Crustal Density Structure Research Articles

The lithospheric structure of Greenland from a stepwise forward and inverse modelling approach

Summary Greenland’s tectonic history is complex, and the resulting lithospheric structure is, although extensively researched, not well constrained. In this study, we model the lithospheric structure of Greenland in a consistent, integrated framework with three steps. First, we build a lithospheric background model by forward modelling, adjusted to gravity gradient data and shear wave velocities from a regional tomography model. Subsequently, we jointly invert for the upper crustal density and susceptibility structure by minimizing the gravity residuals and magnetic total field anomaly misfit. The last modelling step searches for upper crustal thermal parameters to fit our model to the most recent geothermal heat flow predictions for Greenland. Finally, we present 3D models of the density, temperature and velocity structure for the lithosphere as well as thermal parameters and susceptibilities for the upper crust. Our model also includes the depth of the Moho and LAB in Greenland. A comparison between inverted crustal parameters and surface geology shows a clear correlation. The novelty of our model is that all these results are consistent with each other and simultaneously explain a wide range of observed data.

Deep structural characteristics and dynamic significance of the Southeastern margin of the North China Craton: Insights from gravity/GNSS/seismic observations

The North China Craton is a natural laboratory for studying thinning, transformation, and replacement of ancient lithospheres. Its density structure and isostatic state of the crust provide key information for a better understanding of the deep tectonic characteristics and dynamic mechanism. In this study, we analyze observed seismic and gravity/GNSS co-located data, and explore the tectonic characteristics and dynamic significance in the southeastern margin of the North China Craton, using isostatic gravity analysis based on Airy theory and crustal density structure inversions. The results indicate that depth of the Moho interface in the research area ranges between 32 km and 41 km. The crust gradually thins from west to east, and the changing trend of the upper, middle, and lower crust is similar to the Moho interface. There has been significant replacement of the deep mantle and lower crust in the research area, especially where fault zones are located. In addition, the overall area is in a positive isostatic state, indicating that the crust is on an upward trend. The lithosphere on the east side is basically in the state of isostasy, but with a slight tendency for Moho subsidence, indicating a potential thickening of the lithosphere in this region.

Crustal density structure investigation of the East China Sea and adjacent regions using wavenumber domain 3D density imaging method

The East China Sea, situated at the intersection of the Eurasian, Philippine Sea, and Pacific plates, is characterized by complex geology influenced by tectonic phenomena such as plate movements, volcanism, faults, and uplifts. Crustal density structure inversion provides a thorough understanding of the region's geological history as well as Earth's dynamical evolution, providing critical insights into seismic disaster mitigation, resource exploration, marine environmental protection, and maritime safety. The inversion process, on the other hand, presents challenges in data quality, quantity, model complexity, uncertainty, and computational resources. With the advancement of next-generation satellite gravity measurements and developing inversion techniques, the inversion of marine crustal density structures promises to be more precise and comprehensive. We explored the density distribution in the East China Sea and surrounding areas using an innovative wavenumber domain three-dimensional density imaging method along with high-precision global satellite gravity data. By overcoming data quality and computing resource constraints, wavenumber domain three-dimensional density imaging has transformed the discipline of marine geophysics, successfully delivering accurate density distributions in the study area. We were able to get a more precise and comprehensive characterization of the crustal density structure by combining high-precision satellite gravity data and cutting-edge imaging methods. Our investigation has unveiled previously unknown details about density distribution in the East China Sea and its environs. The East China Sea shelf displays smooth low-density perturbations at 18 km depth, whereas the trench–arc–basin region exhibits increasing density perturbations. Notably, the Okinawa Trough, which is surrounded by the Tokara Volcanic Ridge and the Ryukyu Trench, displays strong positive anomalies with a north–northeastern to northeastern orientation. In contrast, the Ryukyu Ridge and the Philippine Sea Basin exhibit smaller negative values and substantial northwestward positive density trends, respectively. These findings indicate diverse material distribution, which provides important insights into the area’s geological evolution and tectonic processes. This study adds new insights into density distribution in the East China Sea and adjacent regions, offering information on the geological complexity of the region. The research lays the groundwork for future research on crustal dynamics and enhances the field of marine geophysics and related disciplines.Graphical abstract

Gravity-Seismic Joint Inversion of Lithospheric Density Structure in the Qiongdongnan Basin, Northwest South China Sea

Abstract Qiongdongnan Basin (QDNB), located at the northwestern corner of the South China Sea (SCS), is a key juncture between the extensional tectonic regime in the northern continental margin and the shear tectonic regime in the western continental margin. Analyzing the crustal density structure and tracking the thermodynamic controlling factors are effective approaches to reveal the nonuniform breakup process of the northwestern SCS. Herein, focusing on the obvious tectonic deformation with distinct eastern and western parts in the QDNB, we present the crustal density structures of five profiles and identify the high-density anomaly related to the synrifting mantle underplating and postrifting magmatic intrusions. The crustal density model was constructed from the Bouguer gravity anomaly, ocean bottom seismic profiles, and multichannel seismic reflection profiles. The northern part of QDNB, with normal crustal density, lower surface heat flow of <55 mW/m2, and limited extension factor of 1.25–1.70, is recognized as the initial nonuniform extension continental crust. The mantle underplating beneath the QDNB is identified as a high mantle density of 3.30–3.40 g/cm3 and a high lower crustal density of 2.92–2.96 g/cm3, which is usually recognized by the high-velocity layers in the northeastern margin of SCS. The magmatic intrusions are identified as the high-density bodies ranging from 3.26 g/cm3 at the base to 2.64 g/cm3 at the top, which become stronger from the west to east. The central part of Xisha Trough is featured by the cooling of the heavily thinned lower crust in the final continental rifting stage, which is close to the cold and rigid oceanic crust. Lateral variations in the deep magmatic anomaly should be the crucial factor for the nonuniform breakup process in the northwestern margin of SCS.

Improved Gravity Inversion Method Based on Deep Learning with Physical Constraint and Its Application to the Airborne Gravity Data in East Antarctica

This paper aims to solve the limitations of traditional gravity physical property inversion methods such as insufficient depth resolution and difficulties in parameter selection, by proposing an improved 3D gravity inversion method based on deep learning. The deep learning network model is established using the fully convolutional U-net network. To enhance the generalization ability of the sample set, the large-scale training set and test set are generated by the random walk, based on the forward theory. Founded on the traditional loss function’s definition, this paper introduces an improvement incorporating a physical constraint to measure the degree of data fitting between the predicted and the real gravity data. This improvement significantly boosted the accuracy of the deep learning inversion method, as verified through both a single model and an intricate combination model. Finally, we applied this improved inversion method to the gravity data from the Gamburtsev Subglacial Mountains in the interior of East Antarctica, obtaining a comprehensive 3D crustal density structure. The results provide new evidence for the presence of a dense crustal root situated beneath the central Gamburtsev Province near the Gamburtsev Suture.

Structure and dynamics of the Ecuador Fracture Zone, Panama Basin

SUMMARYIn this study, multiple geophysical data types are used to investigate the structure and dynamics of the Ecuador Fracture Zone—a complex multistranded strike-slip fault system located in the Panama Basin. Gravity modelling reveals a 25–30-km-wide region of ∼3-km-thick, low-density crust beneath this system and an anomalously low-density region in the uppermost mantle. Along both edges, the transition to the ‘normal’ structure and thickness oceanic crust formed at both the Ecuador and Costa Rica Rifts is abrupt. Within the Ecuador Fracture Zone itself, normal faults bound the median ridges. These faults traverse the entire thickness of accumulated sediment and offset the seabed, while sediment layer geometries document multiple phases of relative uplift, with the most recent phase still ongoing. Active extensional faulting, with an approximately spreading ridge-parallel strike, is also observed in 6–7 Ma Costa Rica Rift crust. The median ridges and the transverse ridge at the eastern edge of the Ecuador Fracture Zone also have contrasting crustal density structures. Both median ridges have a lower density crust than between the intervening valleys, while the transverse ridge crust has an equivalent thickness and density structure to that formed at the Costa Rica Rift. The active median valley basement-cutting normal faults allow seawater ingress and alternation of the crustal footwall, and also flow to mantle depth where, based on gravity modelling, 30–50 per cent serpentinization of mantle peridotite occurs. The resulting serpentinite-driven buoyancy acts as the primary control on the observed median ridge relative vertical tectonism. In contrast, the relative uplift of the transverse ridge results from lithospheric flexure in response to a change in spreading direction between the Ecuador and Costa Rica Rifts. Contrary to the widely accepted assumption that fracture zones are tectonically inactive systems, the Ecuador Fracture Zone provides evidence of extension, serpentinization due to ongoing hydrothermal circulation and relative uplift.

Remaining non-isostatic effects in isostatic-gravimetric Moho determination—is it needed?

SUMMARY For long time the study of the Moho discontinuity (or Moho) has been a crucial topic in inferring the dynamics of the Earth's interior, and with profitable result it is mapped by seismic data, but due to the heterogeneous distribution of such data the quality varies over the world. Nevertheless, with the advent of satellite gravity missions, it is today possible to recover the Moho constituents (i.e. Moho depth; MD and Moho density contrast; MDC) via gravity observations based on isostatic models. Prior to using gravity observations for this application it must be stripped due to the gravitational contributions of known anomalous crustal density structures, mainly density variations of oceans, glacial ice sheets and sediment basins (i.e. stripping gravity corrections). In addition, the gravity signals related mainly with masses below the crust must also be removed. The main purpose of this study is to estimate the significance of removing also remaining non-isostatic effects (RNIEs) on gravity, that is, gravity effects that remain after the stripping corrections. This is carried out by using CRUST19 seismic crustal model and employing Vening Meinesz–Moritz (VMM) gravimetric-isostatic model in recovering the Moho constituents on a global scale to a resolution of 1° × 1°. To reach this goal, we present a new model, named MHUU22, formed by the SGGUGM2 gravitational field, Earth2014 topography, CRUST1.0 and CRUST19 seismic crustal models. Particularly, this study has its main emphasis on the RNIEs on gravity and Moho constituents to find out if we can modify the stripping gravity corrections by a specific correction of the RNIEs. The numerical results illustrate that the RMS differences between MHUU22 MD and the seismic model CRUST1.0 and least-squares combined model MOHV21 are reduced by 33 and 41 per cent by applying the NIEs, and the RMS differences between MHUU22 MDC and the seismic model CRUST1.0 and least-squares combined model MDC21 are reduced by 41 and 23 per cent when the above strategy for removing the RNIEs is applied. Hence, our study demonstrates that the specific correction for the RNIEs on gravity disturbance is significant, resulting in remarkable improvements in MHUU22, which more clearly visualize several crustal structures.

The spreading rate dependence of the distribution of axial magma lenses along mid-ocean ridges

Seismically imaged axial melt lenses (AMLs) are seen almost everywhere along the axis of fast-spreading ridges but at only a few localized segment centers on slow-spreading ridges. Standard models assuming that AMLs form when melt percolating upward pools where freezing produces an impermeable cap do not explain this fundamental observation. To tackle this long-standing problem, we combine a crustal density model and a thermal model with a recent mechanical model for sill formation. The mechanical model predicts that AMLs form below the axial lithosphere but only if the average density of the axial brittle lithosphere is not greater than the magma density. For standard thermal models, crustal density structures inferred from seismic velocity data and normal crustal thicknesses, AMLs are found to be stable along all of a ridge segment for spreading rates greater than about 50 mm/y. To explain slow-spreading observations, we assume that a share of the melt produced by the mantle upwelling all along a segment is focused to the segment center. Some of this melt partially crystallizes, releasing latent heat, before the evolved magma flows along the axis to build the crust away from the segment center. This "extra" heat, beyond what is supplied by the magma that builds the crust near the segment center, results in the lithosphere thin enough for stable melt lenses at the segment center. Our results are consistent with observations and offer a quantitative explanation of the marked difference in the distribution of AMLs along fast- versus slow-spreading centers.

Crustal density structure and flexure mechanism of the Tarim Basin, China, constrained by latest in situ gravity observations

AbstractThe crustal density structure in the Tarim Basin, NW China in response to the plate flexure is so far poorly constrained. We present new insights into the crustal density structure of the Tarim Basin using gravity/GNSS data from 728 mobile stations. The results suggest that the basin is not a uniform unit but spliced by at least two independent blocks. The high Bouguer gravity anomaly to the middle‐south basin corresponds to convex crustal strata. Basing on the results of flexure analysis, we conclude that the crustal lithosphere beneath the Tarim Basin might have been suffered from the upwelling intrusion of deep material that weakened the strength and made the structure convex‐upward, and subsequently deformed under the vertical and horizontal loading.

Localization of large intraplate earthquakes along faulted density-contrast boundaries: Insights from the 2017 Mw6.5 Botswana earthquake

The major controls on the localization of deep crustal intraplate earthquakes remain enigmatic due to their deep hypocentral depths and rarity of coseismic surface ruptures. Here, we investigate the 3-D crustal density structure of the 2017 Mw 6.5 Botswana earthquake epicentral region, a strong lower-crustal (∼24–29 km) event which is suspected to have reactivated a Precambrian structure via normal faulting. We perform a 3-D inversion of the gravity data using published geological constraints, then integrate the resulting density model with aftershock hypocenter distribution and use as constraints in a 3-D thermo-mechanical geodynamic model implemented in ASPECT. Our results reveal crustal blocks of density anomalies, with the aftershocks clustering along a prominent NW-trending, NE-dipping density contrast separating a high-density (>2708 kg/m3) footwall and lower-density (2670–2700 kg/m3) hanging wall blocks. Additionally, a secondary density contrast boundary in the hanging wall coincides with a splay of aftershock clusters at depth. Our observations suggest that the 2017 Mw6.5 Botswana earthquake nucleated near a fault intersection in the lower crust and is associated with brittle normal faulting reactivation of a long-lived basement fault that follows a prominent deep-reaching density contrast boundary. Further, as demonstrated by geodynamic modeling results, we argue that in regions of low tectonic loading rates, where stress perturbations are high enough, faulted crustal-scale density contrast boundaries are preferential concentrators of strain that may localize intraplate earthquakes.

Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge

Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).

Moho depth and crustal density structure in the Tibetan Plateau from gravity data modelling

Although a large number of geophysical experiments have been performed in the Tibetan plateau and its surroundings, our knowledge and understanding of the uplift and deformation in the Tibetan Plateau caused by the India-Asia collision is still incomplete. Gravity method is inevitable and indispensable tool to investigate the evolution of the Tibetan Plateau due to the environmental complexity. In this study, images of the Moho depth and density disturbances in the Tibetan plateau are derived from the separated regional and local gravity anomalies, respectively. The results show that the Moho depth and density present distinct change from north to south in the central and western Tibetan Plateau, but both are relatively featureless in the eastern region. Moho depth sublinearly increases from the south to north and exceed 70 km at the Bangong-Nujiang suture, interpreted as the downward bending of the Tibetan crust due to the subductions of the Indian plate to the south and Asian plate to the north. The significant low density occurs in the middle and lower crust beneath Himalayas, suggesting the absence of the eclogites under the fast subducting effect of the cold Indian plate. In contrast, the high density may suggest the probable presence of eclogites underneath the Lhasa and Qiangtang terranes, where the mantle convection and thickened crust would provide environment of the high temperatures and pressures for eclogite formation. Low density presents underneath the Jinsha suture, indicating that the partial melting occurs due to the upwelling of asthenospheric material.

A Thermo‐Compositional Model of the African Cratonic Lithosphere

AbstractRecently, the continually increasing availability of seismic data has allowed high‐resolution imaging of lithospheric structure beneath the African cratons. In this study, S‐wave seismic tomography is combined with high resolution satellite gravity data in an integrated approach to investigate the structure of the cratonic lithosphere of Africa. A new model for the Moho depth and data on the crustal density structure is employed along with global dynamic models to calculate residual topography and mantle gravity residuals. Corrections for thermal effects of an initially juvenile mantle are estimated based on S‐wave tomography and mineral physics. Joint inversion of the residuals yields necessary compositional adjustments that allow to recalculate the thermal effects. After several iterations, we obtain a consistent model of upper mantle temperature, thermal and compositional density variations, and Mg# as a measure of depletion, as well as an improved crustal density model. Our results show that thick and cold depleted lithosphere underlies West African, northern to central eastern Congo, and Zimbabwe Cratons. However, for most of these regions, the areal extent of their depleted lithosphere differs from the respective exposed Archean shields. Meanwhile, the lithosphere of Uganda, Tanzania, most of eastern and southern Congo, and the Kaapvaal Craton is thinner, warmer, and shows little or no depletion. Furthermore, the results allow to infer that the lithosphere of the exposed Archean shields of Congo and West African cratons was depleted before the single blocks were merged into their respective cratons.

Crustal and upper mantle density structure below the Indian Ocean Geoid Low based on 3-D constrained potential field modelling: Inferences on causative sources

The Indian Ocean Geoid Low (IOGL) in the Central Indian Ocean is the largest geoid depression on the globe. Spectral decomposition of geoid anomalies reveals that a significant fraction (up to ~90%) of the IOGL is due to mass anomalies lying within the lower mantle (~ > 700 km). The crustal density structure and Moho depth variations obtained from 3-D gravity inversion along with the upper mantle density structure derived from SL2013sv tomographic model are used to compute the total geoid anomaly up to 700 km depth and compare it with degree-10 residual geoid anomaly map. Comparison of these two maps necessitated the presence of relatively shallow low- density sources within the upper mantle that were not resolved in the 3-D modelling. The presence of two anomalous sources: i) low S-wave velocities at the base of the LAB (4.4 km/s) and ii) an anomalous source of Vs = −1.0% at a depth of 320–400 km within the sub-lithospheric mantle observed in the tomographic model accounted for this difference. We model the geometry of these low-density anomalous bodies using 2-D joint gravity-geoid modelling, which reveals the total contribution of density structure up to the upper mantle depths (700 km) accounts for only 10% of the IOGL whereas the remaining part is primarily caused by mass anomalies in the lower mantle probably related to high-density subducted slabs or plume sources at the core-mantle boundary as proposed by earlier studies.

Gravimetry and petrophysics for defining the intracratonic and rift basins of the Western-Central Africa zone

The global gravity field obtained from the observations of the satellite Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) satellite offers new opportunities in defining density variations of earth’s crust and mantle, allowing new insights into the structure of specific geologic features. The Central African Rift is a key feature in understanding the dismemberment of Gondwana, and we contribute to defining the crustal density structure underlying the rift. The presence of a narrow and up to 12 km deep basin implies crustal stretching allowed the sediment to accumulate, but a key question is whether the stretching processes also affected the deeper layers of the crust or was limited to the upper crust. The study area includes a subbasin of the greater Chad sag basin, which extends over 1500 × 1500 km and occupies the center of North-Central Africa, shared between the countries of Chad, Sudan, Nigeria, Niger, Algeria, Libya, and Cameroon. We find that the rifting affected the lower crust of the West African Rift, and we evaluate evidence for a 1500 km long and several km thick magmatic crustal intrusion presumably associated with underplating and crustal thinning. We estimate that the stretching factor must be at least 1.5 and had affected the entire crust. To our knowledge, the identification of a continuous body of magmatic intrusions is new and has been only possible through the recent global gravity field. The magmatism has altered the thermal conditions from the time of emplacement on, and it is relevant for the maturation of hydrocarbons present in the sediments. The timing of the magmatism is presumably tied to two pulses of volcanism documented in the rift, associated with the first postrift phase from 96 to 88 Ma and the second postrift phase from 23 Ma up to the Quaternary.

Crustal Density Structure of the Jiuzhaigou Ms7.0 Earthquake Area Revealed by the Barkam-Jiuzhaigou-Wuqi Gravity Profile.

The Barkam–Jiuzhaigou–Wuqi gravity profile extends across the Jiuzhaigou Ms7.0 earthquake (in 2017) zone and passes through several historical big earthquakes’ zones. We have obtained Bouguer gravity anomalies along the profile composed of 365 gravity observation stations with Global Positioning System (GPS) coordinates, analyzed the observed data and inverted subsurface density structure. The results show that the Moho depth has a big lateral variation from southwest to northeast, which shallows from 57 km to 43 km with maximum variation up to 14 km within 800 km. The most acute depth change of the Moho is in the boundary region between the Bayan Har block and West Qinling–Qilian block. According to our analysis, it is related to the eastward movement of the Bayan Har block. There are three main pieces of evidence that support it: (1) Density is higher in the east of the Bayan Har block and smaller in the west, which is the same as seismic activity; (2) Two thin low-density layers exist in the upper and middle crust of the Bayan Har block, which may promote inter-layer slip and the Jiuzhaigou Ms7.0 earthquake occurred in the boundary area of the two low-density layers, where the crustal density and Moho surface fluctuate sharply; (3) the GPS velocity field in the southwestern part gravity profile is significantly larger than that of the northeastern part, which is consistent with the density structure. Our studies also suggest that the large undulation of the Moho prevents the movement of the Bayan Har block, and strain is prone to accumulate here. The dynamic background analysis of the crust in this area indicates that the Moho surface uplifts in the West Qinling–Qilian block, which decelerates the eastern migration of material on the Qinghai–Tibet Plateau, and leads to the weak tectonic activity of the north part of the Bayan Har block.

Density Structure of the Rümker Region in the Northern Oceanus Procellarum: Implications for Lunar Volcanism and Landing Site Selection for the Chang'E‐5 Mission

AbstractThe Rümker region, located in the northern Oceanus Procellarum, is the candidate landing region for China's Chang'E‐5 lunar sample‐return mission. The area is geologically complex and known for its volcanic activity. To understand the volcanism of the Rümker region, we investigate the 3D crustal density structure of this region using Bouguer gravity data from the Gravity Recovery and Interior Laboratory mission. We applied derivative filters and a 3D density inversion to resolve both the shallow and deep structures in the subsurface. Results indicate that the Rümker region exhibits different modes of emplacement. The Mons Rümker volcanic complex is fed by an intrusion‐like structure at a depth of ~6–18 km and contains high‐density basaltic materials of >3,000 kg/m3. A quasi‐circular mass anomaly that was identified in a previous study with high gravity amplitude (~130 mGal) and high density (> 3,000 kg/m3) was also documented in the study region. The quasi‐circular mass anomaly feature represents deeper and thicker buried mare basalts with a bowl‐shaped geometry formed by an impact event. Based on our geophysical analysis, we propose four landing site candidates for the Chang'E‐5 mission that satisfy the geological and geophysical criteria for maximum scientific return.

Crustal density structure of the northwestern Iranian Plateau

We present a new 2D crustal-scale model of the northwestern Iranian plateau based on gravity–magnetic modeling along the 500 km long China–Iran Geological and Geophysical Survey in the Iranian plateau (CIGSIP) seismic profile across major tectonic provinces of Iran from the Arabian plate into the South Caspian Basin (SCB). The seismic P-wave receiver function (RF) model along the profile is used to constrain major crustal boundaries in the density model. Our 2D crustal model shows significant variation in the sedimentary thickness, Moho depth, and the depth and extent of intra-crustal interfaces. The Main Recent Fault (MRF) between the Arabian crust and the overriding central Iran crust dips at approximately 13° towards the northeast to a depth of about 40 km. The geometry of the MRF suggests about 150 km of underthrusting of the Arabian plate beneath central Iran. Our results indicate the presence of a high-density lower crustal layer beneath Zagros. We identify a new crustal-scale suture beneath the Tarom valley between the South Caspian Basin crust and Central Iran and the Alborz. This suture is associated with sharp variation in Moho depth, topography, and magnetic anomalies, and is underlain by a 20 km thick high-density crustal root at 35–55 km depth. The high-density lower crust in Alborz and Zagros may be related to partial eclogitization of crustal roots below about 40 km depth. The gravity and magnetic models indicate a highly extended continental crust for the SCB crust along the profile. Low observed magnetic susceptibility of the Kermanshah ophiolites likely indicates that the ophiolite rocks only form a thin layer that has been thrust over the sedimentary cover.

Control of Volcanic Activity by Crustal Structure: Inference from the Izu‐Bonin‐Mariana and Northeast Japan Arcs

AbstractVolcanic activities in the Izu‐Bonin‐Mariana (IBM) oceanic arc and the Northeast Japan (NEJ) continental arc are different in spite of rather constant subduction rates of the same Pacific plate. IBM volcanoes are much more voluminous than NEJ volcanoes; IBM magmas are predominantly basaltic whereas NEJ magmas are andesitic. We propose that the crustal density structures play a key role in those differences. The crustal density structures were estimated based on seismic P‐wave velocity structure and thermodynamic calculation, suggesting that the neutral buoyancy level of a primitive, mantle‐derived basalt, where a magma chamber forms, is shallower in the IBM arc than the NEJ arc. If a magma that differentiates and attains water oversaturation could erupt at the surface, a larger amount of crystallization should occur for the magma in a deeper chamber. This idea explains that erupting magma in the IBM arc is more voluminous and more mafic than those in the NEJ magma.

Crustal thickness beneath Atlas region from gravity, topographic, sediment and seismic data

The Atlas region in northwest Africa is characterized by the Quaternary volcanism and elevated topography with past complex tectonic mitigation between the African and European plates. Geodynamics of this atypical region has left indubitably imprints in crustal architectonics, mainly regarding the crustal thickness as well as the crustal density structure. The knowledge of crustal thickness variations is of a significant interest, since it provides a crucial constraint to geodynamic and geophysical modelling of this region. In this study, we use gravity, topographic, bathymetric and sediment data together with results of seismic surveys to image the Moho topography beneath the Atlas region. The Bouguer gravity anomalies used for a gravimetric Moho recovery are obtained from the free-air gravity anomalies after subtracting the gravitational contributions of topography, bathymetry and sediments. The regional gravimetric Moho inversion constrained on seismic data is carried out by applying a regularized inversion technique based on Gauss-Newton's formulation of improved Bott's method, while adopting Earth's spherical approximation. The numerical result reveals relatively significant Moho depth variations in the Moroccan Atlas, with minima of approximately 24 km along continental margins of the Mediterranean Sea and maxima exceeding 51 km beneath the Rif Cordillera. The Moho depth beneath the West African Craton varies from 32 km in its southern margin to 45 km beneath the Middle Atlas. The Tell Atlas is characterized by the shallow Moho depth of approximately 22 km and further deepening to 42 km towards the northern edge of the Aures Mountains. Our findings indicate a limited tectonic shortening of the High Atlas with the crustal thickness mostly within 36–42 km. Topographic discrepancies between the Rif Cordillera and the Atlas Mountains suggest that the hypothesis of isostatic compensation cannot be fully established.