南海北部区域构造和陆壳向洋壳的转化

1.海底扩张和陆壳大洋化,均能形成洋壳。陆壳转化为过渡壳是大洋化的必经阶段。 2.航磁测区内的中央海盆,具有类似于大洋中海底扩张形成的对称磁异常条带。 3.在西沙北裂谷、莺歌海裂谷型拗陷,以及属于断陷盆地性质的珠一、珠二拗陷等地,有沿着断裂上升的类似于洋壳成分的地幔物质喷溢或在地壳上层侵位,那里的地壳都有不同程度的减薄,属于过渡型地壳。西沙北裂谷的“莫霍面”,比相邻的南北两侧高出约10公里。 4.在南海北部发生多中心微型扩张和大洋化。     

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.     

Crustal structure in the Kamchatkan segment of the pacific transition zone

Based on deep seismic sounding data, a velocity model of the Earth's crust has been developed for the Kamchatkan segment of the Pacific transition zone. The velocity difference in the structure of the continental and oceanic blocks of the Earth's crust is shown. It has been revealed that these crustal blocks join each other along the deeply inclined fault zone. This zone is located 120 km northwest of the axis of the deep-sea trench. It separates the high-velocity block of eastern peninsulas and bays of Kamchatka from the low-velocity block of the Shatsky Ridge. The latter may be considered to be a contact zone between the continental and oceanic crust.The crust of the regions of Recent volcanic activity in Kamchatka has a number of specific features, such as: a complex heterogeneous structure of the basement; relatively high velocity values in the crust and low values in the upper mantle; anomalous behaviour and velocity inversions related to the complex alternation of sedimentary-volcanogenous and intrusive rocks and zones of hydrothermal alterations; and the possible presence of magma chambers of different types within the crust and the crust-mantle transition zone.     

Segmentation of the southern Mariana back-arc spreading center

SeaBeam multibeam bathymetry obtained during cruise SO-69 of research vessel (R/V) Sonne defines the segmentation and structure of ∼ 300 km of the Mariana back-arc spreading center south of the Pagan fracture zone at 17°33'N. Eight ridge segments, ranging from 14 to 64 km in length, are displaced as much as 2.7–14.5 km by both right- (predominantly) and left-lateral offsets and transform faults. An axial ridge commonly occupies the middle portion of the rift valley and rises from 200 to 700 m above the adjacent sea floor, in places shoaling to a water depth of 3200 m. An exception is the 60-km-long segment between 16°58' and 17°33'N where single peaks only a few tens of meters high punctuate the rift axis. Photographic evidence and rock samples reveal the presence of mostly pillow lavas outcropping on the axial ridges or peaks whereas the deeper parts of the rift valley floor (max. depth 4900 m) are heavily to totally sedimented. Abundant talus ramps along fault scarps testify to ongoing disruption of the crust. Lozenge-shaped collapse structures are covered by layers of sediment up to tens of centimeters thick on the rift valley floor. The presence of discrete volcanic ridges in the southern Mariana back-arc spreading region suggests that emplacement of oceanic crust at this slow spreading center occurs by `multi-site' injection of magma. Along-axis variations in length, crestal depth, and size of the axial ridges can be best explained by different stages in the cyclicity of magma supply along-axis.     

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.     

Magnetic Evidence for Volcanism at Eastern Continental Margin of India: Juxtaposition with Elan Bank (Southern Indian Ocean)

The rifted Eastern Continental Margin of India (ECMI) has evolved as a result of breakup of East Gondwanaland. Previous geophysical studies of the continental margin have not elucidated upon its volcanic nature. Magnetics plays a useful role in the study of continental margins, particularly in identifying the volcanic units. The aeromagnetic map of the offshore Mahanadi basin of ECMI displays a conspicuous linear anomaly along the continental shelf. A comprehensive study of the published aeromagnetic, marine magnetic and gravity data of the offshore Mahanadi basin reveals the existence of a seaward dipping volcanic unit in the offshore Mahanadi basin bordering the Hinge zone. This inference suggests that the ECMI is a volcanic rifted margin. The study further indicates the deepening of the basement towards the sea. In addition, the existing geological studies on the ECMI demarcated the probable limit of the continental crust by studying the basement detached tectonic style of the sedimentation in sub-surface configuration of the East coast basins of India. The probable continental crustal limit, the Hinge zone, and the inner edge of the presently inferred volcanic unit conform to one another spatially in the offshore Mahanadi region. These features characterize the inferred volcanic body as seaward dipping reflectors (SDRs) that usually occur at the rifted continental margins. The deepening of the basement towards the sea and the presence of the volcanic body on the continental margin are indicative of the transitional nature of the crust. It is generally accepted that Antarctica and India were juxtaposed before the breakup of Gondwanaland. But the microcontinents in the southern Indian Ocean are neglected in the reconstruction of Gondwanaland continents. The recent studies of the discovery of continental crust within the Elan Bank (EB) microcontinent show that the EB was contiguous with the East coast of India before the breakup of Gondwanaland. Moreover, it is reported that the upper igneous crust of the EB consists of a 2–3 km thick layer of accumulated lava flows originating from the Kerguelen hotspot. An estimate shows that the total volume of volcanic and plutonic component of the Elan Bank is about 0.3 million cubic kilometers. The present inference of a volcanic body from the offshore Mahanadi basin is in agreement with the above observations of the juxtaposition of EB with ECMI.     

Structure and evolution of passive margins: The plume model again

Anomalous plateaus and structures of passive Atlantic margins are often characterized by seaward-dipping reflectors at their seaward flank and by anomalous high-velocity bodies at the base of the crust. Some deep reflection measurements show a termination or a thinning of continental lower-crustal reflective layers when approaching the continental slope. These and other observations are compatible with the model of a rising plume in a surrounding of at least two low-viscosity layers, one in the continental lower crust and one in the asthenosphere. The peak of magmatic-volcanic activity coincides with the very beginning of spreading. As a consequence of laterally intruding magmas, a certain oceanization of the lower crust below the continental slope, the adjacent shelf and especially under anomalous plateaus takes place.     

The role of rifting in the evolution of the continental margin of Eastern Asia: Geophysical evidence

The role of rift processes is analysed in the structural evolution of the continental margins of Eastern Asia including the Indo-China Peninsula and North China plain. Paleoreconstructions were made for the Indo-China Peninsula to characterize individual stages of rifting covering the Late Cretaceous-Eocene, Oligocene-Middle Miocene and Late Pliocene-Early Quaternary epochs. The rifting of continental margins occurred synchronously with spreading processes in marginal seas, whereas the formation of rift structures in the North China plain was concurrent with the formation of a deep-water basin of the Philippine Sea. The development of asthenospheric diapire led to crustal extension and was responsible for the formation of rift structures in marginal seas and continental margins.     

Age and provenance of the target materials for tektites and possible impactites as inferred from Sm-Nd and Rb-Sr systematics

Sm-Nd and Rb-Sr analyses of tektites and other impactites can be used to place constraints on the age and provenance of target materials which were impact melted to form these objects. Tektites have large negative ε^Nd(0) values and are uniform within each tektite group while the ε^Sr(0) are large positive values and show considerable variation within each group. Chemical, trace element, and isotopic compositions of tektites are consistent with production by melting of sediments derived from old terrestrial continental crust. Each tektite group is characterized by a uniform Nd model age,T_CHUR^Nd, interpreted as the time of formation of the crustal segment which weathered to form the parent sediment for the tektites: (1) ～1.15 AE for Australasian tektites; (2) ～1.91 AE for Ivory Coast tektites; (3) ～0.9 AE for moldavites; (4) ～0.65 AE for North American tektites, and (5) ～0.9 AE for high-Si irghizites. Sr model ages,T_UR^Sr, are variable within each group reflecting Rb-Sr fractionation and in the favorable limit of very high Rb/Sr ratios, approach the time of sedimentation of the parent material which melted to form the tektites. Australasian tektites are derived from ～0.25 AE sediments, moldavites from ～0.0 AE sediments, Ivory Coast tektites from ～0.95 AE sediments. Possible parent sediments of other tektite groups have poorly constrained ages. Our data on moldavites and Ivory Coast tektites are consistent with derivation from the Ries and Bosumtwi craters, respectively. Irghizites are isotopically distinct from Australasian tektites and are probably not related. Sanidine spherules from a Cretaceous-Tertiary boundary clay have initial ε^Nd ～ +2; ε^Sr ～ +5 and are not derived from old continental crust or meteoritic feldspar. They may represent a mixture of basaltic oceanic crust and sediments, implying an oceanic impact. These isotopic results are also consistent with a volcanic origin for the spherules.     

Asymmetric rifting of the northern Mariana Trough

The evolution of rifting in the northern Mariana Trough was studied, based on single-channel seismic reflection profiles and heat flow. The rift showed structural asymmetry. The northernmost part of the Mariana Trough at 24°N, just south of Minami-Iwojima Island, is now in an incipient rifting stage and shows a half-graben structure. The arc crust just behind the volcanic front is cut by a few major east-dipping normal faults. The major faults extend southward behind the Hiyoshi seamounts around 23°30'N. The rift develops to a full-graben stage at ∼ 23°N, where the width of the trough increases to 80 km. The trough is comprised of several faulted and tilted blocks of island-arc crust. Maximum subsidence occurs along a row of small grabens on the eastern margin of the trough. These grabens are separated by arc volcanoes, and their depths increase southward from 2500 m at 23°20'N to 4500 m at 22°N. The strike of each graben is north-northwest–south-southeast, which is close to the trend of the remnant West Mariana Ridge, but oblique to the active Mariana arc. Crustal extension becomes concentrated along the eastern margin of the trough as rifting progresses. The transition from rifting to sea floor spreading may occur at ∼ 22°N, where the width of the trough is ∼ 120 km. The possible spreading center lies along the southern extension of the grabens on the eastern margin. The period of back-arc rifting before spreading begins is estimated to be less than 3 million years. Heat flow is asymmetric in the rift. High heat flow was observed only in or close to the row of grabens along the eastern margin of the trough. The asymmetric pure shear extension model fits the observed heat flow distribution better than the simple shear extension model.     

青藏高原P波速度层析成像与岩石圈结构

利用中国西部地震台网的数据,通过体波层析成像反演了青藏高原及邻域的三维P波速度结构.根据地壳和上地幔的速度变化和构造特征,重点讨论了下地壳流动、地幔上涌、岩石圈减薄以及与藏北新生代火山岩和藏南裂谷系的关系等问题.分析表明,青藏高原中、下地壳平均速度偏低,低速区主要分布在拉萨和羌塘块体内部,随着深度的增加逐渐扩大到松潘—甘孜块体.上述低速区之间多被高速带分隔,暗示地壳中、下部的韧性变形被限制在特定的区域,不太适于产生贯穿整个青藏高原的大规模横向流动.此外,地幔上涌也并非普遍发生于整个青藏高原,而是集中在羌塘、松潘—甘孜以及喜马拉雅东构造结附近,导致上述区域的岩石圈地幔较薄,并且伴生火山活动和岩浆作用.此外,由于印度大陆岩石圈在向北俯冲,板片下沉过程中引起地幔上涌,热流物质有可能上升进入地壳,这一作用对藏北新生代火山岩和藏南裂谷系的形成以及中、下地壳的韧性变形产生了明显的影响.     

山西裂谷带地壳岩石波速的研究

通过实验建立了岩石成分与波速的关系,波速和岩石的各向异性,矿物相变在波速曲线中的反映以及实验温度和压力对波速的影响,尤其得到了高温下的新资料.用实验数据结合地球物理、地质资料建立了山西裂谷带与相邻太行山隆起区地壳各层圈的物质组成,对比说明,两构造单元下地壳的岩石组成是不同的.最后分析了盆地下地壳中部的组成、水和环境条件,认为低速带是由岩石部分熔融造成的.     

Mean crustal velocity: a critical parameter for interpreting crustal structure and crustal growth

Mean crustal velocity is a critical parameter for genesis of continental crystalline crust because it is a function of mean crustal composition and therefore may be used to resolve continental crustal growth in space and time. Although the best values of mean crustal velocity are determined from wide-angle reflection measurements, most studied here necessarily come from vertical averages in crustal refraction determinations. The mode of 158 values of mean crustal velocity is 6.3 km/s, a velocity which corresponds to a mean crustal composition of granodiorite to felsic quartz diorite; Archean crust may be slightly more mafic. Mean crustal velocities range from 5.8 to 7.0 km/s. The lowest values invariably are found in thermally disturbed rift zones and the highest values correspond to velocities in gabbro. Velocities in island arcs may be as low as 6.0 km/s but are typically 6.5–6.9 km/s which corresponds to andesitic composition; estimates of island arc composition are andesitic. If values of mean crustal velocity are not biased, this observation suggests that continental crust did not grow simply by addition of island arc material. Possibilities are that crust formed from fusion of island arcs and was later changed to more felsic composition by addition of material from the mantle or that the late Archean episode of major crustal growth did not involve processes similar to younger island arcs. Some crustal blocks might be changed in composition and thickness by such processes as underplating, interthrusting, necking and sub-crustal erosion. Specially designed experiments are suggested to determine this parameter so critical for understanding genesis of continental crust.     

Non-axial breaching of a rift valley: Evidence from the Lord Howe Rise and the southeastern Australian margin

Present models of continental breakup envisage the formation of a rift valley which undergoes a protracted period of tectonism and eventual seafloor spreading in the axial part of the rift valley. This results in evidence of pre-breakup tectonism on most Atlantic-type margins in the form of normal blockfaults beneath the continental slope. The southeastern margin of the Australian continent has an unusually steep continental slope and shows little evidence of tectonism associated with the rift valley stage of development. The margin was formed by separation of the Lord Howe Rise and Australia during a phase of seafloor spreading in the Tasman Sea which lasted from about 80 to 60 m.y. B.P. Marine geophysical data over the central Lord Howe Rise indicate a contrast between the western and eastern part of of this structure. The western part shows faulted, rough basement topography, disturbed overlying sediments, and a relatively quiet magnetic field. The eastern part shows a smooth basement surface, undisturbed overlying sediments, and a high-amplitude, high-frequency magnetic field. It is suggested that the whole of the pre-breakup rift valley remained attached to the Lord Howe Rise. This explains the absence of rift valley structures within the eastern continental margin of Australia and implies non-axial breaching along the western boundary fault of a pre-Tasman Sea rift valley.     

山西地堑形成的力学模式及山西地震带的特点

山西地堑系又名汾渭地堑系,是新生代发育的小型大陆裂谷带,它有着大陆裂谷带的主要特征.本文根据裂谷和地堑是断裂带发展和控制结果的观测事实,应用断裂力学方法研究了山西地堑形成的力学机制.本文提出一个三维力学模式,分析了由断裂带扩展形成地堑的力学过程,并计算了山西地堑的 Z 形图案中的剪切段与拉伸段之间的夹角,该角度的大小与区域应力场方向、原始断裂带深度和长度的比值,以及断裂带周围介质的力学性质等因素有关,同时还发现,拉张区的总体方向总是指向区域主压应力作用方向.因此,可以根据拉张区的总体方向来确定区域主压应力方向;反之,也可以根据区域主压应力方向来判断拉张段的取向.本文同时分析了由地震资料得到的结果并进行了比较.此外,我们还应用本文提出的方法研究了国际上一些有名的地堑,如莱茵地堑、贝加尔湖地堑和北美洲西部的利奥格兰特裂谷等,这些地堑系的拉张区和剪切区的空间分布特征,也能得到较好地解释.      

Asymmetric back-arc spreading, heat flux and structure associated with the Central Volcanic Region of New Zealand

The Central Volcanic Region of New Zealand is an active back-arc basin developed within continental lithosphere, and therefore offers a rare opportunity to study back-arc extension from land-based observations. Two parameters related to the heat output from the Central Volcanic Region are of particular interest. Firstly, the average heat flow for the eastern half of the Central Volcanic Region is about 800 mW/m²—in order to maintain this heat flow over geological time periods an efficient mass-transfer of heat is required. Secondly, the observed asymmetry in the pattern of heat output, coupled with the tectonic erosion of blocks of continental crust from the eastern axial ranges into the Central Volcanic Region, suggests that the process currently in progress at the eastern margin of the Region is asymmetric spreading with concomitant thermal differentiation of continental crust into its silicic and basic components.     

俄罗斯贝加尔湖-日本仙台断面地震波速结构及其地质意义

本文以中俄、俄日学者合作所得到的地球物理资料为主,结合其它相关地质-地球物理数据,组构了俄罗斯贝加尔湖-日本仙台(BS)4000 km长断面,用于区域性大尺度地研究东北亚洲地壳结构和一系列地质构造问题.研究BS断面地震波速结果表明:(1)西伯利亚板块和黑龙江板块地壳结构变化较大,并可分为上、中、下部地壳,欧亚板块东部陆缘带地壳结构较简单,基本两分.贝加尔裂谷带下部地壳厚度比松辽盆地的薄约7 km,而上部地壳则相反,前者的比后者的厚约9 km.两个裂谷带在Moho界面之下的波速分布差异也较大.(2)结合前人认识,综合分析认为,贝加尔裂谷带属主动式裂谷,松辽盆地属于混合型裂谷.贝加尔裂谷形成动力主要来自地球构造圈B″层物质上涌所形成的地幔热柱的垂向作用,由BLV带佐证,松辽盆地形成动力主要来自太平洋板块斜向俯冲的中远程效应.(3)日本国所位于的西太平洋岛弧带是多地震带,除了太平洋板块俯冲产生的浅部效应、地壳中断裂与流体的直接作用等因素,本文指出仙台等速块的物性条件是岛弧带的主要不稳定因素.同时指出需要关注日本东海岸深约30~40 km的大级次地震的发生.     

Swarms in Andaman Sea,India — a seismotectonic analysis

The seismotectonic characteristics of 1983–1984, 1993 and 2005 swarms in Andaman Sea are analysed. These swarms are characterised by their typical pulsating nature, oval shaped geometry and higher b values. The migration path of the swarms from north to south along the Andaman Spreading Ridge is documented. While the first two swarms are located along existing mapped rift segments, the 2005 swarm appears to have generated a new rift basin along 8°N. The analysis and supporting evidences suggest that these swarms were generated by intruding magmatic dyke along the weak zones in the crust, followed by rifting, spreading and collapse of rift walls. CMT solutions for 2005 swarm activity indicate that intrusion of magmatic dyke in the crustal weak zone is documented by earthquakes showing strike slip solution. Subsequent events with normal fault mechanism corroborate the rift formation, collapse and its spreading.     

Propagating rift during the opening of a small oceanic basin: The Protector Basin (Scotia Arc,Antarctica)

The opening of oceanic basins constitutes one of the key features of Plate Tectonics because it determines the rifting and displacement of the continental crustal blocks. Although the mechanisms of development of large oceans are well known, the opening and evolution of small and middle size oceanic basins have not been studied in detail. The Protector Basin, located in the southern Scotia Sea, is a good example of a small oceanic basin developed between two thinned continental blocks, the Pirie Bank and the Terror Rise, poorly studied up to now. A new set of multibeam bathymetry, multichannel seismic reflection, and gravity and magnetic anomaly profiles obtained on the SCAN 2001 cruise led us to determine that the Protector Basin probably opened during the period comprised between C5Dn (17.4 Ma) and C5ACn–C5ABr chrons (13.8 Ma), forming a N–S oriented spreading axis. The end of spreading is slightly younger to the north. The start of spreading is clearly diachronous, with the most complete set of chrons up to C5Dn in the southern profile, C5Cn in the middle section and only up to C5ADn in the northern part of the basin. The spreading axis propagated northwards during the basin development, producing the wedge shape of the basin. In addition, at the NE part of the basin, a reverse fault developed in the border of the Pirie Bank after basin opening accentuates the sharp northern end. Moreover, the northwestern part of the Pirie Bank margin is an extremely stretched continental crust with N–S elongated magnetic anomalies related to incipient oceanic southward propagating spreading axes. The Protector Basin shows the oldest evidence of E–W continental stretching and subsequent oceanic spreading during Middle Miocene, related with the eastward development of the Scotia Arc that continues up to Present. The relative rotation of continental blocks during the development of small sized oceanic basins by continental block drifting favoured the opening of wedge shape basins like the Protector Basin and conjugate propagating rifts.     

Comparison of rhyolites from continental rift,continental arc and oceanic island arc: Implication for the mechanism of silicic magma generation

We discuss the chemical compositions of rhyolites from three distinct tectonic settings: (i) the continental rift from Ethiopia (both Oligocene–Miocene and Quaternary rhyolites); (ii) the early Miocene continental arc of Japan (the Mt Wasso rhyolites related to the rifting of the Japan Sea); and (iii) the oceanic Izu–Bonin Island Arc. The comparison reveals that the oceanic island arc rhyolites have high contents of CaO, Al₂O₃, and Sr, and extremely low abundance of trace elements including K₂O. In contrast, the Ethiopian continental rift rhyolites are characterized by low contents of CaO, Al₂O₃, and Sr, and high contents of K₂O, and are enriched in the whole range of trace elements. The continental arc Mt Wasso rhyolites are apparently low in Nb content, although they display similar chemical trends to those of the Ethiopian rhyolites. This obvious difference in the chemical signatures of the rhyolites from the three tectonic settings is the consequence of their derivation from different sources. The implication of this result is that fractional crystallization processes were dominant in the rift‐related rhyolites both from continental rift and continental arc regardless of the prevailing tectonic setting and the nature of the crust (age, thickness, composition), whereas the oceanic island arc rhyolites may form through partial melting of young, mafic crust.