2013 Vol. 32, No. 12

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Recent progress in submarine geosciences in China
JIN Xianglong
2013, 32(12): 1-3. doi: 10.1007/s13131-013-0389-x
In China submarine geosciences represents a newly established discipline of oceanography, focusing on the oceanic lithosphere, and its interface with the hydrosphere and biosphere. Recently, supported by the National High Technology Research and Development Program and other high-tech development projects, significant progress has been made in the development of advanced technologies and equipment. This enables the scientists in China to carry out explorations of the international seabed area in the Pacific Ocean and on the Southwest Indian Ridge. In addition, they have been active in the research activities associated the mid-ocean ridges and western Pacific marginal seas. It is anticipated that this research field will continue to be highly fruitful in the near future.
Holocene shelf-coastal sedimentary systems associated with the Changjiang River:An overview
2013, 32(12): 4-12. doi: 10.1007/s13131-013-0390-5
The fate of the terrestrial sediment supplied by rivers is a critical issue for understanding the patterns of Holocene environmental change on continental shelves. The East China Sea is a typical broad continental shelf with abundant sediment supply from large rivers. Here, a variety of sedimentary records were formed during the Holocene period. The sedimentary systems associated with these records have unique characteristics in terms of spatial distribution, material composition, deposition rate and the timing of deposition, which are related to active sediment transport processes induced by tides and waves, shelf circulations and sediment gravity flows. The sedimentary records thus formed are high resolution slices, i.e., each record has a temporal resolution of up to 100-10-1 a, but only covers a limited part of the Holocene time. In terms of the spatial distribution, these records are scattered over a large area on the shelf. Further studies of these systems are required to understand the underlying process-product relationships. In particular, the mid-Holocene coastal deposits on the Jiangsu coast, the early to middle Holocene sequences of the Hangzhou Bay, as well as the Holocene mud deposits off the Zhejiang-Fujian coasts, should be investigated in terms of the material supply (from both seabed reworking during the sea level rise event and river discharges), transport-accumulation processes, the sediment sequences and the future evolution of the sedimentary systems. Advanced numerical modeling techniques should be developed to meet the needs of these studies.
Joint land-sea seismic survey and research on the deep structures of the Bohai Sea areas
HAO Tianyao, YOU Qingyu, LIU Lihua, LV Chuanchuan, XU Ya, LI Zhiwei, ZHAO Chunlei, ZHENG Yanpeng, LIU Chenguang, HAN Guozhong
2013, 32(12): 13-24. doi: 10.1007/s13131-013-0383-4
This paper presents the survey and research work of two land-sea profiles in the Bohai Sea, China, carried out in 2010-2011, including the seismic sources on land and in the sea, the ocean bottom seismographs (OBS) and their recovery, the coupling of OBS and the environment noise in sea area, the data quality of OBSs, and the result of data analysis. We focused on the investigation of crustal structures revealed by the two NE\EW-trending joint land-sea profiles. In combination with the Pn-velocity distribution and gravity-magnetic inversion results in the North China Craton, we propose that the undulation of the Moho interface in the Bohai and surrounding areas is not strong, and the lithospheric thinning is mainly caused by the thinning of its mantle part. The research result indicates that obvious lateral variations of Moho depth and seismic velocity appear nearby all the large-scale faults in Bohai Sea, and there is evidence of underplating and reforming of the lower crust by mantle material in the Bohai area. However, geophysical evidence does not appear to support the "mantle plume" or "delamination" model for the North China Craton destruction. The crustal structure of the Bohai Sea revealed "a relatively normal crust and obviously thinned mantle lid", local velocity anomalies and instability phenomena in the crust. These features may represent a combined effect of North China-Yangtze collision at an early stage and the remote action of Pacific plate subduction at a late stage.
Heat flow measurements on the Lomonosov Ridge, Arctic Ocean
XIAO Wentao, ZHANG Tao, ZHENG Yulong, GAO Jinyao
2013, 32(12): 25-30. doi: 10.1007/s13131-013-0384-3
Heat flow was measured on the Lomonosov Ridge during the 5th Chinese National Arctic Expedition in 2012. To derive the time-temperature curve, resistivity data were transformed to temperature by the resistivity-temperature program. Direct reading and linear regression methods were used to calculate the equilibrium temperature, which were regressed against the depth of the probes in sediment to derive the geothermal gradient. Then, heat flow was calculated as the product of geothermal gradient and thermal conductivity of sediments. The heat flow values on the basis of the two methods were similar (i.e., 67.27 mW/m2 and 63.99 mW/m2, respectively). The results are consistent with the measurements carried out at adjacent sites. The age of the Lomonosov Ridge predicted by the heat flow-age model was 62 Ma, which is in accordance with the inference that the ridge was separated from Eurasia at about 60 Ma.
Stress field modeling of northwestern South China Sea since 5.3 Ma and its tectonic significance
YANG Fengli, ZHOU Zuyi, ZHANG Na, LIU Ning, NI Bin
2013, 32(12): 31-39. doi: 10.1007/s13131-013-0385-2
Tectonically, the northwestern South China Sea (SCS) is located at the junction between three micro-plates, i.e., the Indochina, South China and Zhongsha-Xisha micro-plates, and involves three basins, i.e., the Yinggehai Basin, the Qiongdongnan Basin and Xisha Trough in the east, and the Zhongjiannan Basin in the south. Since the Pliocene (5.3 Ma), the Yinggehai Basin has experienced repeated accelerating subsidence, high thermal fluid, and widely developing mud-rich overpressure chambers, abundant mud diapers and crust-mantle mixed CO2. While a large central canyon was developed in the Qiongdongnan Basin, new rift occurred in the Xisha Trough. These characteristics demonstrate a single tectonic unit for the northwestern SCS, for which we have undertaken stress field modeling to understand its plate deformations and sedimentary responses. Our results demonstrate that an extension tectonic event occurred after 5.3 Ma in the Yinggehai-Qiongdongnan-Xisha trough area, which is characterized by thinner crust (<16 000 m), half-graben or graben structural style and thicker sedimentary sequences (>3 500 m). A new rift system subsequently was developed in this area; this event was mainly driven by the combined effects of different movement velocity and direction of the three micro-plates, and the far-field effect of the continental collision between the Indian Plate and the Tibetan Plateau, and subduction of the Pacific Plate underneath the Eurasian Plate.
Geochemistry and petrogenesis of Quaternary volcanism from the islets in the eastern Beibu Gulf: evidence for Hainan plum
LI Naisheng, YAN Quanshu, CHEN Zhihua, SHI Xuefa
2013, 32(12): 40-49. doi: 10.1007/s13131-013-0386-1
Some of the islets in the eastern Beibu Gulf are covered by Quaternary volcano strata. The rock samples from these islets mainly consist of quartz tholeiites (at Shenjiandao), olivine tholeiites (at Linshidao and Xieyangdao) and alkali basalts (at Yangpubi and Jianshidao), and basically represent four periods of the Quaternary volcanism of Hainan Island and its adjacent regions. Except for the samples from Shenjiandao, most of the Quaternary volcanics of these islets belong to alkali magma series. The trace element characteristics of all of these samples show they are OIB (oceanic island basalt) -like, which implies that their deep geodynamic setting may be related to a mantle plume. The Sr-Nd-Pb isotopic compositions show that the mantle source beneath the Quaternary strata can be regarded as a result of binary mixing between a depleted, DMM (depleted MORB mantle)-like source and an enriched mantle type 2 (EM2). The EM2 may be originated from the Hainan mantle plume, and has been metasomatized by carbonaceous fluids released from ancient recycled oceanic crust at an asthenospheric mantle level. These features, together with typical trace element ratios, reflect that the parent magma was not subjected to crustal contamination during its ascent to the surface. This study provides further petrological and geochemical evidence for the existence of the Hainan mantle plume.
Mantle melting beneath the Southwest Indian Ridge: signals from clinopyroxene in abyssal peridotites
WANG Wei, CHU Fengyou, ZHU Jihao, DONG Yanhui, YU Xing, CHEN Ling, LI Zhenggang
2013, 32(12): 50-59. doi: 10.1007/s13131-013-0387-0
The mineral chemistry and texture of clinopyroxenes in peridotite from the Kingkong tectonic zone of the Southwest Indian Ridge segment in an effort to constrain mantle melting beneath this slow-spreading ridge are reported. There are three types of clinopyroxenes in the abyssal peridotites: coarse-grained, intergranular and exsolved. The compositional variations among these three types suggest that the coarse-grained clinopyroxene is a mantle-derived source. The Al, Na and Ti contents and the Na/Ti ratio of the coarse-grained clinopyroxene may be used to monitor the degree of partial melting, combined with the contradistinction with Spinel Cr#, which is calculated to be between 7.9% and 14.9%, and may represent low degrees of melting in the global ocean ridge system. The along-axis compositional variations in the coarse-grained clinopyroxene suggest that the degree of partial melting is primarily controlled by the transform faults on both sides of the ridge. Nonetheless, the northwestern side of the ridge may be affected by a hypothesised detachment fault as documented by the calculated P-T conditions. Simultaneously high Na and low Ti contents in the coarse-grained clinopyroxene points to mantle heterogeneities along the ridge axis.
Component characteristics of organic matter in hydrothermal barnacle shells from Southwest Indian Ridge
HUANG Xin, ZENG Zhigang, CHEN Shuai, YIN Xuebo, WANG Xiaoyuan, ZHAO Huijing, YANG Baoju, RONG Kunbo, MA Yao
2013, 32(12): 60-67. doi: 10.1007/s13131-013-0388-z
In 2008-2009, hydrothermal barnacle and sediment samples were collected from the Southwest Indian Ridge during a survey of the China Ocean Mineral Resources R&D Association (COMRA). Samples were analyzed by gas chromatography-mass spectrometer (GC-MS), revealing the main organic constituents of hydrothermal barnacle and sediment to be fatty acids and alkylbenzenes. N-alkanes which possessed obvious even carbon advantage were also detected in hydrothermal sediment. The high concentrations of aromatic compounds might be the result of macromolecular thermal alteration. Microorganism in the submarine hydrothermal ecosystem, especially those related to sulfur metabolism, might be the source of the high concentrations of fatty acids detected in these samples. In high temperature and high pressure hydrothermal environments, n-alkanes which possessed obvious even carbon advantage might originate from thermal alteration of carboxylic acids and other lipid compounds.
TEM investigations of South Atlantic Ridge 13.2°S hydrothermal area
TAO Chunhui, XIONG Wei, XI Zhenzhu, DENG Xianming, XU Yixian
2013, 32(12): 68-74. doi: 10.1007/s13131-013-0392-3
According to the exploration contract about polymetallic sulfides in the SWIR (Southwest Indian Ridge) signed by China with the International Seabed Authority, to delineate sulfide minerals and estimate resource quantity are urgent tasks. We independently developed our first coincident loop Transient Electromagnetic Method (TEM) device in 2010, and gained the TEM data for seafloor sulfide at South Atlantic Ridge 13.2 ° S in June 2011. In contrast with the widely applied CSEM (Marine controlled-source electromagnetic) method, whose goal is to explore hydrocarbons (oil/gas) of higher resistivity than seawater from 102 to 103 m below the sea floor, the TEM is for low resistivity minerals, and the target depth is from 0 to 100 m below the sea floor. Based on the development of complex sulfide geoelectrial models, this paper analyzed the TEM data obtained, proposing a new method for seafloor sulfide detection. We present the preliminary trial results, in the form of apparent resistivity sections for both half-space and full-space conditions. The results correspond well with the observations of the actual hydrothermal vent area, and the detection depth reached 50-100m below the bed, which verified the capability of the equipment.
Seismic phases from the Moho and its implication on the ultraslow spreading ridge
ZHANG Jiazheng, ZHAO Minghui, QIU Xuelin, LI Jiabiao, RUAN Aiguo
2013, 32(12): 75-86. doi: 10.1007/s13131-013-0393-2
The Moho interface provides critical evidence for crustal thickness and the mode of oceanic crust accretion. The seismic Moho interface has not been identified yet at the magma-rich segments (46°-52°E) of the ultra-slow spreading Southwestern Indian Ridge (SWIR). This paper firstly deduces the characteristics and domains of seismic phases based on a theoretical oceanic crust model. Then, topographic correction is carried out for the OBS record sections along Profile Y3Y4 using the latest OBS data acquired from the detailed 3D seismic survey at the SWIR in 2010. Seismic phases are identified and analyzed, especially for the reflected and refracted seismic phases from the Moho. A 2D crustal model is finally established using the ray tracing and travel-time simulation method. The presence of reflected seismic phases at Segment 28 shows that the crustal rocks have been separated from the mantle by cooling and the Moho interface has already formed at zero age. The 2D seismic velocity structure across the axis of Segment 28 indicates that detachment faults play a key role during the processes of asymmetric oceanic crust accretion.
The morphotectonics and its evolutionary dynamics of the central Southwest Indian Ridge (49° to 51°E)
LIANG Yuyang, LI Jiabiao, LI Shoujun, RUAN Aiguo, NI Jianyu, YU Zhiteng, ZHU Lei
2013, 32(12): 87-95. doi: 10.1007/s13131-013-0394-1
The morphotectonic features and their evolution of the central Southwest Indian Ridge (SWIR) are discussed on the base of the high-resolution full-coverage bathymetric data on the ridge between 49°-51°E. A comparative analysis of the topographic features of the axial and flank area indicates that the axial topography is alternated by the ridge and trough with en echelon pattern and evolved under a spatial-temporal migration especially in 49°-50.17°E. It is probably due to the undulation at the top of the mantle asthenosphere, which is propagating with the mantle flow. From 50.17° to 50.7°E, is a topographical high terrain with a crust much thicker than the global average of the oceanic crust thickness. Its origin should be independent of the spreading mechanism of ultra-slow spreading ridges. The large numbers of volcanoes in this area indicate robust magmatic activity and may be related to the Crozet hot spot according to RMBA (residual mantle Bouguer anomaly). The different geomorphological feature between the north and south flanks of the ridge indicates an asymmetric spreading, and leading to the development of the OCC (oceanic core complex). The tectonic activity of the south frank is stronger than the north and is favorable to develop the OCC. The first found active hydrothermal vent in the SWIR at 37°47'S, 49°39'E is thought to be associated with the detachment fault related to the OCC.
ADCP measurements of suspended sediment flux at the entrance to Jiaozhou Bay, western Yellow Sea
WANG Yaping, GAO Shu
2013, 32(12): 96-103. doi: 10.1007/s13131-013-0395-0
The purpose of the present contribution is to explore the technique to use Acoustic Doppler Current Profilers (ADCPs) for suspended sediment flux measurements, which may be applied to coastal embayment environments such as estuaries and tidal inlets for sediment exchange and budget studies. Based on tidal cycle measurements from the entrance of Jiaozhou Bay, Shandong Peninsula, eastern China, statistical relationships between the suspended sediment concentration (SSC) and ADCP echo intensity output are established. Echo intensity data obtained during an ADCP survey along two cross-sections during a spring tidal phase were transformed into SSC data. The ADCP current velocity and SSC data were then used to calculate the flux of fine-grained sediment. The results show that net sediment transport at the entrance is directed towards the open sea, with an order of magnitude of 103 t per spring tidal cycle; hence, although Jiaozhou Bay is a low SSC environment, the tidally induced suspended sediment transport can be intense.
Characteristics of silicon and oxygen isotopic compositions of basalts near East Pacific Rise 13°N
WANG Xiaoyuan, ZHAO Huijing, ZENG Zhigang, YIN Xuebo, CHEN Shuai, MA Yao
2013, 32(12): 104-108. doi: 10.1007/s13131-013-0396-z
In this study, 13 groups of silicon and oxygen isotopes and major elements of the basalts near the East Pacific Rise 13°N are used to study the fractionation of silicon and oxygen isotopes. Among these data, δ30Si values of basalts vary from -0.4‰ to 0.2‰ with a mean value of δ30Si of (-0.18±0.22)‰. The δ18O values range from 4.1‰ to 6.4‰ with a mean δ18O value of (+5.35±0.73)‰. Since the δ30Si values increase in the series of basalt-basaltic andesite- andesite, and δ18O values display a positive correlation with the SiO2 content, we propose that the fractionation of silicon and oxygen isotopes is influenced by the SiO2 content in igneous rocks. Compared with the igneous rocks from Manus Basin with clinopyroxene as their dominant mineral phase, MORBs in this study containing olivine and plagioclase as primary minerals have lower δ18O and δ30Si values, indicating that the fractionation of silicon and oxygen isotopes is also affected by different Si-O bridges in silicate minerals. Furthermore, our samples from the EPR are defined as E-MORB based on K/Ti ratios. Probably, the difference in δ30Si and δ18O between our samples and a normal MORB are cause by the enriched components in E-MORBs.
Distribution of large-scale detachment faults on mid-ocean ridges in relation to spreading rates
YU Zhiteng, LI Jiabiao, LIANG Yuyang, HAN Xiqiu, ZHANG Jie, ZHU Lei
2013, 32(12): 109-117. doi: 10.1007/s13131-013-0394-1
Large-scale detachment faults on mid-ocean ridges (MORs) provide a window into the deeper earth. They have megamullion on their corrugated surfaces, with exposed lower crustal and upper mantle rocks, relatively high residual Bouguer gravity anomaly and P-wave velocity, and are commonly associated with oceanic core complex. According to 30 detachment faults identified on MORs, we found that their distances to the axis mostly range from 5 to 50 km, half-spreading rates range from 6.8 to 17 mm/a, and activity time ranges from recent to 3 Ma. Most of the detachment faults are developed on the slow spreading Mid-Atlantic Ridge (MAR) and ultra-slow spreading Southwest Indian Ridge (SWIR), with the dominant half-spreading rates of 7-13 mm/a, especially 10-13 mm/a. Furthermore, they mostly occur at the inside corner of one segment end and result in an asymmetric seafloor spreading. The detachment faults on MORs are mainly controlled by the tectonism and influenced by the magmatism. Long-lived detachment faults tend to be formed where the ridge magma supply is at a moderate level, although the tectonism is a first-order controlling factor. At the slow spreading ridges, detachment faults tend to occur where local magma supply is relatively low, whilst at the ultra-slow spreading ridges, they normally occur where local magma supply is relatively high. These faults are accompanied by hydrothermal activities, with their relationships being useful in the study of hydrothermal polymetallic sulfides and their origin.
Petrophysical characteristics of rocks and sulfides from the SWIR hydrothermal field
TAO Chunhui, WU Tao, JIN Xiaobing, DOU Bingjun, LI Huaiming, ZHOU Jianping
2013, 32(12): 118-125. doi: 10.1007/s13131-013-0367-4
Study of petrophysical properties of rocks in seafloor hydrothermal fields has great significance for investigation of seafloor hydrothermal activities, especially for polymetallic sulfides prospecting. In the present study, based on the current experimental conditions, we conducted systematic experiments to measure the magnetic susceptibility, electrical resistivity, porosity, density, as well as acoustic wave velocity of seafloor rocks and sulfides. Subsequently, we measured the physical characteristics of hydrothermal sulfides, basalts and peridotites which were collected from newly discovered seafloor hydrothermal fields at 49.6°E, 50.5°E, 51°E, 63.5°E, and 63.9°E of the Southwest Indian Ridge (SWIR). Previously available and newly collected data were combined to characterize the physical differences between polymetallic sulfides and rocks. We also discussed the impact of hydrothermal alteration on the bedrock and demonstrated how these petrophysical properties of rocks can help in geophysical prospecting of seafloor hydrothermal fields as indicators.
Methods and procedures to determine the outer limits of the continental shelf beyond 200 nautical miles
WU Ziyin, LI Jiabiao, JIN Xianglong, FANG Yinxia, SHANG Jihong, LI Shoujun
2013, 32(12): 126-132. doi: 10.1007/s13131-013-0389-y
This paper establishes techniques and methods to determine a variety of boundaries associated with 200 nautical miles beyond the continental shelf. The methods, based on topography, slope and second-derivative profile integrated analysis, are now able to identify automatically the foot of the continental slope (FOS). By analyzing the sedimentary profile, the points of 1% sediment thickness are recognized. Through the intersection, cut, deletion and mergence calculation of the extrapolated data set of fixed-point series, the method succeeds in generating automatically the extrapolated boundaries, including the FOS+60 M line, the 350 M line, and the 2 500 m+100 M line. In addition, based on the automatic analysis of the topographic profile, it can be applied to determine rapidly the points of maximum water depth. Taking the northern Okinawa Trough (OT) as an example, these methods are used to calculate and examine the boundaries included in the Submission by the People's Republic of China Concerning the Outer Limits of the Continental Shelf beyond 200 Nautical miles in Part of the East China Sea (ECS); the boundaries thus derived have a solid scientific and rational basis.
Preliminary considerations on the potential influence of submarine fans on marine delimitation
QIU Wenxian, JIN Xianglong, SCHOFIELD Clive, LI Mingbi
2013, 32(12): 133-142. doi: 10.1007/s13131-013-0391-4
The complex issues associated with marine delineation for the outer limit of continental shelf and the boundary delimitation have provoked considerable attention among researchers in a variety of academic circles, particularly in the juristic filed and the geo-science field. In the present contribution, we start from an overview of submarine fans, as one of common types of sediment-deposit bodies in the ocean, and explore the related geological features which may be of relevance to the marine delimitation in accordance with the 1982 United Nations Convention on the Law of the Sea. We carry out a comparative assessment of certain significant geological features of submarine fans, using the Bengal fan as an example, which is the biggest submarine fan in the world and represents an important factor in the maritime boundary dispute between the neighboring states currently. The relationship between the special geological bodies and the international principle in the 1982 United Nations Convention can be established by combining geological and juristic analyses. This preliminary observation on the effect of submarine bodies both on the marine entitlement and boundary delimitation indicates that it is important for the international society to appropriately deal with this problem so that the marine right of any state can be protected.