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 rid ge in the study area rises to similar to3.5 km above the seafloor of adjace nt ocean basins. Basement is covered by sediments up to 0.8 km thick but ex posed at some locations. Seismic refraction and gravity models of the ridge show that the crustal thickness is similar to 22 km, greater than the aver age thickness (7 km) of oceanic crust estimated in the Central Indian and W harton Basins. The ridge is compensated by flexure of layers 2A, 2B, and 3A to the extent of similar to2 km and by a 12 km thick deep crustal body (la yer 3B) of underplated material. Upper crustal velocities (layer 2A) beneat h the ridge are lower, up to 3.4 km s(-1) than in adjacent basins because o f weathering process and hydrothermal circulation of the waters in the tect onized 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 wi thin the lithosphere. Steep downward faulting of similar to2 km along the e astern flank of the Ninetyeast Ridge possibly originated because of compres sional and extensional stresses applied along the 89 degreesE 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 la yer 3A are exposed on the eastern flank of the Ninetyeast Ridge and 90 degr eesE FZ, respectively They are consistent with basalt, dolerite, and gabbro rocks, collected from the fault scarps. The central part of the ridge betw een 1 degreesN and 11 degreesS has uniformly deepened by similar to0.7 km a nd is bounded by major sublatitudinal deep-seated fractures. We interpret t hat the deepened structural block and associated fractures resulted from th e activity of the intense deformation of the central Indian Ocean.