Mathilde Cannat

The morphology of the flanks of the Southwest Indian Ridge holds a record of seafloor formation and abyssal hill generation at an ultraslow spreading rate. Statistical analysis of compiled bathymetry and gravity data from the flanks of the Southwest Indian Ridge from 54°E to 67°E provides estimates of abyssal hill morphologic character and inferred crustal thickness. The extent of the compiled data encompasses a spreading rate change from slow to ultraslow at ∼24 Ma, a significant inferred variation in sub‐axis mantle temperature, and a patchwork of volcanic and non‐volcanic seafloor, making the Southwest Indian Ridge an ideal and unique location to characterize abyssal hills generated by ultraslow spreading and to examine the effect of dramatic spreading rate change on seafloor morphology. Root mean square abyssal hill height in ultraslow spreading seafloor ranges from ∼280 m to ∼320 m and is on average ∼80 m greater than found for slow‐spreading seafloor. Ultraslow spreading abyssal hill width ranges from ∼4 km to ∼12 km, averaging ∼8 km. Abyssal hill height and width increases west‐to‐east in both slow and ultraslow spreading seafloor, corresponding to decreasing inferred mantle temperature. Abyssal hills persist in non‐volcanic seafloor and extend continuously from volcanic to non‐volcanic terrains. We attribute the increase of abyssal hill height and width to strengthening of the mantle portion of the lithosphere as the result of cooler sub‐axial mantle temperature and conclude that abyssal hill height is primarily controlled by the strength of the mantle component of the lithosphere rather than spreading rate.

We investigate the magnetic signature of volcanic and nonvolcanic seafloor areas along the Southwest Indian Ridge between 61°E and 67°E and analyze their relationship with crustal thickness variations and past to present ridge segmentation. This part of the Southwest Indian Ridge is an end‐member for the global ridge system in terms of low melt supply, thin crust, and ultraslow spreading rates. It is characterized by large expanses of seafloor that show no evidence for a volcanic upper crustal layer. We find that variations of intrinsic magnetization and thickness of the basaltic extrusive layer, where it is present, dominate the present‐day along‐axis crustal magnetization. Off‐axis, the magnetization contrast is on average higher for volcanic seafloor than for smooth nonvolcanic topography, indicating that the contribution of the basaltic upper crustal layer to the production of magnetic anomalies remains important. However, magnetic anomalies that record past magnetic polarity events are found almost everywhere in the survey area, even over domains that lack a volcanic upper crustal layer, arguing thus for the contribution of other sources. We propose that both gabbros and serpentinized peridotites contribute to these anomalies. Although not systematic, and weak over most parts of the survey area, an induced component of magnetization is clearly present in some nonvolcanic seafloor domains. Serpentinized peridotites are the likely carriers of this induced magnetization component.

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.

The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh‐looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along‐axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid‐Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra‐slow spreading ridges. The correlation between the distribution of fresh‐looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along‐axis variations of the gravity‐derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh‐looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra‐slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.

The Southwest Indian Ridge (SWIR) to the east of the Melville Fracture zone receives anomalously low volumes of melt on average. However, a small number of ridge segments appear to receive more melt than this regional average. We use off‐axis bathymetry, gravity, and magnetic data to show that this melt distribution pattern, quite distinct from what is observed at the Mid‐Atlantic Ridge (MAR), has been a characteristic of the easternmost SWIR for at least the past 10 myr. We also show that segments of the easternmost SWIR are substantially shorter lived than most segments of the MAR. Melt distribution in our SWIR study area is therefore both more focused and more variable in time than at the MAR. We tentatively propose a mechanism by which strong and transient melt‐focusing events could be initiated by a localized increase in the volume of melt supplied by the melting mantle to the base of the axial lithosphere, causing thermal thinning of this lithosphere and along‐axis melt migration. These two processes may combine to effectively focus larger volumes of melt toward the center of future thick crust segments. Rapid melt extraction by dikes that feed large volcanic constructions on the seafloor, followed by tectonic disruption of these volcanic constructions by deep‐reaching faults, may then cool the axial lithosphere back to its original thickness and end the melt‐focusing events. The easternmost SWIR is also characterized by a common departure from isostatic compensation of seafloor topography and by a pronounced asymmetry of crustal thickness and seafloor relief between the two ridge flanks. At the faster spreading MAR, similar characteristics are found near the ends of ridge segments. We propose that spreading at the ultra‐slow SWIR during periods when the melt supply is low (i.e., most of the time for the easternmost SWIR) is dominated by large offset asymmetric normal faulting, with significant flexural uplift of the footwalls. Faults face either north or south, and changes in fault polarity are frequent, both along axis and along flow lines (i.e., with time). Producing large faults and maintaining high uncompensated reliefs require the axial lithosphere to be thick, a predictable characteristic for this ultra‐slow ridge, which has an anomalously low regionally averaged melt supply.