Mantle Bouguer Anomaly Research Articles

Variation of crustal thickness in the Philippine Sea deduced from three‐dimensional gravity modeling

Abstract Crustal thickness of the northern to central Philippine Sea was gravimetrically determined on the simple assumption of four layers: seawater, sediments, crust and lithospheric mantle, with densities of 1030, 2300, 2800 and 3300 kg/m3, respectively. As for the correction of the regional gravity variation, a 15 km difference of the lithospheric thickness with a density difference of 50 kg/m3 against the asthenosphere below between both sides of the Kyushu‐Palau Ridge was taken into consideration. Mantle Bouguer anomalies were calculated on the assumption of constant crustal thickness of 6 km, and then the crustal thickness was obtained by three‐dimensional gravity inversion method. The results show occurrence of thin crust areas with a thickness of approximately 5 km in the southern part and at the western margin of the Shikoku Basin and also of thick crust areas in the northwestern and northeastern parts of the Parece Vela Basin. We suggest that these are because of the variation of magma supply at the time of sea floor spreading in the Shikoku and Parece Vela Basins, which is possibly related to the variation of spreading rate and enhanced magmatism near the past arc volcanic fronts. The results further show the occurrence of crust thinner than 5 km in the northeastern part of the West Philippine Basin, of crust thicker than 15 km in the Amami Plateau, the Daito and Oki‐Daito Ridges, and also in the northern part of Kyushu‐Palau Ridge, whereas the southern part of the Kyushu‐Palau Ridge the crust is thicker than 10 km. It was also inferred that small basins in the Daito Ridge province have the thinnest oceanic crust of less than 5 km in the Kita‐Daito Basin.

New structural and geochemical observations from the Pacific‐Antarctic Ridge between 52°45′S and 41°15′S

We investigated the morphology and structure of the Pacific‐Antarctic Ridge between 52°45′S and 41°15′S during the Pacantarctic2 cruise using multibeam echosounder together with gravity measurements and dredges. Analysis of the bathymetric, gravity and geochemical data reveal three ridge segments separated by overlapping spreading centers south of the Menard transform fault (MTF) and five segments north of it. Calculation of the cross‐sectional area allows quantification of the variation in size of the axial bathymetric high. Together with the calculation of the mantle Bouguer anomaly, these data provide information about variations in the temperature of the underlying mantle or in crustal thickness. Areas with hotter mantle are found north and south of the MTF. Geochemical analyses of samples dredged during the survey show a correlation of high cross‐sectional area values and negative mantle Bouguer anomalies in the middle of segments with relatively less depleted basalts.

The isostatic compensation of the Azores Plateau: A 3D admittance and coherence analysis

The compensation of the Azores Plateau is re-examined by means of a 3D admittance study involving computations of the admittance between the bathymetry and (1) the geoid, (2) the free air gravity and (3) the mantle Bouguer anomalies. The geoid to bathymetry relationship, rather than being analysed in the space domain, is studied in the spectral domain. In this way, the information related to the flexural response of the lithosphere is retained in the geoid to bathymetry signal and can be compared to that of the gravity to bathymetry. We find that the anomalously shallow depths of the Azores Plateau are not due to dynamic forces sustained by mantle upwelling. The Plateau is supported by a thickened crust, which mainly results from large volumes of accreted extrusives and consequent deflection of the underlying elastic plate. Both the free air gravity and the mantle Bouguer admittance point to a flexural isostatic model with a Moho depth of 12 km and an elastic thickness in the range of 3–6 km. However, the analysis of the coherency between bathymetry and the mantle Bouguer anomaly indicates that buoyant material at the base of the crust partially accounts for the uplift of the Plateau. In our best-fitting model, the average elastic thickness is 4 km and the amplitude of the buoyant load at the Moho is 1 / 3 of that of the surface volcanic load. Buoyancy forces at the Moho may arise from underplated material resulting from subcrustal plutonism.

Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies

We have compiled extensive gravity and bathymetry data for the whole Mariana Trough, which were collected during several Japanese scientific cruises over the last few years. This study aims to clarify the lateral distribution of the local differences in geochemical signatures, which have been observed locally in the Mariana Trough. Shipboard free‐air gravity anomaly data from eight Japan Agency for Marine‐Earth Science and Technology (JAMSTEC) cruises were compiled with those crossover errors of 2.85 mgal. Mantle Bouguer anomalies (MBA) were calculated by subtracting the predictable gravity signal due to the seawater/crust and crust/mantle density boundaries. The crustal thickness variation along the spreading axis was estimated from the MBA. Different features in crustal thickness, its variation, and segment length for each segment, allow us to identify four distinct regional differences in magmatic activity along the spreading axis of the Mariana Trough. Segment in region A (to the north of 20°35′N) shows the largest sectional dimensions of crust along the axis and it is probably affected by an additional supply from island arc magma sources. A variety of crustal thickness values and of along‐axis crustal thickness variations in region B (between 15°38′N and 20°35′N) suggests two types of segments. One is similar to a slow spreading ridge segment that has a plume‐like mantle upwelling under the spreading axis, and the other is a magma‐starved segment. Region C (between 14°22′N and 15°38′N) is a less magmatic region (individual crustal thickness averages of 3.4–4.1 km). Region D (to the south of 14°22′N) has higher individual crustal thickness averages of 5.9–6.9 km, suggesting higher magmatic activity with a sheet‐like mantle upwelling under the spreading axis. Different features in the MBA for off‐axis areas suggest that these four regions have existed since the Mariana Trough started spreading. Moreover, comparison between our results of crustal thickness and previous geochemical results indicates that less magmatic spreading segments with thin crust, which are locally distributed in both regions B and C, probably result from mantle source depleted of water and incompatible elements. This suggests that lateral compositional variation of water and incompatible elements exists on a segment scale in the mantle source beneath the spreading axis of the Mariana Trough.

Variations in upper crustal structure due to variable mantle temperature along the Southeast Indian Ridge

There is a systematic variation in axial morphology and axial depth along the Southeast Indian Ridge (SEIR) with distance away from the Australian Antarctic Discordance, an area of cold uppermost mantle. Since spreading rate (72–76 mm/yr) and mantle geochemistry appear constant along this portion of the SEIR, the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature. In this study, we report results from a multichannel seismic investigation of on‐axis crustal structure along this portion of the SEIR. Three distinct forms of ridge crest morphology are found within our study area: axial highs, rifted axial highs, and shallow axial valleys. Axial highs have a shallow (∼1500 m below seafloor (bsf)) magma lens and a thin (∼300 m) layer 2A along the ridge crest. Rifted axial highs have a deeper (∼2100 m bsf) magma lens and thicker (∼450 m) layer 2A on‐axis. Beneath shallow axial valleys, no magma lens is imaged, and layer 2A is thick (∼450 + m). There are step‐like transitions in magma lens depth and layer 2A thickness with changes in morphology along the SEIR. The transitions between the different modes of axial morphology and shallow structure are abrupt, suggesting a threshold‐type mechanism. Variations in crustal structure along the SEIR appear to be steady state, persisting for at least 1 m.y. Portions of segments in which a magma lens is found are characterized by lower relief abyssal hills on the ridge flank, shallower ridge flank depths, and at the location of along‐axis Mantle Bouguer Anomaly (MBA) lows. The long‐wavelength variation in ridge morphology along the SEIR from axial high segments to the west to axial valley segments to the east is linked to the regional gradient in mantle temperature. Superimposed on the long‐wavelength trend are segment to segment variations that are related to the absolute motion of the SEIR to the northeast which influence mantle melt production and delivery to the ridge.

Crustal structure and rift flank uplift of the Adare Trough, Antarctica

The Adare Trough, located 100 km northeast of Cape Adare, Antarctica, represents the extinct third arm of a Tertiary spreading ridge between East and West Antarctica. It is characterized by pronounced asymmetric rift flanks elevated up to over 2 km above the trough's basement, accompanied by a large positive mantle Bouguer anomaly. On the basis of recently acquired seismic reflection and ship gravity data, we invert mantle Bouguer anomalies from the Adare Trough and obtain an unexpectedly large oceanic crustal thickness maximum of 9–10.5 km underneath the extinct ridge. A regional positive residual basement depth anomaly between 1 and 2.5 km in amplitude characterizes ocean crust from offshore Victoria Land to the Balleny Islands and north of Iselin Bank. The observations and models indicate that the mid/late Tertiary episode of slow spreading between East and West Antarctica was associated with a mantle thermal anomaly. The increasing crustal thickness toward the extinct ridge indicates that this thermal mantle anomaly may have increased in amplitude through time during the Adare spreading episode. This scenario is supported by a mantle convection model, which indicates the formation and strengthening of a major regional negative upper mantle density anomaly in the southwest Pacific in the last 50 million years. The total amount of post‐26 Ma extension associated with Adare Trough normal faulting was about 7.5 km, in anomalously thick oceanic crust with a lithospheric effective elastic thickness (EET) between 3.5 and 5 km. This corresponds to an age between 3 and 5 million years based on a thermal boundary layer model and supports a scenario in which the Adare Trough formed soon after spreading between East and West Antarctica ceased, confined to relatively weak lithosphere with anomalously thick oceanic crust. There is little evidence for major subsequent structural activity in the Adare trough area from the available seismic data, indicating that this part of the West Antarctic Rift system became largely inactive in the early Miocene, with the exception of minor structural reactivation which is visible in the seismic data as offsets up to end of the early Pliocene.

Correlated helium, neon, and melt production on the super-fast spreading East Pacific Rise near 17°S

We report new helium and neon isotopic compositions in dredged basalt glasses from the superfast spreading East Pacific Rise (EPR) between 13° and 23°S. The 3He/ 4He ratios vary from 8.0 to 11.0 times the atmospheric value (Ra) [ 4He/ 3He between 65,700 and 90,300], with the least radiogenic values in samples from near 16.75°S. Atmospheric contamination corrections on neon isotopes are made using step heating and extrapolated 21Ne/ 22Ne ratios (to a solar 20Ne/ 22Ne value of 13.8). The lowest corrected 21Ne/ 22Ne ratios are found near 16.75°S, and are consistent with the helium isotopes in suggesting a less degassed mantle source. The EPR at 17°S is unusual in displaying such isotopic anomalies in the absence of any known hotspot. The minima in 4He/ 3He and 21Ne/ 22Ne correspond to extremes in Sr, Nd and Pb isotope ratios, but the length scale of the helium and neon anomalies are shorter along-axis than the peaks for the other isotopic ratios. The minimum in 4He/ 3He is observed from 16° to 18°S (a distance of ∼220 km), whereas the elevated Sr and Pb values, and lower 143Nd/ 144Nd, are observed between 16° and 20.7°S (a distance of ∼500 km); neon isotope anomalies are observed on an intermediate length scale along-axis. Unradiogenic helium and neon values correlate with low mantle Bouguer anomalies, and maxima in the axial cross-sectional area, all of which are attributed to higher melt production near 17°S. A conceptual model that fits the observations includes melting of an entrained heterogeneity beneath the EPR at 17 °S. The relationship between helium and neon isotopes and along-axis geophysical characteristics (and by inference melt distribution) is probably related to the extreme incompatibility of helium and neon. High concentrations of 3He and 21Ne (extrap) are found in the glasses near 17°S, which is consistent with higher noble gas concentrations in the anomalous mantle.

Interaction between the Mid‐Atlantic Ridge and the Azores hot spot during the last 85 Myr: Emplacement and rifting of the hot spot‐derived plateaus

Multiple‐ and single‐beam bathymetric data are compiled over the Azores plateau to produce a 1 km × 1 km grid between latitudes 32°N and 49°N and longitudes 22°W and 43°W. Mantle Bouguer anomalies are then calculated from this grid and the satellite‐derived gravity. These grids provide new insights on the temporal and spatial variations of melt supply to the ridge axis. The elevated seafloor of the Azores plateau is interpreted as resulting from the interaction of a mantle plume with the Mid‐Atlantic Ridge (MAR). The presence of a large region of elevated seafloor associated with a thick crust between the Great Meteor Seamounts and the Azores platform on the Africa plate, and less developed conjugate structures on the North America plate, favors genetic relations between these hot spot‐derived structures. This suggests that a ridge‐hot spot interaction has occurred in this region since 85 Ma. This interaction migrated northward along the ridge axis as a result of the SSE absolute motion of the Africa plate, following a direction grossly parallel to the orientation of the MAR. Kinematic reconstructions from chron 13 (∼35 Ma) to the present allow a proposal that the formation of the Azores plateau began around 20 Ma and ended around 7 Ma. A sharp bathymetric step is associated with the beginning of important melt supply around 20 Ma. The excess of melt production is controlled by the interaction of the ridge and hot spot melting zones. The geometry and distribution of the smaller‐scale features on the plateau record episodic variations of the hot spot melt production. The periodicity of these variations is about 3–5 Myr. Following the rapid decrease of widespread volcanism, the plateau was subsequently rifted from north to south by the Mid‐Atlantic Ridge since 7 Ma. This rifting begins when the MAR melting zone is progressively shifted away from the 200‐km plume thermal anomaly. These results bear important consequences on the motion of the Africa plate relative to the Azores hot spot. They also provide an explanation to the asymmetric geochemical signature of the Azores hot spot along the Mid‐Atlantic Ridge.

Axial magnetic anomalies over slow-spreading ridge segments: insights from numerical 3-D thermal and physical modelling

The axial magnetic anomaly amplitude along Mid-Atlantic Ridge segments is systematically twice as high at segment ends compared with segment centres. Various processes have been proposed to account for such observations, either directly or indirectly related to the thermal structure of the segments: (1) shallower Curie isotherm at segment centres, (2) higher Fe-Ti content at segment ends, (3) serpentinized peridotites at segment ends or (4) a combination of these processes. In this paper the contribution of each of these processes to the axial magnetic anomaly amplitude is quantitatively evaluated by achieving a 3-D numerical modelling of the magnetization distribution and a magnetic anomaly over a medium-sized, 50 km long segment. The magnetization distribution depends on the thermal structure and thermal evolution of the lithosphere. The thermal structure is calculated considering the presence of a permanent hot zone beneath the segment centre. The 'best-fitting' thermal structure is determined by adjusting the parameters (shape, size, depth, etc.) of this hot zone, to fit the modelled geophysical outputs (Mantle Bouguer anomaly, maximum earthquake depths and crustal thickness) to the observations. Both the thermoremanent magnetization, acquired during the thermal evolution, and the induced magnetization, which depends on the present thermal structure, are modelled. The resulting magnetic anomalies are then computed and compared with the observed ones. This modelling exercise suggests that, in the case of aligned and slightly offset segments, a combination of higher Fe-Ti content and the presence of serpentinized peridotites at segment ends will produce the observed higher axial magnetic anomaly amplitudes over the segment ends. In the case of greater offsets, the presence of serpentinized peridotites at segment ends is sufficient to account for the observations.

Crustal thickness in Antarctica from CHAMP gravimetry

CHAMP, flying at an altitude of about 400 km, is the first of a new generation of satellites dedicated to Earth gravity field observation. The high-quality data have generated new gravity field models: EIGEN-1S in 2001, and EIGEN-2S more recently. The gravitational potential is decomposed into spherical harmonic coefficients and in this study we use the free air gravity anomalies reconstituted up to degree 60, at zero altitude. The anomalies for the Antarctic continent range from −57 to 65 mGal. We have modeled the gravity effect from the ice, the ocean and the bedrock, using a 666 km cut-off filter to simulate the resolution obtained by CHAMP. Computing the differences between this terrain effect and the CHAMP map provides a map of the Bouguer anomalies. Because of the dominant influence of the crust, we first used a crustal thickness model from seismology. This gives a map of the mantle Bouguer anomalies, the range of which is still large (between −255 and 216 mGal) indicating imperfections in the crust model. By appealing to isostasy we then imposed the condition that this mantle Bouguer anomaly should vanish and therefore solve for a new resulting crustal thickness. This gravity-based crust model gives thicknesses from 8.5 to 42.6 km in the zone of interest. There is a good general agreement with seismological models, but our models shows more detail, particularly in the western part of the continent. These details are in agreement with geological studies.

Spatial and temporal distribution of seismicity along the northern Mid‐Atlantic Ridge (15°–35°N)

A detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern Mid‐Atlantic Ridge between 15° and 35°N is presented. Two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones. The seismically active zone extends ∼20 km to either side of the ridge axis, consistent with earlier results from studies of fault morphology. Along the axis, hydrophone‐recorded activity shows important variations. Areas with intense and persistent seismic activity (stripes) stand in sharp contrast to areas that lack seismicity (gaps). The regions of persistent activity are a new observation at mid‐ocean ridges. In general, the patterns of seismically active/inactive regions are also recognized in the 28‐year teleseismic record, implying that these patterns are maintained at timescales between a few years and a few decades. We find no simple relationship between individual segment variables (e.g., length or trend of the segment, maximum offset of discontinuities, or along‐axis change in mantle Bouguer anomaly (MBA) and water depths) and number of hydrophone‐recorded events. There does appear to be a correlation between axial thermal structure and seismicity. Regions of low and high numbers of events would thus correspond to thinner (hotter) and thicker (colder) lithosphere, respectively. Seismicity may reflect thermal structure at short timescales (decadal or longer), while relief and inferred crustal thickness may integrate this structure over longer periods of time (order of 1 Myr).

Morphology and tectonics of the Mid‐Atlantic Ridge, 7°–12°S

We present swath bathymetric, gravity, and magnetic data from the Mid‐Atlantic Ridge between the Ascension and the Bode Verde fracture zones, where significant ridge–hot spot interaction has been inferred. The ridge axis in this region may be divided into four segments. The central two segments exhibit rifted axial highs, while the northernmost and southernmost segments have deep rift valleys typical of slow‐spreading mid‐ocean ridges. Bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since ∼1 Ma. Mantle Bouguer anomalies (MBAs) derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes, suggesting that mantle flow has been established beneath the new spreading centers for at least ∼1 Myr. Inversion of gravity data indicates that crustal thicknesses vary by ∼4 km along axis, with the thickest crust occurring beneath a large axial volcanic edifice. Once the effects of lithospheric aging have been removed, a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis‐parallel seismic line than a model that includes additional along‐axis variations in mantle temperature. Both geophysical and geochemical data from the region may be explained by the melting of small (<200 km) mantle chemical heterogeneities rather than elevated temperatures. Therefore, there may be no Ascension/Circe plume.

Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system

Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian–Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85–100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

The Southwest Indian Ridge between 49°15′E and 57°E: focused accretion and magma redistribution

Bathymetric, gravity, magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow-spreading Southwest Indian Ridge (SWIR, full rate 14 mm/yr) between 49°15′E and 57°E. This analysis reveals that the segmentation defined by along-axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along-axis variations of backscattering strength, mantle Bouguer anomaly (MBA) and amplitude of the central magnetic anomalies (CMA). At axial discontinuities with offsets larger than 15 km, thin crust and reduced volcanic production are suggested by the occurrence of MBA highs, almost non-existent CMA and 50% lower backscattering strength relative to the segment centres. By contrast, smaller non-transform discontinuities, with offsets smaller than 15 km, correspond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities. These small axial discontinuities bound low-relief bathymetric segments (500–700 m), corresponding to weak or no MBA lows (amplitude <11 mGal), and robust high-relief segments (>1000 m), corresponding to large MBA lows (amplitude >30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as melt migration and/or crustal plumbing from adjacent high-relief segments. Pronounced MBA lows at high-relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle. These spreading cells are spaced by about 100 km along the SWIR axis. We suggest that the spacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow-spreading ridges than on slow-spreading ridges.

Crustal structure, accretionary processes and rift propagation: a gravity study of the intermediate-spreading Valu Fa Ridge, Lau Basin

The Valu Fa Ridge is an intermediate‐spreading (full rate of 60 mm yr−1) ridge located in the Lau Basin. In 1995 this ridge was surveyed using a multidisciplinary, geophysical approach to image crust and upper mantle structure, with the aim of investigating the processes of oceanic crustal accretion in a back‐arc tectonic environment. As part of this experiment a network of gravity profiles was acquired, together with seismic, magnetic, swath bathymetry and controlled‐source electromagnetic data. Presented in this paper are the results of forward modelling of a subset of the acquired gravity profiles, two oriented ridge‐perpendicular and one ridge‐parallel, using the preferred seismic models of Turner . (1999) as a basis of initial model construction. In addition, the gravity data set in its entirety has been used to calculate the mantle and residual mantle Bouguer anomalies with the aim of investigating variability in crustal structure, both density and layer thickness, and the nature of the underlying upper mantle. Of particular interest are the overlapping spreading centre between the Central and Northern Valu Fa Ridges, where seismic modelling implies a generally thickened crust and a magma chamber located beneath the overlap basin rather than separate chambers supplying each ridge, and the propagating rift tip and associated basin‐bounding pseudo‐fault. Modelling results suggest that the pre‐ and post‐rift crusts have different compositional origins, with lower densities required > 12 km off‐axis to fit the observed free‐air gravity anomaly. The locations of the transitions into regions of lower density correspond with those of Turner . (1999) derived from seismic modelling, which in turn correspond in location to the rift‐related pseudo‐fault identified by Wiedicke & Collier (1993). Calculation and interpretation of the mantle and residual mantle Bouguer anomalies also confirms the lower off‐axis densities and indicates a general increase in crustal thickness northwards towards the overlapping spreading centre between the Central and Northern Valu Fa Ridge segments. The presence of thicker crust, corresponding to a region of negative mantle and residual mantle Bouguer anomaly, implies that the region surrounding the overlapping spreading centre has been in the recent geological past, or is presently, the site of an increased magma supply. The residual mantle Bouguer anomaly also reveals features related directly to off‐axis lateral variation in density and layer thickness, associated with the southward propagation of the Valu Fa Ridge into the Havre Trough. Modelling of the north–south variation in crustal thickness along‐axis shows that the anomaly trend can be explained simply by thicker crust beneath each of the Valu Fa Ridge segments and the overlapping spreading centre between the Northern and Central Valu Fa Ridges. The observation of segment‐centred crustal thickening is consistent with models of ridge segmentation that suggest that melt influx is located towards segment centres and that segment length is controlled by the maximum lateral extent to which this melt can flow ( Macdonald . 1988 ; Tolstoy . 1993 ). However, the overlapping spreading centre remains anomalous from these models in that it also appears to be associated with thicker crust and an increased melt supply.

Evidence from gravity anomalies for interactions of the Marion and Bouvet hotspots with the Southwest Indian Ridge: effects of transform offsets

The ultra-slow spreading Southwest Indian Ridge (SWIR) presents a unique environment to study the interactions between hotspots and ridges with highly segmented geometry. Using recently available satellite free-air gravity and shipboard bathymetry data, we obtain mantle Bouguer (MBA) and residual mantle Bouguer anomalies (RMBA) by removing from free-air gravity the attractions of seafloor topography, sediment thickness variations, a reference crust, and theoretically predicted effects of lithospheric cooling. The Bouvet hotspot, previously observed to be associated with anomalous bathymetry and geochemistry near the Bouvet triple junction, has an MBA axial gravity low of ∼100 mGal, implying pronounced localized crustal thickening. Off-axis, the RMBA lows along previously calculated Bouvet hotspot tracks are variable in amplitude, suggesting the possibilities that Bouvet flux varies in time or that hotspot magmatism is enhanced by proximity to a spreading center. Along-axis geophysical anomalies suggest that the Marion hotspot has a significant effect on accretionary processes in the central portion of the SWIR. Importantly, the Marion axial anomaly appears to be compartmentalized between the Andrew Bain and Discovery II fracture zones, implying that transform offsets play a significant role in governing the distribution of plume material in a highly segmented, ultra-slow spreading system.

Crustal structure of the Southwest Indian Ridge at the Atlantis II Fracture Zone

The Southwest Indian Ridge is a slow spreading end‐member of the mid‐ocean ridge system. The deepest borehole penetrating the lower oceanic crust, Ocean Drilling Program hole 735B, lies on the eastern transverse ridge of the Atlantis II Fracture Zone at 57°E. A wide‐angle seismic survey in the vicinity of the borehole reveals a crustal structure that is highly heterogeneous. To the east of Atlantis Bank, on which hole 735B is located, the crust consists of a 2–2.5 km thick high‐velocity‐gradient oceanic layer 2 and a 1–2 km thick low‐velocity‐gradient layer 3. The transform valley has a 2.5–3 km thick crust with anomalously low velocities interpreted to consist largely of highly serpentinized mantle rocks. The seismically defined crust is thickest beneath the borehole, where layer 2 is thinner and the lower crust is inferred to contain 2–3 km of partially serpentinized mantle. The seismic velocity models are consistent with gravity data which show weak residual mantle Bouguer anomalies because the regions of thinner crust have lower crustal densities. Stress variations deduced from mass balances between the transform valley floor and the adjacent transverse ridges are much larger than the likely threshold for lithospheric failure and therefore indicate that the relief is supported dynamically. The variation of crustal thickness with spreading rate defined by data from the Southwest Indian Ridge and elsewhere is consistent with models of melt generation in which the upwelling mantle is cooled by conductive heat loss at very slow spreading rates, resulting in reduced melt generation under the spreading axis. Large segment‐scale variations in crustal thickness suggest subcrustal along‐axis migration of melt toward segment centers.

The Pacific-Antarctic Ridge–Foundation hotspot interaction: a case study of a ridge approaching a hotspot

The Foundation hotspot-Pacific-Antarctic Ridge (PAR) system is the best documented case of a fast spreading ridge approaching a hotspot and interacting with it. The morphology, crustal structure inferred from gravity anomalies and the chemical composition of the lavas of the axial area of the PAR show evidence of the influence of the hotspot, that is presently located roughly 35 km west of the spreading ridge axis. Along-axis variation in the Mantle Bouguer anomaly is about 28 mGal, corresponding to a crustal thickening of 1.5 km where the hotspot is nearer to the PAR. Anomalous ridge elevation is 650 m and the along-axis width of the chemical anomaly is 200 km. A comparison of these axial parameters with those derived for other ridge-hotspot systems, suggests that the amount of plume material reaching the ridge axis is smaller for the Foundation-PAR system. This implies a weaker connection between the plume and the ridge. Cumulative effects of a fast spreading rate and of a fast ridge-hotspot relative motion can be responsible for this weak plume-ridge flow. The flow from the hotspot may be less efficiently channelled towards the ridge axis when a fast ridge is rapidly moving towards a hotspot.

A systematic analysis of the Mid‐Atlantic Ridge morphology and gravity between 15°N and 40°N: Constraints of the thermal structure

Multibeam bathymetry data obtained along a 2400 km long section of the Mid‐Atlantic Ridge (MAR) from 15°N to the Azores platform (40°N) and satellite‐derived gravity data were used to calculate the mantle Bouguer anomaly (MBA) along this portion of the MAR. Both data sets were used to determine the relations between gravity anomalies and topographic variations and discuss these in terms of thermal difference. A long‐wavelength influence of the Azores hot spot is characterized by a gentle, continuous slope of the average ridge axial depth and a general gradient in the along‐axis MBA profile. This thermal influence of the Azores hot spot controls a systematic southward propagation of the spreading segments at least to 26°30′N. South of 26°30′N, the direction of the segment propagation is controlled by the local difference in thermal state between adjacent segments. Except on the Azores platform, the systematic along‐axis 11–90 km long wavelength segmentation is independent of the long‐wavelength influence of the Azores. At the segment center, the axial morphology is linked to the thermal state of the segments between: (1) “Hotter segments” characterized by a smooth axial morphology, a well‐defined shallow ”inner valley”, high ΔMBA and a long length;(2) “colder segments” which present a rough axial morphology with a deep, wide and well‐defined rift valley, a low ΔMBA and a small length. For ”hotter segments” the formation of the abyssal hills is mainly due to a magmato‐tectonic cycle over periods of 0.3 to 1 Myr, whereas on “colder segments” the axial morphology is mainly controlled by a tectonic rift valley formation. We propose that these different segment types correspond to a temporal evolution of the rift valley morphology over periods of several million years.