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The formation and evolution of a backarc basin are linked to the dynamics of the subduction system. The opening of the central Mediterranean basins is a well-documented example of backarc extension characterized by short-lived episodes of spreading. The underlying reasons for this episodicity are obscured by the complexity of this subduction system, in which multiple continental blocks enter the subduction zone. We present results from three-dimensional numerical models of laterally varying subduction to explain the mechanism of backarc basin opening and the episodic style of spreading. Our results show that efficient backarc extension can be obtained with an along-trench variation in slab buoyancy that produces localized deformation within the overriding plate. We observe peaks in the trench retreating velocity corresponding first to the opening of the backarc basin, and later to the formation of slab windows. We suggest that the observed episodic trench retreat behavior in the central Mediterranean is caused by the formation of slab windows.

International audience; Since several decades, the processes allowing for the subduction of the continental lithosphere less dense than the mantle in a collision context have been widely explored, but models that are based upon the premise that slab pull is the prominent driver of plate tectonics fail. The India–Asia collision, where several episodes of continental subduction have been documented, constitute a case study for alternative views. One of these episodes occurred in the early collision time within the Asian plate where continental lithosphere not attached to any oceanic lithosphere subducted southward in front of the Indian lithosphere during its northward subduction that followed the oceanic subduction of the Tethys ocean. This process, known as collisional subduction, has a counter-intuitive behavior since the subduction is not driven by slab pull. It has been speculated that the mantle circulation can play an important role in triggering collisional subduction but a detailed, qualitative analysis of it is not available, yet. In this work we explore the southward subduction dynamics of the Asian lithosphere below Tibet by means of analogue experiments with the aim to highlight how the mantle circulation induces or responds to collisional subduction. We found that during the northward oceanic subduction (analogue of Tethys subduction) attached to the indenter (Indian analogue), the main component of slab motion is driven vertically by its negative buoyancy, while the trench rolls back. In the mantle the convective pattern consists in a pair of wide convective cells on both sides of the slab. But when the indenter starts to bend and plunge in the mantle, trench motion reverses. Its advance transmits the far field forces to two upper plates (Asian analogues). The more viscous frontal plate thickens, and the less viscous hinterland plate, which is attached to the back wall of the box, subducts. During this transition, a pair of sub-lithospheric convective cells is observed on both sides of the Asian analogue slab, driven by the shortening of the frontal plate. It favors the initiation of the backwall plate subduction. Such subduction is maintained during the entire collision by a wide cell with a mostly horizontal mantle flow below Tibet, passively advecting the Asian analogue slab. Experimental results suggest that once the tectonic far-field force related to the forward horizontal motion becomes dominant upon the buoyancy forces, trench advancing and the transmission of the tectonic force to the upper and backwall plates are promoted. This peculiar condition triggers the subduction of the backwall plate, despite it is light and buoyant.

The evolution of the Gibraltar Arc and the whole western Mediterranean region is still highly debated by geologists and geophysicists. This work is based on a recent geodynamic model proposing a lateral change in subduction polarity of the Ligurian Tethys oceanic domain to explain its formation and evolution since 46 Myr. The aim of this study is to reproduce and analyze the curvature evolution of the Rif-Gibraltar-Betic slab in such tectonic scenario by means of numerical modelling. The 3D numerical model is carried out via the Underworld framework that solves the Stokes flow equations using Finite Elements combined with a particle-in-cell approach, thus the discretization combines a standard Eulerian Finite Element mesh with Lagrangian particles. The model setup consists of two oceanic plates with a visco-plastic rheology subducting into the viscous upper mantle in opposite directions. In the present-day Alboran Basin region, the plate dips to the southeast with the trailing edge fixed to the Iberian margin, whereas in the present Algerian Basin region the plate dips to the northwest and the trailing edge is fixed to the African margin. To initiate subduction a small slab perturbation is initially imposed. In addition, we include a continental African plate segment west of the present Alboran Basin region, with different shear strength values. We study the influence of the lateral side plates on the trench curvature and the geodynamic consequences in terms of trench velocities, stress distribution, mantle flow, and plate deformation produced by the proximity between the opposite retreating slabs., This work is part of the projects ALPIMED (PIE-CSIC-201530E082) and MITE (CGL2014-59516-P). We also thank to the project AECT-2017-3-0008 of the Barcelona Supercomputing center (BSC-CNS).

Subduction trench curvature is generally attributed to the Earth's sphericity. Although, observed trench curvature in Nature are usually larger than those imposed by Earth radius. Other factors potentially influencing curvature are the mantle flow (produced by some other geodynamic process), internal heterogeneities in the subducting plate (e.g. variations in plate age and consequently in its thickness), the presence of crustal features (e.g. seamounts or plateaus), the orientation of the main plate structures (parallel/perpendicular to the trench) and the geometry of the upper plate. The role of a weak lithosphere on a lateral side perpendicular to the strike of the retreating subduction, e.g. continent/transfer zones, is unexplored. We study the influence of this boundary condition in a retreating subduction setting by varying its kinematic nature using analog modelling. A continental block is considered in one side of the plate, and the kinematic nature of the contact with the oceanic plate is varied considering: 1) fully attached, 2) half attached, and 3) completely free. To first order, lateral boundary perpendicular to the trench introduces an asymmetry in the trench curvature. The magnitude of this asymmetry is proportional to the resistance of the transfer zone. This kinematic resistance slows down the subduction of the oceanic plate near the contact and enhances the subduction in the free lateral side, thus affecting the retreat velocities along the trench. This work has implications for present and geologic past subduction zones where asymmetric trench and laterally varying retreat velocities have been observed/postulated. This is a SUBITOP (674899-SUBITOP-H2020-MSCA-ITN-2015) contribution., This is a SUBITOP (674899-SUBITOP-H2020-MSCA-ITN-2015) contribution.

In this study we present the results of numerical and analogue experiments of a double subduction system characterized by two adjacent slabs retreating in opposite directions. Such a tectonic scenario has been proposed to occur in several regions of the Earth, most recently in the Westernmost Mediterranean. The basic setup consists of two oceanic plates with a linear viscous rheology descending into the upper mantle. Both plates are fixed at their trailing edge to enforce roll-back behavior during subduction driven by a Rayleigh-Taylor instability. The two retreating plates interact with each other as a result of subduction-induced mantle counter-flow. Previous laboratory experiments, based on viscous syrup (representing the mantle) and silicone putty (representing the plates), show that the mantle flow induced by both plates is asymmetrical producing variations of rollback velocities and lateral separation between plates. Additionally, a high-resolution finite difference 3D numerical model (I3ELVIS code) is carried out with the identical material parameters and geometry as for the analogue model. The combination of both experiments allows us to numerically quantify the physical parameters that characterize the evolution of the system such as trench velocities, strain rates, and stresses. We also study how boundary conditions and scaling for analogue modelling may affect the numerical results., This work is part of the projects ALPIMED (PIE-CSIC-201530E082) and MITE (CGL2014-59516-P). We also thank to the project AECT-2017-3-0008 of the Barcelona Supercomputing center (BSC-CNS).

Opposite subduction retreating in adjacent plate segments has been proposed in several regions of the Earth and particularly in the Westernmost Mediterranean (Verg´es et al., 2012; Casciello et al., 2015). Recent numerical experiments show a strong interaction between the induced mantle flows of each retreating plate (Kir´aly et al., 2016). In this work we show the result of a series of analogue models based on viscous syrup (representing the mantle) and silicone putty (representing the subducting plate), which have been designed to simulate the evolution of a double subduction system. The basic setup contains a pair of plates subducting in opposite directions. The plates are fixed at their trailing edge to enforce slab rollback behaviour and subduction is started by deflecting manually the leading edge of the plate (i.e., initial slab pull, phase 1). Different setups were designed to test the influence of two variables on the system: i) the width of the plates, that varies from 10 cm to 30 cm (1 cm in model corresponds to 60 km in nature) and ii) the lateral distance between the two subducting plates, that varies from 10 to 0.5 cm. Our results show that trench velocities increase during the stage of approaching trenches (phase 2) and then decrease after trenches pass each other (phase 3). On the other hand, the trench curvature increases linearly during the entire evolution whereas the lateral distance increases along time, indicating that effective lateral stresses are produced associates with the asymmetry of toroidal flows. This behaviour indicates a strong interaction between the stresses produced by the two retreating slabs that propagate through the mantle flow, which in turn depends on the initial plate separation.

The goal of this work is to understand the dynamics of a subduction system characterized by two adjacent subducting plates with opposite retreating directions as recently proposed for the Westernmost Mediterranean. A series of analogue models based on viscous syrup (representing the mantle) and silicone putty (representing the subducting plate) have been designed to simulate the evolution of a double subduction system. The basic setup contains a pair of plates subducting in opposite directions. The plates are fixed at their back edge to enforce a slab rollback behavior and subduction is started by deflecting manually the leading edge of the plate (i.e. initial slab pull, phase 1). Different setups were designed to test the influence of two variables on the system: i) the width of the plates, that varies from 10 cm to 30 cm (1 cm in model corresponds to 60 km in nature) and ii) the lateral distance between the two subducting plates, that varies from 10 to 0.5 cm. Our results show that trench velocities increase during the stage of approaching trenches (phase 2) and then decrease after trenches pass each other (phase 3). This behavior indicates an interaction of the mantle flows produced by the two retreating slabs. On the other hand, the trench curvature increases linearly during the entire evolution and the lateral distance between plates remains constant along time, indicating that no effective lateral stress is produced when the opposing plates have similar dimensions. In addition, we have reproduced numerically some of the laboratory experiments., This work is part of the projects WE-ME (PIE-CSIC-201330E111) and MITE (CGL2014-59516-P). We also thank to the project AECT-2016-1-0002 of the Barcelona Supercomputing center (BSC-CNS).

""At oceanic margins, syn-convergent exhumation, subduction erosion, and inter-plate coupling are intimately related, but ample questions remain concerning their interaction and individual mechanisms. To analyze these interactions for a thick-skinned, visco-elastic wedge, we focus on properly modeling stresses, energies, and topographies at the inter-plate and wedge bounding interfaces using a Coulomb frictional contact algorithm. In this innovative plane-strain, free surface, Lagrangian finite element model, fault dynamics is modulated by retreating subduction. Subduction is dynamically driven by slab-pull due to a slab sinking in a semi-analytic, computationally favorable approximation of three-dimensional induced mantle flow. Nodal trajectories show that continuous underthrusting of a slab induces a steady state corner flow through forced underplating and subsequent trenchward extrusion due to gravitational spreading. This flow pattern confirms early-proposed models of syn-orogenic deep-seated rock exhumation propelled by coexisting extension and continuous shortening at depth. A distinct reduction in upward flowing material and accompanying decrease of exhumation velocities, to millimeters per year as observed in nature, is induced by a diversion of orogenic wedge material toward the mantle once a subduction channel is formed. The key parameter affecting model evolution and spontaneous formation of a subduction channel is basal friction, which modulates the amount of erosion. However, formation of a subduction channel entrance needs to be ensured through the deformability of the overriding plate, which is influenced by applied pressure at the overriding plate tip and material properties. The down dragging of the overriding plate is sufficient above a threshold inter-plate shear stress of about 2–7 MPa. ""

One of the major issues of the evolution of continental lithospheres is the detachment of the lithospheric mantle that may occur under certain conditions and its impact on the surface. In order to investigate the dynamics of continental delamination, we performed a parametric study using physically scaled laboratory models. The adopted setup is composed of a three-layers visco-elastic body (analog for upper crust, lower crust, lithospheric mantle) locally thickened/thinned to simulate a density anomaly (lithospheric root) and an adjacent weak zone, lying on a low viscosity material simulating the asthenosphere. The results emphasize the interplay between mantle flow, deformation, surface topography and plate motion during a three-phases process: (1) a slow initiation phase controlled by coupling and bending associated with contraction and dynamic subsidence, (2) lateral propagation of the delamination alongside with extension and a complex topographic signal controlled by coupling and buoyancy, while poloidal mantle flow develops around the tip of the delaminating lithospheric mantle, and (3) a late phase characterized by a counterflow that triggers retroward motion of the whole model. A semiquantitative study allows us to determine empirically two parameters: (1) an initiation parameter that constrains the propensity of the delamination to occur and correlates with the duration of the first stage, (2) a buoyancy parameter characterizing the delamination velocity during late stages and therefore its propensity to cease. Finally, we point out similarities and differences with the Sierra Nevada (California, USA) in terms of topography, deformation and timing of delamination.

A variety of atypical plume-like structures and focused upwellings that are not rooted in the lower mantle have recently been discussed, and seismological imaging has shown ubiquitous small-scale convection in the uppermost mantle in regions such as the Mediterranean region, the western US, and around the western Pacific. We argue that the three-dimensional return flow and slab fragmentation associated with complex oceanic subduction trajectories within the upper mantle can generate focused upwellings and that these may play a significant role in regional tectonics. The testable surface expressions of this process are the outsidearc alkaline volcanism, topographic swell, and low-velocity seismic anomalies associated with partial melt. Using three-dimensional, simplified numerical subduction models, we show that focused upwellings can be generated both ahead of the slab in the back-arc region (though ~five times further inward from the trench than arc-volcanism) and around the lateral edges of the slab (in the order of 100 km away from slab edges). Vertical mass transport, and by inference the associated decompression melting, in these regions appears strongly correlated with the interplay between relative trench motion and subduction velocities. The upward flux of material from the depths is expected to be most pronounced during the first phase of slab descent into the upper mantle or during slab fragmentation. We discuss representative case histories from the Pacific and the Mediterranean where we find possible evidence for such slab-related volcanism.

Subduction zones are not static features, but trenches retreat (roll back) or advance. Here, we investigate the dominant dynamic controls on trench migration by means of two- and three-dimensional numerical modeling of subduction. This investigation has been carried out by systematically varying the geometrical and rheological model parameters. Our viscoplastic models illustrate that advancing style subduction is promoted by a thick plate, a large viscosity ratio between plate and mantle, and a small density contrast between plate and mantle or an intermediate width (w 1300 km). Advancing slabs dissipate 45% to 50% of the energy in the system. Thin plates with relatively low viscosity or relatively high density, or wide slabs (w 2300 km), on the other hand, promote subduction in the retreating style (i.e., slab roll-back). The energy dissipated by a retreating slab is 35% to 40% of the total dissipated energy. Most of the energy dissipation occurs in the mantle to accommodate the slab motion, whereas the lithosphere dissipates the remaining part to bend and ‘‘unbend.’’ With a simple scaling law we illustrate that this complex combination of model parameters influencing trench migration can be reduced to a single one: plate stiffness. Stiffer slabs cause the trench to advance, whereas more flexible slabs lead to trench retreat. The reason for this is that all slabs will bend into the subduction zone because of their low plastic strength near the surface, but stiff slabs have more difficulty ‘‘unbending’’ at depth, when arriving at the 660-km discontinuity. Those bent slabs tend to cause the trench to advance. In a similar way, variation of the viscoplasticity parameters in the plate may change the style of subduction: a low value of friction coefficient weakens the plate and results in a retreating style, while higher values strengthen the plate and promote the advancing subduction style. Given the fact that also on Earth the oldest (and therefore probably stiffest) plates have the fastest advancing trenches, we hypothesize that the ability of slabs to unbend after subduction forms the dominant control on trench migration

Laboratory models have been conducted to improve our understanding of the role that the resistance of the slab to bending and its coupling to the ambient mantle play in subduction dynamics over geological time scales. Our models are set up with a viscous plate of silicone (lithosphere) subducting under negative buoyancy in a viscous layer of glucose syrup (mantle). For our study, the lithosphere/upper mantle viscosity contrast has been systematically varied, from ~10 to ~105 in order to explore the parameter space between weak and strong slab dynamics. We found that subduction is characterized by a retreating mode for viscosity ratios N104, by the coexistence of a retreating mode and an advancing mode for viscosity ratios between ~104 and ~102, and quasi-stationary, Rayleigh–Taylor like behaviour for ratios b102. By combining our experimental results and kinematic data from current subduction zones in four reference frames which differ in the amount of net rotation, we infer that a lithosphere/upper mantle viscosity contrast of 150–500 is necessary to obtain realistic trench/subducting plate velocity ratios as well as the variability of subduction styles observed in nature.

A combination of statistical studies on present-day subduction zones and three-dimensional (3D) laboratory models is performed with the aim to clarify the way that plate kinematics control the geometry of the slab and the overriding plate deformation in subduction zones. In 3D laboratory models, the analogue of a two layer linearly viscous lithosphere–upper mantle system is achieved by means of silicon putty glucose syrup tank experiment. The subducting and overriding plate velocities are systematically changed by exploring the variability field of natural plate kinematics. Both statistical and modelling approaches recognize the importance of overriding plate motion on subduction process behavior: (1) trenches migrate at a rate close to the overriding plate motion, but always move slower than the overriding plates. The mechanism at work is a direct consequence of “slab anchoring” opposed by both lithosphere and mantle viscous resistance and is responsible for overriding plate deformation and slab geometry variability. (2) An overriding plate shortens when the overriding plate moves toward the trench and conditions that are favourable for overriding plate extension are created when the overriding plate moves away from the trench. (3) Shallow and steep dips are found if the overriding plate moves toward and away from the trench, respectively.