Daniel Sauter

Modes of seafloor generation at a melt-poor ultraslow-spreading ridge

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years.

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

Abstract 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 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.

Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge

We use bathymetry, gravimetry, and basalt composition to examine the relationship between spreading rate, spreading obliquity, and the melt supply at the ultraslow spreading Southwest Indian Ridge (SWIR). We find that at regional scales (more than 200 km), melt supply reflects variations in mantle melting that are primarily controlled by large-scale heterogeneities in mantle temperature and/or composition. Focusing on adjacent SWIR regions with contrasted obliquity, we find that the effect of obliquity on melt production is significant (about 1.5 km less melt produced for a decrease of 7 mm/a to 4 mm/a in effective spreading rates, ESR) but not enough to produce near-amagmatic spreading in the most oblique regions of the ridge, unless associated with an anomalously cold and/or depleted mantle source. Our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated, thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply. To explain why very oblique SWIR regions nonetheless have large outcrops of mantle-derived ultramafic rocks and, in many cases, no evidence for axial volcanism (Cannat et al., 2006; Dick et al., 2003), we develop a model which combines melt migration along axis to more volcanically robust areas, melt trapping in the lithospheric mantle, and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure. These dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections. The resulting segmentation pattern, with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections, is not specific to oblique ridge regions. It is also observed along the SWIR and the arctic Gakkel Ridge in orthogonal regions underlain by cold and/or depleted mantle.

Segmentation and Morphotectonic Variations Along a Super Slow-Spreading Center: The Southwest Indian Ridge (57° E-70° E)

Bathymetric data along the Southwest Indian Ridge (SWIR) between 57°E and 70° E have been used to analyze the characteristics of thesegmentation and the morphotectonic variations along this ridge. Higheraxial volcanic ridges on the SWIR than on the central Mid-Atlantic Ridge(MAR) indicate that the lithosphere beneath the SWIR axis that supportsthese volcanic ridges, is thicker than the lithosphere beneath the MAR. Astronger/thicker lithosphere allows less along-axis melt flow andenhances the large crustal thickness variations due to 3D mantle upwellings.Magmatic processes beneath the SWIR are more focused, producing segmentsthat are shorter (30 km mean length) with higher along-axis relief (1200 mmean amplitude) than on the MAR. The dramatic variations in the length andamplitude of the swells (8–50 km and 500–2300 m respectively),the height of axial volcanic ridges (200–1400 m) and the number ofvolcanoes (5–58) between the different types of segments identifiedon the SWIR presumably reflect large differences in the volume, focusing andtemporal continuity of magmatic upwelling beneath the axis. To the east ofMelville fracture zone (60°42′ E), the spreading center isdeeper, the bathymetric undulation of the axial-valley floor is less regularand the number of volcanoes is much lower than to the west. The spreadingsegments are also shorter and have higher along-axis amplitudes than to thewest of Melville fracture zone where segments are morphologically similar tothose observed on the central MAR. The lower magmatic activity together withshorter and higher segments suggest colder mantle temperatures withgenerally reduced and more focused magma supply in the deepest part of thesurvey area between 60°42′ E and 70° E. The non-transformdiscontinuities show offsets as large as 70 km and orientations up toN36° E as compared to the N0° E spreading direction. We suggest thatin regions of low or sporadic melt generation, the lithosphere neardiscontinuities is laterally heterogeneous and mechanically unable tosustain focused strike-slip deformation.

Magmato-tectonic cyclicity at the ultra-slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On-axis deep tow side scan sonar data are used together with off-axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra-slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off-axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato-tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato-tectonic cycle is a pseudoperiodic process that lasts ~0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long-lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

A survey of the Southwest Indian Ridge axis between Atlantis II fracture zone and the Indian Ocean Triple Junction : Regional setting and large-scale segmentation

The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30′ E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction.

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.

MODMAG, a MATLAB program to model marine magnetic anomalies

Identifying marine magnetic anomalies is the most common way to date the ocean floor. Although the technique of magnetic anomaly identification has not changed since the 1960s, a forward modeling software that is easy to use, fast and automatic, without abstruse parameters, was lacking. We present a user-friendly MATLAB-based interface, called MODMAG, which allows one to perform forward modeling of marine magnetic anomalies resulting from several successive spreading periods with different spreading rates and asymmetric spreading possibly alternating with axial jumps. The main advantage of our program is that the management of the magnetized bodies resulting from such successive spreading periods is not the users responsibility. Spreading parameters can be set easily for the picking of the marine magnetic anomalies. Non-specialist geophysicists or geologists can therefore easily identify marine magnetic anomalies with the help of MODMAG.

From slow to ultraslow: A previously undetected event at the Southwest Indian Ridge at ca. 24 Ma

Changes in plate motion are thought to be recorded in the trend of fracture zones, even though fracture zones provide no information about the spreading rate. Using newly compiled published and unpublished magnetic data from the Southwest Indian Ridge, we calculated fi nite rotation poles for A13, A8, and A6, from which we determined a 50% decrease in spreading rate from slow to ultraslow at ca. 24 Ma not accompanied by a signifi cant change in spreading direction. This spreading rate decrease is concurrent with changes in plate motions at only two of the four adjoining plate boundaries. Finally, we discuss the possible relationships of this event with other absolute or relative plate motion events that occurred at ca. 24 Ma at the global scale.

Heterogeneity in southern Central Indian Ridge MORB: Implications for ridge–hot spot interaction

Between the Rodrigues Triple Junction and the Marie Celeste fracture zone, basalts from the Central Indian Ridge (CIR) exhibit an enrichment in incompatible elements that increases in intensity northward. In addition, H2O/TiO2, Al[8], and Dy/Yb ratios increase, while Na[8] remains unchanged and Fe[8] decreases. Evolution of the enriched magma appears to be affected by elevated water contents, which lower the mantle solidus, thereby increasing the initial depth of melting, as well as delaying plagioclase crystallization. However, the enrichment affecting the northern samples is not a just function of hydrous mantle melting and crystallization. Instead of trending toward a small melt fraction from the mantle, as predicted by hydrous melting models, the CIR samples lie on a mixing line between N-MORB and a source component that closely resembles present-day Reunion hot spot lavas. Thus it appears that while hydrous melting and crystallization affect the CIR, the enriched and wet mantle originates from the Reunion hot spot, where it migrates eastward toward the CIR, against the direction of motion of the lithosphere.