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Abstract: A change in contractional polarity occurs when the direction of tectonic transport switches along strike. This switch is conditioned by lateral variations in rheology or inherited asymmetries, such as contrasts in structure or changes in the polarity of subduction zones. The parameters controlling contractional polarity changes are less understood in situations when the strain is transferred at large distances from indenters. Analysing this type of strain transfer is critical for understanding the mechanics of thrusting in fore- or back-arc settings of orogenic areas. Comparison of crustal-scale analogue modelling with the inversion of the Black Sea back-arc and the formation of the New Guinea–New Britain fore-arc suggest that far-field changes in contractional polarity are related to rheological contrasts across inherited normal faults. The initial extension creates rheological weak zones that localize the subsequent far-field contractional deformation along groups of thrusts with opposite vergence along the strike of the system. The largest amount of far-field contractional deformation is recorded in the transfer zone located between the two indenters moving in opposite directions and is particularly high when inverting oblique extensional systems. [Copyright &y& Elsevier]

Abstract: The balance between extension and contraction in back-arc basins is very sensitive to a number of parameters related to on-going subduction and collision processes. This leads to complex back-arc geometries, where a lateral transition between crustal blocks with contrasting rheologies is often recorded. One good example is the back-arc region of the Balkanides–Pontides orogens, where lateral variations in rheologies are observed between the Balkanides–Moesian block and the Pontides–Western Black Sea Basin. The latter opened during Cretaceous–Eocene, and has been inverted together with the former starting during late Middle Eocene. The inversion generated contrasting geometries along the orogenic strike, with a narrow zone of high deformation in the Balkanides–Moesia region, wide areas of thrusting with low offsets in the Pontides–Western Black Sea Basin and a transitional zone characterized by highly curved geometries. This overall type of inversion is investigated here by the means of analogue modelling testing the role of inherited crustal geometries during inversion. Our modelling suggests that the contrasting architecture of inverted structures observed in the Balkanides–Pontides domain are the result of pre-existing crustal stretching geometries of various blocks inherited from the Cretaceous–Eocene extension. The stretched and weak back-arc basins can transfer contraction deformation at large distances, explaining structures derived by observational studies. The collisional deformation recorded in the Pontides was transmitted at large distances that are in the range of the contraction structures observed in the centre and northern part of the Western Black Sea. In the light of analogue modelling results we argue that the Western Black Sea was a rheologically weaker domain when compared with the adjacent western onshore at the beginning of the inversion, in contrast with previous results derived from numerical modelling studies that argued for a strong West Black Sea domain at the beginning of inversion. [Copyright &y& Elsevier]

Abstract: The surface expression of a lateral polarity change of continental mantle lithosphere subduction has been studied by using lithosphere-scale physical models. Key parameters investigated were: the degree of lateral coupling between adjacent domains of opposing subduction polarity, the width of the zone separating the domains, and the lithosphere geometry and rheology. The model results illustrate an asymmetric lithospheric structure induced by deformation of the downgoing plates, which have been separated by a narrow transition zone. A wide and symmetric orogenic wedge overlying a region of thickened mantle lithosphere and hampered subduction characterizes this transition zone. In addition, interaction between the neighboring subduction domains caused downbending of the upper plates and resulted in the lateral termination of crustal structures and lowering of surface topography. The lateral extent of interaction between the domains strongly depends on the degree of coupling between the domains, the rheology of the mantle lithosphere and the amount of bulk shortening. The modelling results have major implications on the interpretation of seismic and tomographic data from the European Alps in terms of the crust and lithosphere geometries. It appears that an observed lateral change of subduction polarity at mantle depth can explain the variations of wedge build-up between the Western/Central and Eastern Alps. [Copyright &y& Elsevier]

Abstract: Fundamental aspects of mountain building such as the dominant deformation mechanism, strain localisation, and topography development in response to the presence of decoupling zones, either along boundaries of weak and strong lithospheres or within weak domains of continental lithosphere, are studied by analogue lithospheric models. Scenarios are investigated where a weak part of the lithosphere is bordered on either side by stronger lithospheres. Geometries of the weak zone vary from box- to wedge- to graben shaped representing lateral strength variations as a consequence of earlier deformation phases. The rheologic and geometric configuration at the onset of shortening is therefore considered as representative for continental collision and intra-plate settings. The modelling results show that the geometric and topographic evolution of mountain belts through time is sensitive to the presence of decoupling horizons along plate or tectonic boundaries. This is because the decoupled boundaries localise strain, control the dominant deformation mechanism and hence the locus of uplift and subsidence. Accordingly decoupled boundaries favour deformation by thrusting and particularly, in the case of an initially wedge-shaped weak zone, the development of mountain belts with overall triangular shape. In contrast, lithospheres with coupled boundaries dominantly deform by buckling irrespective of the initial geometry of the weak zone. As this deformation mechanism affects all model domains it produces a distinguishable pattern of uplift and subsidence controlled by buckling. Strong decoupling at mid-crustal levels within the weak zone favours vertical strain partitioning and the formation of plateau shaped mountains. The modelling results suggest that natural collisional systems should respond to an increase in plate coupling through time by widespread uplift of the mountain belt including its foreland basins, which were formerly created under decoupled conditions. Due to the absence of decoupling zones, like plate contacts, weak domains in intra-plate settings are predicted to deform dominantly by buckling. [Copyright &y& Elsevier]

Abstract: The role of the plate boundary in a continental collisional setting is investigated by lithospheric-scale analogue models. Key variables in this study are the degree of coupling at the plate interface and along the Moho of the lower plate as well as the geometry of the plate contact. They control the onset of intra plate deformation, orogenic architecture, amount of mantle lithosphere subduction and basin development. In all experiments, deformation initiates at the plate interface by the formation of a pop-up structure. A vertical plate boundary with respect to the shortening direction results in buckling of the lithosphere, whereas experiments with an inclined plate boundary show underthrusting and foreland basin development without orogenic wedge formation. Continental collision and coinciding mantle lithosphere subduction may occur only if the lower crust of the foreland plate is weak enough promoting crust–mantle decoupling. During decoupling the weak lower crust beneath the orogen thickens significantly by ductile flow as it detaches from the down going mantle lithosphere. This lower crustal thickening effects the distribution of upper crustal deformation and topography. Subduction of weak lower crust is favored when the weak plate interface has a significant thickness (~15km in nature) and a high amount of shortening is applied. Increasing coupling at the plate interface through time leads to intra plate deformation by thickening and gentle folding and influences surface uplift and subsidence above the plate interface. The transition from a mechanically decoupled plate boundary with a significant amount of mantle lithosphere subduction towards stronger plate coupling resulting in intra plate deformation and topography development can be recorded in for instance the Caucasus, the Colombian Cordillera, the Pyrenees and the Alps. Thickening of the lower crust as portrayed for the Western Alps does not demand a strong, frictional-type behavior of the lower crust, but can also be the consequence of ductile processes. [Copyright &y& Elsevier]

The initiation and subsequent evolution of intra-plate orogens, resulting from continental plate interior deformation due to transmission of stresses over large distances from the active plate boundaries, is controlled by lateral and vertical strength contrasts in the lithosphere. We present lithospheric-scale analogue models combining 1) lateral strength variations in the continental lithosphere, and 2) different vertical rheological stratifications. The experimental continental lithosphere has a four-layer brittle–ductile rheological stratification. Lateral heterogeneity is implemented in all models by increased crustal strength in a central narrow block. The main investigated parameters are strain rate and strength of the lithospheric mantle, both playing an important role in crust–mantle coupling. The experiments show that the presence of a strong crustal domain is effective in localizing deformation along its boundaries. After deformation is localized, the evolution of the orogenic system is governed by the mechanical properties of the lithosphere such that the final geometry of the intra-plate mountain depends on the interplay between crust–mantle coupling and folding versus fracturing of the lithospheric mantle. Underthrusting is the main deformation mode in case of high convergence velocity and/or thick brittle mantle with a final asymmetric architecture of the deep lithosphere. In contrast, lithospheric folding is dominant in case of low convergence velocity and low strength brittle mantle, leading to the development of a symmetric lithospheric root. The presented analogue modelling results provide novel insights for 1) strain localization and 2) the development of the asymmetric architecture of the Pyrenees. [ABSTRACT FROM AUTHOR]