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

Pinaki C Bandopadhyay,Douwe J.J Van Hinsbergen,Debaditya Bandyopadhyay,Alexis Licht,Eldert L Advokaat,Alexis Plunder,Biswajit Ghosh,Arnab Dasgupta,João P Trabucho-Alexandre

doi:10.1016/j.gr.2021.10.011

Abstract

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.

Highlights

Tuffs rich in juvenile grains are exclusively present in the Namunagarh Grit of South Andaman Island, whereas Namunagarh Grit sandstones from Middle and North Andaman are tuffaceous
With paleomagnetic data suggesting that the paleolatitude of the West-Burma Block in Late Cretaceous to Eocene time was similar as Sumatra (Westerweel et al, 2019), it is possible that this block, or arc rocks associated with subduction below the West Burma Block, formed a source from the west, rather than the north of the Andaman Islands
We study the provenance of the clastic sedimentary formations, through sediment petrography, geochemistry, and U–Pb detrital zircon age spectra, and evaluate their formation and depositional environment in context of recent paleomagnetic findings that suggest that the West Burma Block occupied an equatorial paleolatitude from the Late Cretaceous to Eocene, much farther south than previously assumed, and at a similar latitude as the Andaman Ophiolites in the Sumatra forearc

Summary

Methods

We collected medium to fine-grained sandstones from exposures of the Namunagarh Grit Formation and the East and West Andaman Flysch, distributed along the 700 km long belt from North Andaman to Great Nicobar (Table 1). Range and average concentrations of major oxides, trace elements and Rare Earth elements (REE) from bulk samples were determined on a selection of 33 samples that include nine sandstone and three mudstone samples from East Andaman Flysch, seven sandstone samples from West Andaman Flysch (five from the Andaman and two from the Great Nicobar Island), and eleven sandstone and three mudstone samples from Namunagarh Grit. Zircon U-Pb age dating was performed on five samples from the West Andaman Flysch (four from the Andaman and one from the Great Nicobar Island), one from the Namunagarh Grit

Results

Discussion

Conclusion