Geophysical characteristics of the Ninetyeast Ridge–Andaman island arc/trench convergent zone

Abstract

The convergence tectonics of the Ninetyeast ridge (NER), upon the Andaman island arctrench system is examined through an analysis of ETOPO2 bathymetry, satellite-derived free air gravity and seismic data. Oblique subduction and the buoyancy forces arising from subduction of the NER render the subduction processes near the Andaman arc highly complex. The bathymetric expression of the NER is visible up to Lat. 10 o N but seismic reflection data indicate that it extends up to about Lat.17 o N. The gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion. There is a prominent break in the continuity of the trench gravity low, where the NER seems to impinge upon the island arc. Further, a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc, associated with the forearc basin, is a dominant feature of the gravity map. An offset in the continuity of this strong negative anomaly occurs at about the same latitude where the NER seems to be converging upon the island arc. Seismic reflection data indicate that the NER is very close to the trench. Flexural modeling of the gravity anomalies for the subducting Indian ocean lithosphere, loaded by sediments and the NER, indicate that the NER is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself. We infer that the en-echelon block structure of the NER in the proximity of the convergent zone is a consequence of complex strike-slip and subduction related tectonic forces.

Lithosphere structure and upper mantle characteristics below the Bay of Bengal

“…As the volcanism leading to the emplacement of the 85°E and Ninetyeast ridges in the BOB are older (> 80 Ma), the swells associated with the initial volcanism will have subsided. Earlier derived crustal models show the presence of thicker crust with and without underplating below these ridges (Detrick and Watts, 1979;Mukhopadhyay and Krishna, 1995;Subrahmanyam et al, 1999;Grevemeyer et al, 2001;Krishna et al, 2001b;Krishna, 2003;Subrahmanyam et al, 2008;Radhakrishna et al, 2010). The present analysis revealed that both ridges, in general, are earlier investigators for the ridge south of 2ºN revealed that this part of the ridge had evolved due to frequent ridge jumps in the vicinity of the Kerguelen hot spot during the rapid northward migration of the Wharton spreading ridge (Royer et al, 1991;Krishna et al, 1999;Sager et al, 2010;Krishna et al, 2012).…”

“…Additionally, for the purpose of 2-D flexural analysis over the aseismic ridges, we used several short E-W seismic reflection profiles across the 85°E Ridge (Bastia et al, 2010;Radhakrishna et al, 2012;Krishna et al, 2014;Rao and Radhakrishna, 2014) and across the Ninetyeast Ridge (Curray et al, 1982;Gopala Rao et al, 1997;Subrahmanyam et al, 2008;Maurin and Rangin, 2009 …”

“…In the case of Ninetyeast Ridge, we further removed the subduction related forebulge from the reconstructed topography. Here the maximum forebulge (w b ) has been estimated based on the method of Harris and Chapman (1994) by assuming a Te of 35 km for the subducted portion of the Indian plate (after Subrahmanyam et al, 2008) and it varies between 190-250 m at a distance of 100 km from the trench axis.…”

S‐velocity characterization of the crust and upper mantle structure beneath the Bay of Bengal

“…The crust beneath the 85E is between 18 and 22 km thick (~20 km thick), that is, it is thicker by~15 km than adjacent normal oceanic crust, which is 5 km thick (Subrahmanyam et al, 2001). This suggestion was followed by other authors, who proposed the presence of a thicker crust below the 90E (Detrick & Watts, 1979;Grevemeyer et al, 2001;Krishna et al, 2001;Mukhopadhyay & Krishna, 1995;Subrahmanyam, Gireesh, Chand, Kamesh Raju, & Gopala Rao, 2008). As the anomalies associated with the 90E are small, Bowin (1973) suggested also the existence of some way of compensation at depth (as the 85E compensation).…”

Tectonic and volcanic implications of a cratered seamount off Nicobar Island, Andaman Sea

“…Based on the known volcanoes in the region, there is a distinct gap in active volcanoes from 06°N to 12°N. Subduction of the base of the Ninety-east ridge was suggested as one of the possible reasons for this gap in volcanic activity (Subrahmanyam et al, 2008). It is interesting to note the occurrence of several earthquake swarms (S1-S7, Fig.…”