Negative Bouguer Anomalies Research Articles

Timing of the Martian core dynamo

Using the radial component magnetic data acquired by Mars Global Surveyor during its mapping phase, a reliable magnetic anomaly map of Mars is derived. Many weak but reliable magnetic anomalies are now detected in the northern lowlands. Their source bodies have likely been partially demagnetized by the lowlands formation processes, and there has been no core field in the later times to remagnetize the bodies. No consistent anomalies are observed in the Hellas basin, implying that the core dynamo has been inactive at least since the impact event. The topography, the positive Bouguer anomaly at the center, and the strong negative Bouguer anomalies over the outskirts of Thaumasia plateau suggest that the plateau was the site of a large impact basin prior to the formation of the plateau. The lack of magnetic anomalies in the central part and the strong magnetic anomalies over the outskirts of the plateau resemble the magnetic signature of Hellas basin, implying a preexisting giant impact basin beneath the plateau and the absence of the core field since the impact time. The lack of magnetic edge effects of Valles Marineris, the absence of thermal demagnetization of Tharsis plain by the shield volcanoes, and the substantial distance between the paleomagnetic poles and the present rotation axis imply that a major part of Tharsis bulge was formed in the absence of the core field.

Upper mantle structure beneath eastern Siberia: Evidence from gravity modeling and mantle petrology

The spatial distribution of large‐scale lithospheric domains and the boundaries between them may control the emplacement of large ore bodies, and as such, regional mapping of the lithosphere is relevant to mineral exploration. In this study we combine potential‐field geophysical data and mantle petrology to map major lithospheric structures on the eastern part of the Siberian platform. The platform consists of several Archean and Proterozoic terranes that have been mapped from regional magnetic data and basement exposures in the Anabar shield. We use garnet and chromite concentrates from a chain of Paleozoic to Mesozoic kimberlites across the platform to construct mantle sections, which show significant lateral variation in rock type distribution within the lithospheric mantle. These lateral variations correspond to the terranes mapped at the surface and indicate that the terrane boundaries are translithospheric. Archean terranes are underlain by depleted Archean lithosphere more than 200 km thick, while the Proterozoic terranes are underlain by thinner and less depleted lithosphere. Geophysical data show more strongly negative Bouguer anomalies and a more heterogeneous magnetic anomaly pattern over the Archean terranes than on the Proterozoic terranes. The pattern of the gravity data reflects the lateral variation in mantle composition beneath the terranes, as shown by mantle‐petrology studies. We invert gravity and topography data to estimate the flexural strength, or elastic thickness (Te), of the lithosphere across the area. Although on a stable Precambrian craton, the Te is relatively low (<30 km) across most of the area, suggesting a relatively weak lithosphere comparable to that of tectonically much younger areas around the world. A 150‐km‐wide zone of very weak lithosphere (Te < 10 km) runs N‐S across the western part of the study area. This weak zone coincides with a zone of thickened lower crust, and abnormally high sub‐Moho P wave velocities which suggest anisotropy in the upper mantle. The kimberlite fields in the Archean part of the platform are localized on the flanks of this zone of weak lithosphere. We suggest that the low‐Te zone may be a mantle shear zone which has been a preferred conduit for the emplacement of magmas into the lower crust and later has controlled the emplacement of kimberlites in the study area.

Pull-down basin in the central part of Japan due to subduction-induced mantle flow

The driving force for the basin subsiding against isostatic balance in and around Lake Biwa in the Kinki district, Japan is discussed. The lake region is characterized by strong negative Bouguer anomalies, especially by a steep horizontal gradient zone of gravity anomaly running along the western margin of the lake. The large negative anomaly (>50 mgal) cannot be explained by low-density sediments beneath it. A down-warping structure extending to the Moho depth should be taken into account. This conjecture has been strongly supported by a short-period receiver function imaging, which shows a clear offset of about 8 km for the Moho discontinuity under the steep gravity gradient zone. A question arises as to what is the driving force to create such a large down-warping structure. We consider that the subduction of the shallow-dipping slab under the region (Philippine Sea Slab) may cause crustal deformation by dragging the viscous mantle downward. In order to verify this model, we simulated the induced mantle flow due to the subduction of the Philippine Sea Slab and the pressure distribution on the crust–mantle boundary. This numerical experiment showed that the induced flow makes a strong negative pressure zone under the lake region if the slab has a vertical offset along the direction of subduction. This offset of the slab is consistent with plate models deduced from hypocentral distributions and Sp phases of the deep-focus earthquakes.

Hot fingers in the mantle wedge: new insights into magma genesis in subduction zones

Quaternary volcanoes in the Northeast Japan arc can be grouped into 10 volcanic clusters striking transverse to the arc; these have an average width of 50 km, and are separated by parallel gaps 30–75 km wide. This clustering of volcanic centres, topographic profiles, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the Japan Sea side of the volcanic arc are closely correlated. All these observations may be related to locally developed hot regions within the mantle wedge that have the form of inclined, 50 km wide fingers. Each of the 10 fingers recognised extends from deep mantle (>150 km) below the back-arc region towards the shallower mantle (∼50 km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. The volcanic basement along the fingers has been uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity at the surface. Although volcanic activity is rare along the Japan Sea coast, tomographic results show that hot, low-velocity mantle fingers exist within the mantle wedge. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers; these have not yet been erupted, but have accumulated at the Moho discontinuity.

Crustal thicknesses in SE Brazilian Shield by receiver function analysis: Implications for isostatic compensation

The Brazilian Lithosphere Seismic Project (BLSP, a joint project by University of São Paulo and Carnegie Institution, 1992–1999) operated more than 20 temporary broadband stations in the southeastern Brazilian shield. The area, a transect ∼1000 km long and 300 km wide, covers different geological provinces: the Precambrian São Francisco craton, the adjacent Brasiliano (700–500 Ma) fold belts, and the Paraná basin of Paleozoic origin. Crustal thicknesses were estimated for 23 sites using receiver functions. For each station, receiver functions were stacked for different sets of earthquakes according to azimuth and distance. The P‐to‐S Moho converted phase was clearly identified at most sites. Crustal thicknesses were estimated using an average crustal P wave velocity of 6.5 km/s. Poisson's ratio of 0.23 (Vp/Vs = 1.70) was used for the São Francisco craton and adjacent fold belt (based on travel times from small, local earthquakes) and 0.25 was used for the Paraná basin and coastal belt. Crustal thicknesses ranged from 35–47 km. Although there is a clear inverse correlation between topography and Bouguer gravity anomalies in the study area, Moho depths show the opposite pattern from that expected: areas of low topography and less negative Bouguer anomalies, such as the Paraná basin, have thicker crust (40–47 km) compared with the high elevation areas of the craton and fold belt (37–43 km). Two hypothesis are proposed to explain the data: (1) A lower density, by 30–40 kg/m3, in the lithospheric mantle under the Archean block of the São Francisco craton relative to the Proterozoic lithosphere is responsible for maintaining the high elevations in the plateau area. Relatively low density and high P wave velocity are compatible with a depleted (low FeO) composition for the Archean lithosphere. (2) Alternatively, if the density contrasts between Archean and Proterozoic lithospheres are smaller than the values above, then the crust beneath the Paraná basin must be more dense than that of the craton. Higher crustal density and high Poisson's ratio would be consistent with magmatic underplating in the lower crust beneath the Paraná basin, as inferred from other studies.

Distribution of Quaternary volcanoes in the Northeast Japan arc

Clustering of volcanic centers, low-velocity regions in the mantle wedge and local negative Bouguer gravity anomalies along the rear of the volcanic arc are closely correlated in Northeast Japan. These observations may bebest explained by the presence of inclined, finger-like, hot regions in the mantle wedge of the subduction zone. These fingers, perpendicular to the arc-trench system in the Northeast Japan arc, have an average separation of ∼80 km and an average width of 50 km. Each of ten fingers extends from the deep mantle (>150 km) below the back-arc region toward the shallower mantle (∼50 km) beneath the volcanic front. Quaternary volcanoes are built immediately above the hot mantle fingers. Volcanic basements are uplifted by repeated injection of magmas into the crust, accompanied by Quaternary volcanic activity on the surface. Although volcanic activity is rare along the Japan Sea coast, the hot mantle fingers exist within the mantle wedge as evidenced by tomographic results. The negative Bouguer anomalies at the rear of the volcanic arc could be caused by magmas supplied from the hot mantle fingers, which have not yet been erupted, but may have accumulated at the Moho discontinuity.

Resolving source parameters through pseudo potential anomalies

Ambiguity is inherent in the interpretation of all geopotential anomalies. The ambiguity can possibly be reduced by generating pseudo potential anomalies and comparing them with the observed anomalies. A 0.25° digitized gravity anomaly data set of the Indian subcontinent and adjoining regions was used to generate a pseudo magnetic anomaly map at an altitude of 400 km. This map was compared with the MAGSAT scalar anomaly map and there seems to be a good reproduction of MAGSAT map over the Himalayas. However, anomalies over east and west coasts show latitudinal shifts of about 2° when compared to MAGSAT anomalies. The southern peninsular of India is characterized by strong negative Bouguer anomalies, possibly due to the bias of Indian ocean geoid low. The pattern of pseudo magnetic map as obtained from gravity data differs from that of MAGSAT suggesting different sources of gravity and magnetic anomalies over southern India.

Rifting of Africa and pattern of mantle convection beneath the African plate

Abstract The Mesozoic and Cenozoic tectonic setting of the African continent is characterized by an extensional stress regime, as is evident from the evolution of the West, Central and East African rift systems, the opening of the Indian and Atlantic oceans and the development of Tethys domain, as well as from the results of geophysical investigations. The most prominent feature of the Bouguer gravity anomaly map of Africa is a great belt of negative Bouguer anomalies which extends across the entire African continent from Ethiopia in the East to Benguela in the West. In conjunction with the Red Sea spreading zone this belt appears to encircle a “centre” in Cameroon. It probably corresponds to a distinct zone of lithospheric thinning and extension. The entire African continent is characterized by higher-than-normal elevations. It is located inside the African residual geoid high. Earthquake fault-plane solutions confirm the extensional character of the present tectonic regime of eastern and southern Africa where normal faulting predominates. The average orientation of T axes is N-S in South Africa, NW-SE in Zambia, Mozambique and Madagascar, E-W in Kenya and Ethiopia, and NE-SW in the Red Sea region. From north to south, the orientation of T axes rotates systematically in a clockwise sense by 130° and describes a radial pattern centred on equatorial West Africa. This indicates that the African lithosphere has been subject to continent-wide radial extension around a hypothetical West African “spreading centre”, referred to as the African Spreading Centre A (ASC-A), which coincides with the geographical centre of the African plate. Shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements. In the case of Africa, the continent-wide regularities in the pattern of lithospheric extension, the elevated position of the continent, as well as the evolution of the mid-oceanic ridge system surrounding Africa, provide distinct boundary conditions for the pattern of large-scale mantle flow beneath the African plate. It is proposed that relatively warm and less dense mantle material rises below Africa, forming a single mega-plume or a number of plumes. At the base of the African lithosphere, the ascending material diverges and flows radially away from the centre of ascent, corresponding to the ASC-A. This ascending flow forms part of a very large mantle convection cell which occupies the Eurafrican hemisphere. The descending branch of this convection cell is located at a distance of 80–90° from the ASC-A. The mid-oceanic ridges surrounding the African continent are located between the upwelling and the downwelling branches of this convection cell, at a distance of 50–60° from the ASC-A. Upper mantle flow below the sea-floor spreading axes is uni-directional and horizontal and is directed away from the ASC-A.

Tomographic estimates of sub-Moho seismic velocities in Fennoscandia and structural implications

Pn and Sn velocity estimates are derived for the sub-Moho structure beneath Fennoscandia using a tomographic conjugate gradient scheme. Observational data stem from local earthquake recordings of crustal phases Pg, Sg (Lg) and the sub-crustal phases Pn and Sn by the Fennoscandian seismograph network. Unknowns are Pn and Sn velocities and event hypocenter parameters. Crustal thicknesses are assumed as known and are not estimated, whereas velocities are allowed to vary. The most prominent P and S velocity anomalies are found in the northwest (inland Lofoten) and southwest (Møre) of Norway and, for shear waves, also in central Finland. There is a good correlation between the velocity anomalies in Norway and observed negative Bouguer anomalies, indicating the presence of low-density sub-Moho material in these areas. Geologically the Pn and Sn velocity anomalies appear to be associated with geodynamic processes tied to the opening of the Norwegian-Greenland Sea.

Crustal structure of the Mamfe basin, West Africa, based on gravity data

The Mamfe basin is the smallest and most southerly of three side rifts associated with the Benue Trough. It extends 130 km east from the Benue Trough into Cameroon as far as the Cameroon Volcanic Line and until recently it was the only rift devoid of geophysical studies to indicate its crustal structure and mode of formation. Between 1984 and 1986 three gravity surveys were made over the basin and adjacent basement areas of Nigeria and Cameroon to reveal that the western half of the basin is associated with a broad positive Bouguer anomaly at least 100 km wide and amplitude in excess of 20 mGal (200 gu). Superimposed on this gravity “high” is a negative Bouguer anomaly which closely correlates with the area of sedimentary cover. Using the crustal thickness and density contrasts determined from seismological and gravity studies over the Yola side rift, located farther north, two-dimensional modelling of the regional and residual gravity fields have been attempted. The regional “high” has been modelled as a reduction in crustal thickness of about 8 km beneath the basin and the residual “low” as a sedimentary basin reaching at least 3 km in depth at the Nigeria/Cameroon border. The eastern half of the Mamfe basin has a gravity expression to indicate no large sedimentary thickness. Thus the gravity expression of the western end of Mamfe basin is similar in character to that found elsewhere within the West African rift system. Crustal models indicate that extension perpendicular to the Mamfe basin could be at most 19 km with geological data indicating only small amounts of compression/shear deformation within the basin during the late Cretaceous times. This lack of deformation probably results from the Mamfe basin orientation being more or less perpendicular to the Benue Trough and Foumban (or Central African) shear zone along which most of the shear deformation has occurred during and since Cretaceous times.

Controls on rifting in Africa and the regional tectonic model for the Nigeria and East Niger rift basins

Since early Mesozoic times, three phases of rifting have occurred in Africa and are related to distinct phases of breakup of Gondwana. These contrasting rift episodes have provided an insight to the extent to which plate tectonic processes and more localised mechanical anisotropy processes within the African lithosphere have influenced rifting. Gravity modelling of the Nigeria and East Niger rift basins shows the extent and nature of the broad (regional) positive Bouguer anomaly associated with these rifts. The removal of this regional anomaly allows the delineation of the (residual) negative Bouguer anomaly which reflects the lateral extent and thickness of Mesozoic and Cenozoic sediments. This residual anomaly is interpreted using iterative three dimensional modelling, by incorporating a density-depth function derived from well logs. Results indicate that an extensive basin complex exists in eastern Niger with sedimentary sections greater than 7 km in depth and are in good agreement with the aeromagnetic data. A simple estimate of crustal extension across the East Niger rift basin indicates that up to 58 km of crustal stretching has occurred, placing an upper limit on the amount of sinistral strike-slip movement required within the Benue Trough to open the East Niger rift. A similar strike-slip and extension rift geometry is observed elsewhere in the West and Central African rift system which indicates that the Cretaceous period was an important time for strike-slip tectonics. Changes in the differential opening of the Central and South Atlantic Oceans during the Cretaceous can adequately explain the large strike-slip displacements and associated rift basins in West and North Central Africa and are considered here to more closely reflect the initial rift processes that shaped the continental margin of Africa than those associated with the modern East African rift system.

Gravity and vertical magnetic gradient investigations of a localised area of the Benue Trough, Nigeria

Gravity and vertical magnetic gradient studies were carried out in Lokpanta-Lekwesi area in the Lower Benue Trough to investigate the local structures which might be associated with the Lead-Zinc mineralisation as evident in the adjacent area (Ishiagu Pb-Zn mineralisation). For this reason 134 gravity and vertical magnetic gradient readings were taken at intervals of 250 m along major roads and track routes that connect widely separated parts of the 25 km 2 area. The instruments used are the Canadian Scintrex CG-2 gravimeter (No. 318), the American Paulin altimeter and the Proton Precession Magnetometer. The geology of the areacomprises the Ezeaku Formation, and the Awgu Formation, their ages ranging from Middle to Upper Cretaceous. The study was used to delineate two igneous intrusions in the Ezeaku Formation. The intrusive bodies show high positive Bouguer anomalies which ranged from 0.5 mgal to 3.3 mgal in contrast to the negative Bouguer anomaly of the country rock that ranged from 0 to − 3.0 mgal. The vertical magnetic gradient method identified the two intrusive bodies as negative anomalies having vertical magnetic gradient values ranging from −5.0 to −25.0 gammas per meter in contrast to 5.0 to 10.0 gammas per meter values of the country rock. The Profiles were interpreted using computer assisted evaluation procedure of non-linear optimization and interactive techniques. Based on two dimensional model, the minimum depth computations to the upper surfaces of the anomalies A and B by gravity method are 60mm and 160 m respectively while the vertical magnetic gradient method gave a minimum depth estimates of the anomaly A as 75 m and that of B as 180 m.

On the geotectonic nature of the Fen-Wei rift system

The Fen-Wei rift system, 1200 km long and 10–100 km wide, is situated in the northern part of East China, following a zigzag path trending NNE-SSW. It cuts the Shanxi-Shaanxi Plateau (average altitude 2 km) reaching depths of 5–6 km, and is filled with Cenozoic sediments (thickness 3–4 km). Accompanying the mainly vertical displacement of the marginal fault zones, the rift system extends laterally up to 7 km (greatest in the southern part). Earthquakes with foci 10–30 km deep are frequent; M = 8. Eruptions mainly of alkaline basalts took place 24 times in the Cenozoic. Heat flow appears to be rather high as marked by 17 hot springs. Negative Bouguer anomalies are dominant, reaching a maximum of −200 mGal. The crust is 38 km thick on average, 2–4 km thinner than that of the marginal uplift zones. Possibly a mantle arched belt exists below. In the upper mantle, V p = 7.95 km/ s. Originally the rift system was part of the broad ancient China platform. In the Early Jurassic, the latter underwent activation and became the North China diwa (geodepression) region ∗ ∗ The term “diwa” is derived from Chinese words meaning geodepression, being a special kind of intermountain-structural basins occurring mostly on the ancient platform region, filled chiefly with mollasoid sediments. A diwa region is marked with high relief and by basins, diwa as negative units intercalated with ranges as positive units called “geodomes”. It is a post-platform mobile region, the third geotectonic element of the continental crust besides géosynclinal and platform regions, proposed by the author in 1959 (Chen, 1959, 1960b, 1965, 1980, etc). Its synonym is “activated region” proposed by the author in 1956, because it is formed by the activation of the platform (Chen 1956, 1960a). . In the Cenozoic, the Fen-Wei rift system formed as a result of contraction of the crust in this region combined with creeping of asthenospheric material. It may be referred to the crevice type of post-platform continental rift zones in E.E. Milanovsky's classification.

Late Caledonian granitoids and timing of deformation in the Iapetus suture zone of eastern Ireland

AbstractLate Caledonian granitoids, (c. 400 Ma) in the zone of the Iapetus suture provide a datum against which current models for the relative timing of deformation can be tested. One such granitoid adjacent to the proposed suture ‘line’ in eastern Ireland is now buried by an Upper Palaeozoic cover. It is identified geophysically by a negative Bouguer anomaly with no magnetic signature, and geologically by a hypabyssal dyke swarm and hornfels metamorphism. The timing of intrusion of the granitoid is shown to have occurred during the continuing end-Silurian/early-Devonian deformation. Other members of this widespread suite in Ireland show features consistent with diapiric intrusion during the later stages of the deformation. This evidence brackets the age of regional deformation as continuing during granite emplacement and cooling (c. 400 Ma). The unifying characteristics of the straddling granitoid suite, coupled with a sinistrally transpressive deformation, in this broad suture zone are interpreted in terms of a continental collision which climaxed in late-Silurian/early-Devonian time.

A gravity study of the precambrian rocks in the malumfashi area of Kaduna State, Nigeria

Gravity measurements in the Malumfashi area of Nigeria show large negative Bouguer anomalies ranging from — 33 mGal to — 65 mGal and free-air anomalies distributed about a mean of +15 mGal. A regional anomaly ranging from about −56 mGal in the southwestern part to about −44 mGal in the northeastern part of the area is tentatively correlated with a gradual thinning of the crust from southwest to northeast. Negative residual anomalies in the area correlate well with granite bodies thus confirming the acidic (low density) nature of the Older Granites. Positive residual anomalies of values up to about +12 mGal are associated with the schist bodies in the area. From two-dimensional modelling using approximate density values, it is estimated that the schists in the area vary in thickness from 4 to 5 km and the granite bodies have thicknesses of the order of 4 km. High-gravity gradients of up to 6 mGal/km at some contact areas between the schists and granitic bodies are attributed to sharp contacts and magmatic sloping is thought to be the most likely mode of emplacement of the granites in the area. The Bouguer anomalies observed over the schists have a maximum of only − 34 mGal. These values are considered too low to allow the existence of more mafic rocks at depth and the schists in the Malumfashi area are therefore believed to have evolved in an ensialic environment.

Structure and late cenozoic evolution of the upper lithosphere in Southwest Tuscany (Italy)

Geological and geophysical data on southwest Tuscany are reviewed in order to define the structure and evolution of the upper lithosphere from the Miocene to the Quaternary. Petrologic studies reveal the existence, below all of Tuscany, of Hercynian and older polyphased metamorphic rocks and of Hercynian granite, whose top is an important seismic reflecting horizon. The basement is characterized by NE-SW trending structures, in contrast with the main NW-SE “Alpine” structures of the uppermost levels. The heat flow map shows two broad areas with values higher than 80 mW/m 2, reaching maximum values of 10.5 and 15 H.F.U. in the geothermal areas, which are also characterized by negative Bouguer anomalies. A Landsat study revealed a NE-SW band of subcircular structures passing through Larderello and coinciding with a regional fault system and a steep rise in the Moho. Petrologic, geochemical and radiometric data on the Tuscan igneous rocks show that partial melting took place in the Tuscan crust at different levels and to varying degrees from the Miocene to Quaternary, producing a continuous “Alpine” granitic layer. The known Tuscan intrusive bodies and two batholiths below the Larderello and Mt. Amiata geothermal fields represent culminations of the “Alpine” granite. The rise of the Tuscan magmas was closely correlated to a post-Tortonian tensional tectonics and followed its N-E migration. Tensional tectonics started after the last compressional phase (10–11 Ma B.P.) as a consequence of the anticlockwise rotation of Italy, the opening of the Tyrrhenian Sea and the swelling of the mantle below southwest Tuscany.

Oxygen and strontium isotopic characteristics of calc-alkalic volcanic rocks from the central and western Japan arcs: Evaluation of contribution of crustal components to the magmas

18O/ 16O and 87Sr/ 86Sr ratios were determined for Quaternary calc-alkalic volcanic rocks from six volcanic rock suites in the central and western Japan arcs. The δ 18O values relative to SMOW and 87Sr/ 86Sr ratios range from +6.3 to +9.9 0/ 00 and 0.70357 to 0.70684, respectively. Both the O- and Sr-isotopic compositions are higher than those for island-arc primitive magmas and their differentiates. The isotopic compositions of the calc-alkalic rocks cannot be derived by a simple fractional crystallization of the primitive magmas. On the other hand, the 18O- and 87Sr-enrichment is confined to the rock suites located in well-developed island arcs having thick continental-type crust with low or negative Bouguer anomalies. Involvement of 18O- and 87Sr-rich crustal material in the magma formation is suggested. The isotopic compositions vary remarkably within individual rock suites as well as from volcano to volcano. The data points in δ 18O vs. 87Sr/ 86Sr plot accord with a mixing model between primitive magmas and crustal material of dioritic composition on an average, assuming their comparative Sr contents. The primitive magmas involved could not be low-Sr tholeiites, but magmas more or less enriched in incompatible elements including Sr, which correspond to high-alkali tholeiites or alkali basalts and their evolved magmas. The nature of the primitive magmas seems to change from tholeiitic to more alkalic with progressing island-arc evolution. Mixing of crust-derived melts is more plausible than assimilation of solid-rocks for involving 20 to 30% crustal material in the magmas along simple mixing curves. Isotopic variations between the rock suites are ascribed to variable Sr concentration radio of the end-members, variable isotopic compositions of crustal material or variable mixing ratio of the end-members. Extremely high-δ 18O rocks with moderate increase in 87Sr/ 86Sr ratio suggest another mixing process in shallower magma chambers between andesite magmas and metasedimentary rocks having high δ 18O and 87Sr/ 86Sr values but low Sr content. Subsequent fractional crystallization of once-derived magmas would be the prominent process for the rock suites showing gradual increase in 18O up to 1 0/ 00 with uniform 87Sr/ 86Sr ratios.

Continental rifting: Progress and outlook

Increased research activity on continental rifts has led to a flood of new data in the last 15 years, but there is little consensus about the basic mechanisms and causes of rifting. The well‐known association of taphrogenic features—doming, faulting, and volcanism—is widely regarded as the result of asthenosphere convection and lithosphere thinning. This concept is supported by observations of long‐wavelength negative Bouguer anomalies, and of subnormal Pn velocities in the upper mantle, resulting from convective mass and heat transfer into the base of the lithosphere. Doming, and in some instances faulting, is the result of isostatic adjustment to the resulting expansion. The crust of rifts is abnormally thin (70–80% of normal) and contains high‐density rocks assumed to be primarily intrusive basic igneous rocks. The collapse of rift lloors may be the consequence of stress‐induced lateral extension in the upper crust, possibly assisted by a mass excess of mafic intrusions in the rifted crust. Anomalous electrical conductivity (low) and heat flow (high) are the result of heating and magmatic and hydrothermal activity.

Gravity profiles across the Samail Ophiolite, Oman

Four hundred and sixty gravity stations have been established over the Samail ophiolite and adjacent units exposed in the Oman Mountains. The Bouguer anomaly pattern follows more or less parallel to ophiolite exposures. Along the coastal areas the Bouguer anomaly values range from +20 to +140 mGal and increase northward. Four gravity profiles across the ophiolite are correlated with known geology and structure. The main gravity profile extending 325 km northward from Ibra to Muscat and into the Gulf of Oman shows two positive Bouguer anomalies (+85 over the southern folded ophiolite nappe and +110 mGal near the shoreline at Muscat) that are associated with two nappes of tabular form. The nappes are nearly continuous, and there is no evidence that they are rooted in the mantle. The gravity high across the southern nappe is centered over the axis of the synform (maximum nappe thickness); its position over the gabbro‐peridotite contact rather than over the peridotite suggests extensive serpentinization of the peridotite. At the southern end of the profile, in the Ibra Dome area, the negative Bouguer anomaly (−10 to −15 mGal) is interpreted as an Infracambrian salt piercement similar to those in the Zagros fold belt. Geologic modeling of the main gravity profile provides a good correlation in terms of known thicknesses of the mappable ophiolite units. The nature of the gravity anomaly across the continental margin indicates a transition from ∼42‐km‐thick continental crust to a thinner (16–18 km) oceanic crust. The two gravity profiles in the north Oman region show a doublet of positive residual Bouguer anomalies (+50 to +90 mGal). The anomalies correlate well with imbricated ophiolite exposures, a result of low‐angle thrusting caused by strong compressive forces. In the absence of imbricate thrusting, the gravity profile in the central region has only one large positive residual Bouguer anomaly (+130 mGal). The anomalies of all the four profiles are further analyzed to elucidate the nappe‐transport distances.

Interpretation of African gravity and its implication for the breakup of the continents

To obtain a more quantitative interpretation of the ‘great negative Bouguer anomaly’ over Africa, a study is made of all the gravity data for the whole of the African continent. Histograms from 1°×1° means are presented for heights, free air, and Bouguer anomalies. The modal values are found to be 400 m, +70±5.1 gu (gravity unit; equal to 0.1 mGal or 10−6 m/s2) and −480 gu, respectively. These are considered to be the best estimates for Africa away from the regions of rifting and are used to construct a ‘standard African lithosphere’ with an assumed thickness of 100 km. The large‐scale Bouguer anomalies are then interpreted as being due to variations in the thickness of this assumed standard lithosphere rather than to the traditional compensation at the base of the crust. On this assumption, the relative density contrast at the lithosphere‐asthenosphere boundary is found to be −0.05. After corrections for the effects of light volcanics and sediments, a massive replacement of the lithosphere is required beneath much of the east African rift system. The interpretation requires: (1) a wide variation in lithospheric structure beneath different parts of the rift system, (2) that the lithosphere be thinnest beneath the eastern rift, and (3) that the thinning increases from south to north. The ‘lesser negative Bouguer anomaly’ associated with the Adrar‐Hoggar‐Tibesti‐Jebel Marra chain of volcanoes is treated in a similar way, and the replacement of the lithosphere is shown to be correspondingly less. Possible implications for the mechanism of rifting and break up of the continents are given.