone in the west and a Carpathian Embayment Ocean in the east. It was only during the Late Paleogene-Early Miocene that the PKB rocks were integrated into the accretionary prism that formed at the front of the eastward-extruded ALCAPA units. This interpretation therefore supports the existence of a major paleotransform fault zone in the Neo-Tethys during the Triassic-Early Paleogene. This paleotransform had been previously suggested to explain the observed reversal in obduction and subduction at the junction between the eastern-southern Alps and the Carpathians-Dinarides. © 2015 Académie des sciences.

Le Pourhiet L.,CNRS Paris Institute of Earth Sciences
Bulletin de la Societe Geologique de France | Year: 2013

After giving a complete analytical solution for the strain softening model associated to Mohr-Coulomb non associated elasto-plastic flow rule (MC-model), the paper demonstrates that this rheology possesses a finite limit load which allows solving for strength drop as a boundary value problem. The MC-model produces a non-dimensional strength drop, which depends on three parameters: the orientation of the shear band versus the least principal stress axis outside the band α0, the peak friction angle φ and the dilatation angle ψ. The maximum reduction of strength obtained with that strain-softening model is on the order of the confining stress p0. For this weakest regime, the effective friction of the shear band drops from μini= 0.85 at peak to μss= 0.64 at the end of the softening phase. In this model, which considers thick shear bands, the weakest regime is not obtained for an orientation corresponding to the exact Coulomb orientation. Instead, the orientation of the weakest shear zone systematically deviates from the coulomb orientation by an angle, which rises with its internal friction angle. The characteristic shear strain needed to achieve steady state is quantified semi analytically and in the range of parameters valid for Earth, this strain is found to be of the order of 7-8%. These numbers are typical of what is observed in the laboratory, which give us confidence on that MC-model is a good and probably the simplest model to localize strain in numerical codes aimed at modeling the brittle part of the Earth.

Tribovillard N.,Lille University of Science and Technology | Algeo T.J.,University of Cincinnati | Baudin F.,CNRS Paris Institute of Earth Sciences | Riboulleau A.,Lille University of Science and Technology
Chemical Geology | Year: 2012

Patterns of uranium-molybdenum covariation in marine sediments have the potential to provide insights regarding depositional conditions and processes in paleoceanographic systems. Specifically, such patterns can be used to assess bottom water redox conditions, the operation of metal-oxyhydroxide particulate shuttles in the water column, and the degree of water mass restriction. The utility of this paleoenvironmental proxy is due to the differential geochemical behavior of U and Mo: (1) uptake of authigenic U by marine sediments begins at the Fe(II)-Fe(III) redox boundary (i.e., suboxic conditions), whereas authigenic Mo enrichment requires the presence of H 2S (i.e., euxinic conditions), and (2) transfer of aqueous Mo to the sediment may be enhanced through particulate shuttles, whereas aqueous U is unaffected by this process. In the present study, we examine U-Mo covariation in organic-rich sediments deposited mostly in the western Tethyan region during oceanic anoxic events (OAEs) of Early Jurassic to Late Cretaceous age. Our analysis generally confirms existing interpretations of redox conditions in these formations but provides significant new insights regarding water mass restriction and the operation of particulate shuttles in depositional systems. These insights will help to address contentious issues pertaining to the character and origin of Mesozoic OAEs, such as the degree to which regional paleoceanographic factors controlled the development of the OAEs. © 2011 Elsevier B.V.

Sanloup C.,CNRS Paris Institute of Earth Sciences
Chemical Geology | Year: 2016

Knowing the density of silicate liquids at high pressure is essential to answer questions relevant to the presence of magmas at depth, whether that be in the present Earth or in its earliest times, during differentiation of the planet. Melts have unique physical and chemical properties, which vary as a function pressure, and chemical composition. The focus here will be on in situ measurements of the density of magmas, with a presentation of the available methods and of the main results obtained so far, including why some magmas may be trapped at depth. Understanding the macroscopical physical properties of magmas requires an accurate microscopic structural description. Structural descriptions of compressed magmas are becoming more widely available, from experiments and from theoretical calculations. These structural inputs are used to understand the compression mechanisms at stake in the densification of magmas, e.g. the collapse of voids, coordination increase for the major cations, and bond compressibility. These densification processes profoundly affect not only the physical properties of the melt, but also its chemical properties, i.e. the way element partition between the magma and a metallic melt or between the magma and crystals. © 2016 Elsevier B.V.

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