Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

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A geomagnetic record over the last 3.5 million years from deep-tow magnetic anomaly profiles across the Central Indian Ridge

International audienceHigh-resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short-lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep-tow (-500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well-dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short-wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific-Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small-scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near-bottom results support the existence of three geo-magnetic features between the Gauss-Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. I n subchron within the Gauss chron. This study emphasizes the potential of deep-tow magnetic surveys in detecting fluctuations in geo-magnetic field intensity and, in particular, short-lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum

The internal structure and geotectonic setting of the Xade and Tsetseng complexes in the western most part of the Kaapvaal Craton

The Xade Complex is an unexposed Y-shaped body, approximately 100 km long and 25 km wide, located close to the western margin of the Kaapvaal craton in Botswana. The complex is characterized by large coincident magnetic and gravity anomalies. It is completely covered by varying thicknesses of Kalahari sediments as well as by Karoo strata, which means that detailed analysis of high resolution airborne magnetic data, ground gravity data and limited seismic data are essential in interpreting the internal configuration of the complex. An earlier interpretation of the first airborne magnetic survey of Botswana (Reeves, 1978) coupled with subsequent drilling discovered the Xade Complex and showed that it is made up of mafic and ultramafic rocks. However, the limited amount of drilling did not provide sufficient information to either interpret in detail its internal geology or its regional geotectonic setting (Meixner and Peart, 1984). New 2D and 3D gravity and magnetic field modelling have constrained the geometry of the complex as a syncline defined by folded mafic lavas and high-level sub-volcanic mafic sheets. The Xade Complex lies within a graben that forms the north-south arm of a triple junction with the faulted western margin of the Kaapvaal Craton. The focal point of the triple junction coincides with an inflection of the cratonic margin and is the likely site of the feeder zone to the mafic lavas of the Xade Complex. The Tsetseng Complex is shown to be an internally layered, magnetite-bearing gabbro

Les anomalies magnétiques marines (contraintes sur les variations courtes périodes des processus de l'accrétion et de l'intensité du champ magnétique terrestre)

PARIS7-Bibliothèque centrale (751132105) / SudocSudocFranceF

Focused volcanism and growth of a slow spreading segment (Mid-Atlantic Ridge, 35°N)

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Crustal thickness of V-shaped ridges south of the Azores: Interaction of the Mid-Atlantic Ridge (36ø-39øN) and the Azores hot spot

We thank the officers and crew of the R/V L'Atalantef or their assistanced uringt he Suda9oresC ruise, and the rest of the scientific party for data gathering, processing and discussions. We also thank J.P. Canales, J. Dafiobeitia, E. Gfftcia, and G. Ito for discussions of various aspects of this work and comments on the manuscript. Reviews by G. Kent and of R. S. White, and reccomendationbsy the AssociateE ditor E. Klein, contributedt o the final manuscript. M. Muller assisted on the thermal calculations . The GMT software [Wesseland Smittl 1991] was extensively used in this studyInternational audienceV-shaped ridges propagating along the Mid-Atlantic Ridge axis south of the Azores and Iceland hot spots indicate that ridge-hot spot interactions produce temporal and spatial variations in melt supply to the ridge axis. Estimates of relative crustal thickness variations associated with the ridges south of the Azores hot spots, based on gravity and bathymetry data collected during the SudAqores cruise (1998), provide constraints on the rate of propagation of these melt anomalies and on the variations in melt production along the axis and in time. The maximum apparent crustal thickness along the Azores V ridge is-14 km near the Azores, decreasing to normal crustal thickness of-6 km toward the south. This crustal thickness variation may be explained by enhanced melt production associated with the propagation of a mantle temperature anomaly that initiated-10 Myr ago at the Azores hot spot. The temperature anomaly decreased as it propagated southward, reaching ambient mantle temperatures at the present time at its predicted location under the axis. The excess melt was emplaced on axis forming discrete, shallow (<1000 m) oceanic plateaus (•100 km in diameter at •37.5øN) that are isostatically compensated. The numerous seamounts, lack of normal faults, and smooth basement at the summit of these plateaus suggest high effusion rates that persisted for-5 Myr or less, with little or no tectonic strain. As the melt anomaly propagated along axis, the magmatic activity at the plateaus ceased, resulting in rifting of the plateau and onset of normal seafloor spreading. The variations in crustal thickness inferred for the V ridges south of the Azores are at least twice that inferred for the Iceland structures. In both cases the V ridges record temporal variations in temperature and/or mantle flux that affect melt production under the ridge axis, but the fluctuations are larger for the Azores than for the Iceland hot spot

Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

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