Moho Discontinuity Research Articles

Crustal Anisotropy from the Birefringence of P-to-S Converted Waves: Bias Associated with P-Wave Anisotropy

Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it.

Moho depths beneath the European Alps: a homogeneously processed map and receiver functions database

Abstract. We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.

Deep crustal structures with reverse time migration applied to offshore wide-angle seismic data: Equatorial and North-West Brazilian margins

We image the Moho discontinuity and deep crustal layers by applying the Reverse Time Migration (RTM) method to wide-angle seismic (WAS) data that were acquired along two different profiles in NW and SE offshore Brazil, where the spacing between OBS is 12.5 km and 150 m between shots. The application of this method is quite uncommon to ocean bottom seismometers (OBS) data due to the OBS wide spacing deployment and low folds. We analyze the effectiveness of the RTM method when applied to the reflectivity of the WAS data. We imaged the structures by cross-correlating the forward and backward wavefields given by the acoustic seismic equation. This allowed us to use each interface crossed by a ray as both a source and a receiver. The velocity models used to perform RTM were previously obtained by applying a procedure of two-dimensional forward ray-tracing followed by a damped least-squares travel time inversion. The results obtained have an unexpected large contribution from the wavefield traveling as refractions within the Earth and, because of that, we can talk about recovering refractors from the RTM results. We obtained strong and continuous refractors for depths of 7–15 km that correspond to the basement and the Moho discontinuity. As we move landwards, the refractors that correspond to the Moho discontinuity disappear due to the deepening of the surface. Finally, we investigate how the slight change on the velocity model influences the result of the RTM method. The obtained results are promising for a wide range of applications at a crustal scale seismic exploration, with wide-angle seismic data.

Depth distribution of Moho model and tectonic patterns in South China Sea and adjacent areas

This study aims to investigate the depth distribution of Mohorovicic discontinuity (Moho) and its relationship with the tectonic pattern of the South China Sea and adjacent areas. To achieve this, the spatial characteristics of the full tensor gravity gradient data are analyzed to identify 17 large and deep faults and to divide the study area into 9 tectonic units with distinct geological structures. Using a three-dimensional (3D) interface inversion method, the Moho depth is determined, constrained by the Moho depth information obtained from sonar-buoy detection and submarine seismograph detection profiles. By analyzing the relationship between the distribution characteristics of Moho and tectonic units, the study summarizes the trend, relief, gradient of Moho, and crustal properties in the study area. Additionally, the seismically constrained Moho undulation combined with the gravity data, gravity gradient anomalies and unconstrained 3D correlation imaging are employed to study the crustal structure of the South China Sea, investigate the vertical and horizontal changes of the crustal structure, and reveal the large-scale crustal and regional structure of the South China Sea. Through the coupling analysis of shallow and deep structures, the study reveals that the gravity gradient anomalies and 3D correlation imaging are consistent with the variations of the Moho depth, indicating the presence of a trench-island arc-back arc basin system and the distribution of continental crust, oceanic crust, and transitional crust in the South China Sea.

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.

Crustal Structure beneath SLY1 Station from Joint Inversion of Receiver Function and Surface Wave Dispersion Analyses, Sulaymaniyah Area, NE Iraq

The crustal structure beneath the Sulaymaniyah area in northeast Iraq which is located in a seismically active zone, was investigated and the results were presented in this study. Teleseismic and regional earthquakes recorded by the Sulaymaniyah broadband seismic station (SLY1) were used to conduct the P-wave receiver functions and Rayleigh wave group velocities using Computer Programs in Seismology (CPS). Joint inversion of receiver functions and surface waves dispersion curves were used to infer the crustal structure below the station. The waveforms were analyzed for 72 earthquakes with epicentral distances ranging from 30º to 90º that occurred during the period from October 2015 to Oct. 2021 with magnitudes ≥ 7Mw. The generated crustal model shows that the sedimentary cover in the area is around 6km with a P-wave velocity of 5.4817km/sec and Moho discontinuity around 48km with a P-wave velocity of 8.0560km/sec. These outcomes indicate that the crystalline crustal thickness at Sulaymaniyah station is 42 km with a P-wave velocity ranging from 5.4817km/sec to 8.0560km/sec. There are two sharp discontinuities around 20km and 38km with P-wave velocities of 5.9182km/sec and 7.1485km/sec respectively. It is believed that these are the discontinuities between the upper, middle, and lower crust.

Heat flow modelling of the Punta del Este Basin (offshore Uruguay) and its correlation with structural crustal domains

The Punta del Este Basin is located at the offshore of Uruguay, and it is considered an underexplored basin with a high potential for holding hydrocarbon plays. Geologically, it constitutes an aborted rift related to the Gondwana fragmentation process, and subsequent opening of the Atlantic Ocean (Upper Jurassic - Lower Cretaceous). The stratigraphic and tectonic evolution comprises four phases from the Paleozoic pre-rift stage to present-day post-rift stage (Neogene/Quaternary). Due to the estimated hydrocarbon potential of this basin, exploratory activities have been intensified in the last decades, based on the acquisition of a significant amount of geophysical data which allowed a better comprehension of the basin's geology. Previous studies related to this basin were mainly oriented to the stratigraphy, structural framework, and petroleum systems comprehension. This research is focused on the thermal evolution of the Punta del Este Basin and its correlation with the crustal geological structure. For this purpose, a heat flow model was developed in order to understand thermal evolution through geological time. A 2D heat flow modelling based on pseudo wells was carried out to research and reconstruct the paleo heat flow. Bottom-hole temperatures (BHTs), seabed heat flow measurements, and Moho discontinuity depth interpretations have been employed in the reconstruction of heat flow evolution through time. A structural analysis based on seismic interpretation allowed to identify the Moho discontinuity and define four crustal domains in the study area: proximal (unattenuated crust), necking, hyperextended (stretched and thinned transition crust), and oceanic crust. The thermal model shows a strong correlation between heat flow and crustal thickness. In this way, the proximal domain develops low heat flow values while the distal sector shows higher values, due to the strong influence of the rifting event in crust thinning.

Contrasting lithospheric geophysical structure of the Black Sea subbasins: Relevance to testing geotectonic models for this mega depression

We present an integrated analysis of the geophysical parameters of the lithosphere of Black Sea basin obtained as a result of the interpretation of magnetic, gravity, thermal, deep seismic sounding and seismic-tomographic data. It first demonstrates inherent significant differences in geophysical parameters of lithosphere in the Western and Eastern Black Sea subbasins existing from the prerift stage. The set of reviewed parameters responsible for formation of the present-day lithosphere includes types of crust, depths to acoustic basement, configuration of subbasins, depths to Moho, heat flow and relief of the thermal-asthenosphere boundary (LAB), trends of main deep faults of the crystalline crust, their kinematic types, occurring linear magnetic anomalies, velocity pattern of subcrustal mantle. The above-mentioned parameters of the lithosphere are indicators of the age, geodynamics and driving mechanisms for opening of a subbasins. Oblique trends of the subbasins and the topography of Moho discontinuity in the west and east domains, oblique striking of pre-rifting Istanbul zone and the Shatsky Ridge and different trends of deep faults in the crystalline crust suggest distinct lithospheric structure existing from pre-opening of the Black Sea subbasins and different geodynamical conditions of its formation. The Odesa-Sinop-Ordu deep fault zone as a direct continuation of the Golovanivskaya suture zone of the Ukrainian shield and its slope, probably is of the Precambrian age. So it could be the tectonic boundary between two segments of pre-rift continental crust and between future subbasins. The examples illustrate how indicated parameters of the lithosphere can geophysically confirm the basic ideas of available models for geodynamics of the Black Sea.

Crustal velocity structure in the South Yellow Sea revealed by the joint tomographic inversion of reflected and refracted seismic waves

The crustal velocity structure in the South Yellow Sea (SYS) Basin is crucial for understanding the basin’s geological structure and evolution. OBS (ocean-bottom station) data from the OBS2013 line have been used to determine the crustal velocity structure in the SYS. The velocity model of the upper crust in the northern SYS was determined using first-arrival traveltime tomography. The model showed a higher resolution shallow crustal velocity structure but a lower resolution middle-lower crustal velocity structure. The crustal velocity structure, together with the Moho discontinuity in the SYS Basin, was also constructed using a human–computer interactive traveltime simulation, and the result was highly dependent on the prior knowledge of the operator. In this study, we reconstructed a crustal velocity model in the SYS Basin using a joint tomographic inversion of the traveltime and its gradient data of the reflected and refracted waves picked from the OBS data. The resolution of the inverted velocity structure from shallow-to-deep crust was improved. The results revealed that the massive high-velocity body below the Haiyang Sag of the Jiaolai Basin extends to the Qianliyan Uplift in the SYS; the low-velocity Cretaceous strata directly cover the pre-Sinitic metamorphic rock basement of the Sulu orogenic belt; and the thick Meso-Paleozoic marine strata are retained beneath the Meso–Cenozoic continental strata in the northern depression. The Moho depth in the SYS Basin ranges from 28 to 32 km.

Enigmatic crustal and upper mantle structure in the NE Sino-Korean Craton based on nuclear explosion seismic data

We interpret the crustal and upper mantle structure along ∼2500 km long seismic profiles in the northeastern part of the Sino-Korean Craton (SKC). The seismic data with high signal-to-noise ratio were acquired with a nuclear explosion in North Korea as source. Seismic sections show several phases including Moho reflections (PmP) and their surface multiple (PmPPmP), upper mantle refractions (P), primary reflections (PxP, PL, P410), exceptionally strong multiple reflections from the Moho (PmPPxP), and upper mantle scattering phases, which we model by ray-tracing and synthetic seismograms for a 1-D fine-scale velocity model. The observations require a thin crust (30 km) with a very low average crustal velocity (ca. 6.15 km/s) and exceptionally strong velocity contrast at the Moho discontinuity, which can be explained by a thin Moho transition zone (< 5 km thick) with strong horizontal anisotropy. We speculate that this anisotropy was induced by lower crustal flow during delamination dripping. An intra-lithospheric discontinuity (ILD) at ∼75 km depth with positive velocity contrast is probably caused by the phase transformation from spinel to garnet. Delayed first arrivals followed by a long wave train of scattered phases of up to 4 s duration are observed in the 800–1300 km offset range, which are modelled by continuous stochastic velocity fluctuations in a low-velocity zone (LVZ) below the Mid-Lithospheric Discontinuity (MLD) between 120 and 190 km depth. The average velocity of this LVZ is about 8.05 km/s, which is much lower than the IASP91 standard model. This LVZ is most likely caused by rocks which are either partially molten or close to the solidus, which explains both low velocity and the heterogeneous structure.

An updated view of the Italian seismicity from probabilistic location in 3D velocity models: The 1981–2018 Italian catalog of absolute earthquake locations (CLASS)

We have built a complete catalog of three-dimensional (3D) hypocenter locations of earthquakes recorded in Italy from 1981 to 2018. Our catalog includes more than 420,000 events relocated by inverting a newly integrated dataset of ∼5.0 million P and ∼ 3.5 million S wave arrival times recorded by the Italian National Seismic Network and other permanent seismological networks operating in Italy. Available magnitudes are associated with earthquake locations from the most recent datasets and bulletins of Italian seismicity. Earthquakes are located in an updated 3D tomographic model of Italy obtained by including the Moho discontinuity and the seismic velocities of the Ionian subduction zone. We used a probabilistic, non-linear earthquake location code which provides complete information of the hypocenter solution uncertainties. Quality estimators of earthquake locations are analyzed a posteriori with an original criterion for a quantitative classification of the results, allowing users to select seismic events belonging to consistent quality classes and giving a more controlled image of the Italian instrumental seismicity for tectonic and geodynamical studies. The resulting catalog gives a new, coherent view of the spatial and temporal distribution of Italian seismicity. By selecting well constrained located events we construct the new Italian Crustal Seismogenic Layer (CSL) with a good spatial resolution, allowing us to show a comparison between seismogenic thicknesses and Moho geometry distribution. We finally present some examples of seismicity distribution in selected areas of Italy at regional and local scale relating the relocated events of our catalog with available multidisciplinary information from geology, geochemistry, geodynamical models, and historical seismicity.

A new approach to construct 3-D crustal shear-wave velocity models: method description and application to the Central Alps

We develop a new inversion approach to construct a 3-D structural and shear-wave velocity model of the crust based on teleseismic P-to-S converted waves. The proposed approach does not require local earthquakes such as body wave tomography, nor a large aperture seismic network such as ambient noise tomography, but a three-component station network with spacing similar to the expected crustal thickness. The main features of the new method are: (1) a novel model parametrization with 3-D mesh nodes that are fixed in the horizontal directions but can flexibly vary vertically; (2) the implementation of both sharp velocity changes across discontinuities and smooth gradients; (3) an accurate ray propagator that respects Snell’s law in 3-D at any interface geometry. Model parameters are inverted using a stochastic method composed of simulated annealing followed by a pattern search algorithm. The first application is carried out over the Central Alps, where long-standing permanent and the temporary AlpArray Seismic Network stations provide an ideal coverage. For this study we invert 4 independent parameters, which are the Moho discontinuity depth, the Conrad discontinuity depth, the P-velocity change at the Conrad and the average Vp/Vs of the crust. The 3-D inversion results clearly image the roots of the Alpine orogen, including the Ivrea Geophysical Body. The lower crust's thickness appears fairly constant. Average crustal Vp/Vs ratios are relatively higher beneath the orogen, and a low-Vp/Vs area in the northern foreland seems to correlate with lower crustal earthquakes, which can be related to mechanical differences in rock properties, probably inherited. Our results are in agreement with those found by 3-D ambient noise tomography, though our method inherently performs better at localizing discontinuities. Future developments of this technique can incorporate joint inversions, as well as more efficient parameter space exploration.

Moho reflections within seismic noise autocorrelations

The noise recorded at a seismic station has information about some properties of the medium such as their discontinuities. The depth of these changes in velocity can be retrieved using auto-correlation and stacking methods applied to the seismic noise. We developed a new method to constrain Moho reflections hidden within the autocorrelation of seismic noise. This is done by filtering the data in different frequency bands (each one defined by a central frequency), applying the auto-correlation and stacking process per central frequency and finally stacking the frequencies of interest. Synthetic tests showed that phases associated with a discontinuity at depth arrive at the same lag time across different frequencies, while side lobes and constructive interactions of the random noise have time shifts in the lag time. We tested the method using data from the MASE array installed in Central Mexico and the results show a good agreement with previous works. We found that exploring the frequency content with different bandwidth filters and stacking only those with useful information helps to improve the SNR and to find the phase associated with the Moho discontinuity at depth.

The role of gravitational body forces in the development of metamorphic core complexes

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands.

Crustal Imbrication in an Alpine Intraplate Mountain Range: A Wide‐Angle Cross‐Section Across the Spanish‐Portuguese Central System

AbstractIntraplate ranges are topographic features that can occur far from plate boundaries, the expected position of orogens as described in the plate tectonics theory. To understand the lithospheric structure of intraplate ranges, we focused on the Spanish‐Portuguese Central System (SPCS), the most outstanding topographic feature in the central Iberian Peninsula. The SPCS is an Alpine range that exhumes Precambrian‐Paleozoic rocks and is located at &gt;200 km from the northern border of the Iberian microplate. Here, we provide a P‐wave velocity model based on wide‐angle seismic reflection/refraction data of the central SPCS (Gredos sector). Our results show: (a) a layered lithosphere characterized by three major interfaces: Conrad, Mohorovicic, and Hales discontinuities, (b) an asymmetry of the crust‐mantle boundary under the SPCS, (c) the extent of the Variscan batholith forming the main outcrops of Gredos, and (d) the thinning of the lower crust toward the south. This model suggests that the exhumation of the SPCS basement was driven by a south‐vergent thick‐skinned thrust system, developed in the southern part of the SPCS and that promoted crustal imbrication and a Mohorovicic discontinuity's offset under the SPCS. Thus, the deformation mechanisms of the crust seem to be controlled by the presence of the late‐ to post‐Variscan granitoids that assimilated the Variscan mid‐crustal detachment creating a new rheological boundary. This tectonic structure allowed the formation of Alpine crustal‐scale thrust systems that eased coupled deformation of the upper and lower crust, leading to limited underthrusting of both crustal layers.

Intracontinental deformation of the Tianshan Orogen in response to India-Asia collision

How the continental lithosphere deforms far away from plate boundaries has been long debated. The Tianshan is a type-example of ongoing lithospheric deformation in an intracontinental setting. It formed during the Paleozoic accretion of the Altaids and was rejuvenated in the Cenozoic, which might be a far-field response to the India-Asia collision. Here we present seismic images of the lithosphere across the central Tianshan, which were constructed from receiver functions and Rayleigh wave dispersions along a N–S-trending linear seismic array. We observe an extensively deformed lithosphere in the Tianshan with inherited, structurally controlled brittle deformation in the shallow crust and plastic deformation near the Moho. We find that earlier multiple accretionary structures were preserved in the crust, which was deformed by pure-shear shortening in the south and thick-skinned tectonics in the north but was limitedly underthrusted by surrounding blocks. A balanced cross-section of Moho discontinuities supports the concept that intracontinental deformation in the Tianshan intensified synchronously with the direct contact between the underthrusting Indian slab and the Tarim Craton in the Late Miocene (~10 Ma). These findings provide a robust and unified seismic model for the Tianshan Orogen, and confirm that effective delivery of the India-Asia collision stress induced the rejuvenation of this intracontinental orogen.

Two-dimensional electrical resistivity structure of the crust and upper mantle across the North-South Gravity Lineament in NE China

The North-South Gravity Lineament (NSGL) is a large anomalous gravity zone that traverses several tectonic units with different properties in northeastern China. Its deep structural features and formation mechanism have long been a topical area of deep geophysical research. This study presents a magnetotelluric profile that extends from the Dongwuqi to Tongliao in Inner Mongolia and passes through geological structures such as the NSGL. Dimensional analysis of the profile data reveals good overall 2D features. 2D nonlinear conjugate gradient inversion of the profile data produced a model of the deep electrical structure. Horizontally, the electrical structure along the profile displays different characteristics in different blocks. The NSGL serves as a boundary for the change in the depth of the Moho discontinuity. Specifically, the Moho discontinuity is deep on the west side of the NSGL and shallow on its east side. Overall, the thickness of the lithosphere varies nonsignificantly, except in the western part of the profile, where the presence of a large low-resistivity anomaly in the shallow layer obscures the boundary of the lithosphere. The large high-resistivity anomaly in the center of the profile may be an indication that the partial delamination of the lithosphere caused by the convection of mantle materials and the upwelling of high-density mantle materials under the action of the westward subduction of the Pacific plate were the major causes of large positive gravity anomalies on the east side of the NSGL. The west side of the NSGL was predominantly controlled by the closure of the Mongol-Okhotsk Ocean, resulting in an increase in the thickness of the crust, which led to the formation of large negative gravity anomalies. These two factors collectively led to the formation of the NSGL.

Coupling mechanism of basin and orogen in the Tianshan Orogenic Belt since Late Mesozoic: Influence on the source–sink process of sandstone‐type uranium mineralization

The coupling mechanism of the Tianshan Orogenic Belt (TOB) and the basins on its sides (Junggar, Turpan–Hami, Tarim, Yili, etc.) since the Late Mesozoic was complex and multi‐levelled. Based on gravity, magnetism, Moho discontinuity, and lithospheric velocity structure of the TOB, together with the structural and sedimentary data of the basins, a model is proposed to reflect the influence of the basin–orogen coupling on the regional source–sink processes of sandstone‐type uranium mineralization since the Late Mesozoic. Generally, there are four main stages of tectonic movements that have changed the coupling mechanism of basin and orogeny: including the Early Yanshanian Movement, the Late Yanshanian Movement, the Middle Himalayan Movement, and the Late Himalayan Movement. These four movements together controlled the mineralization of the sandstone‐type uranium deposits in the TOB together. The mineralization of sandstone‐type uranium deposits in the TOB can be divided into three metallogenic stages: the ore‐bearing formation stage (T‐J2), the tectonic reversal‐related metallogenic stage (J3‐N1), and the epigenetic regenerating ore‐preserving stage (N2‐Q). The tectonic reversal‐related metallogenic stage can be subsequently divided into three sub‐stages: the pre‐metallogenic stage (J3), the sub‐metallogenic stage (K), and the main metallogenic stage (E‐N1). The associated uplift led to the deformation of the mineralized strata in the basins, while the reducing agent migration along faults enhanced the reduction environment within the strata, which affected the enrichment and distribution of the uranium mineralization in the strata. Meanwhile, the recharge and the change of flow state of groundwater also contributed to the later transformation and mineralization. The intra‐continental subduction caused deep lithospheric buckling and depression in the TOB, accompanied by local‐scale mantle convection, which accounts for both tectonic and geodynamic settings for the regional basin–orogen coupling mechanism. In addition, the far‐field effect of the India–Eurasian collision also provided the driving force for the sandstone‐type uranium mineralization in the TOB.

The Moho, slab and tomography of the East Japan forearc derived from seafloor S-net data

We investigate the 3-D P and S wave velocity structure of the East Japan forearc using a large number of high-quality arrival-time data of local earthquakes recorded by a permanent seafloor seismic network (S-net) and the land-based Kiban seismic network. Geometries of the subducting slab boundary and the Conrad and Moho discontinuities under the forearc are refined by referring to results of previous studies including active-source seismic soundings and seismicity located by the S-net. The obtained P and S wave velocity images are quite similar to each other, suggesting that they reflect robust features of the 3-D seismic structure. The subducting Pacific slab exhibits high velocities, whereas significant low-velocity (low-V) anomalies are revealed in the overlying Okhotsk plate beneath the forearc, which may reflect accretionary materials containing abundant fluids. The subducting oceanic crust atop the Pacific slab is revealed clearly as a thin low-V layer extending from the trench axis down to ~100 km depth under the volcanic front. Our results of earthquake relocation using the 3-D velocity model indicate that the double seismic zone in the subducting Pacific slab occurs from the trench axis down to ~180 km depth. The occurrence and focal mechanisms of the double seismic zone are caused by slab deformation during its subduction, such as slab bending or unbending, as well as other processes including hydration, dehydration, compositional variations, and phase changes in the subducting slab.

Lithospheric Thermal Structure in Jinggangshan City: Implications for High Geothermal Background

Jinggangshan City, which is located in the southwestern part of Jiangxi Province, is rich in hot springs. However, lack of geothermal studies has limited the exploration and utilization of the geothermal resources in this city. This study estimated the terrestrial heat flow in Jinggangshan City through well logging and analyses of rock thermal conductivity and heat production. Based on this and the surrounding geoscience transect and the Crust 1.0 model, this study constructed a one-dimensional lithospheric thermal structure by solving the steady-state heat conduction equation. Furthermore, the deep temperature distribution in Jinggangshan City was obtained to better understand the geodynamic condition of the geothermal resources in this city. The results show that the heat flow in this city is around 83.52 mW/m2, indicating the high heat background for the formation of geothermal resources. According to the lithospheric thermal structure of Jinggangshan City, the mantle contributes more to the terrestrial heat flow (qm/qc &amp;gt; 1) than the crust. The temperature of the Mohorovicic discontinuity (the Moho) is 671.7°C, which is consistent with that below the Tanlu deep fault (620–690°C). Moreover, the calculated depth of the Curie surface (585°C) is 27 km, which is consistent with the Curie isotherm depth estimated from aeromagnetic data. This consistency verifies the validity of the lithospheric thermal structure of Jinggangshan City constructed in this study. In summary, the high heat background plays an important role in the formation of geothermal resources in Jinggangshan City.