Morphotectonic and hydrothermal activity of Central Indian Ridge.

Abstract

Korea Institute of Ocean Science and Technology has conducted high resolution mapping with geological sampling in middle portion of the Central Indian Ridge between 7°S and 17°S. On the basis of a series of cruise, we defined the three first-order segments which offset the MCIR spreading axis.These first-order segments are able to subdivided into seven second-order segments by non-transform discontinuities (NTDs) with <30 km offset. The area is characterized by asymmetric ridge geometry. Through the surveys, the eleven ocean core complexes (OCCs) were identified, which is remarked by the gentle-dome structure and corrugation structure. The basement rocks of OCCs are deep seated mantle rocks, which are put adjacent to seafloor basaltic rocks at off-axis. In particular, four OCCs in the area are situated on ridge-transform intersection (RTI) but seven OCCs are adjoining to NTDs. The two OCCs are identified with the uplifted and elongated-shape NTDs of which host rocks are the serpentinite and/or gabbroic affinities. Some OCC is featured by a septal structures and either tip of the NTD link with other OCCs at axial valley flanks. The OCC at 15˚30S shows also a septum structure. This OCC in NTD offsets first-order segment to two second-order segments. The eastern and western part of the NTD disclose the gabbroic rocks and serpentinite, respectively.The observed OCCs frequently show the hydrothermal plum signals. Through the dredge haul, hydrothermal sulfides were also sampled at some OCC. The sulphides are featured by Cu-bearing massive sulfides (or their relatives). The fragments-like samples indicate low-temperature Zn-Ba chimneys. On the basis of bathymetric-mapping surveys, the OCCs were more intensively investigated to identify the evidences of hydrothermal activities. Subsequently we often recognized great methane concentration (~45 nmol/l) at water column above OCCs. The light-scattering values in water column was little bit low. High methane contents but little particulates in the water column reflect that hydrothermal fluids might have been generated from interaction between ultramafic rocks and seawaters.