Lithosphere-asthenosphere Boundary Research Articles

Stress change in southwest Japan due to the 1944–1946 Nankai megathrust rupture sequence based on a 3-D heterogeneous rheological model

The Nankai trough has repeatedly experienced large megathrust earthquakes at intervals of 100–200 years. The inland region of southwest (SW) Japan has a seismically active period from 50 years before to 10 years after megathrust earthquakes. To assess the activities of inland earthquakes after megathrust earthquakes, we need to quantitatively evaluate the postseismic stress accumulation on the inland source faults considering plausible viscoelastic relaxation. Recent studies have shown the importance of low-viscosity layers along the lithosphere-asthenosphere boundary (LAB layer) in postseismic deformation. In the present study, we focus on the most recent ruptures, the 1944 M7.9 Tonankai and the 1946 M8.0 Nankai earthquakes, estimating the 4-year stress change on the source faults in SW Japan in a forward modeling approach. The 1944–1946 megathrust rupture sequence was followed by severe ~ M7 inland earthquakes, such as the 1945 M6.8 Mikawa and 1948 M7.1 Fukui earthquakes. For stress calculation, we used a highly detailed finite element model incorporating the actual topography and the plausible viscoelastic underground structure from past studies. The calculated inland stress field shows the dominance of the coseismic change during the 1944 and 1946 earthquakes and little contribution from viscoelastic relaxation. In contrast, viscoelastic relaxation has a significant effect on stress in the slab, indicating the importance of quantifying the viscosity of the LAB layer. Based on the calculated stress, we evaluated the change in the Coulomb failure stress (ΔCFS) on each source fault. The ΔCFS is generally positive on the strike-slip faults east of 135°E due to the 1944 rupture. In contrast, the ΔCFS on the faults west of 135°E, including the Median Tectonic Line segments, became positive due to the 1946 rupture. The occurrence of the damaging earthquakes in 1945 and 1948 can be explained by the calculated ΔCFS. The ΔCFS on the recent earthquake faults of the recent damaging earthquakes such as the 2016 M7.2 Kumamoto earthquake is generally negative, suggesting the delay in stress accumulation. The ΔCFS on the source faults of the intra-slab earthquakes differ significantly as large as tens of kilopascals depending on the viscosity of the LAB layer.Graphical

Seismic evidence for melt-rich lithosphere-asthenosphere boundary beneath young slab at Cascadia

The Lithosphere-Asthenosphere Boundary (LAB) beneath oceanic plates is generally imaged as a sharp seismic velocity reduction, suggesting the presence of partial melts. However, the fate of a melt-rich LAB is unclear after these plates descend into the mantle at subduction zones. Recent geophysical studies suggest its persistence with down-going old and cold slabs, but whether or not it is commonly present remains unclear, especially for young and warm slabs such as in the Cascadia subduction zone. Here we provide evidence for its presence at Cascadia in the form of a large (9.8±\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm$$\\end{document}1.5%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document}) decrease in shear-wave velocity over a very small (<3 km) depth interval. Similarly large and sharp seismic velocity reduction at the bottom of both old and young slabs, as well as along the base of oceanic plates before subduction, possibly represents widespread presence of melts. The melt-rich sub-slab LAB may strongly influence subduction dynamics and viscoelastic earthquake cycles.

Mapping Global Lithospheric Mantle Pressure‐Temperature Conditions by Machine‐Learning Thermobarometry

AbstractComprehending the temperature distribution within Earth's lithospheric mantle is of paramount importance for understanding the dynamics of Earth's interior. Traditional mineral‐based thermobarometers effectively constrain temperature and pressure for particular compositions, but their application is limited at the global scale. Here, we trained machine‐learning (ML) algorithms on 985 published high‐temperature and high‐pressure experiments for use as thermometers and barometers to overcome the limitations of classic methods. We compared our ML models to classic thermobarometers to assess the accuracies of predicted pressures and temperatures. The comparison shows that the ML models outperform classic methods and better fit various mineral pairs. Global application of the ML models unveils mantle conditions beneath cratons. Furthermore, depths to the lithosphere‐asthenosphere boundary (LAB) calibrated based on the ML thermobarometry results are generally deeper by ∼40 km than those derived geophysically, implying the existence of melt‐bearing or hydrated mineral zones at the LAB.

Deep lithosphere-coupled surface deformation beneath Southeastern Brazil from amphibious magnetotellurics

The southeastern Brazilian onshore-offshore margin and underlying lithosphere are investigated by three-dimensional magnetotelluric imaging using 202 high-quality broadband and long-period amphibious MT-dataset, orthogonally traversing the regional surface deformation hills of Serra da Mantiqueira and Serra do Mar, as well as the coast-parallel onshore-offshore lineaments. Three-dimensional conductivity model reveals an electrically heterogeneous crust (up to 40 km) in the onshore Brasília and Ribeira Orogenic Belts, and ∼ 2–10 km thick-conductive-sedimentary-wedge beneath the offshore Santos Basin laying on top of a stretched and thinner resistive crust. Regionally, the lithosphere is found ∼130 km thick beneath the most northwestern part of the Brasília Orogenic Belt which is thinning out towards the deep-water Santos Basin (∼70 km). This thinning coincides with the lithosphere-asthenosphere boundary in recent seismic tomography results, reflecting the current state of the uplifted asthenosphere post-West Gondwana break-up. A major steeply dipping sub-lithospheric conductor, in conjunction with a confined asthenosphere upwelling (or lithospheric thinning), has been observed underneath Embu-Paraíba do Sul domain (Ribeira belt), which may be somehow associated with the continental scale rifting, surface uplift and the Cretaceous São Paulo-Rio de Janeiro dykes swarm. Our findings provide valuable insights into the formation of southeast Brazilian highlands and its association with mantle plume-induced lithospheric delamination (or removal) and the occurrence of alkaline magmatism, after reactivation of some pre-existing weak zones.

Crust and upper mantle S wave velocity structure in eastern Turkey based on ambient noise tomography

Eastern Turkey is an ideal location to study continental collision, which is manifested here by the presence of extensive volcanism and rapid tectonic uplift. To better understand the relationship between these surface phenomena and underground structures, we collect continuous waveform data from 121 seismic stations in the region to investigate the S wave velocity structure of the crust and upper mantle using ambient noise tomography. We obtain Rayleigh wave dispersion curves for periods between 5 and 100 s based on frequency-Bessel (F-J) transform dispersion analysis. We then perform quasi-Newton inversion to calculate the S wave velocity structure between 0 and 200 km. The results indicate the presence of two high-velocity anomalies: one in the southern Black Sea that may represent stable and cold lithospheric mantle and another near Lake Van that may represent the subduction remnant of the southern branch of the Neo-Tethys Ocean. In addition, the results demonstrate that the overall velocity in the study area is lower than the global average and reveal extensive upper mantle low-velocity zones (UMLVZs) which represent asthenospheric materials, and mid-crustal low-velocity zones (MCLVZs). In this study, we propose two possible formation mechanisms of the MCLVZs: one is partial melting, and the other is the formation of low-velocity compositional layering due to complex refining and differentiation processes and subsequent continent reworking. Considering that the spatial distribution of the MCLVZs and UMLVZs is well correlated with regional volcanic activity, the MCLVZs and UMLVZs may provide the driving force and are the material sources for volcanism in the area. Furthermore, the extremely thin and weak lithospheric mantle lid and the exceedingly shallow lithosphere asthenosphere boundary (LAB) indicate that the lower part of the lithosphere has delaminated and that the asthenosphere intrudes into a very shallow location, which then supports regional tectonic uplift.

Multiscale structures of crust-mantle beneath the South China block and their geodynamic implication

The South China block is an ideal location in which to study the impact of paleo-Pacific plate subduction on the upper mantle and crustal structures of the overriding plate, as well as the mechanism of lithospheric thinning. By integrating data from permanent seismic stations of the China Seismic Network and a dense seismic array at Xuefeng Mountain, this study employed P- and S-wave receiver functions (RFs) to image multiscale structures of crust-mantle across the South China block, thereby providing constraints for a comprehensive understanding of its tectonic evolution. The main findings are as follows: (1) The thickness of lithosphere and crust gradually decreases from northwest (NW) to southeast (SE). (2) Beneath the Xuefeng Mountain uplift zone, the P-to-S conversion from the 660 km discontinuity arrives earlier than that predicted from a one-dimensional model, other weak discontinuities can be traced in the upper mantle, including the lithosphere-asthenosphere boundary, the crust becomes thinner, and the Conrad discontinuity becomes deeper. Referring to previous results from tomography and geological reconstructions, we deduce that lithospheric delamination might have occurred beneath the Xuefeng Mountain uplift zone, and the delaminated lithosphere penetrated the upper mantle and reached the mantle transition zone.

Geodetic signature of a weak Lithosphere-Asthenosphere Boundary in postseismic deformation of large subduction earthquakes

The rheology of the seismically imaged Lithosphere-Asthenosphere Boundary (LAB) is an important new question in plate tectonics. If the LAB is associated with ponding of partial melts as inferred from many seismic studies, it should have a low viscosity that minimizes resistance to plate motion and subduction. Here, we propose that recently observed postseismic enhanced landward motion (ELM) in coastal areas outside the rupture zone of large subduction earthquakes provides the first geodetic evidence for a low-viscosity LAB beneath subducting oceanic lithosphere. Using three-dimensional viscoelastic finite element models, we demonstrate that a low-viscosity LAB readily leads to postseismic ELM over broad coastal areas far beyond the rupture zone. If the LAB thickness is assumed to be 10 km, key spatiotemporal characteristics of the observed ELM can be well explained by an LAB viscosity of 5 × 1016 Pa s, orders of magnitude lower than typical asthenospheric viscosities. A thicker LAB such as 20 km with a higher viscosity may also explain the postseismic ELM but is incompatible with existing, albeit limited, near-field vertical deformation observations. The small thickness and the low viscosity of the model LAB are consistent with inferences from seismological observations and support the notion of ponding melts. Our preliminary weak-LAB models also predict diagnostic deformation features that can be tested by future onshore and offshore geodetic observations. Whether a weak LAB is ubiquitously present before and after plate subduction and how its presence and spatial heterogeneity impact postseismic and plate-scale deformation are important research targets for future investigations.

The Pannon LitH2Oscope magnetotelluric array in the Pannonian Basin

AbstractThe Pannonian Basin is one of the best natural laboratories in the world to study the lithospheric response to continental extension and subsequent tectonic inversion. Here we address the topic of lithospheric structure by a combined geochemical and magnetotelluric analysis, which has been carried out in the framework of the Pannon LitH2Oscope project. The main objective was to detect the resistivity distribution over the entire lithosphere by magnetotelluric measurements, considering the lithological resistivity properties and relate the results to the structure and evolution of the Pannonian Basin. The Pannon LitH2Oscope MT array was used to estimate the depth of the Lithosphere-Asthenosphere Boundary (LAB), considering the legacy MT data and compared to previous estimates for the region. Using the MT and geomagnetic response functions, major structural zones of the Pannonian basin, such as the Mid-Hungarian Shear Zone or fault systems like the Makó Trough and the Békés Basin, were also imaged. In addition, we used the apparent resistivity soundings to compare 1D resistivity models computed from geochemistry and obtained from field MT measurements. This comparison provided new constrains for the composition, fluid and melt content variations at the local lithosphere-asthenosphere boundary. The Pannon LitH2Oscope MT dataset and the results presented in this paper provide input for more complex 3D inversions and further investigations of the lithospheric structure in the Carpathian-Pannonian region.

Contribution of carbonatite and recycled oceanic crust to petit-spot lavas on the western Pacific Plate

Abstract. Petit-spot volcanoes, occurring due to plate flexure, have been reported globally. As the petit-spot melts ascend from the asthenosphere, they provide crucial information of the lithosphere–asthenosphere boundary. Herein, we examined the lava outcrops of six monogenetic volcanoes formed by petit-spot volcanism in the western Pacific. We then analyzed the 40Ar/39Ar ages, major and trace element compositions, and Sr, Nd, and Pb isotopic ratios of the petit-spot basalts. The 40Ar/39Ar ages of two monogenetic volcanoes were ca. 2.6 Ma (million years ago) and ca. 0 Ma. The isotopic compositions of the western Pacific petit-spot basalts suggest geochemically similar melting sources. They were likely derived from a mixture of high-μ (HIMU) mantle-like and enriched mantle (EM)-1-like components related to carbonatitic/carbonated materials and recycled crustal components. The characteristic trace element composition (i.e., Zr, Hf, and Ti depletions) of the western Pacific petit-spot magmas could be explained by the partial melting of ∼ 5 % crust bearing garnet lherzolite, with 10 % carbonatite flux to a given mass of the source, as implied by a mass-balance-based melting model. This result confirms the involvement of carbonatite melt and recycled crust in the source of petit-spot melts. It provides insights into the genesis of tectonic-induced volcanoes, including the Hawaiian North Arch and Samoan petit-spot-like rejuvenated volcanoes that have a similar trace element composition to petit-spot basalts.

On-chip very low strain rate rheology of amorphous olivine films

Recent observations made by the authors revealed the activation of stress induced amorphization and sliding at grain boundary in olivine [1], a mechanism which is expected to play a pivotal role in the viscosity drop at the lithosphere-asthenosphere boundary and the brittle-ductile transition in the lithospheric mantle. However, there is a lack of information in the literature regarding the intrinsic mechanical properties and the elementary deformation mechanisms of this material, especially at time scales relevant for geodynamics. In the present work, amorphous olivine films were obtained by pulsed laser deposition (PLD). The mechanical response including the rate dependent behavior are investigated using a tension-on-chip (TOC) method developed at UCLouvain allowing to perform creep/relaxation tests on thin films at extremely low strain rates. In the present work, strain rate down to 10−12 s−1 was reached which is unique. High strain rate sensitivity of 0.054 is observed together with the activation of relaxation at the very early stage of deformation. Furthermore, digital image correlation (DIC), used for the first time on films deformed by TOC, reveals local strain heterogeneities. The relationship between such heterogeneities, the high strain rate sensitivity and the effect of the electron beam in the scanning electron microscope is discussed and compared to the literature.

Lithospheric weakening by a small-scale plume and its geodynamic implications

The process of small-scale mantle plume emplacement within the continental lithosphere influences the dynamic topography and architecture of the lithosphere-asthenosphere boundary (LAB). Furthermore, plume emplacement alters the rheology of the crust and lithospheric mantle through melt-driven, mechanical and thermal weakening. Previous studies of plume emplacement within the lithosphere have described these outcomes, albeit without considering plume-driven weakening. In this study, we quantitatively evaluated the effects of small-scale plume-driven predefined weakening of the continental lithospheric mantle and crust on dynamic topography and LAB depth changes using a series of two-dimensional numerical models. Model calculations showed that small plume-driven predefined weakening led to short-wavelength dynamic topography and LAB depth reduction through both mechanical and thermal erosion of the lithospheric mantle by the plume. An increase in the degree of weakening decreased the lithospheric strength and reduced the LAB depth, thereby transforming the short-wavelength topography from depressions to elevations as the extensional kinematics shifted to compression. Our model findings explain the topography, lithospheric strength, and LAB depth constrained from natural small-scale plume emplacement zones.

Deep electrical imaging of the extinct oceanic ridge in the southwestern sub-basin of the South China Sea

The resistivity structure of an extinct mid-ocean ridge is significant in understanding the evolution of a mid-ocean ridge from its spreading phase to its dying phase. The magnetotelluric (MT) method is a crucial tool in studying the deep resistivity structure as it is sensitive to resistivity which is affected by heat and allows for imaging of the electrical properties of the mantle. While modern electromagnetic data has enhanced our understanding of the deep structure of rapidly expanding and ultra-slow expanding mid-ocean ridges, the deep electrical structure below extinct mid-ocean ridges has not been studied extensively.In July 2020, marine MT instruments were deployed in the southwest subbasin of the South China Sea to study the resistivity structure below a stalled mid-ocean ridge. The study found that the imaged thickness of the lithospheric lid (&amp;gt;100Ωm) varies between 20 and 90 km, exhibiting a positive correlation with its age. The melt ascent channel is closed below the stalled mid-ocean ridge, and the melt falls back and forms a small melt trap below the dead mid-ocean ridge. In the northwest survey line of oceanic ridge, huge low-resistivity anomalies (&amp;lt;1Ωm), located between 80km and 160 km depth. In the southeast survey line of oceanic ridge, there is a slightly smaller low-resistivity anomalies (&amp;lt;1Ωm). These results indicate that partial melt continues to exist after the cessation of spreading at the mid-ocean ridge. The lithosphere-asthenosphere boundary (LAB) and the ocean basin of the South China Sea experienced a certain magmatic transformation during the cooling and falling process. According to the resistivity, temperature, pressure, and SEO3 model, the melt content is estimated to be approximately 1-12%. The electrical structure of the mantle in the South China Sea is an important basis for studying the current state beneath the ceased spreading mid-ocean ridge.

Shallow sources of upper mantle seismic anisotropy in East Africa

The East African rift overlies one or more mantle upwellings and it traverses heterogeneous Archaean-Paleozoic lithosphere rifted in Mesozoic and Cenozoic time. We re-analyze XKS shear wave splitting at publicly available stations to evaluate models for rifting above mantle plumes. We use consistent criteria to compare and contrast both splitting direction and strength, infilling critical gaps with new data from the Turkana Depression and North Tanzania Divergence sectors of the East African rift system. Our results show large spatial variations in the amount of splitting (0.1–2.5 s), with fast axes predominantly sub-parallel to the orientation of Cenozoic rifts underlain by thinned lithosphere with and without surface magmatism. The amount of splitting increases with lithospheric thinning and magmatic modification. Nowhere are fast axes perpendicular to the rift, arguing against the development of extensional strain fabrics. Thick cratons are characterized by small amounts of splitting (≤0.5 s) with a variety of orientations that may characterize mantle plume flow. Splitting rotates to rift parallel and increases in strength over short distances into rift zones, implying a shallow depth range for the anisotropy in some places. The shallow source and correlation between splitting direction and the shape of upper mantle thin zones suggests that the combination of channel flow and oriented melt pockets contribute > 1 s to the observed splitting delays. Enhanced flow, metasomatism, and melt intrusion at the lithosphere-asthenosphere boundary suggest that fluid infiltration to the base of the lithosphere may facilitate rifting of cratonic lithosphere.

Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observations

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (&amp;lt; 14 s) and teleseismic surface waves at long periods (&amp;gt; 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

Adopting normalized full gradient method for regional‐scale gravity modelling: A case study for Northwestern Iran

AbstractThe normalized full gradient was developed to determine anomalous bodies, such as oil and gas fields or simple geological structures studies. We believe that even in complicated geology, normalized full gradient is practical. We introduce data preprocessing and use step‐by‐step simple‐to‐complicated synthetic tests to develop previous researchers’ ideas for regional‐scale gravity modelling. One of the most important steps of the normalized full gradient is the determination of the N optimum value. We found that prevalent methods such as the standard spectral or maxima method are feasible in simple structures only. So, we have suggested the imaging criteria routine for complicated cases. We trace maximum normalized full gradient responses to detect the normalized full gradient responses at the increasing harmonic numbers as the transition of the extensive part of the anomaly to the sharp part of that. With imaging criteria for the determination of N optimum values, the complicated synthetic test results show the success of the normalized full gradient to understand complicated gravity signals. In the real case, we have studied the Northwestern Iran normalized full gradient model of the Bouguer ground gravity data beneath the seismic profile and prepared a P receiver function depth section to uncover the geometry of the Moho boundary and important interfaces in the crust. We suggest the inferred synthetic model from the Bouguer ground gravity anomaly and P receiver function depth section to normalized full gradient trustworthy test in real cases. According to the synthetic test results, we understand the frame of the normalized full gradient responses in the semi‐real case and truthful responses in the real case. Along with this, we study the second ground gravity profile of Northwestern Iran in a good resolution to uncover the deeper structures. The real case results show the possibility of Moho offset and thinning lithosphere beneath the North Tabriz Fault lithospheric boundary, the possible source of Sahand volcanic centre at the west side of the Moho offset beneath North Tabriz Fault, the deep root of the Sabalan volcanic centre in the lower crust and the lithospheric and asthenospheric wedge with the density contrast beneath Sahand–Sabalan volcanic centres. One of the most important results of our study is the lithosphere–asthenosphere boundary offset and stepped Moho possibility beneath the Talesh Mts next to the South Caspian Basin boundary.

Thermal Evolution of the Lithosphere‐Asthenosphere Boundary Beneath Arc and Its Geodynamic Implications: Depth Variation of Thermal Histories of Mantle Xenoliths From Ichinomegata, Northeast Japan

AbstractQuantitative reconstruction of thermal history recorded in mantle xenoliths is imperative for understanding the temporal change of thermal state and dynamics of the lithosphere‐asthenosphere boundary (LAB). We challenged this problem in the arc settings by examining nine spinel peridotite xenoliths from Ichinomegata maar in the back‐arc side of Northeast Japan Arc. Extensive mineral chemical analyses combined with the derivation depths of the xenoliths revealed a depth‐dependent variation of chemical zoning patterns in olivine and pyroxenes. The depth variation of thermal histories of the Ichinomegata xenoliths was decoded by applying diffusion‐controlled reaction modeling to reproduce the zoning patterns. The decoded thermal events in the order of occurrence are (a) ∼14 million years of cooling causing lithosphere thickening up to ∼55 km depth, (b) subsequent ∼12 thousand years of heating from the underlying asthenosphere resulting in lithosphere thinning up to the depths of ∼40 km, and (c) 1–68 days of heating during xenolith transportation by the host magma. The duration of the lithosphere thickening is consistently explained by the period of the Japan Sea opening. On the other hand, the timescale of the lithosphere thinning is too short to be explained by heat conduction through the ∼15 km thick LAB and requires a more effective heat transportation mechanism such as direct magma injection into the LAB or significant viscosity reduction of the mantle peridotite aided by the pervasive permeable flow of silicate melt.

Sheared Peridotites from Northern Lesotho: Metasomatism-Induced Deformation and Craton Destabilization

Abstract Sheared peridotite xenoliths are snapshots of deformation processes that occur in the cratonic mantle shortly before their entrainment by kimberlites. The process of deformation that caused the shearing has, however, been highly debated since the 1970s and remains uncertain. To investigate the processes involved in the deformation, we have studied 12 sheared peridotites from Late Cretaceous (90 Ma) kimberlites in northern Lesotho, on the southeast margin of the Kaapvaal craton. Various deformation textures are represented, ranging from porphyroclastic to fluidal mosaic. Our sample suite consists of eleven garnet peridotites, with various amounts of clinopyroxene, and one garnet-free spinel peridotite with a small amount of clinopyroxene. All of the peridotites are depleted in Fe, and the Mg# of olivine and orthopyroxene range from 91 to 94. Three groups of sheared peridotites are present and have been identified primarily on the basis of Ca contents of olivine and orthopyroxene. The porphyroclasts preserve pre-deformation P–T conditions of 3.5 to 4.5 GPa and 900°C to 1100°C (Group I), 5 to 5.5 GPa and 1200°C to 1250°C (Group II) and 6 ± 0.5 GPa and 1400 ± 50°C (Group III). Group III samples lie above the 40 mW/m2 conductive geothermal gradient, indicating thermal perturbation prior to deformation. The sheared peridotites from Lesotho were affected by various metasomatic events. Pre-deformation metasomatism, involving melts and fluids, is recorded in the porphyroclasts. In Group II and III samples, the clinopyroxene porphyroclasts have similar compositions to Cr-rich and Cr-poor clinopyroxene megacrysts, respectively, that have previously described from southern African kimberlites. This suggests a relationship between them. Younger pre-deformation metasomatism is preserved in a zoned garnet from Group II (enrichment in Ti, Zr, Y + HREE) and orthopyroxene in a Group I sample. The latter exhibits a complex zonation, with a highly enriched (Fe, Ti) inner rim and a less-enriched outer rim. These enrichments must have occurred shortly before deformation. Metasomatism during deformation is revealed by the complex chemical changes recorded in olivine neoblasts with, depending on the sample, increasing or decreasing contents of Ti, Ca, Al, Cr, Mn and Na. Crystallographic preferred orientations of olivine neoblasts are consistent with bimodal, B, C, E, AG-type fabrics and indicate the presence of a hydrous metasomatic agent. We suggest that, akin to the shallower sheared peridotites (Group I), Groups II and III were influenced by early (proto-)kimberlite melt pulses and propose the following model: (proto-)kimberlitic melts invaded the lower lithosphere. These melts followed narrow shear zone networks, produced by deformation at the lithosphere–asthenosphere boundary, heated and metasomatized the surrounding peridotites and were responsible for megacryst crystallization. Sheared peridotites from close to the melt conduits (Group III) have compositions comparable to Cr-poor megacrysts, while those located at a greater distance (Group II) resemble Cr-rich megacrysts. Reactive infiltration of volatile-rich proto-kimberlite melts caused rheologically weakening of olivine in the lithospheric mantle. The consequence of this positive feedback mechanism of metasomatism, weakening and deformation—due to the high magmatic and metasomatic activity in the Late Cretaceous—is the progressive perforation of the lower Kaapvaal lithosphere by rheologically weak zones and the destruction of the protecting dry and depleted layer at its base. This could have caused the observed thinning and destabilization of the lower lithosphere below the southern margin of the Kaapvaal craton.

The lithosphere–asthenosphere boundary structure at 11–21 Ma from wide-angle seismic data in the equatorial Atlantic Ocean

SUMMARYThe lithosphere–asthenosphere boundary (LAB) separates the rigid lithospheric plate above with the ductile and convective asthenosphere below and plays a fundamental role in plate tectonic processes. The LAB has been imaged using passive geophysical methods, but these methods only provide low-resolution images. Recently, seismic reflection imaging method has provided high-resolution images of the LAB, but imaging of the LAB at younger ages has been difficult. Here, we present the image of the LAB using wide-angle seismic reflection data covering 11–21 Ma old lithosphere in the equatorial Atlantic Ocean. Using ocean bottom seismometers (OBSs), we have observed wide-angle reflections between 150 and 400 km offsets along with crustal and mantle refraction arrivals. We first performed traveltime tomography to obtain the velocity in the crust and upper mantle. The Pn arrivals provide the information about P-wave velocity down to 4 km below the Moho. The disappearance of Pn arrivals beyond 130 km offset suggests that vertical P-wave velocity gradient is negligible or negative below this depth. We extended these velocities down to 90 km depth and then applied two imaging techniques to wide-angle reflection data, namely traveltime mapping of picked reflection arrivals and pre-stack depth migration of full wavefield data. We find that these reflections originate between 34 and 67 km depth, possibly from the LAB system. We have carried out extensive modelling to show that these reflections are real and not artefacts of imaging. Comparison of our results with coincident passive seismological and magnetotelluric results suggests that wide-angle imaging technique can be successfully used to study the lithosphere and the LAB system. We find that the LAB gradually deepens with age, but becomes very deep at 17–19 Ma, which we interpret to be due to the anomalous geology along this part of the profile.

The Accuracy Assessment of Lithospheric Density Models

The Earth’s synthetic gravitational and density models can be used to validate numerical procedures applied for global (or large-scale regional) gravimetric forward and inverse modeling. Since the Earth’s lithospheric structure is better constrained by tomographic surveys than a deep mantle, most existing 3D density models describe only a lithospheric density structure, while 1D density models are typically used to describe a deep mantle density structure below the lithosphere-asthenosphere boundary. The accuracy of currently available lithospheric density models is examined in this study. The error analysis is established to assess the accuracy of modeling the sub-lithospheric mantle geoid while focusing on the largest errors (according to our estimates) that are attributed to lithospheric thickness and lithospheric mantle density uncertainties. Since a forward modeling of the sub-lithospheric mantle geoid also comprises numerical procedures of adding and subtracting gravitational contributions of similar density structures, the error propagation is derived for actual rather than random errors (that are described by the Gauss’ error propagation law). Possible systematic errors then either lessen or sum up after applying particular corrections to a geoidal geometry that are attributed to individual lithospheric density structures (such as sediments) or density interfaces (such as a Moho density contrast). The analysis indicates that errors in modeling of the sub-lithospheric mantle geoid attributed to lithospheric thickness and lithospheric mantle density uncertainties could reach several hundreds of meters, particularly at locations with the largest lithospheric thickness under cratonic formations. This numerical finding is important for the calibration and further development of synthetic density models of which mass equals the Earth’s total mass (excluding the atmosphere). Consequently, the (long-to-medium wavelength) gravitational field generated by a synthetic density model should closely agree with the Earth’s gravitational field.

Broad fault zones enable deep fluid transport and limit earthquake magnitudes

Constraining the controlling factors of fault rupture is fundamentally important. Fluids influence earthquake locations and magnitudes, although the exact pathways through the lithosphere are not well-known. Ocean transform faults are ideal for studying faults and fluid pathways given their relative simplicity. We analyse seismicity recorded by the Passive Imaging of the Lithosphere-Asthenosphere Boundary (PI-LAB) experiment, centred around the Chain Fracture Zone. We find earthquakes beneath morphological transpressional features occur deeper than the brittle-ductile transition predicted by simple thermal models, but elsewhere occur shallower. These features are characterised by multiple parallel fault segments and step overs, higher proportions of smaller events, gaps in large historical earthquakes, and seismic velocity structures consistent with hydrothermal alteration. Therefore, broader fault damage zones preferentially facilitate fluid transport. This cools the mantle and reduces the potential for large earthquakes at localized barriers that divide the transform into shorter asperity regions, limiting earthquake magnitudes on the transform.