ONSET OF INDIAN OBLIQUE CONVERGENCE ALONG THE BURMESE SUBDUCTION MARGIN RECORDED IN UPPER EOCENE SEDIMENTARY DEPOSITS OF CENTRAL MYANMAR

Since the extrusion and rotation of the Indochina Peninsula following the India-Asia collision, the convergence of India relative to Myanmar has been highly oblique. This high obliquity has resulted in the strike-slip partitioning of the 1100-km long forearc basin of central Myanmar into individual pull-apart basins, as well as the early build-up of the Indo-Burman Ranges, the accretionary prism of the Burmese subduction margin. Yet, the timing of Indochina rotation and shift to high oblique convergence remain poorly understood.

Here, we present sedimentological observations and stable isotopic analyses from the Chindwin sub-basin of central Myanmar, where we studied a 800-meter section covering most of the Upper Eocene, dated by geochronology and magnetostratigraphy. Sedimentary facies reflect a basin scale (~50 km wide) barrier-bound estuary similar to those found along the humid tropical coast of west Africa. Microfossils, trace fossils, and sulfur isotopes from lignite beds indicate regular, long-lasting freshwater episodes with complete sealing of the estuary from the sea.

The dimension, long-lasting character of the estuarine system and the presence of numerous sealing episodes are today only found in tectonically-produced estuaries, such as those developed along continental margins deformed by strike-slip tectonics (e.g. San Francisco Bay). Our results suggest that (1) pull-aparting along the Burmese margin was already active during the Late Eocene, and that (2) the accretionary prism was already uplifted close to sea level, to allow eustatic drops to regularly seal the paleo-estuary. These results suggest that the Indian convergence relative to Myanmar was already oblique during the Late Eocene and that Peninsular Indochina was already significantly rotated.