FIELD INVESTIGATION OF INFERRED UPPER OCEANIC CRUSTAL FRAGMENTS EXPOSED IN THE SOUTHERN INDOBURMAN RANGES, WEST COAST OF MYANMAR, NE INDIAN OCEAN

The Indo-Burman Ranges (IBR) of western Myanmar are an active accretionary prism of offscraped Bengal Fan sediments. Ophiolitic rocks are common in large fault zones along the eastern margin of the IBR.

We conducted two field campaigns on Ramree and Cheduba Islands in 2013 and 2014. These islands, offshore of west-central Myanmar, are an actively uplifting older section of the prism formed of deep-water Eocene to Oligocene siliciclastic turbidites (Indo-Burman Flysch) that are overlain by Miocene and younger trench slope strata. In south Ramree Island, strongly folded and sheared ophiolitic rocks are exposed in fault contact with the turbidites. The assemblage consists of strongly sheared serpentinite and greenstone along with bluish silicified limestone and bedded red and green chert that contain Maastrichtian radiolarians. Similar greenstones are exposed along the south coast of Cheduba island as well.

On a regional scale, the turbidite section is deformed into a series of NNW-trending folds with steep limbs (> 50°) that are laterally continuous for tens of km. We have mapped several zones within the turbidites that exhibit "broken formation" styles of deformation, with fold wavelengths less than 1 m and evidence of small-scale thrusting. Some of the strongly deformed shear zones transition into tectonic mélange.

We interpret this assemblage to represent fragments of upper oceanic crust originally stripped off the subducting Indian Ocean plate and accreted prior to the main India-Asia collision in the Eocene when the sediment cover was thin. The Eocene-Oligocene turbidites, erosional debris from the collision zone, were accreted under the ophilolite fragments and further deformed prior to the Miocene. This ophiolitic assemblage is correlative with similar rocks to the south along the Andaman-Nicobar Ridge.