Jean-Marc Lardeaux

HP-LT rocks exhumed during intra-oceanic subduction: the example of the Escambray massif (Cuba).

High-pressure (HP) metabasites from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origin and evolution of the Northern Caribbean boundary plate during the Cretaceous, in a global subduction context. Geochemical and petrological studies of these eclogites reveal two groups with contrasting origins and pre-subduction metamorphic histories. Eclogites collected from exotic blocks within serpentinite (mélange zone) originated from a N-MORB type protolith, do not record pre-eclogitic metamorphic history. Conversely eclogites intercalated in Jurassic metasedimentary rocks (non-mélange zone) have a calc-alkaline arc-like origin and yield evidence for a pre-subduction metamorphic event in the amphibolite facies. However, all the studied Escambray eclogites underwent the same eclogitic peak (around 600 C at 16 kbar), and followed a cold thermal gradient during their exhumation (estimated at around 13.5 C km), which can suggest that this exhumation was coeval with subduction. Concordant geochronological data (Rb ⁄ Sr and Ar ⁄Ar) support that the main exhumation of HP ⁄LT rocks from the Sancti Spiritus dome occurred at 70 Ma by top to SW thrusting. The retrograde trajectory of these rocks suggests that the north-east subduction of the Farallon plate continued after 70 Ma. The set-off to the exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematics. The contrasting origin and ante-subduction history of the analysed samples imply that the Escambray massif consists of different geological units that evolved in different environments before their amalgamation during exhumation to form the present unit III of the massif.

The Moldanubian Zone in the French Massif Central, Vosges/Schwarzwald and Bohemian Massif revisited: differences and similarities

Abstract In order to portray the main differences and similarities between the Northeastern Variscan segments (French Massif Central (FMC), Vosges, Black Forest and Bohemian Massif (BM)), we review their crustal-scale architectures, the specific rock associations and lithotectonic sequences, as well as the ages of the main magmatic and metamorphic events. This review demonstrates significant differences between the ‘Moldanubian’ domains in the BM and the FMC. On this basis we propose distinguishing between the Eastern and Western Moldanubian zones, while the Vosges/Black Forest Mountains are an intermediate section between the BM and the FMC. The observed differences are the result of, first, the presence in the French segment of an early large-scale accretionary system prior to the main Variscan collision and, second, the duration of Saxothuringian/Armorican subduction, which generated long-lived magmatic arc and back-arc systems in the Bohemian segment, while the magmatic activity in the FMC was comparably short-lived.

Late tectonic and metamorphic evolution of the Piedmont accretionary wedge (Queyras Schistes lustrés, western Alps): Evidences for tilting during Alpine collision

This paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material. We present structural data based on the analysis of ductile, brittle-ductile, and brittle deformation features in metapelitic schists and meta-basalts from the Schistes lustres nappe stack in the Piedmont zone of the western Alps. To link the structures with synkinematic pressure and temperature estimates, geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition. Samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale. The early generation of veins associated with boudinage occurred at 415–345 °C and 6–2.5 kbar, and subsequent incipient normal faulting occurred at temperatures of 315–235 °C and pressures <1.7 kbar. The transition from ductile to brittle extension occurred between ca. 27 Ma and ca. 10 Ma. These results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional-scale tilting of the exhumed Schistes lustres tectonic pile. The extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal Alpine zones as the Alpine collision continues.

Thermal and mechanical evolution of an orogenic wedge during Variscan collision; an example in the Maures-Tanneron Massif (SE France)

Abstract Synthesis of structural, petrological and geochronological data for the Maures–Tanneron Massif and its integration in the framework of adjacent massifs (i.e. Sardinia and Corsica) has allowed us to propose a new model of evolution for the southern Variscan belt. After Siluro-Devonian subduction associated with high-pressure–low-temperature (HP/LT) metamorphism M0 (c. 10–15 °C km−1) and subsequent Carboniferous nappes stacking, the belt underwent strong reworking related to back-thrusting. Nappes stacking and back-thrusting were associated with typical Barrovian metamorphism M1 (c. 20–30 °C km−1) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M2 (c. 40–60 °C km−1) and M3 (c. 60–80 °C km−1) during 330–300 Ma in the internal part of the belt. Progressive increase of the thermal gradient is interpreted as a consequence of gravitational instabilities triggered in the partially molten orogenic root. Continuous compressive forces applied to the belt allowed vertical extrusion of the orogenic root in fold-dome structures. The mass transfer is accommodated by orogen-parallel transpressive shearing synchronous with M3 during Late Carboniferous time. The orogenic wedge is characterized by two main tectono-metamorphic units decoupled by a major shear belt: an Internal Zone with migmatites and syntectonic granitoids, where HP relicts have been exhumed, and an External Zone that escaped the late HT event and preserved precious structures.

Timing of geothermal activity in an active island-arc volcanic setting: First 40Ar/39Ar dating from Bouillante geothermal field (Guadeloupe, French West Indies)

Abstract Mineral separates of adularia have been extracted from three samples of highly silicified hydrothermal breccias, newly discovered in the active Bouillante geothermal field (Guadeloupe archipelago), and investigated by 40Ar/39Ar geochronology in order to constrain the timing of geothermal activity in this part of the active Lesser Antilles island arc. The inverse isochron diagram indicates an age of 248±50 ka (2σ) for all adularia from one breccia sample (n=8), with an initial 40Ar/36Ar ratio of atmospheric composition (309±12 (2σ)) attesting that this age is valid. This age is concordant with the weighted mean age of 290±40 ka for the same sample. Adularia from other samples yields concordant ages. The obtained 40Ar/39Ar ages can be related either to the magmatic activity of the Bouillante Volcanic Chain (c. 850–250 ka ago) or to the initiation of the volcanic activity of the active Grande Découverte–Soufrière system (200 ka ago–present day). Our results demonstrate that the Bouillante hydrothermal event is coeval with change in the volcanic pulses previously recognized in the magmatic history of the studied area. The possible duration calculated for this hydrothermal activity requires at least two superposed volcanic pulses to be developed.

The Southeast France basin during Late Cretaceous times: The spatiotemporal link between Pyrenean collision and Alpine subduction

We present and discuss the Late Cretaceous evolution of the Southeast France Basin (SEFB) owing to the Pyrenean and Alpine belts. The available geological data (isopachs maps, boreholes and field data) were integrated in 3D GeoModeller software to build a 3D model of the geometry of the Cenomanian to Campanian sedimentary series of the Late Cretaceous period. Maps, 3D block diagrams and cross-sections extracted from the 3D model reveal a significant eastward marine regression during the Late Cretaceous with an average velocity of 0.5 to 1 cm per year. According to the location of the Late Cretaceous depocenters, two sub-basins are recognized in the SEFB and correspond to “en-échelon” synclines filled by syn-buckling sediments. These events are related to the sub-meridian “Pyrenean-Provence” crustal shortening. During Campanian time, the deepening and the tilting of the SEFB are interpreted as a consequence of the subduction of the Alpine Tethys. The Late Cretaceous SEFB is the prolongation on the European foreland of the Alpine subduction trench.