Seismotectonic and stress distribution in the central Chile subduction zone
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Convergence present very different morphologies depending on the type of the lithosphere involved (oceanic or continental). In the same way, the content of buried material is also different (oceanic crust, continental crust, sediments). Field and metamorphic P-T-t data suggest contrasting exhumation modes for these different materials (e.g., structural location within the orogenic belt, shape of the P-T path, exhumation rates). We herein present several numerical subduction models that are implemented using the thermo-mechanically and thermo-dynamically coupled code PARA(O)VOZ. The results of our models are compared with the available data for the alpine belt, where the continental subduction of the European passive margin followed the oceanic subduction of the Liguro-Piemontese ocean. They show that: (1) Our experiments can successfully reproduce natural data (topography, morphology and P-T-t paths). (2) In the oceanic subduction context, exhumation of sediments within a steady sedimentary accretionary wedge occurs only if the overriding continental plate has a strong lower crust, if the sediments have a high viscosity and/or a low density, and if the convergence rate is slow (30 mm.an-1). Moreover, exhumation of oceanic crust occurs only in the presence of a weak serpentinite layer located below the subducting oceanic crust. (3) In the continental subduction context, our experiments reproduce the reported biphase evolution of exhumation rate of high-pressure rocks, fast at mantle depths (>10 mm.yr-1) and slow at crustal depths (<4 mm.yr-1). Such a bi-phase evolution is more pronounced for slow convergence rates. UHP exhumation in a slow convergence context also requires the presence of a double-layered continental crust for the subducting plate, and leads to self-localization of non-predefined crustal separation zones near the level of the brittle-ductile transition, from which the low-density continental material is exhumed. (4) The syn-convergent exhumation of continental material at the rear of the accretionary wedge is a transient process (< 10 Myr) that is largely controlled by the balance of buoyancy and viscous forces within the depth interval of 35-100 km and by erosion at shallower depths (< 35 km). Our models also indicate that slab break-off does not have a significant impact on the rates of exhumation.
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Some of the fundamental features of plate tectonics are interpreted in connection with the behavior of oceanic crust. It is shown to be likely that the oceanic crust which is produced at the mid-ocean ridge by chemical differentiation may be removed from the downgoing slab by melting at the depth of asthenosphere behind the deep-sea trench. The melting of crustal material after the subduction is made possible by an efficient supply of heat through the well-developed asthenosphere with a low-velocity and high-attenuation of seismic waves. The removal of subducted oceanic crust from the slab is consistent with the positive gravity anomaly behind trenches and the double Benioff zone recently discovered. We propose new type of driving forces of plate motion, which arises from the density contrast between the crust and mantle when the oceanic crust is either created or destructed. The proposed driving mechanism is consistent with the non-uniform size and shape of individual plates, the migration of mid-ocean ridges and compressional intraplate stress, while these facts are difficult to understand in the framework of conventional models. A continuous accumulation of basaltic magma beneath the trench-arc system results in a catastrophic overflow of material, which corresponds to back-arc spreading. The picture presented in this paper explains the evolution of marginal basins that is characterized by the presence of remnant arcs, the changes in stress field and the dip angle of the slab, and the anomalous depth-age relationships.
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Using laboratory experiments, we investigate the dynamics of the collisional process that follows the closure of an oceanic basin. The evolution of these experiments systematically shows four successive episodes of deformation, which correspond to (1) the initiation of oceanic subduction, (2) a mature period of oceanic subduction, (3) an episode of continental subduction, during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates, and (4) continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds. We observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere. The slab‐pull force, in turn, depends on the amount of subducted oceanic material, on the thickness of the convective mantle, and also on the rheology of the slab. Our experiments, indeed, suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle, decreasing the slab‐pull level and triggering the rapid onset of collision. We observe two possible modes of slab deformation: slab break‐off and development of viscous instabilities. We define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation. In all cases, oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km, prior to the onset of continental collision.
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We investigate how subduction may be triggered by continental crust extension at a continental margin. The large topography contrast between continental and oceanic domains drives the spreading of continental crust over oceanic basement. Subduction requires the oceanic plate to get submerged in mantle, so that negative buoyancy forces may take over and drive further descent. This is promoted by two mechanisms. Loading by continental crust bends the oceanic plate downwards. Extension in the continental domain induces crustal thinning, which acts to raise mantle above the oceanic plate. In this model, the width of the continental region undergoing extension is an important control parameter. The main physical controls are illustrated by laboratory experiments and simple theory for elastic flexure coupled to viscous crustal spreading. Three governing dimensionless parameters are identified. One involves the poorly constrained oceanic plate buoyancy. We find that the oceanic plate can be thrust to depths larger than 40 km even if it is buoyant, enabling metamorphic reactions and density increase in the oceanic crust. Another parameter is the ratio between the width of the continental extension region and the flexural parameter for the oceanic plate. Initiating subduction is easier if the continent thins over a short lateral distance or if the oceanic plate is strong. The third important parameter is the ratio of oceanic plate thickness to initial continental crust thickness, such that a weak plate and a thick crust do not favour subduction. Thus, the change from a passive to an active margin depends on the local characteristics of the continental crust and is not determined solely by the age and properties of the oceanic lithosphere. It is shown that the spreading of continental crust induces uplift of the margin as the adjacent seafloor subsides. Evidence for the emplacement of continental crust over oceanic basement at passive margins is reviewed.
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