Éditeur(s) : HAL CCSD Ocean Drilling Programm
Résumé : This paper provides a summary of postcruise scientific results from Ocean Drilling Program (ODP) Leg 209 available to date, building upon shipboard observations and syntheses summarized in the Leg 209 Initial Results volume. During Leg 209, 19 holes were drilled at 8 sites along the Mid-Atlantic Ridge from 14°43´ to 15°44´N, mainly in residual mantle peridotite intruded by gabbroic rocks, in order to understand the tectonic and structural processes responsible for formation of oceanic lithosphere with abundant residual peridotite exposed on the seafloor coupled with a relatively low proportion of volcanic rocks. Based on proportions of recovered lithologies, the entire area may be underlain by mantle peridotite with ~20%–40% gabbroic intrusions and impregnations. Impregnated peridotites with olivine + two pyroxenes + plagioclase + spinel that apparently formed in equilibrium probably record crystallization from primitive mid-ocean-ridge basalt at pressures of 0.5–0.6 GPa. Metamorphic equilibria record isobaric cooling to ~1100°C at this pressure. Thus, the conductively cooled thermal boundary layer beneath the Mid-Atlantic Ridge in this region is >15 km thick. Combined crystallization and reaction with residual peridotite formed a series of impregnated peridotites recording increasing Na content at nearly constant Mg#; this process could explain some of the variation in fractionation-corrected Na (e.g., Na = 8.0) observed in mid-ocean-ridge basalts. Clinopyroxene textures and compositions record such impregnation processes, and they are particularly well documented for Site 1274. Other Leg 209 gabbroic rocks formed from extensive crystallization of highly evolved melts, indicating that a substantial proportion of melt entering the thermal boundary layer crystallizes entirely beneath the seafloor, with no volcanic equivalent. Alteration of peridotites occurred over a range of temperatures and is the result of three distinct processes: rock-dominated serpentinization with formation of brucite in olivine-rich lithologies, fluid-dominated serpentinization with formation of magnetite and no brucite, and fluid-dominated talc alteration with addition of SiO2 as well as H2O and oxygen. The latter two processes also exhibit detectable trace element metasomatism that is distinct in its character from the igneous impregnation described in the previous paragraph. Microstructures show that most residual peridotites were not ductilely deformed at temperatures less than ~1200°C. Structural and paleomagnetic data require tectonic rotations of relatively undeformed blocks; some rotations probably exceeded 60° around nearly horizontal axes parallel to the rift axis. Rotations occurred along several generations of high-temperature mylonitic shear zones extending deeper than 15 km depth and numerous faults at lower temperature. Early formed shear zones and faults were passively rotated around later features; such a process could have produced low-angle fault surfaces without slip on low-angle faults. This region provides end-member examples of processes that are common at many or most slow-spreading ridges. Osmium isotope ratios indicate an ancient history of depletion for residual peridotites from the 14°–16°N region along the Mid-Atlantic Ridge. Though depleted Os isotope ratios in peridotite have been reported elsewhere along the global ridge system, the values from this region are among the most depleted. In general, Os isotope ratios from mid-ocean-ridge basalts are systematically more radiogenic than Os isotope ratios from ridge peridotite samples, suggesting a polygenetic heterogeneous source for mid-ocean-ridge basalts. Geochemical studies of zircons from Leg 209 gabbroic rocks and impregnated peridotites, together with other ridge and arc-related zircons, indicate that ridge zircons have systematically lower fractionation-corrected U and Th concentrations compared to arc zircons. This observation provides a tool for interpreting the tectonic provenance of ancient detrital zircons and indicates an arclike provenance for Hadean detrital zircons. Geobiological studies and aerobiological studies were also undertaken during Leg 209. The geobiological work found no measurable microbial enhancement of olivine dissolution rate, possibly because the samples from Leg 209 were sterile. The aerobiological study determined that dust from North Africa, collected from the derrick of the JOIDES Resolution during Leg 209, contains a variety of abundant microorganisms.
Proceedings ODP, scientific results, 209

hal-00407966
https://hal.archives-ouvertes.fr/hal-00407966
DOI : 10.2973/odp.proc.sr.209.001.2007

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Cerro del Almirez massif (Nevado–Filábride Complex, Betic Cordillera, SE Spain) is composed of antigorite (Atg-) serpentinite and chlorite (Chl-) harzburgite separated by a thin reaction front formed in a palaeo-subduction setting. These ultramafic rocks preserve unique prograde structures related to the pre- and peak high-pressure event (1.6–1.9 GPa and 680–710 °C). The oldest subduction-related structures are preserved in Atg-serpentinites: a penetrative S1 foliation and associated L1 stretching lineation that formed in a non-coaxial regime with a top-to-the-W sense of shear. This planar–linear fabric is crosscut by olivine ± Ti-clinohumite veins, formed during the prograde breakdown of brucite and pre-metamorphic clinopyroxene, which form a system of veins decimetres long. They record embrittlement processes due to the release of 6 vol.% of H2O associated with brucite dehydration. The growth of prograde olivine and/or tremolite porphyroblasts is syn- to post-kinematic in relation to the S1 foliation. Further reactions at higher temperature related to the complete breakdown of the Atg (i.e. Atg-out) are post-kinematic to S1. Prograde Chl-harzburgite is crosscut by sets of conjugate zones associated with grain-size reduction of olivine grains. These grain-size reduction zones are interpreted as brittle structures generated by hydrofracturing due to overpressure fluids in a compressional setting at low-differential stresses.Structures related to the exhumation process are mainly preserved in the metasedimentary host rock, where an S2/L2 planar–linear fabric developed within a shear zone dominated by a non-coaxial regime with a top-to-the-W sense of movement in a transpressional regime. Peak metamorphic conditions deduced for the schists are similar in pressure (1.3–1.9 GPa) but lower in temperature (560–590 °C) compared to the ultramafic rocks in contact with them, suggesting a major shear zone at the base of the ultramafic massif during D2 deformation in a right-handed transpressional regime.
ISSN: 0040-1951

hal-01172303
https://hal.archives-ouvertes.fr/hal-01172303
DOI : 10.1016/j.tecto.2014.12.022

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.
ISSN: 0022-3530

hal-01054319
https://hal.archives-ouvertes.fr/hal-01054319
DOI : 10.1093/petrology/egt068

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Lawsonite pseudomorphs are used to identify and distinguish the kinematic records of subduction and exhumation in blueschist-facies rocks from Syros (Cyclades; Greece). Lawsonite is a hydrous mineral that crystallizes at high-pressure and low-temperature conditions. During decompression, lawsonite is typically pseudomorphed by an aggregate dominated by epidote and paragonite. Such aggregates are easily deformable and if deformation occurs after the lawsonite breakdown, the pseudomorphs are difficult to distinguish from the matrix. The preservation of the lawsonite crystal shape, despite complete retrogression, indicates therefore that the host blueschist rock has not been affected by penetrative deformation during exhumation, thus providing indication of strain-free conditions. Therefore, tracking the lawsonite growth and destabilization along the P-T path followed by the rocks during a subduction/exhumation cycle provides information about the subduction/exhumation-related deformation. Using microstructural observations and P-T pseudosections calculated with thermocalc, it is inferred that top-to-the-south sense of shear preserved in lawsonite pseudomorph-bearing blueschists on Syros occurred during the prograde metamorphic path within the lawsonite stability field, and is therefore associated with subduction. On the contrary, the deformation with a top-to-the-north sense of shear is observed in surrounding rocks, where lawsonite pseudomorphs are deformed or apparently lacking. This deformation occurred after the lawsonite breakdown during exhumation. At the regional scale, exhumation-related deformation is heterogeneous, allowing the preservation of lawsonite pseudomorphs in significant volumes of blueschists of the central and southern Cyclades. It is argued that such successive shearing deformation events with opposite senses more likely correspond to an exhumation process driven by slab rollback, in which subduction and exhumation are not synchronous
ISSN: 0263-4929

insu-00841214
https://hal-insu.archives-ouvertes.fr/insu-00841214
DOI : 10.1111/jmg.12035

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPaP∼1.4 GPa and View the MathML sourceT∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.
ISSN: 0012-821X

hal-01243499
https://hal.archives-ouvertes.fr/hal-01243499
DOI : 10.1016/j.epsl.2015.08.026

Éditeur(s) : HAL CCSD
Résumé : International audience
High Pressure - Low Temperature (HP-LT) rocks testify for burying and exhumation of crustal material in subduction zones. The question of their exhumation is nevertheless still unclear. The Aegean domain offers several HP-LT belts. Especially, the Attic-Cycladic Blueschist unit (ACBU) is an example of a HP-LT unit exhumed in a convergent and then in a divergent context. It actually appears in metamorphic core complexes (MCCs) formed in the back-arc of the subduction. The syn-convergence "message" is however difficult to read because of the post-convergence structural and thermal overprint that accompanied the formation of the MCCs. Particularly, the geometry, the timing and the amount of the syn-orogenic exhumation remain unclear. We investigate theses questions with metamorphic petrology (Andros and Ios islands) and phengite population, single grain and in situ 40Ar-39Ar geochronology (Andros, Tinos, Syros and Ios islands). Based on these new constraints and a synthesis of published data, we are able to refine the geometry, the timing and the modes of the exhumation of the ACBU. Data from the whole Cyclades exhibit little differences in the age and P-T conditions at the successive stages of syn-orogenic exhumation. The peak of pressure (500±50 °C-18±1 kbar) has been reached at 50-55 Ma. The onset of the exhumation occurred at 40-45 Ma in blueschist facies conditions, between a basal thrust and a summital detachment. The following decompression was accompanied by cooling until 30-35 Ma (400±50 °C-8±1 kbar). At that time, the boundary conditions switched to extension. The change in boundary conditions is recorded by slowing of the exhumation and isobare heating. The conditions of post-orogenic exhumation differ from an island to another. The onset of post-orogenic exhumation is dated around 30 Ma in the northern Cyclades and 25 Ma in the southern Cyclades. It is related to top-to-the-North deformation in all the studied domain. The end of ductile deformation below the detachments is also diachronous (21 Ma in the North and 18 Ma in the South). Finally, we propose a model for the final steps of the exhumation related to the intrusion of granitic bodies around 14 Ma.
American Geophysical Union, Fall Meeting
San Francisco, United States

insu-00860510
https://hal-insu.archives-ouvertes.fr/insu-00860510
BIBCODE : 2010AGUFM.T23C2301L

Éditeur(s) : HAL CCSD
Résumé : International audience
The Socotra Island belongs to the southern rifted margin of the Gulf of Aden and occupied in Neoproterozoic times a key position to constrain the age and the nature of the largely hidden Neoproterozoic rocks of the Arabian plate. Our integrated field, petrographic, geochemical and geochrono- logical study in the Neoproterozoic rocks recognises three main successive events: (a) high-temperature ductile defor- mation and metamorphism forming probably in a compressive or transpressive regime; (b) mafic to intermediate intrusions as vertical sheets, kilometre-scale gabbro laccoliths, mafic dike swarm and lavas which present mainly a depleted arc signature with some evidences of evolution from an enriched-arc signature; (c) felsic intrusions mainly composed of highly potassic calc-alkaline and pinkish granites dated between 840 and 780 Ma. Relationships between the various petrographic types and U-Pb data suggest that these events occurred during a relatively short time span (80 Ma at max). Earlier high-temperature-low-pressure metamorphism stage as well as geochemical signature of mafic rocks show that development of Cryogenian formations of Socotra were controlled successively by an Andean-arc and a back-arc setting. These features cannot be easily reconciled with those of the Arabian-Nubian shield to the west of Socotra and of the Mozambique Belt to the south. We propose that the Socotra basement was developed at an active margin close to the India block in Cryogenian times.
Arabian Journal of Geoscience

hal-00647487
https://hal.archives-ouvertes.fr/hal-00647487
DOI : 10.1007/s12517-011-0476-3

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbona-ceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar– 39 Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomar-gins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamor-phism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cre-taceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamor-phism are intimately associated with the areas where subcon-tinental mantle rocks have been unroofed or exhumed.
ISSN: 1869-9510

insu-01161826
https://hal-insu.archives-ouvertes.fr/insu-01161826
https://hal-insu.archives-ouvertes.fr/insu-01161826/document
https://hal-insu.archives-ouvertes.fr/insu-01161826/file/PL03457.pdf
DOI : 10.5194/se-6-643-2015

Éditeur(s) : HAL CCSD Geological Society of America
Résumé : International audience
his study describes the structural setting, petrogenesis, and geochronology of a suite of acidic magmatic rocks that are intruded in the metamorphic core of the Tertiary ophiolitic suture zone of the Sabzevar Range, NE central Iran. These intrusive bodies show tabular geometries with solid-state fabrics documenting syntectonic emplacement during crustal shortening. In the total alkalis-silica (TAS) diagram, their compositions define a medium-K calc-alkaline suite, spanning from basaltic andesite to the dacite and rhyolite fields. They show characteristic low MgO (0.15–0.60 wt%) and Ni (<20 ppm), high Sr contents, a negligible Eu anomaly, and extremely fractionated rare earth element (REEs), with high La/Yb and Sr/Y (up to 900) ratios, but very low Yb and Y contents. They also show zircon Hf isotope compositions compatible with a mid-ocean-ridge basalt (MORB)–type oceanic crust. Inverse and forward thermobarometry constrains conditions of magma crystallization in the upper-pressure field of the amphibolite facies (ca. 1.2–1.5 GPa and 750 °C). Integrated U-Pb zircon and 40Ar/39Ar white mica and amphibole geochronology constrains the Sabzevar magmatism to the late Paleocene (at ca. 58 Ma). Genesis of the Sabzevar magmatic suite is interpreted in terms of prograde, wet amphibolite melting during oceanic subduction, within a pressure-temperature range between a plagioclase-out and a hornblende-out boundary. Magma differentiation and high-pressure amphibole fractionation of pristine slab melts are proposed as the dominant factors that imparted the adakite signature in the Sabzevar structural zone. Implications in terms of the regional tectonic scenario are discussed and framed within the advancing and retreating evolution of the Neotethyan subduction during the Mesozoic–Tertiary time span.
ISSN: 0016-7606

hal-01122824
https://hal.archives-ouvertes.fr/hal-01122824
DOI : 10.1130/B30913.1

Éditeur(s) : HAL CCSD
Résumé : High Pressure - Low Temperature (HP-LT) rocks testify for burying and exhumation of crustal material in subduction zones. Their complete exhumation is the result of processes acting during (syn-orogenic) and after (post-orogenic) subduction is active, the latter partly obliterating the first. In the Aegean domain, the Attic-Cycladic Blueschist unit (ACBU) is a HP-LT unit exhumed in metamorphic core complexes (MCCs) in the back-arc domain of the Hellenic subduction zone. This study is an attempt to distinguish features related to syn-orogenic exhumation processes responsible for the first stages of exhumation This syn-orogenic "message" has to be read through the post-convergence structural and thermal overprint that accompanied the formation of the MCCs. Particularly, the geometry, the timing and the amount of the syn-orogenic exhumation has to be determined. We investigate these features with coupled field, metamorphic petrology and radiochronology studies First, based on observations on Ios (southern Cyclades), it has recently been proposed that syn-orogenic exhumation of the ACBU was accommodated by a basal thrust over the Cycladic Basement (CB) [Huet, et al., 2009]. The peak conditions, deduced from pseudosections calculation for the Ios main lithologies, are 500 °C-19 kbar for the ACBU and 550 °C-16 kbar for the CB. Exhumation until 400 °C-8 kbar is considered as syn-orogenic. Second, phengites populations, single grains and in situ 40Ar-39Ar geochronology has been carried out on samples from Tinos and Andros (northern Cyclades), Syros (central Cyclades) and Ios. The results gather in several clusters: 50-55 Ma, 40-45 Ma, 30-35 Ma, and 22-26 Ma. Based on these new P-T-t data and a synthesis of published data, we are able to refine the geometry and the timing of the exhumation of the ACBU. Data from the whole Cyclades exhibit little differences in ages and P-T conditions for the successive stages of exhumation. The peak of pressure (500±50 °C-18±1 kbar) has been reached at 50-55 Ma from Tinos to Ios. The onset of the exhumation occurred at 40-45 Ma in blueschist facies conditions, between a thrust at their bottom and a detachment at their top. The following decompression was accompanied by cooling until 30-35 Ma (400±50 °C-8±1 kbar). At that time, the boundary conditions switched to extension. Finally, we propose a model for the exhumation of HP-LT units accounting for these data. We also consider the rheological implications of the syn-orogenic structuration of the Aegean wedge on the post-orogenic evolution. B. Huet, L. Labrousse, and L. Jolivet [2009] Thrust or detachment? Exhumation processes in the Aegean: Insight from a field study on Ios (Cyclades, Greece), Tectonics, 28, 3, doi:10.1029/2008TC002397.
EGU General Assembly
Vienne, Austria

insu-00839054
https://hal-insu.archives-ouvertes.fr/insu-00839054
BIBCODE : 2010EGUGA..1214098H

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Syn-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.
ISSN: 0040-1951

insu-00411771
https://hal-insu.archives-ouvertes.fr/insu-00411771
https://hal-insu.archives-ouvertes.fr/insu-00411771/document
https://hal-insu.archives-ouvertes.fr/insu-00411771/file/Jolivet-Tectonophysics-2010.pdf
DOI : 10.1016/j.tecto.2009.10.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Subduction infancy corresponds to the first few million years following subduction initiation, when slabs start their descent into the mantle. It coincides with the transient (yet systematic) transfer of material from the top of the slab to the upper plate, as witnessed by metamorphic soles welded beneath obducted ophiolites. Combining structure–lithology–pressure–temperature–time data from metamorphic soles with flow laws derived from experimental rock mechanics, this study highlights two main successive rheological switches across the subduction interface (mantle wedge vs. basalts, then mantle wedge vs. sediments; at ∼800 °C and ∼600 °C, respectively), during which interplate mechanical coupling is maximized by the existence of transiently similar rheologies across the plate contact. We propose that these rheological switches hinder slab penetration and are responsible for slicing the top of the slab and welding crustal pieces (high- then low-temperature metamorphic soles) to the base of the mantle wedge during subduction infancy. This mechanism has implications for the rheological properties of the crust and mantle (and for transient episodes of accretion/exhumation of HP-LT rocks in mature subduction systems) and highlights the role of fluids in enabling subduction to overcome the early resistance to slab penetration.
ISSN: 0012-821X

hal-01383368
http://hal.upmc.fr/hal-01383368
http://hal.upmc.fr/hal-01383368/document
http://hal.upmc.fr/hal-01383368/file/Agard_2016_Plate_interface.pdf
DOI : 10.1016/j.epsl.2016.06.054

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
In south China the Lushan massif forms a topographic high of the South China Block south of the Qinling-Dabie belt. The Lushan massif consists of two main lithotectonic units separated by a major tectonic contact: a Neoproterozoic (upper Sinian)-Paleozoic unit comprising primarily unmetamorphosed sandstones overlies a Paleoproterozoic unit mainly composed of low-pressure, high-temperature gneisses and micaschists. Both units are cut by Cretaceous granitic intrusions. Three primary tectono-metamorphic and magmatic events are recognized. The eastern part of the Lushan massif is cut by a NNE-SSW trending ductile normal fault (D3 deformation) coeval to the emplacement of a 100-110 Ma leucogranite dated by 40Ar/39Ar laserprobe on biotite and muscovite. D2 deformation is responsible for the formation of a decakilometer-scale NE-SW trending upright anticline characterized by NE-SW stretching and NW-SE shortening. The age of this folding event is defined by a 127±1 (2σ) Ma U/Pb titanite date obtained for a syntectonic granodiorite and 40Ar/39Ar ages of 133 Ma for amphibole. This Cretaceous age also corresponds to the 40Ar/39Ar ages of 126 Ma found on syntectonic muscovites at the base of the Sinian unit. An older deformation event, D1, characterized by a top-to-the-NW extensional decollement of the Sinian-Paleozoic series above Proterozoic metamorphic rocks is related to the Triassic tectonics of the Dabieshan. Lastly, in the lower part of Sinian rocks, the occurrence of kyanite cataclased during D1 documents an older, poorly preserved, late Paleozoic-early Mesozoic tectonometamorphic event (Dx) related to a blind thrust in the continental crust of the South China Block in the southern foreland of the Dabieshan.
ISSN: 0278-7407

insu-00717721
https://hal-insu.archives-ouvertes.fr/insu-00717721
https://hal-insu.archives-ouvertes.fr/insu-00717721/document
https://hal-insu.archives-ouvertes.fr/insu-00717721/file/Lin-Tectonics-2000.pdf
DOI : 10.1029/2000TC900009

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Pyrenean peridotites (lherzolites) form numerous small bodies of subcontinental mantle, a few meters to 3 km across, exposed within the narrow north Pyrenean zone (NPZ) of Mesozoic sediments paralleling the north Pyrenean Fault. Recent studies have shown that mantle exhumation occurred along the future NPZ during the formation of the Albian-Cenomanian Pyrenean basins in relation with detachment tectonics. This paper reviews the geological setting of the Pyrenean lherzolite bodies and reports new detailed field data from key outcrops in the Bearn region. Only two types of geological settings have to be distinguished among the Pyrenean ultramafic bodies. In the first type (sedimented type or S type), the lherzolites occur as clasts of various sizes, ranging from millimetric grains to hectometric olistoliths, within monogenic or polymictic debris flow deposits of Cretaceous age, reworking Mesozoic sediments in dominant proportions as observed around the Lherz body. In the second type (tectonic type or T type), the mantle rocks form hectometric to kilometric slices associated with crustal tectonic lenses. Both crustal and mantle tectonic lenses are in turn systematically associated with large volumes of strongly deformed Triassic rocks and have fault contacts with units of deformed Jurassic and Lower Cretaceous sediments belonging to the cover of the NPZ. These deformed Mesozoic formations are not older that the Aptian-early Albian. They are unconformably overlain by the Albian-Cenomanian flysch formations and have experienced high temperature-low pressure mid-Cretaceous metamorphism at variable grades. Such a tectonic setting characterizes most of the lherzolite bodies exposed in the western Pyrenees. These geological data first provide evidence of detachment tectonics leading to manle exhumation and second emphasize the role of gravity sliding of the Mesozoic cover in the preorogenic evolution of the Pyrenean realm. In the light of such evidence, a simple model of basin development can be inferred, involving extreme thinning of the crust, and mantle uprising along a major detachment fault. We demonstrate coeval development of a crust-mantle detachment fault and generalized gravitational sliding of the Mesozoic cover along low-angle faults involving Triassic salt deposits as a tectonic sole. This model accounts for the basic characteristics of the precollisional rift evolution in the Pyrenean realm.
ISSN: 0278-7407

hal-00512818
https://hal.archives-ouvertes.fr/hal-00512818
DOI : 10.1029/2009TC002588