Timing of Formation and Obduction of the Andaman Ophiolite

Abstract

The Andaman ophiolite forms part of the ophiolite chain that encircles the northern and eastern boundaries of the Indian plate. Unlike its northern counterparts, it occurs on an accretionary prism, located at the convergent margin of Indian and Burma plates. It not only forms part of the basement of the accretionary complex but also occurs as thrust sheets on outer-arc sediments. The timings of formation and emplacement/obduction of this ophiolite had remained tentative. Here, we present results of our effort to date these events by ¹⁴⁷Sm–¹⁴³Nd and ⁴⁰Ar–³⁹Ar methods and understand their implications for the evolution of the Andaman subduction zone. Our geochemical data suggest that the ophiolite had formed in a supra subduction zone setting. The whole-rock ¹⁴⁷Sm–¹⁴³Nd isochron age of the ophiolite suggests a formation age of 98 ± 8 (2σ) Ma, which is consistent with the existing age data and with the age of the oceanic crust located beneath the Andaman volcanic arc. The ⁴⁰Ar–³⁹Ar apparent age spectra for rocks that occur along the faulted margin of the east coast of the south and middle Andaman Islands, though disturbed, yield plateaus for low temperature steps (<900 °C) having concordant ages with an average of 0.9 ± 0.3 (2σ) Ma, the timing of argon resetting. This timing points to thermal metamorphism during a thrusting event, which most likely was responsible for obduction of the entire eastern belt ophiolite. Based on our results and stratigraphy of the Andaman Islands, we propose a tectonic evolutionary model for the Andaman Accretionary Prism that involves twofold ophiolite obduction, one prior to 55 Ma and the other at ~0.9 Ma.