PROVENANCE OF SEDIMENTS IN THE ANDAMAN ACCRETIONARY PRISM: IMPLICATIONS FOR PALEOGENE DRAINAGE IN SOUTH ASIA

The Andaman-Nicobar Islands of India are part of an accretionary complex formed on the outer-arc ridge of the Sunda-Andaman subduction zone. The ridge is an imbricate stack of fault slices consisting of Cretaceous seafloor ophiolites and Paleocene to Holocene terrigenous and pelagic sediments. Deposited along the convergent margin of Indian and Eurasian plates, the sedimentary rocks of these islands are believed to have recorded the evolutionary history of the Indo-Myanmar region including the eastern Himalayas. Geochemical and Sr-Nd isotopic ratio studies were carried out on the samples collected from all the siliciclastic formations, spread over the entire Andamans, to determine the sources of the terrigenous sediments and their transport pathways. New age constraints were placed based on Sr-isotope stratigraphy of carbonate formations and ⁴⁰Ar/³⁹Ar dating of tuff horizons – which put the entire sedimentary sequence of the accretionary wedge in the age bracket of 72.0 to 0.73 Ma. Our results show that ε_Nd(0) of the Paleocene-Eocene trench-slope sediments (Mithakhari Group) vary from -8.1 to +4.2 with a mode at -2.0 (⁸⁷Sr/⁸⁶Sr = 0.70497 to 0.71554) whereas that of the overlying Oligocene fore-arc sediments (Andaman Flysch Group) range from -12.7 to -9.4 with a mode at -10.0 (⁸⁷Sr/⁸⁶Sr =0.71548 to 0.73049). The sediments of Mithakhari Group, very clearly, are primarily derived from the ophiolitic basement and/or magmatic arc (> 80%) with minor contributions from the continental sources. These continental sources started dominating (20 - 40%) during the deposition of the Andaman Flysch Group. Since this trend coincides with the timing of attainment of critical height of the Himalayas and establishment of the Indian monsoon system, we are inclined to believe that the enhanced weathering and erosion of the Himalayan rocks generated the continental sediments that ultimately got deposited in the Indo-Burman-Andaman fore-arc basin. This would mean that there existed active drainage system in the eastern Himalayas, possibly the proto-Brahmaputra, which provided the transport pathways.