40Ar/39Ar geochronology of Andaman Ophiolite: Evidence for a Pleistocene mega thrusting event within the Andaman-Nicobar Accretionary Wedge

Abstract:

A geochronological study the Andaman ophiolite was performed along a ~120 km corridor on the eastern margin of the Andaman Islands, which forms part of the Andaman-Nicobar accretionary wedge. This Cretaceous ophiolite sequence occurs as imbricate thrust wedges overlying the Paleogene flysch and Neogene pelagic sediments. Incremental heating ⁴⁰Ar/³⁹Ar dating of three pillow basalts and a peridotite samples reveals that the ophiolite has a composite thermal history. Apparent age spectra of all the samples suggest a clear two stage evolution, with the high temperature steps (1000 to 1400^oC) and the lower temperature steps (400-950^oC) showing a staircase pattern and a plateau-like pattern, respectively. The apparent ages of the highest temperature steps vary from 135 Ma to 97 Ma, which appear to suggest multiple crystallization ages. Interestingly, the plateau-like spectra for the lower temperature steps yield indistinguishable ages, although with very high errors (as high as 67% at 1s), across all the samples. The isochron ages too overlap with the plateau-like ages. Thus the weighted average of the plateau-like ages of 0.9±0.3 (2s) Ma most likely represents the timing of the latest thermal resetting. Based on the above information we conclude that while different units of the Andaman ophiolite may have been formed at different times, all had seen a major thermal resetting. Considering the position of these rocks vis-à-vis the structural configuration of the Andaman-Nicobar accretionary wedge we infer that there was a mega thrusting event in the region at ~0.9 Ma which was possibly responsible for exhumation of the current outcrops of the ophiolite sequence along the east coast of the Andman Islands.