Subduction-induced Mantle Flow Research Articles

The Nature of Mantle Flow May Depend on the Type of Slab Subducting

Researchers tease apart the links between slabs and mantle flow near subduction zones, upending some traditional views of subduction-induced mantle flow.

The role of subduction and gravitational collapse in driving extension in the Aegean Sea

The Aegean domain has experienced lithospheric deformation during continuous northward subduction at the Hellenic trench. Early Cenozoic shortening and thickening were followed by late Cenozoic extension and thinning. Thickened lithosphere, in particular its crust, has excess gravitational potential energy, which is thought to promote orogenic collapse (i.e., gravity spreading). In the Aegean domain, release of this potential energy and collapse of the lithosphere has been proposed as the first-order driver for extension of the overriding plate. Here we show, using dynamically self-consistent analogue experiments of subduction and gravity spreading, that the subducting plate significantly influences overriding plate deformation, despite the overriding plate being positively buoyant. With the introduction of a subducting plate, the trench retreat velocity and overriding plate extension rate increase, which is in contrast with a continuous decline during gravity spreading. In addition, ∼E-W trending normal faults emerge, as opposed to arc-parallel faults trending ∼ENE-WSW to ∼ESE-WNW and arc-normal extensional zones that result from gravity spreading only. Similarly, the subducting plate causes overriding plate displacement to be unidirectional (trenchward), in accordance with the present-day geodetic velocity field, whereas gravity spreading causes a radially outward-directed displacement field. As the decline in trench retreat rate, radial spreading, dominantly arc-parallel normal faults, and arc-normal extensional zones are not observed in nature, this implies that gravity spreading cannot be the sole driver of extensional deformation in the overriding plate. Indeed, the models suggest that subduction-induced mantle flow is the dominant driver of overriding plate deformation because it imposes a basal shear stress on the overriding plate, dragging and extending it trenchward in a unidirectional manner. Our experimental results corroborate earlier studies that highlight the important role of subduction-induced mantle flow in driving overriding plate deformation. Additionally, we find that this role is more pronounced in experiments with a thicker brittle layer.

Accelerated subduction of the western Pacific Plate promotes the intracontinental uplift and magmatism in late Jurassic South China

Widespread fold and thrust belt and magmatism that occurred in the Mesozoic South China Block (SCB) have generated much debate on their dynamic mechanism related to the western Pacific Plate. Unlike a delayed Andean orogen, the SCB contains large areas of magmatic rocks coinciding with the absence of Upper Jurassic strata, which is not consistent with a flat-slab subduction geometry, and no consensus model has been suggested. To obtain more general insight into the effects of paleo-Pacific Plate subduction on the SCB, thermal-fluid-solid simulations of the subduction process are conducted. The models involved varied subduction rates and lithospheric viscosities to investigate their effect on the subduction style, mantle flow and overriding plate deformation. Low-rate subduction easily forms a subduction geometry with a higher angle, while high-rate subduction causes the slab to break off and drip downward. Fast subduction produces a quasi-horizontal mantle flow and a poloidal flow at the front of the mantle wedge, whereas slow subduction produces two poloidal flows developed on both sides of the subducting slab. The strain tensor of the overriding plate coincides well with the gradient of the horizontal mantle flow rate. A high subduction rate and low lithospheric viscosity promote the deformation of the overriding plate and are further attributed to mantle flow. Combining these results with global plate reconstructions, we find that accelerated subduction of the paleo-Pacific Plate resulted in the Late Jurassic uplift of the SCB. The low viscosity of the overriding plate caused by multiphase orogenic deformation promoted the thinning of the lithosphere due to induced mantle flow, which was associated with the intrusion of magmas and foundering of the subducted slab in a compressional regime. At the end of the Late Jurassic, subduction-induced mantle flow transformed from westward to eastward. Concurrently, the subducted slab began to steepen, and widespread Cretaceous magmatism occurred.

Hydrothermal Vent Complexes Control Seepage and Hydrocarbon Release on the Overriding Plate of the Tyrrhenian-Ionian Subduction System (Paola Basin)

Active fluid seeps have been described in a wide range of geological environments and geodynamic contexts, which include continental shelves of non-volcanic passive margins and accretionary wedges. Fluids seeping in hybrid volcanic-sedimentary basins, characterized by the presence of magmatic intrusive complexes, have always received less attention. We detected and imaged dozens of distinct gas flares, as high as 700 m, on the continental slope of the Paola Basin in the southeastern Tyrrhenian Sea, at 550–850 m water depth. The sedimentary basin is surrounded by Pleistocene active and inactive volcanoes and volcanic-intrusive complexes, which formed in the back-arc basin of the Calabrian subduction zone, in response to subduction-induced mantle flow. Gas flares develop above pockmarks, craters and mud flows that form over and along the scarps of mound structures and correspond to seismic zones of free gas accumulation in the sub-seafloor. Here, methane-derived siderite shows enrichment in δ13C and δ18O isotopes likely related to methanogenesis and intermittent venting of deep-sourced CO2. Multichannel seismic reflection data showed that the gas flares develop in correspondence of doming and diapirism apparently originating from the top of the Messinian evaporites and nearby magmatic sills, that are present in the lower part of the Plio-Quaternary succession. These diapiric structures can be related to seafloor hydrothermal vent complexes fed by the igneous intrusions. Our data suggest that the vent complexes acted as fluid migration pathways and gas conduits, which at times are bounded by deep-rooted normal faults, leading to post-explosive near-surface microbial activity and seep carbonate formation. Fluids being mobilised by magmatism in the study area include: hydrocarbons and hydrothermal fluids generated at depth, interstitial water expelled during formation of polygonal faults. The close spatial correlation between seafloor seep manifestations, fluid migration pathways in the sub-surface involving part of the Messinian units and igneous features indicates that magmatic activity has been the main driver of fluid flow and can have a long-term effect in the southern Tyrrhenian Sea.

Subduction-controlled mantle flow and seismic anisotropy in South America

Seismic anisotropy records both the past and present deformation inside the solid Earth. In the mantle, seismic anisotropy is mainly attributed to the lattice preferred orientation (LPO) of mineral fabrics, caused by the shear deformation due to mantle flow. However, contributions from different tectonic processes remain debated, and a single geodynamic model that simultaneously explains the observed mantle structures and various seismic anisotropy measurements is still lacking. Here, we present a model for the Cenozoic subduction history in South America using a geodynamic simulation constrained by both past plate reconstructions and present mantle seismic structures. With a recently developed software package DRexS, we further predict azimuthal seismic anisotropy at different depths and generate synthetic shear wave splitting (SWS) measurements using the resulting mantle flow. Our results provide a good match to both depth-dependent surface wave anisotropy and various land-based SWS records. We find that the dominant control on seismic anisotropy in South America comes from subduction-induced mantle flow, where anisotropy below the subducting Nazca Plate aligns with plate-motion-induced Couette flow and that below the overriding South American Plate follows slab-induced Poiseuille flow. This large-scale mantle flow can be diverted by secondary slabs, such as that below the Antilles subduction zone. In contrast, the contribution to SWS from fossil continental anisotropy and from the effects due to mantle flow modulation by lithosphere thickness variation are minor. Upper-mantle fast seismic anomalies beneath the southern Atlantic margin should have close-to-neutral buoyancy in order to satisfy the observed seismic anisotropy.

The role of deep subduction in supercontinent breakup

The breakup of continents and their subsequent drifting plays a crucial role in the Earth's periodic plate aggregation and dispersal cycles. While continental aggregation is considered the result of oceanic closure during subduction, what drives sustained divergence in the following stages remains poorly understood. In this study, thermo-mechanical numerical experiments illustrate the single contribution of subduction and coupled mantle flow to the rifting and drifting of continents. We quantify the drag exerted by subduction-induced mantle flow along the basal surface of continental plates, comparing models of lithospheric slab stagnation above the upper-lower mantle boundary with those where slabs penetrate into the lower mantle. When subduction is upper-mantle confined, divergent basal tractions localise at distances comparable to the effective upper mantle thickness (~500km), causing the opening of a marginal basin. Instead, subduction of lithosphere in the lower mantle reorganises the flow into a much wider cell localising extensional stresses at greater distances from the trench (~3000km). Sub-continental tractions are higher and more sustained over longer time periods in this case, and progressively increase as the slab sinks deeper. Although relatively low, basal-shear stresses when integrated over large plates, generate tension forces that may exceed the strength of the continental lithosphere, eventually leading to breakup and opening of a distal basin. The models illustrate the emergence of a similar mechanism, which results in the formation of back-arc basins above upper-mantle confined subduction, and scales to much larger distances for deeper subduction. Examples include the Atlantic Ocean formation and drifting of the South and North American plates during the Mesozoic-Cenozoic Farallon plate subduction.

Does subduction-induced mantle flow drive backarc extension?

Abstract Backarc extension is a characteristic feature of many narrow subduction zones. Seismological and geochemical studies imply the occurrence of mantle flow around the narrow subducting slabs. Previous 3D models suggested that backarc extension is related to subduction-induced toroidal mantle flow. The physical viability of this mechanism, however, has never been tested using laboratory-based geodynamic models. In this work, we present dynamic laboratory models of progressive subduction in three-dimensional (3D) space that were carried out to test this mechanism. To achieve this, we have used a stereoscopic Particle Image Velocimetry (sPIV) technique to map simultaneously overriding plate deformation and 3D subduction-induced mantle flow underneath and around an overriding plate. The results show that the strain field of the overriding plate is characterized by the localization of an area of maximum extension within its interior (at 300–500 km from the trench). The position of maximum extension closely coincides (within ∼2 cm, scaling to 100 km) with that of the maximum trench-normal horizontal mantle velocity and velocity gradient measured at a scaled depth of 15–25 km below the base of the overriding plate, and the maximum horizontal gradient of the vertical mantle velocity gradient. We propound that in narrow subduction zones backarc extension in the overriding plate is mainly a consequence of the trench-normal horizontal gradients of basal drag force at the base of the overriding plate. Such shear force gradients result from a horizontal gradient in velocity in the mantle below the base of the lithosphere induced by slab rollback. Calculations based on our models indicate a tensional horizontal trench-normal deviatoric stress in the backarc region scaling to ∼28.8 MPa, while the overriding plate trench-normal stress resulting from the horizontal component of the trench suction force is about an order of magnitude smaller, scaling to ∼2.4–3.6 MPa.

Evolution of 3-D subduction-induced mantle flow around lateral slab edges in analogue models of free subduction analysed by stereoscopic particle image velocimetry technique

We present analogue models of free subduction in which we investigate the three-dimensional (3-D) subduction-induced mantle flow focusing around the slab edges. We use a stereoscopic Particle Image Velocimetry (sPIV) technique to map the 3-D mantle flow on 4 vertical cross-sections for one experiment and on 3 horizontal depth-sections for another experiment. On each section the in-plane components are mapped as well as the out-of-plane component for several experimental times. The results indicate that four types of maximum upwelling are produced by the subduction-induced mantle flow. The first two are associated with the poloidal circulation occurring in the mantle wedge and in the sub-slab domain. A third type is produced by horizontal motion and deformation of the frontal part of the slab lying on the 660 km discontinuity. The fourth type results from quasi-toroidal return flow around the lateral slab edges, which produces a maximum upwelling located slightly laterally away from the sub-slab domain and can have another maximum upwelling located laterally away from the mantle wedge. These upwellings occur during the whole subduction process. In contrast, the poloidal circulation in the mantle wedge produces a zone of upwelling that is vigorous during the free falling phase of the slab sinking but that decreases in intensity when reaching the steady-state phase. The position of the maximum upward component and horizontal components of the mantle flow velocity field has been tracked through time. Their time-evolving magnitude is well correlated to the trench retreat rate. The maximum upwelling velocity located laterally away from the subducting plate is ∼18–24% of the trench retreat rate during the steady-state subduction phase. It is observed in the mid upper mantle but upwellings are produced throughout the whole upper mantle thickness, potentially promoting decompression melting. It could thereby provide a source for intraplate volcanism, such as Mount Etna in the Mediterranean, the Chiveluch group of volcanoes in Kamchatka and the Samoan hotspot near Tonga.

Bifurcation of the Yellowstone plume driven by subduction-induced mantle flow

In the northwestern United States, two neighbouring volcanic chains—the Snake River Plain and High Lava Plains—decrease in age in opposing directions. Laboratory experiments show that diverging volcanic chains can form where vigorous mantle circulation beneath a subduction zone causes an upwelling mantle plume to split into two.

Hot mantle upwelling across the 660 beneath Yellowstone

P-to-s receiver functions mapped to depth through P and S body-wave tomography models image continuous 410 and 660km discontinuities beneath the area covered by USArray prior to the year 2011. Mean depths to the 410 and 660km discontinuities of 410 and 656km imply a mantle transition zone that is about 4km thicker than the global average and hence has a slightly cooler mean temperature and/or enhanced water content. Compared to the mean 660 depth beneath this ~2000km wide area, the 660 beneath the Yellowstone hotspot is deflected upward by 12–18km over an area about 200km wide. This is the most anomalous shallowing of the 660 imaged and its horizontal extent is similar to the area where P and S tomography image low-velocity mantle extending from the top of the transition zone to about 900km depth. Together, these results indicate a high-temperature, plume-like upwelling extending across the 660. The depth of 410km discontinuity beneath the Yellowstone region is within 5km of the mean depth implying that the plume is vertically heterogeneous and possibly discontinuous. Tomography indicates a similar vertically heterogeneous thermal plume. The irregular plume structure may be intrinsic to the dynamics of upwelling through the transition zone, or distortion may be caused by subduction-induced mantle flow. Topography of the 410 and 660 confirms that subducted slabs beneath the western U.S. are highly segmented, as inferred from recent tomography studies. We find no evidence of regionally pervasive velocity discontinuities between 750 and 1400km depth. The plume's depth of origin within the lower mantle remains uncertain.

Upper mantle anisotropy beneath central and southwest Japan: An insight into subduction-induced mantle flow

Analysis of seismic anisotropy in the crust and mantle wedge above subduction zones gives much information about the dynamic processes inside the Earth. For this reason, we measure shear wave polarization anisotropy in the crust and upper mantle beneath central and southwestern Japan from local shallow, intermediate, and deep earthquakes occurring in the subducting Pacific slab. We analyze S phases from 198 earthquakes recorded at 42 Japanese F-net broadband seismic stations. This data set yields a total of 980 splitting parameter pairs for central and southwestern Japan. Dominant fast polarization directions of shear waves obtained at most stations in the Kanto–Izu–Tokai areas are oriented WNW–ESE, which are sub-parallel to the subduction direction of the Pacific plate. However, minor fast polarization directions are oriented in NNE–SSW directions being parallel to the strike of the Japan Trench, especially in the north of Izu Peninsula and the northern Tokai district. Generally, fast directions obtained at stations located in Kii Peninsula and the Chubu district are oriented ENE–WSW, almost parallel to the Nankai Trough, although some fast directions have NW–SE trends. The fast directions obtained at stations in northern central Honshu are oriented N–S. Delay times vary considerably and range from 0.1 to 1.25 s depending on the source depth and the degree of anisotropy along the ray path. These lateral variations in splitting character suggest that the nature of anisotropy is quite different between the studied areas. Beneath Kanto–Tokai, the observed WNW–ESE fast directions are probably caused by the olivine A-fabric induced by the corner flow. However, the slab morphology in this region is relatively complicated as the Philippine Sea slab is overriding the Pacific slab. This complex tectonic setting may induce lateral heterogeneity in the flow and stress state of the mantle wedge, and may have produced NNE–SSW orientations of fast directions. The ENE–WSW fast directions in Kii Peninsula and the Chubu district are more coherent and may be partly induced by the subduction of the Philippine Sea plate. The N–S fast directions in northern central Honshu might be produced by the trench-parallel stretching of the wedge due to the curved slab at the arc–arc junction.

Thinning of continental backarc lithosphere by flow-induced gravitational instability

Many continental backarcs have thin (∼ 60 km) lithosphere for 100s of km behind the volcanic arc, even where there has been no significant extension. One mechanism to produce thin backarc lithosphere is through gravitational thinning of normal thickness lithosphere by subduction-related mantle flow. The stability of backarc mantle lithosphere is examined using thermal–mechanical models of subduction of an oceanic plate beneath continental lithosphere with an initial thickness of 120 km and a thermal structure similar to average Phanerozoic continental lithosphere. Subduction-induced mantle flow shears the base of the backarc lithosphere, producing lateral density perturbations. Owing to the non-Newtonian lithosphere rheology, shearing also reduces the effective viscosity of the lowermost lithosphere, enabling the density perturbations to become gravitationally unstable. The perturbations develop into rapidly-growing downwellings which result in removal of lower backarc lithosphere on timescales of 5–10 Ma. Conductive heating and shearing of the remaining lithosphere results in a second, more muted, phase of gravitational instability and thinning. Lower lithosphere instability is enhanced by higher subduction rates, weaker intrinsic rheology, higher compositional density, and hotter initial thermal structure. The numerical model results are in good agreement with a buoyancy stability analysis, which includes the vertical gradients in temperature and strain rate through the lithosphere. As both rheology and density depend on lithosphere composition, significant thinning may be restricted to continental mantle lithosphere that is fertile and contains a small amount of water. To produce a final thickness comparable to that observed without lithospheric contraction or extension, it is necessary to have a weak lithosphere rheology, an initial thermal structure hotter than average Phanerozoic continental geotherms, or a combination of the two. The region of thin lithosphere at the northern Cascadia backarc coincides with terranes that were accreted to the North American craton. The known fertile composition of the backarc lithosphere may allow it to be thinned, while the drier, more refractory craton lithosphere is resistant to thinning.

Pull-down basin in the central part of Japan due to subduction-induced mantle flow

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone. A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.

The Devonian to Permian subsidence mechanisms in basins of the East-European platform

Tectonic subsidence analysis of five major sedimentary basins of the East European platform reveals two episodes of rapid downward motion since the Devonian. The mechanisms of basin evolution are examined to explain the rapid subsidence of the basins. Models of these mechanisms are compared with seismic, gravity, heat flow and Devonian magmatism observations from the Timan-Pechora, Pre-Ural, Pre-Caspian, Dnieper-Donets and Moscow basins. The models assume that the post-Devonian development of the basins has been affected by one or more of the following processes: stretching of the lithosphere and thermal decay, mineralogical phase transition in the crust, magmatism and eclogitization-induced mantle flow in the uppermost mantle, subduction under the platform and continental collision. While some features of basin evolution are in good agreement with one of the mechanisms, no specific model can explain all features of the platform basins. Magmatism-eclogitization mechanisms are likely to be responsible for the Devonian subsidence of the Dnieper-Donets, Timan-Pechora, and Pre-Caspian basins, whereas the subduction-induced mantle flow may explain the Devonian to Permian subsidence of the Moscow basin and Pre-Ural depression.

Platform subsidence behind an active subduction zone

NUMERICAL models of viscous mantle flow indicate that a broad subsidence should occur above subduction zones1–4. This 'platform subsidence' is predicted to cause an asymmetrical depression 500–1,000 km wide, with a maximum depth of ∼2 km if filled with water. Direct evidence linking platform subsidence to mantle flow has remained elusive, however. Here we offer evidence from western New Zealand for a complete link in both space and time between subduction initiation and platform subsidence. Because of the broad wavelength (>200 km) of the observed 1,500-m subsidence it is unlikely that the subsidence is due to a flexural foredeep. The observed subsidence does, however, match in both wavelength and amplitude that predicted by a model of subduction-induced mantle flow. The occurrence of platform subsidence has implications not only for understanding mantle convection, but also for explaining geoid anomalies over active margins5 and patterns of continental submergence6 and sedimentation7 that could otherwise be interpreted as a eustatic sea level change8.