The interplay between subduction and lateral extrusion: A case study for the European Eastern Alps based on analogue models
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Intra-continental subduction is of special importance for studying the formation of intra-continental orogens, crust-mantle structural evolution, and the far-field effects of continental collision, whose mechanism is still a matter of discussion. In this work, we investigated the role of pre-existing weak zones and the continental lithospheric rheological layering in the formation and evolution of the intra-continental subduction based on a 2D finite element numerical technique. The model results indicate that the deeper the intra-continental weak zone is and the faster the convergence velocity is, the more likely it is to develop into a new intra-continental subduction. Altering the rheological strength of the overriding plate may not have a substantial impact on the intra-continental subduction mode when the depth of the pre-existing weak zone is larger than half of the lithospheric thickness. In contrast, the lithospheric rheological strength is closely related to the continental collision system's deformation style: Models with a weaker overriding plate are inclined to delaminate continuously under collision, whereas a strong overriding plate results in the subducting plate's roll-back. The reactivation of the suture that runs deep into the lithosphere as a result of the Indian-Asian continental collision could be one of the crucial factors controlling the formation of the south-dipping subduction under the North Pamir.
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Geoscientists use plate tectonics to explain many aspects of both continental evolution and evolution of the planet as a whole. The subduction of material at convergent plate boundaries forms a fundamental component to the theory of plate tectonics. Plates, continents, subduction zones, and spreading centers all exhibit motion and geometric evolution, so to try and resolve the past geometries of the planet, geologists have utilized plate tectonic reconstructions. Here we present a three‐dimensional image of the subducted Indo‐Australian plate below southeast Asia and show that the geometry of the subducted slab at depth is intimately related to the geometric evolution of SE Asia over the past 50 Ma, including the collision of India with the Asian continent. We show how the once semicontinuous subducting Indo‐Australian plate has been segmented during collision between India, Australian, and the subduction margin to the north. Thus we have found that the geometry of the subducted plate should form a key component to the interpretation of the evolution of Earth's surface, as complexities and evolution of the subducted plate are manifest in the evolution of the overriding plate.
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When tectonic plates converge, one plate gets buried (subducted) below the other. The subducted plate can be of continental origin and part of that plate may return (exhume) to the Earth’s surface driven by deep Earth and surface processes as documented in the Mediterranean region. Such continental burial-exhumation cycles influence ... read more the architecture of mountain belts, which are an expression of a dynamic planet. This thesis aims at unravelling how continental subduction zones evolve through time by using a wide range of methodologies. Field geological investigations in eastern (Greece) and western (SE-Spain) Mediterranean regions portray the tectonic burial and exhumation of rocks during the Africa-Eurasia convergence and the changes they undergo. Combining field geological observations with petrological investigations and radiometric dating of rocks allowed to better understand the structural and temporal evolution of continental subduction zones in the studied regions. The results show that the rocks can be subject to multiple burial-exhumation cycles during Africa-Eurasia convergence and that mechanical heterogeneities control the place where deformation occurs within the buried continental crust. The subduction of continents below oceanic plates leads to the emplacement of a heavy oceanic plate, called ophiolite, on top of light continental plate. Numerical simulations were used to understand how this process of obduction works. The results show that the subducted continental crust is squeezed upwards against the overlying oceanic plate which then breaks apart, leading to the emplacement of the ophiolites far away (up to hundreds of kilometres) from their origin. show less
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Earth's lithosphere is characterized by the relative movement of almost rigid plates as part of global mantle convection. Subduction zones on present‐day Earth are strongly asymmetric features composed of an overriding plate above a subducting plate that sinks into the mantle. While global self‐consistent numerical models of mantle convection have reproduced some aspects of plate tectonics, the assumptions behind these models do not allow for realistic single‐sided subduction. Here we demonstrate that the asymmetry of subduction results from two major features of terrestrial plates: (1) the presence of a free deformable upper surface and (2) the presence of weak hydrated crust atop subducting slabs. We show that assuming a free surface, rather than the conventional free‐slip surface, allows the dynamical behavior at convergent plate boundaries to change from double‐sided to single‐sided. A weak crustal layer further improves the behavior towards steady single‐sided subduction by acting as lubricating layer between the sinking and the overriding plate. This is a first order finding of the causes of single‐sided subduction, which by its own produces important features like the arcuate curvature of subduction trenches.
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<p>Conventional plate tectonics envisages simple continental breakup with clean splitting of supercontinents and subsequent orderly widening of oceans by seafloor spreading about a central ridge. No sooner was this paradigm proposed when the clear, first-order misfit of intraplate and large-volume volcanism was highlighted. That was quickly accommodated by adding an additional degree of freedom into the theory of Earth dynamics, i.e., ad hoc mantle plumes. Although this simple picture was adequate in the early years of plate tectonics, the subsequent rapid accumulation of vast datasets of ever-more-precise observations has rendered a theory of such simplicity no longer tenable. Simple plate tectonics can now serve only as a basic canvas on which the complexities of the real world must be painted. There is no better region for illustrating this than the Northeast Atlantic Realm which illustrates the full range of complexities. After a history of tectonic unrest spanning several 100 Myr true continental breakup, involving fracture of the entire lithosphere and ocean widening via sea-floor spreading, finally proceeded. However, geological complications are on at least an equal level to features arguably amenable to description by simple plate tectonics. Spreading ridges developed by propagation through continental lithosphere comprising a collage of cratons separated by orogenic belts. Where these propagators met insurmountable barriers the extension demanded by local kinematics could only be accommodated by diffuse continental extension. Continual changes occurred in the direction of regional extension and these resulted in local tectonic instabilities manifest in lateral ridge migrations, jumps, and parallel-ridge-pair extension. Extreme, magma-assisted continental extension, together with intense volcanism, formed lava-capped transitional crust. As a consequence the true extent of continental crust under the oceans is unclear. The geophysical characteristics of transitional crust are ambiguous in terms of physical properties. This presents a challenge to mapping continental material in the oceans, a problem that can be mitigated by joint interpretation with gravity, heatflow and geochemical data. Known continental blocks in the ocean include the array of blocks west of the British continental shelf (the Hatton-, George Bligh-, Lousy-, Bill Bailey&#8217;s- and Faroe Bank Highs, and Wyville-Thompson- and Fugl&#248;y Ridges), the Jan Mayen Microplate Complex, the Greenland-Iceland-Faroe Ridge and likely others that remain to be found. All of the above complexities in the solid Earth have profoundly affected the natural environment in the region, especially the oceans and the biosphere, and must be taken into account in predictions of future evolution of the natural environment.</p>
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Significance Subduction, the process by which tectonic plates sink into the mantle, is a fundamental tectonic process on Earth, yet the question of where and how new subduction zones form remains a matter of debate. In this study, we find that a divergent plate boundary, where two plates move apart, was forcefully and rapidly turned into a convergent boundary where one plate eventually began subducting. This finding is surprising because, although the plate material at a divergent boundary is weak, it is also buoyant and resists subduction. This study suggests that buoyant, but weak, plate material at a divergent boundary can be forced to converge until eventually older and denser plate material enters the nascent subduction zone, which then becomes self-sustaining.
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Abstract Plate reconstruction studies show that the Neotethys Ocean was closing due to the convergence of Africa and Eurasia toward the end of the Cretaceous. The period around 75 Ma reflects the onset of continental collision between the two plates as convergence continued to be taken up mostly by subduction of the Neotethys slab beneath Eurasia. The Owen transform plate boundary in the northeast accommodated the fast northward motion of the Indian plate relative to the African plate. The rest of the plate was surrounded by mid‐ocean ridges. Africa was experiencing continent‐wide rifting related to northeast‐southwest extension. We aim to quantify the forces and paleostresses that may have driven this continental extension. We use the latest plate kinematic reconstructions in a grid search to estimate horizontal gravitational stresses (HGSs), plate boundary forces, and the plate's interaction with the asthenosphere. The contribution of dynamic topography to HGSs is based on recent mantle convection studies. We model intraplate stresses and compare them with the strain observations. The fit to observations favors models where dynamic topography amplitudes are smaller than 300 m. The results also indicate that the net pull transmitted from slab to the surface African plate was low. To put this into context, we notice that available tectonic reconstructions show fragmented subduction zones and various colliding micro‐continents along the northern margin of the African plate around this time. We therefore interpret a low net pull as resulting from either a small average slab length or from the micro‐continents' resistance to subduction.
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Continental subduction takes place in the final stage of subduction when all oceanic lithosphere is consumed and continental passive margin is pulled into the mantle. When the overriding plate is oceanic, dense forearc oceanic lithosphere might be obducted onto light continental crust forming an ophiolite (Tethyan-style ophiolite obduction). Four-dimensional dynamic analog subduction models have been constructed to evaluate the mechanical feasibility of continental subduction and forearc oceanic lithosphere obduction on top of continental crust. The roles of continental crust thickness, passive margin length, subducting lithosphere thickness, and overriding plate thickness were investigated to determine the maximum continental subduction depth, maximum forearc obduction distance, and forearc deformation during continental subduction. Our buoyancy-driven experiments indicate that deep continental subduction occurs in most circumstances (down to ~560 km) and that obduction of dense oceanic forearc lithosphere on top of light continental crust is mechanically feasible. Maximum obduction distances are relatively small (~26–37 km) but are sufficient to explain obduction of short ophiolite sheets, such as observed in New Caledonia. When including the thin (5–10 km thick) accretionary wedge of off-scraped deep sea sediments, oceanic crust, and mantle, then maximum obduction distances are much larger, ~60–160 km, sufficient to account for the obducted Northland Allochthon in New Zealand. Results indicate that increasing continental crust thickness decreases continental subduction depth, whereas increasing passive margin length and subducting lithosphere thickness increases continental subduction depth. Notably, during continental subduction, backarc extension continues, while forearc deformation (shortening) increases moderately compared to the preceding phase of normal (oceanic) subduction.
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