Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential.Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift.During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature.There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality.In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Recôncavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge.Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.     

南极布兰斯菲尔德海峡及邻区地壳结构反演及构造解析

南极布兰斯菲尔德海峡及邻区是南极半岛海域火山、地震等新构造运动最活跃的地区,由于前人对资料处理解释的差异,导致盆地的构造格局仍部分存疑。本文以研究区的卫星重力数据为基础,以多道反射地震和部分岩性资料为约束,采用重震联合反演方法构建了三条横跨研究区的地壳结构剖面,并进一步研究布兰斯菲尔德海峡盆地的地壳结构。研究结果表明布兰斯菲尔德海峡盆地莫霍面深度为33—38km。菲尼克斯板块俯冲消减下沉至南设得兰岛弧之下,导致南设得兰海沟的俯冲带后撤,产生3—4km厚的岩浆混染地壳,密度为2.9g/cm~3。分析认为受板块运动和弧后扩张影响,沿布兰斯菲尔德海峡盆地扩张脊分布的海底火山裂隙式喷发,并进一步导致盆地的持续性扩张。     

The Kerguelen Province revisited: Additional constraints on the early development of the Southeast Indian Ocean

The Kerguelen Province, consisting of two oceanic plateaus (Kerguelen, Broken Ridge) and three basins (Enderby, Labuan and Diamantina), covers a large area of ocean floor in the southeast Indian Ocean. As very few magnetic anomalies have been identified in this area and only a few basement ages from the Kerguelen Plateau are known, reconstruction models of the Kerguelen Province are not well constrained. In an effort to gain more understanding about the evolution of this area, we have used satellite gravity to identify additional fracture zones. As they are likely to be associated with high frequency and low amplitude gravity anomalies, we have computed the vertical derivative map instead of the regular satellite gravity map. Using this approach, we have identified a series of fracture zones in the Enderby Basin, which are aligned with the Mesozoic fracture zones in the Perth Basin and converge to the Kerguelen Fracture Zone. In the conjugate Bay of Bengal, we traced an equivalent pattern of fracture zones which, together, better constrain the early evolution of this part of the Indian Ocean. Synthesis of these images and the other available data from the Kerguelen Province, suggests that the spreading of India from both Australia and Antarctica is closely related. Spreading between the three continents appears to have begun about the same time, in the early Cretaceous and thus, the accretion of some parts of the Kerguelen Province must have occurred before the onset of the quiet magnetic period at 118 Ma. At about 96–99 Ma, when the spreading direction in the Indian Ocean had changed into a N-S direction, it also took place throughout the Kerguelen Province. We find that previously proposed slow spreading in the Diamantina Zone and Labuan Basins, between 96–99 Ma and the initiation of the Southeast Indian Ridge at 43 Ma, could not have taken place. Furthermore, we suggest that there is growing evidence that the same is true for spreading in the eastward continuation of the Diamantina Zone and Labuan Basin, between Australia and Antarctica. Initiation of spreading in this area is likely to be contemporaneous with the spreading in the Kerguelen Province and, thus, older than 96–99 Ma. This revised version was published online in November 2006 with corrections to the Cover Date.     

台西盆地乌丘屿凹陷新生代构造演化特征

以区域地质、地震等资料为基础,系统研究了台西盆地乌丘屿凹陷构造特征及其形成演化。台西盆地的发育受欧亚板块、印度板块、太平洋板块和菲律宾海板块4大板块共同作用的影响。中生代晚期,台西盆地区域应力场从挤压转为松弛,地壳拉张减薄。新生代初期拉张形成裂谷,乌丘屿凹陷是在此背景下发育而成东断西超的半地堑式陆缘断陷。乌丘屿凹陷的构造发育与演化过程,可分为4个阶段,分别为中生代晚期的裂前阶段、古新世至渐新世的断陷阶段、中新世的坳陷阶段和上新世至第四纪的区域沉降阶段。     

Preliminary attempt to characterize the rotation of seafloor in the Pito Deep area of the Easter Microplate using a submersible magnetometer

Several mechanisms have been proposed to account for the rotation of the nearly north-south abyssal hill fabric formed on the East Pacific Rise north of the Easter Microplate to the nearly east-west trends in the northern microplate interior. Proposed mechanisms include rigid microplate rotation, transform fault – parallel shear, and bookshelf faulting during the transfer of lithosphere from the Nazca Plate to the microplate. We used a submersible magnetometer on a NAUTILE dive program to measure the magnetic vector rotation of a pillow basalt and dike spur near Pito Deep, the present location of the tip of the propagating rift system that created the microplate. Our results, although too limited to draw strong conclusions from, suggest clockwise rotations of the seafloor magnetic vectors inconsistent with the transform-parallel shear model, and larger than can be explained solely by rigid microplate rotation.     

Structural evolution of Malay Basin,its link to Sunda Block tectonics

The Malay Basin is located offshore West Malaysia in the South China Sea, within north central region of 1st order Sunda Block. The basin developed partly as a result of tectonic collisions and strike-slip shear of the Southeast Asia continental slabs, as the Indian Plate collided into Eurasia, and subsequent extrusion of lithospheric blocks towards Indochina. The Sunda Block epicontinental earliest rift margins were manifested by the Palaeogene W–E rift valleys, which formed during NW–SE sinistral shear of the region. Later Eocene NW–SE dextral shear of (2nd order) Indochina Block against East Malaya Block rifted open a 3rd order Malay Basin. Developed within it is a series of 4th order N–S en-echelon ridges and grabens. The grabens and some ridges, sequentially, host W–E trending 5th order folds of later compressional episodes. The Malay Basin Ridge and Graben Model explains the multi-phased structural deformation which started with, the a) Pre-Rift Palaeo/Mesozoic crystalline/metamorphic Basement, b) Synrift phase during Paleogene, c) Fast Subsidence from Late Oligocene to Middle Miocene, d) Compressional inversion of first Sunda fold during Late Miocene, and e) Basin Sag during Plio-Pleistocene with mild compressional episodes. The subsequent Mio-Pliocene folding history of Malay Basin is connected to the collision of Sunda Block against subducting Indian–Australian Plate. This Neogene Sunda tectonics, to some degree after the cessation of South China Sea spreading, is due to the diachronous collision along the 1st order plate margins between SE Asia and Australia.     

Geological structure and prospect of the oil and gas bearing of the Falkland Islands region

In the region of the Falkland Islands, resulting from the breaking up of Gondwana and the separation of the South American and African continents, a series of rift basins was formed, such as, the Austral (Magallanes), Falkland (Malvinas), South Falkland (Malvinas), East Falkland (Malvinas), North Falkland (Malvinas), and San Julian. The sedimentary cover of the basins is represented by Phanerozoic deposits 4–7 km thick. The principle productive series are the sandstones of the Springhill Formation (Jurassic-Late Cretaceous), as well as the Tertiary mainly turbidite deposits. There are numerous types of traps dome, stratigraphic, lithologic, tectonically screened, and other types. These basins, except for the North Falkland and San Julian, form a specific producing trend extending from the west to east, which is represented by sandstones of the Springhill Formation (Jurassic-Late Cretaceous), by the maturate source rock Inoceramus Formation, and by high-quality reservoirs and caprock. This trend contains all the prerequisites for highly productive stratum; therefore, the oil and gas bearing potential of these basins is significant and the exploration works will be successful.     

The Agulhas Ridge, South Atlantic: The Peculiar Structure of a Fracture Zone

The Agulhas Ridge is a prominent topographic feature that parallels the Agulhas-Falkland Fracture Zone (AFFZ). Seismic reflection and wide angle/refraction data have led to the classification of this feature as a transverse ridge. Changes in spreading rate and direction associated with ridge jumps, combined with asymmetric spreading within the Agulhas Basin, modified the stress field across the fracture zone. Moreover, passing the Agulhas Ridge’s location between 80 and 69 Ma, the Bouvet and Shona Hotspots may have supplied excess material to this part of the AFFZ thus altering the ridge’s structure. The low crustal velocities and overthickened crust of the northern Agulhas Ridge segment indicate a possible continental affinity that suggests it may be formed by a small continental sliver, which was severed off the Maurice Ewing Bank during the opening of the South Atlantic. In early Oligocene times the Agulhas Ridge was tectono-magmatically reactivated, as documented by the presence of basement highs disturbing and disrupting the sedimentary column in the Cape Basin. We consider the Discovery Hotspot, which distributes plume material southwards across the AAFZ, as a source for the magmatic material.     

南海东北部陆缘地壳结构特征及下地壳高速层成因

南海东北部陆缘构造演化信息丰富,对于理解南海的演化过程至关重要。本文收集了南海东北部的深反射地震和海底广角地震成果剖面,提取地壳和下地壳高速层的厚度结果,并结合水深、重磁异常和岩石圈的流变学等地质地球物理资料,对南海东北部的地壳减薄特征、吕宋-琉球转换板块边界的性质和下地壳高速层的分布及成因进行了分析和讨论。南海东北部的地壳减薄在横向和垂向上都存在不均匀性,以下地壳减薄为主,在台西南盆地存在极端减薄地壳;南海北缘的白云凹陷、西沙海槽和西缘的中建南盆地也存在类似的极端减薄地壳,且都与刚性地块共轭或邻近,推测刚性地块的存在导致地壳初始破裂时下地壳流动和地幔上隆是局部出现地壳极端减薄的主要原因。吕宋-琉球转换板块边界两侧在海底地形、新生代反射和重磁异常等方面均存在差异,与中生代岛弧引起的高磁异常大角度相交,其可能是中生代古特提斯构造域向太平洋构造域转换的边界断裂。下地壳高速层在南海东北部广泛发育,结合其分布特征和波速比V_p/V_s的分布区间,认为其是多期次岩浆底侵形成的铁镁质基性岩。     

An evolving ridge system around the Indian Ocean triple junction

A 2°×2° map of spreading centres and fracture zones surrounding the Indian Ocean RRR triple junction, at 25.5°S, 70°E, is described from a data set of GLORIA side-scan sonar images, bathymetry, magnetic and gravity anomalies. The GLORIA images show a pervasive fabric due to linear abyssal hills oriented parallel to the two medium-spreading ridges (the Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR)). A cuvature of the fabric occurs along fracture zones, which are also located by lows in the bathymetry and gravity data and by offsets between magnetic anomalies. The magnetic anomalies also record periods of asymmetric spreading marking the development of the fracture zones, including the birth, at anomaly 2A, of a short fracture zone 50 km north of the triple junction on the CIR, and its death near the time of the Jaramillo anomaly. In some localities, a fine-scale fabric corresponds to a coarser fabric on the opposite flank of the CIR, possibly indicating a persistent asymmetry in the faulting at the median valley walls if the fabric has a tectonic and not a volcanic origin. A plate velocity analysis of the triple junction shows that both the CIR and Southwest Indian Ridge (SWIR) are propagating obliquely; the CIR appears to form an oblique trend by segmenting into a series of almost normally-oriented segments separated by short-offset fracture zones. For the last 4 m.y., the abyssal hill lineations indicate that the CIR segment immediately north of the triple junction has been spreading with an average 10° obliquity. The present small 5 km offset of the centres of the CIR and SEIR median valleys (Munschy and Schlich, 1989) is shown to be the result of this obliquity and a 30% spreading asymmetry between anomaly 2 and the Jaramillo on the CIR segment immediately north of the triple junction.     

Three-dimensional SeaMARC II,gravity, and magnetics study of large-offset rift propagation at the Pito Rift,Easter microplate

We present results from a SeaMARC II bathymetry, gravity, and magnetics survey of the northern end of the large-offset propagating East Rift of the Easter microplate. The East Rift is offset by more than 300 km from the East Pacific Rise and its northern end has rifted into approximately 3 Ma lithosphere of the Nazca Plate forming a broad (70–100 km) zone of high (up to 4 km) relief referred to as the Pito Rift. This region appears to have undergone distributed and asymmetric extension that has been primarily accommodated tectonically, by block faulting and tilting, and to a lesser degree by seafloor spreading on a more recently developed magmatic accretionary axis. The larger fault blocks have dimensions of 10–15 km and have up to several km of throw between adjacent blocks suggesting that isostatic adjustments occur on the scale of the individual blocks. Three-dimensional terrain corrected Bouguer anomalies, a three-dimensional magnetic inversion, and SeaMARC II backscatter data locate the recently developed magmatic axis in an asymmetric position in the western part of the rift. The zone of magmatic accretion is characterized by an axis of negative Bouguer gravity anomalies, a band of positive magnetizations, and a high amplitude magnetization zone locating its tip approximately 10 km south of the Pito Deep, the deepest point in the rift area. Positive Bouguer gravity anomalies and negative magnetizations characterize the faulted area to the east of the spreading axis supporting the interpretation that this area consists primarily of pre-existing Nazca plate that has been block faulted and stretched, and that no substantial new accretion has occurred there. The wide zone of deformation in the Pito Rift area and the changing trend of the fault blocks from nearly N-S in the east to NW-SE in the west may be a result of the rapidly changing kinematics of the Easter microplate and/or may result from ridge-transform like shear stresses developed at the termination of the East Rift against the Nazca plate. The broad zone of deformation developed at the Pito Rift and its apparent continuation some distance south along the East Rift has important implications for microplate mechanics and kinematic reconstructions since it suggests that initial microplate boundaries may consist in part of broad zones of deformation characterized by the formation of lithospheric scale fault blocks, and that what appear to be pseudofaults may actually be the outer boundaries of tectonized zones enclosing significant amounts of stretched pre-existing lithosphere.     

Rifting of oceanic crust at Endeavor Deep on the Juan Fernandez microplate

The development of an anomalously deep rift appears to be a common characteristic of the evolution of microplates along the East Pacific Rise, including the Galapagos, Easter, and Juan Fernandez microplates. We investigate crustal rifting at Endeavor Deep on the Juan Fernandez microplate using bathymetry, gravity and side scan sonar data. An initial phase of lithospheric extension accompanied by extensive subsidence results in the formation of a very deep rift valley (up to 4 km of relief, 70 km long and 20 km wide). Morphological observations and gravity data derived from GEOSAT satellite altimetry show the subsequent initiation of crustal accretion and development of a mature spreading center. Recent models of the kinematics of microplate rotation allow the amount of opening across Endeavor Deep over the past 1 m.y. to be quantified. We develop a simple mechanical model of rifting involving block faulting and flexural response to explain the gravity signature over the rift valley. The Bouguer gravity anomaly is asymmetric with respect to the surface topography and requires that a shallow-dipping fault on the western wall of the valley dominate the extension at Endeavor Deep. Consideration of three similar microplate rift valleys leads us to suggest that asymmetric rifting is the characteristic process forming microplate deeps.     

南海海盆的形成演化探讨

根据“陆缘扩张”理论,利用古地磁数据,结合地质、地球物理资料,对南海海盆的成因机制和演化过程进行探讨。结果得出:南海的多期多轴扩张及其形成演化,是在欧亚板块、太平洋板块和印度板块的联合作用下,中、新生代南海周缘的微板块和岛弧的相互运动以及南海海盆构造应力场不断变化的情况下逐渐完成的。     

南海西南次海盆被动陆缘构造单元划分及伸展演化过程

南海西南次海盆被动陆缘洋陆转换带位于陆缘强烈伸展区,蕴含着岩石圈临界伸展破裂和洋盆扩张过程的丰富信息。本文利用多道地震剖面和重力异常数据,对西南次海盆被动陆缘构造单元进行划分,研究陆缘南、北部洋陆转换带结构构造特征,探讨陆缘伸展演化过程。多道地震剖面资料显示,北部洋陆转换带发育有裂陷期断陷和向海倾斜的掀斜断块;南部发育有低角度正断层控制的裂陷期断陷、海底火山以及局部隆起;从陆到洋方向,重力异常值变化明显。根据上述结果南海西南次海盆被动陆缘划分为近端带、洋陆转换带和洋盆三个构造单元,分别对应了其伸展演化过程的三个阶段：前裂谷阶段、陆缘裂陷阶段和海底扩张阶段。     

Active oceanic spreading in the northern North Fiji Basin: Results of the NOFI cruise of R/V L'Atalante (newstarmer project)

The South Pandora and the Tripartite Ridges are active spreading centers located in the northern part of the North Fiji Basin. These spreading centers were surveyed over a distance of 750 km during the NOFI cruise of R/V L'Atalante (August–September 1994) which was conducted in the frame of the french-japanese Newstarmer cooperation project. SIMRAD EM12-dual full coverage swath bathymetric and imagery data as well as airgun 6-channel seismic, magnetics and gravity profiles were recorded along and offaxis from 170°40 E to 178° E. Dredging and piston coring were also performed along and off-axis. The axial domain of the South Pandora Ridge is divided into 5 first-order segments characterized by contrasted morphologies. The average width of the active domain is 20 km and corresponds either to bathymetric highs or to deep elongated grabens. The bathymetric highs are volcanic constructions, locally faulted and rifted, which can obstruct totally the axial valley. The grabens show the typical morphology of slow spreading axes, with two steep walls flanking a deep axial valley. Elongated lateral ridges may be present on both sides of the grabens. Numerous volcanoes, up to several kilometers in diameter, occur on both flanks of the South Pandora Ridge. The Tripartite Ridge consists of three main segments showing a sigmoid shape. Major changes in the direction of the active zones are observed at the segment discontinuities. These discontinuities show various geometrical patterns which suggest complex transform relay zones. Preliminary analysis of seismic reflection profiles suggest that the Tripartite Ridge is a very young feature which propagates into an older oceanic domain characterized by a significant sedimentary cover. By contrast, a very thin to absent sedimentary cover is observed about 100 km on both flanks of the South Pandora Ridge active axis. The magnetic anomaly profiles give evidence of long and continuous lineations, parallel to the South Pandora Ridge spreading axis. According to our preliminary interpretation, the spreading rate would have been very low (8 km/m.y. half rate) during the last 7 Ma. The South Pandora and Tripartite Ridges exhibit characteristics typical of active oceanic ridges: (1) a segmented pattern, with segments ranging from 80 to 100 km in length; (2) an axial tectonic and volcanic zone, 10 to 20 km wide; (3) well-organized magnetic lineations, parallel to the active axis; (4) clear signature on the free-air gravity anomaly map. However, no typical transform fault is observed; instead, complex relay zones are separating first-order segments.     

南海北缘新生代盆地沉积与构造演化及地球动力学背景

南海北缘新生代沉积盆地是全面揭示南海北缘形成演化及与邻区大地构造单元相互作用的重要窗口。通过对盆地沉积-构造特征分析,南海北缘新生代裂陷过程显示出明显的多幕性和旋转性的特点。在从北向南逐渐迁移的趋势下,东、西段裂陷过程也具有一定的差异,西部裂陷活动及海侵时间明显早于东部,裂陷中心由西向东呈雁列式扩展。晚白垩世-早始新世裂陷活动应是东亚陆缘中生代构造-岩浆演化的延续,始新世中、晚期太平洋板块俯冲方向改变导致裂陷中心南移,印度欧亚板块碰撞效应是南海中央海盆扩张方向顺时针旋转的主要原因。     

Stress field modeling of northwestern South China Sea since 5.3 Ma and its tectonic significance

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 ~rough. 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 sedimen- tary responses. Our results demonstrate that an extension tectonic event occurred after 5.3 Ma in theYingge- hal-Qiongdongnan-Xisha trough area, which is characterized by thinner crust C〈16000 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 veloc- ity 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.     

Gravity surveys over the Reykjanes Ridge and between Iceland and the Faeroe Islands

A regional survey of the southern Reykjanes Ridge (52°N to 57°N) shows an irregular topography: a rift valley which is only partly recognizable as such, with varying azimuth and some fracturezone-like interruptions. The survey also comprised gravity and magnetic measurements.The course of the axis as well as the perpendicular fractures show up well in the free air anomalies as relative troughs within an area of positive free air gravity (Figure 5). There is no indication of density variations within the topographic masses.The anomaly pattern of total magnetic intensity indicates the exact position of the rift axis and a bifurcation at about 55°N. From the parallel magnetic anomalies south of 55°N (Figure 2) a spreading rate can be deduced of 1.10 cm/yr perpendicular to the rift axis (Figure 3). This spreading rate is at the same time the plate movement involved.A survey of the Iceland-Faeroe Ridge with a 3–5 miles grid shows large gravity and magnetic anomalies over a smooth topography, indicating large pockets of light material, probably of volcanic origin. These areas have normal magnetization. Positive gravity anomalies forming a ring structure along the 200 m isobath are characterized by reversed magnetization.The dissimilarity in morphology, seismicity and inner structure between the two ridges that intersect in Iceland suggest that there is no relation between the two phenomena.Paper presented at the meeting of the International Gravity Commission, Paris, on September 8, 1970.     

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.     

塔斯曼海构造特征及其演化过程研究

塔斯曼海位于西南太平洋地区,处于印度-澳大利亚板块和西兰板块之间,大地构造背景复杂。该地区是全球油气资源勘探的重点海域之一,但是国内对该地区的研究相当匮乏。本文根据塔斯曼海海域的自由空气重力异常对塔斯曼海海域的构造单元进行了划分,前人关于塔斯曼海的研究主要集中在Resolution海岭北部,我们认为塔斯曼海的范围应包括Resolution海岭以南,麦夸里海岭以西,塔斯曼断裂带以东的区域（即南部次盆）。结果显示,塔斯曼海域及邻区包括3个一级构造单元:东澳大利亚陆缘、西兰板块和塔斯曼海盆,且塔斯曼海盆可进一步划分为西部次盆、东部次盆和南部次盆。本文基于塔斯曼海域90 Ma以来的洋壳年龄数据编制了构造演化图,将塔斯曼海的形成演化过程分为4个阶段:（1）中生代陆内裂谷期（90~83 Ma BP）;（2）塔斯曼海扩张阶段（83~61 Ma BP）;（3）塔斯曼海北部扩张停止阶段（61~52 Ma BP）;（4）塔斯曼海南部改造阶段（52 Ma BP至今）。