Chapter 13 Anatomy of the Andaman–Nicobar subduction system from seismic reflection data

Abstract

The Andaman–Nicobar subduction system is the northwestern segment of the Sunda subduction system, where the Indian Plate subducts beneath the Sunda Plate in a nearly arc-parallel direction. The entire segment ruptured during the 2004 great Andaman–Sumatra earthquake (Mw=9.3). Using recently acquired high-resolution seismic reflection data, we characterize the shallow structure of the whole Andaman–Nicobar subduction system from west to east, starting from the nature of the subducting plate in the Bay of Bengal to back-arc spreading in the Andaman Sea. We find that the Ninety-East Ridge is overlain by thick continental margin sediments beneath the recent Bengal Fan sediments. The boundary between these two sedimentary units defines the plate interface. We observe evidence of re-activation of fracture zones on the subducting plate beneath the forearc, influencing the morphology of the upper plate. The forearc region, which includes the accretionary wedge, the forearc high and the forearc basin, is exceptionally wide (250 km). We observe an unusually large bathymetric depression within the forearc high. The forearc high is bounded in the east by a normal fault, whereas the forearc basin contains an active backthrust. The forearc basin is floored by the continental crust of Malayan Peninsula origin. The active sliver strike-slip fault lies in a deep basin, created during the rifting of the forearc continental crust and the Malayan Peninsula. The sliver fault connects with the Great Sumatra Fault in the south and with the Sagaing Fault in the north, via the Andaman Sea spreading centre and a large transform fault in the Andaman Sea.

Late Cenozoic seismic stratigraphy of the Andaman Forearc Basin, Indian Ocean

“…The EMF is a marginal normal fault with easterly dips, whereas the DF comprises of a collective thrust faulted units adjoining IB. The uplift of the IB appears to have occurred episodically, which agrees with previous studies (Curray 2005;Singh and Moeremans 2017). We infer that it may have undergone a preliminary uplift in the Early Oligocene time followed by deformational activities since the Mid-Miocene.…”

“…3,4,5,6,7). Stratigraphic interpretations from the Sumatra Forearc region (Beaudry and Moore 1985;Berglar et al 2008;Cochran 2010;Moeremans and Singh 2015;Singh and Moeremans 2017) and recent IODP drilling results (Clemens et al 2015;McNeill et al 2016) show that the igneous basement underneath the southern part of the Andaman-Sumatra Forearc is overlain by the Paleogene meta-sediments. In the present study, from the northern parts of the Andaman-Sumatra Forearc, the acoustic basement is interpreted to be at * 6.5-7 s TWT deep.…”

“…3, 4). The geological ages of the key identified horizons were obtained based on correlation with the well data from the AFB (Goli and Pandey 2014) and other published information (Curray 2005;Cochran 2010;Singh and Moeremans 2017). The prominent horizons on the seismic sections represent major unconformities from the Neogene to the Recent period.…”

“…A rather oblique convergence resulted in a strike-slip fault along the trench axis, formation of forearc and back-arc basins and seafloor spreading (Curray et al 1979;Curray 2005). The tectonics of the Andaman-Nicobar region has been discussed in detail by several researchers (Rodolfo 1969;Curray et al 1979;Kamesh Raju et al 2004Curray 2005;Singh et al 2013;Moeremans et al 2014;Singh and Moeremans 2017;and references therein).…”

“…The stratigraphy and structural evaluation of the forearc off northern Sumatra to Java is fairly well documented (Beaudry and Moore 1985;Izart et al 1994;Susilohadi et al 2005;Mosher et al 2008;Berglar et al 2008Berglar et al , 2010Moeremans et al 2014;Singh and Moeremans 2017). In contrast, the seismic stratigraphy of the Andaman Forearc region and its structural details have not been precisely described so far, due to the lack of adequate high-quality depth-controlled data.…”