Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge

Kumar, Ratheesh RT and Windley, BF and Rajesh, VJ and Santosh, M (2013) Elastic thickness structure of the Andaman subduction zone: Implications for convergence of the Ninetyeast Ridge. In: Journal of Asian Earth Sciences, 78 . pp. 291-300.

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Abstract

We use the Bouguer coherence (Morlet isostatic response function) technique to compute the spatial variation of effective elastic thickness (T-e) of the Andaman subduction zone. The recovered T-e map resolves regional-scale features that correlate well with known surface structures of the subducting Indian plate and the overriding Burma plate. The major structure on the India plate, the Ninetyeast Ridge (NER), exhibits a weak mechanical strength, which is consistent with the expected signature of an oceanic ridge of hotspot origin. However, a markedly low strength (0< T-e <3 km) in that region, where the NER is close to the Andaman trench (north of 10 N), receives our main attention in this study. The subduction geometry derived from the Bouguer gravity forward modeling suggests that the NER has indented beneath the Andaman arc. We infer that the bending stresses of the viscous plate, which were reinforced within the subducting oceanic plate as a result of the partial subduction of the NER buoyant load, have reduced the lithospheric strength. The correlation, T-e < T-s (seismogenic thickness) reveals that the upper crust is actively deforming beneath the frontal arc Andaman region. The occurrence of normal-fault earthquakes in the frontal arc, low Te zone, is indicative of structural heterogeneities within the subducting plate. The fact that the NER along with its buoyant root is subducting under the Andaman region is inhibiting the subduction processes, as suggested by the changes in trench line, interrupted back-arc volcanism, variation in seismicity mechanism, slow subduction, etc. The low T-e and thinned crustal structure of the Andaman back-arc basin are attributed to a thermomechanically weakened lithosphere. The present study reveals that the ongoing back-arc spreading and strike-slip motion along the West Andaman Fault coupled with the ridge subduction exerts an important control on the frequency and magnitude of seismicity in the Andaman region. (C) 2013 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Publication:	Journal of Asian Earth Sciences
Publisher:	Elsevier Science
Additional Information:	Copyright of this article belongs to Elsevier Science.
Keywords:	Ninetyeast Ridge; Bouguer Coherence; Elastic Thickness; Hot Spot; Frontal Arc
Department/Centre:	Division of Mechanical Sciences > Centre for Earth Sciences
Date Deposited:	10 Jan 2014 08:29
Last Modified:	10 Jan 2014 08:29
URI:	http://eprints.iisc.ac.in/id/eprint/48098

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