Jean-Christophe Sempéré

Segmentation and morphotectonic variations along a slow-spreading center: The Mid-Atlantic Ridge (24°00′ N– 30°40′ N)

Analysis of Sea Beam bathymetry along the Mid-Atlantic Ridge between 24°00′ N and 30°40′ N reveals the nature and scale of the segmentation of this slow-spreading center. Except for the Atlantis Transform, there are no transform offsets along this 800-km-long portion of the plate boundary. Instead, the Mid-Atlantic Ridge is offset at intervals of 10–100 km by nontransform discontinuities, usually located at local depth maxima along the rift valley. At these discontinuities, the horizontal shear between offset ridge segments is not accommodated by a narrow, sustained transform-zone. Non-transform discontinuities along the MAR can be classified according to their morphology, which is partly controlled by the distance between the offset neovolcanic zones, and their spatial and temporal stability. Some of the non-transform discontinuities are associated with off-axis basins which integrate spatially to form discordant zones on the flanks of the spreading center. These basins may be the fossil equivalents of the terminal lows which flank the neovolcanic zone at the ends of each segment. The off-axis traces, which do not lie along small circles about the pole of opening of the two plates, reflect the migration of the discontinuities along the spreading center.The spectrum of rift valley morphologies ranges from a narrow, deep, hourglass-shaped valley to a wide valley bounded by low-relief rift mountains. A simple classification of segment morphology involves two types of segments. Long and narrow segments are found preferentially on top of the long-wavelength, along-axis bathymetric high between the Kane and Atlantis Transforms. These segments are associated with circular mantle Bouguer anomalies which are consistent with focused mantle upwelling beneath the segment mid-points. Wide, U-shaped segments in cross-section are preferentially found in the deep part of the long-wavelength, along-axis depth profile. These segments do not appear to be associated with circular mantle Bouguer anomalies, indicating perhaps a more complex pattern of mantle upwelling and/or crustal structure. Thus, the long-recognized bimodal distribution of segment morphology may be associated with different patterns of mantle upwelling and/or crustal structure. We propose that the range of observed, first-order variations in segment morphology reflects differences in the flow pattern, volume and temporal continuity of magmatic upwelling at the segment scale. However, despite large first-order differences, all segments display similar intra-segment, morphotectonic variations. We postulate that the intra-segment variability represents differences in the relative importance of volcanism and tectonism along strike away from a zone of enhanced magma upwelling within each segment. The contribution of volcanism to the morphology will be more important near the shallowest portion of the rift valley within each segment, beneath which we postulate that upwelling of magma is enhanced, than beneath the ends of the segment. Conversely, the contribution of tectonic extension to the morphology will become more important toward the spreading center discontinuities. Variations in magmatic budget along the strike of a segment will result in along-axis variations in crustal structure. Segment mid-points may coincide with regions of highest melt production and thick crust, and non-transform discontinuities with regions of lowest melt production and thin crust. This hypothesis is consistent with available seismic and gravity data.The rift valley of the Mid-Atlantic Ridge is in general an asymmetric feature. Near segment mid-points, the rift valley is usually symmetric but, away from the segment mid-points, one side of the rift valley often consists of a steep, faulted slope while the other side forms a more gradual ramp. These observations suggest that half-grabens, rather than full-grabens, are the fundamental building blocks of the rift valley. They also indicate that the pattern of faulting varies along strike at the segment scale, and may be a consequence of the three-dimensional, thermo-mechanical structure of segments associated with enhanced mantle upwelling beneath their mid-points.

Bathymetry of the mid-atlantic ridge, 24°-31°N: A map series

This paper presents a series of eleven maps of the bathymetry of a 900 km long section of the crestal region of the Mid-Atlantic Ridge. Along with a twelfth key map, this series defines the morphology of fifteen discrete spreading segments and shows convincingly that no transform faults exist between the Kane and Atlantis fracture zones. The publication of these multi beam bathymetry data with a contour interval of 50 m and at a scale of 30 inches per degree of longitude is intended to allow easy access by a broad community of marine earth scientists to this unique and powerful data set.

Morphology and tectonics of the Australian-Antarctic Discordance between 123° E and 128° E

The Australian-Antarctic Discordance (AAD) is an anomalously deep and rugged zone of the Southeast Indian Ridge (SEIR) between 120° E and 128° E. The AAD contains the boundary between the Indian Ocean and Pacific Ocean isotopic provinces. We have analyzed SeaMarc II bathymetric and sidescan sonar data along the SEIR between 123° E and 128° E. The spreading center in the AAD, previously known to be divided into several transform-bounded sections, is further segmented by nontransform discontinuities which separate distinct spreading cells. Near the transform which bounds the AAD to the east, there is a marked change in the morphology of the spreading center, as well as in virtually every measured geochemical parameter. The spreading axis within the Discordance lies in a prominent rift valley similar to that observed along the Mid-Atlantic Ridge, although the full spreading rate within the AAD is somewhat faster than that of slow-spreading centers (~ 74 mm a−1 vs. 0–40 mm a−1). The AAD rift valleys show a marked contrast with the axial high that characterizes the SEIR east of the AAD. This change in axial morphology is coincident with a large (~ 1 km) deepening of the spreading axis. The segmentation characteristics of the AAD are analogous to those of the slow-spreading Mid-Atlantic Ridge, as opposed to the SEIR east of the AAD, which exhibits segmentation characteristics typical of fast-spreading centers. Thus, the spreading center within and east of the AAD contains much of the range of global variability in accretionary processes, yet it is a region free from spreading rate variations and the volumetric and chemical influences of hotspots. We suggest that the axial morphology and segmentation characteristics of the AAD spreading centers are the result of the presence of cooler than normal mantle. The presence of a cool mantle and the subsequent diminution of magma supply at a constant spreading rate may engender the creation of anomalously thick brittle lithosphere within the AAD, a condition which favor, the creation of an axial rift valley and of thin oceanic crust, in agreement with petrologic studies. The morphologies of transform and non-transform discontinuities within the Discordance also possess characteristics consistent with the creation of anomalously thick lithosphere in the region. The upper mantle viscosity structure which results from lower mantle temperatures and melt production rates may account for the similarity in segmentation characteristics between the AAD and slow-spreading centers. The section of the AAD which overlies the isotopic boundary is associated with chaotic seafloor which may be caused by an erratic pattern of magmatism and/or complex deformation associated with mantle convergence. Finally, the pattern of abyssal hill terrain within a portion of the AAD supports previous models for the formation of abyssal hills at intermediate- and slow-spreading ridges, and provides insights into how asymmetric spreading is achieved in this region.

Three-dimensional inversion of marine magnetic anomalies: Implications for crustal accretion along the Mid-Atlantic Ridge (28°–31°30′ N)

We present magnetic field data collected over the Mid-Atlantic Ridge in the vicinity of the Atlantis Fracture Zone and extending out to 10 Ma-old lithosphere. We calculated a magnetization distribution which accounts for the observed magnetic field by performing a three-dimensional inversion in the presence of bathymetry. Our results show the well-developed pattern of magnetic reversals over our study area. We observe a sharp decay in magnetization from the axis out to older lithosphere and we attribute this decay to progressive low temperature oxidation of basalt. In crust which is ∼ 10 Ma, we observe an abrupt increase in magnetic field intensity which could be due to an increase in the intensity of magnetization or thickness of the magnetic source layer. We demonstrate that because the reversal epoch was of unusually long duration, a two-layer model comprised of a shallow extrusive layer and a deeper intrusive layer with sloping polarity boundaries can account for the increase in the amplitude of anomaly 5. South of the Atlantis Fracture Zone, high magnetization is correlated with bathymethic troughts at segment end points and lower magnetization is associated with bathymetric highs at segment midpoints. This pattern can be explained by a relative thinning of the magnetic source layer toward the midpoint of the segment. Thickening of the source layer at segment endpoints due to alteration of lower oceanic crust could also cause this pattern. Because we do not observe this pattern north of the fracture zone, we suggest it is a result of the nature of crustal formation process where mantle upwelling is focused. South of the fracture zone, reversals along discontinuity traces only continue to crust ∼2 Ma old. In crust >∼2 Ma, we observe bands of high, positive magnetization along discontinuity traces. We suggest that within the discontinuity traces, a high, induced component of magnetization is produced by serpentinized lower crust/upper mantle and this masks the contribution of basalts to the magnetic anomaly signal.

The Mid-Atlantic Ridge between 29°N and 31°30′N in the last 10 Ma

Abstract The segmentation of the Mid-Atlantic Ridge between 29°N and 31°30′ N during the last 10 Ma was studied. Within our survey area the spreading center is segmented at a scale of 25–100 km by non-transform discontinuities and by the 70 km offset Atlantis Transform. The morphology of the spreading center differs north and south of the Atlantis Transform. The spreading axis between 30°30′N and 31°30′N consists of enechelon volcanic ridges, located within a rift valley with a regional trend of ∼ 040°. South of the transform, the spreading center is associated with a well-defined rift valley trending ∼ 015°. Magnetic anomalies and the bathymetric traces left by non-transform discontinuities on the flanks of the Mid-Atlantic Ridge provide a record of the evolution of this slow-spreading center over the last 10 Ma. Migration of non-transform offsets was predominantly to the south, except perhaps in the last 2 Ma. The discontinuity traces and the pattern of crustal thickness variations calculated from gravity data suggest that focused mantle upwelling has been maintained for at least 10 Ma south of 30°30′ N. In contrast, north of 30°30′N, the present segmentation configuration and the mantle upwelling centers inferred from gravity data appear to have been established more recently. The orientation of the bathymetric traces suggests that the migration of non-transform offsets is not controlled by the motion of the ridge axis with respect to the mantle. The evolution of the spreading center and the pattern of segmentation is influenced by relative plate motion changes, and by local processes, perhaps related to the amount of melt delivered to spreading segments. Relative plate motion changes over the last 10 Ma in our survey area have included a decrease in spreading rate from ∼ 32 mm a −1 to ∼ 24 mm a −1 , as well as a clockwise change in spreading direction of 13° between anomalies 5 and 4, followed by a counterclockwise change of 4° between anomaly 4 and the present. Interpretation of magnetic anomalies indicates that there are significant variations in spreading asymmetry and rate within and between segments for a given anomaly time. These differences, as well as variations in crustal thickness inferred from gravity data on the flanks of spreading segments, indicate that magmatic and tectonic activity are, in general, not coordinated between adjacent spreading segments.

Evidence for variable upper mantle temperature and crustal thickness in and near the Australian-Antarctic Discordance

Abstract The Southeast Indian Ridge (SEIR) in and near the Australian-Antarctic Discordance (AAD) exhibits, at a constant spreading rate, almost the full range of the many geophysical and geochemical parameters characteristic of the ‘slow’ Mid-Atlantic Ridge and ‘fast’ East Pacific Rise. We used satellite-derived gravity data, in combination with SeaMARC II bathymetry in and near the AAD, to examine regional density variations in the upper mantle beneath the AAD. Through three-dimensional gravity analysis, we found that at least two end-member models satisfy the gravity observations: regional crustal thickness variations of at least 3 km along the SEIR near the AAD or a temperature anomaly of the order of 150°C in the upper mantle beneath the SEIR. These new observations, combined with other geophysical and geochemical characteristics of the Australian-Antarctic Discordance, provide further evidence that the temperature structure of a mid-ocean ridge is a controlling factor, in addition to spreading rate, in the crustal accretionary process. Numerical models of mantle flow beneath mid-ocean ridges offer one means of investigating the dynamic effect of a variable upper mantle temperature on the accretionary process. Our results indicate that temperature is important, especially at intermediate and slower spreading rates, where thermal effects can dominate mantle flow beneath a mid-ocean ridge and result in increasing crustal production with decreasing spreading rate. At the constant, intermediate spreading rate of 37 mm/yr, characteristic of the SEIR in and near the AAD, our numerical models show that significant crustal thinning (2–4 km) can occur with relatively small variations in upper mantle temperature, all else being equal. Thus, combined with our end-member gravity models, these observations and results suggest that both anomalously cool upper mantle and thin crust exist beneath the AAD.

The structure and segmentation of the Southeast Indian Ridge

Abstract The Southeast Indian Ridge (SEIR) spreads at a relatively narrow range of intermediate rates (59–75 km/Ma) but exhibits the full range of slow to fast spreading morphology and segmentation. Satellite gravity data reveal transitions in the structure of the spreading center where it is influenced by the Amsterdam and Kerguelen hotspots and at the Australian–Antarctic Discordance (AAD). Although the spreading rate between the hotspots and the AAD is nearly constant, the ridge exhibits a variety of distinct styles of morphology and segmentation not observed at fast or slow spreading centers. Recently, collected multibeam bathymetry data reveal a transition from East Pacific Rise style overlapping axial highs near 92°E to Mid-Atlantic Ridge style axial valleys with non-transform offsets near 116°E. The intervening segmentation is characterized by propagating offsets coexisting with stationary transforms which exhibit different degrees of temporal stability. Currently, there are 10 transform offsets between the hotspots and the AAD but only five of these have persisted since seafloor spreading stabilized at 35 Ma. The other five appear to have formed since 35 Ma and several more have disappeared by transform shortening or coalesced by along-axis propagation. There is a transition from monotonic offset propagation near the hotspots to oscillatory propagation approaching the AAD. This change in offset stability corresponds to transitions in depth, axial morphology and offset structure. Through much of the transitional region, higher order segmentation is characterized by en-echelon offsets of a diffuse spreading axis that generally lacks a well defined neovolcanic zone. Since the spreading rate is nearly constant, the regional variation in axial morphology and segmentation appears to be controlled by an upper mantle thermal gradient — possibly a result of flux of asthenosphere from the hotspots to the AAD. This is consistent with the gradual increase in average ridge flank depths along this part of the plate boundary but segment scale changes in axial depth reveal spatio-temporal variability in the dynamic topography that are not preserved on older lithosphere. Intrasegment transitions in axial morphology and en-echelon offsets within first order segments suggest that local variations in mantle thermal structure introduce short-lived instabilities in higher order segmentation and dominate the short term evolution of the plate boundary.

A detailed magnetic study of the Reykjanes Ridge between 63°00′N and 63°40′N

Immediately southwest of Iceland, the Reykjanes Ridge consists of a series ofen échelon, elongate ridges superposed on an elevated, smooth plateau. We have interpreted a detailed magnetic study of the portion of the Reykjanes Ridge between 63°00′N and 63°40′N on the Icelandic insular shelf. Because the seafloor is very shallow in our survey area (100–500 m), the surface magnetic survey is equivalent to a high-sensitivity, nearbottom experiment using a deep-towed magnetometer. We have performed two-dimensional inversions of the magnetic data along profiles perpendicular to the volcanic ridges. The inversions, which yield the magnetization distribution responsible for the observed magnetic field, allow us to locate the zones of most recent volcanism and to measure spreading rates accurately. We estimate the average half spreading rate over the last 0.72 m.y. to have been 10 mm/yr within the survey area. The two-dimensional inversions allow us also to measure polarity transition widths, which provide an indirect measure of the width of the zone of crustal accretion. We find a mean transition width on the order of 4.5±1.6 km. The observed range of transition widths (2 to 8.4 km) and their mean value are characteristic of slow-spreading centers, where the locus of crustal accretion may be prone to lateral shifts depending on the availability of magmatic sources. These results suggest that, despite the unique volcanotectonic setting of the Reykjanes Ridge, the scale at which crustal accretion occurs along it may be similar to that at which it occurs along other slow-spreading centers. The polarity transition width measurements suggest a zone of crustal accretion 4–9 km wide. This value is consistent with the observed width of volcanic systems of the Reykjanes Peninsula. The magnetization amplitudes inferred from our inversions are in general agreement with NRM intensity values of dredge samples measured by De Boer (1975) and ourselves. Our thermomagnetic measurements do not support the hypothesis that the low amplitude of magnetic anomalies near Iceland is the result of a high oxidation state of the basalts. We suggest that the observed reduction in magnetic anomaly amplitude toward Iceland may be the result of an increase in the size of pillows and other igneous units.

A three-dimensional gravity study of the Rodrigues Triple Junction and Southeast Indian Ridge

Abstract Three-dimensional analysis of gravity and bathymetry data is used to investigate the density structure of the Rodrigues Triple Junction and the first segment of the Southeast Indian Ridge south of the triple junction. The distribution of mantle Bouguer and residual gravity anomalies suggests that focused upwelling of mantle is occurring along the nearly co-linear Central Indian and Southeast Indian Ridge limbs of the triple junction. In contrast, the mantle Bouguer anomaly over the Southwest Indian limb of the triple junction shows little variation despite nearly 4 km of topographic relief within this segment of the Southwest Indian Ridge. The absence of a significant mantle Bouguer anomaly over the Southwest Indian Ridge near the triple junction suggests that the rift valley observed on this segment is not completely compensated by thinning oceanic crust; only ∼ 1 km of crustal thinning within the axial valley is expected, based on our gravity analysis. Further, no clear long-wavelength thermal signal associated with lithospheric growth can be associated with the Southwest Indian Ridge in this region. These observations imply that focused upwelling is not occurring on this segment of the Southwest Indian Ridge and require a primarily dynamic compensation mechanism for the extreme axial topography. Our analysis indicates that the Rodrigues Triple Junction system is dominated by mantle upwelling associated with the Southeast and Central Indian Ridges.

GRAVITY ANOMALIES, FLEXURE OF AXIAL LITHOSPHERE, AND ALONG-AXIS ASTHENOSPHERIC FLOW BENEATH THE SOUTHEAST INDIAN RIDGE

Abstract Mantle Bouguer and residual gravity anomaly patterns consistent with two- and three-dimensional mantle flow and/or crustal thickness variability, are observed juxtaposed within the five first-order segments of the Southeast Indian Ridge (SEIR) surveyed between 98° and 112°E. Within each segment ‘bulls-eye’ or ‘triangular’ patterns of mantle Bouguer and residual gravity anomalies, consistent with focused mantle upwelling and/or three-dimensional crustal thickness patterns, occur systematically toward the western ends of segments which adjoin significant left-stepping offsets in the ridge axis. The eastern ends of these segments and those that abut smaller, right-stepping transforms display a more two-dimensional anomaly character. Axial morphology also varies along strike; axial highs coincide with focused gravity anomalies, while rifted highs occur elsewhere. These observations imply that more magmatically robust extension, more vigorous mantle upwelling, and/or thicker crust characterize the western parts of those segments adjacent to significant ridge offsets. These trends are mirrored east of the deepest portions of the Australian-Antarctic Discordance (AAD), where more vigorous upwelling is located toward the eastern end of segment B5. In addition to the usual interpretations in terms of crustal thickness and/or mantle temperature, we suggest that the systematics of topographic style and gravity anomaly character provide further evidence for along-axis asthenospheric flow beneath the SEIR. In western portions of segments between 98° and 112°E, sub-axial flow toward the AAD causes asthenosphere to be drawn from beneath older lithosphere to the west, thereby increasing the region of mantle upwelling and melting supplying the ‘upstream’ portion of the ridge axis.