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The transition area between the Pannonian Basin and the East Carpathians is the subject of considerable tectonic research in Central Europe because of its geological diversity, especially for Lithosphere–Asthenosphere Boundary determinations. The major objective of this study is to investigate the Earth’s lithospheric structure and in particular to determine the Lithosphere–Asthenosphere Boundary from the Pannonian Basin, through the Transylvanian Basin to the Carpathian Bend area. We recorded six new deep magnetotelluric soundings and used two archive measurements to complete the information and to put additional constraints on the depth of the Lithosphere–Asthenosphere Boundary. In the part of the discussions we provide a brief overview of the existing various methodologies and Lithosphere–Asthenosphere Boundary determinations for the wider Carpathian–Pannonian region and Europe and comparison with new magnetotelluric results. The lowest Lithosphere–Asthenosphere Boundary depth was detected in the Pannonian Basin (~50 km). Our results indicate in addition that the Lithosphere-Asthenosphere Boundary beneath the Transylvanian Basin (~70–80 km) is not thick and much thinner than those of the European and Moesian Platforms. The additional geophysical information (geomagnetic data and phase tensor results) detected the presence of deep well conductive zones towards the Pannonian Basin, the Bogdan Vodă–Dragoș Vodă fault and the East Carpathians. This could be explained by the elevated position of the well conductive asthenosphere in the Pannonian Basin and deep and presumably fluid rich tectonic zones associated with the Bogdan Vodă–Dragoș Vodă fault and the East Carpathians. The phase tensors highlighted that the most complex tectonic zones are present in the vicinity of the East Carpathians (where the average Lithosphere–Asthenosphere Boundary depth is >100 km), which is in line with the relatively young age of the mountain belt and the very complex nature of collisional orogens. [ABSTRACT FROM AUTHOR]

Apart from the lithosphere-asthenosphere boundary (LAB), mid-lithospheric discontinuities (MLDs) in thick and old continental lithospheres appear to play an important role in global plate tectonics. Initiation of intra-continental subduction, delamination of the lower continental lithospheric mantle and removal of cratonic roots are likely to occur along MLDs. Here we introduce the 'pargasosphere' hypothesis which could account for the origin of both boundaries. The observation that pargasitic amphibole is stable even at very low bulk 'water' concentration (~a few hundreds ppm wt.) implies that the solidus of the shallow upper mantle (<3 GPa) is usually the pargasite dehydration solidus at ~1100 °C. In young continental and oceanic lithosphere (<70 Ma) this solidus defines the LAB. The LAB separates the deeper, partial melt bearing asthenosphere from the shallower melt barren lithosphere, explaining their contrasting rheology. In old continents pargasite breaks down at the sub-solidus pargasite dehydration boundary at ~3 GPa and liberates 'water'-rich fluids. This latter process may be responsible for the formation of MLDs. The occurrence of partial melts or fluids beyond the pargasite stability field can explain commonly observed geophysical anomalies associated with the LAB and MLDs. We present forward modelled variations of shear wave velocity and resistivity at the LAB and MLDs for idealised lithospheric columns. These columns are constructed based on the 'pargasosphere' hypothesis and geotherms corresponding to continental lithospheres with various tectono-thermal ages. The 'pargasosphere' hypothesis offers a number of other empirically testable implications. For instance, cooling asthenosphere beneath young extensional continental and oceanic lithosphere settings can be the source of surface CO 2 emanations even at locations distant from areas with active volcanoes. The Vrancea zone (Eastern Europe) appears to be a suitable site for testing the 'pargasosphere' hypothesis for elucidating the origin of intermediate-depth earthquakes (70–300 km) and to explain the delamination of the lower continental lithospheric mantle. [Display omitted] • 'Pargasosphere' refers to part of the upper mantle where pargasite is stable. • The rheology of the shallow upper mantle (<100 km) is controlled by pargasite. • The upper limit of pargasite stability defines mid-lithosphere discontinuities. • Along this horizon, the lower part of the continental lithosphere can delaminate. • In the cooling asthenosphere, formation of pargasite can lead to CO 2 emission. [ABSTRACT FROM AUTHOR]