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For instance, an analytical model 1 has been proposed to stress the importance of a background mantle flow in influencing slabs geometry. This model suggests that the dip angle of subducted slabs are strongly controlled by a large-scale flow imposed within the mantle by tectonic plates moving in their observed geometry and, more importantly, the slabs are orientated as if they were responding passively to the flow driven by the surface motion of the plates.
Moreover, this model shows how important the decoupling role of a Low Viscosity Layer LVZ between lithosphere and mantle would be. In fact, the match between the direction of mantle flow and the direction of the subducted slab, given by the trend in earthquakes hypocentres, is good for most of the subduction zones and is usually improved by the inclusion of this decoupling level 1.
More recently, a purely mechanical physical model 6 has been used to investigate influences of a regional convective i. These modelling results have been applied to explain geophysical observations in some regional subduction settings NE-Japan, Central America. Further numerical thermomechanical models 8 have been used to understand how much of the slab-dip variability found in nature can be attributed to the interaction between the slab and a background mantle flow. The number of salient features of mantle-lithosphere interaction that these models do succeed in reproducing provides useful insights.
However, these thermomechanical numerical models did not employ realistic free upper surface condition, which is crucial for subduction zones asymmetry 36 , and used highly simplified rheology of the mantle that is in conflict with available geochemical, experimental and theoretical data 37 — Hence, the validation of the mantle wind hypothesis, through a realistic state of the art numerical thermomechanical model, stands as a challenge that motivated our work.
Here, we explore and integrate the same effects of a priori defined mantle flow on the subduction zone morphology and slab dynamics, improving the modelling by means of self-consistent two-dimensional rheologically realistic thermomechanical numerical experiments with a free surface.
In these experiments an oceanic plate sinks beneath a continental plate under the control of non-Newtonian temperature-, pressure- and strain rate-dependent viscous-plastic rheologies with viscosity magnitude ranging from 10 18 and 10 25 - Supplementary Table S1 in a fully thermodynamically coupled model accounting for mineralogical phase changes The use of mantle wind, in conjunction with a weak asthenospheric layer, produce sustained asymmetric subduction for the majority of models run in this study see Supplementary Information Figs 5 — 10 and Table S2.
The temporal evolution of two end-member models, one with discordant mantle flow and one with concordant mantle flow with respect to the subduction polarity, demonstrate the defining features of the evolution of all models. In the following sections we describe our end-member models, divided by direction of the mantle flow with respect to the subduction polarity.
Subduction initiates via slip along the interplate weak zone see Supplementary Fig. S3 for further details of the model setup. As some of the initial plate boundary interface material gets subducted, it is replenished by material from the upper layer of the subducting plate, and the hinge begins to retreat.
Before the slab tip flattens in the transition zone, the slab is pushed backwards and downwards by the flow. The hinge continues to retreat, allowing the spontaneous formation of back-arc extension at a distinct location with respect to the trench, and causing an uplift of the asthenospheric mantle.
The subduction hinge moves away from the upper plate and subduction rates are faster than the convergence rates 12 , leading to a faster recycling of the lithosphere into the mantle Hinge retreat correlates with the intensity of the mantle wind, regardless of subducting plate strength or age-dependent density.
Age variations of the subducting oceanic plate have indeed negligible effects on subduction dynamics Supplementary Fig.
S4 and Supplementary Table S2 — model 7 , thus suggesting that mantle wind intensity is a more critical subduction parameter compared to the slab age. Panel a shows a W-directed slab. All numerical models present pre-defined rightward subduction polarity; therefore this model was mirrored for better comparison with nature.
In panel b a slab along E or NE-subduction zone is designed. In each model a horizontal mantle flow is imposed, having concordant or opposite direction with respect to the subduction polarity. During incipient collision, a large volume of weak crustal material is interposed between the plates. The negative pressure gradient caused by mantle flow and dense retreating slab favor sucking of the mantle into the accretionary prism.
As collision proceeds, the rheologically weak part of the crust is scraped off from the retreating lithosphere by the mantle wedge above the subducting slab that acts as a backstop moving in the opposite direction to convergence.
In this case the subduction hinge is moving toward the upper plate, which is set under compression, and the subduction rates are slower than the convergence rates The strong correlation between topography and slab dip angle with corresponding variations in mantle flow direction suggests a strong relationship with the underlying subduction dynamics.
This should be the reason why this kind of subductions have the highest mountain ranges i. Notably, the shallow subduction also produces slower recycling of the oceanic lithosphere into the mantle As in the previous model, the age of the subducting plate has subordinate effects on subduction dynamics: low dip angle can be reached either by younger and older oceanic plates, despite having different values of Activation Volume and isobaric Thermal Expansion Supplementary Table S2.
The persistence of a mantle flow is therefore crucial to determine the dip angle of the slab and the state of stress within the upper plate. At the onset of collision, the buoyancy of the continental crust slows down the convergence rate. As collision continues, crustal material is accreted at the margin, raising the topography and thus building up the compressional stresses within the upper plate.
The final structure is that of a narrow and thick collisional zone delimitated by a shallow-dipping slab and characterized by diffuse deformation. These results thus suggest that when these heterogeneities that are, concordant mantle wind and a weak asthenospheric layer are combined in a single model, the dynamics of the subducting plate and the topography evolution of the overriding plate can all be reconciled.
Results demonstrate that symmetric changes in the mantle wind direction have an impact on the total force propagated to the upper plate and influences the plate motions, dips and vertical stresses, reflected as a topographic high in the overriding plate.
Our numerical models suggest that the dip of the slab consistently changes as a function of intensity of the mantle wind, whereas the presence or absence of the LVZ is demonstrated to only play a collateral role Supplementary Figs S6 and S7. The resulting dip angle associated to a horizontal mantle flow mimics the natural data, providing a different clue to explain the global asymmetry of slab dip.
Comparing our models with a global compilation of slab dips Fig. The catalogue in Fig. These can be used as representative for W- and E- to NE-directed subduction zones spread out in opposite Pacific Ocean sides.
Also, looking at tomography 42 , 43 considering the segment of the slab on which seismicity is plotted and other seismicity data 44 — 46 , a quite good correlation between slab dip and subduction polarity can be observed. Exceptions are for Northeastern-Japan, Java and Central America, these latter two being more likely related to the obliquity of the slab direction with respect to the main convergence direction. The northern-Japan setting is peculiar because the subduction hinge is now converging relative to the upper Eurasian plate, hence inverting the previous slab retreat relative to the upper plate and the contemporaneous opening of the Japan Sea This picture shows our two models in green , compared with a compilation of the slab dip measured along cross-sections perpendicular to the trench of most subduction zones.
Each line represents the mean trace of the seismicity along every subduction. Dominant down-dip compression occurs in the W-directed intraslab seismicity, whereas down-dip extension prevails along the opposed E- or NE-directed slabs. The W-directed slabs are, on average, dipping In this figure the differences in topography and state of stress between the upper plates of both models can be seen.
Although a possible link between slab dip, deformation of the overriding plate and trench motion has been already proposed 2 , 5 , our results could be useful to explain some of the different features between Andean- and Mariana-type subductions in a different way.
In the past 2 , differences between these two subduction types were explained by ablation extent during subduction process. In such settings most of the crust and the lithospheric mantle are completely subducted instead of being involved in the accretionary prism building.
Some authors 5 , 47 , then, used seismicity data coming from deeper and shallower parts of the slab, separately, to study relations between slab dip and subduction direction. However, regional monocline dip reveal the same asymmetry between W-directed an E- to NE-directed subduction zones Another issue that should be considered when evaluating relationship between subduction zones direction and slab dip, is that most of subductions worldwide show an arcuate geometry 7 , 48 ; therefore slabs could form different angles with subduction direction and, consequently, they have different dip angles according to their obliquity with respect to the main convergence direction.
Furthermore, some works 5 consider trench-perpendicular migration velocities but, doing this, an assumption is made: in fact, when having an oblique trench with respect to subduction direction the hinge moves obliquely too, due to stress deviation from the convergence direction and strain partitioning. Moreover, hinge velocity cannot be measured in a precise way, especially when calculated with respect to the mantle 7. However, here we analyse the possibility that a global feature —such as the mantle wind— could be the first-order controlling parameter of slab dip and stress regime within the upper plate 7.
In fact, it has to be considered that extension within the overriding plate, in the two subduction end-members, has different geologic origins: along W-directed subduction zones, back-arc spreading occurs as a consequence of slab rollback and of the asthenospheric replacement for the retreated lithosphere as it can be seen also from our W-directed subduction model , whereas for E-NE-directed subductions back-arc basins open in few places where the upper plate lithosphere is split into two sub-plates that have different velocity with respect to the same lower plate Moreover, the first one are characterized by fast back-arc opening, widely distributed throughout the upper plate and eventually arriving to oceanization e.
The subduction of the Nazca plate beneath the Andes provides a key test for this analysis. In fact, in the kinematic model of a slow net rotation of the lithosphere driven only by the slab negative buoyancy, beneath the Nazca plate and the slab of the south America cordillera, the mantle flow should be westerly directed, providing a steep slab.
However, as shown in the numerical modelling presented here, the shallow dip of the Andean slab is consistent with an eastward mantle flow even beneath the Nazca plate as evident by shear wave splitting results 49 and providing indirect support for the fast lithospheric rotation rates relative to the mantle The origin of the hypothesized global mantle wind is largely enigmatic and goes beyond the scope of our study.
As a result, a larger fraction of the asthenospheric mantle material has to move to the east, thus creating a global mantle wind. Because earthquakes can only occur when a rock is deforming in a brittle fashion, subduction zones can cause large earthquakes.
If such a quake causes rapid deformation of the sea floor, there is potential for tsunamis , such as the earthquake caused by subduction of the Indo-Australian Plate under the Euro-Asian Plate on December twenty-sixth, that devastated the areas around the Indian Ocean.
Small tremors that cause small, nondamaging tsunamis occur frequently. Outer rise earthquakes occur when normal faults oceanward of the subduction zone are activated by flexture of the plate as it bends [6] into the subduction zone. The Samoa earthquake of is an example of this type of event. Displacement of the sea floor caused by this event generated a six-metre tsunami in nearby Samoa. Anomalously deep events are a characteristic of subduction zones which produce the deepest quakes on the planet.
Earthquakes are generally restricted to the shallow, brittle parts of the crust, generally at depths of less than twenty kilometres. However, in subduction zones, quakes occur at depths as great as seven hundred kilometres. These quakes define inclined zones of seismicity known as Wadati-Benioff zones , after the scientists who discovered them, which trace the descending lithosphere.
Seismic tomography has helped detect subducted lithosphere in regions where there are no earthquakes. Some subducted slabs seem not to be able to penetrate the major discontinuity in the mantle that lies at a depth of about kilometres, whereas other subducted oceanic plates can penetrate all the way to the core-mantle boundary. The great seismic discontinuities in the mantle - at and kilometre depth - are disrupted by the descent of cold slabs in deep subduction zones.
Subducting plates can bring island arcs and sediments to convergent margins. This material often does not subduct with the rest of the plate, but instead is accreted to the continent in the form of exotic terranes. These cause crustal thickening and mountain-building. This accretion process is thought by many geologists to be the source of much of western North America and of the uplift that produced the Rocky Mountains.
Subduction typically occurs at a moderately steep angle right at the point of the convergent plate boundary. However, anomalous shallower angles of subduction are known to exist as well some extremely steep. Subduction zones are important for several reasons [ citation needed ] :. Subduction zones have also been considered as possible disposal sites for nuclear waste , in which the action of subduction itself would carry the material into the planetary mantle , safely away from any possible influence on humanity or the surface environment.
However, this method of disposal is currently banned by international agreement. Search this site. Subduction From Wikipedia, the free encyclopedia. Redirected from Subduction zone. Contents [ hide ]. Main article: Orogeny. Theory on origin [ edit ] Although the process of subduction as it occurs today is fairly well understood, its origin remains a matter of discussion and continuing study. Earthquakes and tsunamis [ edit ] The strains caused by plate convergence in subduction zones cause at least three different types of earthquakes.
Orogeny [ edit ] Main article: Orogeny.
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