Focused magmatism versus amagmatic spreading along the ultra‐slow spreading Southwest Indian Ridge: Evidence from TOBI side scan sonar imagery

Abstract

[1] The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40 0 E and 65°40 0 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 alongaxis 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 G 3

Ocean rises are products of variable mantle composition, temperature and focused melting

“…3b). Considering the idealized two-dimensional triangular melting regime beneath an ocean ridge, a good deal of mantle rock is removed from the melting column as the mantle rises up to the base of the crust; consistent with the evidence for focused flow of melt in the shallow mantle beneath ocean ridges ( [31][32][33] ). Thus, models for MORB genesis must take this into consideration.…”

Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

“…The rocks exposed on the seafloor are mainly basalts, with some gabbros, pyroxenites and serpentinized peridotites [12]. Sauter et al [18,19] and Cannat et al [20] discussed the melt supply processes based on gravitational, magnetic and petrological data. They found that at the ultraslow-spreading ridges, the melt may migrate horizontally along the axis and mantle upwelling may be focus at some specific sections [18][19][20].…”

“…Sauter et al [18,19] and Cannat et al [20] discussed the melt supply processes based on gravitational, magnetic and petrological data. They found that at the ultraslow-spreading ridges, the melt may migrate horizontally along the axis and mantle upwelling may be focus at some specific sections [18][19][20]. Moreover, the ridge spreading and hydrothermal activities may interact with hot spots such as the Marion, Bouvet, Del Cano and Crozet plateau [21,22].…”

Tectonic and magmatic segmentation of the Global Ocean Ridge System: a synthesis of observations

“…In this context, the pronounced along-axis topographic and MBA gradients associated with the volcanoes may define the boundaries of more magmatic second-order spreading segments. Alternatively, several authors have proposed that melt produced in the mantle beneath the adjacent nearly amagmatic domains could be focused for over tens of kilometres toward the volcanoes (Cannat et al 1999Okino et al 2002;Sauter et al 2004;Standish et al 2008;Cochran 2008). In this case, the second-order segments in these ultraslow ridge regions would also include part of the amagmatic domains adjacent to axial volcanos.…”