Deciphering large-scale superposed fold systems at shallow crustal levels in collision zones: insights from the Marguareis Massif (southwestern Alps)

We present and discuss the results of a field-based approach including accurate geological mapping and micro- to map-scale structural analysis to highlight the finite strain pattern recorded in Marguareis Unit, a massif deformed at shallow crustal levels at the boundary between Maritime and Ligurian Alps. We describe superposed tectonic structures developed under low-grade metamorphic conditions during the Alpine collision and nowadays exceptionally well recorded in the area of interest. We demonstrate that the structural frame of the Marguareis Unit results from superposition of fourfold systems, later segmented, but without significant displacements, by brittle faults

How many subductions in the Variscan orogeny? Insights from numerical models

We developed a 2D numerical model to simulate the evolution of two superposed ocean-continent-ocean subduction cycles with opposite vergence, both followed by continental collision, aiming to better understand the evolution of the Variscan belt. Three models with different velocities of the first oceanic subduction have been implemented. Striking differences in the thermo-mechanical evolution between the first subduction, which activates in an unperturbed system, and the second subduction, characterised by an opposite vergence, have been enlighten, in particular regarding the temperature in the mantle wedge and in the interior of the slab. Pressure and temperature (P-T) conditions predicted by one cycle and two cycles models have been compared with natural P-T estimates of the Variscan metamorphism from the Alps and from the French Massif Central (FMC). The comparative analysis supports that a slow and hot subduction well reproduces the P-T conditions compatible with data from the FMC, while P-T conditions compatible with data of Variscan metamorphism from the Alps can be reproduced by either a cold or hot oceanic subduction models. Analysing the agreement of both double and single subduction models with natural P-T estimates, we observed that polycyclic models better describe the evolution of the Variscan orogeny

Permian high-temperature metamorphism in the Western Alps (NW Italy)

During the late Palaeozoic, lithospheric thinning in part of the Alpine realm caused high-temperature low-to-medium pressure metamorphism and partial melting in the lower crust. Permian metamorphism and magmatism has extensively been recorded and dated in the Central, Eastern, and Southern Alps. However, Permian metamorphic ages in the Western Alps so far are constrained by very few and sparsely distributed data. The present study fills this gap. We present U/Pb ages of metamorphic zircon from several Adria-derived continental units now situated in the Western Alps, defining a range between 286 and 266 Ma. Trace element thermometry yields temperatures of 580-890°C from Ti-in-zircon and 630-850°C from Zr-in-rutile for Permian metamorphic rims. These temperature estimates, together with preserved mineral assemblages (garnet-prismatic sillimanite-biotite-plagioclase-quartz-K-feldspar-rutile), define pervasive upper-amphibolite to granulite facies conditions for Permian metamorphism. U/Pb ages from this study are similar to Permian ages reported for the Ivrea Zone in the Southern Alps and Austroalpine units in the Central and Eastern Alps. Regional comparison across the former Adriatic and European margin reveals a complex pattern of ages reported from late Palaeozoic magmatic and metamorphic rocks (and relics thereof): two late Variscan age groups (~330 and ~300 Ma) are followed seamlessly by a broad range of Permian ages (300-250 Ma). The former are associated with late-orogenic collapse; in samples from this study these are weakly represented. Clearly, dominant is the Permian group, which is related to crustal thinning, hinting to a possible initiation of continental rifting along a passive margin

A relict blueschist in meta-ophiolite from the central Norwegian Caldedonides-discovery and consequences

Lithos, v. 60, p. 1-19, 2002International audienc

New Advances in W-Mediterranean Early Neolithic Pottery Sourcing and Technology,

International audienc

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

International audienceSynthesis 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 M 0 (c. 10– 15 8C km 21) 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 M 1 (c. 20–30 8C km 21) starting at 360 Ma that progressively evolved to higher temperature metamorphisms M 2 (c. 40– 60 8C km 21) and M 3 (c. 60– 80 8C km 21) 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 M 3 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

Lithospheric shear zones and mantle-crust connections

cited By 88A crustal-scale ductile shear zone network in the Precambrian granulite-facies crust of Madagascar is examined to determine the nature of the connections between the mantle and lower crust. Based on three independent data sets - field and satellite mapping, C- and O-isotope geochemistry and gravimetry - this crust is divided into three zones: (1) outside of shear zones; (2) minor shear zones that are <140 km long and 7 km wide; and (3) major shear zones that are >350 km long (up to 1000 km) and 20-35 km wide. The mantle is uplifted by about 10 km beneath the major shear zones. The major shear zones are rooted in and are inferred to be controlled by the mantle; they directly tapped mantle-derived CO2. The small-scale minor shear zones were controlled by crustal processes and focused crustally derived H2O-rich±CO2 fluids. The regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context. The propagation of these mechanical instabilities promoted and channelled fluid flow. These major Pan-African shear zones thinned the crust and were reactivated during the subsequent drifting of Madagascar and opening of the Indian Ocean during Jurassic to Cretaceous times. They also controlled many of the brittle fault zones in the overlying sedimentary basins. Mantle-rooted large-scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

Discovery of Lower Cretaceous synmetamorphic thrust tectonics in French Lesser Antilles (La Désirade Island, Guadeloupe): Implications for Caribbean geodynamics

International audienceLocated east of Guadeloupe, the island of La Désirade exhibits the oldest rocks of the Lesser Antilles arc and the eastern Caribbean plate. An old magmatic basement is composed of (1) late Jurassic ophiolitic complex with meta-basaltic pillow lavas and interbedded radiolarites, (2) acid igneous complex comprising meta-quartz-diorite and meta-rhyolitic lavas flows, and (3) meta-diabasic/microdioritic dyke swarm complex. We present and discuss the discovery of synmetamorphic thrust tectonics in this island. Based on detailed structural analysis we evidenced two main compressive events. The first event (D1) is characterized by pervasive folding associated with thrust development. In the northeastern part of the island, a major thrust fault, the Grand Abaque Thrust, has been discovered which displaces for several kilometers the acid igneous complex onto the ophiolitic unit. The second deformation event (D2) is featured by a main pervasive cleavage S2 observed at the regional scale associated with upright folding (F2) and by the development of two sets of conjugated steeply dipping strike-slip shear zones, dextral 130°N and sinistral 20°N striking, respectively. Microstructural observations indicate that Greenschist facies metamorphism is clearly contemporaneous with the development of D1 and D2 superimposed tectonic structures. Ar/Ar geochronology was performed on two bulk of adularia minerals sampled in the northeast volcanic complex within a deformed zone related to a D2 event. These analyses yield well-defined and concordant plateau ages at 106.2 ± 1.7 Ma and 107.2 ± 1.8 Ma. Thus, shortening and thickening tectonics occurred during Lower Cretaceous (Albian). In the available framework of Caribbean geodynamics, the synmetamorphic thrust tectonics we discovered in the French Lesser Antilles is related to the collision between an Aptian-Albian oceanic plateau and the Andean-Cordilleran east-dipping subduction zone. Our results support subduction polarity reversal during Lower Cretaceous

Metamorphic structure of the Western and Ligurian Alps

Mitt. Österr. Miner. Ges., v. 149, p. 125-144, 2004International audienc