Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The anisotropic single crystal seismic properties are reviewed in the light of recent experimental and theoretical determinations. Although considerable progress has been made on the determination of single crystal properties, data are still lacking, particularly for the temperature derivatives of transition zone and lower mantle phases. The common types of LPO of olivine, opx and cpx are presented together with their associated seismic properties. It is emphasized that simple seismic symmetry pattern of upper mantle rocks are a direct result of the interaction of olivine, opx and cpx. Using the standard structural frame (X lineation, Z pole to foliation) for typical LPOs of olivine, opx and cpx have the maximum Vp parallel to X, in the XZ plane and parallel to Y respectively. Destructive interference occurs between these minerals and hence P-wave anisotropy should be sensitive to the aggregate composition. For shear wave splitting (dVs) typical olivine and opx LPOs result in similar patterns with the maximum dVs in the YZ plane and the fast split shear wave (Vs1) polarized parallel to the foliation. A typical cpx LPO on the other hand produces destructive interference as the max dVs is close to X. By comparison with experiments and numerical simulations, it is estimated that upper mantle samples have an olivine LPO strength which recorded shear strain gamma of between 0.25 and 2.0. Pyrolite and piclogite models are compared with global transverse isotropic models. The slowly reducing P-wave anisotropy in the first 200 km can be explained by a model with constant composition and LPO strength. The sharp decrease in the observed anisotropy in the global models cannot be explained by the transformation of opx to cpx at 300 km, it is proposed that this decrease is due to a reduction in LPO strength from 200 to 350 km at the base of the lithosphere.
Earth's Deep Interior: Mineral Physics and Tomography From the Atomic Scale to the Global Scale

ISBN : 9780875909752
hal-01391524
http://hal.univ-reunion.fr/hal-01391524
http://hal.univ-reunion.fr/hal-01391524/document
http://hal.univ-reunion.fr/hal-01391524/file/Mainprice_AGUmonog_DM_GB_WBI_2000.pdf

Éditeur(s) : HAL CCSD Public Library of Science
Résumé : International audience
Objectives To investigate the association between priapism in men with sickle cell anemia (SCA) and hemorheological and hemolytical parameters. Materials and Methods Fifty-eight men with SCA (median age: 38 years) were included; 28 who had experienced priapism at least once during their life (priapism group) and 30 who never experienced this complication (control group). Twenty-two patients were treated with hydroxycarbamide, 11 in each group. All patients were at steady state at the time of inclusion. Hematological and biochemical parameters were obtained through routine procedures. The Laser-assisted Optical Rotational Cell Analyzer was used to measure red blood cell (RBC) deformability at 30 Pa (ektacytometry) and RBC aggregation properties (laser backscatter versus time). Blood viscosity was measured at a shear rate of 225 s(-1) using a cone/plate viscometer. A principal component analysis was performed on 4 hemolytic markers (i.e., lactate dehydrogenase (LDH), aspartate aminotransferase (ASAT), total bilirubin (BIL) levels and reticulocyte (RET) percentage) to calculate a hemolytic index. Results Compared to the control group, patients with priapism exhibited higher ASAT (p = 0.01), LDH (p = 0.03), RET (p = 0.03) levels and hemolytic indices (p = 0.02). Higher RBC aggregates strength (p = 0.01) and lower RBC deformability (p = 0.005) were observed in patients with priapism compared to controls. After removing the hydroxycarbamide-treated patients, RBC deformability (p = 0.01) and RBC aggregate strength (p = 0.03) were still different between the two groups, and patients with priapism exhibited significantly higher hemolytic indices (p = 0.01) than controls. Conclusion Our results confirm that priapism in SCA is associated with higher hemolytic rates and show for the first time that this complication is also associated with higher RBC aggregate strength and lower RBC deformability.
ISSN: 1932-6203

hal-01390971
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01390971
DOI : 10.1371/journal.pone.0154866
PUBMED : 27145183

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The nucleation, propagation, and associated folding of ramp-flat-ramp normal faults were analyzed from field examples developed in a brittle/ductile multilayer sequence of the Huércal-Overa Basin (SE Spain). Gently dipping sandy silt layers, which display a low cohesive strength (C0 = 7 kPa, μ = 34°), favor the development of extensional detachments. A tectonic origin instead of a possible gravitational origin is supported by the perpendicularity between the paleoslope direction of the fluvial-deltaic environment inferred from imbricated pebbles, and the senses of movement deduced from fault slicken-lines. The link between high-angle normal faults (HANFs) --formed at different levels in the layered sequence-- with horizontal fault segments comes to develop ramp-flat-ramp normal faults with associated roll-over in the hanging wall. Observed extensional duplexes are formed by parallel detachments connected through synthetic Riedel faults. These Riedel faults would produce the back-rotation of the individual blocks (horses), i.e., extensional folding of the originally subhorizontal layers. There is no correlation between the analyzed ramp-flat-ramp normal faults, accommodating south-southeastward extension during Serravallian-lower Tortonian, and either the regional Alpujarride/Nevado-Filabride west-directed extensional shear zone or the top-to-the-north detachments within Alpujarride units, which are clearly sealed by Serravallian-lower Tortonian sediments. Therefore, the studied normal faults are restricted to the brittle/ductile multilayer fluvio/deltaic sequence and accommodate moderate late extension instead of belonging to a large crustal extensional system connected with a regional detachment at depth. Therefore, the basin formed in a moderate crustal thickness context where small and medium-scale extensional systems were subordinate structures. These natural examples support the development of low-angle normal faults at very shallow crustal levels in multilayer sequences with suitable rheological conditions.
ISSN: 0278-7407

hal-00750352
https://hal.archives-ouvertes.fr/hal-00750352
DOI : 10.1029/2012TC003130

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We present Contact Dynamics discrete element simulations of the earthquake-triggered Jiufengershan avalanche, which mobilized a 60 m thick, 1.5 km long sedimentary layer, dipping similar to 22 degrees SE toward a valley. The dynamic behavior of the avalanche is simulated under different assumptions about rock behavior, water table height, and boundary shear strength. Additionally, seismic shaking is introduced using strong motion records from nearby stations. We assume that seismic shaking generates shearing and frictional heating along the surface of rupture, which, in turn, may induce dynamic weakening and avalanche triggering; a simple "slip-weakening'' criterion was adopted to simulate shear strength drop along the rupture surface. We investigate the mechanical processes occurring during triggering and propagation of an avalanche mobilizing shallowly dipping layers. Incipient deformation forms a pop-up structure at the toe of the dip slope. As the avalanche propagates, the pop-up deforms into an overturned fold, which overrides the surface of separation along a decollement. Simultaneously, uphill layers slide at high velocity (125 km/h) and are folded and disrupted as they reach the toe of the dip slope. The avalanche foot forms a wedge that is pushed forward as deformed rocks accrete at its rear. We simulated five cross sections across the Jiufengershan avalanche, which differ in the geometry of the surface of separation. Topographic and simulated surface profiles are similar. The friction coefficient at the surface of separation determined from back analysis is abnormally low (mu(SS) = 0.2), possibly due to lubrication by liquefied soils. The granular deposits of simulated earthquake- and rain-triggered avalanches are similar.
ISSN: 0148-0227

hal-00420882
https://hal.archives-ouvertes.fr/hal-00420882
DOI : 10.1029/2008JF001075

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Quartz is a common crustal mineral that deforms plastically in a wide range of temperatures and pressures, leading to the development of different types of crystallographic preferred orientation (CPO) patterns. In this contribution we present the results of an extensive modeling of quartz fabric transitions via a viscoplastic self-consistent (VPSC) approach. For that, we have performed systematic simulations using different sets of relative critical resolved shear stress of the main quartz slip systems. We have performed these simulations in axial compression and simple shear regimes under constant Von Mises equivalent strain of 100% (γ = 1.73), assuming that the aggregates deformed exclusively by dislocation glide. Some of the predicted CPOs patterns are similar to those observed in naturally and experimentally deformed quartz. Nevertheless, some classical CPO patterns usually interpreted as result from dislocation glide (e.g. Y-maxima due to prism < a > slip) are clearly not developed in the simulated conditions. In addition we reported new potential preferred orientation patterns that might happen in high temperature conditions, both in axial compression and simple shear. We have demonstrated that CPOs generated under axial compression are usually stronger that those predicted under simple shear, due to the continuous rotation observed in the later simulations. The fabric strength depends essentially on the dominant active slip system, and normally the stronger CPOs result from dominant basal slip in < a >, followed by rhomb < a > and prism [c] slip, whereas prism < a > slip does not produce strong fabrics. The opening angle of quartz [0001] fabric used as a proxy of temperature seems to be reliable for deformation temperatures of ~ 400 °C, when the main slip systems have similar behaviors.
ISSN: 0040-1951

hal-01115674
https://hal.archives-ouvertes.fr/hal-01115674
DOI : 10.1016/j.tecto.2014.08.011

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Fractures or discontinuities perpendicular to the least stress σ3 were generated in a synthetic rock analogue (granular, frictional, cohesive, and dilatant) material in axisymmetric extension tests. These fractures are of two types defined by the mean stress σ. When σ is very small, the fractures form through the mode I cracking at tensile σ3 equal to the material tensile strength. The fracture walls have smooth surfaces. At higher σ, these surfaces become rougher, with the topography features forming faint or delicate plumose patterns. The amplitude of the plumose topography increases with σ. The axial stress σ3 at fracturing reduces in magnitude and changes in sign with the σ increase. Thus σ3 orthogonal discontinuities can form at compressive σ3. SEM observations show that these discontinuities are deformation localization bands where the material is characterized by the heterogeneous decohesion and volume and porosity increase due to dilatancy. The band thickness is several grain sizes. At formation, the bands are not opened, so they are not mode I fractures. They become fractures with plumose fractography after the separation of the sample parts along the band. The formation mechanism of these discontinuities or fractures is not completely clear, but it is suggested that it represents a running constitutive instability in the form of dilatancy banding (with further σ increase the bands become inclined to σ1, i.e., shear). The similarity between the experimentally generated plumose surface fractures and natural joints is discussed, and it is suggested that they can be formed as propagating constitutive instabilities.
ISSN: 0148-0227

hal-00585846
https://hal.archives-ouvertes.fr/hal-00585846
DOI : 10.1029/2010JB008104

Éditeur(s) : HAL CCSD
Résumé : A section of ancient fast-spreading ocean crust was sampled at Wadi Gideah, located in the Wadi Tayin Massif of the Oman ophiolite. This sample suite was used for generating a coherent data set combining various petro-geochemical and structural investigations, with the aim of building a reference section for advancing our understanding of crustal accretion processes at fast-spreading mid-ocean ridges. Additionally, this study focuses on one outcrop that represents part of the fossil axial melt lens (AML).Major and trace element analyses reveal a trend of chemical evolution up section in the lower crust, with a marked difference between the layered gabbros and the foliated gabbros above. Petrological modeling shows that the chemical evolution up section can be produced by hydrous fractionation crystallization, with melt H2O contents of ~0.8 wt% and 0.8 to 1.2 wt% for the lower and upper crust, respectively. Together with the observed very steep bulk Zr/Hf vs. Zr gradient, high F and Cl content of magmatic amphibole, general Nb-Ta depletion of melts compared with NMORB, and high Sr87/Sr86 ratio compared with modern fast-spread crust, this suggests that Wadi Gideah layered gabbros were accreted by crystallization of ascending melts in sills, in a context of subduction initiation. The distinct changes in chemical trends and average grain size at the layered to foliated gabbro transition are tentatively explained by enhanced hydrothermal cooling deeper than the AML. The gabbro/dike transition displays complex structural and lithological relationships that, together with fractional crystallization modeling, point to episodic vertical movements of the AML. Plagioclase crystallographic preferred orientations (CPO) measured over the whole gabbro section are always consistent with magmatic flow. The CPO strength generally increases downward, with more scattering in the layered gabbro section. From the top to the bottom of the layered gabbros, CPO become progressively more prolate, possibly reflecting increasing shear induced by active mantle flow underneath.Wadi Gideah lower crust recorded a formation history consistent with a hybrid accretion model, combing several processes such as downward magmatic flow and/or upward melt migration in the upper foliated gabbros, sill intrusions in the layered gabbros, and deep hydrothermal circulation. The latter was presumably focused in channels, preserved today as several, up to 100 m wide zones of extensively altered former layered gabbro, cross-cutting the magmatic layering. These metagabbros display significantly higher Sr87/Sr86 ratio, late stage magmatic phases, and evidence for high temperature partial melting.
Une coupe d'ancienne croute océanique rapide a été échantillonnée dans le Wadi Gideah, situé dans le Massif de Wadi Tayin de l'Ophiolite d'Oman. Cette série d'échantillons a permis de générer un jeu de données cohérent, combinant des études pétro-géochimiques et structurales, dans le but de construire une coupe de référence pour mieux contraindre notre compréhension des processus d'accrétion crustale aux dorsales océaniques rapides. Ce travail intègre aussi l'étude détaillée d'un affleurement représentant en partie la lentille magmatique axiale (LMA) fossile.Les analyses d'éléments majeurs et traces révèlent une évolution chimique dans la croûte inférieure, avec une nette différence entre les gabbros lités et les gabbros foliés sus-jacents. La modélisation pétrologique montre que l'évolution chimique vers le haut peut être produite par cristallisation fractionnée hydratée, avec des teneurs en H2O des magmas de ~0.8 wt% dans la croute inférieure et 0.8 to 1.2 wt% dans la croute supérieure. Combiné aux forts gradients de Zr/Hf vs. Zr, aux teneurs élevées de F et Cl dans les amphiboles magmatiques, à l'appauvrissement général en Nb-Ta des magmas comparé aux NMORB, et aux rapports Sr87/Sr86 élevés comparés à la croûte rapide moderne, ceci suggère que les gabbros lités du Wadi Gideah ont été formés par la cristallisation des magmas ascendants dans des sills, dans un contexte d'initiation de subduction. Les changements de signature géochimique et de taille de grain à la transition entre gabbros lités et gabbros foliés témoignent possiblement d'un fort refroidissement hydrothermal plus profond que la LMA. La transition complexe filonien/gabbro montre des relations structurale et lithologiques complexes, qui, combinées o la modélisation de la cristallisation fractionnée, traduisent les mouvements verticaux épisodiques de la LMA. Les orientations préférentielles cristallographiques (OPC) des plagioclases mesurées sur l'ensemble de la coupe dans les gabbros témoignent toujours d'une déformation magmatique. L'intensité des OPC a tendance à augmenter vers le bas, avec une plus forte variabilité dans les gabbros lités. Dans les gabbros lités, les OPC sont de plus en plus linéaires vers le bas, résultant possiblement d'un cisaillement plus fort induit par l'écoulement actif du manteau sous-jacent.La croûte inférieure dans le Wadi Gideah a enregistré une histoire compatible avec un modèle d'accrétion hydride, combinant plusieurs processus tels que l'écoulement magmatique vers le bas et/ou la migration de liquides magmatiques vers le haut dans les gabbros foliés supérieurs, l'intrusion de sills dans les gabbros lités, et une circulation hydrothermale profonde. Cette dernière était probablement concentrée dans des chenaux, préservés aujourd'hui sous la forme de plusieurs zones, dont l'épaisseur atteint 100 m, de gabbros lités fortement altérés, recoupant le litage magmatique. Ces métagabbros ont des rapports Sr87/Sr86 élevés, comportent des phases magmatiques tardives et montrent des évidences de fusion partielle.
https://hal.archives-ouvertes.fr/tel-01302675

tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675
https://hal.archives-ouvertes.fr/tel-01302675/document
https://hal.archives-ouvertes.fr/tel-01302675/file/Th%C3%A8se_Tim-Mueller_2015.pdf

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : Dehydration reactions result in minerals’ replacement and a transient fluid-filled porosity. These reactions involve interface-coupled dissolution–precipitation and might therefore lead to fixed crystallographic orientation relations between reactant (protolith) and product phases (i.e. topotaxy). We investigate these two phenomena in the dehydration of a foliated antigorite (atg) serpentinite by comparing the crystallographic preferred orientation (CPO) developed by olivine (ol), orthopyroxene (opx) and chlorite (chl) during high-pressure antigorite and chlorite dehydration in piston-cylinder experiments and in natural samples recording the dehydration of antigorite (Cerro del Almirez, Betic Cordillera, Spain). Experiments were performed under undrained conditions resulting in fluid-filled porosity and in strong CPO of the prograde minerals, controlled by the pre-existing antigorite CPO in the reactant foliated serpentinite. The orientation of a ol,opx and c∗chl is parallel to c∗atg from the protolith. The Cerro del Almirez samples show similar, locally well-developed topotactic relations between orthopyroxene, chlorite and antigorite, but the product CPOs are weaker and more complex at the thin section scale. In contrast to the experiments, olivine from natural samples shows a weak correlation between b ol and the former c∗atg. We relate the strengthening of local topotactic relations and the weakening of the inherited CPO at a larger scale in natural samples to compaction and associated fluid migration. Microstructural features that might be related to compaction in the natural samples include: (1) smooth bending of the former foliation, (2) gradual crystallographic misorientation (up to 16°) of prismatic orthopyroxene due to buckling by dislocation creep, (3) inversion of enstatite to low clinoenstatite (P21/c) along lamellae and (4) brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite. The coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction. An lower bound for the strain rates in the order of 10−12 to 10−13 s−1 and shear stresses of 60–70 MPa are estimated based on creep data. Lower shear stresses (20–40 MPa) are retrieved using a theoretical approach. These data point to slow compaction (and fluid extraction) in nature if the system is not perturbed by external forces, with rates only marginally higher than the viscoplastic deformation of the solid matrix.
ISSN: 0010-7999

hal-01174158
https://hal.archives-ouvertes.fr/hal-01174158
DOI : 10.1007/s00410-015-1129-4

Éditeur(s) : HAL CCSD American Physical Society
Résumé : International audience
Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.
ISSN: 1539-3755

hal-00594891
https://hal.archives-ouvertes.fr/hal-00594891
https://hal.archives-ouvertes.fr/hal-00594891/document
https://hal.archives-ouvertes.fr/hal-00594891/file/Estrada2010ReSubmitV2.pdf
ARXIV : 1105.4418

Éditeur(s) : HAL CCSD Elsevier
Résumé : The depth of the oceanic lithosphere–asthenosphere boundary (LAB), as inferred from shear wave velocities, increases with lithospheric age, in agreement with models of cooling oceanic lithosphere. On the other hand, the distribution of radial anisotropy under oceanic plates is almost age-independent. In particular, radial anisotropy shows a maximum positive gradient at a depth of ∼70 km, which, if used as a proxy, indicates an age-independent LAB depth. These contrasting observations have fueled a controversy on the seismological signature of the LAB. To better understand the discrepancy between these observations, we model the development of lattice preferred orientation (LPO) in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate-driven mid-ocean ridge flows. The model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms. We find that an age-independent distribution of radial anisotropy is the natural consequence of these simple flows. The depth and strength of anisotropy is further controlled by the deformation regime – dislocation or diffusion creep – experienced by crystals during their ascent towards, and subsequent motion away from, the ridge axis. Comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume. Although not excluded, additional mechanisms proposed to explain some geophysical signatures of the LAB, such as the presence of partial melt or changes in water content, are not required to explain the radial anisotropy proxy. Our prediction, that the age-independent radial anisotropy proxy marks the transition to flow-induced asthenospheric anisotropy, provides a way to reconcile thermal, mechanical and seismological views of the LAB.
ISSN: 0012-821X

hal-01553243
https://hal.archives-ouvertes.fr/hal-01553243
DOI : 10.1016/j.epsl.2016.12.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study reports petrostructural observations in the pargasite and phlogopite-bearing Finero peridotite massif (Italian Western Alps), which suggest that the pervasive foliation in this massif was formed by deformation concomitant with percolation of hydrous Si-rich melts: (1) diffuse contacts, but systematic parallelism between the pyroxenitic layers and the foliation of the peridotite (2) strong shape and crystal preferred orientations (SPO and CPO), but subhedral or interstitial shapes and weak intracrystalline deformation of the hydrous phases, (3) CPO, but interstitial shapes of the pyroxenes, (4) very coarse olivine grain sizes, which are correlated to the olivine abundance, and (5) elongated shapes, but weak intracrystalline deformation, and extremely weak and highly variable CPO of olivine. The pervasive deformation of the Finero peridotite occurred therefore under conditions that allowed coexistence of H2O–CO2-bearing melts, pargasite, and spinel, that is, temperatures of 980–1080 °C and pressures <2 GPa. The petrostructural observations suggest that the presence of hydrous melts results in accommodation of large amounts of deformation by stress-controlled dissolution–precipitation and advective transport of matter by the melts and in fast grain boundary migration in olivine. By consequence, it produces significant rheological weakening. Water contents in olivine are <4 ppm wt., implying limited contribution of hydration of olivine to weakening. In addition, the analysis of protomylonites composing the external domains of the shear zones that overprint the pervasive foliation indicates that the transition to melt-free conditions results in enhanced contribution of dislocation creep to the deformation. The associated increase of the peridotites' strength leads to onset of strain localization. The latter is not correlated to the local abundance in pargasite or phlogopite, implying that crystallization of amphiboles or phlogopite, even at concentrations of 25 vol.%, does not produce rheological weakening in the upper mantle.
ISSN: 0012-821X

hal-01667879
https://hal.archives-ouvertes.fr/hal-01667879
DOI : 10.1016/j.epsl.2017.08.015

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Deep crustal and mantle rocks are exhumed in core complex mode of extension in three types of structures: metamorphic core complexes, oceanic core complexes and magma poor passive margins. Using available analogue and numerical models and their comparison with natural examples, the present paper reviews the mechanical processes involved in these different types of extensional setting. Three main aspects are considered: i) the primary role of lithosphere rheology, ii) the lithosphere-scale patterns of progressive deformation that lead to the exhumation of deep metamorphic or mantle rocks and iii) the initiation and development of detachment zones. Crustal core complexes develop in continental lithospheres whose Moho temperature is higher than 750 °C with “upper crust-dominated” strength profiles. Contrary to what is commonly believed, it is argued from analogue and numerical models that detachments that accommodate exhumation of core complexes do not initiate at the onset of extension but in the course of progressive extension when the exhuming ductile crust reaches the surface. In models, convex upward detachments result from a rolling hinge process. Mantle core complexes develop in either the oceanic lithosphere, at slow and ultra-slow spreading ridges, or in continental lithospheres, whose initial Moho temperature is lower than 750 °C, with “sub-Moho mantle-dominated” strength profiles. It is argued that the mechanism of mantle exhumation at passive margins is a nearly symmetrical necking process at lithosphere scale without major and permanent detachment, except if strong strain localization could occur in the lithosphere mantle. Distributed crustal extension, by upper crust faulting above a décollement along the ductile crust increases toward the rift axis up to crustal breakup. Mantle rocks exhume in the zone of crustal breakup accommodated by conjugate mantle shear zones that migrate with the rift axis, during increasing extension.
ISSN: 0040-1951

insu-01610186
https://hal-insu.archives-ouvertes.fr/insu-01610186
https://hal-insu.archives-ouvertes.fr/insu-01610186/document
https://hal-insu.archives-ouvertes.fr/insu-01610186/file/brun-tect-2017.pdf
DOI : 10.1016/j.tecto.2017.09.017

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We conducted triaxial compression experiments to investigate the effects of pressure, temperature and strain rate on the rheology of talc rocks. The tests were carried out at T from 25 to 860 degrees C, P from 0.1 to 300 MPa, and <(epsilon) over dot> from 0.3 to 30 x 10-5 s(-1). In addition to being very weak, there are other important differences between the mechanical behavior of talc and that of most other silicates. Deformation at all temperatures up to dehydration was accommodated by a combination of crystal plasticity, frictional sliding, and cataclasis. A transition from localized to distributed deformation occurred at P = 300 MPa and T = 400 degrees C, but this transition is ill-defined as both distributed and localized deformation were observed at P = 300 MPa and T = 600 degrees C. Unlike most silicates, both the coefficient of internal friction and the coefficient of (sliding) friction are very low and nearly equal to each other. Temperature enhances plastic deformation (kinking), and inter- and intra-granular microcracks are observed parallel to the (001) planes at all conditions. Voids are created by delamination along (001) planes at kink-band boundaries. Full crystal plasticity was not achieved under any of the conditions tested, i.e., the von Mises criterion was never satisfied. However, the strength of the aggregates remained much less than the confining pressure. These unusual properties are likely a manifestation of the pronounced mechanical anisotropy of talc at the grain scale. At T >= 750 degrees C dehydration is observed, but only along shear zones. These results suggest that feedbacks between deformation and reaction kinetics could control fluid flow in fault zones. The presence of talc can promote significant weakening and strain localization in the oceanic lithosphere, the subducting plate and the overlying mantle wedge. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00492706
https://hal.archives-ouvertes.fr/hal-00492706
DOI : 10.1016/j.epsl.2008.02.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We report preliminary results of a seismological experiment, SEISMOTANZ'07, which consisted in the deployment of a local network (35 stations) in the East African Rift System (EARS), North Tanzania, during 6 months in 2007. We compare two earthquake sequences (Gelai and Manyara) occurring respectively in the southern end of the Kenya rift and in the North Tanzanian Divergence (NTD). Only distant of 150 km, their triggering mechanisms are different. None of the sequences depicts typical swarm or mainshock-aftershock patterns. They highlight the change in the magmatic/tectonic nature of the rift where the eastern branch of the EARS enters the Tanzanian craton. The similar shape and long-axis of the elongate sequences emphasize the preferred locus of active strain release along NE-SW discontinuities which probably root at depth into steep Proterozoic shear zones. At Gelai, the deformation is dominated by aseismic process involving slow slip on normal fault and dyke intrusion within the upper crust Calais_et_al_2008. The spatial and temporal earthquake distribution indicates a possible correlation between the Gelai crisis and the eruption of the nearby Oldoinyo Lengai volcano. At Manyara, the sequence is more uncommon, revealing a long-lasting seismic activity deepely rooted (20-35 km depth) possibly related to stress loading transmitted laterally. The yield stress envelope modelled from the depth frequency distribution of earthquakes in the NTD is consistent with the presence of a mafic lower crust and further supports the strength increase of the rifted crust from south Kenya to the NTD.
ISSN: 0899-5362

hal-00420599
https://hal.archives-ouvertes.fr/hal-00420599
DOI : 10.1016/j.jafrearsci.2009.09.005