Paleogeography of the West Burma Block and the eastern Neotethys Ocean: Constraints from Cenozoic sediments shed onto the Andaman-Nicobar ophiolites

The Andaman and Nicobar ophiolites, in the forearc of the western Sunda subduction zone, underwent enigmatic, rapid Cenozoic vertical motions: shallow-water sediments with abundant arc debris characterize the middle Paleocene–middle Eocene and are under- and overlain by significantly deeper sediments. Recent paleomagnetic results revealed a near-equatorial paleolatitude of the West Burma Block and the associated subduction zone, at a similar latitude as the Andaman forearc until the early Eocene, providing a new avenue toward explaining the unusual stratigraphy. Here, we studied the provenance of the clastic sediments of the Andaman-Nicobar accretionary ridge using petrography, geochemistry, and detrital zircon geochronology. We found that the Paleocene-Eocene Namunagarh Grit is likely to be derived from a then proximal, 60 Ma old arc that was likely located in the ocean to the north (present-day east) of the West Burma Block, west of Andaman-Nicobar. The Oligocene–lower Miocene East Andaman Flysch contains West Burma Block debris that traveled much farther and mixed with sediments derived from Sundaland. The West Andaman and Great Nicobar Flysch have an additional Himalayan source consistent with derivation from the downgoing plate. We interpret this history as reflecting the late Paleocene–early Eocene collision of the West Burma Block, likely then part of the Australian Plate, with the Andaman forearc causing uplift and proximal sedimentation shed from the colliding arc. Subsequent northward motion of the West Burma Block caused subsidence of the Andaman forarc and N-S opening of the Andaman Sea, which opened a pathway for Sundaland-derived sediments to reach the Andaman ophiolites. The recently proposed high Cenozoic mobility of the West Burma Block remains to be reconciled in detail with geological observations in Myanmar and Sundaland, but our results show that this scenario provides ample opportunity to explain the previously enigmatic stratigraphic evolution of the Andaman and Nicobar Islands.