Abstract

The Andaman Basin extends 1200 km from Burma to Sumatra and 650 km from the Malay Peninsula to the Andaman and Nicobar islands. The eastern portion of the basin is the Malay continental margin, a 250 km wide shelf which was cut into Malayan Paleozoic and Mesozoic rocks during the Early and Middle Tertiary. Subsequent folding and faulting modified the shelf, forming at least two major terraces, the Sumatra Shelf-basin and a 650 km long continental slope. Only a discontinuous veneer of Cenozoic sediments covers the continental margin, except for the Sumatra Shelf-basin, in which over 1 km of sediment has collected.

The Andaman-Nicobar Ridge, western boundary of the basin, consists of an Upper Cretaceous serpentinite-ophiolite-radiolarite core overlain by Paleocene to Miocene graywackes and shales at least 3000 m thick. Between the ridge and the Malay continental margin lies the Central Andaman Trough, two elongate basaltic seamounts 220 km long, and a complex system of rift valleys and associated smaller volcanic seamounts. Maximum depths of 4400 m in the Andaman Basin are located in the major branch of the rift system, the 700 km long Nicobar Rift Valley which is the proper boundary between the Andaman-Nicobar Ridge and the Central Andaman Trough.

Sediments in the Central Trough are 1.5 km thick, surprisingly thin in view of the huge Irrawaddy River sediment load which enters the north end of the basin and is trapped in the basin by surface currents. Large volumes of detritus which must have been produced during the Tertiary planation of the Malay shelf also cannot be accounted for by the minor Andaman Basin fill. The great thicknesses of pre-Late Miocene Tertiary sediments in the Andaman-Nicobar Ridge were derived from the northeast, an area now occupied by submerged youthful rift topography. It is concluded that the sediments derived from planation of the Malay shelf were shed into an adjacent Tertiary trough which was subsequently molded into the Andaman-Nicobar Ridge during Oligocene to Miocene time. The continental margin and the Andaman-Nicobar Ridge were then rifted apart by Late Miocene to Recent movements which formed the Andaman Basin as a rhombochasm. Geophysical data support this interpretation.

The entire southeast Asian margin is marked by evidences of south-southeastward movement. Dextral strike-slip along Sumatra and sinistral strike-slip along the Philippine Rift are documented in the literature. The Java Trench along the front of the block has been interpreted as a product of thrusting. Parallelism of the Philippine Trench and Philippine Rift is understandable when the trench is interpreted as a rhombochasm.

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