The non-transform discontinuity on the Central Indian Ridge at 11°S: The transtensional basin formation and hydrothermal activity

The bathymetric and magnetic survey, hydrocasting and seabed sampling have been carried out in the middle portion of the Central Indian Ridge (MCIR) between 7°S and 17°S. The MCIR constitutes six first-order segments and seven second-order segments with four non-transform discontinuities (NTDs) and twelve ocean core complexes (OCCs). These segments are characterized by asymmetric accretion that corresponds to about 70% of the surveyed MCIR segment. One of the outstanding NTD in the area is a basin like NTD3-1 at 11°S (50km in length) which strike at 035°, approximately 45° oblique in a clockwise direction to the orientation of two adjoining second-order segments. The hydrothermal activity is recognized at the tips of NTD3-1. No abyssal hills paralleling to basin-shape NTD3-1 are observed. Anomalous depth of the basin, lack of positive magnetic anomaly across the basin and rare seismic activities in the basin floor suggests that extensional tectonism with a sparse volcanism is the dominant process occurring along the NTD3-1. Based on the previous researches that the counterclockwise rotation of ridge is predominant in the area, the region of NTD3-1 largely accommodates shear strain by left-lateral sense motion and consequently forms a transtensional basin, i.e., a pull-apart basin. The strong and frequent hydrothermal plume signals, and highly tectonized rocks in both tips of the NTD3-1 are reflective of the dilation zones or tensional fractures accompanied by the pull-apart basin formation. It is the first identification of a pull-apart basin associated with hydrothermal activity in the Central Indian Ridge.