Sedimentary Record of Paleodeformation of the Saint Martin Anticline Reveals the Interaction Between Tectonics, Sedimentation Processes and Relative Sea-level Changes: Ganges-Brahmaputra Delta Burma Arc Collision, SE Bangladesh

Abstract:

Along the Ganges-Brahmaputra Delta Burma Arc collision zone, the Indian plate is converging obliquely with the Burma arc with a shortening GPS rate of 14 mm/yr. In this region tectonics, huge sediment supply from the erosion of the Himalayas (>1 GT/yr) and relative sea level changes interact. The extremely thick sediments of the Ganges-Brahmaputra Delta and Fan (~20 km) are being gradually accreted into a very wide thrust-fold belt along this subduction/collision zone. This interaction has led to the formation of a fold-and-thrust belt and wide accretionary prism that is exposed on land and on the shelf of the Burma forearc. Sedimentary sequence and structure document the seismo-tectonic and sedimentary evolution of St Martin’s Island, which is an expression of an anticline in the outer part of the accretion forearc. During late Pleistocene sea-level low stands, the anticline and most of the rest of the shelf were exposed. The anticline ridge could then grow, receiving little erosion by local drainages. At some point during sea level rise, the coast advanced over the ridge and bevelled it. Then sea-level continued to rise and new sediment deposited forming an angular unconformity. A low-relief unconformity, in fact, separates the folded strata below from overlying strata of Holocene age. Where now exposed on the island above sea level, the unconformity is ~8000 y old. At that time, sea level was 30 m below the current level and thus rapid tectonic uplift is necessary to account for its current elevation. Dead coral heads of the species Porities that populate the coast were dated with U-Th (Mondal et al., 2013). They document a megathurst rupture and ~ 2m uplift that occurred during the Great Arakan earthquake in 1762 and further earthquakes are likely at ~ 1100 and 800 AD.

Studies of the 1762 rupture suggest that the anticline rises during megathrust earthquakes (~2m) and subsides in the interseismic period (a few mm/yr; Steckler and Mondal 2014). Radiocarbon ages that are being obtained from marine fossils recovered from trenches in the southern and northern parts of the island will be coupled to coral uplift studies. They will permit us to better constrain the relation between erosion and sedimentation of the anticline, as well as its growth and subsidence during coseismic and interseismic periods.