How many rifts are there in West Africa?

The West African Rift System has, for the last ten years, been thought to consist of five interconnected rifts extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected rifts in West Africa; the Lower Benue Rift which extends to the northeast from the Gulf of Guinea to a triple junction near Chum, and the Gongola and Yola Rifts which extend to the north and east, respectively, from the Chum triple junction. These three rifts opened during the earlier part of the Mesozoic and were subsequently filled with Cretaceous sediments. The evidence for two further rifts, the Ati Rift and the Fort Archambault Rift which were thought to extend to the northeast and southeast, respectively, from a triple junction at the eastern end of the Yola Rift, does not stand up to re-examination.The “Ati Rift” was thought to follow a major linear positive gravity anomaly which had been mapped beneath the Quaternary sediments of the Chad Basin. The main gravity anomaly is separated from the Yola Rift by over 300 km and is probably due to a linear body of basic volcanic or volcano-clastic rocks associated with a suture of Pan-African age. Within the gap, between the main anomaly and the Yola Rift, there are three localised positive anomalies which relate to a gabbro of Precambrian age, a band of dense meta-sediments within the Basement Complex and an acid igneous complex of Palaeogene age. The anomaly as a whole is therefore a sequence of unrelated anomalies, none of which are due to features of Mesozoic age.The “Fort Archambault Rift” was thought to follow a major linear negative gravity anomaly which has been mapped beneath the Quaternary sediments of the Chad Basin. To a large extent the negative anomaly overlies the fosse de Baké-Birao (Baké-Birao Basin) which is itself part of a far larger structure that extends, parallel to the southern margin of the West African Rift System, from Douala on the Gulf of Guinea to Birao near the C.A.R. frontier with Sudan. The Douala-Birao Structure may possibly be loosely related to the West African Rift System in that it would appear to be a compressional structure formed at the same time as the Coniacian-Santonian phase of folding which is observed in the West African rifts. However, the two structures are clearly separated from each other and are quite different in character and to a lesser extent in age.     

Ray tracing through models of the East African Rift

In this paper seismic rays are traced through proposed models of the East African Rift and the predicted travel-time residuals are compared to those observed at a number of African seismic stations. The velocity models are based on published gravity models of the East African Rift and empirical velocity-density relationships. Searle's (1970) revision of the models first proposed by Girdler et al. (1969), comprising a low-density, low-velocity asthenolith that partly replaces and thins the continental lithosphere beneath East Africa, is found to be compatible with most of the observed travel-time residuals. Results from the ray tracing suggest that the model may be improved by increasing the volume of normal mantle material between the two branches of the rift. Some of the interesting travel-time residuals associated with anomalous material away from the rift are also discussed.     

Age progressive volcanism in the Comores Archipelago, western Indian Ocean and implications for Somali plate tectonics

The Comores Islands together with the Tertiary volcanic province of northern Madagascar form a sublinear trend of alkali olivine basalt shield volcanoes across the northern entrance of the Mozambique Channel. Potassium-argon dating of shield-building lavas confirms an eastward increase in age of volcanism along the chain, consistent with a hotspot origin for the lineament. The velocity of the Somali plate over the mantle magma source is 50 mm/yr.

We use the distribution of ages along the Comores-Madagascar chain in conjunction with existing age data for the Reunion-Mascarene Plateau hotspot track to model the absolute motion of the Somali plate for the last 10 m.y. We calculate the relative motion across the East African Rift by subtracting the Somali plate absolute motion from African plate absolute motion during this period. The model predicts 320 km of total separation across the East African Rift during the past 10 m.y. which is greater than has been estimated from surface geological evidence

The geometry of older portions of the Comores and Reunion trends indicates that there was no significant relative motion between the African and Somali plates prior to about 10 m.y. ago.     

The geology of the angareb ring dike complex,northwestern ethiopia

The Angareb Ring Dike Complex is located in Begemder Province in northwestern Ethiopia, 55 kilometers NNW of the provincial capital Gondar. The structure was first identified in 1973 from Landsat 1 satellite imagery analysis of the East African Rift System. The complex is exposed on the floor of the Tana Graben, a feature bearing structural affinity to the East African Rift. A post-early Miocene age is ascribed to the complex. Association with other as yet univestigated circular features may define a volcanic province of early continental rifting in Ethiopia. Contained within the ring structure is a heterogenous tuff breccia of over 100 cubic kilometers in volume, composed of massive agglomorates, ash flows, welded tuffs, and subaqueous tuffs. The tuff is interpreted as filling a subsiding caldron above an eviscerating magma chamber. Mineralogy and chemical analyses depict a fractionating basaltic magma of transitional-mildly alkaline composition. Partial melting of the Precambrian basement is envisioned to explain the subalkaline granites. Comparison of chemical data with regional petrochemical interpretations suggests that the complex represents the beginning of central vent activity in the waning stages of Tertiary volcanism in Ethiopia.     

Continental break-up: The East African perspective

Cynthia Ebinger discusses the evolution of continental rifts through the example of the East African Rift system, in the Bullerwell Lecture, given at the European Union of Geosciences meeting in Nice, April 2004.     

Possible extensions of the Narmada-Son lineament towards Murray Ridge (Arabian Sea) and the eastern syntaxial bend of the Himalayas

Geophysical data contiguous with the Narmada-Son lineament suggests its possible extension westward into the Arabian Sea and eastward up to the Shillong Plateau. The airborne magnetic anomaly map of the north Arabian Sea delineates a linear trend of magnetic anomalies in line with the Narmada-Son lineament. This group of magnetic anomalies, spreading over 20°N to 22°N, starts near the west coast of India at 21°N, 69°30′E and extends to the Murray Ridge. The tectonic feature represented by this group of magnetic anomalies is buried by a thick layer of sediments. This westward extension of the lineament is also reflected in the average Bouguer gravity anomaly map of the Indian Ocean. Towards the east, the gravity and magnetic data delineate a subsurface linear tectonic feature which extends in line with this lineament to the eastern syntaxial bend. These various geophysical signatures further suggest the lineament to be a typical rift-like structure. The tectonic implications of the lineament, which extends from the western to the eastern margins of the Indian plate, is discussed.     

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).     

Combined Gravimetric–Seismic Crustal Model for Antarctica

The latest seismic data and improved information about the subglacial bedrock relief are used in this study to estimate the sediment and crustal thickness under the Antarctic continent. Since large parts of Antarctica are not yet covered by seismic surveys, the gravity and crustal structure models are used to interpolate the Moho information where seismic data are missing. The gravity information is also extended offshore to detect the Moho under continental margins and neighboring oceanic crust. The processing strategy involves the solution to the Vening Meinesz-Moritz’s inverse problem of isostasy constrained on seismic data. A comparison of our new results with existing studies indicates a substantial improvement in the sediment and crustal models. The seismic data analysis shows significant sediment accumulations in Antarctica, with broad sedimentary basins. According to our result, the maximum sediment thickness in Antarctica is about 15 km under Filchner-Ronne Ice Shelf. The Moho relief closely resembles major geological and tectonic features. A rather thick continental crust of East Antarctic Craton is separated from a complex geological/tectonic structure of West Antarctica by the Transantarctic Mountains. The average Moho depth of 34.1 km under the Antarctic continent slightly differs from previous estimates. A maximum Moho deepening of 58.2 km under the Gamburtsev Subglacial Mountains in East Antarctica confirmed the presence of deep and compact orogenic roots. Another large Moho depth in East Antarctica is detected under Dronning Maud Land with two orogenic roots under Wohlthat Massif (48–50 km) and the Kottas Mountains (48–50 km) that are separated by a relatively thin crust along Jutulstraumen Rift. The Moho depth under central parts of the Transantarctic Mountains reaches 46 km. The maximum Moho deepening (34–38 km) in West Antarctica is under the Antarctic Peninsula. The Moho depth minima in East Antarctica are found under the Lambert Trench (24–28 km), while in West Antarctica the Moho depth minima are along the West Antarctic Rift System under the Bentley depression (20–22 km) and Ross Sea Ice Shelf (16–24 km). The gravimetric result confirmed a maximum extension of the Antarctic continental margins under the Ross Sea Embayment and the Weddell Sea Embayment with an extremely thin continental crust (10–20 km).     

Membrane tectonics and the East African Rift

It is proposed that the East African Rift System is a propagating fracture system caused by membrane stresses in the lithosphere. The membrane stresses are due to the northward movement of the African plate on a non-spherical earth. The theory predicts that the interior of the African plate should be in tension and that the membrane stresses within it should be sufficient to fracture the lithosphere. The location of the rift system within the plate and the magnitude of the crustal extension observed along the rift are both consistent with such a hypothesis. The southward migration of the onset of rift volcanism with time is expected as the continent continues to change latitude and the tension fracture propagates southward. Once initiated volcanism along such fracture continues as long as the lithosphere stays in tension.     

Teleseismic delay times,Bouguer anomalies and inferred thickness of the African lithosphere

Teleseismic P-wave travel time residuals (delay times) have been determined for 38 African seismograph stations using a total of 104 presumed Russian underground nuclear explosions. The delay times (T) for seismograph stations situated on Precambrian crust within the interior of Africa, where surface erosion is minimal, appear to be linearly related to both the station elevation (E) and Bouguer anomaly (B) such that: T=(1.12±0.32)E?(1.81±0.44)(1) T=?(0.013±0.005)B?(1.81±0.60)(2) where T is in seconds,E in kilometres and B in milligals. Equation 2 has been used to predict delay times more generally using the smoothed Bouguer anomaly map of Africa. The delay time map is tentatively used to derive a map of lithosphere thickness which takes into account lateral variations of velocity within both the lithosphere and asthenosphere. The lithospheric thickness model indicates a major zone of thin lithosphere (or thick asthenosphere) is associated with the East African Rift system and its continuation into southern Africa. Concomitant seismicity and volcanism further suggest that incipient separation of the continental plate is taking place along the axis of this zone.     

中国东部大兴安岭重力梯级带域地球物理场特征及其成因

综合利用7条地学断面（GGT）资料研究了大兴安岭重力梯级带附近的壳幔地球物理特征模式.分析了形成上述地球物理特征的3种因素:东亚大陆边缘周边三大板块运动、地幔流运动和地幔热柱。对大兴安岭重力梯级带的重力异常的正演拟合结果表明，壳幔物质密度不均匀和莫霍界面超伏造成该带的重力异常，地应力场的综合作用产生了该重力梯级系列地球物理特征。最后，探讨了大兴安岭重力梯级带成因机制，提出了以“挤”、“涌”为动力的“三结点模型”。     

Evidence for a new regional NW–SE fault and crustal structure in Tunisia derived from gravity data

A new Tunisian gravity map interpretation based on the Gaussian filtered residual anomaly, total horizontal gradient, and Moho discontinuity morphology established from gravity data exhibit a new regional northwest–southeast fault extending from Eastern Kairouan to Ghardimaou (Algeria–Tunisia Boundary). It presents a horizontal gradient maximum lineament that terminates the north–south Jurassic structures in the Kairouan plain. Further, this interpretation reveals other known fault systems and crustal structures in Tunisia. The new regional northwest–southeast fault constitutes with the north–south axis and Gafsa–Jefara faults the deepest faults coinciding with the Moho flexures, which had an important role in their initiation. They constitute the border intra‐continental crust faults of the Mesozoic rift. The newly recognized deep fault has critical implications for mineral and petroleum perspectives.     

Geophysical evidence for a failed Jurassic rift and triple junction in Kenya

The interpretation of new and pre-existing aeromagnetic survey data for Kenya, in conjunction with published gravity data, indicates the existence of a palaeo-triple junction of Jurassic age in eastern Kenya. Two arms, represented by the Mombasa coast and the Somali coast respectively, developed into a part of the Indian Ocean. The third arm, which is now concealed by a cover of Quaternary sediments and volcanic rocks, remains as a rifted, sediment-filled trough extending at least as far northwest as the presently active East Africa Rift in Lake Turkana. It has a remarkable similarity, both in scale and geometry, with the Benue Trough of the Niger Delta. As with the Niger Delta, the present Kenyan coastline is not indicative of the true continental margin, as extensive sedimentation has occurred beyond the continental edge in the region of the triple junction since dispersal. Delineation of the true continental margin from aeromagnetic evidence allows pre-drift Madagascar to be re-assembled in closer proximity to the coast than indicated in many reconstructions though the geological evidence supporting this pre-drift position is still controversial.     

Crustal structure of the Mizuho Plateau, East Antarctica from geophysical data

Various kinds of geophysical surveys have been carried out in the Mizuho Plateau, East Antarctica by the Japanese Antarctic Research Expeditions (JARE). The correlation between the high-level gravity anomaly and the bedrock elevation is examined along a route where both data are sufficient to permit deriving a crustal model from gravity, radio-echo sounding and explosion seismic data. The bedrock elevation usually correlates well with the high-level gravity anomaly. However, along the traverse route S-H-Z from the Syowa to Mizuho stations, the bedrock elevation has a weak negative correlation with the high-level gravity anomaly. Such a weak negative correlation is attributed to the deeper part of the crust.The crustal structure between the Syowa and Mizuho stations is modeled from the gravimetric data and the radio-echo sounding of bedrock elevations, so as to fit the P-wave velocity structure derived from the data of explosion seismic experiments. Then the structure is extended from Syowa Station seaward across Lützow-Holm Bay and from Mizuho Station southeastwards inland, where only gravimetric data are available. Thus, a crustal section about 600 km long is obtained on a margin of East Antarctica. The depth of the Moho increases by about 7 km from Syowa Station to the point Y200 (71° 46′S, 48° 56′E), about 500 km from the coast. A graben-like structure is obtained along the line across Lützow-Holm Bay. This suggests that both sides of the bay are bounded by faults.     

关于中国海陆莫霍面深度图编绘的思考

莫霍面是地壳和上地幔的分界面,是一个重要的密度界面.布格重力异常与莫霍面深度之间具有紧密的联系,利用重力异常反演莫霍面深度成是研究莫霍面深度和编制莫霍面深度图的主要手段之一.本文总结了前人编制莫霍图的方法和结果,并加以分析讨论.提出应用布格重力异常反演中国海陆莫霍面深度的方法与技术,并考虑到中国海、陆构造的一体性,进行海陆统一编图以展示莫霍面的海、陆演化与构造关系.     

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.     

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.     

东海陆架盆地及其周边海域地质、地球物理场特征

分东海陆架盆地及其周边海域的重、磁场特征、对研究区域的地球物理场资料进行了小波变换与多尺度分布与计算，计算了研究了海域重，磁资料的1－4阶小波变换逼近、细节、根据分析与计算可知，在东海陆架盆地及其周边域，重、磁异常的主要特征没有内陆盆地那样明显，但盆地内的绝大部分地区均位于异常相对缓变区，在盆地内部有时有局部异常的圈闭及剧变区，但总的特征为缓变的，而周边区域多为相对剧变区，但也存在局部区域的缓变, 、磁场特征表明，盆地基底相对盆地周边区域主要由密度较大，磁性中的岩石组成，因此盆地中部以高正磁异常为主，布格重力异常 对周边地区高，东海陆架盆地及其周边海域的重力场的主要走向为NNE向，局部重力异常较为平缓，磁异常走向没有重力异常明显，但主要也为NNE向，重、磁场的分布特征表明，东海陆架盆地及其邻域的地质构造为大陆架构造的延伸。     

Deep structure of the Betic Cordillera derived from the interpretation of a complete Bouguer anomaly map

A complete Bouguer-anomaly map of the Betic Cordillera and surrounding areas has been compiled from 2995 land-gravity observations. The measurements used to construct the map come from previous surveys as well as from a survey carried out specially for this project.A large negative anomaly reaching −145 mGal is centered on the intra-mountain basins of Loja-Granada and Baza-Guadix, and systematically displaced from the region of greatest topographic relief. This gravity low is interpreted as produced by the thickeming of the earth's crust at a principal contact between two domains of continental crust having different origins and geological evolution.Three gravity profiles selected to investigate the deep structure of the crust in the Betic Cordillera are presented and discussed.