NEW CONSTRAINTS ON THE EVOLUTION OF THE ANDAMAN FOREARC RIDGE

The Andaman Islands to the east of the Andaman Java trench belong to the submarine forearc ridge between Western Burma and the Indonesian island-arc system. The region is characterized by rapidly changing systems of subduction and accretion linked to the collision history of India with Asia. The Andaman forearc stratigraphy begins with an oceanic sequence represented by ophiolite and ocean floor sediments that have been interpreted to represent units of an accretionary prism formed by an imbricate stack of east dipping fault slices. The chronology of thrusting and uplift is poorly known due to a lack of robust biostratigraphy and thermochronological constraint. Here, we present new constraint based on detrital thermochronometry that identify key periods of thrusting and uplift. The earliest identifiable event occurred in the Early Eocene, and is based on detrital apatite and zircon fission-track (FT) results from the Mithakhari Group that record contemporaneous arc-related volcanism at ~55 Ma. This timing coincides with a Ypresian age for limestone clasts found within the Mithakhari Group derived from erosion of the uplifting ophiolite and its sedimentary cover. We ask whether it is coincidental that this inversion occurs at the same time as hard collision between India and Asia. Overlying the Mithakhari Group is the Andaman Flysch, a laterally continuous succession of turbidites of unknown thickness. Biostratigraphic evidence is poor but indicates an Oligocene age. Apatite FT analysis, which constrain turbidite thickness to < 2km, record two source age components at ~38 Ma and ~25 Ma. The older age mode at 38 Ma is also found in the detrital zircon data. These results show that the Andaman Flysch cannot be older than the Late Oligocene and that thrusting involving the Andaman Flysch must have taken place after this but prior to deposition of the mainly volcanic Mio-Pliocene Archipelago Group that once blanketed the Andaman region. A final phase of inversion began in the Pliocene linked to opening of the Andaman Sea.