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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 (

In contractional (subduction/collisional) settings, convergence is accommodated by the formation of thin- and thick-skinned thrust and fold belts. The transmission of such deformation over larger distances into orogenic foreland areas is influenced by the inherited rheological characteristics of continental lithosphere. Lateral rheological variations parallel to the strike of continental foreland areas creates contrasting geometries and sequences of deformation that interact during orogenic build-up. We investigate the far-field transmission of strain within a continental lithosphere characterized by a laterally variable rheology through physical analogue modelling. Rheological weak crustal zones were introduced at distance from an advancing backstop to study the progressive along strike linkage and interference of structures during contraction. The results reveal that rheologically weak crustal zones localise far-field contractional deformation. When the size of weak zones, by means of their horizontal extend to depth ratio, is large, deformation localises at the boundaries of the weak zone where they lead to the formation of large-offset faults. Subsequently the faults migrate along-strike into areas that are rheologically stronger. When the size of the weak zone is reduced, a large-scale contractional step-over forms in orogenic forelands, where rheologically contrasting domains transmit out-of-sequence deformation by a gradual migration of thrust offsets and fold amplitudes along their strike. These results show that crustal scale orogenic step-overs do not always reflect variations in the geometry of the plate boundary (indenter) or along-strike gradients in shortening rates. Such features may also form in response to variations in rheology, as the ones created by inherited extensional basins situated at large distances from plate boundaries in the orogenic foreland.

Extensional basins often show along-strike variability in terms of fault geometries, basement structures, subsidence, and thermal evolution. This is particularly pronounced when extension reactivates preexisting suture zones with opposing dip directions, that is, opposite polarities, which creates wide strike-slip transfer zones. We have studied this extensional variability by means of thermomechanical lithospheric-scale 3-D numerical modeling. We conducted a series of experiments to model the extension of a thick lithosphere simulating a young orogenic area containing segmented suture zones inherited from former opposing subduction polarities, implemented as rheologically weak inclined layers with opposite dips. Numerical experiments demonstrate that the initial subduction sutures are reactivated by two long-lived lithospheric-scale detachment faults, which remain active until the onset of oceanic spreading. The opposite polarity of these faults causes their migration toward each other by asymmetric mantle lithosphere extension and thermal accretion. Crustal kinematics shows the formation of extensional detachment faults and high-offset listric normal faults. These structures undergo gradual tilting during their footwall exhumation, while the rheological weak suture layers are redistributed beneath the crust. The results demonstrate an active interaction along the strike of the system between the two opposite dipping weak zones, where fault segments mechanically interact. During extension, the maximum horizontal fault offset switches the apparent sense of shear in the separating strike-slip transfer zone. This kinematics explain the rapid change in the sense of shear in major strike-slip transfer zones or transform faults, such as observed during the extension of the Pannonian back-arc basin.

The tectonic evolution of sedimentary basins is the intrinsic result of the interplay between lithospheric stresses, lithospheric rheology, and thermal perturbations of the lithosphere–upper mantle system. The thermomechanical structure of the lithosphere exerts a prime control on its response to basin-forming mechanisms, in both extension and compression. Tectonic reactivation has strongly affected the structure and fill of many sedimentary basins. The long-lasting rheological memory of the lithosphere appears to play a far more important role in basin reactivation than hitherto assumed. The temporal evolution of the strength of continents and the spatial variations in stress and strength at continental margins, rifts, and orogenic belts govern the mechanics of basin development in time and space. Polyphase deformation is a common feature of many sedimentary basin systems. Compressional reactivation of extensional basins during their postrift phase appears to occur in many intraplate rifts and passive margins, reflecting temporal and spatial changes in the orientation and magnitude of the intraplate stress regime. Similarly, foreland basins are frequently characterized by preorogenic extension. The actual subsidence patterns of these polyphase systems are often more complex than predicted by end-member models that only consider the basin formation mechanism. Folding of the lithosphere, involving positive and negative deflections, appears to be of more importance in the large-scale deformation of intraplate domains than hitherto realized. In the intraplate domain of continental Europe that was thermally perturbed by Cenozoic upper mantle plume activity, lithospheric folding, for instance, plays an important role and strongly affects the pattern of vertical motions, in terms of both the basin subsidence and the uplift of broad arches. Tectonic processes operating during basin formation and during the subsequent deformation of basins can generate significant differential topography in basin systems. In view of the close link between erosion of topographic highs and sedimentation in subsiding areas, constraints are needed on uplift and coeval subsidence to validate quantitative process-oriented models for the evolution of sedimentary basins. Integration of analog and numerical modeling provides a novel approach to assess the feedback mechanisms between deep mantle, lithospheric, and surface processes.

A 2D gravity and magnetic data model has been constructed along a 71 km densely observed profile, called DACIA PLAN GRAV MAN's. The profile crosses part of the nappe pile of the south-eastern Carpathians and includes the seismically active Vrancea Zone and was acquired with the objective to illuminate the basement structure and affinity in this area. The modelling approach was to create an initial model from well constrained geological information, integrate it with previous seismic ray tracing and tomographic models and then alter it outside the a priori constraints in order to reach the best fit between observed and calculated potential field anomalies. The results support a realignment of the position of the TTZ (Tornquist-Teisseyre Zone), the profound tectonic boundary within Europe that separates Precambrian cratonic lithosphere of the East European Craton (EEC) from younger accreted lithosphere of Phanerozoic mobile belts to its west. The TTZ is shown to lie further to the south-west than was previously inferred within Romania, where it is largely obscured by the Carpathian nappes. The crust of the EEC beneath the south-eastern Carpathians is inferred to terminate along a major crustal structure lying just west of the Vrancea seismogenic zone. The intermediate depth seismicity of the Vrancea Zone therefore lies within the EEC lithosphere, generally supporting previously proposed models invoking delamination of cratonic lithosphere as the responsible mechanism.