Segmentation and morphotectonic variations of the Central Indian Ridge (21°10′S–22°25′S)

Abstract

Bathymetric, magnetic, and petrographic data have been used to analyze the segmentation and the morphotectonic variations on the Central Indian Ridge (CIR) between 21°10′S and 22°25′S. Volcanoes, abyssal hills, and steep slopes, corresponding to fault scarps and to volcanic constructional slopes, have been obtained by image processing methods applied to Sea Beam data. In the survey area, segmentation of the CIR is similar to that of a slow spreading ridge. The plate boundary morphology is defined by a linear zone composed of an ensemble of median ridges and elongate depressions. Hourglass‐shaped valleys are bounded by second and third‐order discontinuities located at local along‐axis depth maxima (400–700 m depth variations) and marked by oblique extensional basins and en échelon jogs, respectively. Large‐throw faults are prevalent toward segment ends and suggest greater amagmatic extension in these places. Along the axial valley, the petrographic characteristics of the dredged basalts are correlated with the morphotectonic segmentation and suggest different magma reservoirs. The morphology and structure of the rift valley reflect the three‐dimensional structure of the axial lithosphere and the geometry of mantle upwelling beneath the spreading center. Abyssal hills are bounded by both inward and outward facing faults in ∼1.5 Ma crust, indicating that the formation of abyssal hills is not solely resulting from a combination of volcanic and tectonic processes within the central part of the ridge but that tectonic processes may also occur as far as 20–40 km from the axis by initiating or reactivating some faults at the outward facing slope of the abyssal hills. Reconstructions of the plate boundary geometry through time, using abyssal hills as isochrons, provide evidence to show that the nontransform discontinuity, located at 21°47′S, with an offset as small as 2–3 km, is associated with V‐shaped off‐axis traces in the form of alignments of disrupted abyssal hill lineations. These reconstructions suggest that a small ridge jump is responsible for the initiation of this discontinuity and that this latter migrates away from areas of higher melt delivery. Three‐dimensional inversion of magnetic data shows that this discontinuity has been initiated between 0.780 and 0.984 Ma and suggests postaccretionary crustal block rotations within the discordant zones.