Widespread tectonic extension at the Central Indian Ridge between 8°S and 18°S

Abstract

The middle part of the Central Indian Ridge (MCIR) between 8°S and 18°S is representative of mid-ocean ridges in the Indian Ocean but has not previously been systematically surveyed. Here we present results from the first high-resolution mapping survey over both on- and off-axis sections of the MCIR including multibeam bathymetry, magnetics, hydrocasting, and seabed sampling. The 700-km-long MCIR consists of six first-order segments that are offset by >30km along well-developed transform faults. Three of the first-order segments are further divided into seven second-order segments with <30km offset along non-transform discontinuities. We have recognized for the first time 11 prominent ocean core complexes (OCCs). These occur at nearly all segment ends, corresponding to an occurrence every 60km of the surveyed ridge. Seafloor spreading model studies using magnetic reversals show that the MCIR is a slow-spreading ridge with average full opening rates ranging from 33.7 to 45.1mm/yr, increasing from north to south. The highly curved and intermittent axial ridge geometry, rugged flank fabric, variation in the depth and width of the middle valley, and the frequent occurrences of ocean core complexes and non-transform discontinuities demonstrate that asymmetric accretionary processes are dominant along the ridge. The spreading rate symmetry combined with morphotectonic features, reveal that the MCIR segments developed mainly via tectonic extension with little magmatism. Segments with asymmetric accretion controlled by tectonic extension makes up ~96% of the MCIR, whereas symmetric accretion controlled by robust magmatism make up <4%. Hydrothermal plumes with high methane concentrations occur frequently over the OCCs. This finding indicates that abundant OCCs exposed by detachment faults lead to extensive hydrothermal circulation at off-axis areas and that detachment faults are the primary fluid path for hydrothermal fluid circulation at off-axis regions. Serpentinization of mantle-derived rock at OCCs may be one of the major sources of heat and methane in off-axis areas.