Magnetic anomalies of offshore Krishna-Godavari basin,eastern continental margin of India

The marine magnetic data acquired from offshore Krishna-Godavari (K-G) basin, eastern continental margin of India (ECMI), brought out a prominent NE-SW trending feature, which could be explained by a buried structural high formed by volcanic activity. The magnetic anomaly feature is also associated with a distinct negative gravity anomaly similar to the one associated with 85°E Ridge. The gravity low could be attributed to a flexure at the Moho boundary, which could in turn be filled with the volcanic material. Inversion of the magnetic and gravity anomalies was also carried out to establish the similarity of anomalies of the two geological features (structural high on the margin and the 85°E Ridge) and their interpretations. In both cases, the magnetic anomalies were caused dominantly by the magnetization contrast between the volcanic material and the surrounding oceanic crust, whereas the low gravity anomalies are by the flexures of the order of 3–4 km at Moho boundary beneath them. The analysis suggests that both structural high present in offshore Krishna-Godavari basin and the 85°E Ridge have been emplaced on relatively older oceanic crust by a common volcanic process, but at discrete times, and that several of the gravity lows in the Bay of Bengal can be attributed to flexures on the Moho, each created due to the load of volcanic material.     

遥感影像地表温度反演与地热资源预测——以石家庄地区为例

热红外遥感技术可以反演地表温度信息,可在地热资源预测方面发挥重要作用.研究基于京津冀地区地热成藏模式,利用单窗算法反演出石家庄地区2015年3月6日地表温度,结合夜间热红外影像、遥感构造解译结果和剩余重力异常数据,综合分析,相互论证,圈定1处山地对流型地热远景区和2处沉积盆地型地热远景区.其中,平山县寺沟村昼夜地温值均...     

The crustal structure of the Sahel Basin (eastern Tunisia) determined from gravity and geothermal gradients: implications for petroleum exploration

An analysis of Bouguer gravity anomaly data and geothermal gradient data obtained from bottom hole and drill stem tests temperature is used to determine the crustal structure of the Sahel Basin in eastern Tunisia and its role in the maturation and location of the large number of oil and gas fields in the region. The regional Bouguer gravity anomaly field is dominated by gradual increase in values from the northwest to southeast and is may be caused by crustal thinning as revealed by regional seismic studies. In addition, higher geothermal gradients in the same region as the Bouguer gravity anomaly maximum add an additional constraint for the existence of crustal thinning in the region. A detailed analysis of the Bouguer gravity anomaly data was performed by both upward continuation and horizontal gradients. These two techniques were combined to show that the study area consists of two structural regions: (1) the North–South Axis (NOSA)–Zeramedine region which is characterized by northwest-dipping, northeast-striking faults, thicker crust (30–31 km) and low geothermal gradients, and (2) the Mahres–Kerkennah region which is characterized by vertical, northwest-striking faults, thinner crust (28–29 km) and higher geothermal gradients. The correlation of a variety of features includes mapped and geophysically defined faults, volcanic rocks, a thinned crust and high geothermal gradients within the same location as known oil and gas fields indicate that the faults are a major factor in the location of these petroleum accumulations.     

Structural mapping based on potential field and remote sensing data,South Rewa Gondwana Basin,India

Intracratonic South Rewa Gondwana Basin occupies the northern part of NW–SE trending Son–Mahanadi rift basin of India. The new gravity data acquired over the northern part of the basin depicts WNW–ESE and ENE–WSW anomaly trends in the southern and northern part of the study area respectively. 3D inversion of residual gravity anomalies has brought out undulations in the basement delineating two major depressions (i) near Tihki in the north and (ii) near Shahdol in the south, which divided into two sub-basins by an ENE–WSW trending basement ridge near Sidi. Maximum depth to the basement is about 5.5 km within the northern depression. The new magnetic data acquired over the basin has brought out ENE–WSW to E–W trending short wavelength magnetic anomalies which are attributed to volcanic dykes and intrusive having remanent magnetization corresponding to upper normal and reverse polarity (29N and 29R) of the Deccan basalt magnetostratigrahy. Analysis of remote sensing and geological data also reveals the predominance of ENE–WSW structural faults. Integration of remote sensing, geological and potential field data suggest reactivation of ENE–WSW trending basement faults during Deccan volcanism through emplacement of mafic dykes and sills. Therefore, it is suggested that South Rewa Gondwana basin has witnessed post rift tectonic event due to Deccan volcanism.     

Insight into the structure of the Upper Rhine Graben and its basement from a new compilation of Bouguer Gravity

A compilation of gravity data from the Upper Rhine Graben (URG) is presented that includes all the main data sources from its German and French parts. This data is used to show that the URG consists of, at least, two arc-shaped and asymmetric rift units that tectonically are the basic building blocks of the graben. In this sense the URG does not differ from other continental rifts, such as the African rifts. This division should replace the now classical geomorphologic division of the URG into three segments, based on their different trends. Moreover, the gravity suggests that the faults in the central and southern segments are continuous and have the same trend, appearing to respond as a single kinematic unit. Changes in the gravity field in the graben are shown to reflect not only the structure of the graben, but also the highly variable composition of the basement. In this respect, the URG is quite different from some other Tertiary continental rifts, where possible changes in the composition of the basement are mostly masked in the gravity field by the effect of the overlying low-density sediments. This characteristic is used to study the extent of some of the main basement units that underlie the graben.     

Neotectonic transpressional intraplate deformation in eastern Eurasia: Insights from active fault systems in the southeastern Korean Peninsula

Transpression occurs in response to oblique convergence across a deformation zone in intraplate regions and plate boundaries. The Korean Peninsula is located at an intraplate region of the eastern Eurasian Plate and has been deformed under the ENE–WSW maximum horizontal compression since the late Pliocene. In this study, we analyzed short-term instrumental seismic (focal mechanism) and long-term paleoseismic (Quaternary fault outcrop) data to decipher the neotectonic crustal deformation pattern in the southeastern Korean Peninsula. Available (paleo-)seismic data acquired from an NNE–SSW trending deformation zone between the Yangsan and Ulleung fault zones indicate spatial partitioning of crustal deformation by NNW–SSE to NNE–SSW striking reverse faults and NNE–SSW striking strike-slip faults, supporting a strike-slip partitioned transpression model. The instantaneous and finite neotectonic strains, estimated from the focal mechanism and Quaternary outcrop data, respectively, show discrepancies in their axes, which can be attributed to the switching between extensional and intermediate axes of finite strain during the accumulation of wrench-dominated transpression. Notably, some major faults, including the Yangsan and Ulsan fault zones, are relatively misoriented to slip under the current stress condition but, paradoxically, have more (paleo-)seismic records indicating their role in accommodating the neotectonic transpressional strain. We propose that fluids, heat flow, and lithospheric structure are potential factors affecting the reactivation of the relatively misoriented major faults. Our findings provide insights into the accommodation pattern of strain associated with the neotectonic crustal extrusion in an intraplate region of the eastern Eurasian Plate in response to the collision of the Indian Plate and the subduction of the Pacific/Philippine Sea Plates.     

Seismic ray direction anomalies and relative residuals of earthquakes recorded at Gauribidanur array

Slowness and azimuthal anomalies provide valuable information about lateral inhomogeneities within the crust and mantle of the earth. Over 300 earthquakes (distance range 14°–36° and azimuth 0°–360°) recorded at Gauribidanur seismic array (GBA) in southern India, were analysed using adaptive processing techniques. Slowness anomalies upto 1·3 sec/deg and azimuthal anomalies upto 8° have been observed in the present analysis. Slowness anomaly patterns for Java trench, Mid-Indian oceanic ridge earthquakes are more consistent as compared to the events originating in the Himalayan and Hindukush regions. A significant feature of the azimuthal anomaly pattern was the distinct absence of any positive anomalies from earthquakes occurring in mid-oceanic ridge. These anomalies have also been analysed as a function of epicentral distance and are mainly attributed to the transition zones occurring between 400–700 km depth ranges in the Indian upper mantle regions. Relative residuals between the stations of GBA have very little dependence on azimuth and distance. An anomalous structure beneath the array in the direction of the Java trench region (azimuth 116–126°) has been postulated on the basis of large systematic slowness vectors observed.     

Major and trace elements of abyssal peridotites: evidence for melt refertilization beneath the ultraslow-spreading Southwest Indian Ridge (53° E segment)

This study examines the geochemistry of major and trace elements of abyssal peridotites from the Southwest Indian Ridge (SWIR) (53° E amagmatic segment), to determine the influence of mafic melts on mantle peridotites during melt extraction. The results show a great geochemical variability in the ~90 km-long ridge segment, with a degree of mantle melting ranging from 4% to 24%. An ancient melting event may explain the presence of highly depleted peridotites at the ultraslow-spreading ridge. The 53° E segment peridotites show enrichment of light rare earth elements (LREEs) (average La_N/Sm_N = 1.87) and significant positive anomaly of U and Pb normalized to primitive mantle (PM). The positive correlations between LREEs (La, Ce, Pr, Nd) and high field strength elements (HFSEs; e.g. Nb and Zr) suggest that the enrichment of LREEs is caused by melt refertilization, which is also supported by prevalent magmatic microstructures in the peridotites. The melt refertilization model shows that the addition of 0.02–2.7% basaltic melts to peridotites can be responsible for the LREE enrichment. We suggest that the positive anomaly of U is probably attributed to fluid alteration whereas the enrichment of Pb is probably attributed to both melt refertilization and fluid alteration. Melt refertilization in the 53° E segment peridotites may be a result of melt–rock reaction and crystallization of melts trapped in peridotites. These processes may be enhanced by increased melt permeability in the mantle owing to the refractory peridotites produced by ancient melting and the decreasing efficiency of melt extraction in the cold and thick lithosphere at the 53° E ridge segment. The presence of melt refertilization implies that melt extraction is incomplete in the ridge mantle, which may be one of the reasons for the extremely thin and irregular variation of the crustal thickness at ultraslow-spreading ridges.     

Geophysical signatures over and around the northern segment of the 85°E Ridge,Mahanadi offshore,Eastern Continental Margin of India: Tectonic implications

The nature and origin of the subsurface 85°E Ridge in the Bay of Bengal has remained enigmatic till date despite several theories proposed by earlier researchers. We reinterpreted the recently acquired high quality multichannel seismic reflection data over the northern segment of the ridge that traverses through the Mahanadi offshore, Eastern Continental Margin of India and mapped the ridge boundary and its northward continuity. The ridge is characterized by complex topography, multilayer composition, intrusive bodies and discrete nature of underlying crust. The ridge is associated with large amplitude negative magnetic and gravity anomalies. The negative gravity response across the ridge is probably due to emplacement of relatively low density material as well as ∼2–3 km flexure of the Moho. The observed broad shelf margin basin gravity anomaly in the northern Mahanadi offshore is due to the amalgamation of the 85°E Ridge material with that of continental and oceanic crust. The negative magnetic anomaly signature over the ridge indicates its evolution in the southern hemisphere when the Earth’s magnetic field was normally polarized. The presence of ∼5 s TWT thick sediments over the acoustic basement west of the ridge indicates that the underlying crust is relatively old, Early Cretaceous age.The present study indicates that the probable palaeo-location of Elan Bank is not between the Krishna–Godavari and Mahanadi offshores, but north of Mahanadi. Further, the study suggests that the northern segment of the 85°E Ridge may have emplaced along a pseudo fault during the Mid Cretaceous due to Kerguelen mantle plume activity. The shallow basement east of the ridge may have formed due to the later movement of the microcontinents Elan Bank and Southern Kerguelen Plateau along with the Antarctica plate.     

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.     

Satellite Gravity Anomalies and Their Correlation with the Major Tectonic Features in the South China Sea

The South China Sea (SCS) is a region of interaction among three major plates: the Pacific, Indo-Australian and Eurasian. The collision of the Indian subcontinent with the Eurasian plate in the northwest, back-arc spreading at the center, and subduction beneath the Philippine plate along Manila trench in the east and the collision along Palawan trough in the south have produced complex tectonic features within and along the SCS. This investigation examines the satellite-derived gravity anomalies of the SCS and compares them with major tectonic features of the area. A map of Bouguer gravity anomaly is derived in conjunction with available seafloor topography to investigate the crustal structure. The residual isostatic gravity anomaly is calculated assuming that the Cenozoic sedimentary load is isostatically compensated. The features in the gravity anomalies in general correlate remarkably well with the major geological features, including offsets in the seafloor spreading segments, major faults, basins, seamounts and other manifestations of magmatism and volcanism on the seafloor. They also correlate with the presumed location of continental-oceanic crust boundary. The region underlain by oceanic crust in the central part of the SCS is characterized by a large positive Bouguer gravity anomaly (220–330 mgal) as well as large free-air and residual isostatic anomalies. There are, however, important differences among spreading segments. For example, in terms of free-air gravity anomaly, the southwest section of mid-ocean has an approximately 50 km wide belt of gravity low superimposed on a broad high of 45 mgal running NW–SE, whereas there are no similar features in other spreading segments. There are indications that gravity anomalies may represent lateral variation in upper crustal density structure. For instance, free air and isostatic anomalies show large positive anomalies in the east of the Namconson basin, which coincide with areas of dense volcanic material known from seismic surveys. The Red River Fault system are clearly identified in the satellite gravity anomalies, including three major faults, Songchay Fault in the southwest, Songlo Fault in the Northeast and Central Fault in the center of the basin. They are elongated in NW–SE direction between 20±30'N and 17°N and reach to Vietnam Scarp Fault around 16°30'N. It is also defined that the crustal density in the south side of the Central Basin is denser than that in the north side of the Central Basin.     

GIS-based analysis of the Stream Length-Gradient Index for evaluating effects of active tectonics: a case study of Enfidha (North-East of Tunisia)

Our study aims at calculating morphometric indices through the recourse to the Digital Elevation Model (DEM) and its attributes (hydrographic networks, watersheds, and reliefs). This quantitative measurement is used for the morphotectonical analysis of the Enfidha area (North-East of Tunisia) in the GIS environment and statistical platform. The Enfidha area is selected to analyze and identify neotectonic morphostructures on basis of the morphological evolution of the landscape. The coastal Enfidha plain is part of the Tunisian Sahel that is marked by its instability and frequent earthquakes that prove the presence of the recent deformation and active subsurface faults. This morphometric modeling in the geological investigation area reflects the morphotectonic spatial evolution in the Quaternary basins areas. Stream length (SL) proposed by Hack (1973) and denominated by Etchebehere et al. (2004) Revista do Instituto de Geociências USP, Série Científica, São Paulo 4(2):43–56 as the RDE index (Relation Declivity Extension) was used for application in the studies of neotectonic approach. RDE index is one of the morphotectonical indices that designates the tectonics anomalies in rivers. The variability in (RDE) values can indicate both a lithology resistance towards erosion and major active subsurface faults or low slope. Our geostatistical method is required to obtain RDE index maps based on lithological filtrations and value interpolations which are calculated in a drainage line network. This method calculates, measures, and validates spatial distribution of hack RDE index through a high statistical accuracy in a quantitative anomalies study. The RDE anomaly maps of the study area mirror the concurrence between anomaly zones, major active subsurface E-W faults, and local geological discontinuities related to neotectonical activities that affect the hydrographic network. This study proves that the RDE index can be especially used to detect the influence of active large-scale faults on the landscape evolution.     

Development of the negative gravity anomaly of the 85°E Ridge,northeastern Indian Ocean – A process oriented modelling approach

The 85°E Ridge extends from the Mahanadi Basin, off northeastern margin of India to the Afanasy Nikitin Seamount in the Central Indian Basin. The ridge is associated with two contrasting gravity anomalies: negative anomaly over the north part (up to 5°N latitude), where the ridge structure is buried under thick Bengal Fan sediments and positive anomaly over the south part, where the structure is intermittently exposed above the seafloor. Ship-borne gravity and seismic reflection data are modelled using process oriented method and this suggest that the 85°E Ridge was emplaced on approximately 10–15 km thick elastic plate (Te) and in an off-ridge tectonic setting. We simulated gravity anomalies for different crust-sediment structural configurations of the ridge that were existing at three geological ages, such as Late Cretaceous, Early Miocene and Present. The study shows that the gravity anomaly of the ridge in the north has changed through time from its inception to present. During the Late Cretaceous the ridge was associated with a significant positive anomaly with a compensation generated by a broad flexure of the Moho boundary. By Early Miocene the ridge was approximately covered by the post-collision sediments and led to alteration of the initial gravity anomaly to a small positive anomaly. At present, the ridge is buried by approximately 3 km thick Bengal Fan sediments on its crestal region and about 8 km thick pre- and post-collision sediments on the flanks. This geological setting had changed physical properties of the sediments and led to alter the minor positive gravity anomaly of Early Miocene to the distinct negative gravity anomaly.     

Sediment thickness map of the Indian region 79°E–86°E, 2°S–8°S based on satellite gravity interpretation

Satellite free air gravity anomalies over the Indian ocean region 79°E–86°E, 2°S–8°S were obtained from the website http://topex.ucsd.edu and a contour map was compiled. Five profiles of the anomaly have been interpreted in terms of two-dimensional structures in the ocean. Thickness of sediments lying on the oceanic crust determined from the interpretation of gravity profiles were used to compile an isopach map of the region 79°E–86°E, 2°S–8°S. This map in combination with one of the isopach maps compiled by previous workers, provides information regarding the thickness of sediments up to 6° S. According to this map sediment thickness varies from ～600 m over the middle part of the region to ～800 m further south, indicating that thinning of sediments in the middle part of the region is only localized. Information provided by this gravity study may be useful in planning detailed seismological studies to delimit the outer edge of the continental margin of Sri Lanka, defined according to the United Nations Convention of the Law of the Sea (UNCLOS).     

Geophysical evidence for a large impact structure on the Falkland (Malvinas) Plateau

A large, roughly circular structural basin is recognised on the Falkland (Malvinas) Plateau to the NW of West Falkland (Gran Malvina) Island (S 51°00′, W 62°00′). The basin, seen in seismic‐reflection profiles and evident as a large negative gravity anomaly, has a diameter of ~250 km. The age of the basin is estimated to be Late Palaeozoic. It is completely buried by younger sediments and has no topographic expression on the sea floor. We propose that the basin and geophysical anomalies, especially the combination of a large circular negative gravity anomaly with a rim of positive anomalies, and a marked circular series of positive magnetic anomalies in the same area, may be best explained by the presence of a large buried impact structure.     

Evidence of dislocations on the southern slope of the nebrodi-madonie mountains (northern Sicily, Italy): Their neotectonics implications

The investigated area is located on the southern slope of the NebrodiMadonie Mts. (northern Sicily, Italy). It was studied in order to obtain neotectonic evidence by means of fracture measurements, morphotectonic and hydrographical patterns, aerial photographs and satellite analysis. The results were compared with the well-known geological structures.This area could be considered an outstanding crustal part of Sicily, as here we find alignments of earthquake foci and a deep gradient of gravity anomalies, testifying to an important isostatic imbalance. This analysis confirms the observed structural picture of surface tectonics. Rose diagrams elaborated from fieldwork and aerial lineations show distinctive E—W trends which conform to the directions of the main faults.From a chronological viewpoint faults breaks, aerial photographs and satellite lineations seem to indicate that the E—W trend is the less dislocated and, therefore, the most recent.     

甘肃白银厂铜多金属矿田重磁异常及构造特征北大核心CSCD

甘肃白银厂铜多金属硫化物矿田是受陆缘弧环境火山机构及其同生断裂控制的典型火山岩赋矿块状硫化物矿床(VHMS)。为了进一步探讨该矿床重磁场特征及与构造和矿床的关系,通过对区内的重磁场异常数据进行位场分离、小波分析和基于张量数据的三维欧拉反褶积自动确定地质体位置和埋藏深度的定量反演计算。结果表明,研究区布格重力场具有西部高、东部低的特征,其磁场可划分为4个磁场区;局部异常按一定分布规律呈圆形或似圆形正负相间分布,不同尺度的重磁细节异常图在一定程度上反映出引起重磁异常的地质异常体具有一定的延深且分布稳定,而且这些地质异常体具有向深部复合的趋势。经位场分离后正负相间的剩余重磁异常分布范围和分布特征大致反映了白银厂奥陶纪中酸性火山岩建造构造为一个继承性的火山穹窿构造,东、西部不同的重磁异常特征说明东西部的火山喷发具有不同的基底、源区和喷发方式。发育NW向、NE向、近NS向、NEE向等4组断裂构造,近NS向断裂F3、F4、F5、F6、F7形成时间稍晚于NEE向断裂,其与NEE向断裂F1、F2共同形成了研究区棋盘网格状的构造分布特征,这两组断裂为研究区内主要的控岩、控矿断裂。火山机构的分布明显受断裂构造控制,火山口集中分布在深大断裂、大断裂或两组断裂的交汇处。矿田内各矿床及成矿有利地段均处于研究区中部低缓重磁异常场内正负磁异常变化的梯度带内。本次研究为控矿要素研究和开展找矿预测工作提供了丰富、翔实的地球物理资料。     

Features of deeper structure of Magnitogorsk synclinorium,from geophysical data

The Magnitogorsk synclinorium, a major structure of the southern Urals, has pronounced positive gravity anomalies associated with numerous greenstone intrusives. It is bounded on the west by a gravity low which corresponds with the Ural-Tau and Bashkir anticlinorium and on the east by gravity minima corresponding to the East-Uralian anticlinorium with its numerous intrusives. The relationship between gravity anomalies and major south Uralian structure indicates the effect of deeper structure conditions. Magnetic data correlates with the gravity results in that the western and eastern zones have quiescent negative magnetic fields and the Magnitogorsk synclinorium shows a strongly differentiated magnetic field. Positive magnetic anomalies are related to ultrabasic and basic intrusives, though some large magnetic maxima correspond to lower Carboniferous acidic and alkalic intrusives. Gravity and magnetic anomalies give some idea of deeper structure of the area. Copper pyrite deposits are associated with linear magnetic anomalies and associated zones of crushing. Regionally, these deposits are definitely associated with positive gravity anomaly zones. Geologic and geophysical maps are included, showing major geologic features and related magnetic and gravity anomaly zones. — C. E. Sears.     

Asymmetry in oceanic crustal structure of the South China Sea basin and its implications on mantle geodynamics

ABSTRACT

We investigated the oceanic crustal structure and lithospheric dynamics of the South China Sea (SCS) basin through a comprehensive analysis of residual gravity anomaly and bathymetry combined with seismic constraints and interpretation from geodynamic modelling. We first calculated the residual mantle Bouguer anomaly (RMBA) of the oceanic crustal regions of the SCS by removing from free-air gravity anomaly the predicted gravitational attractions of water-sediment, sediment-crust, and crust-mantle interfaces, as well as the effects of lithospheric plate cooling, using the latest crustal age constraints including IODP Expedition 349 and recent deep-tow magnetic surveys. We then calculated models of the gravity-derived crustal thickness and calibrated them using the available seismic refraction profiles of the SCS. The gravity-derived crustal thickness models correlate positively with seismically determined crustal thickness values. Our analysis revealed that the isochron-averaged RMBA are consistently more negative over the northern flank of the SCS basin than the southern conjugate for magnetic anomaly chrons C8n (~25.18 Ma) to C5Dn (~17.38 Ma), implying warmer mantle and/or thicker crust over much of the northern flank. Computational geodynamic modelling yielded the following interpretations: (1) Models of asymmetric and variable spreading rates based on the relatively high-resolution deep-tow magnetic analysis would predict alternating thicker and thinner crust at the northern flank than the southern conjugate, which is inconsistent with the observed systematically thicker crust on the northern flank. (2) Models of episodic southward ridge jumps could reproduce the observed N-S asymmetry, but only for crustal age of 23.6–20 Ma. (3) Southward migration of the SCS ridge axis would predict slightly thinner crust at the northern flank, which is inconsistent with the observations. (4) Models of higher mantle temperatures of up to 25–50°C or >2% less depleted mantle sources on the northern flank could produce large enough anomalies to explain the observed N-S asymmetries.     

中国大陆东部新构造期北西向断裂带的初步探讨

前人对中国大陆东部新构造期 (N—Q)活动的北东向断裂 (带 )已有很多研究。笔者根据地震构造、新构造和火山活动等资料并结合Pn波速结构研究结果 ,分析了该区新构造期的北西向活动断裂 ,初步得出 :北西向断裂具有成带性 ,可分 7条北西向断裂带 ;它们有鲜明的新生性 ;均为地壳构造带 ;与北东向断裂带呈共轭关系 ,是在先存构造基础上于新构造应力场作用下正在发育的一套地壳共轭剪切破裂系统。