Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In this study, we report new U-Pb zircon ages and amphibole and biotite 40Ar-39Ar cooling ages for igneous and metaigneous rocks from the eastern portion of the Borborema Province (NE Brazil). We combine these results with preliminary thermobarometric estimates and existing data to place constraints on the timing and duration of the Brasiliano-Pan-African Orogeny in northeastern Brazil and on the subsequent late- to post-orogenic history. The Alcantil orthogneiss contains a complex zircon population, but the youngest concordant analyses yielded an age of 632 ± 17 Ma (2σ). These grains have low Th/U ratios (<0.1), and the age of 632 Ma is therefore interpreted as dating the metamorphic event responsible for the acquisition of the gneissic fabric. In the Caruaru biotite amphibole orthogneiss, concordant analyses provided an age of 618 ± 4 Ma (2σ), which we relate to emplacement of the protolith of the Caruaru orthogneiss. Together with available U-Pb data from the literature, the 632-600 Ma age range is regarded as dating development of a regional flat-lying foliation coeval with high-grade metamorphism (upper amphibolite facies conditions). The 586 ± 3 Ma age (2σ) of the Panelas pluton is similar to those of other granitic plutons associated with a voluminous plutonism of high-K calc-alkaline to shoshonitic nature that occurred at 590-580 Ma. This plutonism slightly predated or was synchronous with development of large strike-slip shear zones. Amphibole grains yielded 40Ar-39Ar ages ranging from 584 ± 4 Ma to about 530 Ma, suggesting that regional temperatures remained near the closing temperature of this mineral (≈500 °C) for a long time. Most 40Ar-39Ar biotite ages range from 552 to 533 Ma, but two samples yielded plateau ages of 519 ± 5 Ma and 509 ± 5 Ma. The inferred rather slow cooling of the studied region possibly reflects low rates of unroofing, allowing persistence of a protracted thermal anomaly caused by the massive injection of magmas at the end of the Neoproterozoic.
ISSN: 0895-9811

hal-00745698
https://hal.archives-ouvertes.fr/hal-00745698
DOI : 10.1016/j.jsames.2012.06.003

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
This study investigates three-dimensional flow modes of orogenic plateaus by means of physical modeling. Experiments consist of shortening two contiguous lithospheres of contrasting strength, one being a weak plateau-type lithosphere and the other a strong craton-type lithosphere. The lateral boundaries are either totally confined or allow escape toward a lateral foreland on one side. Two synconvergence flow regimes are distinguished, which are governed by the balance between the gravity potential and the strength of the plateau crust and the resistance of its lateral foreland. The first regime implies transversal (orogen-normal) injection of plateau lower crust into the collision zone as a result of confinement of the plateau by an increasingly stiffer lateral boundary. As a precursor mechanism to channel flow, transversal injection responds to downward thickening of the plateau crust that is forcedly extruded into the orogenic wedge. The second regime is that of collapse-driven lateral escape of the plateau. This regime is established after a threshold is attained in the interplate coupling in the collision zone, which allows the gravity potential of the plateau to overcome the resistance of its lateral boundary. Under the collapse-driven escape regime (orogen parallel), such as that governing Tibet during the last 13 Ma, most of the convergence in the plateau and the top and rear of the collisional wedge is transformed into lateral flow and extension.
ISSN: 0148-0227

hal-01229107
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01229107
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01229107/document
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01229107/file/2015JB012110_revised.pdf
https://hal-univ-rennes1.archives-ouvertes.fr/hal-01229107/file/SupplementaryMaterial_final.pdf

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 Springer Verlag
Résumé : International audience
The ENE-WSW Autun Shear Zone in the northeastern part of the French Massif Central has been interpreted previously as a dextral wrench fault. New field observations and microstructural analyses document a NE-SW stretching lineation that indicates normal dextral motions along this shear zone. Further east, similar structures are observed along the La Serre Shear Zone. In both areas, a strain gradient from leucogranites with a weak preferred orientation to highly sheared mylonites supports a continuous Autun-La Serre fault system. Microstructural observations, and shape and lattice-preferred orientation document high-temperature deformation and magmatic fabrics in the Autun and La Serre granites, whereas low- to intermediate-temperature fabrics characterize the mylonitic granite. Electron microprobe monazite geochronology of the Autun and La Serre granites yields a ca. 320 Ma age for pluton emplacement, while mica 40Ar-39Ar datings of the Autun granite yield plateau ages from 305 to 300 Ma. The ca. 300 Ma 40Ar-39Ar ages, obtained on micas from Autun and La Serre mylonites, indicate the time of the mylonitization. The ca. 15-Ma time gap between pluton emplacement and deformation along the Autun-La Serre fault system argue against a synkinematic pluton emplacement during late orogenic to postorogenic extension of the Variscan Belt. A ductile to brittle continuum of deformation is observed along the shear zone, with Lower Permian brittle faults controlling the development of sedimentary basins. These results suggest a two-stage Late Carboniferous extension in the northeastern French Massif Central, with regional crustal melting and emplacement of the Autun and La Serre leucogranites around 320 Ma, followed, at 305-295 Ma, by ductile shearing, normal brittle faulting, and subsequent exhumation along the Autun-La Serre transtensional fault system.
ISSN: 1437-3254

insu-00596918
https://hal-insu.archives-ouvertes.fr/insu-00596918
https://hal-insu.archives-ouvertes.fr/insu-00596918/document
https://hal-insu.archives-ouvertes.fr/insu-00596918/file/Article_Morvan_v_INIST.pdf
DOI : 10.1007/s00531-011-0673-z

Éditeur(s) : HAL CCSD Cambridge University Press (CUP)
Résumé : International audience
This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia-Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj-Sirjan, Kermanshah and Urumieh-Dokhtar magmatic arcs), the P-T-t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW-NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115-85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene-Eocene (60-40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30-0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj-Sirjan Zone: 20-15 Ma, High Zagros: ~12-8 Ma; Simply Folded Belt: 5-0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.
ISSN: 0016-7568

insu-00616582
https://hal-insu.archives-ouvertes.fr/insu-00616582
https://hal-insu.archives-ouvertes.fr/insu-00616582/document
https://hal-insu.archives-ouvertes.fr/insu-00616582/file/692.full.pdf
DOI : 10.1017/S001675681100046X

Éditeur(s) : HAL CCSD
Résumé : 1 p.
International audience
The 12th May 2008 Wenchuan earthquake in the Longmen Shan occurred on a large thrust fault largely inherited from an Indosinian structure itself probably controlled by an older structural heritage of the South China block continental margin. Within the whole northeast Tibet region, such a structural inheritance has had a major impact on the Tertiary deformation. It appears of primary importance to assess the pre-Tertiary tectonic evolution of the main blocks involved to understand the actual deformation in the eastern edge of Tibet. Over the past decades, the Proterozoic to Cenozoic tectonic, metamorphic and geochronologic history of the Longmen Shan and Songpan Garzê area have been largely studied. We present a synthesis of the tectonic evolution of the Songpan Garzê fold and thrust belt from Triassic to present. The Songpan-Garzê belt was formed during closure of a wide oceanic basin ?lled with a thick (5 to 15 km) sequence of Triassic ?yschoid sediments. Closure of the basin due to Triassic subduction involved strong shortening, intense folding and faulting of the Triassic series. A large-scale décollement, that presently outcrops along the eastern boundary of the belt (Danba area), allowed the growth of a wide and thick accretionary wedge. It develops in the Paleozoic and Triassic series and separates the accretionary prism from an autochthonous crystalline basement which shares many similarities with the basement of the Yangtze Craton (0.7-0.9 Ga). To the north and northwest, below the thickened Triassic series of the belt, the composition (oceanic or continental) of the basement remains unknown. During the Indosinian orogeny the emplacement of orogenic granites (220 - 150 Ma) was associated to crustal thickening. The isotopic composition of granitoids shows that their magma source were predominantly derived from melting of the proterozoic basement with varying degrees of sedimentary material and negligible mantle source contribution. In the Danba area, the décollement outcrops in a large tertiary antiform with a NNW-SSE axis. It has been exhumed too in the hanging wall of the NE-SW faults of the Tertiary Longmen-Shan belt that marks the present day transition from the Tibetan plateau to the Sichuan basin. These faults have episodically absorbed signi?cant shortening since the Late Triassic. The amount and precise timing of post-triassic deformation are dif?cult to constrain especially because of the dif?culty to isolate the tertiary thermochronological signal from the protracted late Triassic - Cretaceous thermal history. Nonetheless it is generally accepted that Jurassic - Cretaceous tectonism did not modi?ed the general Triassic architecture of eastern Tibet contrarily to the Tertiary deformation. The long-term cooling histories obtained on Mesozoic granites and on the metamorphic series of the Danba dome are very similar showing a very slow and regular cooling during Jurassic andCretaceous, con?rming the absence of major tectonic event between c.a. 150 and 30 Ma. Low temperature thermochronology data indicate that ?nal exhumation and cooling occurred in theTertiary with an acceleration between 10 and 5 Ma along the major tectonic structures. Within the Longmen Shan range, a total denudation of 7 to 10 km is estimated for the late Cenozoic period. Similar amounts of late Tertiary denudation have been estimated along an east-west section across the Xianshuihe fault
Asian climate and tectonics seminar
Utrecht, Netherlands

insu-00610298
https://hal-insu.archives-ouvertes.fr/insu-00610298
DOI : 10.1016/j.jseaes.2010.03.008

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
Coupling between erosion and tectonics is thought to play a determinant role in orogenic evolution. Here, we investigate the interplay in this coupling between the assumed erosion law and the crustal rheology at the margin of a collisional plateau, like the Himalaya of Central Nepal. Lithospheric deformation is calculated over a time scale of 100 kyr by a 2D finite element model that incorporates the rheological layering of the crust and the main features of the convergence across the range. For the upper boundary condition, two surface processes were tested: a linear diffusion model and a 1D1/2 integrative model including fluvial incision along the fluvial network and hillslope erosion by landsliding. Model results and their sensitivity to the chosen combinations of erosion law and crustal properties are discussed in light of the constraining geologic and geomorphologic observations. In contrast with the conclusions of Cattin and Avouac [2000] , where a compliant quartz-rich crustal rheology and diffusion law were required, we combine a composite quartz-diabase rheology for the crust with fluvial incision erosion law to account for erosion and elevation profiles across the Himalaya of Central Nepal. More generally, it is proposed that, because of the interplay between the dominant denudation conditions and the rheology of the crust, both well documented erosion rates and processes can provide significant constraints on crustal properties within an active orogen.
ISSN: 0094-8276

hal-00788697
https://hal.archives-ouvertes.fr/hal-00788697
https://hal.archives-ouvertes.fr/hal-00788697/document
https://hal.archives-ouvertes.fr/hal-00788697/file/godard_et_al.pdf
DOI : 10.1029/2004GL021006