Subduction subsidence of the leading edge of a lithospheric plate into aesthenosphere. It occurs when either two oceanic plates or one oceanic plate and one continental plate converge forming an elongate zone in which one lithosphereic plate descends beneath another. A subduction zone is typically characterised by an oceanic trench, lines of volcanoes and crustal deformation leading to mountain building. The deep oceanic trenches are the most active geological structures on the surface of the earth, developing adjacent to the volcanic island-arcs and along the continental margins. These trenches constitute linear zones along which moving oceanic plates slip under the stationary or comparatively slow moving continental plate and plunge deep into the upper mantle where they are consumed or destroyed. Subduction zones are characterised by shallow, intermediate and deep seismicity, negative gravity anomaly and very low heat flow. The trenches are the sites where sediments accumulate partly scraped off from the underthrusting oceanic plate and subsequently compressed and deformed into mountains.

The northward moving Indian Ocean Plate is striking and disappearing into two subduction zones, one along the Marakan coast of Pakistan and Iran in the west and the other along the Java-Andaman margin. Andaman subduction zone continues northward along Patkai-Naga Arakan orogenic belt predominantly of Tertiary sediments and represents southeastern extension of the Timor-Mentawei sedimentary islands. The Andaman-Nicobar sedimentary island chain is constituted of Cretaceous flysh, radiolarian chert and pelagic limestone. These are implanted with ophiolites and opiolitic melange overlain by Eocene to Oligocene flysch and Neogene shallow water sediments derived from the Tenasserim-Malaysia belt in the east-dipping nappes which ride over the Recent Bengal Fan sediments. The structural design of the Andaman arc is very similar to that of the Patkai-Arakan range being characterised by a highly deformed, by steep east dipping thrusts delimited by ophiolitic melange and ultramafics. The Upper Cretaceous ophiolites and the sediments represent the sea floor material scraped off the underthrusting Indian Plate. The seismicity of the Andaman-Indonesia arc shows a well-defined Benioff zone, with shallow, intermediate and deep hypocentres distributed from southwest to northeast. [ASM Woobaidullah]