Subduction

'Subduction' The process of subsidence of the leading edge of a lithospheric plate into aesthenosphere is known as subduction.The earth crust is categorized as oceanic and continental. The oceanic crust of the Earth is different from its continental crust. The oceanic crust is 5 km to 10 km thick and is composed primarily of basalt, diabase, and gabbro. The continental crust is typically from 30 km to 50 km thick, and it is mostly composed of less dense rocks such as granite. Because of convection in the underlying plastic, although non-molten, upper mantle and asthenosphere, the lithosphere is broken into tectonic plates that move.

Subduction 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.

The Bengal Basin is gradually being encroached on by the arcuate Indo-Burma ranges, almost 230 km wide active orogenic belt associated with eastward subduction of the Indian plate below Myanmar. Folds and thrust faults in the Indo-Burma ranges trend north south consistent with this eastward subduction. Earthquake data however, suggest that the basement of the Indian plate below the Indo-Burma ranges is moving north. [ASM Woobaidullah] [Woobaidullah, ASM Professor of Geology, Dhaka University]