A possible Caledonide arm through the Barents Sea imaged by OBS data

The assembly of the crystalline basement of the western Barents Sea is related to the Caledonian orogeny during the Silurian. However, the development southeast of Svalbard is not well understood, as conventional seismic reflection data does not provide reliable mapping below the Permian sequence. A wide-angle seismic survey from 1998, conducted with ocean bottom seismometers in the northwestern Barents Sea, provides data that enables the identification and mapping of the depths to crystalline basement and Moho by ray tracing and inversion. The four profiles modeled show pre-Permian basins and highs with a configuration distinct from later Mesozoic structural elements. Several strong reflections from within the crystalline crust indicate an inhomogeneous basement terrain. Refractions from the top of the basement together with reflections from the Moho constrain the basement velocity to increase from 6.3 km s⁻¹ at the top to 6.6 km s⁻¹ at the base of the crust. On two profiles, the Moho deepens locally into root structures, which are associated with high top mantle velocities of 8.5 km s⁻¹. Combined P- and S-wave data indicate a mixed sand/clay/carbonate lithology for the sedimentary section, and a predominantly felsic to intermediate crystalline crust. In general, the top basement and Moho surfaces exhibit poor correlation with the observed gravity field, and the gravity models required high-density bodies in the basement and upper mantle to account for the positive gravity anomalies in the area. Comparisons with the Ural suture zone suggest that the Barents Sea data may be interpreted in terms of a proto-Caledonian subduction zone dipping to the southeast, with a crustal root representing remnant of the continental collision, and high mantle velocities and densities representing eclogitized oceanic crust. High-density bodies within the crystalline crust may be accreted island arc or oceanic terrain. The mapped trend of the suture resembles a previously published model of the Caledonian orogeny. This model postulates a separate branch extending into central parts of the Barents Sea coupled with the northerly trending Svalbard Caledonides, and a microcontinent consisting of Svalbard and northern parts of the Barents Sea independent of Laurentia and Baltica at the time. Later, compressional faulting within the suture zone apparently formed the Sentralbanken High.     

Lower–Middle Jurassic foraminiferal and ostracode biostratigraphy of the Barents Sea shelf

The Barents Sea shelf is an attractive target as a prospective large petroleum province. Further development of geological and geophysical exploration in the area requires high-resolution biostratigraphic constraints and update stratigraphic charts. The zonal succession of Lower and Middle Jurassic assemblages of foraminifers and ostracodes of the Barents Sea fits well the division for northern Siberia based on correlated independent Jurassic and Cretaceous zonal scales on all main microfossil groups, of which some scales were suggested as the Boreal Zonal Standard. The stratigraphic range of the Barents Sea microfossil assemblages has been updated through correlation with their counterparts from northern Siberia constrained by ammonite and bivalve data. Joint analysis of foraminiferal and ostracode biostratigraphy and lithostratigraphy of the sections allowed a revision to the stratigraphic position and extent of lithological and seismic units. The discovered similarity in the Lower and Middle Jurassic lithostratigraphy in the sections of the Barents Sea shelf and northern Siberia, along with their almost identical microfossil taxonomy, prompts similarity in the Early and Middle Jurassic deposition and geological histories of the two areas.     

Structural–Tectonic Features of the Northeastern Barents Plate from Numerical Modeling of Potential Fields

Structural–geological inhomogeneities in the northeastern Barents Sea are zoned based on an analysis of various components of the gravity and magnetic fields. The objects revealed in the basement and sedimentary cover of the Barents Sea Plate form anomalies in potential fields at coexisting complex geological structures and contrasting petrophysical properties. Cluster analysis reveals the fault-marked boundaries of individual blocks in the basement. A numerical model of faults in the sedimentary cover and basement of the Barents Sea Plate is constructed.     

关于南海北部特提斯的讨论

南海及邻区处于欧亚大陆与冈瓦纳古陆拼合带的东南端,是特提斯构造域和濒太平洋构造域交汇的重要地区.特提斯缝合带沿金沙江-哀牢山构造带进入南海,人们从而认为南海可能存在特提斯洋遗迹,并认为缝合带存在于磁静区中.本文通过对南海北部陆坡地球物理资料的解释结果,包括重力、磁力、海底地震和深反射地震数据,以及区域地质特征分析,研究了南海北部陆缘高磁异常带和磁静区的成因.结果表明高磁异常带是中白垩世时期古太平洋板块转向俯冲形成的陆缘火山弧,当时存在古俯冲带.磁静区经历了后期大陆边缘张裂和古南海和南海的打开,并经历了高温热物质的底辟作用,使得地壳拉张减薄,居里面抬升形成磁静区.经历了南海的扩张后,原始的俯冲带可能已经向南迁移到南海南部或者已经俯冲消失,其中也不存在缝合带.      

Late Mesozoic magmatism and Cenozoic tectonic deformations of the Barents Sea continental margin: Effect on hydrocarbon potential distribution

The paper is focused on the two tectonic-geodynamic factors that made the most appreciable contribution to the transformation of the lithospheric and hydrocarbon potential distribution at the Barents Sea continental margin: Jurassic-Cretaceous basaltic magmatism and the Cenozoic tectonic deformations. The manifestations of Jurassic-Cretaceous basaltic magmatism in the sedimentary cover of the Barents Sea continental margin have been recorded using geological and geophysical techniques. Anomalous seismic units related to basaltic sills hosted in terrigenous sequences are traced in plan view as a tongue from Franz Josef Land Archipelago far to the south along the East Barents Trough System close to its depocentral zone with the transformed thinned Earth’s crust. The Barents Sea igneous province has been contoured. The results of seismic stratigraphy analysis and timing of basaltic rock occurrences indicate with a high probability that the local structures of the hydrocarbon (HC) fields and the Stockman-Lunin Saddle proper were formed and grew almost synchronously with intrusive magmatic activity. The second, no less significant multitectonic stress factor is largely related to the Cenozoic stage of evolution, when the development of oceanic basins was inseparably linked with the Barents Sea margin. The petrophysical properties of rocks from the insular and continental peripheries of the Barents Sea shelf are substantially distinct as evidence for intensification of tectonic processes in the northwestern margin segment. These distinctions are directly reflected in HC potential distribution.     

Pleistocene sediments of the eastern Barents Sea (Central Deep and Murmansk Bank): Communication 2. Lithological composition and formation conditions

Llithology of massive diamictons was studied in two areas of the eastern Barents Sea using cores and geophysical data. These sediments dominate in the Pleistocene section as two seismostratigraphic complexes (SSC): Upper Weichselian (SSC III) and locally distributed Lower Weichselian (SSC V). Diamictons of these complexes represent tills produced by the geological activity of the Pleistocene Novaya Zemlya and Scandinavian ice sheets. The Upper Weichselian glacial sequence is laterally heterogeneous. It includes two seismic facies represented by ordinary (overconsolidated) tills (they also constitute SSC V) and a spacious moraine of the specific type with the normally consolidated sediments (they avoided compaction by the ice load) and certain lithological specifics. The last glacial sediments were formed in a specific subglacial setting similar to the sediments under fast ice streams of Antarctica. However, the specific features allow us to define these sediments as a new (Barents Sea) facies of tills related to zones of intense basal melting of glaciers.     

Study of Residual Basin and Tectonolayering Based on Airborne Gravity and Magnetic Data

This paper takes the South Yellow Sea as an example to show a new method for comprehensive geological-geophysical research such as residual basin and tectonolayering using airborne gravity and magnetic data in China. Based on airborne gravity and magnetic data, by measuring and analyzing stratigraphic density and susceptibility, the depths to the pre-Sinian magnetic basement top, the pre-Jurassic top and the Cenozoic bottom, are obtained by forward and inverse methods constrained by seismic and drilling data; and furthermore, the residual thicknesses of the Sinian–Triassic, the Jurassic–Cretaceous and the Cenozoic are calculated. Based on airborne gravity and magnetic anomalies, the faults in the pre-Sinian magnetic basement, the Jurassic-Cretaceous and the Cenozoic are respectively interpreted by the qualitative and quantitative methods. On the basis of the above study, and combining regional important tectonic events, four tectonolayers are divided in the vertical succession in South Yellow Sea, namely the pre-Sinian magnetic basement, the Sinian-Triassic, the Jurassic-Cretaceous and the Cenozoic. The result shows that there are thick Cenozoic and Jurassic-Cretaceous strata and thin residual Sinian-Triassic strata in the Suzhong-Huangnan depression area, but there are thin or only sporadic Mesozoic-Cenozoic terrestrial strata and thick Sinian-Triassic marine strata reserved in Subei-Huangzhong uplift area and Sunan-Wunansha uplift area. The four tectonolayers are very different in structures as well as distributions in plane.     

Crustal character and thickness over the Dangerous Grounds and beneath the Northwest Borneo Trough

New deep reflection seismic, bathymetry, gravity and magnetic data have been acquired in a marine geophysical survey of the southern South China Sea, including the Dangerous Grounds, Northwest Borneo Trough and the Central Luconia Platform. The seismic and bathymetry data map the topography of shallow density interfaces, allowing the application of gravity modeling to delineate the thickness and composition of the deeper crustal layers. Many of the strongest gravity anomalies across the area are accounted for by the basement topography mapped in the seismic data, with substantial basement relief associated with major rift development. The total crustal thickness is however quite constant, with variations only between 25 and 30 km across the Central Luconia Platform and Dangerous Grounds. The Northwest Borneo Trough is underlain by thinned crust (25–20 km total crustal thickness) consistent with the substantial water depths. There is no evidence of any crustal suture associated with the trough, nor any evidence of relict oceanic crust beneath the trough. The crustal thinning also does not extend along the complete length of the trough, with crustal thicknesses of 25 km and more modeled on the most easterly lines to cross the trough. Modeled magnetic field variations are also consistent with the study area being underlain by continental crust, with the magnetic field variations well explained by irregular magnetisations consistent with inhomogeneous continental crust, terminating at the basement unconformity as mapped from the seismic data.     

Crustal structure and transform margin development south of Svalbard based on ocean bottom seismometer data

The Barents Sea is located in the northwestern corner of the Eurasian continent, where the crustal terrain was assembled in the Caledonian orogeny during Late Ordovician and Silurian times. The western Barents Sea margin developed primarily as a transform margin during the early Tertiary. In the northwestern part south of Svalbard, multichannel reflection seismic lines have poor resolution below the Permian sequence, and the early post-orogenic development is not well known here. In 1998, an ocean bottom seismometer (OBS) survey was collected southwest to southeast of the Svalbard archipelago. One profile was shot across the continental transform margin south of Svalbard, which is presented here. P-wave modeling of the OBS profile indicates a Caledonian suture in the continental basement south of Svalbard, also proposed previously based on a deep seismic reflection line coincident with the OBS profile. The suture zone is associated with a small crustal root and westward dipping mantle reflectivity, and it marks a boundary between two different crystalline basement terrains. The western terrain has low (6.2–6.45 km s⁻¹) P-wave velocities, while the eastern has higher (6.3–6.9 km s⁻¹) velocities. Gravity modeling agrees with this, as an increased density is needed in the eastern block. The S-wave data predict a quartz-rich lithology compatible with felsic gneiss to granite within and west of the suture zone, and an intermediate lithological composition to the east. A geological model assuming westward dipping Caledonian subduction and collision can explain the missing lower crust in the western block by subduction erosion of the lower crust, as well as the observed structuring. Due to the transform margin setting, the tectonic thinning of the continental block during opening of the Norwegian-Greenland Sea is restricted to the outer 35 km of the continental block, and the continent–ocean boundary (COB) can be located to within 5 km in our data. Distinct from the outer high commonly observed on transform margins, the upper part of the continental crust at the margin is dominated by two large, rotated down-faulted blocks with throws of 2–3 km on each fault, apparently formed during the transform margin development. Analysis of the gravity field shows that these faults probably merge to one single fault to the south of our profile, and that the downfaulting dominates the whole margin segment from Spitsbergen to Bjørnøya. South of Bjørnøya, the faulting leaves the continental margin to terminate as a graben 75 km south of the island. Adjacent to the continental margin, there is no clear oceanic layer 2 seismic signature. However, the top basement velocity of 6.55 km s⁻¹ is significantly lower than the high (7 km s⁻¹) velocity reported earlier from expanding spread profiles (ESPs), and we interpret the velocity structure of the oceanic crust to be a result of a development induced by the 7–8-km-thick sedimentary overburden.     

Locations of nonstructural hydrocarbon traps on the shelf of the northern Barents Sea

This paper considers the results of summarized integrated geophysical investigations that were carried out from 2006 to 2012. The investigations included common depth point (CDP) seismic reflection survey, over water gravity survey, and differential hydromagnetic exploration with a total work scope of 30 000 linear kilometers. The deep structural tectonic plan, the structural and lithofacies features of the sedimentary cover section on the basic reflecting boundaries, and the features of the seismogeological complexes and seismic sections on a depth scale have been studied, and geological oil-and-gas zoning of the Northern Barents shelf has been made. Seventy-nine local anticlinal highs have been revealed, and the zones with potential nonstructural hydrocarbon traps have been determined. Due to the lack of huge anticlinal highs in the northern Barents Sea region, nonstructural traps are of interest in studying and replacing the mineral raw material base of Russia, as well as for arranging marine exploration.     

南海东北部磁静区深部构造及成因模式

2001年7-8月,国家重点基础研究发展规划项目（973）的二级课题《中国边缘海前新生代基底特征及其构造格局》与两岸合作课题《南海东北部与台湾间的构造及地球物理研究》合作在南海东北部地区开展了重、磁、震资料的海上采集航次,获取了大量珍贵数据,其中973NH-01与MLTW测线垂直穿越了南海东北部传统意义上的磁静区。以这两条测线的重磁资料为主,结合地震解释剖面,对磁静区的重磁异常特征、重磁基底及深部构造进行了深入的分析和研究,并结合前人工作对磁静区的成因模式提出了新的解释,认为磁静区位于南海北部被动大陆边缘的洋陆交界带（COB）,独特的动力构造背景导致其下伏的地幔高热物质上涌并沿构造薄弱带侵入成岩墙,进而造成区域居里面抬升,磁性层减薄,磁异常减弱而形成磁静区。     

Concerning tectonics and the tectonic evolution of the Arctic

The particularities of the current tectonic structure of the Russian part of the Arctic region are discussed with the division into the Barents–Kara and Laptev–Chukchi continental margins. We demonstrate new geological data for the key structures of the Arctic, which are analyzed with consideration of new geophysical data (gravitational and magnetic), including first seismic tomography models for the Arctic. Special attention is given to the New Siberian Islands block, which includes the De Long Islands, where field work took place in 2011. Based on the analysis of the tectonic structure of key units, of new geological and geophysical information and our paleomagnetic data for these units, we considered a series of paleogeodynamic reconstructions for the arctic structures from Late Precambrian to Late Paleozoic. This paper develops the ideas of L.P. Zonenshain and L.M. Natapov on the Precambrian Arctida paleocontinent. We consider its evolution during the Late Precambrian and the entire Paleozoic and conclude that the blocks that parted in the Late Precambrian (Svalbard, Kara, New Siberian, etc.) formed a Late Paleozoic subcontinent, Arctida II, which again “sutured” the continental masses of Laurentia, Siberia, and Baltica, this time, within Pangea.     

广西区域岩石物性特征及其地质意义

广西区域岩石物性调查,给出了区域岩石的密度、磁性、电性特征及其时空分布规律,划分了密度界面,明确了磁性基底的客观存在,构思了岩石层序地球物理模型,提出了不同时代花岗岩的磁性变化特征,建立了隐伏在岗岩的重力低预测模式.在区域构造填图、隐伏花岗岩填图、小盆地油气田构造研究、固定矿产成矿带预测以及某些基础地质研究等方面,具有重要意义.     

Maximum extent of the Eurasian ice sheets in the Barents and Kara Sea region during the Weichselian

Based on field investigations in northern Russia and interpretation of offshore seismic data, we have made a preliminary reconstruction of the maximum ice-sheet extent in the Barents and Kara Sea region during the Early/Middle Weichselian and the Late Weichselian. Our investigations indicate that the Barents and Kara ice sheets attained their maximum Weichselian positions in northern Russia prior to 50 000 yr BP, whereas the northeastern flank of the Scandinavian Ice Sheet advanced to a maximum position shortly after 17 000 calendar years ago. During the Late Weichselian (25 000-10 000 yr BP), much of the Russian Arctic remained ice-free. According to our reconstruction, the extent of the ice sheets in the Barents and Kara Sea region during the Late Weichselian glacial maximum was less than half that of the maximum model which, up to now, has been widely used as a boundary condition for testing and refining General Circulation Models (GCMs). Preliminary numerical-modelling experiments predict Late Weichselian ice sheets which are larger than the ice extent implied for the Kara Sea region from dated geological evidence, suggesting very low precipitation.     

基于磁力资料的珠江口盆地构造单元分布特征

珠江口盆地是南海北部陆坡最大的前新生代沉积盆地,油气资源丰富,由于新的地球物理资料未得以充分应用等问题导致盆地内部构造单元分布特征存在诸多不同看法,这些问题制约了盆地的进一步勘探和开发。本文以最新实测1∶10万高精度航磁数据为基础,采用切线法对研究区航磁异常深度进行反演计算,结合钻井、地震及南海北部陆域物性资料研究珠江口盆地磁性基底分布特征。在充分调研珠江口盆地已有构造单元划分的基础上,以盆地磁性基底展布特征为基础,结合断裂、区域构造等对珠江口盆地内部构造单元进行研究。研究表明:珠江口盆地磁性基底深度在0～9 km之间,磁性基底呈"三隆两坳"构造格局,整个坳陷区具有"南北分带,东西分块"的特征; NE向深大断裂为控盆断裂,常为盆地二级构造单元的边界,NW向断裂常控制次一级构造单元并影响其展布形态。     

Caledonide development offshore–onshore Svalbard based on ocean bottom seismometer, conventional seismic, and potential field data

The western Barents Sea and the Svalbard archipelago share a common history of Caledonian basement formation and subsequent sedimentary deposition. Rock formations from the period are accessible to field study on Svalbard, but studies of the near offshore areas rely on seismic data and shallowdrilling. Offshore mapping is reliable down to the Permian sequence, but multichannel reflection seismic data do not give a coherent picture of older stratigraphy. A survey of 10 Ocean Bottom Seismometer profiles was collected around Svalbard in 1998. Results show a highly variable thickness of pre-Permian sedimentary strata, and a heterogeneous crystalline crust tied to candidates for continental sutures or major thrust zones. The data shown in this paper establish that the observed gravity in some parts of the platform can be directly related to velocity variations in the crystalline crust, but not necessarily to basement or Moho depth. The results from three new models are incorporated with a previously published profile, to produce depth-to-basement and -Moho maps south of Svalbard. There is a 14 km deep basement located approximately below the gently structured Upper Paleozoic Sørkapp Basin, bordered by a 7 km deep basement high to the west, and 7–9 km depths to the north. Continental Moho-depth range from 28 to 35 km, the thickest crust is found near the island of Hopen, and in a NNW trending narrow crustal root located between 19°E and 20°E, the latter is interpreted as a relic of westward dipping Caledonian continental collision or major thrusting. There is also a basement high on this trend. Across this zone, there is an eastward increase in the V_P, V_P/V_S ratio, and density, indicating a change towards a more mafic average crustal composition. The northward basement/Moho trend projects onto the Billefjorden Fault Zone (BFZ) on Spitsbergen. The eastern side of the BFZ correlates closely with coincident linear positive gravity and magnetic anomalies on western Ny Friesland, apparently originating from an antiform with high-grade metamorphic Caledonian terrane. A double linear magnetic anomaly appears on the BFZ trend south of Spitsbergen, sub-parallel to and located 10–50 km west of the crustal root. Based on this correlation, it is proposed that the suture or major thrust zone seen south of Svalbard correlates to the BFZ. The preservation of the relationship between the crustal suture, the crustal root, and upper mantle reflectivity, challenges the large-offset, post-collision sinistral transcurrent movement on the BFZ and other trends proposed in the literature. In particular, neither the wide-angle seismic data, nor conventional deep seismic reflection data south of Svalbard show clear signs of major lateral offsets, as seen in similar data around the British Isles.     

Potential field modelling of the Baltica–Avalonia (Thor–Tornquist) suture beneath the southern North Sea

Magnetic anomaly maps of the Trans-European Suture Zone (TESZ) highlight the contrast between the highly magnetic crust of Baltica and the less magnetic terranes to the SW of the suture. Although the TESZ is imaged on gravity maps, anomalies related to postcollisional rifting and reactivated rift structures tend to dominate.

Seismic and potential field data have been used to construct 2 -D crustal models along three profiles crossing the Baltica–Avalonia suture in the southern North Sea (SNS). The first of these models lies along a transect assembled from reflection line GECO SNST 83-07 and refraction profile EUGENO-S 2; the other two models are coincident with MONA LISA profiles 1 and 2. Additional structural information and density information for the cover sequence is available from released wells, while magnetic susceptibility values are compatible with values measured from borehole core samples.

Magnetic anomalies related to the suture are interpreted as due to magnetic Baltican basement of the Ringkøbing-Fyn High dipping SW beneath nonmagnetic Avalonian basement underlying the western part of the SNS. Low-amplitude, long-wavelength magnetic anomalies occurring outboard of the suture are interpreted as due to a mid-crustal magnetic body, possibly a buried magmatic complex. This might represent the ‘missing’ arc related to inferred southward subduction of the Tornquist Sea, or an exotic element emplaced during the collision between Avalonia and Baltica. The present model supports an imbricated structure within Baltica as indicated by the latest reprocessing of the MONA LISA seismic data.     

南黄海北部航空重力场特征及主要地质认识

南黄海北部重力场信息丰富、梯级带发育、异常特征明显,充分反映了该区隆坳构造格局、断裂展布等地质特征。综合研究认为: NE向断裂构成了南黄海北部主体构造格架,嘉山-响水断裂、南黄海北缘断裂共同构成了苏鲁造山带南部边界; 依据航空重磁资料新发现的NW向宫家岛深大断裂对南黄海北部基底构成、岩浆岩分布具有重要的控制作用; 通过重磁联合反演,发现在南黄海北部坳陷的东北凹陷存在着前寒武系稳定的结晶基底; 航空重力资料表明,胶莱盆地向东延伸进入南黄海,在海域内其最大沉积厚度可达3 km。上述地质认识和发现为南黄海北部海洋区域地质调查、油气资源调查及重大基础地质问题的解决提供了借鉴。     

西沙地块构造—地层特征及其记录的南海洋盆演化过程

西沙地块是南海岩石圈地壳拉伸减薄过程中发育于深水区的陆块,其保存了陆缘演化的重要信息。文章研究以西沙地块作为研究对象,基于研究区地质和地球物理资料,开展了地壳结构、盆地构造—地层分析和断层活动特征等研究。研究发现,西沙地块与其周缘的凹陷地壳结构具有显著的差异,西沙地块地壳厚度较大,发育了高角度断层控制的小型断陷盆地,基底断层活动一直可持续到T60地震界面发育时（~23 Ma）;而西沙地块周缘发育的是规模较大的拆离断层及其控制的强烈减薄陆壳。结合区域动力学事件,研究认为渐新世早期拆离断层在南海西北次海盆的活动导致了西沙地块北部的岩石圈地壳的减薄,而中新世早期拆离断层在南海西南次海盆构造位置的活动使西沙地块与南沙地块分离。文章研究成果不仅深化了西沙地块裂解规律的认识,而且对该区的油气勘探具有启示意义。     

中国东部海域地质特征及一些重要大地构造界线在海区延伸的地质地球物理证据

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