冲绳海槽弧后张裂构造及其动力机制讨论

在总结、讨论冲绳海槽弧后张裂构造的地形地貌、地球物理场和动力背景特征的基础上,归纳出海槽北、中、南三段之间的差异,以及它们在陆坡、槽底、岛坡上的不同表现特征.槽底雁行排列的地堑及断层斜切入陆坡,伴随的断块隆眷作用往南加强,使得钓鱼岛一赤尾屿隆起带区别于其北边的陆架外缘隆起带,吕宋岛向台湾的碰撞挤压引起的旋张活动加强了海槽南段的地壳拉张,从北往南岛坡侧都可以追踪出双列岛孤特性.海槽内部构造现象不能为笼统的构造动力背景所能完全解释,需要对各个典型中央地堑的具体深入探测研究.鉴于地堑、地垒、断层以及它们所界定的构造单元和它们内部发生的岩浆作用各有特点,最后讨论了各个地段中央地堑的构造属性、动力要素的差异及变化规律,以期为海槽构造动力的进一步深入研究提供借鉴参考.     

南海重力异常的沉积层密度改正及其对区域构造特征分析的意义

南海位于太平洋板块、印澳板块和欧亚板块交汇处,自晚中生代以来历经张裂作用、海底扩张以及印藏碰撞、菲律宾海板块西向运动等构造事件的叠加改造,不仅形成了复杂多样的构造格局,而且堆积了厚薄不均的沉积层.为了考察沉积层密度改正对利用重力资料分析南海不同尺度构造特征的影响,本文利用南海各区域不同深度沉积层的地震波速度及钻孔密度等数据,建立了沉积层与沉积基底密度差随深度变化的二次函数关系式,并基于该关系式,计算了南海沉积层相对基底密度低而产生的重力异常值.结果显示,南海沉积层的重力异常值在海盆区介于-40~-60 mGal,而在堆积巨厚沉积物的莺歌海盆地可达到-135 mGal;相对于空间重力异常、布格重力异常,经沉积层重力异常改正后的地壳布格重力异常更能突出深部不同尺度的密度结构和莫霍面的起伏特征,其总水平导数模更突显了南海西北部红河断裂带的海上延伸;利用谱分析技术估算岩石圈强度时,经沉积层重力异常改正的地壳布格重力异常数据获得的岩石圈有效弹性厚度值更为符合地质实际,特别是在长条形的巨厚沉积区如莺歌海盆地和马来盆地.分析表明,重力异常的沉积层密度改正对揭示南海构造特征具有重要的意义.     

A new contribution to the geology of the Egyptian Red Sea shelf using geophysical data

The study examines the Egyptian Red Sea shelf and throws more light on the structural set-up and tectonics controlling the general framework of the area and nature of the crust. Herein, an integrated study using gravity and magnetic data with the available seismic reflection lines and wells information was carried out along the offshore area. The Bouguer and reduced-to-pole aeromagnetic maps were processed and reinterpreted in terms of rifting and plate tectonics. The qualitative interpretation shows that the offshore area is characterized by positive gravity everywhere that extremely increases towards the centre of the graben, supporting the presence of an intrusive zone below the axial/main trough. The gravity data were confirmed by the presence of high magnetic amplitudes, magnetic linearity and several dipoles concentrated along the rift axis for at least 250 km. The lineament analysis indicates widespread of the Erythrean (Red Sea) trend that was offset/cut by transform faults in the NE direction (Aqaba). The tectonic model suggests the presence of one tensional (N65°E) and two compressional (N15°W, N30°W) phases of tectonism, resulted in six cycles of deformations, classified into three left lateral (N35°E, N15°E and N–S) and three right lateral (N85°W, N45°W and N60°W). The basement relief map reveals a rough basement surface that varies in depth between 1 and 5.6 km. It outlines several offshore basins, separated from each other by ridges. The models show that the basement consists of tilted fault blocks, which vary greatly in depth and composition and slopes generally to the west. They indicate that the coastal plain is underlain by acidic basement blocks (continental crust) with no igneous activity while suggesting elevated basic materials (oceanic crust) below the rift axis. The study suggests that northern Red Sea forms an early stage of seafloor spreading or at least moved past the late stage of continental rifting.     

St. John's Island (Red Sea): a new geophysical model and its implications for the emplacement of ultramafic rocks in fracture zones and at continental margins

New gravity and magnetic data from the northern Red Sea reveal the extent of the large gravity anomaly (164 mgal) and the presence of significant magnetic anomalies over St. John's Island. Spectral transformation and three-dimensional potential-field modelling delineate the surface configuration and vertical extent of the causative body and the enormous density contrast required (1.2 g/cm³) suggests that it is composed of unserpentinised peridotite (density 3.4 g/cm³) to a depth of at least 8 km.St. John's Island is uniquely located, not only at a passive continental margin but also within a fracture zone at the transition from plate separation by seafloor spreading to extension by lithospheric attenuation. This precludes several suggested mechanisms for the emplacement of ultramafic bodies in fracture zones.Thermal contraction, serpentinite diapirism and changes in the poles of rotation do not seem possible mechanisms in this tectonic environment and the emplacement is most probably related to the spreading readjustment necessary to create a continent-to-continent fracture zone. A post-Mesozoic age of emplacement, associated with the onset of continental rifting and the rejuvenation of a pre-existing continental fracture, seems most probable.     

Dynamics of continental rift propagation: the end-member modes

An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.     

Lithophile elements in Huronian low-Ti continental tholeiites from Canada, and evolution of the Precambrian mantle

Huronian basalts from central Ontario, Canada, dated at about 2450 Ma and associated with an early rifting episode, are classified as siliceous, low-TiNb tholeiites. They display strong enrichment in large-ion lithophile (LILE) and light rare earth (LREE) elements compared to modern oceanic lavas. The tectonic setting and geochemistry resemble Mesozoic rift-related low-Ti flood basalts, including the Ferrar Group of Antarctica, and the Parana and equivalent Etendeka volcanics of south Brazil and Namibia, respectively. High LILE/LREE ratios are also similar to subduction-related island arc tholeiites, and it is suggested that enrichment of the Huronian lithospheric mantle source occurred through ancient subduction of crustal material, probably during formation and consolidation of the Archean continental crust.Melting models suggest that Huronian subcontinental mantle source compositions, derived from least contaminated, aphyric, mafic end-members, had already undergone a complex evolution, including withdrawal of Archean basalts and hydrous enrichment in incompatible components. Despite several subsequent melting episodes and a second, probably magmatic, enrichment event, however, many aspects of the Huronian source signature were preserved, and appeared in later basaltic products of this mantle mass. Keweenawan volcanics, for example, dated at about 1100 Ma, preserve low P, Zr, Ti and HREE abundances.     

南极洲东部普里兹湾海域重磁场特征及地壳结构

普里兹湾位于南极洲东部大陆边缘,其深部地壳结构特征对认识白垩纪冈瓦纳古陆裂解和新生代大陆边缘形成具有重要意义.本文利用重磁、多道反射地震、声纳浮标折射地震和ODP钻井数据对普里兹湾海域的深部地壳结构进行了研究.研究结果显示,普里兹凹陷表现为典型的盆地负重力异常特征,其沉积基底较深,而在四夫人浅滩为高幅重力正异常,其沉积基底普遍抬升.在大陆架中部存在SW-NE向条带状基底的抬升,且呈朝NE向逐渐变深的趋势.在中大陆架外侧,均衡残余重力异常呈V字形负异常条带状分布,其两翼分别与四夫人浅滩和弗拉姆浅滩外的大陆坡相连.该异常带在大陆架中部向陆的偏移可能是由于古大陆架边缘的地形影响,推测其与普里兹冲积扇同属于洋陆过渡带向陆的部分,在重力模拟剖面表现为地壳向海逐渐减薄.普里兹冲积扇的地壳厚度较薄,平均为6 km,最薄处可达4.6 km,并且根据洋陆过渡带向海端的位置,推测可能属于接近洋壳厚度的过渡壳.重力异常分区的走向与兰伯特地堑在普里兹湾的构造走向基本一致,可能主要反映了二叠纪-三叠纪超级地幔柱对普里兹湾的裂谷作用的影响.该区域的自由空间重力异常和均衡残余异常均表现为超过100×10^-5m/s²的高幅正异常特征,可能由位于大陆架边缘的巨厚沉积体负载在高强度岩石圈之上的区域挠曲均衡作用所导致,可能与该区域第二期裂谷期之后的沉积间断以及快速进积加厚的演化过程有关.普里兹湾磁力异常的走向与重力异常明显不同,大致可分为东北高幅正异常区和西南低幅异常区.重磁异常在走向上的差异反映高磁异常主要来源于岩浆作用形成的铁镁质火成岩的影响,并且岩浆作用的时代不同于基底隆升的时代,而可能形成于前寒武纪或者南极洲和印度板块裂谷期间(白垩纪).     

Tectonics of South China continent and its implications

This paper aims at exploring the tectonic characteristics of the South China Continent(SCC)and extracting the universal tectonic rules from these characteristics,to help enrich the plate tectonic theory and better understand the continental dynamic system.For this purpose,here we conduct a multi-disciplinary investigation and combine it with the previous studies to reassess the tectonics and evolution of SCC and propose that the tectonic framework of the continent comprises two blocks,three types of tectonic units,four deformation systems,and four evolutionary stages with distinctive mechanism and tectonic characteristics since the Neoproterozoic.The four evolutionary stages are:(1)The amalgamation and break-up of the Neoproterozoic plates,typically the intracontinental rifting.(2)The early Paleozoic and Mesozoic intracontinental orogeny confined by plate tectonics,forming two composite tectonic domains.(3)The parallel operation of the Yangtze cratonization and intracontinental orogeny,and multi-phase reactivation of the Yangtze craton.(4)The association and differentiation evolution of plate tectonics and intracontinental tectonics,and the dynamic characteristics under the Meso-Cenozoic modern global plate tectonic regime.     

Convection pattern and stress system under the African plate

Studies on tectonic forces from satellite-derived gravity data have revealed a subcrustal stress system which provides a unifying mechanism for uplift, depression, rifting, plate motion and ore formation in Africa. The subcrustal stresses are due to mantle convection. Seismicity, volcanicity and kimberlite magmatism in Africa and the development of the African tectonic and magnetic features are explained in terms of this single stress system. The tensional stress fields in the crust exerted by the upwelling mantle flows are shown to be regions of structural kinship characterized by major concentration of mineral deposits. It is probable that the space techniques are capable of detecting and determining the tectonic forces in the crust of Africa.     

东秦岭造山带莫霍面展布与碰撞造山带深部过程的关系

根据P波走时反演重建的三维速度图像,研究东秦岭造山带莫霍面的展布性态结果表明,在华北板块南缘潼关-登封-阜阳-线、商丹主缝合带北侧卢氏-奕川-方城-信阳-线和扬子板块北缘佛坪-陨西-武当山-枣阳-线莫霍面沿着造山带走向呈带状隆起而介于这三条带间,莫霍面均不同程度地下陷因此,东秦岭造山带在岩石圈缩短方向上莫霍面的展布目前仍然存在着很大的非均一性结合造山带地质、岩石地球化学和同位素年代学综合分析,认为造成莫霍面这一展布格局,主要与该碰撞造山带在不同演化时期各岩石构造单元中发生不同性态的岩石圈-软流圈和壳-幔间物质与能量的相互作用方式不一所造成的大陆动力学过程不同有关加上碰撞期后造山带深部岩石圈均衡在不同岩石构造单元中的差异,形成了东秦岭造山带目前莫霍面的展布。     

长乐—南澳断裂带两侧地壳结构差异的地震-重力联合反演

长乐一南澳断裂带出露于福建沿海地区,由于海陆过渡带的特殊性,地球物理探测受到许多限制,难以获得由陆到海的清晰而准确的深部构造形态.2014年福建省地震局采集了横跨长乐一南澳断裂带的广角反射/折射剖面(HX-6),由于观测系统的缺陷和原始资料信噪比等问题,单纯使用地震数据反演长乐一南澳断裂带的深部地壳结构有很强的不确定性,无法解答断裂带两侧地壳结构存在何种差异,影响了对断裂带构造属性和区域构造演化的正确认识.基于岩石波速和密度有良好的对应关系,地震-重力联合反演可以有效降低多解性.本文采用地震走时拟合和重力异常拟合同步进行的方法,利用最新采集的高质量P波地震走时数据与高精度实测重力数据,反演得到了连城一厦门一金门外海剖面的二维地壳波速-密度结构模型.联合反演结果显示:长乐一南澳断裂带两侧地壳厚度差约3 km,壳内分层结构和上地幔顶部波速-密度无显著变化,推断长乐一南澳断裂带是华南地块正常陆壳和台湾海峡减薄陆壳的分界,本研究结果为进一步研究该区深部构造环境和长乐一南澳断裂带的地球动力学意义提供了新的地球物理学证据.     

南海大陆边缘动力学:科学实验与研究进展

国家重点基础研究发展计划(973)项目(2007CB411700)首次在南海南部大陆边缘及西南次海盆开展长排列大震源多道地震、海底地震仪(OBS)折射/反射地震等的综合地球物理探测,结合地质构造、地球化学、动力模拟等的综合研究,形成如下重要认识:南海海盆新生代发生了早、晚两期海底扩张.早期扩张发生于33.5～25 Ma...     

Change in tectonic force inferred from basin subsidence: Implications for the dynamical aspects of back-arc rifting in the western Mediterranean

A method has been developed that allows temporal changes in tectonic force during rift basin formation to be inferred from observed tectonic subsidence curves and has been applied to the Gulf of Lions (the Provençal Basin) and the Valencia Trough in order to gain some understanding of the dynamical aspects of back-arc basin rifting in the western Mediterranean Sea. Two distinct tectonic force regimes active at different times during the evolution of each of these back-arc basins are identified. The first, which can be seen in both basins, is characterized by tensional forces that gradually abate with time to vanish some ～ 20 my after the onset of rifting. The magnitude of tectonic force required to initiate the rifting process is significantly greater in the Valencia Trough than in the Provençal Basin. Subsequently, the dynamic development of these back-arc basins differs. In the Provençal Basin, there is a renewal of force, with extensional deformation concentrated in the central part of the rift whereas, in the Valencia Trough, the second tectonic force regime is inferred to be one that causes compression that subsequently relaxes. Such temporal patterns of tectonic force are interpreted to be related to the causative driving processes, allowing constraints to be placed on the transient interaction between the overriding and subducting plates in a back-arc setting. The models also allow inferences to be made about the rheological structure of the lithosphere. A significant variation of initial crustal thickness is inferred for the Provençal Basin but not for the Valencia Trough. In both basins, a wet rheology is required in order to initiate rifting given currently accepted bounds on tectonic force magnitudes; adoption of a dry rheology leads to insufficiently high strain rates for significant lithosphere extension in both cases.     

南海北部琼东南盆地地层结构与地壳伸展特征

琼东南盆地发育于前新生代基底之上,作为南海被动大陆边缘一部分,记录了南海北部裂陷盆地结构及其演化.利用最新钻井、反射地震、重力等资料,分析新生代盖层和前新生代基底地壳结构,建立盆地地层结构模型,然后计算全盆地地壳伸展变化特征.结果表明:新生代地层序列的盆地充填由西向东逐渐减薄,古近纪、新近纪以及第四纪期间(45 Ma~现今)最后沉积中心呈现逐渐向西或西南迁移趋势.下地壳局部表现为地震速度偏高(厚度2~4 km,v_P>7.0 km·s^-1,水平延伸范围约为40~70 km).重震联合模拟显示这里存在密度偏高特征,推测存在可能与张裂晚期和扩张早期岩浆物质底侵或混合到伸展程度较低的大陆地壳有关.计算获得的前新生代基底地壳厚度由在弱展区域陆架区约25 km,在减薄最大区域中央坳陷为3 km.伸展系数(β)最高值大于6.0出现在中央坳陷,低值小于2.0在坳陷南北两侧,说明地壳在盆地中央拉伸比较剧烈.     

Accordant Summit Levels (ASLs) and Their Tectonic Deformation in the Denudational Areas of the North China Rift Zone

The North China rift zone,including the North China Plain rift zone and the middle Shanxi graben-rift zone,is a continental rift;geomorphologically,its most common and important characteristics are the multiple accordant summit levels (ASLs) in the denudational area.In this study three methods based on geomorphic analysis,that is,(1) unified topographic profiles,(2) cyclic knickpoints along longitudinal river profiles,and (3) depth contours of river downcutting,are used to identify the ASLs and analyze their tectonic deformation.The formation time of the ASLs is determined by geological and geomorphological evidence,the correlation with the sedimentary facies in the Plain rift zone,and K-Ar ages of basalts on the ASLs,indicating the rates of tectonic uplift.The North China rift zone has five ASLs,of which the highest and oldest ASL I is the remnant of a fossil peneplain formed before rifting and the other four lower and younger ones are pediplain-typed formed contemporaneously with rifting.Study of ASLs     

Amount of clockwise rotation of Southwest Japan — fan shape opening of the southwestern part of the Japan Sea

Paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 Ma and 112 Ma from Yamaguchi and Go¯river areas in the central part of Southwest Japan. The new data, together with those of younger igneous rocks published previously, define the change of paleomagnetic field direction during the late Mesozoic/ Cenozoic period for Southwest Japan. The paleomagnetic direction from this area has pointed 56 ± 3° clockwise from the expected field direction estimated from APWP (apparent polar wandering path) of the whole of Eurasia during the period between 100 and 20 Ma. In comparison with the expected one from the eastern margin of Eurasia (Korea, China, Siberia), the Cretaceous field direction of Southwest Japan shows the clockwise deflection by 44–49°. These results establish that while the eastern margin of Eurasia, including Southwest Japan, was rotated more or less with respect to the main part of Eurasia during last 100 Ma, Southwest Japan was rotated clockwise through more than 40° with respect to the eastern margin of Eurasia since 20 Ma. The large amount of rotation for Southwest Japan implies that it is rotated by an opening of the southwestern part of the Japan Sea, which widens northeastward (fan shape opening). The tectonic feature of Southwest Japan and the Japan Sea is analogous to that of Corso-Sardinia and the Ligurian Sea in the Mediterranean, indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim.     

The isostatic gravity anomaly: key to the evolution of the ocean-continent boundary at passive continental margins

Isostatic gravity highs bordering the passive continental margins are interpreted as resulting from oceanic basement highs. These basement elevations are relics of the transient phenomenon of a higher ridge axis elevation during early rifting. The steep landward gradient in the isostatic gravity field, generally associated with a magnetic edge effect anomaly, delineates the boundary between oceanic and continental basement.     

Frequency characteristics and far-field effect of gravity perturbation before earthquake

We used high-pass filtering and the Fourier transform to analyze tidal gravity data prior to five earthquakes from four superconducting gravity stations around the world. A stable gravitational perturbation signal is received within a few days before the earthquakes. The gravitational perturbation signal before the Wenchuan earthquake on May 12, 2008 has main frequency of 0.1–0.3 Hz, and the other four have frequency bands of 0.12?0.17 Hz and 0.06?0.085 Hz. For earthquakes in continental and oceanic plate fault zones, gravity anomalies often appear on the superconducting gravimeters away from the epicenter, whereas the stations near the epicenter record small or no anomalies. The results suggest that this kind of gravitational perturbation signals correlate with earthquake occurrence, making them potentially useful earthquake predictors. The far-field effect of the gravitational perturbation signals may reveal the interaction mechanisms of the Earth’s tectonic plates. However, owing to the uneven distribution of gravity tide stations, the results need to be further confirmed in the future.     

3-D Simulation of Steady Plate Subduction with Tectonic Erosion: Current Crustal Uplift and Free-Air Gravity Anomaly in Northeast Japan

Free-air gravity anomaly in plate subduction zones, characterized by island-arc high, trench low and outer-rise gentle high, reflects the cumulative effects of long-term crustal uplift and subsidence. In northeast Japan the island-arc high of observed free-air gravity anomaly takes its maximum about the eastern coastline. On the other hand, the current vertical crustal motion estimated from geological and geomorphological observations shows a gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Such a discrepancy in spatial patterns between the free-air gravity anomaly and current vertical crustal motion can be ascribed to a change in the mode of crustal uplift and subsidence associated with the initiation of tectonic erosion at the North American-Pacific plate interface. We developed a realistic 3-D simulation model of steady plate subduction with tectonic erosion in northeast Japan on the basis of elastic/viscoelastic dislocation theory. Through numerical simulations with this model we found that simple steady plate subduction brings about the crustal uplift characterized by island-arc high with its maximum about the eastern coastline, while steady plate subduction with tectonic erosion, which is represented by the landward retreat of the plate interface, brings about gentle uplift in the land area and steep subsidence in the sea area with the neutral point near the eastern coastline. Therefore, if we suppose that tectonic erosion started 3–4 million years ago after the long duration of simple steady plate subduction, we can consistently explain both patterns of free-air gravity anomaly and current crustal uplift in northeast Japan.     

The fissure swarm of the Askja volcanic system along the divergent plate boundary of N Iceland

Divergent plate boundaries, such as the one crossing Iceland, are characterized by a high density of subparallel volcanic fissures and tectonic fractures, collectively termed rift zones, or fissure swarms when extending from a specific volcano. Volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events, when magma intrudes fractures to form dikes and even feeds fissure eruptions. We mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern Iceland. The study area is ~1,800 km², located within and north of the Askja central volcano. The style of fractures changes with distance from Askja. Close to Askja the swarm is dominated by eruptive fissures. The proportion of tectonic fractures gets larger with distance from Askja. This may indicate that magma pressure is generally higher in dikes close to Askja than farther away from it. Volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3–4 identified calderas in Askja. The average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity. As this deviation decreases gradually northward, we suggest that the effect of the triple junction of the North American, Eurasian and the Hreppar microplate is a likely cause for this deviation. Shallow, tectonic earthquakes in the vicinity of Askja are often located in a relatively unfractured area between the fissure swarms of Askja and Kverkfjöll. These earthquakes are associated with strike-slip faulting according to fault plane solutions. We suggest that the latest magma intrusions into either the Askja or the Kverkfjöll fissure swarms rotated the maximum stress axis from being vertical to horizontal, causing the formation of strike-slip faults instead of the dilatational fractures related to the fissure swarms. The activity in different parts of the Askja fissure swarm is uneven in time and switches between subswarms, as shown by a fissure swarm that is exposed in an early Holocene lava NW of Herðubreið but disappears under a younger (3500–4500 BP) lava flow. We suggest that the location of inflation centres in Askja central volcano controls into which part of the Askja fissure swarm a dike propagates. The size and amount of fractures in the Kollóttadyngja lava shield decrease with increasing elevation. We suggest that this occurred as the depth to the propagating dike(s) was greater under central Kollóttadyngja than under its flanks, due to topography.