Showing papers on "Slab published in 2018"

A Rigorous Method of Calculating Exfoliation Energies from First Principles

Abstract: The exfoliation energy, the energy required to peel off an atomic layer from the surface of a bulk material, is of fundamental importance in the science and engineering of two-dimensional materials. Traditionally, the exfoliation energy of a material has been obtained from first-principles by calculating the difference in the ground-state energy between (i) a slab of N atomic layers (N ≫ 1) and (ii) a slab of N – 1 atomic layers plus an atomic layer separated from the slab. In this paper, we prove that the exfoliation energy can be obtained exactly as the difference in the ground-state energy between a bulk material (per atomic layer) and a single isolated layer. The proposed method is (i) tremendously lower in computational cost than the traditional approach because it does not require calculations on thick slabs, (ii) still valid even if there is a surface reconstruction of any kind, (iii) capable of taking into account the relaxation of the single exfoliated layer (both in-plane lattice parameters and ...

Basal continental mantle lithosphere displaced by flat-slab subduction

Abstract: During the subhorizontal (flat) subduction of an ocean–lithosphere plate, the overlying continental plate is deformed far inland of the plate boundary. In addition, arc magmatism, which is caused by melting in the asthenospheric wedge during steep subduction, wanes or ends when the subduction is subhorizontal. The observed upper-plate deformation patterns have been explained by an end load exerted at the plate boundary or by stress transmitted into the overlying plate along the top of the flat slab. Here we present numerical thermal–mechanical models of flat-slab subduction that show the flattening of the slab results in a compression of the continental plate through end loading. The advancing flat slab scrapes off the lowermost 20–50 km of continental mantle lithosphere. The displaced continental mantle lithosphere fills the asthenospheric wedge, ending arc-type melting. If the displaced material is buoyant, it accumulates in a growing keel that migrates ahead of the slab; if it is dense, the displaced material sinks with the slab. Flat-slab removal renews the asthenospheric wedge and arc magmatism, and leaves a step in the lithosphere–asthenosphere boundary and/or a keel that consists of displaced continental mantle lithosphere. A fossil keel and a fossil step, formed during Laramide flat subduction, are preserved below the western United States. Continental mantle lithosphere is scraped from the base of the overriding plate by the underlying oceanic slab during flat subduction, according to numerical thermal–mechanical models.

Puzzling features of western Mediterranean tectonics explained by slab dragging

Abstract: The recent tectonic evolution of the western Mediterranean region is enigmatic. The causes for the closure of the Moroccan marine gateway prior to the Messinian salinity crisis, for the ongoing shortening of the Moroccan Rif and for the origin of the seismogenic Trans-Alboran shear zone and eastern Betics extension are unclear. These puzzling tectonic features cannot be fully explained by subduction of the east-dipping Gibraltar slab in the context of the regional relative plate motion frame. Here we use a combination of geological and geodetic data, as well as three-dimensional numerical modelling of subduction, to show that these unusual tectonic features could be the consequence of slab dragging—the north to north-eastward dragging of the Gibraltar slab by the absolute motion of the African Plate. Comparison of our model results to patterns of deformation in the western Mediterranean constrained by geological and geodetic data confirm that slab dragging provides a plausible mechanism for the observed deformation. Our results imply that the impact of absolute plate motion on subduction is identifiable from crustal observations. Identifying such signatures elsewhere may improve the mantle reference frame and provide insights on subduction evolution and associated crustal deformation.

Oxidising agents in sub-arc mantle melts link slab devolatilisation and arc magmas

Abstract: Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42− in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/∑Fe in spinel record a S6+–Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.

Proto-South China Sea Plate Tectonics Using Subducted Slab Constraints from Tomography

Abstract: The past size and location of the hypothesized proto-South China Sea vanished ocean basin has important plate-tectonic implications for Southeast Asia since the Mesozoic. Here we present new details on proto-South China Sea paleogeography using mapped and unfolded slabs from tomography. Mapped slabs included: the Eurasia-South China Sea slab subducting at the Manila trench; the northern Philippine Sea Plate slab subducting at the Ryukyu trench; and, a swath of detached, subhorizontal, slab-like tomographic anomalies directly under the South China Sea at 450 to 700 km depths that we show is subducted ‘northern proto-South China Sea’ lithosphere. Slab unfolding revealed that the South China Sea lay directly above the ‘northern Proto-South China Sea’ with both extending 400 to 500 km to the east of the present Manila trench prior to subduction. Our slab-based plate reconstruction indicated the proto-South China Sea was consumed by double-sided subduction, as follows: (1) The ‘northern proto-South China Sea’ subducted in the Oligo–Miocene under the Dangerous Grounds and southward expanding South China Sea by in-place ‘self subduction’ similar to the western Mediterranean basins; (2) limited southward subduction of the proto-South China Sea under Borneo occurred pre-Oligocene, represented by the 800–900 km deep ‘southern proto-South China Sea’ slab.

Slab stagnation due to a reduced viscosity layer beneath the mantle transition zone

Abstract: The linear structures of seismically fast anomalies, often interpreted as subducted slabs, in the southern Asia and circum-Pacific lower mantle provided strong evidence for the whole mantle convection model. However, recent seismic studies have consistently shown that subducted slabs are deflected horizontally for large distances in mantle transition zone in the western Pacific and other subduction zones, suggesting that the slabs meet significant resistance to their descending motion and become stagnant in the transition zone. This poses challenges to the whole mantle convection model and also brings the origin of stagnant slabs into question. Here, using a global mantle convection model with realistic spine–post-spinel phase change (−2 MPa K−1 Clapeyron slope) and plate motion history, we demonstrate that the observed stagnant slabs in the transition zone and other slab structures in the lower mantle can be explained by the presence of a thin, weak layer at the phase change boundary that was suggested by mineral physics and geoid modelling studies. Our study also shows that the stagnant slabs mostly result from subduction in the past 20–30 million years, confirming the transient nature of slab stagnation and phase change dynamics on timescales of tens of millions of years from previous studies. Slab stagnation in the transition zone is explained by a thin, weak layer and is transient on timescales of tens of millions of years, according to a global mantle convection model that includes phase changes and plate motion history.

Carbonate Transfer during the Onset of Slab Devolatilization: New Insights from Fe and Zn Stable Isotopes

Abstract: Long-term carbon cycling is a subject of recent controversy as new mass balance calculations suggest that most carbon is transferred from the slab to the mantle wedge by fluids during subduction, limiting the efficiency of carbon recycling to the deep mantle. Here, we examine the large scale mobility of carbon during subduction using new isotopic tracers sensitive to H–C–O–S–Cl fluids, namely iron and zinc stable isotopes, in samples interpreted to represent residual slab (Queyras, Western Alps) and sub-arc mantle (Kohistan, Himalaya). We show that during subduction there are several stages of carbonate precipitation and dissolution at metasomatic interfaces between metasedimentary and ultramafic rocks in the slab. During the early stages of subduction, before the slab reaches the 300–400C isotherms, the infiltration of sediment-derived fluids into ultramafic lithologies enhances carbonate precipitation in antigorite-bearing serpentinites. Carbonate storage in serpentinites, therefore, acts as a temporary reservoir of carbon in subduction zones. This episode is accompanied by a decrease in serpentinite iron isotope composition (d56Fe), due to interaction with low-d56Fe sediment-derived fluids, and an increase in the concentrations of fluid-mobile elements (e.g. B, Li, As). At higher temperatures (>400C), carbonate is leached from the serpentinites by fluids. This is accompanied by a decrease in serpentinite zinc isotope composition (d66Zn) which we interpret as the release of a carbonate-bearing fluid with an isotopically heavy d66Zn signature. Thermodynamic modelling shows that the sudden change in fluid carbon mobility is due to a decrease in the aCO2 of the fluids released during slab prograde metamorphism, which shifts from sediment- to serpentinite-dominated dehydration. This demonstrates that slab fluids bearing oxidized carbon (e.g. CO2), associated with isotopically light Fe, heavy Zn and fluid-mobile elements, can be released before the slab reaches eclogite facies P-T conditions. These observations provide strong evidence for the mobility of carbon in fluids during the early stages of subduction. Moreover, the fluids released will act as a potential metasomatic agent for the fore-arc mantle (or slab/mantle interface). The observation of carbonate-bearing metamorphic veins in the Himalayan sub-arc mantle with complementary light d56Fe and heavy d66Zn signatures provides further support for the large scale transfer of both sulphate- and carbonate-bearing fluids during the early stages of subduction. This suggests that the fore-arc may have an important role in delivering water, sulfur and carbon to the source of arc-magmas.

Deep embrittlement and complete rupture of the lithosphere during the M w 8.2 Tehuantepec earthquake

Abstract: Subduction zones, where two tectonic plates converge, are generally dominated by large thrust earthquakes. Nonetheless, normal faulting from extensional stresses can occur as well. Rare large events of this kind in the instrumental record have typically nucleated in and ruptured the top half of old and cold lithosphere that is in a state of extension driven by flexure from plate bending. Such earthquakes are limited to regions of the subducting slab cooler than 650 °C and can be highly tsunamigenic, producing tsunamis similar in amplitude to those observed during large megathrust events. Here, we show from analyses of regional geophysical observations that normal faulting during the moment magnitude Mw 8.2 Tehuantepec earthquake ruptured the entire Cocos slab beneath the megathrust region. We find that the faulting reactivated a bend-fault fabric and ruptured to a depth well below the predicted brittle–ductile transition for the Cocos slab, including regions where temperature is expected to exceed 1,000 °C. Our findings suggest that young oceanic lithosphere is brittle to greater depths than previously assumed and that rupture is facilitated by wholesale deviatoric tension in the subducted slab, possibly due to fluid infiltration. We conclude that lithosphere can sustain brittle behaviour and fail in an earthquake at greater temperatures and ages than previously considered. Geophysical observations of the 2017 Tehuantepec earthquake suggest that oceanic lithosphere can sustain brittle behaviour and rupture in an earthquake at greater depths than previously assumed.

The Impact of Slab Rollback on Earth's Surface: Uplift and Extension in the Hinterland of the North American Cordillera

Abstract: Slab rollback processes alter the intraplate force balance and buoyancy of the overriding plate, driving surface uplift or extension. From ca. 55–24 Ma, Farallon slab rollback produced migrating volcanism and sedimentation across the western United States as stress on the North American plate transitioned from subduction-driven compression to widespread extension. Hypotheses regarding rollback-driven surface deformation differ widely in timing and magnitude. Here we combine hydrogen isotope ratios with high-resolution geochronology to show that a high-elevation plateau extended westward from the Sevier-Laramide fold-thrust belt across Utah and eastern Nevada prior to slab rollback. Quantitative paleoelevation estimates show that this plateau had obtained over 80% of peak paleoelevations by middle Eocene. Slab rollback, heating, and lithospheric delamination generated 400–600 m of Oligocene surface uplift. Concurrent extension limited overall uplift and rollback-induced mantle flow likely contributed to the propagation of upper crustal extension that formed the Basin and Range province. Plain Language Summary Changes in surface elevations are critical to understanding the formation and evolution of mountains and basins across ancient landscapes. Slab rollback—retreat and steepening of the downgoing tectonic plate during subduction—can transform Earth’s surface by altering the stresses on the overriding plate. By measuring the ratios of hydrogen isotopes in ancient rainwater, preserved for millions of years in volcanic ash, we can estimate past elevations. Here we measure past elevations across the Basin and Range province of Utah and Nevada to reconstruct topography before and during slab rollback. Results show that a high elevation plateau extended westward from the western edge of the Rocky Mountains across Utah and eastern Nevada prior to slab rollback. Slab rollback generated a shortlived interval of 400–600 m of surface uplift, much less than has been previously estimated. Instead, subsequent processes resulting from rollback likely contributed to collapse of the high plateau and the widespread extensional faulting that created the characteristic topography of the modern Basin and Range province.

Slab narrowing in the Central Mediterranean: the Calabro-Ionian subduction zone as imaged by high resolution seismic tomography

Abstract: A detailed 3D image of the Calabro-Ionian subduction system in the central Mediterranean was obtained by means of a seismic tomography, exploiting a large dataset of local earthquakes and computing algorithms able to build a dense grid of measure nodes. Results show that the slab is continuous below the southern sector of the Calabro-Peloritan Arc, but the deformation processes developing at its edges are leading to its progressive narrowing, influencing tectonics and magmatism at the surface, and with possible stress concentration in the tip zones. In the southwest, the deformation occurring at a free slab edge lead to propagation of a vertical lithospheric tear in the overriding plate, which extends along a NW-SE fault system (Aeolian-Tindari-Letojanni) up to about 30 km into the Ionian Sea; further southeast, the lithosphere appears only flexed and not broken yet. In the northeast, the slab seems to break progressively, parallel to the trench. Finally, northwest of Mt. Etna, the tomography highlights low VP that can be related to an upwelling of deep mantle material likely flowing laterally through a window opened by the complete slab detachment.

Full scale laboratory testing of ballast and concrete slab tracks under phased cyclic loading

Abstract: Full-scale laboratory-based testing is used to compare the long-term settlement performance of a precast concrete slab track section to a ballasted track (with concrete sleepers) resting on a compacted substructure. The railway track substructure is constructed from a 1.2 m deep combined subgrade and frost protection layer, according to modern high-speed rail standards such as those specified in Germany. Phased cyclic loading is then used to simulate the primary loading mechanism of a train after 3.4 million load cycles representing many years’ worth of train passages. Displacement transducers, earth pressure cells and accelerometers are employed to determine the permanent settlement, the cyclic displacement, transient stresses and vibrations of the track. The equipment, loading combinations, material properties and experimental displacement results are presented and compared. The results indicate that the ballasted track experienced 20 times more settlement when compared to the concrete slab track under the same loading conditions, even though the ballasted track was tested at a slightly higher compacted state due to the concrete slab track test being conducted first.

Western US volcanism due to intruding oceanic mantle driven by ancient Farallon slabs

Abstract: The origin of late Cenozoic intraplate volcanism over the western United States is debated. One important reason is the lack of a clear understanding of the mantle dynamics during this volcanic history. Here we reconstruct the mantle thermal states beneath North America since 20 million years ago using a hybrid inverse geodynamic model with data assimilation. The model simultaneously satisfies the past subduction kinematics, present mantle tomographic image and the volcanic history. We find that volcanism in both the Yellowstone volcanic province and the Basin and Range province corresponds to a similar eastward-intruding mantle derived from beneath the Pacific Ocean and driven mostly by the sinking Farallon slab below the central-eastern United States. The hot mantle that forms the Columbia River flood basalt and subsequent Yellowstone–Newberry hotspot tracks first enters the western United States through tears within the Juan de Fuca slab. Subsequent coexistence of the westward asthenospheric flow above the retreating Juan de Fuca slab and eastward-propagating mantle beyond the back-arc region reproduces the bifurcating hotspot chains. A similar but weaker heat source intrudes below the Basin and Range around the southern edge of the slab, and can explain the diffuse basaltic volcanism in this region. According to our models, the putative Yellowstone plume contributes little to the formation of the Yellowstone volcanic province. Volcanism in the western US may result from warm oceanic mantle beneath the Pacific Ocean being drawn eastwards by mantle flow induced by the sinking of Farallon slabs, according to numerical model simulations.

Imaging the Eastern Trans‐Mexican Volcanic Belt With Ambient Seismic Noise: Evidence for a Slab Tear

Abstract: The eastern sector of the Trans-Mexican Volcanic Belt (TMVB) is an enigmatic narrow zone that lies just above where the Cocos plate displays a sharp transition in dipping angle in central Mexico. Current plate models indicate that the transition from flat to steeper subduction is continuous through this region, but the abrupt end of the TMVB suggests that the difference in subduction styles is more likely to be accommodated by a slab tear. Based on a high-resolution shear wave velocity and radial anisotropy model of the region, we argue that a slab tear within South Cocos can explain the abrupt end of the TMVB. We also quantify the azimuthal anisotropy beneath each seismic station and present a well-defined flow pattern that shows how mantle material is being displaced from beneath the slab to the mantle wedge through the tear in the subducted Cocos plate. We suggest that the toroidal mantle flow formed around the slab edges is responsible for the existence of the volcanic gap in central Mexico. Moreover, we propose that the temperature increase caused by the influx of hot, less dense mantle material flowing through the tear to the Veracruz area may have significant implications for the thermomechanical state of the subducted slab and explain why the intermediate-depth seismicity ends suddenly at the southern boundary of the Veracruz basin. The composite mantle flow formed by the movement of mantle material through the slab tears in western and southern Mexico may be allowing the Cocos plate to roll back in segments. Plain Language Summary The Trans-Mexican Volcanic Belt (TMVB) is a prominent and enigmatic feature of the subduction system in Mexico. Its volcanic style diversity and oblique orientation to the trench are explained by the large along-strike variations in the subduction parameters of the Rivera and Cocos plates. However, the abrupt termination of the TMVB on its eastern end with the Pico de Orizaba volcano is puzzling as the current slab model suggests that the transition of the Cocos flat-slab geometry to normal subduction is smooth through this region. There is evidence that suggests that a tear in the slab might be developing, but it is unclear how this feature can support the unusually large topographic gradient that connects the volcanic high peaks with the Veracruz basin just south of the volcanic front. To provide further insight into the transition anatomy of this portion of the subducted slab and its relation with surface topography, we present a detailed and unified model of the velocity structure of the crust and uppermost mantle of central Mexico.