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 Elsevier
Résumé : International audience
The formation age of the Siberian cratonic mantle is not well established as yet. Re–Os data on various mantle-derived materials have shown that it contains Archaean components, but the reported Re-depletion ages show a broad variation range (3.4 to 1 Ga) and are commonly ≤ 2 Ga for peridotite xenoliths. We report Hf and Nd isotope data for cpx and garnet separated from nine refractory spinel and garnet peridotite xenoliths from the Udachnaya kimberlite. The cpx from low-opx spinel harzburgites show extremely high εHf values, from + 607 to + 2084, which testify to long-term evolution of these rocks with high Lu/Hf ratios, consistent with their residual origin and near absence of post-melting enrichments in the Lu–Hf system. Such high εHf values are unusual for cpx from other cratonic peridotites and are higher than those reported for depleted cpx in off-cratonic peridotites. The clinopyroxenes from low-opx spinel harzburgites yield Hf model ages from 1.9 to 1.7 Ga while the cpx from high-opx spinel harzburgites yield Hf model ages from 3.0 to 1.9 Ga. When plotted together, they define a Lu/Hf isochron with an age of 1.80 Ga, which we consider as a robust estimate of the formation age (melt extraction event) because it is obtained on residual rocks that show no evidence for HREE and Hf enrichments and because the model ages for three out of four individual samples are similar to each other. The cpx have high εNd of + 94 to + 123, which are inconsistent with their low Sm/NdPM of < 1 and yield no meaningful age estimates. The consistently high, positive εNd in these cpxs can be interpreted in terms of long-term evolution of refractory peridotites with high Sm/Nd, followed by relatively recent LREE enrichments.We infer that a significant part of the lithospheric mantle in the central Siberian craton may have been formed during a major event (or a series of events) at around 1.8 Ga. Older ages reported for the central Siberian craton may refer to less common materials from cratonic or other domains formed in the Archaean that were later incorporated into the cratonic roots. The transition from the “Archaean” to “modern” tectonic regimes in Siberia and possibly elsewhere may have taken place at 1.8–1.9 Ga rather than at ~ 2.5 Ga, i.e. in the second half of the Paleoproterozoic rather than at the Archaean–Proterozoic boundary, at which time the asthenospheric mantle became generally too cold to experience high-degree melting on a large scale. The ~ 1.8 Ga formation age of the Siberian cratonic mantle is coeval with that for a major part of the ancient continental crust in the central Siberian craton. The temporal crust–mantle links may be explained either by the generation of the initial source materials for continental crust in the same melting event that formed the residual peridotites or, alternatively, by subduction and melting of pre-existing proto-lithosphere destabilized by a major mantle upwelling that formed the residual mantle.
ISSN: 0009-2541

hal-01115519
https://hal.archives-ouvertes.fr/hal-01115519
DOI : 10.1016/j.chemgeo.2014.10.018

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500 m thick highly strained metamorphic rock units found beneath myloniticbanded peridotites at the base of large-scale ophiolites, as exemplified in Oman. Metamorphic soles aremainly composed of metabasalts deriving from the downgoing oceanic lithosphere and metamorphosed up togranulite-facies conditions by heat transfer from the mantle wedge. Pressure–temperature peak conditions areusually estimated at 1.00.2 GPa and 800100ºC. The absence of HP–LT metamorphism overprint implies thatmetamorphic soles have been formed and exhumed during subduction infancy. In this view, metamorphic soleswere strongly deformed during their accretion to the mantle wedge (corresponding, now, to the base of the ophiolite).Therefore, metamorphic soles and banded peridotites are direct witnesses of the dynamics of early subductionzones, in terms of thermal structure, fluid migration and rheology evolution across the nascent slab interface.Based on fieldwork and EBSD analyses, we present a detailed (micro–) structural study performed on samplescoming from the Sumeini window, the better-preserved cross-section of the metamorphic sole of Oman. Largedifferences are found in the deformation (CPO, grain size, aspect ratio) of clinopyroxene, amphibole and plagioclase,related to mineralogical changes linked with the distance to the peridotite contact (e.g., hardening due to theappearance of garnet and clinopyroxene).To model the incipient slab interface in laboratory, we carried out 5 hydrostatic annealing and simple-shear experimentson Griggs solid-medium apparatus. Deformation experiments were conducted at axial strain rates of 10-6s-1. Fine-grained amphibolite was synthetized by adding 1 wt.% water to a (Mid-Ocean Ridge) basalt powder asa proxy for the metamorphic sole (amphibole + plagioclase + clinopyroxene garnet assemblage). To synthetizegarnet, 2 experiments were carried out in hydrostatic conditions and with deformation at 800C with confiningpressure of 2 GPa. Another simple-shear experiment has been carried out at 800C and 1 GPa with fined-grainednatural garnet. With the aim of mimicking the early slab interface (between the metamorphic sole and bandedperidotites at the base of the ophiolite), 2 simple-shear deformation experiments with 2 layers have been carriedout at 800C and confining pressure of 1 GPa. The bottom layer was made of hydrated basalt powder and the toplayer was made of olivine.Fined-grained garnet-free amphibolite is significantly weaker than dunite but the appearance of harder mineralsin the amphibolite (garnet and clinopyroxene) has major implications on its rheological evolution. These resultsallow liking field observations of strain localization at the interface to the metamorphic sole formation
European Geosciences Union General Assembly 2016
Vienne, Austria

insu-01309394
https://hal-insu.archives-ouvertes.fr/insu-01309394

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
Large earthquakes nucleate at tectonic plate boundaries, and their occurrence within a plate's interior remains rare and poorly documented, especially offshore. The two large earthquakes that struck the northeastern Indian Ocean on 11 April 2012 are an exception: they are the largest strike-slip events reported in historical times1, 2 and triggered large aftershocks worldwide3. Yet they occurred within an intra-oceanic setting along the fossil fabric of the extinct Wharton basin, rather than on a discrete plate boundary4, 5, 6, 7, 8. Here we show that the 11 April 2012 twin earthquakes are part of a continuing boost of the intraplate deformation between India and Australia that followed the Aceh 2004 and Nias 2005 megathrust earthquakes, subsequent to a stress transfer process recognized at other subduction zones9, 10. Using Coulomb stress change calculations, we show that the coseismic slips of the Aceh and Nias earthquakes can promote oceanic left-lateral strike-slip earthquakes on pre-existing meridian-aligned fault planes. We further show that persistent viscous relaxation in the asthenospheric mantle several years after the Aceh megathrust explains the time lag between the 2004 megathrust and the 2012 intraplate events. On a short timescale, the 2012 events provide new evidence for the interplay between megathrusts at the subduction interface and intraplate deformation offshore. On a longer geological timescale, the Australian plate, driven by slab-pull forces at the Sunda trench, is detaching from the Indian plate, which is subjected to resisting forces at the Himalayan front6, 8, 11.
ISSN: 0028-0836

hal-00767569
https://hal.archives-ouvertes.fr/hal-00767569
DOI : 10.1038/nature11520

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present evidence that GPS velocity estimates of plate motions and fault slip rates agree to within uncertainties with geologic estimates during the most recent phase of the geologic evolution of the E Mediterranean region (post-Late Miocene). On this basis, we use the GPS differential velocities to estimate the timing of initiation of the principal structures in NW Turkey, the N Aegean Sea, and central Greece, including, the Marmara Sea, the Gulfs of Evia (GoE) and Corinth (GoC). and the Kephalonia Transform fault (KTF). We interpret these ages to indicate that the North Anatolian fault propagated across the N Aegean, opening the GoE and GoC and initiating the KTF, during the past 1-4 Ma. We further suggest that Aegean extension that was earlier more distributed across the Aegean Basin became focused on this new fault system allowing the southern Aegean and Peloponnisos to translate SW with little internal deformation, as observed today with GPS. This change in tectonic configuration may account for the clear geologic evidence for crustal thinning throughout the S Aegean in apparent contradiction with low present-day strain rates. We further show that the low present-day strain rate along the southern edge of the Aegean micro-plate requires substantial aseismic slip along the plate interface below Crete, consistent with the low level of historic, subduction-type earthquakes along this segment of the subduction zone.
ISSN: 0040-1951

hal-00512634
https://hal.archives-ouvertes.fr/hal-00512634
DOI : 10.1016/j.tecto.2009.05.027

Éditeur(s) : HAL CCSD
Résumé : National audience
The Lesser Antilles subduction zone has produced no recent strong thrust earthquakes, making it difficult to quantify the seismic hazard from such events. The Lesser Antilles arc has a low subduction rate and an accretionary wedge that is very wide at its southern end. To investigate the effect of the wedge on seismogenesis, numerical models of forearc thermal structure were constructed along six transects perpendicular to the arc in order to determine the thermally predicted width of the seismogenic zone. The geometry of each section is constrained by published seismic profiles and crustal models derived from gravity and seismic data and by earthquake hypocenters at depth. A major constraint on the deep part of the model is that mantle temperature beneath the volcanic arc should achieve a temperature of 1,100°C to generate partial melts. Predicted surface heat flow is compared to the available heat flow observations. Thermal modeling results indicate a systematic southward increase in the width of the seismogenic zone, more than doubling in width from north to south and corresponding to a dramatic southward increase in forearc width (distance from the arc to the deformation front of the accretionary wedge). The minimum width of the seismogenic zone (distance between the intersections of the subduction interface with the 150°C and 350°C isotherms) increases from about 80 km, north of 16°N, to 230 km, at 13°N. The maximum width (between the 100°C and 450°C isotherms) ranges from about 150 km in the north to up to 320 km in the south. This large variation in the width of the seismogenic zone is a consequence of the increasing width of the accretionary wedge to the south, caused by the increased thickness of sediment on the subducting plate. There is good agreement between the thermally predicted seismogenic limits and the sparse distribution of recorded thrust earthquakes, which are observed only in the northern portion of the arc. Possible scenarios for mega-thrust earthquakes are discussed. Depending on the segment length (along-strike) of the rupture plane, the occurrence of an event of magnitude 8-9 cannot be excluded.
Bulletin Société Géologique de France

insu-00814056
https://hal-insu.archives-ouvertes.fr/insu-00814056
DOI : 10.2113/gssgfbull.184.1-2.47

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : In this study, we investigate the Lesser Antilles forearc basin, focusing on the late Pliocene to Pleistocene sedimentary archives in order to track the occurrence of extreme events triggered by enhanced subduction-related tectono-volcanic activity. We identify late Piacenzian deposits covering a major regional erosional surface, displaying sedimentary dykes and large marine boulders embedded in a mixed continental–marine matrix, characteristic of tsunamites. We interpret this episode of platform emersion and the successive cataclysmic deposits as resulting from enhanced tectonic activity at the interface of the subduction zone, synchronous with the initiation of the Lesser Antilles volcanic arc. We then discuss the implications in terms of the mechanical behaviour of the Lesser Antilles subduction zone.
ISSN: 0954-4879

hal-01571956
https://hal.archives-ouvertes.fr/hal-01571956
DOI : 10.1111/ter.12271

Éditeur(s) : HAL CCSD The Geochemical Society of Japan
Résumé : International audience
The Western Cordillera of Ecuador consists of Cretaceous crustal fragments of oceanic plateaux and superimposed insular arcs, which were accreted to the northwestern South American margin during the Late Cretaceous and Paleocene. Slices of high-grade metabasites, ultramafic rocks, gabbros and basalts, unmetamorphosed radiolarian cherts and scarce garnet-bearing metasediments were randomly exhumed along Miocene to Recent transcurrent faults crosscutting the Western Cordillera. The basalts show geochemical characteristics of oceanic plateau basalts (flat REE patterns, La/Nb = 0.85). The gabbros differ from the basalts in having lower REE levels, positive Eu anomalies, and negative Nb and Ta anomalies; they are interpreted as resulting from arc magmatism. The amphibolites and banded amphibolites have major and trace element chemistry similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or to cumulate gabbros. The granulite shares with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and εNdi values (+7.6). Continent-derived metasediments are depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly. Foliated lherzolites, melagabbronorites and pyroxenites consist of serpentinized olivine + cpx + opx ± Ca-plagioclase. Lherzolites, melagabbronorites and pyroxenites are LREE depleted with positive Eu anomalies, while the harzburgite displays a U-shaped REE pattern. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chondritic and primitive mantle values). The ultramafic rocks represent fragments of depleted mantle, deformed cpx-rich cumulate, and continental lithospheric mantle or mantle contaminated by subduction-fluid. Except the scarce quartz-rich metasediments, all these rocks likely represent remnants of accreted oceanic crustal fragments and associated depleted mantle. Since these samples were randomly sampled at depth by the fault, we propose that the Western Cordillera and its crustal root are mainly of oceanic nature.
ISSN: 0016-7002

insu-00679228
https://hal-insu.archives-ouvertes.fr/insu-00679228

Éditeur(s) : HAL CCSD
Résumé : We present high-resolution bulk sedimentary δ 15N data from the southern edge of the present-day oxygen minimum zone of the eastern South Pacific. The record is interpreted as representing changes in water column nitrogen removal during the last 70,000 years. We found significant fluctuations in the isotopic signal that suggest major reorganizations of the oxygen minimum zone at millennial timescales. These fluctuations were not related to other millennial-scale changes like the Northern Hemisphere's Dansgaard-Oeschger climate swings or local changes in primary productivity, so appear to be dictated by the Southern Hemisphere's climate rhythm. This is preliminarily corroborated by an overall agreement between our δ 15N data and the sedimentary proxy of ice sheet dynamics in Patagonia, which is in turn correlated with surface water properties at the midlatitude subduction region of the eastern South Pacific intermediate waters. Finally, potential implications on late Quaternary changes in atmospheric CO2 concentrations are discussed.
Paleoceanography / PALEOECEANOGRAPHY

hal-00407260
https://hal.archives-ouvertes.fr/hal-00407260
DOI : 10.1029/2006PA001308

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500m thick tectonic slices welded beneath most of the large-scale ophiolites.They typically show a steep inverted metamorphic structure where the pressure and temperature conditions ofcrystallization increase upward (from 500100ºC at 0.50.2 GPa to 800100ºC at 1.00.2 GPa), with isogradssubparallel to the contact with the overlying ophiolitic peridotite. The proportion of mafic rocks in metamorphicsoles also increases from the bottom (meta-sediments rich) to the top (approaching the ophiolite peridotites). Thesesoles are interpreted as the result of heat transfer from the incipient mantle wedge toward the nascent slab (associatedwith large-scale fluid transfer and possible shear heating) during the first My of intra-oceanic subduction(as indicated by radiometric ages). Metamorphic soles provide therefore major constraints on early subductiondynamics (i.e. thermal structure, fluid migration and rheology along the nascent slab interface).We present a detailed structural and petrological study of the metamorphic sole from 4 major cross-sections alongthe Oman ophiolite. We show precise pressure–temperature estimates obtained by pseudosection modelling andEBSD measurements performed on both the garnet-bearing and garnet-free high-grade sole. Results allow quantificationof the micro-scale deformation and highlight differences in pressure–temperature–deformation conditionsbetween the 4 different locations, showing that the inverted metamorphic gradient through the sole is not continuousin all locations.Based on these new constraints, we suggest a new tectonic–petrological model for the formation of metamorphicsoles below ophiolites. This model involves the stacking of several homogeneous slivers of oceanic crust leadingto the present-day structure of the sole. In this view, these thrusts are the result of rheological contrasts between thesole and the peridotite as the plate interface progressively cools down. These slivers later underwent several stagesof retrogression (partly mediated by ascending fluids from the slab) from amphibolite- to prehnite/pumpellite-faciesconditions.
Geophysical Research Abstracts
Vienne, Austria

insu-01309369
https://hal-insu.archives-ouvertes.fr/insu-01309369

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
This paper investigates the orientation and sources of stress in the forearc of the Cascadia subduction zone in southwest British Columbia, using Bayesian inversion results from focal mechanism data and comparing results with GPS derived short-term strain rates. The subduction margin in this region includes a change in orientation from N-S in Washington State to NW-SE in British Columbia. Over 1000 focal mechanisms from North American crustal earthquakes have been calculated to identify the dominant style of faulting, and ∼600 were inverted to estimate the principal stress orientations and the stress ratio. Our results indicate the maximum horizontal compressive stress orientation changes with distance to the trench, from approximately margin-normal along the coast to approximately margin-parallel 100-150 km inland from the coast. Comparing stress orientations with GPS data, we relate the margin-normal stress direction to subduction-related strain rates due to the locked interface between the North American and Juan de Fuca plates just west of Vancouver Island. Further from the margin the plates are coupled less strongly, and the margin-parallel maximum horizontal compressive stress in the North American Plate relates to the northward push of the Oregon Block, which is also observed in the horizontal shortening direction of the residual strain rates, after the subduction component is removed.
ISSN: 0148-0227

hal-00853734
https://hal.archives-ouvertes.fr/hal-00853734
DOI : 10.1029/2010JB008003

Éditeur(s) : HAL CCSD
Résumé : International audience
Two opposing visions of the Aegean backarc tectonics implicitly contain contrasting images of the rheological behaviour of the lihosphere, supported by different sets of observations. The propagation of the NAF and extension in the Corinth Rift suggest a strongly localising rheology whereas the formation of MCC in the Cyclades and the Rhodope suggests instead a more viscous behaviour. This paradox was the seminal question addressed by the ANR-EGEO programme (2007-2010) that leads to the following conclusions: (1) The exhumation of the Cycladic MCC's is accommodated by the N-dipping North Cycladic Detachment System (NCDS) that partly reworks the Vardar suture and at the base by a series of thrusts, including the basal contact of the Cycladic Blueschists (CBS) over the Cycladic Basement (CB). The activity of LANF is due to the reactivation of pre-existing discontinuities such as thrusts or earlier detachments and much less to the interaction with granitic plutons. (2) Exhumation in the Cyclades has proceeded in two stages: (a) Eocene syn-orogenic exhumation within the subduction lower/upperplate interface while the post-orogenic Rhodope MCC formed further north, (b) Oligo-Miocene post-orogenic extension in an MCC mode coeval with syn-orogenic exhumation in Crete and the Peloponnese. (3) Both stages were associated with slab retreat, as early as the Eocene with an acceleration at 30-35 Ma. (4) The localisation of the presently active steeply-dipping normal faults on the southern margin of the Corinth Rift may have been preceded by a partial reworking of thrusts and syn-orogenic detachments by shallow-dipping decollements. (5) The localisation of the NAF in the Marmara Sea region and the Northern Aegean Sea is now accurately dated between 5.3 and 5 Ma Ma using the erosion and deposition surfaces that mark the Messinian salinity Crisis and nannofossils ages. (6) The formation of MCCs in the Cyclades resulted in the draining of the low viscosity lower crust from the northern Aegean Sea and led to an easier coupling between upper mantle and upper crust, easing the localisation and propagation of the NAF. (7) A comparison between crustal and mantle finite strain suggests that mantle flow due to slab retreat controls the stretching of the crust. Our model involves progressive slab retreat and mantle flow below the overriding plate inducing the observed succession of tectonic episodes with a progressive localisation of extension due to several slab tearing events. There is no paradox in the tectonic history and rheological behaviour of the Aegean lithosphere but an evolution of the kinematic boundary conditions due to slab deformation at depth.
American Geophysical Union
San Francisco, United States

insu-00860518
https://hal-insu.archives-ouvertes.fr/insu-00860518
BIBCODE : 2010AGUFM.T13G..04J

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