Deep seismic sounding data reveal the crustal structures beneath Zoigê basin and its surrounding folded orogenic belts

Songpan-Garze massif is located at the turning position of tectonics from the nearly west-east direction to the nearly north-south direction in the northeastern margin of Tibetan Plateau,with Zoigê basin in the centre of the massif.In this paper,we build a crustal structure model of Zoigê basin and its surrounding folded orogenic belts using the deep seismic sounding data in this region.We also discuss structures and properties of the basement in Zoigê basin,tectonic relations between Zoigê upland basin and its surrounding folded orogenic belts,crustal deformation and thickening in the northeastern margin of Tibetan Plateau,and decoupling and relaxing processes in the crust.The results indicate that a special "Mesozoic basement" is formed of Triassic rocks with high density (2.65-2.75 g/cm3) and high velocity (5.6 km/s) in Zoigê basin.Songpan-Garze tectonic massif was transformed into two types of tectonic units with different crustal structures,i.e.,relatively stable Zoigê upland basin and active folded orogenic belts around the basin,in the course of the crustal material of Tibetan Plateau flowing eastward and obstructed by surrounding stable blocks.The thickening of the crust in the northeastern margin of Tibetan Plateau mainly occurred in the mid and lower crust,and the structure characterized by low velocities and multiple reflectors obviously appears in the folded orogenic belts around Zoigê basin.It implies that the mid and lower crust underwent a strong tectonic deformation in the folded orogenic areas.The thickness of the crust is about 50 km in Zoigê basin and the folded orogenic belts at the both southern and northern sides of Zoigê basin.The "Mountain root" cannot be identified.It is inferred that during the later orogenic period the eastwards flowing deep materials moved clockwise along the relatively relaxing southern side around the eastern tectonic knot under the obstructing of surrounding rigid massifs,and it resulted in the strong stretching action of the folded orogenic belts around Zoigê basin.     

ew Breakthrough in Research of Framework and Faults in Basement of Turfan Sag

The deep information of the Turfan sag was extracted and analyzed through the re-processing of the magneto-gravitational data of the Turfan sag in the Turfan-Hami basin. It is considered that the basement faults have played an important role in the controlling of the framework, lithology and the distribution of volcanic rocks in the basement of the Turfan sag. The deep crystalline basement and the upper Hercynian folded basement were studied part by part in the sag through the combined data of aeromagnetic and electric methods. It is revealed that the Huoyanshan fault is steep in the upper and lower parts but gentle in the middle, displaying a “S” type texture, and discovered that there are at least a row of local structures in the down-thrown block of the Huoyanshan fault, through the CEMP prospecting in Huoyanshan. The result is very important for the studying of the Turfan sag as a whole.     

The scientific connotation of oil and gas formations under deep fluids and organic-inorganic interaction

As a relatively stable craton block in the earth system, the petroliferous basin is influenced by the evolution of the earth system from the early development environment of source rocks, hydrocarbon formation, and reservoir dissolution to hydrocarbon accumulation or destruction. As a link between the internal and external factors of the basin, deep fluids run through the whole process of hydrocarbon formation and accumulation through organic-inorganic interaction. The nutrients carried by deep fluids promote the bloom of hydrocarbon-generating organisms and extra addition of carbon and hydrogen source, which are beneficial to the development of high-quality source rock and enhancement of the hydrocarbon generation potential. The energy carried by the deep fluid promotes the early maturation of the source rock and facilitates the hydrocarbon generation by activation and hydrogenation in high-mature hydrocarbon sources. The dissolution alteration of carbonate rocks and clastic reservoirs by CO_2-rich deep fluids improves the deep reservoir space, thus extending the oil and gas reservoir space into greater depth. The extraction of deeply retained crude oil by deep supercritical CO_2 and the displacement of CH_4 in shale have both improved the hydrocarbon fluidity in deep and tight reservoirs. Simultaneously, the energy and material carried by deep fluids(C, H, and catalytic substances) not only induce inorganic CH_4 formation by Fischer-Tropsch(F-T) synthesis and "hydrothermal petroleum" generation from organic matter by thermal activity but also cause the hydrothermal alteration of crude oil from organic sources. Therefore, from the perspective of the interaction of the earth's sphere, deep fluids not only input a significant amount of exogenous C and H into sedimentary basins but also improve the reservoir space for oil and gas, as well as their enrichment and accumulation efficiencies.     

The deep structures and oil-gas prospect evaluation in the Qiangtang Basin, Tibet

The Qiantang Basin is now one of the topics of general interest in petroleum exploration in China. This paper reports a comprehensive study of geophysical and geological survey data recently obtained in this area and, combined with INDEPTH-3 deep survey results, comes to the following conclusions： 1） The hydrocarbon source formations, reservoirs, and overlying strata and their association within the basin are quite good, local structures are developed, and, therefore, the region is favorable for forming and preserving oil and gas accumulations. Faults are not a fatal problem. The future main target strata are the middle-deep structural strata composed of Upper-Triassic and middle Jurassic rocks; 2） A new classification has been made for second-order tectonic sequences inside the basin to disavow the central Qingtang uplift. It is noted that the main structures at the surface are orientated NW-SE and the crustal structure can be described as three depressions, three risees, and one deep depression, of which the prospective zone with the most potential is the inner main subsided belt and its two sides; 3） Comparatively intensive interaction between the crust and mantle and volcanic and thermal activities in the northern basin play a very important role in petroleum evaluation. The southern deeper sedimentation and less thermal activity make this area a more perfect zone for oil exploration; 4） Currently, the most important objective is determining the physical properties of the deep strata, the status of oil and gas accumulations, the source of the hydrocarbons, and the relationship between the upper and lower structures; and 5） The Lunpola Tertiary basin may be favorable for oil accumulations because petroleum may migrate from marine strata on two sides.     

Crustal electric structure of Haiyuan arcuate tectonic region in the northeastern margin of Qinghai-Xizang Plateau, China

Through the analysis and 2-D inversion for the 5 profiles in Haiyuan arcuate tectonic region (105°～107°E,36°～37.5°N) in the northeastern margin of Qinghai-Xizang Plateau, we have obtained the electric structure within a range of 160 km in width (east-west) and 60 km in depth in the studied area. The results show that the crustal electric structure can be divided into 6 sections, corresponding respectively to Xiji basin (Ⅰ), Xihuashan-Nanhuashan uplift (Ⅱ), Xingrenbu-Haiyuan basin (Ⅲ), Zhongwei-Qingshuihe basin (Ⅳ), Zhongning-Hongsibu basin (Ⅴ) and west-margin zone of Ordos (Ⅵ) from the southwest to the northeast. The crustal electric structure is characterized by a broom-shaped pattern, which scatters to the northwest and shrinks to the southeast. The structures in the top part of Haiyuan arcuate tectonic region are complete and large, however, they diminish from the arc top to the northwest and southeast ends. In the depth from 0 km to 10 km, the resistivity is high in the sections Ⅱ and Ⅵ, but relatively low in the other four sections, showing a similar pattern of basin depression. The electrical basement in the section Ⅲ is the deepest, displaying a "dustpan" shape that is deep in the southwest and shallow in the northeast. A series of discontinuous zones with high conductivity exist in the middle-lower crust in Haiyuan arcuate tectonic region, which is possibly related to the moderate and strong earthquakes in the region. The resistivity distribution in the focal area of the 1920 Haiyuan earthquake is significantly heterogeneous with an obviously high conductivity zone near the hypocenter regime.     

Thermal-rheological structure of the lithosphere beneath Jiyang Depression: Its implications for geodynamics

Thermal regime of the lithosphere is the scenario of the lithospheric thermal evolution, and the thermo-mechanical state of lithosphere definitively controls its deformation style and mechanism. Better understanding of the lithospheric deep thermal-rheo- logical structure of sedimentary basin will shed light on the formation and evolution dynamic process of the basin. Surface tectonics is the response of the deep structure, and is controlled by the lithospheric ther-mal-rheological properties.…     

Velocity structure and active fault of Yanyuan-Mabian seismic zone―The result of high-resolution seismic refraction experiment

The authors processed the seismic refraction Pg-wave travel time data with finite difference tomography method and revealed velocity structure of the upper crust on active block boundaries and deep features of the active faults in western Sichuan Province. The following are the results of our investigation. The upper crust of Yanyuan basin and the Houlong Mountains consists of the superficial low-velocity layer and the deep uniform high-velocity layer, and between the two layers, there is a distinct, and gently west-dipping structural plane. Between model coordinates 180-240 km, P-wave velocity distribution features steeply inclined strip-like structure with strongly non-uniform high and low velocities alternately. Xichang Mesozoic basin between 240 and 300 km consists of a thick low-velocity upper layer and a high-velocity lower layer, where lateral and vertical velocity variations are very strong and the interface between the two layers fluctuates a lot. The Daliang Mountains to the east of the 300 km coordinate is a non-uniform high-velocity zone, with a superficial velocity of approximately 5 km/s. From 130 to 150 km and from 280 to 310 km, there are extremely distinct deep anomalous high-velocity bodies, which are supposed to be related with Permian magmatic activity. The Yanyuan nappe structure is composed of the superficial low-velocity nappe, the gently west-dipping detachment surface and the deep high-velocity basement, with Jinhe-Qinghe fault zone as the nappe front. Mopanshan fault is a west-dipping low-velocity zone, which extends to the top surface of the basement. Anninghe fault and Zemuhe fault are east-dipping, tabular-like, and low-velocity zones, which extend deep into the base-ment. At a great depth, Daliangshan fault separates into two segments, which are represented by drastic variation of velocity structures in a narrow strip: the west segment dips westward and the east segment dips eastward, both stretching into the basement. The east margin fault of Xichang Mesozoic basin features a strong velocity gradient zone, dipping southwestward and stretching to the top surface of the basement. The west-dipping, tabular-like, and low-velocity zone at the easternmost segment of the profile is a branch of Mabian fault, but the reliability of the supposition still needs to be confirmed by further study. Anninghe, Zemuhe and Daliangshan faults are large active faults stretching deep into the basement, which dominate strong seismic activities of the area.     

Paleoseismic ruptures and evolution of a small triangular pull-apart basin on the Zemuhe fault

Trenching is a primary technique on paleoseismology to reveal evidence of surface deformation produced by large earthquakes.A good trenching site requires completeness of geologic recording on paleoseismic events and corresponding reliable dating from radiocarbon samples.Based on three-dimension trenching,we show a structure of a small triangular pull-apart basin at the Daqingliangzi section on the Zemuhe fault,then explore interrelation between paleoseismic surface rupturing and evolution of the pull-apart basin,and give a corresponding identification model.Sedimentary boundary of the pull-apart basin is tightly bounded by two branch faults,which produced multiple paleoseismic events with deformation of some large fissures in sequence.Strata are thinner at north of the pull-apart basin,however thicker at south.These above characteristics show that evolution of the pull-apart basin is a continuous sedimentation process accompanying extensional deformation produced by multiple paleoseismic events.Small pull-apart basins are favorable sites for trenching and paleoseismic study on active strike-slip faults.     

Geometrical Study of Detachment Depth of Tilted Blocks

In an extensional basin,the process of hanging wall block rotation above a detachment fault zone is controlled by block geometry,the shape of the main normal faults,and the detachment depth.A formula for calculating the detachment depth of a tilted block is established in geometrical aspect from the tilting angle of the rotational block,dip angle of the main boundary fault,and the sedimentary thickness in the basin.The calculated result shows that the detachment depth of the tilted block,controlled by a rotational planar normal fault in the Linxian basin,on the eastern slope of Taihangshan Mountain,North China,is about 12.5 km,which is consistent with the analytic result of seismic data in the area.The value is similar to the estimated depth of the usual brittle ductile transition in a rift zone.The geometrical model presented in this paper,however,can provide a range of structural patterns of hanging wall and the detachment depth for reference in hydrocarbon exploration and earthquake processes.     

High-resolution crustal structure of the Yinchuan basin revealed by deep seismic reflection profiling: implications for deep processes of basin

The Yinchuan basin, located on the western margin of the Ordos block, has the characteristics of an active continental rift. A NW-striking deep seismic reflection profile across the center of Yinchuan basin precisely revealed the fine structure of the crust. The images showed that the crust in the Yinchuan basin was characterized by vertical stratifications along a detachment located at a two-way travel time(TWT) of 8.0 s.The most outstanding feature of this seismic profile was the almost flat Mohorovicˇic′ discontinuity(Moho) and a high-reflection zone in the lower crust. This sub-horizontal Moho conflicts with the general assumption of an uplifted Moho under sedimentary basins and continental rifts, and may indicate the action of different processes at depth during the evolution of sedimentary basins or rifts.We present a possible interpretation of these deep processes and the sub-horizontal Moho. The high-reflection zone, which consists of sheets of high-density, mantlederived materials, may have compensated for crustal thinning in the Yinchuan basin, leading to the formation of a sub-horizontal Moho. These high-density materials may have been emplaced by underplating with mantlesourced magma.     

Annual variation of sea surface height,dynamic topography and circulation in the South China Sea

TOPEX/Poseidon satellite altimetry data from 1993 to 1999 were used to study mean annual variation of sea surface height anomaly (SSHA) in the South China Sea (SCS) and to reproduce its climatological monthly surface dynamic topography in conjunction with historical hydrographic data. The characters and rules of seasonal evolution of the SCS dynamic topography and its upper circulation were then discussed. Analyses indicate that annual variation of the SCS large-scale circulation could be divided into four major phases. In winter (from November to February), the SCS circulation is mainly controlled by double cyclonic gyres with domination of the northern gyre. Other corresponding features include the Kuroshio intrusion from the Luzon Strait and the northeastward off-shelf current in the area northwest off Kalimantan Island. The double gyre structure disassembled in spring (from March to April) when the northern gyre remains cyclonic, the southern gyre becomes anticyclonic, and the general circulation pattern shows a dipole. There is no obvious large-scale closed gyre inside the SCS basin in both summer (from May to July) and autumn (from August to October) when the SCS Monsoon Jet dominates the circulation, which flows northeastward across the SCS. Even so, circulation patterns of these two phases diverse significantly. From May to July, the SCS monsoon jet flows northward near the Vietnam coast and bends eastward along the topography southeast off Hainan Island at about 18°N forming an anticyclonic turn. It then turns northeastward after crossing the SCS. From August to October, however, the monsoon Jet leaves the coast of Vietnam and enters interior of the basin at about 13°N, and the general circulation pattern becomes cyclonic. The Kuroshio intrusion was not obvious in spring, summer and autumn. It is suggested from these observations that dynamic adjustment of the SCS circulation starts right after the peak period of the prevailing monsoon.     

Numerical study of mean flow patterns in the South China Sea and the Luzon Strait

Previous investigations have suggested that wind stress curl, the balance of influx- and outflux-induced upwelling, as well as a positive vorticity source fed from the left flank of the Kuroshio are all possible mechanisms that contribute to a persistent cyclonic gyre in the South China Sea (SCS). Studies have also suggested that the loop current that forms from the Kuroshio intrusion in the Luzon Strait, similar to the Loop Current in the Gulf of Mexico (GOM), has rarely been observed in the northern SCS. In this research, an idealized numerical model driven by annual mean wind stress was adopted to investigate the relative importance of dynamic processes that control the mean flow pattern of Kuroshio in the Luzon Strait and regulate circulation in the SCS. An analysis of results drawn from numerical experiments suggests that the three mechanisms are of approximately equal importance in the formation of the persistent cyclonic gyre in the northern SCS. Unlike the Gulf Stream which enters the Gulf of Mexico through the Yucatan Channel, the two topographic ridges that align nearly meridionally in the Luzon Strait keep the Kuroshio flowing roughly northward without distinct intrusion into the SCS. Unsurprisingly, an anticyclonic loop current similar to the Gulf Stream pathway in the GOM was barely observed in the northern SCS.     

A model study of the summer Southeast Vietnam Offshore Current in the southern South China Sea

The western boundary current in the southern South China Sea (SCS) in summer does not always flow northward along the Indo-China Peninsula, it leaves the southeast coast of peninsula around 10–14°N, forming a strong eastward jet called “Vietnam Coastal Current” or “Southeast Vietnam Offshore Current” (SVOC). It is known that the wind stress curl is the major driving factor responsible for this current. In this paper, we carry on the study of the separation position, strength and forming time of this current. A connected single-layer/two-layer model is employed here to study these problems. According to the numerical experiments and analyses of the vorticity dynamics, it is found that, the local wind stress curl (including the northern cyclonic and the southern anticyclonic wind forcing curl), the nonlinear term, the topographic effect, the planetary vorticity advection and the water exchange between the SCS and Java Sea via the Sunda Shelf have an important effect on both the position where this current leaves the coast and its strength; when there is an inflow via the Sunda Shelf, the current is stronger and the separation position is more northward; whereas the water stratification, the coastline and the inflow of Kuroshio have little effect on its separation. In fact, two opposite flowing currents, the northward SVOC and the southward western branch of the cyclonic eddy to its north near the Indo-China Peninsula, collide with each other, and the strength of these two currents determine the separation position of the SVOC. Origin of the SVOC may be driven by the local negative wind stress curl in the middle SCS in mid-spring, this current flows along the coast of the Indo-China Peninsula and leaves the coast at high latitude, flowing northeastward; once the local positive wind stress curl near the northern Indo-China Peninsula or the negative one near the southern Indo-China Peninsula is large enough, this current will begin to leave the coast at low latitude.     

Evolution of an anticyclonic eddy southwest of Taiwan

Satellite images of sea-surface temperature, surface chlorophyll a concentration, and sea-level anomaly, together with ocean reanalysis data of Asia and Indian–Pacific Ocean (AIPOcean1.0), are utilized to study the three-dimensional characteristics and evolution of an anticyclonic warm eddy adjacent to the southwest coast of Taiwan during October and November 2006. Originated from the Kuroshio intrusion in the Luzon Strait, but unlike previously found westward moving anticyclonic eddies (AE) in the northeastern South China Sea, this AE was so close to the Taiwan coast and stayed where it was formed for over 1 month until it dissipated. Energy analysis is utilized to study the evolution process of the AE, and it shows that the barotropic instability (BTI) and baroclinic instability introduced by the Kuroshio intrusion flow appear to be the main energy sources for the AE. Periodical enhancement/relaxation of local northeasterly monsoon and its associated negative wind stress curl modify the current patterns in this region, reinforce the intraseasonal variability of the Kuroshio intrusion flow, and act together with Kuroshio to form the AE. Eddies detected from AIPOcean1.0 data also show that AEs are most likely to be generated southwest of Taiwan during the transition period of summer monsoon to winter monsoon, and generally, the BTI of Kuroshio intrusion contributes more than the direct wind stress work to the increase of the eddy kinetic energy for the generation and growth of the AEs.     

1997—1998年El-Nio至La-Nia期间东海黑潮的变异

基于日本“长风号”调查船在 1 997与 1 998年 1 0个航次的CTD资料 ,采用改进逆方法及改进动力计算方法对东海黑潮的流速、流量进行计算 .1 997年 5月出现了El Ni no现象 ,东海黑潮流量在 1 997年夏季减少 ,1 997年东海黑潮的平均流量也减少 .在 1 997年 1月与 6— 7月 ,即El Ni no现象出现前后 ,东海环流的流态有些不同 .在 1 998年 4至 1 1月黑潮在PN断面出现多流核心的结构 ,特别在 1 0— 1 1月出现 3个流核心 ,黑潮主流核的位置秋季时东移 .1 995年与 1 998年都是东海黑潮异常年 ,这些异常现象可能与冲绳岛以南出现的反气旋涡的强度变化以及从El Ni no现象过渡到La Ni na现象有关     

1997—1998年El-Ni?o至La-Nia期间东海黑潮的变异

基于日本“长风号”调查船在1997与1998年10个航次的CTD资料，采用改进逆方法及改进动力计算方法对东海黑潮的流速、流量进行计算. 1997年5月出现了El-Ni?o现象，东海黑潮流量在1997年夏季减少，1997年东海黑潮的平均流量也减少. 在1997年1月与6—7月，即El-Ni?o现象出现前后，东海环流的流态有些不同. 在1998年4至11月黑潮在PN断面出现多流核心的结构，特别在10—11月出现3个流核心，黑潮主流核的位置秋季时东移. 1995年与1998年都是东海黑潮异常年，这些异常现象可能与冲绳岛以南出现的反气旋涡的强度变化以及从El-Ni?o现象过渡到La-Ni?a现象有关.     

Summer circulation structure and formation mechanism in the Beibu Gulf

Due to limited in situ data and diagnostic numerical models, the summer circulation structure and formation mechanism in the Beibu Gulf have always been in controversy in the past 50 years. Therefore, a new three-dimensional hindcast model was built within the northwestern South China Sea(SCS), forced with the daily averaged wind, heat flux, lateral flux, as well as tidal harmonic and eight major rivers discharges. And the east boundary was set up far away off the Qiongzhou Strait(QS). Lastly, the model results were consistent with not only the synchronous observation data from the project 908 but also the historical observed data. As a result, the summer circulation structure was revealed that the southern Gulf was occupied by an anticyclonic eddy whereas the northern Gulf was dominated by a cyclonic gyre. Although the circulation major structure was stable, its area and strength had yearly and monthly oscillation. The other three sensitive experiments indicated that the circulations in the southern and northern Gulf were driven by the SCS circulation and monsoon wind, respectively. After the theoretical analysis of the potential vorticity budget, it was further revealed the circulation in the northern Gulf was driven by the positive wind stress curl in summer. Besides, the river discharge was also significant as the vertical circulation had two layer structures outside the mouth of the Red River. Generally, this work calls for the further research on other subjects, such as ocean biogeochemical or marine fisheries.     

赤道MJO活动对南海夏季风爆发的影响

利用1979—2013年NCEP/DOE再分析资料的大气多要素日平均资料、美国NOAA日平均向外长波辐射资料和ERSST月平均海温资料,分析赤道大气季节内振荡(简称MJO)活动对南海夏季风爆发的影响及其与热带海温信号等的协同作用.结果表明,赤道MJO活动与南海夏季风爆发密切联系,MJO的湿位相(即对流活跃位相)处于西太平洋位相时,有利于南海夏季风爆发,而MJO湿位相处于印度洋位相时,则不利于南海夏季风爆发.赤道MJO活动影响南海夏季风爆发的物理过程主要是大气对热源响应的结果,当MJO湿位相处于西太平洋位相时,一方面热带西太平洋对流加强使潜热释放增加,导致处于热源西北侧的南海—西北太平洋地区对流层低层由于Rossby响应产生气旋性环流异常,气旋性环流异常则有利于西太平洋副热带高压的东退,另一方面菲律宾附近热源促进对流层高层南亚高压在中南半岛和南海北部的建立,使南海地区高层为偏东风,从而有利于南海夏季风建立;当湿位相MJO处于印度洋位相时,热带西太平洋对流减弱转为大气冷源,情况基本相反,不利于南海夏季风建立.MJO活动、孟加拉湾气旋性环流与年际尺度海温变化协同作用,共同对南海夏季风爆发迟早产生影响,近35年南海夏季风爆发时间与海温信号不一致的年份,基本上是由于季节转换期间的MJO活动特征及孟加拉湾气旋性环流是否形成而造成,因此三者综合考虑对于提高季风爆发时间预测水平具有重要意义.     

Investigating mesoscale eddy characteristics in the Luzon Strait region using altimetry

Ocean Dynamics - The circulation in the northern South China Sea (SCS) strongly responds to anticyclonic eddies that shed from the Kuroshio intruding across Luzon Strait. An eddy tracking algorithm...     

利用多年卫星测高资料研究南海上层环流季节特征

利用10年高精度卫星测高海面高异常网格资料,联合EGM96稳态海面地形模型,构成南海海域合成海面地形的时间序列,并计算了各个时期的南海表层地转流场. 利用卫星跟踪漂流浮标观测结果与相应时期南海地转流场进行对比验证,结果显示本文结果可以很好地反映南海海域一些中小尺度的环流特征. 根据南海各季节多年平均表层环流场结构,对南海环流周年变化规律和季节特征进行了初步的探讨. 研究结果表明,南海表层环流始终处在不断演变过程之中,在时间和空间上都表现出明显的多尺度特征.