A common deep source for upper-mantle upwellings below the Ibero-western Maghreb region from teleseismic P-wave travel-time tomography

Upper-mantle upwellings are often invoked as the cause of Cenozoic volcanism in the Ibero-western Maghreb region. However, their nature, geometry and origin are unclear. This study takes advantage of dense seismic networks, which cover an area extending from the Pyrenees in the north to the Canaries in the south, to provide a new high-resolution P-wave velocity model of the upper-mantle and topmost lower-mantle structure. Our images show three subvertical upper-mantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc, which appear to be rooted beneath the upper-mantle transition zone (MTZ). Two other mantle upwellings beneath the eastern Rif and eastern Betics surround the Gibraltar subduction zone. We propose a new geodynamic model in which narrow upper-mantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc rise from a laterally-propagating layer of material below the MTZ, which in turn is fed by a common deep source below the Canaries. In the Gibraltar region, the deeply rooted upwelling interacts with the Gibraltar slab. Quasi-toroidal flow driven by slab rollback induces the hot mantle material to flow around the slab, creating the two low-velocity anomalies below the eastern Betics and eastern Rif. Our results suggest that the Central Atlantic plume is a likely source of hot mantle material for upper-mantle upwellings in the Ibero-western Maghreb region

Geodynamic models of short-lived, long-lived and periodic flat slab subduction

© 2021 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.Flat slab subduction has been ascribed to a variety of causes, including subduction of buoyant ridges/plateaus and forced trench retreat. The former, however, has irregular spatial correlations with flat slabs, while the latter has required external forcing in geodynamic subduction models, which might be insufficient or absent in nature. In this paper, we present buoyancy-driven numerical geodynamic models and aim to investigate flat slab subduction in the absence of external forcing as well as test the influence of overriding plate strength, subducting plate thickness, inclusion/exclusion of an oceanic plateau and lower mantle viscosity on flat slab formation and its evolution. Flat slab subduction is reproduced during normal oceanic subduction in the absence of ridge/plateau subduction and without externally forced plate motion. Subduction of a plateau-like feature, in this buoyancy-driven setting, enhances slab steepening. In models that produce flat slab subduction, it only commences after a prolonged period of slab dip angle reduction during lower mantle slab penetration. The flat slab is supported by mantle wedge suction, vertical compressive stresses at the base of the slab and upper mantle slab buckling stresses. Our models demonstrate three modes of flat slab subduction, namely short-lived (transient) flat slab subduction, long-lived flat slab subduction and periodic flat slab subduction, which occur for different model parameter combinations. Most models demonstrate slab folding at the 660 km discontinuity, which produces periodic changes in the upper mantle slab dip angle. With relatively high overriding plate strength or large subducting plate thickness, such folding results in periodic changes in the dip angle of the flat slab segment, which can lead to periodic flat slab subduction, providing a potential explanation for periodic arc migration. Flat slab subduction ends due to the local overriding plate shortening and thickening it produces, which forces mantle wedge opening and a reduction in mantle wedge suction. As overriding plate strength controls the shortening rate, it has a strong control on the duration of flat slab subduction, which increases with increasing strength. For the weakest overriding plate, flat slab subduction is short-lived and lasts only 6 Myr, while for the strongest overriding plate flat slab subduction is long-lived and exceeds 75 Myr. Progressive overriding plate shortening during flat slab subduction might explain why flat slab subduction terminated in the Eocene in western North America and in the Jurassic in South China

Control of slab width on subduction-induced upper mantle flow and associated upwellings: Insights from analog models

The impact of slab width W (i.e., trench-parallel extent) on subduction-induced upper mantle flow remains uncertain. We present a series of free subduction analog models where W was systematically varied to upscaled values of 250–3600 km to investigate its effect on subducting plate kinematics and upper mantle return flow around the lateral slab edges. We particularly focused on the upwelling component of mantle flow, which might promote decompression melting and could thereby produce intraplate volcanism. The models show that W has a strong control on trench curvature and on the trench retreat, subducting plate, and subduction velocities, generally in good agreement with previous modeling studies. Upper mantle flow velocity maps produced by means of a stereoscopic particle image velocimetry technique indicate that the magnitude of the subduction-induced mantle flow around the lateral slab edges correlates positively with the product of W and trench retreat velocity. For all models an important upwelling component is always produced close to the lateral slab edges, with higher magnitudes for wider slabs. The trench-parallel lateral extent of this upwelling component is the same irrespective of W, but its maximum magnitude gets located closer to the subducting plate in the trench-normal direction and it is more focused when W increases. For W ≤ 2000 km the upwelling occurs laterally (in the trench-parallel direction) next to the subslab domain and the mantle wedge domain, while for W ≥ 2000 km it is located only next to the subslab domain and focuses closer to the trench tip, because of stronger poloidal flow in the mantle wedge extending laterally

Introduction to the special issue celebrating 200 years of geodynamic modelling

Since the first published laboratory models from Sir James Hall in 1815, analogue and numerical geodynamic modelling have become widely used as they provide qualitative and quantitative insights into a broad range of geological processes. To celebrate the 200th anniversary of geodynamic modelling, this special issue gathers review works and recent studies on analogue and numerical modelling of tectonic and geodynamic processes, as an opportunity to present some of the milestones and recent breakthroughs in this field, to discuss potential issues and to highlight possible future developments

Thermo-Mechanical Numerical Modeling of the South American Subduction Zone: A Multi-Parametric Investigation

The South American subduction zone is enigmatic due to its fast subducting plate consumption, strong oscillation in subducting plate velocity, and deep mantle subduction. A key to help understand these subduction zone characteristics is through studying its dynamics with buoyancy-driven numerical modeling that uses independent variables to best approximate the dynamics of the real subduction system. We conduct a parametric investigation on the effect of upper mantle rheology, subduction interface yield stress and slab thermal weakening. To constrain those model variables, we attempt to find best-fits by comparing our model outcomes with the present-day slab geometry and estimates of Cenozoic velocities in the center of the South American subduction zone. Key ingredients that need to be reproduced are low slab dip angles close to the surface, steeper lower mantle dip angles, strong oscillation of the fast Farallon-Nazca subducting plate velocity and a progressive decrease in trench retreat rate during long-term subduction. We include these ingredients to define a model fitting score using 10 criteria. Our best fitting models involve a weak subduction zone interface (yield stress of ∼20 MPa) and significant slab thermal weakening to attain the fast Farallon-Nazca subducting plate velocity and to better reproduce the subduction partitioning since 48 Ma due to reduced shear stresses resisting downdip slab sinking and reduced slab bending resistance. Furthermore, a non-Newtonian upper mantle promotes slab folding and realistic oscillation of the subducting plate velocity. Whether and how this slab folding process induces temporal variations in Andean deformation remains an open question

Effect of Plate Length on Subduction Kinematics and Slab Geometry:Insights From Buoyancy-Driven Analog Subduction Models

Subduction style is controlled by a variety of physical parameters. Here we investigate the effect of subducting plate length on subduction style using laboratory experiments of time-evolving buoyancy-driven subduction in 3-D space. The investigation includes two experimental sets, one with a lower (~740) and one with a higher (~1,680) subducting plate-to-mantle viscosity ratio (ηSP/ηM). Each set involves five models with a free-trailing-edge subducting plate and variable plate length (20–60 cm, scaling to 1,600–4,800 km), and one model with a fixed-trailing-edge subducting plate representing an infinitely long plate. Through determining the contact area between subducting plate and underlying mantle, plate length affects the resistance to trenchward motion of the subducting plate and thus controls the partitioning of the subduction velocity (vS) into the subducting plate velocity (vSP) and trench velocity (vT). This subduction partitioning thereby determines the subduction style by controlling the dip angle of the slab tip once it first touches the 660 km discontinuity. The low ηSP/ηM models display two subduction styles. Short plates (≤40 cm) induce a higher subduction partitioning ratio (vSP/vS), promoting trench advance and slab rollover geometries, whereas longer plates (≥50 cm) lead to a lower vSP/vS, producing continuous trench retreat and backward slab draping geometries. In contrast, the high ηSP/ηM models exclusively show trench retreat with draping geometries, as the high ηSP/ηM enables less slab bending before its tip touches the 660 km discontinuity. Our study indicates that future modeling work should consider the effects of plate length on the style and evolution of subduction

Overriding Plate Deformation and Topography During Slab Rollback andSlab Rollover: Insights From Subduction Experiments

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Overriding Plate Deformation and Topography During Slab Rollback and Slab Rollover: Insights From Subduction Experiments

Some subduction zones in nature show mainly overriding plate (OP) extension and low topography, and others show mainly shortening and elevated topography. Here we investigate how end-member subduction modes (trench retreat with slab rollback and trench advance with slab rollover) affect overriding plate deformation (OPD), topography, and mantle flow with time-evolving three-dimensional fully-dynamic analog models using particle image velocimetry. We conduct two sets of experiments, one of which is characterized by trench retreat, and the other characterized by trench advance. Experiments showing continuous trench retreat experience overall OP extension, while experiments dominated by trench advance experience overall shortening. Both subduction modes present fore-arc shortening and intra-arc extension. Our experiments indicate that the overall OPD is mainly driven by the horizontal mantle flow at the base of the OP inducing a viscous drag force (FD), and is determined by the horizontal gradient of the horizontal mantle shear rate (Formula presented.), which controls the horizontal trench-normal gradient in FD. Furthermore, a large-scale trenchward OP tilting and overall subsidence are observed in the experiments showing continuous trench retreat, while a landward OP tilting and an overall uplift are observed during long-term trench advance. The two types of topography during the two different subduction modes can be ascribed to the downward component of the large-scale trenchward mantle flow and the upward component of the landward mantle flow, respectively, and thus represent forms of dynamic topography. Our models showing trench advance provide a possible mechanism for OPD and topography at the Makran subduction zone

Geodynamic models of Indian continental flat slab subduction with implications for the topography of the Himalaya-Tibet region

The slab structure and high elevation of the Himalaya-Tibet region and their underlying mechanisms have been widely discussed. Many studies interpret a flat slab segment of Indian continental lithosphere located below the overriding plate, but interpretations of the northward extent of the flat slab differ substantially, with minimum estimates placing the boundary at the northern margin of the Himalaya (Indus-Yarlung Tsangpo suture), and maximum estimates placing it at the northern boundary of Tibet. In this study, we investigate for the first time if a flat slab segment of subducted buoyant Indian continental lithosphere below the Himalaya-Tibet region is geodynamically feasible and we quantify its northward extent, as well as its contribution to the high topography of the region. We conduct three large-scale fully-dynamic (buoyancy-driven) analogue experiments to simulate the subduction of the Indian continent. Our preferred, and geodynamically most feasible, model shows a continental flat slab extending northward up to ~ 320 km from the Himalayan thrust front, in agreement with recent estimates. Furthermore, it suggests that the positively buoyant flat slab segment of the Indian continent contributes some ~ 1.5–2 km to the high topography of the Himalaya-Southern Tibet region by providing an upward force to elevate the overriding Eurasian plate.</p