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Marine gravimeter has been proved to be the primary technique to efficiently obtain middle-to-short wavelength signals of the earth’s gravity field in geodesy, geodynamics and marine sciences research. In recent years, some prototypes of inertial platform and strapdown marine gravimeters have been developed, where the inertial platform gravimeter systems include CHZ-II and ZL11, and strapdown gravimeter systems include SAG-2M and SGA-WZ. In order to validate the performance of these marine gravimeter prototypes, a synchronous test with the widely used gravimeters GT-2M and LCR arranged on the same vessel was carried out in the north of South China Sea. All the data are processed according to the survey standard flow, and the performance is estimated by analyzing the errors of the repeat lines and the crossover points under the same environment. The compared results show that all the six gravimeters can meet the precision requirement of marine gravity survey. Meanwhile, the precision results of the improved gravimeters can get close to the precision of gravimeter GT-2M, higher than the precision gravimeter LCR.

Manganese nodules are a potential source of critical metals such as Cu, Ni, and Co and are widely distributed on the abyssal plains of the global oceans. A polymetallic nodule metallogenic belt with a heterogeneous and spatially clustered nodule distribution was recently discovered in NW Pacific inter-seamount basin (NPIB) areas. However, the geological processes that regulate the nodule occurrence in that region are unresolved. Here, we report on the characteristics of a high-density field of manganese nodules in the abyssal plain north of Suda Seamount. Ship-borne multibeam bathymetric data reveal a typical seamount sector-collapse topography characterized by radial lineaments of debris channels and ridges formed by rapid debris-avalanche flow. Backscatter data linked with underwater observation indicate that manganese nodules are more concentrated (50%–80% areal coverage) along the main body of the debris apron compared to adjacent neighboring areas (

The equivalent magnetization (EM) and mantle Bouguer anomaly (MBA) were calculated along the ultraslow-spreading Mohns Ridge axis in the Norwegian-Greenland Sea. The magnetic anomaly and the associated EM were compared with the bathymetry, MBA, seismically determined crustal structure and geochemical data at both the inter-segment scale (>60 km) and the intra-segment scale (20–60 km). At the inter-segment scale, the magnetic highs at the segment centers are independent of the MBA. Of the 13 segments, 9 with magnetic anomalies >700 nT coincide with axial volcanic ridges identified from multibeam bathymetry maps, which suggests that the magnetic highs at the segment centers may be more associated with the extrusive lavas rather than the amount of magma supply. With few exceptions, the magnetic anomaly lows associated with MBA highs at the segment ends increase from south to north. This trend might be explained by thickened extrusive basalts and/or more serpentinized peridotites at the segment ends in the north. At the intra-segment scale, the most prominent features are the decreases in the magnetic anomalies and associated EMs from the segment centers to the ends. The intra-segment magnetic anomalies have positive and negative correlations with the bathymetry and MBA, respectively. The magnetic signal modeled by the seismically determined layer 2A with an assumed constant magnetization is remarkably consistent with the observed magnetic anomaly, which strongly suggests that the thickness of the extrusive basalts dominates the magnetic structure in each segment along the Mohns Ridge. In general, the thickness of the extrusive basalts dominates the magnetic structure along the Mohns Ridge, whereas the contributions from serpentinized peridotites may be significant at the segment ends and may produce long-wavelength magnetic variations. The magnetic data can be used as an indicator of the thickness of the extrusive basalts within segments along the ultraslow-spreading Mohns Ridge.

Polymetalic sulfide is the main product of sea-floor hydrothermal venting, and has become an important sea-floor mineral resources for its rich in many kinds of precious metal elements. Since 2007, a number of investigations have been carried out by the China Ocean Mineral Resources Research and Development Association (COMRA ) cruises (CCCs) along the Southwest Indian Ridge (SWIR). In 2011, the COMRA signed an exploration contract of sea-floor polymetallic sulfides of 10 000 km2 on the SWIR with the International Seabed Authority. Based on the multibeam data and shipborne gravity data obtained in 2010 by the R/V Dayang Yihao during the leg 6 of CCCs 21, together with the global satellite surveys, the characteristics of gravity anomalies are analyzed in the Duanqiao hydrothermal field (37°39′S, 50°24′E). The “subarea calibration” terrain-correcting method is employed to calculate the Bouguer gravity anomaly, and the ocean bottom seismometer (OBS) profile is used to constrain the two-dimensional gravity anomaly simulation. The absent Moho in a previous seismic model is also calculated. The results show that the crustal thickness varies between 3 and 10 km along the profile, and the maximum crustal thickness reaches up to 10 km in the Duanqiao hydrothermal field with an average of 7.5 km. It is by far the most thicker crust discovered along the SWIR. The calculated crust thickness at the Longqi hydrothermal field is approximately 3 km, 1 km less than that indicated by seismic models, possibly due to the outcome of an oceanic core complex (OCC).

Although the East China Sea (ECS) is widely accepted as the offshore extension of the South China Block (SCB), an alternate hypothesis is that the ECS basement is a buoyant allochthon that collided with Eurasia in the Cretaceous. In order to constrain the Mesozoic collision position, or the Mesozoic suture zone, wide-angle reflection/refraction seismic profiles (ECS2017 and ECS2019) were carried out in 2017 and 2019 to map the crustal structure of the ECS and clarify the possible Mesozoic suture zone. Crustal structures are generated by both a ray-tracing forward method (RayInvr) and a travel time tomographic inversion (Tomo2D). The resulting velocity models show significant lateral variations along these two seismic profiles. An extended continental crust gradually decreases in thickness from 30 km in the Zhemin Volcanic Belt (ZMVB) to 15 km in the Continental Shelf Basin (CSB) with lateral velocity variations of 4.40–7.15 km/s and 4.30–6.90 km/s, respectively, indicating differences in crustal structure between the ZMVB and the CSB. A 50 km-wide high-velocity anomaly with high density and uplifted Moho is imaged between the ZMVB and the CSB. Combined with the Mesozoic subduction and collision zone of the exotic ECS basement by previous studies, we infer that the high-velocity anomaly marks the position of the Mesozoic suture zone, which is associated with magma intrusion due to upwelling molten through the detached slab with the rollback of the Paleo-Pacific subduction slab.

Geophysical features and crustal structure of the East China Sea basement were revealed based on geophysical inversion analysis with recent gravity and magnetic data. Our results show great differences between the East China Sea continental shelf basins and the Zhemin Volcanic Belt in the coastal South China Block with respect to geophysical features and crustal structures. These areas are separated by distinct geophysical and crustal changes, approximately along a line 100 km from the coastline of China. We hypothesize that the East China Sea basement is a buoyant allochthon in the Palaeo-Pacific Plate, and that the East China margin was an Andean-type active continental margin until the collision of the East China Sea basement with the South China Block in the Late Cretaceous, which jammed the Late Cretaceous Trench, terminating the subduction and the related granitoid magmatism. The East China continental margin was dominated by dextral transtension with limited magmatism due to the NNW-trend motion of the Palaeo-Pacific Plate with a reduced convergence rate until the Middle Eocene. The renewed westward subduction of the present-day Pacific Plate replaced the East China Andean-type continental margin with a Western Pacific-type one. The detailed reconstructed tectonic model for the East China Sea since the Mesozoic is presented in this paper.

The basalt-hosted Wocan Hydrothermal Field (WHF), located on the NW slope of an axial volcanic ridge at a depth of ∼3000 m at 6°22′N on the slow-spreading Carlsberg Ridge, northwest Indian Ocean, was discovered in 2013 during Chinese DY28th cruise. Preliminary investigations show that the field consists of two hydrothermal sites: Wocan-1, which shows indications for recent high-temperature hydrothermal activity, is located near the peak of the axial volcanic ridge at a water depth of 2970–2990 m, and Wocan-2 site, located at a water depth of 3100 m, ∼1.7 km to the northwest of Wocan-1. The recovered hydrothermal precipitates can be classified into four groups: (i) Cu-rich chimneys; (ii) Cu-rich massive sulfides; (iii) Fe-rich massive sulfides; and (iv) silicified massive sulfides. We conducted mineral texture and assemblage observation and Laser-ablation ICP-MS analyses of the hydrothermal precipitates to study the mineralization processes. Our results show that there are distinct systematic trace element distributions throughout the different minerals in the four sample groups. In general, chalcopyrite from the group (i) is enriched in Pb, As, Mo, Ga, Ge, V, and Sb, metals that are commonly referred to as medium- to low-temperature elements. In contrast these elements are present in low contents in the chalcopyrite grains from other sample groups. Selenium, a typical high-temperature metal, is enriched in chalcopyrite from groups (ii) and (iv), whereas Ag and Sn are enriched only in some silicified massive sulfides. As with chalcopyrite, pyrite also shows distinct trace element associations in grains with different habitus. The low-temperature association of elements (Pb, Mo, Mn, U, Mg, Ag, and Tl) is typically present in colloform/framboidal pyrite, whereas the high-temperature association (Se, Co, and Bi) is enriched in euhedral pyrite. Sphalerite in the groups (i) and (iii) at Wocan-1 is characterized by high concentrations of Ga, Ge, Pb, Cd, As, and Sb, indicating that sphalerite in these sample groups likely precipitated at intermediate temperatures. Early bornite, which mainly occurs in the central part of the Cu-rich chimney, is typically enriched in Sn and In compared to the other minerals. In contrast, late bornite that likely formed during increasing interaction of hydrothermal fluids with cold, oxygenated seawater has low Sn and In, but significantly higher concentrations of Ag, Au, Mo and U. Digenite, also forming in the exterior parts of the samples during the late stages of hydrothermal fluid venting, is poor in most trace elements, except Ag and U. The notable Ag enrichment in the late-stage mineral assemblages at both Wocan-1 and Wocan-2 may therefore be related to lower temperatures and elevated pH. Our results indicate that Wocan-1 has experienced a cycle of heating with Cu-rich chimney growth and subsequent cooling, followed by late seafloor weathering, while Wocan-2 has seen intermediate- to high-temperature mineralization followed by intense silicification of sulfides. Seafloor weathering processes or mixing of hydrothermal fluids with seawater during the waning stages of hydrothermal fluid flow result in significant redistributions of trace elements in sulfide minerals.

Near-bottom magnetic prospecting is considered to be an efficient method for investigating inactive hydrothermal areas and the study of the spatial structure of hydrothermal systems. Furthermore, geophysical forward modeling is widely used to simulate the anomalous characteristics of geological bodies. To understand the magnetic and magnetic structure features of hydrothermal sulfide deposits at mid-ocean ridges, we built 3D forward models for both mafic- and ultramafic-hosted hydrothermal sulfide deposits to simulate the near-bottom magnetic field. Our modeling results showed a low amplitude magnetic anomaly above the mafic-hosted hydrothermal sulfide deposits, and a high amplitude magnetic anomaly above the ultramafic-hosted deposits. These features allow us to identify and classify the host rocks of hydrothermal sulfide deposits. Moreover, we can recognize the edge of the magnetic anomalies using the intensity of the spatial differential vector method, considering variables such as the width of the alt...

Spreading rate is a primary factor of mantle melting and tectonic behavior of the global mid-ocean ridges. The spreading rate of the Gakkel ridge decreases gradually from west to east. However, the Gakkel ridge can be divided into four thick-and-thin zones with varying crustal thicknesses along ridge axis. This phenomenon indicates that mantle melting of the Gakkel ridge is not a simple function of spreading rate. Mantle temperature, water content, mantle composition, and other factors are important in crustal accretion processes. Based on gravity-derived crustal thickness and wet melting model, we estimate that the mantle potential temperatures of the four zones are 1 270, 1 220, 1 280, and 1 280°C (assuming that mantle water content equals to global average value), with corresponding mantle water contents of 210, 0, 340, and 280 mg/kg (assuming that mantle potential temperature is 1 260°C), respectivly. The western thinned crust zone is best modeled with low mantle temperature, whereas the other zones are mainly controlled by the enhanced conduction caused by the slower spreading rate. Along the Gakkel ridge, the crustal thickness is consistent with rock samples types. Predominated serpentinized peridotite and basalt are found in the area with crustal thickness 〈1.5 km and 〉2.5 km, respectively. The rock samples are including from basalt to peridotite in the area with crustal thickness between 1.5 and 2.5 km. Based on this consistency, the traditional magmatic accretion zone accounted for only 44% and amagmatic accretion accounted for 29% of the Gakkel ridge. The amagmatic accretion is a significant characteristic of the ultra-slow spreading ridge.

The Tilt-Euler deconvolution based on the first derivatives of the tilt angle is advanced from the routine Euler deconvolution and it is widely used. It estimates both the horizontal location and the depth of magnetic bodies without the known structural index. Since the derivatives of the tilt angle cannot be calculated when the first horizontal derivatives of the magnetic anomaly are equal to zero, there are singularities in the inversion from the Tilt-Euler method. We present an improved method for the Tilt-Euler deconvolution, which avoids the occurrence of singularities in the calculation and reduces the unstable factors in the inversion. Since the Euler methods should be implemented in a window, we apply the standard deviation of the Euler solutions as the measure of dispersion to determine the optimal window size. Both the model test and the field application show that the solutions of the improved Tilt-Euler deconvolution are more concentrated than that of the Tilt-Euler deconvolution and, subsequently, result in a more robust interpretation. In the field application, the results of the proposed method imply that the source body is probably a dike with limited extent.

In order to study the pre-Cenozoic tectonics of the northern South China Sea (SCS), we have calculated the reduction to the pole (RTP) anomalies, apparent magnetization and pseudo-gravity anomalies using the latest shipboard magnetic data in the north of the South China Sea. The density and magnetic susceptibility along three OBS profiles were determined by inversion calculation. The result shows that the high magnetic anomaly belt (HMAB) of the Dongsha Uplift (DSU) is the continuation of the volcanic belt in the coastal area of Zhejiang and Fujian Provinces, and is dislocated by the NW trending NW fault F10 which may be the old transformation boundary. The NW trending fault F2 is the south boundary of the HMAB, and limits the north boundary of underplating. The fault F3 is a week zone before the crustal extension and thinning, and deep thermal states are different on both sides, which may reveal the location of the Late Mesozoic subduction. The magnetization of magnetic layer reduces in the magnetic quiet zone between faults F2 and F3. We ascribe the reducing magnetization to the hydrothermal alteration while high temperature mantle materials being underplated. However, on the south side of F3, the magnetization increases and magnetic source blocks of the Magnetic Quiet Zone (MQZ) are fragmented. The cause of weak positive magnetic anomalies on the surface is due to the suppression between positive and negative magnetic anomalies under oblique magnetization. The unique characteristics of the magnetic layer in this area may indicate the existence of “ancient oceanic crust”.

The Philippine Basin, surrounded by a series of oceanic trenches, is an independent deep ocean basin in the West Pacific Ocean. Its middle part is divided into three marginal sea sub-basins by the Kyushu-Palau and West Mariana Ridges, namely, the West Philippine Basin, the Shikoku and Parece Vela Basins and the Mariana Trough. This paper, through the analysis of the geomorphologic features and gravity and magnetic characteristics of the basin and identification of striped magnetic anomalies, suggests that the entire Philippine Basin developed magnetic lineation of oceanic nature, and therefore, the entire basin is of the nature of oceanic crust. The basin has developed a series of special geomorphic units with different shapes. The KPR runs through the entire Philippine Basin. From the view of geomorphologic features, the KPR is a discontinuous seamount chain (chain-shaped seamounts) and subduction beneath the Japanese Island arc at the Nankai Trough which is the natural boundary between the basin and the Japanese Island arc. At the positions of 25°N, 24°N, 23°N and 18°N, obvious discontinuity is shown, which belongs to natural topographic discontinuity. Therefore, the KPR is topographically discontinuous.

Summary Vessel magnetism is always an important factor of reducing precision of marine magnetic survey. Since the traditional model of vessel magnetism (TMVM) only depends on the magnetic heading, the corrected magnetic data are still possessed of systematic error and up-and-down slope, bringing about difficulties of data fusion for cumulating cruises. Variation of vessel induction magnetism are smallest for the network of lines towards geomagnetic northwest and northeast, and difference of vessel magnetism are smallest between the geomagnetic eastward and westward lines, but largest between the geomagnetic northward and southward lines. Therefore, suppressing the influences of vessel magnetism needs to take measures at the design of survey lines. The comprehensive model of vessel magnetism (CMVM) can take all factors into consideration, such as cable length, geomagnetic heading, geomagnetic total intensity, declination and inclination. Cross-over adjustment of main and check-up lines with different cable length will reliably separate magnetic anomaly from vessel magnetism of the model, realizing comprehensive correction of vessel magnetism. The example indicates that vessel permanent magnetism, especially its negative vertical component, could make the influences of vessel magnetism change a lot with latitude, which need to be removed with calculation and correction of the CMVM.