Eclogitic metamorphism in the Alpine far-west: petrological constraints on the Banchetta-Rognosa tectonic unit (Val Troncea, Western Alps)

The Banchetta-Rognosa tectonic unit (BRU), covering an area of 10 km2 in the upper Chisone valley, consists of two successions referred to a continental margin (Monte Banchetta succession) and a proximal oceanic domain (Punta Rognosa succession) respectively. In both successions, Mesozoic meta-sedimentary covers discordantly lie on their basement. This paper presents new data on the lithostratigraphy and the metamorphic evolution of the continental basement of the Monte Banchetta succession. It comprises two meta-sedimentary sequences with minor meta-intrusive bodies preserving their original lithostratigraphic configuration, despite the intense Alpine deformation and metamorphic re-equilibration. Phase equilibrium modeling points to a metamorphic eclogitic peak (D1 event) of 20–23 kbar and 440–500 °C, consistent among three different samples, analyzed from suitable lithologies. The exhumation P–T path is characterized by a first decompression of at least 10 kbar, leading to the development of the main regional foliation (i.e. tectono-metamorphic event D2). The subsequent exhumation stage (D3 event) is marked by a further decompression of almost 7–8 kbar associated with a significant temperature decrease (cooling down to 350–400 °C), implying a geothermal gradient compatible with a continental collision regime. These data infer for this unit higher peak P–T conditions than previously estimated with conventional thermobarometry. The comparison of our results with the peak P–T conditions registered by other neighboring tectonic units allows to interpret the BRU as one of the westernmost eclogite-facies unit in the Alps.

A heterogeneous subcontinental mantle under the African–Arabian Plate boundary revealed by boron and radiogenic isotopes

The northern and northwestern margins of the Arabian Plate are a locus of a diffuse and long-lasting (early Miocene to Pleistocene) Na-alkali basaltic volcanism, sourced in the asthenosphere mantle. The upwelling asthenosphere at the Africa–Arabia margin produces very limited magma volumes in the axial zone. Therefore, portions of hot, fertile mantle continue their eastward migration and are stored at shallower depths under the 100-km thick Arabian lithosphere, which is much thinner than the African one (≈175 km): this causes the occurrence and 20-Ma persistence of magma supply under the study area. Erupted basalts sampled a continuous variation of the mantle source, with a striking correlation among temperature, pressure and isotopic composition shifting between two end members: a 100 km-deep, more depleted source, and a 60 km-deep, more enriched one. In particular, we observed an unusual variation in boron isotopes, which in the oceanic domain does not vary between more depleted and more enriched mantle sources. This study shows that, at least in the considered region, subcontinental mantle is more heterogeneous than the suboceanic one, and able to record for very long times recycling of shallow material.

Subduction and roll-back of narrow oceanic slabs: Back-arc basin modelling of the Carpathians subduction zone

<p>The Carpathians subduction system evolved similarly to many Mediterranean systems where extensional back-arc basins and separate large sag basins develop in the overriding plate. The evolution of such basins can be explained in the context of roll-back of narrow oceanic slabs. Their evolution is linked to extensional and sag back-arc basins, retreating orogenic systems and slab detachment. A recent example of slab detachment can be studied by the Vrancea slab beneath the SE Carpathians.<br>Significant effort has been dedicated to modelling such Mediterranean-style subduction systems, and in most cases the model was set up with a narrow oceanic domain, which has an increased difficulty to create rollback due to reduced buoyancy of the slab.<br>Our approach is to use a two-dimensional thermo-mechanical numerical model that introduces an inherited oceanic domain, which adds to the younger, narrow ocean developed in the later stages.<br>Our model can produce sustained subduction of the oceanic slab associated with roll-back and slab detachment. In most of our models a retro-arc sag basin develops, which can be interpreted as the Transylvanian Basin. This sag basin is one of the most consistent features of our model. At larger distances from the subduction zone, the extensional back-arc of the Pannonian basin can be modelled by introducing an lithospheric weakness zone, which represents a suture zone inherited from a previous orogenic evolution. Such a suture zone is compatible with the overall orogenic evolution of the Alps-Carpathians-Dinarides system. We furthermore discuss the limitations of our 2D modeling in the overall 3D settings of the Carpathians system and possibilities of future integration.</p>

Obliquity of the Eastern Mediterranean Sea rifted margins: Reconciling structures and kinematic models.

<p>Orthogonal, oblique and transform rifted margins are defined by the comparison of the structural trend of the margin versus the orientation of the oceanic spreading ridge marked by marine magnetic anomalies. However, when neither transform fault nor marine magnetic anomalies can be identified in the oceanic domain, the determination of the obliquity of extension is delicate and deduced from the architecture of the rifted margins. This setting is illustrated by the Eastern Mediterranean Sea, which is a relic of an oceanic domain, now partly subducted northward underneath Anatolian, Aegean and Calabrian domains. Although the Southern and Eastern margins, from Malta to Lebanon, escaped compressional reactivation during Late Cretaceous and Cenozoic, their potential orthogonal, oblique or transform components have been the subject of extensive debates. Multiple geodynamic scenarios implying different ages and directions of oceanic opening have been proposed suggesting that either the southern or the eastern margins had a transform motion (or highly oblique).</p><p>In this contribution, we investigate the architecture of the different margin segments using 2D and 3D seismic data combined with available stratigraphic records and potential field maps. Based on these observations, we identified and mapped the different rift domains of the Eastern Mediterranean margins, adapting the terminology developed for hyper-extended rifted margins. The Eastern Mediterranean rifted margins are characterized by Mesozoic thick post-rift carbonate platforms developed over moderately thinned continental crust. Distal domains are dominated by thick sedimentary basins (>10 km) where the top basement is barely visible on reflection seismic data. Between the carbonate platform and the distal basin, the transition is always sharp (<30km in width) and marked by large normal faults. The resulting rift domain map highlights different structural trends, which are not coherent with a simple pair orthogonal-transform margins. Moreover, we reconstructed the extensional evolution of the former Northern and Western conjugate margins, which are now integrated in the Alps, Balkanides, Hellenides and Taurides by compiling boreholes and onshore geological data. These fossil margins recorded evidence for different tectonic extensional phases from Permian to Cretaceous.  </p><p>Our preliminary conclusion suggests that poly-phased and poly-directional extension led to distinct breakup ages in the Herodotus and northern Levant Basins. It results in the superposition of extensional structures of different orientations and ages, which inhibit the clear determination of orthogonal, oblique or transform margins. We tentatively explain this architectural complexity by the close position of the East Mediterranean Sea to the migrating rotation pole between Africa and Eurasia during the Mesozoic in relation with the Central Atlantic spreading to the West and the multiple subduction systems of the Neo-Tethys to the North.</p>

Extrusion of subducted crust explains the emplacement of far-travelled ophiolites

Continental subduction below oceanic plates and associated emplacement of far-travelled ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit continental rocks that experienced subduction-related high pressure-low temperature (HP-LT) metamorphism and subsequent exhumation coeval with the emplacement of ophiolites. However, the link between continental subduction dynamics and ophiolite emplacement is poorly understood. Here we combine data collected from ophiolite belts worldwide with thermo-mechanical simulations of continental subduction dynamics to show the causal link between the exhumation of subducted continental crust and ophiolite emplacement. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. This process is fundamental for the formation of a far-travelled ophiolite sheet that is separated from the oceanic domain by the exhumed, HP-LT continental upper crust. Our results indicate that the exhumation of the subducted continental crust and far-travelled ophiolite sheet emplacement are inseparable processes and thus shed light on one of the most mysterious aspect of plate tectonics.

The final closure of the Vardar Ocean: paleomagnetic, AMS and structural results

<p>The Vardar zone divides units of African affinity from units of the European margin. It is characterized by extensional opening of an oceanic domain during the Triassic and Jurassic followed by divergent simultaneous obduction of the oceanic litoshphere over the continental units in the Upper Jurassic. However, a stripe of the oceanic domain persisted till the Cretaceous and Paleogene convergence. The remnants of the last closing part of the Vardar ocean are found in the Sava zone.</p><p>In this paper recently published and new paleomagnetic, AMS results in combination with structural observations will be presented from Upper Cretaceous sediments and Oligocene –Lower Miocene igneous rocks representing the areas bordering the Sava zone from the western and eastern sides, respectively and from the upper Cretaceous flysch deposited in the Sava zone.</p><p>In the areas W and E of the Sava zone, respectively, the primary remanences of the igneous rocks point to post-Oligocene CW rotation of about 30°. The sediments carry secondary magnetizations, imprinted during magmatic activity. Compared to the areas flanking it, the sediments of the Sava zone were intensively folded during the Upper Cretaceous and Paleogene and the paleomagnetic signals, which exhibit smeared distribution close to the present N, are of post-folding age. The AMS foliation and bedding planes are sub-parallel, thus the deformation must have been weak. Fold axes and AMS lineations are roughly N-S oriented, pointing to the deformational origin of the AMS lineations. These observations form the Sava zone will be discussed in the context of the post-Oligocene CW rotation of the flanking areas and the general NE-SW orientation of the compressional stress field outside of the zone.</p><p>Acknowledgement. This work was financially supported by the National Development and Innovation Office of Hungary, project K 128625 and by the Ministry of Education and Science of the Republic of Serbia, project 176015.</p>