L'eau et la dynamique lithosphérique

Concentrée à la surface du Globe, l'eau n'en est pas moins présente en grande quantité à l'intérieur de la Terre. Elle y joue un rôle majeur en diminuant la résistance des roches, en permettant leur fusion partielle et en transportant des éléments chimiques. Elle interagit à toutes les échelles d'espaces et de temps dans les profondeurs de la croûte et du manteau, que ce soit pendant les séismes, la formation des chaînes de montagnes ou des rifts. Sans eau, il est probable que la tectonique des plaques serait très différente, voire absente de la dynamique terrestre

Structural and kinematic relationships between Corsica and the Pyrenees-Provence domain at the time of the Pyrenean orogeny

International audienceThe Pyrenees-Provence belt and the Alps were both active in the late Eocene. Alpine Corsica was once a part of the Alps, and the now obducted metamorphic oceanic domain is similar and easily correlated in both areas. Tectonic reconstructions before the Oligo-Miocene opening of the Liguro- Provenc¸al basin show that at the same time, Corsica was located in the hinterland of the Provenc¸al ranges. A late Eocene cross section running from Alpine Corsica to Provence gives an image of a complete mountain belt from an internal domain made of metamorphosed oceanic material (Alpine Corsica) to the foreland fold and thrust with a thin-skinned geometry (Provence). During the late Eocene the intervening basement of western Corsica was thus within this mountain belt, probably thrust onto the European basement. We analyze and interpret the structural pattern and the overall geometry of the Provenc¸al-Corsican domain during late Eocene times in terms of oblique convergence and strain partitioning, within the framework of the Africa- Eurasia convergence. This evolution is integrated in a set of kinematic reconstructions of the western Mediterranean region from 65 Ma to the presen

Mechanics of low-angle extensional shear zones at the brittle-ductile transition

Present address : Institut des Sciences de la Terre d'Orléans UMR 7327, Université d'Orléans, CNRS/INSU, BGRMInternational audienceLow-angle midcrustal ductile shear zones and the related microseismic activity recorded below regions of active extension are seen here as two consequences of strain localization. The feldspar-to-mica reaction which occurs once feldspar grains are fractured is the destabilizing mechanism selected to explain the strain localization. The model problem considered to substantiate these claims is solved by numerical means and combines the simple shear due to the rigid gliding of the upper crust (at the velocity of V s ) and the stretch resulting from the extension of the whole crust (at the velocity V e ). The rheological model accounts for dislocation creep of quartz, feldspar, and mica, the feldspar-to-mica reaction, and its prerequisite, which is the feldspar fracturing detected by the Mohr-Coulomb criterion. The one-dimensional (1-D) solution, which constrains shear bands to be horizontal, shows the depth partitioning in deformation mode between the simple shear of the low-viscosity deep crust and the stretching of the highly viscous midcrust. Strain localization occurs during rapid increase of the shearing velocity V s , corresponding to low values of the velocity ratio V e /V s . The 2-D solution (for V e /V s = 10−3) reveals the development of a periodic system of extensional shear bands, dipping at 30° toward the shearing direction at a depth of 12 to 14 km. Shear bands are formed after less than half a million years at the base of the reaction zone defined by the region where feldspar-to-mica reaction is completed. Shear bands do not propagate to greater depths because the pressure prevents the feldspar from fracturing and thus the reaction to occur. The periodic system of shear bands defines a midcrustal flat weakened zone within which the equivalent shear stress is enhanced by at least a factor of three at the shear band tips. Brittle fracture could thus occur within the midcrustal flat weakened zone, explaining therefore the microseismicity monitored at these depths in regions of active extension

Consequences of progressive eclogitization on crustal exhumation, a mechanical study

The very low water content of the granulitic unit of Holsnøy, in the Bergen Arcs, Norway, caused its partial metastable preservation throughout Caledonian burial end exhumation, leading to the observed mixture of completely eclogitized and uneclogitized rocks. The eclogitization of the granulite-facies protolith led to a density increase ∼10%, but also to a large rheological weakening that resulted in the localization of all deep ductile deformation in the eclogite fraction. We address the consequences on subduction dynamics of the concomitant evolution of large-scale density and rheology during progressive eclogitization of crust by comparing the behaviour of buried crust with various properties in the channel-flow model. Large-scale buoyancy and density are integrated in a single adimensional parameter, the 'exhumation number'α, which describes the capacity of the crust to exhume. For a crust whose eclogitization causes a very large viscosity decrease ('burial resistant' end-member), α goes through a maximum in the partially eclogitized zone, that is, there is a given proportion of eclogite for which the crust is strongly weakened but still significantly lighter than surrounding mantle. For this partially eclogitized crust with α max, the maximum possible downward flow is very low, this zone acts as a bottleneck and all incoming crust in excess is forced backwards and starts exhuming. This return flow zone is bounded downwards in the partially eclogitized zone by α max, while its upper limit propagates upwards at a constant rate. The curve α(eclogite fraction) controls the maximum proportion of eclogite of the crust that can be exhumed. We also demonstrate that large exhumation rates reached by certain UHP units are incompatible with any stationary flow regime, whatever the nature of subducted crust. The additional presence of a weak layer on top of the channel favours burial for narrow channels and exhumation for large ones, but does not qualitatively change the dynamics of the subduction channel. This study shows that deep crustal circulation in subduction zones and exhumation from large depths are controlled by the evolution of large-scale crustal properties with progressive eclogitization

Early Middle Paleozoic Intraplate Orogeny in the Ogcheon Belt (South Korea): A new insight on the Paleozoic buildup of east Asia

Laurent Jolivet est Professeur à l'Université d'Orléans depuis le 1er septembre 2009.International audienceThe polycyclic Ogcheon belt of South Korea represents the boundary between two Precambrian blocks. Investigation on the timing and kinematics of the block agglomeration is of considerable bearing on the understanding of the constitution of the Asian continent. We report here structural evidence showing that the earlier tectonic event dates back to early middle Paleozoic (the "Ogcheon tectonism" or "orogeny"). The ductile piling up of nappes of the Ogcheon belt corresponds to an intracontinental orogeny involving the opening and the subsequent closure of an aborted rift and not of a wide oceanic area. The nappes contain unequivocal evidence for ductile shearing with a northwest over southeast sense of movement. Field evidence includes the presence of S1 and S1-2 foliations bearing an extensive stretching lineation perpendicular to the belt, associated with a strongly noncoaxial deformation regime. The F1 isoclinal folds are commonly "A type" and sheath folds whose axes are parallel to the stretching lineation, whereas the southeast facing F2 recumbent folds have subhorizontal axes parallel to the belt. All lines of evidence (unconformity, superposed deformations, age of the metamorphism, etc...) imply that the D1-2 "Ogcheon tectonism" is pre-middle Carboniferous (Late Silurian-Early Devonian). The subsequent Indosinian (Middle Triassic) F3 upright synfolial folds that trend NNE-SSW and later structures clearly overprint the middle Paleozoic ductile thrusts and isoclinal folds. The early Paleozoic Ogcheon Supergroup is formed of a thick volcanosedimentary pile deposited above an early platform sequence in a rift basin. On the basis of the geochemical characteristics of the metavolcanics, it appears that oceanization did not occur and Ogcheon rift aborted soon after it was formed. We suggest that middle Paleozoic ductile nappe structures have been formed by the structural inversion of the rift extensional features. During the ductile stacking episode, the early formed foliation and isoclinal folds have been folded in a continuum during the same tectonic event. Middle Carboniferous terrigenous formations have been deposited unconformably upon folded and thrust older rocks. A tentative correlation with early Paleozoic Imjingang, Qinling and Cathaysian belts within the Asian continent reveals that Ogcheon rift was emplaced within the South China plate and that Imjingang belt should represent the limit between North and South China blocks in the Korean peninsula

Deciphering orogenic evolution

International audienceDeciphering orogenic evolution requires the integration of a growing number of geological and geophysical techniques on various spatial and temporal scales. Contrasting visions of mountain building and lithospheric deformation have been proposed in recent years. These models depend on the respective roles assigned to the mantle, the crust or the sediments. This article summarizes the contents of the Special Issue dedicated to 'Geodynamics and Orogenesis' following the 'Réunion previous termdesnext termprevious termSciencesnext termprevious termdenext termprevious termlaprevious termTerre' 2010 conference held in Bordeaux, France. Further, based on the example of the Western Alps-Mediterranean domain we emphasize the possibility to integrate long and short term, plate- to sample-scale, datasets in order to constrain orogenic evolution

Kinematics, topography, shortening, and extrusion in the India-Eurasia collision

Laurent Jolivet est Professeur à l'Université d'Orléans depuis le 1er Septembre 2009International audienceWe examine the problem of partitioning between shortening and extrusion in the India-Asia collision since 45 Ma. We compute the amount of shortening expected from the kinematics of India's motion with respect to Eurasia, using the reconstruction at collision time to put bounds on the possible amounts of surface loss within Greater India and within Eurasia. We then compute the amounts of surface loss corresponding to the thickened crust of Tibet and of the Himalayas, assuming conservation of continental crust. The spatial distribution of the topography reveals a large systematic deficit of crustal thickening distributed rather uniformly west of the eastern syntaxis but an excess of shortening east of it. This distribution indicates an important eastward crustal mass transfer. However, the excess mass east of the eastern syntaxis does not account for more than one third to one half of the deficit west of the eastern syntaxis. The deficit may be accounted either by loss of lower crust into the mantle, for example through massive eclogitization, or by lateral extrusion of nonthickened crust. A mass budget of the crust of the Himalayas indicates that lower crust has not been conserved there, but the deficit is so large that local loss in the mantle is unlikely to be the unique cause of the deficit. Alternatively, following Zhao and Morgan [1985], lower crust may have been transferred below the Tibetan crust. We conclude that a combination of possible transfer of lower crust to the mantle by eclogitization and lateral extrusion has to account for a minimum of one third and a maximum of one half of the total amount of shortening between India and Asia since 45 Ma. This conclusion leaves open the possibility that the partitioning between extrusion and loss of lower crust into the mantle on the one hand and shortening on the other hand has significantly changed during the 45 m.y. history of the collision

Right-lateral shear along the Northwest Pacific Margin and the India-Eurasia Collision

International audienceRight-lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull-apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right-lateral shear is a consequence of indentation of Asia by India. As in small-scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left-lateral shear zones (Tien Shan-Baikal-Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re-emphasize the role played in the opening of marginal basins by the Pacific subduction as a free boundary to the east