Slab Morphology Research Articles

Tonga Slab Morphology and Stress Variations Controlled by a Relic Slab: Implications for Deep Earthquakes in the Tonga‐Fiji Region

AbstractDeep focus earthquakes within the Tonga‐Fiji subduction system account for about two‐thirds of the global total and provide significant constraints on slab deformation. The factors controlling the intense deformation remain unclear. Here, we use two‐dimensional, time‐dependent geodynamic models to study the morphology, stress state, and thermal structure of the Tonga‐Fiji subduction zone. The results, consistent with tomographic images and focal mechanisms, demonstrate that collision between a relic slab from the Vanuatu Trench and the Tonga slab may control the steeper dip of the Tonga slab and earthquakes in the mantle transition zone. We suggest that the magnitude 8.2 and 7.9 earthquakes in 2018 mostly ruptured within the warm rim of the Tonga slab and occurred beneath the folding relic slab with high temperatures of at least ∼900°C and ∼1100°C, respectively. The findings support the hypothesis that local slab temperature likely controls rupture of deep earthquakes.

Feedback of Slab Distortion on Volcanic Arc Evolution: Geochemical Perspective From Late Cenozoic Volcanism in SW Japan

AbstractSouthwest Japan is an island arc formed by subduction of the Philippine Sea (PHS) plate. The Quaternary magmatism in this region is characterized by eruptions of high‐Sr andesites and dacites, considered to have been derived by melting of the PHS plate. The loci of these volcanoes spatially coincide with seismic discontinuities of the subducted PHS plate. Thus, the magmatism is interpreted as the result of slab melting at the plate tears. However, the processes that promote slab tearing remain unclear. In this study, we applied geochronological and geochemical analyses to late Cenozoic volcanic rocks in southwest Japan as tracers of slab morphology. Two different magma types, ocean‐island basalt (OIB) and island‐arc basalt (IAB), have occurred over 12 million years (Myr). These two magmas are attributed to different integrations of melts extracted from an originally fertile mantle; the OIBs from high temperature melt (1,300–1,400°C) were extracted at a depth of 40–80 km, whereas the IABs were extracted from a shallower, lower temperature region (30–60 km, 1,200–1,350°C). Secular change in Sr enrichment of IAB likely arose due to a transition of slab‐derived fluids, incorporated into magmas as they formed, from water‐ to melt‐dominant one. Progressive shallowing of the subducted PHS plate is responsible for secular change in the properties of slab‐derived fluids as well as rollback of OIB volcanoes. Production of chemically variable magmas in the Chugoku district is the surface expression of distorting slab morphology by interaction between mantle and the subducting plate.

Orphaning Regimes: The Missing Link Between Flattened and Penetrating Slab Morphologies

Slab orphaning is a newly discovered phenomenological behaviour, where the slab tip breaks off at 660 km depth and is abandoned by its parent slab. Upon orphaning, subduction continues uninterrupted through the lateral motion of the parent slab above 660 km depth. In this work, we present a regime diagram for the range of conditions under which slabs can orphan at mid-mantle depths. We observe that a viscosity jump at 1000 km, separate from the endothermic phase change responsible for the 660 km seismic discontinuity, is necessary for orphaning, as is slab and mantle weakening between 660 and 1000 km. We show that orphan slabs, similar to other deep slab morphologies, can be the end result for a wide range of physical parameters governing slab dynamics: Slab orphaning persists across wide variations in slab dip, slab strength, Clapeyron slope and overriding plate nature. The diversity in orphan slab sizes and orphaning length scales is tied to the orphaning regime space, which describes a hitherto unexplored region between deflected and penetrating deep-subduction modes that can be found in the transition zone between upper and lower mantle. Orphaning provides a simple dynamic link between the well known deflection and penetration, and provides one possible way for subducting slabs to switch from direct penetration to deflection, littering the mantle with abandoned fragments.

Interaction of the Indian and Asian Plates Under the Pamir and Hindu‐Kush Regions: Insights From 3‐D Shear Wave Velocity and Anisotropic Structures

AbstractThe Pamir‐Hindu‐Kush region is widely cited as a best place to study opposing continental subduction on Earth. Yet, subducting slab morphology of the Indian and Asian plates under this region remains elusive. Here we report new shear wave velocity and radial anisotropy models from Rayleigh and Love wave tomography to constrain the geometry of these two slabs. Together with previous tomographic studies, we show that the Indian slab exhibits along‐strike variation with subhorizontally underthrusting into the Pamir and high‐angle subduction under the Hindu‐Kush. We speculate that the Indian slab was probably detached and sank into the mantle transition zone under the Pamir, while break‐off process is ongoing under the Hindu‐Kush. The Asian lithospheric mantle and lower crust are inferred to be subducting southward into the North Pamir. Two opposing subducting slabs could have interacted under the Pamir, resulting in thickening crust, horizontally stretched midcrustal flow, and horizontal mantle flow. These dynamic processes could explain surface geologic observations and potentially constrain the causes of intermediate‐depth seismicity.

Structure and Quantification of Edge Sites of WS2/Al2O3 Catalysts Coupling IR/CO Spectroscopy and DFT Calculations

AbstractIR spectroscopy of CO adsorption (IR/CO) and Density Functional Theory (DFT) calculations on sulfide W/Al2O3 catalyst were investigated for an insight into the sulfide edge sites. Parallel between experimental and theoretical results allow to assign the ν(CO) bands at 2121 and 2066 cm−1 to CO adsorption on M‐ and S‐edge sites of WS2 phase, respectively. Through a careful analysis of CO spectra on well‐selected W catalysts, the molar attenuation coefficients for CO adsorbed on the two exposed edges could be determined. This allowed the morphology of WS2 slabs to be calculated. This work shows that increasing the W loading (6–20 wt% W) changes the slab morphology from an almost pure triangle exhibiting mostly M‐edge to a truncated triangle. On W as well as on Mo sulfide catalysts, IR spectroscopy and DFT calculation show that CO adsorption allows to account for M‐ and S‐edge sites, and thus of the morphology of sulfide slabs. It was shown that increasing the W loading shifts the slab morphology from a triangle exposing predominantly M‐edge towards a more hexagonal shape. For close to monolayer coverage of alumina, Mo and W sulfide slabs present similar shape i. e. truncated triangle. However, edge sites of WS2 slabs clearly exhibit greater stability under H2 atmosphere than Mo sulfided ones.

Topographic Fingerprint of Deep Mantle Subduction

AbstractThe dynamic topography links with the mantle structures at various temporal and spatial scales. However, it is still unclear how it relates to the dynamics of subducting lithosphere when plates reach the mantle transition zone and lower mantle. Seismic tomography images show how slab morphologies vary from sinking subvertically into the lower mantle, to lying flat above the upper‐lower mantle discontinuity, to thickening in the shallow lower mantle. These slab shapes have been considered to be the result of variable interaction of the slab with the upper‐lower mantle discontinuity at ~670 km depth. Previous studies show that periodic deep slab dynamics can explain a variety of enigmatic geological and geophysical observations such as periodic variations of the plate velocities, trench retreat and advance episodes, and the scattered distribution of slab dip angle in the upper mantle. In this study, we use two‐dimensional subduction models to investigate the surface topography expression and its evolution during slab transition zone interaction. Our models show that topography does not depend on slab morphology; indeed, the dynamic topography cannot distinguish between a slab sinking straight into the lower mantle and slab stagnation at the upper‐lower mantle boundary. However, topographic oscillations are related to episodes of the trench advance and retreat, which in turn are linked to the slab folding behavior at transition zone depths. Our results suggest that the surface transient signal observed by geological studies could help to detect deep subduction dynamics.

Slab Morphology Beneath Northern Sumatra Revealed by Regional and Teleseismic Traveltime Tomography

AbstractAn eikonal equation‐based tomography method is used to invert the high‐quality regional and teleseismic traveltime data recorded by 26 broadband seismic stations in northern Sumatra, following which we obtain the P and S wave velocity structures of the crust and mantle down to a depth of 800 km. The results of both P and S wave tomography clearly show that the Indo‐Australian oceanic plate continuously penetrates downward beneath northern Sumatra and the maximum penetration depth varies approximately from 400 km at the northern tip of Sumatra to about 800 km around the southern boundary of our study area. Significant slab folding or bending reported in the literature as the main feature of the subducted slab beneath northern Sumatra is not found in our results. Instead, our tomographic images demonstrate only a less curved slab that mimics the shape of the Sunda Trench and volcanic arc and generally extends over depths from 120 to 450 km. P wave tomography shows broad and pronounced low‐velocity anomalies beneath the island of Sumatra in the lower crust and uppermost mantle. Our model also reveals a slab tear approximately at 120‐km depth, which has been documented in previous studies and considered to be related to the eruption of the Toba supervolcano.

New insight into the support effect on HDS catalysts: evidence for the role of Mo-support interaction on the MoS2 slab morphology

MoS2 catalysts were prepared on Al2O3, SiO2 and TiO2 support. On the oxidic forms, Raman and UV–vis characterizations show the strong Mo-TiO2 interaction and the weak Mo-SiO2 one that leads to a poorer Mo dispersion. MoS2 nanoparticles were then studied by TEM and adsorption of CO followed by infra-red spectroscopy which allows the distinction and quantification of the two stable edges namely the S- and M- edges. It appears that the slab morphology is strongly modified by the support nature: thus the stronger the MoS2-support interaction the higher the S-/M- edge ratio. Finally, MoS2 morphology impacts the catalytic activity in thiophene HDS: (1) an increase of the S-/M- edge ratio leads to the increase of the TOF value for Mo sulfided supported on Al2O3 and SiO2; (2) a specific behavior with high TOF value of mainly M-edge sites and strong hydrogenating properties is underlined for Mo/TiO2.

Subducting slab morphology and mantle transition zone upwelling in double-slab subduction models with inward-dipping directions

SUMMARY Lithospheric plates on the Earth's surface interact with each other, producing distinctive structures comprising two descending slabs. Double-slab subduction with inward-dipping directions represents an important multiplate system that is not yet well understood. This paper presents 2-D numerical models that investigate the dynamic process of double-slab subduction with inward dipping, focussing on slab geometry and mantle transition zone upwelling flow. This unique double-slab configuration limits trench motion and causes steep downward slab movement, thus forming fold piles in the lower mantle and driving upward mantle flow between the slabs. The model results show the effects of lithospheric plate properties and lower-mantle viscosity on subducting plate kinematics, overriding plate stress and upward mantle flow beneath the overriding plate. Appropriate lower-mantle strength (such as an upper–lower mantle viscosity increase with a factor of 200) allows slabs to penetrate into the lower mantle with periodical buckling. While varying the length and thickness of a long overriding plate (≥2500) does not have a substantial effect on slab geometry, its viscosity has a marked impact on slab evolution and mantle flow pattern. When the overriding plate is strong, slabs exhibit an overturned geometry and hesitate to fold. Mantle transition zone upwelling velocity depends on the speed of descending slabs. The downward velocity of slabs with a large negative buoyancy (caused by thickness or density) is very fast, inducing a significant transition zone upwelling flow. A stiff slab slowly descends into the deep mantle, causing a small upward flow in the transition zone. In addition, the temporal variation of mantle upwelling velocity shows strong correlation with the evolution of slab folding geometry. In the double subduction system with inward-dipping directions, the mantle transition zone upwelling exhibits oscillatory rise with time. During the backward-folding stage, upwelling velocity reaches its local maximum. Our results provide new insights into the deep mantle source of intraplate volcanism in a three-plate interaction system such as the Southeast Asia region.

A Visual Survey of Global Slab Geometries With ShowEarthModel and Implications for a Three‐Dimensional Subduction Paradigm

Increasing volumes of digital data, combined with advances in visualization, allow for a new three‐dimensional (3D) conceptualization of modern subduction systems. Here we present 16 visual exploration sessions with the ShowEarthModel program that interrogate modern slab morphologies on Earth. These virtual voyages through the Earth's interior provide a snapshot into the four‐dimensional evolution of plate tectonics, captured by the modern state of slab structure. Examining the modern subduction system in an interactive 3‐D framework provides an improved conceptualization of the morphology of slabs. The movies demonstrate that unlike the modern mid‐ocean ridge system, modern subduction systems are laterally discontinuous features, making lateral slab edges prevalent. The 3‐D visualizations elucidate the manner in which slabs intersect and overlap, processes that occur in at least six of the Earth's major subduction zones. The observed variability within a given subduction zone as well as the physical presence of intersecting slabs suggests that slab gaps and slab windows are common. The 3‐D rendering shows relative differences in the radius of curvature for arcuate slabs, as well as the variability in dip within a given subduction zone. Addressing the complex geometries of subducted lithosphere in this way can place constraints on outstanding questions of subduction dynamics including slab strength, mantle rheology, mantle flow circulation, and plate‐asthenosphere coupling. By conceptualizing the modern subduction system on Earth in 3‐D space, this paper contributes to the paradigm shift from a two‐dimensional (2‐D) to 3‐D framework for interpreting the subduction process.

The engraved monoliths in the Mané er Groez Neolithic passage tomb in Kercado (Carnac, Morbihan)

The site of Kercado, in Carnac, is situated above the valley of Kerloquet and the ria that runs along the beach of Kerdual for 2 km. It comprises a probable low mound, the cairn of Mane er Groez, with a passage grave, and a curvilinear enclosure of about thirty standing stones bordering the south side of the site. Three orthostats in the passage, three other stones in the funerary chamber, as well as the cover slab, bear engraved signs recorded mainly between 1866 and 1927. In addition to the well-known “axe-plough” on the ceiling of the chamber, we can now add a new inventory of engravings, including pairs of polished blades, a ring, bows, boats with crew and variations on the quadrangular symbol.The aim of this article is to present an updated account of this corpus of engravings, and to take slab morphology into consideration, as well as the anomalies that can obscure our perception of these symbols, in order to enhance our understanding of this passage tomb.

The effect of rheological approximations in 3-D numerical simulations of subduction and collision

Subduction and collision zones evolve differently from one another due to different rheological properties, different amounts of regional isostatic compensation, and the different mechanisms by which forces are applied to the convergent plates. The rheology of mantle and lithosphere is known to have the largest influence on the dynamics of subduction and continental collision. However, previous 3-D geodynamic models of subduction/collision processes have used various rheological approximations, making their results difficult to compare, since there is no clear understanding on the extent of these approximations on the dynamics. Here, we test the effect of rheological approximations in a geometrically simplified 3-D model in which the dynamics of subduction and collision is entirely driven by slab-pull (i.e., “free subduction”). The results exhibit a wide range of behaviors depending on the rheological law employed: from linear viscous (LV) to temperature-dependent visco-elasto-plastic (VEP) rheology that takes into account both diffusion and dislocation creep. Rheological approximations exert a strong effect on slab morphology, trench migration rates and topography in the upper plate. For example, we find that slab dynamics varies drastically between end member models. In LV approximations, slabs do not break-off, but instead buckle at the transition zone. The dynamics is dominated by low strain rates and diffuse stresses, and topography is controlled by lateral extrusion of material which homogenizes amplitudes in the upper plate. In simulations with non-linear VEP rheology, slab break-off occurs at the corners of the indenting continent, opening of a back-arc basin is fast and topography has a laterally heterogenous distribution of amplitudes. Opening of a back-arc basin is observed only in simulations with non-linear VEP rheology. When the rheology is approximated with multiple discrete layers, the dynamics also resembles more the non-linear VEP case.

Mantle flow influence on subduction evolution

The impact of remotely forced mantle flow on regional subduction evolution is largely unexplored. Here we investigate this by means of 3D thermo-mechanical numerical modeling using a regional modeling domain. We start with simplified models consisting of a 600 km (or 1400 km) wide subducting plate surrounded by other plates. Mantle inflow of ∼3 cm/yr is prescribed during 25 Myr of slab evolution on a subset of the domain boundaries while the other side boundaries are open. Our experiments show that the influence of imposed mantle flow on subduction evolution is the least for trench-perpendicular mantle inflow from either the back or front of the slab leading to 10–50 km changes in slab morphology and trench position while no strong slab dip changes were observed, as compared to a reference model with no imposed mantle inflow. In experiments with trench-oblique mantle inflow we notice larger effects of slab bending and slab translation of the order of 100–200 km. Lastly, we investigate how subduction in the western Mediterranean region is influenced by remotely excited mantle flow that is computed by back-advection of a temperature and density model scaled from a global seismic tomography model. After 35 Myr of subduction evolution we find 10–50 km changes in slab position and slab morphology and a slight change in overall slab tilt. Our study shows that remotely forced mantle flow leads to secondary effects on slab evolution as compared to slab buoyancy and plate motion. Still these secondary effects occur on scales, 10–50 km, typical for the large-scale deformation of the overlying crust and thus may still be of large importance for understanding geological evolution.

基于线结构光传感器的轨道板几何形貌检测方法

基于线结构光传感器的轨道板几何形貌检测方法

基于线结构光传感器的轨道板几何形貌检测方法

基于线结构光传感器的轨道板几何形貌检测方法

Reconstructing subducted oceanic lithosphere by “reverse‐engineering” slab geometries: The northern Philippine Sea Plate

Subducting slabs commonly acquire complex geometries from the migration of subduction trenches, slab‐mantle interaction, slab tearing, and collision of slabs at depth. Although it is possible to construct three‐dimensional models of subducted slabs using earthquake hypocenter locations and tomographic models, it is often not possible to rigorously test their accuracy. Here we present a methodology for performing such a test, by “reverse‐engineering” the presubduction configuration of a slab of oceanic lithosphere from interpretations of its present‐day morphology. We illustrate our approach for the Ryukyu and Shikoku slabs, northwest Philippine Sea, having simulated them as viscoelastic sheets that we unfolded and “floated” to the surface. The net strain distribution of the floated mesh indicated which parts of the original slab model were geometrically viable (minimal net strain) and which parts of the mesh required additional tears and/or zones of localized ductile extension to have enabled the slab to deform during subduction. In the instance of the Ryukyu and Shikoku slabs, the Palau‐Kyushu and Gagua ridges are shown to have both acted as planes of weakness that broke into major vertical slab tears. These subducted ridges are connected by a trench‐parallel tear that represented the former contact between the Huatung and West Philippine Basins. The fossil spreading center of the Shikoku Basin formed a separate slab window upon subduction along the Nankai Trough. The methodology presented herein is a powerful tool to evaluate complex slab morphologies, infer the locations of slab tears, and therefore reconstruct intricate configurations of subducted oceanic lithosphere.

Trench motion‐controlled slab morphology and stress variations: Implications for the isolated 2015 Bonin Islands deep earthquake

AbstractThe subducted old and cold Pacific Plate beneath the young Philippine Sea Plate at the Izu‐Bonin trench over the Cenozoic hosts regional deep earthquakes. We investigate slab morphology and stress regimes under different trench motion histories with mantle convection models. Viscosity, temperature, and deviatoric stress are inherently heterogeneous within the slab, which we link to the occurrence of isolated earthquakes. Models expand on previous suggestions that observed slab morphology variations along the Izu‐Bonin subduction zone, exhibited as shallow slab dip angles in the north and steeper dip angles in the south, are mainly due to variations in the rate of trench retreat from the north (where it is fast) to the south (where it is slow). Geodynamic models consistent with the regional plate tectonics, including oceanic plate age, plate convergence rate, and trench motion history, reproduce the seismologically observed principal stress direction and slab morphology. We suggest that the isolated ~680 km deep, 30 May 2015 Mw 7.9 Bonin Islands earthquake, which lies east of the well‐defined Benioff zone and has its principal compressional stress direction oriented toward the tip of the previously defined Benioff zone, can be explained by Pacific slab buckling in response to the slow trench retreat.

The deforming Nazca slab in the mantle transition zone and lower mantle: Constraints from teleseismic tomography on the deeply subducted slab between 6°S and 32°S

We present new tomographic models of the Nazca slab under South America from 6°S to 32°S, and from 95 km to lower mantle (895 km) depths. By combining data from 14 separate networks in the central Andes, we use finite-frequency teleseismic P-wave tomography to image the Nazca slab from the upper mantle into the mantle transition zone (MTZ) and the uppermost lower mantle on a regional scale. Our tomography shows that there is significant along-strike variation in the morphology of the Nazca slab in the MTZ and the lower mantle. Thickening of the slab in the MTZ is observed north of the Bolivian orocline, possibly related to buckling or folding of the slab in response to the penetration of a near-vertical slab into the higher-viscosity lower mantle, which decreases the sinking velocity of the slab. South of the orocline, the slab continues into the lower mantle with only minor deformation in the MTZ. In the lower mantle, a similar difference in morphology is observed. North of 16°S, the slab anomaly in the lower mantle is more coherent and penetrates more steeply into the lower mantle. To the south, the slab dip appears to be decreasing just below the 660 km discontinuity. This change in slab morphology in the MTZ and lower mantle appears to correspond to the change in the dip of the slab as it enters the MTZ, from steeply dipping in the north to more moderately dipping in the south.

Trench dynamics: Effects of dynamically migrating trench on subducting slab morphology and characteristics of subduction zones systems

Understanding the mechanisms of trench migration (retreat or advance) is crucial to characterizing the driving forces of Earth’s tectonics plates, the origins of subducting slab morphologies in the deep mantle, and identifying the characteristics of subduction zones systems, which are among the fundamental issues of solid Earth science. A series of numerical simulations of mantle convection, focusing on plate subduction in a three-dimensional (3-D) regional spherical shell coordinate system, was performed to examine subduction zone characteristics, including geodynamic relationships among trench migration, back-arc stress, and slab morphology. The results show that a subducting slab tends to deflect around the base of the mantle transition zone and form a sub-horizontal slab because its front edge (its ‘toe’) is subject to resistance from the highly viscous lower mantle. As the sub-horizontal slab starts to penetrate into the lower mantle from its ‘heel,’ the toe of the slab is drawn into the lower mantle. The results for models with dynamically migrating trenches suggest that trench retreat is the dynamically self-consistent phenomenon in trench migration. The reason for this is that the strong lateral mantle flow that is generated as a sequence of events leading from corner flow at the subduction initiation to return flow of the formation of a sub-horizontal slab in the shallower part of mantle wedge produces the retreat of the subducting slab. In fact, a ‘mantle suction force,’ which is generated in the mantle wedge to fill space left by the retreating subducting plate, is enhanced by the subsequent trench retreat. Even when upwelling flow with significant positive buoyancy originates just above a mantle phase boundary at a depth of 410km (as inferred from independent seismic tomographic, geodynamic, geochemical, and mineral physics), reaches the base of the overriding plate, and the overriding plate is slightly thinned, lithospheric stress tends to be compressed above the upwelling flow. The reason for this is that the strong lateral mantle flow originating from the upwelling flow generates resistance drag force at the base of the overriding plates. This situation may apply to a case of East Asia, under which the typical morphology of sub-horizontal slabs can be seen by seismic tomography. The strong lateral velocity observed in the shallower mantle wedge in the present numerical simulation may account for both the compressional subduction tectonics and back arc compression in the Japan–Kuril–Kamchatka, Aleutian, and South Chile trenches, as well as for weak plate-slab coupling, strong seismic coupling, and the possibility of great earthquakes along these trenches.

Complicated 3D mantle flow beneath Northeast China from shear wave splitting and its implication for the Cenozoic intraplate volcanism

We obtained shear wave splitting measurements from XKS (SKS and SKKS) phases recorded by the NECESSArray project consisting of 127 seismic stations in Northeast China (NEC). The most salient feature of the anisotropy in NE China is the alignment of subduction-parallel fast direction, which can be explained by the background upper mantle flow in the big mantle wedge (BMW). However, the geographical variations of fast direction and delay time, which closely follow the geometry of subducting Pacific slab, suggests that the slab morphology controls the geometry of 3-D mantle flow in the BMW. The most significant finding of this study is the complex anisotropic pattern with most nulls observed in the Changbaishan mountain region (CBM) and southern Songliao basin. At the southern Songliao basin, the small delay times as well as null measurements of shear wave splitting are coincident with the high velocity in the upper mantle down to ~300km from previous studies. The observed anisotropy likely arises from the combination of BMW convection and more localized upper mantle convection, that is, a plume-like mantle upwelling from the mantle transition zone beneath the Changbaishan region triggers the down-welling beneath the southern Songliao basin.