Distribution of stress axes orientation in the Andaman-Nicobar island region: a possible stress model and its significance for extensional tectonics of the Andaman Sea

Abstract

Composite plots of pressure and tension axes of the available focal mechanism solutions of earthquakes are used to investigate the nature and process of the stress system and their bearing on tectonics in the Andaman-Nicobar island region. The composite plots show that the Andaman-Nicobar island arc and the Andaman Sea are subjected to SSW-NNE directed compressive force in response to the NE drifting motion and/or oblique subduction of the Indian plate beneath the Andaman-Nicobar island arc, Stress study also reveals that the Andaman Sea at the same time is experiencing an ESE-WNW tensile stress analogous to active back-arc opening/extension. Based on this stress pattern, a strain ellipse model is proposed to explain the extensional tectonics of the Andaman Sea. It is suggested that the oblique subduction of the Indian plate invokes NE-SW directed compression as well as north-south directed shear stress. These combined stresses are transmitted to or act on the Andaman Sea plate and result in stretching of the Andaman Sea plate in a NW-SE (least stress) direction on the strain ellipse. This stretching leads to NE-SW directed fracture zones, normal to stretching, where ascending magmas are emplaced to form active mid-oceanic spreading ridges, and thus active back-arc extension or opening of the marginal basin takes place, Nearly N-S dominated fracture zones, which form ridge-ridge type transform faults by offsetting the spreading ridges, are thought to originate chiefly along the plane of maximum shear of the strain ellipse that makes an angle of 45° ± 5° with the greatest horizontal stress component ( σ 1) and both have a close resemblance.