Negative Gravity Anomalies Research Articles

Evolution of the intracratonic Officer Basin, central Australia: implications from subsidence analysis and gravity modelling

ABSTRACT The intracratonic basins of central Australia are distinguished by their large negative Bouguer gravity anomalies, despite the absence of any significant topography. Over the Neoproterozoic to Palaeozoic Officer Basin, the anomalies attain a peak negative amplitude in excess of 150 mGal, amongst the largest of continental anomalies observed on Earth. Using well data from the Officer and Amadeus basins and a data grid of sedimentary thicknesses from the eastern Officer Basin, we have assessed the evolution of these intracratonic basins. One‐dimensional backstripping analysis reveals that Officer and Amadeus basin tectonic subsidence was not entirely synchronous. This implies that the basins evolved as discrete geological features once the Centralian Superbasin was dismembered into its constituent basins. Two‐ and three‐dimensional backstripping and gravity modelling suggest that the eastern Officer Basin evolved from a broad continental sag into a region of intracratonic flexural subsidence from the latest Neoproterozoic, when flexure of the lithosphere deepened the northern basin. The results from gravity modelling improve when the crust is thickened beneath the northern margin of the basin and thinned at the southern margin, as has been suggested by recent deep seismic data. The crustal thickening beneath the basin's northern margin abuts the region of greatest topographic relief and is consistent with the observed structure at the edges of many orogenic belts. If the Officer Basin evolved as a foreland‐type basin from the late Proterozoic and has retained those features to the present, then one implication is that in the absence of any significant topography, cratonic lithosphere must be able to support stresses over very long periods of geological time.

Characteristics and development of the flexural forebulge and basal unconformity of Western Taiwan Foreland Basin

The Western Taiwan Foreland Basin formed during the Early Pliocene as the flexural response to the loading of Taiwan orogen on the Eurasian plate. Using more than 4000km of marine multi-channel seismic reflection profiles and 22 exploration wells offshore and on land, we recognized a major unconformity separating the overlying sediments of foreland basin from the underlying passive margin strata and a flexural forebulge located at the distal part of the foreland basin. Those allow us to refine and extend the area of the Western Taiwan Foreland Basin to the middle of the Taiwan Strait. The Western Taiwan Foreland Basin is about 350km long and 150km wide between the west front of the Taiwan orogen and the Chinese craton.Seismic lines show that a narrow zone of uplifted Miocene passive margin sequences at the distal parts of the foreland basin extends approximately parallel to the strike of the deformation front of the Taiwan orogen. The Miocene bulge is generally 200m high, a subtle morphologic/structural feature, and is characterized by an unconformity where Quaternary sediments overlie the eroded Miocene strata. Positive Bouguer gravity anomalies associated with the bulge on the cratonic side suggest that the Chinese continental margin is flexed upward to produce the bulge with higher density materials compared to the negative gravity anomalies of the sediments deposited in the adjacent foreland basin.The base of the foreland basin fills is a regional angular unconformity. The stratigraphic patterns of sediments overlying the unconformity change laterally, towards the Taiwan orogen, into conformity or disconformity of insignificant gap at the coastal areas. In contrast, the chronostratigraphic gap across the unconformity increases cratonward, with the largest gap at flexural forebulge. The increase in stratigraphic gap reflects the progressively younger transgression of the Pliocene–Quaternary orogenic sediments from Taiwan onto the Chinese passive margin combined with progressively older Miocene units subcropping towards the forebulge. The characteristic geometry, stratigraphic pattern and large gap up to 10Ma suggest that the basal unconformity is a flexural forebulge unconformity rather than a type 1 unconformity, mainly due to sea level change.The flexural forebulge and associated basal unconformity mark transition from shelf-slope-rise deposition on a passive margin to deposition in a foreland basin associated with a convergent margin. The flexural unconformity developed during the collision between the Luzon Arc and the Chinese continental margin, reflecting the uplift and erosion of progressively cratonward migrating forebulge.

Ancient geodynamics and global-scale hydrology on Mars.

Loading of the lithosphere of Mars by the Tharsis rise explains much of the global shape and long-wavelength gravity field of the planet, including a ring of negative gravity anomalies and a topographic trough around Tharsis, as well as gravity anomaly and topographic highs centered in Arabia Terra and extending northward toward Utopia. The Tharsis-induced trough and antipodal high were largely in place by the end of the Noachian Epoch and exerted control on the location and orientation of valley networks. The release of carbon dioxide and water accompanying the emplacement of approximately 3 x 10(8) cubic kilometers of Tharsis magmas may have sustained a warmer climate than at present, enabling the formation of ancient valley networks and fluvial landscape denudation in and adjacent to the large-scale trough.

Role of ocean‐continent contrast and continental keels on plate motion, net rotation of lithosphere, and the geoid

Three‐dimensional spherical shell models of mantle convection have been formulated to investigate the effects of ocean‐continent contrast (i.e., different viscosity for continental and oceanic lithospheres) and continental keels on plate motion, net rotation of lithosphere (i.e., degree 1 toroidal plate motion), and the geoid. The models include relatively realistic plate rheology (i.e., strong plate interiors and weak plate margins) and continental keel structure based on seismic models. The mantle flow in the models is driven by slab buoyancy. The models demonstrate that the ocean‐continent contrast and continental keels have minor effects on the long‐wavelength geoid (for degrees 2–5). However, continental keels overriding subducted slabs, for example in North America, can produce large negative gravity anomalies at a regional scale. This may have important implications to interpreting the gravity anomalies in North America. Our models show that when plates bounded by weak plate margins have the same thickness, neither weak plate margins nor the ocean‐continent contrast efficiently excite net rotation of lithosphere, although they excite toroidal motion at higher harmonic degrees. However, plate thickness variations like continental keels excite the net rotation. In spite of their ability to excite the net rotation, continental keels as thick as 300 km cannot provide the necessary coupling to the deep mantle to produce the observed net rotation of lithosphere and slow continental motion if there is a weak asthenosphere underlying them. If the thermomechanic structure of North American continental upper mantle derived from seismic and heat flow studies is representative, our models suggest that the temperature‐ and pressure‐dependent mantle rheology may not produce sufficiently high viscosity below continental keels that is needed to explain the observed plate motion. Our study hints the necessity to include more realistic treatment of plate boundaries in future studies to assess possible effects of plate‐plate coupling on plate motion.

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.

Detection of groundwater conduits in limestones with gravity surveys: data from the area of the Chicxulub Impact crater, Yucatan Peninsula, Mexico.

Small negative gravity anomalies are found in gravity data from along the northwestern shoreline of the Yucatan Peninsula. These anomalies are shown to be due to elongate, shallow anomalous porosity zones in the Tertiary carbonates. These zones are caused primarily by groundwater solution and are presently active conduits for groundwater flow. The association of these small gravity anomalies with known topographic and structural features of the area, which partially overlies the Chicxulub Impact crater, indicates their development was influenced by structures, faults and/or fractures, within the Tertiary and pre-Tertiary carbonates.

Granitic magma ascent and emplacement: neither diapirism nor neutral buoyancy

Abstract It is probable that granitic magma ascent does not result from the intrinsic properties of the magmas. Within the uppermost crust, neither the reduced viscosity nor the density contrast between magma and surroundings are themselves sufficient to induce either low-inertia flow (diapirism) or fracture-induced magma propagation (dyking). Igneous diapirism is intrinsically restricted to the lower, ductile crust. Dyking is therefore the most probable ascent mechanism for granitic magmas that reach shallow crustal levels. A neutral buoyancy level in the crust, at which magma ascent should stall, is never observed. This is demonstrated by coeval emplacement of magmas with different compositions and densities, and the negative gravity anomalies measured over many granitic plutons. We suggest that deformation, through strain partitioning, is necessary to magma ascent. Pluton formation is controlled by local structures and rock types rather than by intrinsic magma properties. As a result of its intermittent character, deformation (both local and regional) induces magma pulses, and this may have important consequences for the chemical homogeneity of intruded magmas.

Structure and geophysics of the Gåsborn granite, central Sweden: an example of fracture-fed asymmetric pluton emplacement

Abstract The emplacement mechanisms of the Palaeoproterozoic Gåsborn granite, a satellite of the Transscandinavian Igneous Belt (TIB), are investigated using an integrated structural and geophysical approach. The pluton is discordant to c . 1.89 Ga folded supracrustal rocks that were deformed and metamorphosed at c . 1.85–1.80 Ga during the Svecokarelian Orogeny. Emplacement occurred at a depth of c . 10 km, within a regime of late Svecokarelian dextral transpression. Deformation of the pluton during cooling resulted in the formation of a variably developed foliation in the granite and deflection of less competent wall-rock units around its western and eastern contacts. Later E-W Sveconorwegian shortening resulted in the formation of shear zones that affect one of the pluton margins and may contribute to a component of the observed wall-rock distortion. The granite is situated above strong NNW-trending linear magnetic and negative gravity anomalies, which are interpreted to correspond to an important early Svecokarelian shear zone. The geophysical data indicate that the pluton is markedly asymmetric and modelling of the residual gravity field suggests that it consists of a deep root zone in the west and a thin sill-like body, which makes up most of the east and south parts of the body. Emplacement of the sill-like part occurred by lateral flow of magma from the root zone accommodated by downwarping of the underlying units. Intrusion of the thicker, discordant west part may have been accommodated by a combination of roof lifting and floor depression, aided by displacement on an active shear zone.

Density structure of the Northeast German basin: 3D modelling along the DEKORP line BASIN96

The key for understanding the dynamics of the Northeast German basin is the knowledge of its present-day structures. Our studies are focused on the complex geometry and evolution of this basin by the aid of numerical models. To support this task, it was necessary to consider and integrate all available geoscientific information. Based on borehole data, depth maps and on results of isostatic modelling we derived a 3D model of the basin structure. A smoothed map of the regional gravity field (after Grosse and Conrad, 1990) completed the data base. By means of 3D gravity modelling, the initial model structure was modified to fit in the geophysical data set.An important characteristic of the Northeast German Basin is the presence of Upper Permian Zechstein salt. The salt domes and walls related to the postdepositional mobilisation of the salt layer cause pronounced negative gravity anomalies. This effect is considered as a central problem in this study. In order to investigate possible causes of gravimetric anomalies, we studied the influence of different crustal depth levels. We applied a 3D gravity stripping approach to eliminate the gravimetric effects caused by sedimentary fill of the basin and to separate density anomalies within the sedimentary fill from the influence of deeper levels in the crystalline crust. Complementary, we calculated the downward continuation of the gravimetric field to the basin floor and compared the outcome with the results obtained by 3D stripping. The good fit between the calculated gravity anomalies and the measured anomalies confirms the applicability of the approach. Additionally, we interpreted the Bouguer anomalies in terms of crustal density distribution and discuss the model concerning its consistency with other geophysical data considering the first results of the seismic reflection experiment DEKORP BASIN96.

Three-dimensional P-wave velocity structure in the upper crust beneath Kuju Volcano, central Kyushu, Japan

Kuju Volcano lies near Aso Caldera at the center of Kyushu Island, western Japan. After a few hundred years of dormancy, a phreatic explosion accompanied by a small ash eruption occurred on 11 October 1995. This study was undertaken to determine the subsurface seismic velocity structure associated with the active magmatic regime in the Kuju volcanic region. The three-dimensional, upper crustal, P-wave velocity structure beneath Kuju Volcano was determined using methods for the simultaneous inversion of P-wave arrival times from local earthquakes in and around the Kuju volcanic region for velocities and hypocentral parameters. Results reveal two shallower low-velocity anomalies located in the northern and southern parts of Kuju Volcano, consistent with the presence of significant negative Bouguer gravity anomalies. In addition, a high-velocity anomaly is located approximately 5 km northwest of Mt. Kuju, one of the domes in Kuju Volcano. Beneath this high-velocity anomaly, a low-velocity anomaly is present. This velocity structure suggests a magmatic regime that has a lid consisting of cooled solid material overlying a chamber of partially molten material.

An integrated modelling study of the central and northern Baikal rift: evidence for non-uniform lithospheric thinning?

We employ a kinematic model of continental extension to assess the lithospheric structure across the Baikal rift zone. The model includes simple shear extension of the upper crust along detachment faults and independent pure-shear thinning of the lower crust and mantle. Model predictions of topography, gravity anomalies and surface heat flow are compared with observations along two sections, one crossing the central Baikal rift and the other across northern Baikal. For the central Baikal profile, topography and gravity anomaly patterns are fit by a uniform extension model: upper crustal extension of approximately 19 km is equalled by a predicted mantle thinning factor δ < 2. In contrast, the high flank topography and strongly negative gravity anomalies across the northern Baikal basin require the approximately 12 km of crustal extension there to be accompanied by much higher mantle thinning factors (δ = 4–6). A model of non-uniform mantle thinning below northern Baikal is supported by P-T estimates for Miocene and Quaternary lherzolite xenoliths from the Vitim volcanic field. The rate of extension has a minor influence on the predicted topography and gravity anomaly patterns. None of the models predicts a significant (> 10 mW m −2) increase in regional surface heat flow. The short-wavelength, large-amplitude variability of the observed surface heat flow requires crustal redistribution processes involving large-scale ground water flow. The modelled topography and gravity anomaly patterns suggest that a possible asthenospheric upwarp is more likely to be situated below the northern than below the central Baikal basin, in contrast with earlier inferences from teleseismic and gravity studies.

Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous

Multichannel seismic reflection, gravity, magnetic, and bathymetric investigations were made along five latitudinal profiles from the eastern shelf of India to the Andaman shelf and four NW‐SE profiles in the Western Basin off Madras (seismics on three latitudinal profiles) in the Bay of Bengal. The trend of the fracture zones, the locations of the magnetic chron 34, and the Cretaceous Magnetic Quiet Zone suggest that Greater India separated from Antarctica after a period of transform motion in the early Cretaceous, that is, about polarity chron M0 (120 Ma) and drifted northwestward. Negative gravity anomalies are associated with basement rises including the 85°E Ridge between the continental margin of India and the Ninetyeast Ridge. Magnetic reversals and northward trend of the 85°E Ridge support a hotspot origin of the ridge and its emplacement most likely after the Cretaceous quiet period. Juxtaposition of high‐amplitude hyperbolic reflections, down‐faulted continental blocks buried under thick sediments, and associated gravity and magnetic anomalies mark the boundary between continental and oceanic rocks at the foot of continental slope, about 80 km seaward of the present continental shelf edge. Eight seismic sequences, as thick as 8.5 km, overlie the early Cretaceous oceanic basement and include four unconformities (lower Eocene, upper Oligocene, upper Miocene, and upper Pleistocene) which have been correlated to the major geologic/tectonic events. We also interpret the late Cretaceous/early Tertiary features on the eastern flank of the buried 85°E Ridge as carbonate reefs. The observation of steep subduction of older (cold) Indian plate beneath the Burmese plate near the Andaman Islands suggests the Sunda Arc in this region as low to intermediate stress subduction zone.

Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model

Research Article| October 01, 1996 Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model Max Rohrman; Max Rohrman 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Peter van der Beek Peter van der Beek 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Search for other works by this author on: GSW Google Scholar Author and Article Information Max Rohrman 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Peter van der Beek 1Faculty of Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1996) 24 (10): 901–904. https://doi.org/10.1130/0091-7613(1996)024<0901:CPDUON>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Max Rohrman, Peter van der Beek; Cenozoic postrift domal uplift of North Atlantic margins: An asthenospheric diapirism model. Geology 1996;; 24 (10): 901–904. doi: https://doi.org/10.1130/0091-7613(1996)024<0901:CPDUON>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Postrift domal uplifts are a distinct feature of the northern North Atlantic rifted margins. Uplifted regions (southern Norway, Finnmark, Svalbard, northern United Kingdom) occur with a typical spacing of ∼ 900 km. The amount of domal uplift is substantial (1–2 km); thermochronological, geomorphological, and seismic stratigraphic data suggest a coincident Neogene onset of uplift, postdating North Atlantic breakup and predating onset of glaciation. The Norwegian domes are characterized by strongly negative gravity anomalies and reduced mantle P-wave and S-wave velocities. Previous models for uplift around the North Atlantic (e.g., glacial erosion, intraplate stresses, secondary convection) fail to explain the above characteristics. We present an asthenospheric diapir model based on a Rayleigh-Taylor instability, wherein a hot “Icelandic” asthenosphere layer meets cold cratonic lithosphere. The generated asthenospheric diapirs and associated partial melting offer a viable explanation for updoming. Application of this model to the south Norwegian dome succesfully explains most of the currently available data. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Natural and human-induced sinkholes in gypsum terrain and associated environmental problems in NE Spain

The central Ebro Basin comprises thick evaporite materials whose high solubility produces typically karstic landforms. The sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground-penetrating radar (GPR) on stream terraces, as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928, 1957, and 1985. The sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development. On stream terraces and cover pediments the most commonly encountered dolines are bowl-shaped in their morphology with both diffuse and scarped edges. In contrast, dolines developed in the gypsiferous silt infilled valleys have a funnel and well-shaped morphology. The diffuse-edged bowl-shaped dolines are developed through the progressive subsidence of the alluvial cover, due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves, resulting in a decrease alluvial density. Future compaction of the alluvial cover will produce surface subsidences. This type of dolines are associated with negative gravity anomalies. In contrast, the scarped-edge dolines are formed by the sudden collapse of a cavity roof. The cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping, and they may subsequently collapse. In this instance, the cavities can be detected by both gravity and GPR anomalies where the voids are not deeper than 4–5 m from the surface. These processes forming sinkholes can be enhanced by man-induced changes in the groundwater hydrologic regime by both inflows, due to irrigation, ditch losses, or pipe leakages, and by outflows from pumping activities.

Semi-quantitative geophysical investigation of permafrost in northern Québec

Both continuous and discontinuous permafrost occur in northern Québec. In contrast with permafrost in western Canada and Alaska, permafrost in eastern Canada contains no massive ice because of the regionally different climatic conditions and soil compositions. Site investigations related to infrastructures such as building foundations, airports, and roads frequently require a knowledge of the spatial distribution of ice content in permafrost since differential thawing can significantly affect the mechanical behavior of the soil. Delineation of permafrost ice content in northern Québec is difficult because of its diffusion into the various unconsolidated deposits. Electrical and gravity surface surveys are appropriate methods for determining the horizontal and vertical extent of such permafrost conditions because the resistivity, IP chargeability, and density parameters are directly affected by the volume ice content and the amount of unfrozen water in the permafrost. This integrated surface geophysical approach was applied at three test sites in the Kangiqsualujjuaq (George River) area to yield a semi-quantitative volumetric estimate rather than a discrete spatial distribution of the excess ice content. The selected test sites at which this integrated method was demonstrated are representative of continental and supratidal cryogenic mounds of different dimensions in which negative residual gravity anomalies, large electrical resistivity contrasts, and near-zero IP chargeability were combined to delineate and characterize the excess ice zones. The relative magnitudes of these parameters are interpreted to yield a semi-quantitative estimate of the ice content in the anomalous regions. Test drill holes at the surveyed sites indicate that the integrated approach can provide useful geotechnical assessments of the integrity of foundations and other structures subject to permafrost environments.

Possible Neoproterozoic-early Paleozoic grabens in Mississippi, Alabama, and Tennessee

Low-intensity magnetic and negative gravity anomalies in northern Mississippi, north-western Alabama, and western Tennessee are viewed as being similar to the magnetic and gravity low associated with the Mississippi Valley graben. The anomalies are interpreted as shallow grabens, and it is proposed that the grabens, like the Mississippi Valley graben, resulted from northwest-southeast extension during Neoproterozoic-early Paleozoic fragmentation of Laurentia. If real, the grabens indicate that extension affected a larger part of Laurentia than has been previously documented. They imply hitherto unsuspected oil and gas potential and, if the area is coupled to the stress field affecting the Mississippi Valley graben, represent structures that warrant evaluation for seismic risk.

Gravity modelling of extensional basins in Southeast Asia

Comparison of free-air gravity over sedimentary basins in Southeast Asia with sediment accumulations in the basins indicates no correlation between gravity and sediment thickness_ This is due to differences in crustal structure under basins in extensional versus convergent regimes. In convergent regimes, thickened crust creates negative gravity anomalies. In extensional regimes, the crust is thinned creating both positive and negative gravity anomalies. We examine the problem of gravity modelling in extensional regimes, using the Malay basin as an example. We find: 1. while gravity modelling is a nonunique process, the use of a priori knowledge (i.e., prior information derived from other sources) in gravity models greatly reduces the range of possible models. In the Malay basin, a priori knowledge indicates the basin should have a -100 mGal anomaly. However, only a -20 mGal anomaly is seen over the basin. This suggests there is a high density intrusion at the base of the crust, producing an offsetting positive anomaly. 2. the gravity field is only a minor constraint in gravity modelling over extensional basins. This is due to the almost complete cancellation of the positive anomaly associated with the high density intrusion at the base of the crust and the negative anomaly associated with the sedimentary basin in the upper crust. 3. gravity modelling can produce useful geologic information if deep crustal structure is accounted for before attempting to model shallow crustal features. This can best be accomplished with the use of a priori knowledge in the modelling process.

Isotopic composition of helium, and CO 2 and CH 4 contents in gases produced along the New Zealand part of a convergent plate boundary

New Zealand straddles an active tectonic boundary between the Indo-Australian and Pacific plates. To the NE and SW oblique convergence of oceanic and continental crusts leads to the establishment of subduction zones; in the center continental crusts collide along a transform boundary. With regard to mantle degassing, and on the basis of chemical and He isotopic analyses of 140 samples from all over New Zealand, four major environments can be distinguished: 1. (1) R/ R A values approaching MORB values are observed in areas of andesitic and rhyolitic volcanism and high-temperature geothermal activity over the center of the North Island. C 3He ratios there vary from <10 × 10 9, typical of volatiles released from the mantle, in the W, to >40 × 10 9, suggesting considerable addition of CO 2 from other than mantle sources, in the E. 2. (2) In areas of recent igneous activity, residual mantle He is extracted from rocks through hydrothermal alteration by percolating groundwater. The gases are low in CO 2 due to conversion to carbonate; any CH 4 present is generated within the crust. C 3He ratios are generally well below those of any magmatic vapor. 3. (3) In the absence of recent igneous activity, but in areas of increased seismicity, mantle He may reach the surface along fractures provided by the movement of the subducting slab in forearc regions or by dilatancy pumping or fault-valving in highly compressional, but seismically active parts of an orogenic plate boundary. 4. (4) Mantle degassing is greatly impeded in areas of crustal thickening as indicated by negative gravity anomalies, low seismicity and rapid uplift. There 3He 4He ratios approach typical crustal values of <0.1 R A . Except in areas of present-day volcanic and geothermal activity, production, transport, storage, and release processes of 3He, 4He, CO 2, and CH 4 appear to be effectively decoupled.

Variation in cross‐sectional area of the axial ridge along the East Pacific Rise: Evidence for the magmatic budget of a fast spreading center

Along the fast and ultrafast spreading East Pacific Rise, the cross‐sectional area of the axial ridge varies significantly over length scales similar to its morphologic segmentation. Using an automated method, we measure the ridge area (volume per kilometer along axis) where there is complete bathymetric coverage. Along 3500 km of the northern and southern East Pacific Rise, our two study areas encompass numerous transform faults, large overlapping spreading centers, small overlapping spreading centers, and smaller discontinuities (first‐, second‐, third‐, and fourth‐order discontinuities). The cross‐sectional area variation mimics the undulation of the ridge crest depth; local area maxima occur toward the middle of segments, and the axial area decreases by 40% or more at first‐ and second‐order discontinuities. Third‐order discontinuities are generally marked by smaller disruptions in the ridge area, and fourth‐order offsets do not systematically correspond with features of the cross‐sectional area profile. The correlation between shallower ridges and larger ridge areas breaks down at some locations because axial cross‐sectional area represents a longer term average of the ridge's magmatic state than axial depth. A correlation between large ridge areas and negative residual gravity anomalies indicates that inflated ridges are underlain by low‐density crust and mantle. Also, a correlation between larger area and higher MgO content of axial basalts suggests that inflated areas generally erupt hotter magmas which are presumably supplied more rapidly to the neovolcanic zone. The cross‐sectional area of the axial ridge appears to correlate with the width of the axis‐centered low‐velocity zone in the crust. These observations, as well as the absence of large, relict axial ridges off‐axis, indicate that the axial ridge originates from buoyancy due to thermal expansion and the presence of melt in the crust and mantle within about 10 km of the rise axis. Portions of the northern East Pacific Rise underlain by a magma chamber reflector generally occur where the ridge cross‐sectional area is greatest; this supports the connection between processes which inflate the axial ridge and those which heat the crust and upper mantle and produce melt. Thus, while the axial high on fast spreading centers resembles a constructional volcano in cross section, it is more like a long, narrow balloon whose cross‐sectional area is a sensitive indicator of magma supply. Using this relationship, we predict with 75% confidence that at least 45% of the unsurveyed northern East Pacific Rise (18°N to 13°N and 9°N to 5°N) and at least 60% of the southern East Pacific Rise (4°S to 23°S) is underlain by a magma chamber reflector.

The West and Central African rift systems: Foreword

This chapter discusses about the West and Central African rift systems. The recognition of these intracontinental Cretaceous rift systems were based on the extensive gravity data base which exists for this region and on data made available by the Petroleum industry for many of the rift basins. This permitted delineating the extensional and strike-slip zones through West, Central, and North Africa. This was helped by understanding that the negative gravity anomalies associated with the sedimentary basins, forming part of this rift system, were superimposed on broad positive anomalies, which are up to 450 km wide and have amplitudes of up to 80 mGal. The spatial and temporal link between the West and Central African rift systems and plate tectonic processes within the Atlantic Ocean was recognized to indicate that their is a strong interaction between oceanic and intra-continental tectonics.