Indian Journal of Marine Sciences Vol. 36(2), June 2007, pp. 105-109 Power law relation of bathymetry and gravity roughness with age of oceanic crust below Ninety East Ridge Abhey Ram Bansal* National Geophysical Research Institute, Uppal Road, Hyderabad – 500 007, India *[E-mail: abhey_bansal@ngri.res.in ] Received 11 October 2006; revised 11 April 2007 In this study altimeter gravity and ship track bathymetry data from National Geophysical Data Center (NGDC) is explored to investigate the relation between roughness of gravity and bathymetry data to the age of oceanic crust over 90°E Ridge. The gravity and bathymetry profiles are selected over the ridge at places of known ages from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). The gravity and bathymetry data are gridded at sampling interval of 1’ 12’’ and profiles are extracted on the DSDP and ODP sites. The roughness of gravity and bathymetry data is computed as the square root of the average squared deviation about a linear trend. The roughness varies as a power –1.31 and -1.59 of age for bathymetry and gravity data. The mean fractal dimension is found to be 1.54 and 1.46 for bathymetry and gravity data. The roughness of bathymetry and gravity data varies with age with different power law. It is also interesting to note that the roughness of bathymetry decreased with age, which indicates modification of bathymetry data in the older (northern) portion of the ridge with continuous depositions of Bengal fan sediments. [Key words: Gravity, bathymetry, roughness and power law, age of oceanic crust, ninety east ridge, ODP, DSDP] Introduction Fig. 1⎯Study area map (after Krishna et al. 6 ) . Afanasy Nikitin Seamount (ANS), Cretaceous Magnetic Quit Zone (CMQZ) and various fracture zones (FZ) i.e. 80°E, Indira, 84.5°E and 86°E are also shown. The Indian Ocean is evolved through the breakup of Eastern Gondwana land into Australia- Antarctica and Greater India in the Late Jurrassic/ Early Cretaceous 1-6 . The Eastern Indian Ocean is overlain by 85 °E and 90° E ridge and many fractures zones (Fig. 1). The 90°E ridge is one of the world’s longest volcanic ridge extends more than 5000 km from 18 °N to 34 °S in N – S direction 7,8 . It emplaced during the northern movement of Indian plate over the Kerguelen hotspot during the period 38 - 82 Ma due to transform motion. The northern part (north of 10 °N) of the ridge is buried under the thick sediments of Bengal fan 4,8 . The ages found from DSDP and ODP sites were monotonically decreasing from north to south whereas ages found from magnetic data did not show such trend 7 . The aim of this study is to establish a relation between roughness of gravity and bathymetry data with age of oceanic crust below the 90°E Ridge. The roughness is independent of precise knowledge of the location of the ridge axis, whereas amplitude and polarity of the ridge axis anomaly depends strongly on the location of the ridge axis 9 . ________________ *Ph.: +91-40-2343700 Fax:- +91-40-23434651