Development of the negative gravity anomaly of the 85°E Ridge, northeastern Indian Ocean – A process oriented modelling approach

Abstract

The 85 • E Ridge extends from the Mahanadi Basin, off northeastern margin of India to the Afanasy Nikitin Seamount in the Central Indian Basin. The ridge is associated with two contrasting gravity anomalies: negative anomaly over the north part (up to 5 • N latitude), where the ridge structure is buried under thick Bengal Fan sediments and positive anomaly over the south part, where the structure is intermittently exposed above the seafloor. Ship-borne gravity and seismic reflection data are modelled using process oriented method and this suggest that the 85 • E Ridge was emplaced on approximately 10-15 km thick elastic plate (Te) and in an off-ridge tectonic setting. We simulated gravity anomalies for different crust-sediment structural configurations of the ridge that were existing at three geological ages, such as Late Cretaceous, Early Miocene and Present. The study shows that the gravity anomaly of the ridge in the north has changed through time from its inception to present. During the Late Cretaceous the ridge was associated with a significant positive anomaly with a compensation generated by a broad flexure of the Moho boundary. By Early Miocene the ridge was approximately covered by the postcollision sediments and led to alteration of the initial gravity anomaly to a small positive anomaly. At present, the ridge is buried by approximately 3 km thick Bengal Fan sediments on its crestal region and about 8 km thick pre-and post-collision sediments on the flanks. This geological setting had changed physical properties of the sediments and led to alter the minor positive gravity anomaly of Early Miocene to the distinct negative gravity anomaly.

Heterogeneous structure of the lithosphere in the Black Sea from a multidisciplinary analysis of geophysical fields

“…The same sign of the gravity field is registered over the buried 85° Ridge in the Indian Ocean [e.g. Sreejith et al, 2011].…”

Lithosphere structure and upper mantle characteristics below the Bay of Bengal

“…As stated earlier, for the coherence analysis, we considered a value of 0.5 for the loading ratio f as the modeled f values elsewhere along the Ninetyeast Ridge range from 0.5 -0.7 (Grevemeyer and Flueh, 2000;Grevemeyer et al, 2001) and 0.3 -0.6 . The present Te values obtained from both 85°E and Ninetyeast ridges are combined with the Te estimates published for the 85ºE Ridge (Sreejith et al, 2011) and for the Ninetyeast Ridge Te values (Watts et al, 2006), however, our estimates are better constrained due to the availability of regional seismic data for the ridge north of 10 o N which is progressively buried below the Bengal Fan sediments.…”

85°E Ridge, India – Constraints on its Development and Architecture

“…Gravity models of the ridge indicate existence of hotspot-related crustal sagging and underplating beneath the ridge (e.g., Subrahmanyam et al, 1999;Sreejith et al, 2011). Subrahmanyam et al (1999) further note the trajectory of the 85 o E Ridge as being slightly oblique to the sea-floor spreading vectors, a setting in which the ridge emplacement must have taken place on relatively young but already evolved ocean floor.…”

“…an abandoned spreading centre (Mishra, 1991); 2. a northward continuation of the 86°E fracture zone (Chaubey et al, 1991;Kent et al, 1992;Ramana et al, 1997); and 3. a hot-spot track (Curray and Munasinghe, 1991;Muller et al, 1993;Murthy et al, 1993;Gopala Rao et al, 1997;Subrahmanyam et al, 1999;Bastia et al, 2010;Sreejith et al, 2011;Radhakrishna et al, 2012). Most of the recent studies have postulated a hot-spot origin of the ridge, although the associated plume remains debated.…”