Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Although subducting slabs undergo a bending deformation that resists tectonic plate motions, the magnitude of this resistance is not known because of poor constraints on slab strength. However, because slab bending slows the relatively rapid motions of oceanic plates, observed plate motions constrain the importance of bending. We estimated the slab pull force and the bending resistance globally for 207 subduction zone transects using new measurements of the bending curvature determined from slab seismicity. Predicting plate motions Using a global mantle flow model, we constrain the viscosity of the bending slab to be at most similar to 300 times more viscous than the upper mantle: stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of similar to 10(-15) s(-1). The constrained bending viscosity (similar to 2 x 10(23) Pa s) is larger than previous estimates that yielded similar or larger bending resistance (here similar to 25% of forces). This apparent discrepancy occurs because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction Of Strong slabs. This gentle bending may ultimately permit plate tectonics on Earth.
ISSN: 0012-821X

hal-00412383
https://hal.archives-ouvertes.fr/hal-00412383
DOI : 10.1016/j.epsl.2008.05.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales. Our models are set up with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (mantle). For our study, the lithosphere/upper mantle viscosity contrast has been systematically varied, from similar to 10 to similar to 10(5) in order to explore the parameter space between weak and strong slab dynamics. We found that subduction is characterized by a retreating mode for viscosity ratios > 10(4), by the coexistence of a retreating mode and an advancing mode for viscosity ratios between similar to 10(4) and similar to 10(2), and quasi-stationary, Rayleigh-Taylor like behaviour for ratios < 10(2). By combining our experimental results and kinematic data from current subduction zones in four reference frames which differ in the amount of net rotation, we infer that a lithosphere/upper mantle viscosity contrast of 150-500 is necessary to obtain realistic trench/subducting plate velocity ratios as well as the variability of subduction styles observed in nature.
ISSN: 0012-821X

hal-00412008
https://hal.archives-ouvertes.fr/hal-00412008
DOI : 10.1016/j.epsl.2008.04.006

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : Approximately 4 yr of campaign and continuous Global Positioning System (GPS) measurements across the Dead Sea fault system (DSFS) in Lebanon provide direct measurements of interseismic strain accumulation along a 200-km-long restraining bend in this continental transform fault. Late Cenozoic transpression within this restraining bend has maintained more than 3000 m of topography in the Mount Lebanon and Anti-Lebanon ranges. The GPS velocity field indicates 4-5 mm yr(-1) of relative plate motion is transferred through the restraining bend to the northern continuation of the DSFS in northwestern Syria. Near-field GPS velocities are generally parallel to the major, left-lateral strike-slip faults, suggesting that much of the expected convergence across the restraining bend is likely accommodated by different structures beyond the aperture of the GPS network (e.g. offshore Lebanon and, possibly, the Palmyride fold belt in SW Syria). Hence, these geodetic results suggest a partitioning of crustal deformation involving strike-slip displacements in the interior of the restraining bend, and crustal shortening in the outer part of the restraining bend. Within the uncertainties, the GPS-based rates of fault slip compare well with Holocene-averaged estimates of slip along the two principal strike-slip faults: the Yammouneh and Serghaya faults. Of these two faults, more slip occurs on the Yammouneh fault, which constitutes the primary plate boundary structure between the Arabia and Sinai plates. Hence, the Yammouneh fault is the structural linkage that transfers slip to the northern part of the transform in northwestern Syria. From the perspective of the regional earthquake hazard, the Yammouneh fault is presently locked and accumulating interseismic strain.
ISSN: 0956-540X

hal-00406620
https://hal.archives-ouvertes.fr/hal-00406620
DOI : 10.1111/j.1365-246X.2006.03328.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : The motion of oceanic plates is commonly related to the subduction of cold and dense oceanic material into the mantle. These models predict plate velocities from subduction velocities but the trench motion is not directly included in the computation. Here, using a recent compilation of a global data set, we found that the motion of trenches (either advancing or retreating with respect to upper plates) scales with their corresponding subducting plates motion. Based on simple experimental tests, we found that subduction of strong slabs inside the upper mantle correctly predicts these kinematic relationships. We deduce that the motion of the trenches represent the surface manifestation of the resistance encountered by the subducting lithosphere to bend and penetrate within the upper mantle.
ISSN: 0012-821X

hal-00407200
https://hal.archives-ouvertes.fr/hal-00407200
DOI : 10.1016/j.epsl.2007.02.016

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
New estimates of trench migration rates allow us to address the dynamics of trench migration and back-arc strain. We show that trench migration is primarily controlled by the subducting plate velocity V-sub, which largely depends on its age at the trench. Using the hot and weak arc to back-arc region as a strain sensor, we define neutral arcs characterized by the absence of significant strain, meaning places where the forces (slab pull, bending, and anchoring) almost balance along the interface between the plates. We show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions: V-t = 0.5V(sub) - 2.3 (in cm a(-1)) in the HS3 reference frame. Deformation occurs when the velocity combination deviates from kinematic equilibrium. Balancing the torque components of the forces acting at the trench indicates that stiff (old) subducting plates facilitate trench advance by resisting bending.
ISSN: 0278-7407

hal-00411731
https://hal.archives-ouvertes.fr/hal-00411731
https://hal.archives-ouvertes.fr/hal-00411731/document
https://hal.archives-ouvertes.fr/hal-00411731/file/Lallemand2008-Tectonics.pdf
DOI : 10.1029/2007TC002212

Éditeur(s) : HAL CCSD
Résumé : International audience
The main biological function of reaction wood is to act as “muscle” for trees, enabling them to control their posture and right stems when they are permanently disturbed from their set-point inclination by gravity or winds. The key property to achieve this function is the development of high mechanical stress during the formation of reaction wood cells, called “maturation strains”. Moreover, this function is basically performed through the asymmetric formation of wood (in quantity or quality) around the tree circumference. This asymmetry enables stems to bend upward or to compensate for the downward bending induced by gravity, as an asymmetric force associated to maturation strains creates a bending moment (i.e. one side of the stem “pulls” the other one). We will discuss the traits (cross sectional size and shape, wood features as G fibers or microfibril angle, eccentric growth even in usual or extreme cases as buttresses, total biomass growth and height of center of mass, stem lean and set-point angle) involved to understand how the posture control works. We will give simple formulas to estimate a “muscle efficiency” from biomechanical modeling. We will give practical methods to estimate all the involved traits, especially maturation strains from direct and indirect measurements, in peripheral functional wood or by retrospective dendrochronological analysis. Then, we will illustrate how tree stem tropisms and especially gravitropism, widely studied by physiologists, are key processes in natural ecosystems, through practical examples from tropical rainforest tree communities and regeneration of European mixed beech forests. Using the previous definition of postural control efficiency, we will show that the cross section size is rapidly a limiting factor, opposing the extreme strategies of flexible small stems, which bend easily but also right themselves quickly, to stiff big stems. Lastly, as wood in living trees ensures general storage, defence, vascular and skeletal functions, we will ask general questions about synergies and trade-offs between posture control and other functions.
Botany 2014 : new frontiers in botany
Boise, United States

hal-01627031
https://hal.archives-ouvertes.fr/hal-01627031

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.
ISSN: 1342-937X

hal-01481263
https://hal.archives-ouvertes.fr/hal-01481263
DOI : 10.1016/j.gr.2016.07.007

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Calabro-Peloritan belt constitutes the eastward termination of the southern segment of the Alpine Mediterranean belt. This orogenic system was built up during the convergence between the Eurasian and the African plates, roughly directed North-South since the Upper Cretaceous. It was subsequently fragmented during the opening of the Western Mediterranean basins since Oligocene times. The curved shape of the Calabro-Peloritan belt was acquired during the opening of the Tyrrhenian basin since Tortonian. The origin, kinematics and significance of the Calabro-Peloritan tectonic pile are still debated. Our data in the Aspromonte Massif of Southern Calabria reveal an Alpine history marked by two main superimposed kinematic regimes. (i) A first phase corresponds to the piling up of basement nappes with a top-to-the-SE vergence, i.e in a direction orthogonal to the belt trend and towards the Adriatic foreland. This external vergence is similar to what is observed in both Northeastern Sicily and Northern Calabria. In Sicily, the age of nappe piling is Alpine, as evidenced by pinched slices of Mesozoïc sediments. In the Aspromonte Massif, thrusting age is less constrained. Our data suggest remnants of late Hercynian structuration before the Alpine stacking. (ii) A second phase corresponds to the thinning of the continental crust, dated at around 30 Ma by both geochronological and stratigraphical data. This extension is mainly localized on two low-angle detachment contacts, with top-to-the-NE displacement. The lower one corresponds to the reworking of the former main nappe contact. The upper one is a large detachment fault cutting across the pile from upper sedimentary levels down to metamorphic basement. Extension of similar Alpine age and similar internal vergence has been already recognized in other parts of the Calabro-Peloritan Arc: in the basement nappes of Northeastern Sicily and in the ophiolitic units of Northern Calabria. Coming back to the original geometry and position of the Calabro-Peloritan belt, before its bending and the opening of the Liguro-Provençal and Tyrrhenian basins, we evidence a homogeneous Oligocene NE-SW extension all along the Calabro-Peloritan segment of the Alpine Mediterranean belt. This tectonometamorphic history is best explained within the framework of the continuous Tertiary westward dipping subduction of the Tethyan oceanic domain below the European active margin and the progressive eastward retreat of the Apennine trench since Oligocene times.
ISSN: 0278-7407

hal-00283182
https://hal.archives-ouvertes.fr/hal-00283182
https://hal.archives-ouvertes.fr/hal-00283182/document
https://hal.archives-ouvertes.fr/hal-00283182/file/Heymes_et_al_2008.pdf
DOI : 10.1029/2007TC002157

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Recent geochemical work shows that subduction-zone serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B–O–H, Sr, Pb) of high-pressure serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones.The serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show undeformed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed serpentinite has very high δ11BSRM951 (from +26‰ to +30‰), low Sr and Pb isotope ratios (87Sr/86Sr = 0.7053–0.7069; 206Pb/204Pb = 18.131–18.205) and low As and Sb contents (0.1 and 0.01 μg/g, respectively). Oxygen and hydrogen isotope compositions are +4.5‰ and −67‰, respectively. In contrast, mylonitic serpentinite shows lower δ11B (from +22‰ to +17‰), significant enrichment in radiogenic Sr and Pb isotopes (87Sr/86Sr up to 0.7105; 206Pb/204Pb up to 18.725), and enrichment in As and Sb (1.3 and 0.39 μg/g, respectively). δ18O of the mylonitic serpentinites reaches values of +5.9‰, whereas δD is comparable with that of undeformed rocks (approximately −70‰). In mylonitic serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component.Our dataset shows that undeformed serpentinite still preserves an oceanic geochemical fingerprint, whereas mylonitic serpentinite is reset in its concentrations of FME and its B, Sr and Pb isotope compositions, due to interaction with sediment- and crust-derived fluids. The environment of this interaction is either compatible with (i) an outer-rise zone setting, with percolation of seawater-derived fluids enriched in sedimentary components into bending-related fault structures, or with (ii) subduction channel domains, where ascending sediment-derived slab fluids infiltrate slices of former oceanic serpentinite accreted to the plate interface domain. Influx of sediment-derived subduction fluids along major deformation zones in serpentinite modifies the element budget of the rocks, with important implications for element recycling and the tectonic history of serpentinite. The B, Sr and Pb isotopic systematics, coupled with FME concentration in serpentinites are particularly helpful geochemical tracers of interaction between different reservoirs in subduction-interface environments, and are more sensitive than the traditionally applied stable oxygen and hydrogen isotope compositions.
ISSN: 0016-7037

hal-01419940
https://hal.archives-ouvertes.fr/hal-01419940
DOI : 10.1016/j.gca.2016.06.034

Éditeur(s) : HAL CCSD
Résumé : Several major earthquakes have struck the 2500 km long Himalayan range during the past century. However, both the maximum size of such earthquakes and the probability of occurrence of a magnitude 9 megaquake are still a mater of debate. In this framework it is of key importance to investigate the lateral segmentation of the Himalayan arc in order to deepen our understanding of the mechanisms that control such events as well as their extents. Besides the remarkably uniform tectonic set-up and the fact that most studies on the Himalayas have focused on Central Nepal, several indications of along strike changes can be gathered.This thesis brings new constraints on lateral variations of crustal structure and of deformation between western Nepal (~80°E) and eastern Bhutan (~92°E).The first part of this thesis focuses on the lateral variations of crustal structure. In the past three years we have carried out four land gravity campaigns. We have established 366 new gravity measurement points to fill the data gaps in central and western Nepal as well as in Bhutan. This new dataset, together with available seismological data, is used to constrain thermo-mechanical models of the bending of the India plate underneath the Himalayas. While the inferred crustal geometry does not exhibit major along-strike variations over central and eastern Nepal, the shorter wavelength flexure of the lithosphere in Bhutan is associated with a weaker upper mantle rheology.The second part focuses on the lateral variations of recent deformation in the Himalayas. Fifteen years of paleoseismological investigations have extended the catalogue of major Himalayan events over the last millennium. Combining these information with secular loading, we assess the spatial and temporal stress changes on the Main Himalayan Thrust along the orogen over the last nine centuries. Our calculations indicate that inter-seismic loading has now nearly overcome the Coulomb stress decrease caused by the great ~1100 medieval earthquake. Our results also point out that the 1897 Shillong plateau earthquake does not have a major influence on the stress accumulated on the Main Himalayan Thrust since then. In order to better characterize active tectonics in the Bhutan Himalayas where no studies were done so far, we carried out a morphotectonic analysis in the south-central part of the kingdom. We show that the same amount of Holocene deformation is accommodated on the frontal thrust in Bhutan as on the neighbouring portions of the Himalayan arc. We also find evidences for two major (M>8) earthquakes on this thrust in the Bhutan Himalayas during the last millennium. Our results therefore show that Bhutan cannot be considered as a seismic gap. They also challenge the interpretation of one single ~1100 medieval mega-event that would have ruptured the Main Frontal Thrust from central Nepal to eastern Bhutan.
Au cours du dernier siècle, plusieurs séismes majeurs ont affecté l'Himalaya. Cependant, la taille maximale de ces événements et la probabilité d'occurrence de méga-séismes avec des magnitudes proches de 9 sont toujours matière à débat. L'étude de la segmentation de l'arc Himalayen est donc primordiale afin de comprendre les mécanismes qui contrôlent ces séismes ainsi que leur extension spatiale. La compréhension du cycle sismique en Himalaya est aujourd'hui essentiellement basée sur des études menées au Népal central, ce qui limite notre connaissance de son fonctionnement tri-dimensionnel. Ce travail de thèse permet d'apporter de nouvelles contraintes sur les variations latérales de la déformation crustale dans la zone comprise entre l'ouest Népal (~80°E) et l'est Bhoutan (~92°E). La première partie de cette thèse est consacrée à l'étude des variations latérales de la structure lithosphérique. Quatre campagnes gravimétriques ont été réalisées entre 2010 et 2012. Elles ont permis d'étendre le réseau gravimétrique Népalais jusqu'à l'ouest Népal et d'établir le premier réseau gravimétrique Bhoutanais. Ce nouveau jeu de données, combiné avec les données sismologiques disponibles, permet de contraindre des modèles thermo-mécaniques de la flexure de la plaque Indienne sous l'Himalaya. Les résultats suggèrent qu'il n'existe pas de variations latérales majeures du comportement mécanique de la lithosphère Indienne entre le centre et l'est Népal contrairement au Bhoutan où une rigidité flexurale plus faible est nécessaire pour expliquer les données.La deuxième partie de cette thèse est consacrée à l'étude des variations latérales des déformations récentes dans le prisme Himalayen. Les études paléosismologiques menées depuis 15 ans en Himalaya ont permis d'étendre le catalogue des événements majeurs sur le dernier millénaire. En intégrant à la fois les contraintes disponibles sur ces séismes majeurs et le chargement séculaire, nous étudions les variations spatiales et temporelles de l'état de contrainte du chevauchemlent Himalayen principal le long de l'arc. Nos calculs montrent que même dans la cas où un méga-séisme se serait produit en ~1100 en Himalaya, le chargement séculaire a quasiment compensé la chute de contrainte associée. Les modèles montrent aussi que le séisme du Shillong ne joue pas un rôle majeur sur les contraintes accumulées sur le chevauchement Himalayen depuis 1897 au niveau du Bhoutan. Enfin, nous présentons les premières contraintes sur la tectonique active au Bhoutan. L'étude morphotectonique réalisée au centre du Sud-Bhoutan a permis de montrer que la majeure partie de la déformation Holocène est accommodée au niveau du front Himalayen, comme au Népal. Nous montrons aussi des évidences de ruptures de surface avec des décalages verticaux de plusieurs mètres associés à deux séismes majeurs sur le dernier millénaire. Ces évidences remettent en cause l'interprétation d'un méga-séisme en ~1100.
https://tel.archives-ouvertes.fr/tel-01002381

NNT : 2013MON20074
tel-01002381
https://tel.archives-ouvertes.fr/tel-01002381
https://tel.archives-ouvertes.fr/tel-01002381/document
https://tel.archives-ouvertes.fr/tel-01002381/file/35970_BERTHET_2013_archivage_cor.pdf