Crustal structure and tectonics of the Ninetyeast Ridge from seismic and gravity studies

Seismic reflection and refraction, gravity, and bathymetric data across and along the central part of the Ninetyeast Ridge were analyzed to determine the crustal structure of the ridge and to understand its tectonics. The ridge in the study area rises to ∼3.5 km above the seafloor of adjacent ocean basins. Basement is covered by sediments up to 0.8 km thick but exposed at some locations. Seismic refraction and gravity models of the ridge show that the crustal thickness is ∼22 km, greater than the average thickness (7 km) of oceanic crust estimated in the Central Indian and Wharton Basins. The ridge is compensated by flexure of layers 2A, 2B, and 3A to the extent of ∼2 km and by a 12 km thick deep crustal body (layer 3B) of underplated material. Upper crustal velocities (layer 2A) beneath the ridge are lower, up to 3.4 km s−1 than in adjacent basins because of weathering process and hydrothermal circulation of the waters in the tectonized blocks. While velocities in deep crustal body (layer 3B) are higher, up to 7.8 km s−1 and seem to be resulted from magmatic rocks accreted within the lithosphere. Steep down ward faulting of ∼2 km along the eastern flank of the Ninetyeast Ridge possibly originated because of compressional and extensional stresses applied along the 89°E Fracture Zone (FZ). The stresses are interpreted as resulting from multiple ridge jumps of Wharton spreading centers during the Late Cretaceous and middle Eocene. The velocity‐depth model suggests that the rocks of layers 2A and 2B and layer 3A are exposed on the eastern flank of the Ninetyeast Ridge and 90°E FZ, respectively. They are consistent with basalt, dolerite, and gabbro rocks, collected from the fault scarps. The central part of the ridge between 1°N and 11°S has uniformly deepened by ∼0.7 km and is bounded by major sublatitudinal deep‐seated fractures. We interpret that the deepened structural block and associated fractures resulted from the activity of the intense deformation of the central Indian Ocean.