Structure Of Lithospheric Mantle Research Articles

The structure of a continental intraplate volcanic system and controls from shear zones: Insights into the central Hoggar Cenozoic volcanic province, Northwest Africa, from electrical resistivity images

Continental intraplate volcanic systems, with their locations far from plate tectonic boundaries, are not well understood: the crustal and lithospheric mantle structure of these systems remain enigmatic and there is no consensus on the mechanisms that cause melt generation and ascent. The Cenozoic saw the development of numerous volcanic provinces on the African plate, including within the Central Hoggar, located in Northwest Africa, part of the Tuareg shield. The magmatic activity began at approximately 34 Ma and continued throughout the Quaternary. In order to understand the origins and potential mechanisms that generated the intraplate volcanic activity in the Central Hoggar we aim to image the subsurface architecture, in terms of electrical resistivity, from the surface to the lithospheric mantle. To do so we use magnetotelluric measurements from 40 locations to generate a 3-D electrical resistivity model, over an area of about 100 km by 160 km. Low-resistivity features (i.e., conductors) are observed in the crust that are narrow, linear structures congruent with the boundaries of terranes and prominent fault zones (e.g., Azrou N’Fad). They likely reflect the Pan-African mega-shear zones, which were reactivated throughout the tectonic evolution of the region. The model reveals that these faults are lithospheric-scale. The low-resistivity features likely represent the signatures of past fluid pathways and mineralization. A deeper low-resistivity feature is observed in the upper lithospheric mantle directly beneath the Manzaz and Atakor volcanic districts. It may represent local, small-scale metasomatism of the sub-continental lithospheric mantle, and low-percent partial melting, that sits above a regional, large-scale asthenospheric upwelling associated with the Hoggar swell. It is likely the origin point of the fluids responsible for the overlying anomalies. The results highlight the control of the lithospheric-scale, mega-shear zones on the spatial distribution of the recent Cenozoic volcanic activity, which was influenced by the location of pre-existing structural weaknesses.

Constraining Earth’s nonlinear mantle viscosity using plate-boundary resolving global inversions

Variable viscosity in Earth's mantle exerts a fundamental control on mantle convection and plate tectonics, yet rigorously constraining the underlying parameters has remained a challenge. Inverse methods have not been sufficiently robust to handle the severe viscosity gradients and nonlinearities (arising from dislocation creep and plastic failure) while simultaneously resolving the megathrust and bending slabs globally. Using global plate motions as constraints, we overcome these challenges by combining a scalable nonlinear Stokes solver that resolves the key tectonic features with an adjoint-based Bayesian approach. Assuming plate cooling, variations in the thickness of continental lithosphere, slabs, and broad scale lower mantle structure as well as a constant grain size through the bulk of the upper mantle, a good fit to global plate motions is found with a nonlinear upper mantle stress exponent of 2.43 [Formula: see text] 0.25 (mean [Formula: see text] SD). A relatively low yield stress of 151 [Formula: see text] 19 MPa is required for slabs to bend during subduction and transmit a slab pull that generates asymmetrical subduction. The recovered long-term strength of megathrusts (plate interfaces) varies between different subduction zones, with South America having a larger strength and Vanuatu and Central America having lower values with important implications for the stresses driving megathrust earthquakes.

The Pannonian-Carpathian-Alpine seismic experiment (PACASE): network description and implementation

The Pannonian-Carpathian-Alpine Seismic Experiment (PACASE) is a collaborative project based on a large, passive seismic network comprising 214 temporary stations. Among the primary aims are the imaging of the Earth’s crustal, lithospheric and upper mantle structure, including joint inversions; monitoring and mapping of seismic activity; and interpretation of the data from seismotectonic and geodynamic perspectives. The base of the cooperation is a high-quality, broadband seismic network covering the very centre of Europe: the Eastern Alpine and Western Carpathian Mountain ranges, the Bohemian Massif, and the sedimentary Molasse and Pannonian Basins. In this overview, we focus on the implementation and achieved field goals of PACASE, such as seismic station configuration, general network organization, data availability and access to the dataset. With selected seismological examples, we demonstrate the good usability of the records of earthquake detection, and a first publication attests to the structural imaging capability of the PACASE data. We assess the background noise level at various stations and its variations in time and space. Our aim is to collect all practical information relevant to serve as a long-term reference for the PACASE.

Stabilization and evolution of the Brazilian subcontinental lithospheric mantle: Insights from garnet xenocrysts and peridotite xenoliths of Três Ranchos kimberlite (APIP, Brazil)

The thermal structure and chemical composition of the subcontinental lithospheric mantle beneath the Alto Paranaíba Igneous Province defined by the major and trace element compositions of garnet and clinopyroxene xenocrysts and mantle peridotite xenoliths entrained in the Cretaceous Três Ranchos kimberlite. Estimated temperatures and pressures indicate a typical cratonic geotherm, with a surface heat flow of ∼ 40 mW/m2 slightly hotter than that at the São Francisco Craton margin (35–36 mW/m2), suggesting lateral heterogeneities in the geothermal gradients. A deep mantle portion (160 km) is recorded by a lherzolite xenolith (5 GPa and 1200 °C). This portion has undergone pervasive interstitial melt percolation of an ultra-alkaline silicate melt and late injection of small silicate melt fractions such as the host carbonated kimberlite. This pervasive migration extended into shallower SCLM sectors at 124–100 km (3.9–3.1 GPa and ∼ 940–810 °C) as evidenced by metasomatized (±phlogopite) peridotite identified by sinusoidal REE garnet compositions. This portion is apparently intercalated at 120–110 km depth (3.7–3.3 GPa and ∼ 850 °C) with strongly depleted peridotites showing garnet and clinopyroxene compositions extremely impoverished in moderately incompatible elements, as a result of extreme ancient, melt extraction. Atop the metasomatized and depleted peridotites, at 100 km depth, a layer of fertile lherzolite is identified by garnet xenocrysts, with 'normal', HREE-enriched, steadily fractionated LREE-depleted patterns. Combining our data with those of the Osvaldo França SCLM points to the occurrence of a deeper sector (>100 km depth beneath Trȇs Ranchos) originally formed by extremely depleted peridotites, successively metasomatized by ultra-alkaline melts; and a shallower sector of the São Francisco craton composed of fertile lherzolites unrelated to ultra-alkaline magmatism. Similar fertile lherzolites are found in the uppermost sectors of the Siberian and Kaapval SCLM, suggesting that they may be a ubiquitous, primary feature, possibly related to the stabilization of the Archean cratons.

The Elusive Congo Craton Margin During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat Flow beneath the Xaudum Kimberlite Province, NW Botswana

Abstract The continental lithospheric mantle (CLM) beneath the southern margin of the Congo craton has remained elusive, mainly because of the thick Phanerozoic sedimentary cover concealing possible kimberlite and lamproite diatremes. In this study, we explore this lithospheric mantle section using major and trace element compositions of mantle-derived clinopyroxene and garnet xenocrysts from kimberlites of the ca. 84 Ma Nxau Nxau cluster in northwest (NW) Botswana, which is part of the poorly known Xaudum kimberlite province extending into northern Namibia. We utilize these data to better understand the thermal and compositional evolution of the lithospheric mantle at the southern margin of the Congo craton. The clinopyroxene population (83 individual grains) comprises Cr-rich and Cr-poor diopsides with variable major (Al2O3, Na2O, Mg#) and incompatible trace element (U, Th, Zr, Hf, Nb, Ta, REEs) compositions. The large garnet population studied (496 individual grains) is dominated by lherzolitic G9 (38%) and ‘megacrystic’ G1 (41%) compositions, with minor contributions from Ti-metasomatized G11 (7%) and eclogitic G3 (6%) cratonic mantle sources. Harzburgitic G10 garnet is very rare (two grains only), consistent with a lherzolite-dominated CLM section in a craton margin position. The eclogitic garnet population has compositions similar to garnet from high-Mg cratonic mantle eclogite xenoliths, and such compositions have recently been interpreted as metasomatic in origin within the mantle xenoliths literature. Pressure–temperature calculations using the single-grain clinopyroxene technique reveal a relatively cold cratonic geotherm of 37–38 mW/m2 for the study region during the Late Mesozoic. For peridotitic garnets, projections of calculated Ni-in-garnet temperatures onto the independently constrained regional conductive geotherm suggest that lherzolite dominates at <145 km depth, whereas high-Ti lherzolitic G11 garnets and ‘megacrystic’ G1 garnets originate mostly from greater depths, reaching down to the lithosphere base at 150 to 210 km depth. The apparent confinement of ‘megacrystic’ G1 garnet to the bottom of the lithosphere suggests formation from infiltrating asthenosphere-derived proto-kimberlite liquids during melt–rock interactions. In general, the data suggest that the CLM beneath NW Botswana is depleted to about 145 km depth, and between 145 and 210 km depth, a thick metasomatized layer is identified, representing the transition into the underlying asthenosphere. A relatively thin lithosphere beneath NW Botswana is consistent with the proposed craton margin setting, especially when compared with the thicker cratonic roots beneath the central regions of the Congo and Kalahari cratons in Angola and South Africa, respectively, reaching down to 250 km depth and possibly even deeper. The compositional dissimilarity between the deepest-derived garnets from kimberlites in NW Botswana (i.e. from the diamond stability field) and garnets that occur as inclusions in diamond from cratons worldwide suggests extensive overprinting of the lowermost cratonic lithosphere by oxidative melt-related metasomatism. This finding, together with the very low diamond grades of the Xaudum kimberlites, points to a diminished diamond potential of the large and mostly unexposed ‘cratonic’ region (e.g. covered by thick desert sand) located between the major diamond mining districts of the Congo craton to the north (e.g. Catoca) and the Kalahari craton to the south (e.g. Orapa and Jwaneng).

Cenozoic subsidence‐driving mechanisms in the southernmost Patagonian basins of Tierra del Fuego and SW Atlantic

AbstractForeland basins are ideal laboratories to examine and quantify forces that contribute to Earth's topography. The interaction of these driving mechanisms (atmospheric, lithospheric and asthenospheric) affects the accumulation and preservation of strata in marine or terrestrial depocentres. For foreland basins that cover thousands of kilometres along orogens, geodynamic processes or lithospheric structure might differ and/or overlap differently along or across strike. The Magallanes‐Austral basin in the southernmost Patagonia serves as a good analogue to analyse the interactions between subcrustal forces and foreland sedimentation. While to the northern part of southern Patagonia, Cenozoic basins were predominantly terrigenous and above sea level; at the southernmost end of Patagonia, sedimentation in the island of Tierra del Fuego was mostly submarine. We analysed in this contribution the southernmost foreland of Patagonia by combining backstripping with reconstruction of flexural and dynamic subsidence. These results were compared with terrestrial records exposed further north of southern Patagonia. We found that, in addition to crustal contributions (as deformation and sedimentation), subcrustal forces are required to accommodate the proximal and distal foreland strata and explain the palaeoenvironmental and subsidence discrepancies that resulted after our analysis. When our models are compared with dynamic topographic curves, strong correlations are observed during the Palaeogene, whereas strong topographic differences occurred in the Neogene. Dynamic topography models in the Neogene have reproduced clear uplift, whereas our residual topography results show equilibrium (close to the orogen) to subsidence values (to the distal foreland). We propose that changes in the lithospheric mantle had to work together with the rest of the tectonics and dynamic forces to match 1‐D backstripping and flexural curves. This suggests that foreland basins in southern Patagonia were controlled differently along strike the southern Andes and that crustal deformation, asthenospheric flows and a heterogeneous lithospheric mantle structure affected the Cenozoic basin evolution.

Systematics of clinopyroxene phenocrysts, megacrysts, and cumulates in Tertiary basalts of southern Slovakia with implications in the structure of lithospheric mantle

Systematics of clinopyroxene phenocrysts, megacrysts, and cumulates in Tertiary basalts of southern Slovakia with implications in the structure of lithospheric mantle

Crustal and lithospheric mantle structure below the Indian shield based on 3-D constrained gravity inversion and Deep Neural Network approach: Geological implications

The Indian shield region comprises of several Archean cratons, Proterozoic mobile belts, Proterozoic-Phanerozoic rift basins, and volcanic provinces and has been significantly modified by various tectono-thermal events and the supercontinental cycles. These overprinting processes and evolution can be understood, to some extent by characterizing the lithosphere structure. In this study, we carried out seismically constrained 3-D nonlinear gravity inversion using tesseroids in the spherical coordinate system to delineate the Moho geometry for the Indian shield. Further, the supervised machine learning approach using Deep Neural Network (DNN) is implemented on the geopotential data to obtain Lithosphere-Asthenosphere Boundary (LAB) across the Indian shield. The inversion results show that the Moho depth varies between 33 and 60 km below the Indian shield with a mean difference of ∼ −0.3 km and a standard deviation of ∼4.9 km relative to the Moho from receiver function data. For the DNN model, the estimated LAB depth values range between 95 and 280 km below the Indian shield. The model gave rise to the coefficient of determination R2 of 0.93, Mean Absolute Error of ∼14 km, and Root Mean Square Error of ∼22 km between the observed and predicted LAB depths. Apart from the general agreement of Moho and LAB depth estimates with previous seismological studies, the present study provides better resolved information of lithosphere structure across the Indian shield. The modelled lithosphere structure therefore will be useful to integrate with the geochronological data in order to to understand the Proterozoic tectonic evolution of the Indian Shield.

Mantle compositional structure and dynamics of the Arabia-Eurasia collision zone: Insights from integrated thermochemical modelling

The tectonic evolution of the Arabia-Eurasia collision zone is enigmatic due to our poor knowledge of the compositional structure of the Zagros keel and the sub-lithospheric mantle beneath the Iranian plateau. Here, we present a thermochemical model for the upper mantle, starting from the Arabian plate toward the Zagros keel (in SW Iran) and the Iranian plateau to the Kopet Dagh Mountains in NE Iran. The crustal and (sub) lithospheric mantle structure is constrained using receiver functions and high-resolution seismic tomography. The crustal density is obtained by converting local Vp velocity profiles to density. The presumed mantle composition/density of different tectonic units has been inferred from the composition of mantle-derived xenoliths in Pleistocene alkaline basalts of NE Iran, mantle xenoliths from Eastern Arabia, and integrated geophysical-petrological modeling of seismic tomography as well as Bouguer and geoid anomalies. Our results indicate that the lower part of the lithosphere in the Arabian plate is ∼ +25 kg/m3 denser than the sub-lithospheric mantle, which allows underthrusting of the Arabian lithosphere beneath the Iranian plateau. The Zagros core–an ultra-positive seismic velocity anomaly in the tomographic models– is modeled assuming a refractory “craton-like” composition, leading to a ∼-60 kg/m3 density contrast with its underlying fertile lithospheric mantle. We hypothesize that at least the upper ∼100 km of the hydrous Arabian lithosphere was compositionally cratonic before the collision-related shortening processes. In contrast, the anhydrous Iranian lithosphere has been refertilized during asthenosphere upwelling and mantle flows. The Iranian plateau is underlain by a cold sub-lithospheric body characterized by ∼ +25 kg/m3 density contrast with respect to the ambient asthenosphere.

Subcontinental lithospheric mantle discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India

SUMMARY We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear wave velocity (Vs) structure of subcontinental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (Phs) is a positive amplitude arrival at ∼10–12 s and has positive moveout with increasing ray-parameter. From joint inversion, the H-D is modelled at a depth range of 90–106 km, with 11–12 per cent Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–99 km, with 6–10 per cent Vs increase. An intralithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude PILDs phase, arriving at 8–8.5 s. This is modelled at a depth range of 66–75 km with Vs increase of 2–9 per cent. We construct 2-D profiles of depth-migrated common-conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3-D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleosubducted oceanic slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.

Insights into the relationship between the Red Sea rift-related structures and the seismo-volcanic activity in Harrat Lunayyir, Saudi Arabia: A seismic tomography study

Harrat Lunayyir is one of the youngest Cenozoic volcanic fields in western Saudi Arabia, which in 2009 experienced a seismic swarm of about 34,000 events with a maximum magnitude of M5.4. Many studies debated about the relationship of a depicted shallow-level magma intrusion with the Red Sea rifting and the channelized northward flow from the Afar mantle plume. In this study, a detailed seismic tomography inversion for the P- and S-wave velocities, and the Poisson’s ratio structures beneath Harrat Lunayyir and the adjacent Red Sea offshore area is presented. The tomographic images showed evidence of magma intrusions beneath the Lunayyir Swarm Area (LSA) characterized by low P- and S-wave velocity and high Poisson’s ratio anomalies down to 21 km depth. These anomalies are in juxtaposition with some high-velocity anomalies attributed to remnants of solidified magma intrusions relevant to previous episodes of volcanicity. Beneath the Red Sea, the lithospheric mantle structure is clearly observed as a high P-wave velocity high-Poisson’s ratio anomaly at a depth of 20–40 km. Traces of high-Vp anomalies with anomalous Poisson’s ratio are observed in the area between the Red Sea and the LSA suggesting that a channel of hot lithospheric mantle material may link the Red Sea rift structure with the LSA. We suggest that the Red Sea drifting and its related processes in the Zabargad Shear Zone might have significantly contributed to the early formation of the Lunayyir volcanic field and could possibly have in an impact on any magmatic activity in the future.

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

The Elusive Southern Margin of the Congo Craton During Gondwana Breakup: Insights from Lithospheric Mantle Structure and Heat-Flow Beneath the 85 Ma Xaudum Kimberlite Province in Nw Botswana

A Gondwanan micro-fragment adjacent to southern granulite terrane of India: Evidence from satellite gravity studies

The Southern Granulite Terrane (SGT) of Peninsular India is a complex collage of Mesoarchean to Late Neoproterozoic – Cambrian tectonic units, which were part of Earth's major supercontinents such as Ur, Columbia, Rodinia, Gondwana and Pangea. We analyzed high-resolution satellite gravity field with a view to demarcate the underlying crustal and lithospheric mantle structure of the SGT. The western part of SGT is characterized by a high-order elongated negative residual gravity anomaly of −80 mGal together with a thicker Moho (43 km) and the lithosphere (~130 km). In comparison, in the central and eastern parts of SGT, the gravity anomalies are distinctly positive, Moho is shallow (~36 km) and the lithosphere is thin (~100 km). An important finding of the present study is the delineation of a fragmented micro-continental block of the SGT, submerged under Indian Ocean, and located about 30 km west of the rifted continental margin of India, which is characterized by moderate crustal seismicity, and high-order gravity low of −80 mGal, identical to the features of western SGT. This distinct micro-block might represent a vestige of the rifting between India and Madagascar at around 90 Ma due to upwelling of the Marion mantle plume and marking the final breakup of the Gondwana assembly.

Thermobarometry of Inclusions: Implications to the Structure of Lithospheric Mantle and Evolution in Time and Diamond Formation

Thermobarometry of Inclusions: Implications to the Structure of Lithospheric Mantle and Evolution in Time and Diamond Formation

Structure of the northwestern North Anatolian Fault Zone imaged via teleseismic scattering tomography

SUMMARYInformation on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.

Variation in Upper Plate Crustal and Lithospheric Mantle Structure in the Greater and Lesser Antilles From Ambient Noise Tomography

AbstractThe crust and upper mantle structure of the Greater and Lesser Antilles Arc provides insights into key subduction zone processes in a unique region of slow convergence of old slow‐spreading oceanic lithosphere. We use ambient noise tomography gathered from island broadband seismic stations and the temporary ocean bottom seismometer network installed as part of the Volatile Recycling in the Lesser Antilles experiment to map crustal and upper mantle shear‐wave velocity of the eastern Greater Antilles and the Lesser Antilles Arc. Taking the depth to the 2.0 km/s contour as a proxy, we find sediment thickness up to 15 km in the south in the Grenada and Tobago basins and thinner sediments near the arc and to the north. We observe thicker crust, based on the depth to the 4.0 km/s velocity contour, beneath the arc platforms with the greatest crustal thickness of around 30 km, likely related to crustal addition from arc volcanism through time. There are distinct low velocity zones (4.2–4.4 km/s) in the mantle wedge (30–50 km depth), beneath the Mona Passage, Guadeloupe‐Martinique, and the Grenadines. The Mona passage mantle anomaly may be related to ongoing extension there, while the Guadeloupe‐Martinique and Grenadine anomalies are likely related to fluid flux, upwelling, and/or partial melt related to nearby slab features. The location of the Guadeloupe‐Martinique anomaly is slightly to the south of the obliquely subducted fracture zones. This feature could be explained by either three‐dimensional mantle flow, a gap in the slab, variable slab hydration, and/or melt dynamics including ponding and interactions with the upper plate.

Global influence of mantle temperature and plate thickness on intraplate volcanism

The thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. <100 km). We observe a negative correlation between shear-wave velocities at depths of 125–175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.

Lithospheric structure and evolution of the North China Craton: An integrated study of geophysical and xenolith data

Determination of the physical and chemical structures of the inaccessible continental lithosphere by comprehensive geophysical and geochemical studies can provide valuable information on its formation and evolution. Extensive studies from various disciplines have revealed complex lithospheric modification of the North China Craton (NCC), but less attention has been paid to an integrated study from different fields. Here we provide an integrated constraint on the lithospheric mantle structure of the NCC by comprehensive semiology, gravity and thermal studies with xenolith data involving depth (levels in the lithosphere), property (chemical and physical), and timing (formation and reworking ages). Our results suggest that the NCC has a relatively heterogeneous lithospheric mantle. Its margins and internal weak zones, especially in the eastern NCC, are generally underlain by the fertile, weakly metasomatized mantle with generally young formation ages. In contrast, its core tends to preserve the refractory, strongly metasomatized mantle with ages roughly coupled to the overlying Archean crust. Such a lithospheric structure shows the preferential modification of the lithospheric mantle in the eastern NCC and in the peripheral regions of the western NCC. The interior of the craton, especially most of the western NCC, remains stable and has been weakly modified.

PRELIMINARY LONG-PERIOD MAGNETOTELLURIC INVESTIGATION AT THE EDGE OF ICE SHEET IN EAST ANTARCTICA

Abstract. The lithospheric mantle structure of the Antarctic continent is of great significance of studying the polymerization and fragmentation mechanism of Gondwana and the plate movement law. Long-period magnetotelluric (LMT) is an important method to study the electrical structure of earth crust and mantle. However, been limited by the bad natural environment and logistics supply difficulties, there is no LMT record of Antarctica before. In 2018, China's 34th Antarctic scientific expedition carried out the LMT survey at the eastern edge of the Antarctic continent with a frequency range of 0.00015 Hz to 0.1 Hz. After the processing and analysis, we get three points as fellow: (1) The lithospheric mantle of Antarctica has a three-dimensional resistivity structure; (2) There are low resistivity regions in the Antarctic mantle, which may be related to thermal activity. (3) It is possible to carry out LMT measurements in eastern Antarctic and more can be done in the future.

3-D thermochemical structure of lithospheric mantle beneath the Iranian plateau and surrounding areas from geophysical–petrological modelling

SUMMARY While the crustal structure across the Iranian plateau is fairly well constrained from controlled source and passive seismic data, the lithospheric mantle structure remains relatively poorly known, in particular in terms of lithology. Geodynamics rely on a robust image of the present-day thermochemical structure interpretations of the area. In this study, the 3-D crustal and upper mantle structure of the Iranian plateau is investigated, for the first time, through integrated geophysical–petrological modelling combining elevation, gravity and gravity gradient fields, seismic and petrological data. Our modelling approach allows us to simultaneously match complementary data sets with key mantle physical parameters (density and seismic velocities) being determined within a self-consistent thermodynamic framework. We first elaborate a new 3-D isostatically balanced crustal model constrained by available controlled source and passive seismic data, as well as complementary by gravity data. Next, we follow a progressively complex modelling strategy, starting from a laterally quasi chemically homogeneous model and then including structural, petrological and seismic tomography constraints. Distinct mantle compositions are tested in each of the tectonothermal terranes in our study region based on available local xenolith suites and global petrological data sets. Our preferred model matches the input geophysical observables (gravity field and elevation), includes local xenolith data, and qualitatively matches velocity anomalies from state of the art seismic tomography models. Beneath the Caspian and Oman seas (offshore areas) our model is defined by an average Phanerozoic fertile composition. The Arabian Plate and the Turan platform are characterized by a Proterozoic composition based on xenolith samples from eastern Arabia. In agreement with previous studies, our results also suggest a moderately refractory Proterozoic type composition in Zagros-Makran belt, extending to Alborz, Turan and Kopeh-Dagh terranes. In contrast, the mantle in our preferred model in Central Iran is defined by a fertile composition derived from a xenolith suite in northeast Iran. Our results indicate that the deepest Moho boundary is located beneath the high Zagros Mountains (∼65 km). The thinnest crust is found in the Oman Sea, Central Iran (Lut Block) and Talesh Mountains. A relatively deep Moho boundary is modelled in the Kopeh-Dagh Mountains, where Moho depth reaches to ∼55 km. The lithosphere is ∼280 km thick beneath the Persian Gulf (Arabian–Eurasian Plate boundary) and the Caspian Sea, thinning towards the Turan platform and the high Zagros. Beneath the Oman Sea, the base of the lithosphere is at ∼150 km depth, rising to ∼120 km beneath Central Iran, with the thinnest lithosphere (&amp;lt;100 km) being located beneath the northwest part of the Iranian plateau. We propose that the present-day lithosphere–asthenosphere topography is the result of the superposition of different geodynamic processes: (i) Arabia–Eurasia convergence lasting from mid Jurassic to recent and closure of Neo-Tethys ocean, (ii) reunification of Gondwanian fragments to form the Central Iran block and Iranian microcontinent, (iii) impingement of a small-scale convection and slab break-off beneath Central Iran commencing in the mid Eocene and (iv) refertilization of the lithospheric mantle beneath the Iranian microcontinent.