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Continental subduction below oceanic plates and associated emplacement of ophiolite sheets remain enigmatic chapters in global plate tectonics. Numerous ophiolite belts on Earth exhibit a far-travelled ophiolite sheet that is separated from its oceanic root by tectonic windows exposing continental crust, which experienced subduction-related high pressure-low temperature metamorphism during obduction. However, the link between continental subduction-exhumation dynamics and far-travelled ophiolite emplacement remains 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 extrusion of subducted continental crust and the emplacement of far-travelled ophiolites. Our results reveal that buoyancy-driven extrusion of subducted crust triggers necking and breaking of the overriding oceanic upper plate. The broken-off piece of oceanic lithosphere is then transported on top of the continent along a flat thrust segment and becomes a far-travelled ophiolite sheet separated from its root by the extruded continental crust. Our results indicate that the extrusion of the subducted continental crust and the emplacement of far-travelled ophiolite sheets are inseparable processes. The interplay between continental subduction exhumation dynamics and the obduction of ophiolite sheets remains enigmatic. Here, the authors show that the extrusion of the subducted continental upper crust triggers the necking and breaking of the oceanic upper plate and leads to far-travelled ophiolite sheet emplacement. [ABSTRACT FROM AUTHOR]

Although many different mechanisms for subduction initiation have been proposed, only few of them are viable in terms of consistency with observations and reproducibility in numerical experiments. In particular, it has recently been demonstrated that intra‐oceanic subduction triggered by an upwelling mantle plume could greatly contribute to the onset and operation of plate tectonics in the early and, to a lesser degree, modern Earth. On the contrary, the initiation of intra‐continental subduction still remains underappreciated. Here we provide an overview of 1) observational evidence for upwelling of hot mantle material flanked by downgoing proto‐slabs of sinking continental mantle lithosphere, and 2) previously published and new numerical models of plume‐induced subduction initiation. Numerical modeling shows that under the condition of a sufficiently thick (>100 km) continental plate, incipient downthrusting at the level of the lowermost lithospheric mantle can be triggered by plume anomalies of moderate temperatures and without significant strain‐ and/or melt‐related weakening of overlying rocks. This finding is in contrast with the requirements for plume‐induced subduction initiation within oceanic or thinner continental lithosphere. As a result, plume‐lithosphere interactions within continental interiors of Paleozoic‐Proterozoic‐(Archean) platforms are the least demanding (and thus potentially very common) mechanism for initiation of subduction‐like foundering in the Phanerozoic Earth. Our findings are supported by a growing body of new geophysical data collected in various intra‐continental areas. A better understanding of the role of intra‐continental mantle downthrusting and foundering in global plate tectonics and, particularly, in the initiation of "classic" ocean‐continent subduction will benefit from more detailed follow‐up investigations. Plain Language Summary: Subduction zones, areas where tectonic plates sink into the Earth mantle, are a vital component of the plate tectonic cycle and result in the opening and closure of oceans over geologic time. Despite major advances in the study of plate tectonics and geodynamics during the last decades, the causes for triggering subduction are still not well understood. In this paper, we review observations and insights from geodynamic models and show that upwelling inside the mantle (i.e., "mantle plumes or hot spots") offer a viable mechanism that takes lithospheric plates back into the deep mantle, thus initiating subduction. However, subduction initiated by mantle plumes within oceanic tectonic plates has rarely been observed in the Phanerozoic history of Earth (541‐0 Ma). In contrast, thick and old continental segments of tectonic plates appear to provide optimal conditions for their participation in plume‐induced sinking (subduction) into the interior of modern Earth in a mode detectable by geophysical imaging in different regions (Europe, Asia, North America). The role of such intra‐continental mantle sinking in global plate tectonics warrants thorough further examination in forthcoming studies. Key Points: We review insights from observational and numerical modeling studies on plume‐induced sinking of lithospheric mantlePlume‐induced mantle downthrusting within continental interiors and at their rims appears to be much more common than hitherto assumedPlausible consequences of intra‐continental mantle sinking for the operation of plate tectonics require further investigation [ABSTRACT FROM AUTHOR]

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]

Tearing of the lithosphere at the lateral end of a subduction zone is a consequence of ongoing subduction. The location of active lithospheric tearing is known as a Subduction-Transform-Edge-Propagator (STEP), and the tearing decouples the down going plate and the part of the plate that stays at the surface. STEPs can be found alongside many subduction zones, such as at the south Caribbean or the northern end of the Tonga trench. For the Caribbean, the San Sebastián/El Pilar fault zone represents the surface expression of the wide STEP fault between the Caribbean and South America, and the active STEP is located near Trinidad. However, what parts of the deeper lithosphere participate in the tearing process is largely unknown. Some constraints on the deformation of the deep part of the lithosphere are available from gravity, which suggests significant lateral variability in densities of the lithospheric mantle to the south of the STEP fault zone. A sub-crustal low-density zone beneath northern South America may result from higher sub-crustal temperatures, such as would arise from an asthenospheric window resulting from a wide STEP fault at depth. Here we investigate what controls the evolution and geometry of the lithospheric STEP.We study the ductile tearing in the process of STEP evolution by physical analogue models, which are dynamically driven by the buoyancy of the subducting slab. In our experiments, the lithosphere as well as asthenosphere are viscoelastic media in a free subduction setup. A stress-dependent rheology plays a major role in localization of strain in tearing processes of lithosphere such as slab break-off. Therefore we developed and tested new analogue materials that can serve as mechanical analogues for the stress-dependent lithosphere rheology, such as has been inferred by rock laboratory test for dislocation creep of olivine. We show the influence of age and integrated strength of the lithosphere and its contrasts across the passive margin, on the timing, depth, and the degree of localization of the tearing process. When tearing of the lithosphere is dominated by ductile deformation, we find that gradual necking of the passive margin precedes tearing. Furthermore, we relate the tearing at depth to deformation at the surface during the evolution of the STEP. [ABSTRACT FROM AUTHOR]

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]