Active faulting on the Ninetyeast Ridge and its relation to deformation of the Indo-Australian plate

The similar to 4500 km long Ninetyeast Ridge (NER) in the northeastern Indian Ocean crosses a broad zone of deformation where the Indo-Australian plate is fracturing into three smaller plates (India, Capricorn, Australia) separated by diffuse boundaries whose extents are poorly defined. New multichannel seismic reflection profiles image active faults along the entire length of the NER and show spatial changes in the style of deformation along the ridge. The northern NER (0 degrees N-5 degrees N) displays transpressional motion along WNW-ESE faults. Observed fault patterns confirm strike-slip motion at the western extent of the April 2012 Wharton Basin earthquake swarm. In the central NER (5 degrees S-8 degrees S), deformation on WNW-ESE-trending thrust faults implies nearly N-S compression. An abrupt change in fault style occurs between 8 degrees and 11 degrees S, with modest, extension characterizing the southern NER (11 degrees S-27 degrees S). Although extension is dominant, narrow zones of faults with strike-slip or compressional character also occur in the southern NER, suggesting a complex combination of fault motions. At all sites, active faulting is controlled by the reactivation of original, spreading-center formed, normal faults, implying that deformation is opportunistic and focused along existing zones of weakness, even when original fault trend is oblique to the direction of relative plate motion. Observed faulting can be interpreted as India-Australia deformation in the northern NER and Capricorn-Australia deformation in the southern NER. The India-Capricorn boundary is directly adjacent to the northern NER and this juxtaposition combined with a different style of faulting to the east of the NER imply that the ridge is a tectonic boundary.