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Metamorphic studies in the cover sequences of the Bitlis complex allow the thermal evolution of the massif to be constrained using metamorphic index minerals. Regionally distributed metamorphic index minerals such as glaucophane, carpholite, relics of carpholite in chloritoid-bearing schists and pseudomorphs after aragonite in marbles record a LT-HP evolution. This demonstrates that the Bitlis complex was subducted and stacked to form a nappe complex during the closure of the Neo-Tethys. During late Cretaceous to Cenozoic evolution the Bitlis complex experienced peak metamorphism of 1.0-1.1 GPa at 350-400°C. During the retrograde evolution temperatures remained below 460°C. 39Ar/40Ar dating of white mica in dif erent parageneses from the Bitlis complex reveals a 74-79 Ma (Campanian) date of peak metamorphism and rapid exhumation to an almost isothermal greenschist stage at 67-70 Ma (Maastrichtian). The HP Eocene flysch escaped the greenschist facies stage and were exhumed under very cold conditions. These single stage evolutions contrast with the multistage evolution reported further north from the Amassia-Stepanavan Suture in Armenia. Petrological investigations and isotopic dating show that the collision of Arabia with Eurasia resulted in an assemblage of different blocks derived from the northern as well as from the southern plate and a set of subduction zones producing HP rocks with diverse exhumation histories. [ABSTRACT FROM AUTHOR]

Abstract: Eclogites and eclogites-facies rocks in mountain belts provide evidence that material in subduction zones material can return from depths of more than 100 km to the surface in the early stages of orogenic processes. Their relationship with lower metamorphic grade rock of i.e. blueschist and greenschist facies can provide information on the late stages of orogenic processes, but are often matter of debate. Petrography of metamorphic rocks in mountain belts has mainly focused on mafic rocks systems. Nevertheless, metamorphic domains of recent mountain belts like the Alps are not only constituted by mafic rocks but also by metasediments that continuously outcrop over very large areas. Such mountain belts are made of a large part of low-temperature metasediments devoid of index minerals classically observed in mafic and quartzo-feldspathic rocks systems, allowing a direct comparison to be made. These metasediments also have various chemical and mineralogical compositions that represent an important geothermobarometric potential. Thus we propose to study relationships between eclogites and less metamorphosed rocks for a significant case that has been long debate (Entrelor area, Western Alps). Despite metasediments continuously outcropping over the whole area, all previous metamorphic studies were carried out on dismembered sequences of mafic rocks. They have evidenced two kind of metamorphic evolution. Three explanations are generally proposed to interpret this feature: incertainties of the thermodynamic data, the loss of HP mineralogy during the exhumation and a late-stage tectonic juxtaposition. By studying simultaneously both eclogites and metasedimentary contry-rocks, we show that rocks association in the Entrelor area can be interpreted as a metamorphic mix. This area consists of eclogites rocks embedded in a blueschist facies matrix mainly made of metapelites. Exhumation of HP metamorphic rocks reveals that different pressure peaks (1.2 GPa at 450 °C vs. 2.3 GPa at 550 °C) were contemporaneous. The different types of rocks have been juxtaposed at a shallow crustal level within a subduction channel. Due to the fact the Western Alps do not reach the mature stage of a colliding belt, as the Central or as the Eastern Alps, the rocks of the Entrelor area can be viewed as an exhumed part of a frozen subduction channel attributed to a metamorphic mixing of rocks having different metamorphic evolution and accreted at great depths. [Copyright &y& Elsevier]

Abstract: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to collision in the Alps. Unlike previous models, which relate the fate of Alpine Tethys solely to relative motions of Africa, Iberia and Europe during opening of the Atlantic, our reconstruction additionally invokes independent microplates whose motions are constrained primarily by the geological record. The motions of these microplates (Adria, Iberia, Alcapia, Alkapecia, and Tiszia) relative to both Africa and Europe during Late Cretaceous to Cenozoic time involved the subduction of remnant Tethyan basins during the following three stages that are characterized by contrasting plate motions and driving forces: (1) 131–84Ma intra-oceanic subduction of the Ligurian part of Alpine Tethys attached to Iberia coincided with Eo-alpine orogenesis in the Alcapia microplate, north of Africa. These events were triggered primarily by foundering of the older (170–131Ma) Neotethyan subduction slab along the NE margin of the composite African–Adriatic plate; subduction was linked by a sinistral transform system to E–W opening of the Valais part of Alpine Tethys; (2) 84–35Ma subduction of primarily the Piemont and Valais parts of Alpine Tethys which were then attached to the European plate beneath the overriding African and later Adriatic plates. NW translation of Adria with respect to Africa was accommodated primarily by slow widening of the Ionian Sea; (3) 35Ma–Recent rollback subduction of the Ligurian part of Alpine Tethys coincided with Western Alpine orogenesis and involved the formation of the Gibraltar and Calabrian arcs. Rapid subduction and arc formation were driven primarily by the pull of the gravitationally unstable, retreating Adriatic and African slabs during slow convergence of Africa and Europe. The upper European–Iberian plate stretched to accommodate this slab retreat in a very mobile fashion, while the continental core of the Adriatic microplate acted as a rigid indenter within the Alpine collisional zone. The subducted lithosphere in this reconstruction can be correlated with slab material imaged by seismic tomography beneath the Alps and Apennines, as well as beneath parts of the Pannonian Basin, the Adriatic Sea, the Ligurian Sea, and the Western Mediterranean. The predicted amount of subducted lithosphere exceeds the estimated volume of slab material residing at depth by some 10–30%, indicating that parts of slabs may be superposed within the mantle transition zone and/or that some of this subducted lithosphere became seismically transparent. [Copyright &y& Elsevier]

Understanding the relationships between density and spatio-thermal variations at convergent plate boundaries is important for deciphering the present-day dynamics and evolution of subduction zones. In particular, the interaction between densification due to mineralogical phase transitions and slab pull forces is subject to ongoing investigations. We have developed a two-dimensional subduction zone model that is based on thermodynamic equilibrium assemblage calculations and includes the effects of melting processes on the density distribution in the lithosphere. Our model calculates the “metamorphic density” of rocks as a function of pressure, temperature and chemical composition in a subduction zone down to 250 km. We have used this model to show how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within the subduction system. These processes are largely neglected by other approaches that reproduce the density distribution within this complex tectonic setting. Our model demonstrates that the initiation of eclogitization (i.e., when crustal rocks reach higher densities than the ambient mantle) of the slab is not the only significant process that makes the descending slab denser and generates the slab pull force. Instead, the densification of the lithospheric mantle of the sinking slab starts earlier than eclogitization and contributes significantly to slab pull in the early stages of subduction. Accordingly, the complex metamorphic structure of the slab and the mantle wedge has an important impact on the development of subduction zones. [ABSTRACT FROM AUTHOR]

The main rationale of the IGCP 667 project is to perform a map (1/10 000 000) of world (ancient and modern) orogens and to represent them along the geological history of the Earth. Therefore, the definition of an orogen is critical and obliges us to go back to the fundamentals of geology by identifying the key elements of an orogen and place them accurately on a map in order to delineate where they sit relative to the cratons boundaries and the pre-existing orogens. This approach will emphasize the steps of evolution of the continents.Issues are multiples and include "accretionary" versus "orogenic" wedges and associated "internal" versus "external" zones, the magmatism associated to oceanic and to continental subduction and the relevance of separating syn- and post-collision magmatism. How to include the molassic basins, since they are successor (unconformable) basins although part of them is involved in the frontal and late deformation. In the concepts of orogens and suture zones, distinction is often made between intracontinental and ocean basins closure, whereas in fact we often observe in basins a transition via a propagator from ocean basin to intra-continental rift. In most cases, reactivation of former plate boundaries is observed and the map will highlight this aspect.Early discussions focused on how the main elements should be represented on such a map. In the current discussions, the legend would involve different type of orogens: (i) the subduction orogen including ophiolites, metamorphism, magmatism and accretionary wedge, (ii) the collisional orogen including metamorphism, syn-collisional magmatism, molassic basin, nappes and (iii) the compression or intracontinental orogen. A reflection will also be made for ancient orogens when plate tectonics did not exist. As we go backward in time, the main elements could be simplified due to a lack of data, a lack of understanding, or a strong erosion, which unroofed the upper layers of the orogen. [ABSTRACT FROM AUTHOR]