Patrick Ledru

The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution

This paper is a synthesis of available data on the Velay dome that include both small- and large-scale lithologic and structural mapping, strain analysis, isotope geochemistry, geochronology and pressure–temperature estimates. The Velay dome, one of the largest granite–migmatite domes of the Variscan Belt, formed during orogenic collapse at around 300 Ma. Its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the Variscan orogenic crust. A first melting stage developed in connection with south-verging thrust zones during the Early Carboniferous, leading to a crustal thickening estimated at 20 km minimum. The involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water-saturated melts. The volume of biotite granite extracted from melt during this period was limited. The second phase of melting, corresponded to generalized melting of gneiss achieved by biotite-dehydration melting reactions and accompanied by the generation of cordierite-bearing granites. At this stage, crustal-scale detachment faults were activeandpartiallyobliteratedtheearlierstructures.Thenewstructureswereprogressivelytiltedtotheverticalatthemarginofthe Velay dome due to the southward and lateral ballooning of the granitic dome. The reconstructed P, T path indicate that the large volume ofmelt produced was a consequence ofa significant increase in temperature at the onset of biotite dehydration melting. At the base of the crust, this melting event is coeval with granulite facies metamorphism associated to underplating of mantle-derived magmas as suggested by the geochemical signature of Late Paleozoic lower crustal xenoliths sampled by Cenozoic volcanoes and withtheisotopicsignatureofthelategraniticintrusions.Accordingly,itisproposedthatasthenosphericupwellingwasresponsible for the temperature increase favoring melting of hydrous minerals. D 2001 Published by Elsevier Science B.V.

Lithologic tomography: an application to geophysical data from the Cadomian belt of northern Brittany, France

Abstract A probabilistic description of subsurface lithologic structures can be established by inverting multidisciplinary geophysical data constrained by geological and geostatistical priors. The methodology is based on the joint modelling of several media properties and on a statistical description of the relationships between them. The information provided by the geophysical data and the geological and geostatistical priors is represented by probability density functions ( pdf ) that are combined into a posterior pdf composed by: (1) a prior pdf in the space of the primary (lithologic) model parameters, (2) a pdf of the secondary (physical) model parameters conditional to the primary model parameters and (3) a joint likelihood function that is the product of the independent likelihood functions for each observed geophysical field. Applying a Markov chain sampling method enables a large sample of joint models to be generated from the posterior pdf . The true configuration of the media is then determined from the representation of models pulled from the chain and the elaboration of statistics from the large sample of posterior joint models. The method was used to invert gravity and magnetic data jointly characterising the mass density field, the magnetic susceptibility field and the lithotype field along two 2-D sections of the geological units in the Cadomian belt of northern Brittany. Besides generating 10 6 joint models consistent with the observations and priors, some features of the joint models and the statistical tomographic images provided additional insights to the geologic configuration of the area. For example, the Main Cadomian Thrust shows an irregular geometry that could have resulted from the belt emplacement and/or from Variscan tectonism, and the Hercynian granitic intrusion shows a deep subsurface continuation. The cosimulation of magnetic susceptibility and mass density inside each lithologic region was performed according to a multivariate Gaussian model or a mixed multivariate Gaussian functions model that was developed specially to describe multimodal distributed properties.

Contrasting styles of deformation during progressive nappe stacking at the southeastern margin of the Bohemian Massif (Thaya Dome)

Abstract Variscan nappe stacking at the southeastern margin of the Bohemian Massif is a polyphase process implying a progressive modification of the deformation regime from thrusting to wrenching, associated change of finite strain axes orientations and progressive downwards migration of thrust, or wrench, planes. A first high temperature deformation phase ( D 1 ) is related to nappe stacking of Moldanubian units in a NW-SE direction; it is recorded only in these units. D 2 corresponds to the piling of the Moldanubian units over the Moravian ones producing internal nappe stacking of the latter and a first medium-temperature mylonitization of Thaya basement rocks. The main feature of this stage is a switch in finite strain axes orientation and northeastward transpressional shearing. Strain partitioning is important within the Moravian nappes and implies alternation of plane strain, oblate, and constrictional, fabrics. D 3 is also a phase connected with strong non-coaxial NE-directed shearing. It is concentrated in zones of weakness and causes reactivation of older structures. The structural pattern strongly depends on the position in the nappe pile: strike-slip tectonics dominate near the resistant eastern Cadomian basement border, foliation-parallel extensional reactivation producing large-scale folds is typical in Moravian nappes, while extensional tectonics prevail in the uppermost Gfohl nappe. Large-scale strain partitioning during D 2 stacking of Moravian nappes and the extension of the upper part of the nappe pile during D 3 reactivation are mechanically controlled by a lateral ramp basement geometry along the Bruno-Vistulian Cadomian foreland.

Strain patterns within the late Variscan granitic dome of Velay, French Massif Central

The Velay granites are intruded as a late Variscan dome in the French Massif Central. Strain patterns within the Velay granites reflect the geometry and kinematics of late Carboniferous deformation, at the site of final emplacement. The internal deformation developed during granitic magmatism is not homogeneously distributed. From inner to outer zones increasing deformation is marked by changes from high-temperature foliations to S-C orthogneisses. Strain concentration at the margins reflects a strength contrast due to cooling and subsequent magma hardening. According to the internal strain field two zones can be distinguished in the Velay dome: (i) a central and southern zone where concentric patterns indicate a southward expansion of granites; and (ii) a northern zone where the lateral expansion is limited and combined with top-to-the-north extensional shearing along the Mont Pilat detachment zone and high-temperature wrenching. These asymmetric strain patterns relate to the expansion and deformation of granites emplaced below a detachment fault, during the uplift of a thickened area. Post-thickening combination of extensional and wrench tectonics is inferred.

Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian–Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Riouperoux–Livet plutono-volcanic units. Micaschist slices have also been inserted within the Riouperoux–Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Riouperoux–Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest–southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt.

Syn-thinning pluton emplacement during Mesozoic extension in eastern Mongolia

[1]xa0This paper documents relationships between deformation and magmatic activity that occurred in the central part of eastern Mongolia during late Mesozoic continental-scale NW-SE extension. Two coarse-grained, biotite-bearing, syn-tectonic intrusions are described. The Nartyn granite that extends over an area greater than 30 by 10 km was emplaced within low-grade metasediments and shows a weak pervasive, magmatic fabric reworked by solid-state deformation along its margins. The northwestern roof of the granite is marked by a normal shear zone, the Choyr Shear Zone, characterized by top-to-the-NW motions. The shear zone is overlain by the Choyr Basin, which is filled with unmetamorphosed continental sedimentary rocks of early Cretaceous ages. From structural and geochronological data, we propose that the Nartyn massif was emplaced as a flat laccolith-type intrusion at ca. 136–130 Ma during crustal thinning. The Altanshiree granite, located ∼140 km east of the Nartyn granite, is a syn-kinematic pluton of similar age (134–128 Ma), also emplaced during crustal thinning. In the Nartyn and Altanshiree areas, extension implies pervasive crustal thinning, combined with limited exhumation. These areas are different from classical metamorphic core complexes, where strong strain localization along detachments induces exhumation of hot middle to lower crust. Results also suggest that early Cretaceous syn-extensional intrusions are an important feature of the tectonic history of eastern Mongolia.