Micro-tectonic constraints on the evolution of the Barles half-window (Digne Nappe, southern Alps). Implications for the timing of folding in the Valensole foreland basin

International audienceThe "Vélodrome" overturned syncline, at the northern margin of the Cenozoic foreland basin of Valensole in SE France, was formed during the Late Cenozoic at the front of the Digne nappe. Microstructural analyses reveal that mesoscale faulting in the molassic series, from the Oligocene "Molasse Rouge" at the base to the middle to late Miocene "Valensole Conglomerates" at the top, partly occurred before the folding, as layer-parallel shortening: the NNE-SSW-directed compression is recorded by two systems of reverse and strike-slip faults, which formed when the strata were still horizontal and were passively tilted as folding occurred. These data suggest that the Vélodrome folding postdates the deposition of the Valensole Conglomerates and occurred in late Miocene-Pliocene times during the emplacement of the Digne nappe. These results are difficult to reconcile with the interpretation of the Vélodrome as a growth fold progressively formed in 10-15 m.y. during the deposition of the Miocene molasses. Structural data collected in the Barles tectonic half-window enable to reconstruct the evolution of the deformation since the Jurassic. The two main phases of shortening, the pre-Oligocene Pyrenean-Provençal and the Mio-Pliocene Alpine phases, are almost homoaxial with a direction of compression trending N-S for the former and NNE-SSW for the later. A late Eocene-basal Oligocene N-S extensional episode is documented between these two phases, probably in relation with the formation of the western European rift system. The direction of extension of the Liassic rifting of the Alpine Tethys is roughly constrained in the NW-SE quadrant. Paleo-stress field reconstruction brings consistent results at the regional scale and proves to be a powerful tool to decipher the evolution of the deformation in a remarkably complicated tectonic setting

Shear band formation and strain localization on a regional scale: Evidence from anisotropic rocks below a major detachment (Betic Cordilleras, Spain)

International audienceRegional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C3' shear bands (or extensional crenulation cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation

Subduction interface processes recorded by eclogite-facies shear zones (Monviso, W. Alps)

International audienceThe Monviso ophiolite Lago Superiore Unit constitutes a well-preserved, almost continuous upper fragment of oceanic lithosphere subducted at c. 80 km depth, thereby providing a unique opportunity to study mechanical coupling processes and meter-scale fluid-rock interactions occurring at such depths in present-day subduction zones. It is made of (i) a variably thick (50-500 m) section of eclogitized basaltic crust (associated with minor calcschist lenses) overlying a 100-400 m thick metagabbroic body and of (ii) a c. 1 km thick serpentinite sole. We herein focus on the three major eclogite-facies shear zones found at the top of the unit, at the boundary between basalts and gabbros, and between gabbros and serpentinites, respectively. Strain localization occurred at lithological interfaces, irrespective of material strength. While ductile deformation dominates along the shear zones, local brittle behaviour is demonstrated by the existence of numerous eclogite breccias of Fe-Ti metagabbros and widespread garnet fractures, possibly linked with intermediate-depth eclogite-facies (micro)seismicity. These m- to hm-sized fragments of Fe-Ti metagabbros were later sheared and disseminated within serpentinite schists along the gabbro-serpentinite boundary (Lower Shear zone; LSZ). Pervasive and focused fluid flow is attested in the LSZ by significant alteration of bulk rock compositions, weakening of the rocks and widespread crystallization of hydrous parageneses. By contrast, the Intermediate Shear zone (ISZ) shows evidence for more restricted, short-range fluid flow. The activity of both the ISZ and LSZ ceased during early lawsonite eclogite-facies exhumation, when deformation localized deeper within the serpentinite sole, allowing for the detachment (and preservation) of this large ophiolitic fragment

Evolution of hydrothermal regime along a crustal shear zone, Tinos island, Greece

International audienceStructural, cathodoluminescence and SIMS δ18O analyses of quartz-calcite veins are combined to constrain the evolution in space and time of fluid infiltration in an exhuming detachment shear zone (Tinos Island, Cyclades, Greece). Careful description of vein arrays shows that the plumbing system evolved into an interconnected network just beneath the ductile-to-brittle transition. Micro-textures of quartz and calcite infilling veins display deformed relics and newly precipitated grains. High-resolution δ18O mapping in relics yield a steady quartz-calcite fractionation of 2 ‰ at ~400 °C, whereas fractionation increases in neo-grains from 2 to 5 ‰ in 190 m toward the fault, or a ~150 °C temperature fall. Cooling is associated with a shift of fluid stable isotope signature from 15 ‰ to 0 ‰ beneath the detachment. Results are interpreted as advective removal of heat by massive infiltration of surface-derived fluids over depths of 10-15 km. Fluid penetration was promoted by the coalescence of late ductile-early brittle veins, as the exhuming footwall crossed the ductile-brittle transition. Only small amounts of fluids penetrated the ductile crust beneath the transitional rheology

Burial and exhumation in a subduction wedge : mutual constraints from thermo-mechanical modelin and natural P-T-t data (Sch. Lustrés, W. Alps)

The dynamic processes leading to synconvergent exhumation of high-pressure low-temperature (HP-LT) rocks at oceanic accretionary margins, as well as the mechanisms maintaining nearly steady state regime in most accretion prisms, remain poorly understood. The present study aims at getting better constraints on the rheology, thermal conductivity, and chemical properties of the sediments in subduction zones. To reach that goal, oceanic subduction is modeled using a forward visco-elasto-plastic thermomechanical code (PARA(O)VOZ-FLAC algorithm), and synthetic pressure-temperature-time (P-T-t) paths, predicted from numerical experiments, are compared with natural P-T-t paths. The study is focused on the well constrained Schistes Lustrés complex (SL: western Alps) which is thought to represent the fossil accretionary wedge of the Liguro-Piemontese Ocean. For convergence rates comparable to Alpine subduction rates (∼3 cm yr−1), the best-fitting results are obtained for high-viscosity, low-density wedge sediments and/or a strong lower continental crust. After a transition period of 3-5 Ma the modeled accretionary wedges reach a steady state which lasts over 20 Ma. Over that time span a significant proportion (∼35%) of sediments entering the wedge undergoes P-T conditions typical of the SL complex (∼15-20 kbar; 350-450°C) with similar P-T loops. Computed exhumation rates (<6 mm yr−1) are in agreement with observations (1-5 mm yr−1). In presence of a serpentinite layer below the oceanic crust, exhumation of oceanic material takes place at rates approaching 3 mm yr−1. In all experiments the total pressure in the accretionary wedge never deviated by more than ±10% from the lithostatic component

Strain localisation in mechanically layered rocks beneath detachment zones: insights from numerical modelling.

International audienceFully dynamic numerical simulations have been designed in order to asses how the orientation of mechanical layering in rocks controls the orientation of shear bands and the depth of penetration of strain in the footwall of detachment zones. Two parametric studies are presented. In the first one, the influence of stratification orientation on the occurrence and mode of strain localisation is tested. The second parametric study shows that results are length-scale independent and that orientation of shear bands is not sensitive to the viscosity contrast or the strain rate. Based on the results, a conceptual model for strain localisation under detachment faults is presented. In the early stages, strain localisation occurs at slow rates by viscous shear instabilities but as the layered media is exhumed, the temperature drops and the strong layers start yielding plastically, forming shear bands and localising strain at the top of the shear zone. Once strain localisation has occured, the deformation in the shear band becomes extremely penetrative but the strength cannot drop since the shear zone has a finite thickness

Along-strike variations of P-T conditions in accretionary wedges and syn-orogenic extension, the HP-LT Phyllite-Quartzite Nappe in Crete and the Peloponnese

International audienceSyn-orogenic detachments in accretionary wedges make the exhumation of high-pressure and low-temperature metamorphic rocks possible with little erosion. The velocity of exhumation within the subduction channel or the accretionary complex, and thus the shape of P-T paths, depend upon the kinematic boundary conditions. A component of slab retreat tends to open the channel and facilitates the exhumation. We document the effect of slab retreat on the shape of P-T paths using the example of the Phyllite-Quartzite Nappe that has been exhumed below the Cretan syn-orogenic detachment during the Miocene in Crete and the Peloponnese. Data show a clear tendency toward colder conditions at peak pressure and during exhumation where the intensity of slab retreat is larger. This spatial evolution of P-T gradient is accompanied with an evolution from a partly coaxial regime below the Peloponnese section of the detachment toward a clearly non-coaxial regime in Crete

Episodic slab rollback fosters exhumation of HP-UHP rocks

International audienceThe burial-exhumation cycle of crustal material in subduction zones can either be driven by the buoyancy of the material, by the surrounding flow, or by both. High pressure and ultrahigh pressure rocks are chiefly exhumed where subduction zones display transient behaviours, which lead to contrasted flow regimes in the subduction mantle wedge. Subduction zones with stationary trenches (mode I) favour the burial of rock units, whereas slab rollback (mode II) moderately induces an upward flow that contributes to the exhumation, a regime that is reinforced when slab dip decreases (mode III). Episodic regimes of subduction that involve different lithospheric units successively activate all three modes and thus greatly favour the exhumation of rock units from mantle depth to the surface without need for fast and sustained erosion

Evolution métamorphique et structurale des métapelites océaniques dans l'orogène alpin : l'exemple des schistes lustrés des Alpes occidentales (Alpes Cottiennes)

Cette étude s'est interessée à l'évolution tectono-métamorphique des schistes lustrés dans le cadre de l'orogène alpin, et à l'évolution pétrographique de détail de ces métapélites. Les conditions métamorphiques déterminées croissent de manière régulière d'ouest en est sur le secteur étudié de 13-14 Kbar / 350\c a 20-21 Kbar / 450-500\c, respectivement. Deux étapes principales d'exhumation, de vergence opposée, sont distinguées. La première, responsable de l'essentiel de l'exhumation des schistes lustrés, est une phase de déformation ductile à vergence est, s'effectuant du faciès schiste bleu au faciès schiste vert. La deuxième phase se marque par la présence de bandes de cisaillement à vergence ouest, espacées à l'échelle du terrain, témoins d'une extension en regime ductile-cassant dont l'intensitè croit vers l'est tout en perturbant peu la zonation métamorphique - elle resulterait des grands bouleversements tertiaires se produisant à l'arrière de la chaine vers 40-35 ma. Une approche integrée des paramètres p-t / deformation / interaction fluide-roche à l'echelle métrique est developpée grâce au référentiel pétrographique que constitue la carpholite. Lors du chemin rétrograde, il est en particulier possible de rendre compte de manière conjointe de l'adaptation de la composition des phengites (contenu en k de 0.9 a 0.6 pfu), des bilans de matière déduits des réactions rétrogrades (20 a 30 mol% d'eau libérée), de la variation de composition de la phase fluide (de 15 a 4 WT% NaCl). L'évolution pétrographique résulterait ainsi au premier ordre d'une équilibration à l'échelle locale, et souligne la validité d'indicateurs métamorphiques tels que les phengites, les pseudomorphoses ou les veines syn-métamorphiques. La quantité d'eau stockée à haute pression et leur position centrale dans l'orogène, suggèrent par ailleurs que ces métapélites ont pu jouer le rôle de tampon et d'échangeur de fluides à grande échelle.pas de résum

Subduction of oceanic lithosphere in the Alps: Selective and archetypal from (slow-spreading) oceans

International audienceThe Alps are amongst the best subduction archives in the world, with abundant blueschists and eclogites preserving fragments of mantle, gabbros, thinned continental margin and pelagic sediments partly within their pre-collisional architecture. But to what extent is the Alpine record representative of the subduction of oceanic lithosphere worldwide? What is its significance, merits and limits for understanding subduction (and exhumation) dynamics? This contribution shows that this record is neither exceptional nor atypical but rather exemplifies the fate of relatively short-lived and small, slow-spreading and slowly closing North Atlantic-type oceans (in this case a ~ 400–700 km-wide domain closed over 60 Ma, at ~ 1 cm/a), whose subducting slabs do not reach below the Mantle Transition Zone. Subducted fragments experienced conditions typical of mature subduction worlwide and show no sign of significant tectonic overpressure. Contrasts in rock recovery with time and space outline distinct subduction dynamics. During the first half of the subduction lifetime (~30 Ma), no subducted oceanic fragments were recovered. A marked difference is observed between metasediment- and mafic/ultramafic-dominated units (S and MUM units). Underplating of S units took place intermittently and preferentially at ~ 30–40 km depth. The MUM units of the Western Alps deeply subducted to ~ 80 km were only recovered late, i.e. within a few Ma at most before continental subduction and initially lied close to the margin. At the scale of the orogen, the recovery of subducted fragments allows to recognize four distinct sectors and demonstrates a strong influence of initial margin architecture and/or continental subduction. Whilst typical of subduction zone thermal regimes, the subduction archive appears to selectively preserve slow-spreading oceans and/or hyperextended margins