A Late Cretaceous Orogen Triggering the Tertiary Rifting of the West Sunda Plate; Andaman Sea Region

Abstract:

Rifted Basins often develop in internal zones of orogenic belts, although the latter may not be easy to unravel. We chose the example of the super-stretched Andaman sea region affected by several stages of rifting in the internal zone of a composite collage of allochthonous terranes. We made use of a set of geophysical, geochronological and structural data to analyze the rifting evolution and reconstruct the previous compressional structures.

- Starting in the late Oligocene the East Andaman Basin opened as a back arc in a right-lateral pull- apart. The rifting propagated Westward to the central Andaman basin in the Middle Miocene, and to the oceanic spreading stage in the Pliocene.

- An early extension occurred in the Paleogene, marked by widespread opening of isolated continental basins onshore Malay Peninsula and offshore Andaman Shelf and Malacca Straits. The rifting was accommodated by LANF’s along preexisting weakness zones such as hinges of folds and granitic batholiths. Continuous extension connected the isolated basins offshore, whereas onshore, the grabens remained confined. There, AFT data show an uplift phase around 30Ma.

In the Late Cretaceous, a major deformation occurred oblique to the pre-existing Indosinian basement fabrics. The convergence was partitioned into thrusting and uplift of the Cretaceous volcanic arc in Thailand and Myanmar, inversion of Mesozoic basins, and coeval wrenching responsible for large phacoid-shaped crustal slivers bounded by wide strike slip fault zones. The slivers share similar characteristics: a thick continental core of lower Paleozoic sedimentary basins units surrounded by Late Cretaceous granitoids. Radiometric data and fission tracks indicate a widespread thermal anomaly in all West Sunda Plate synchronous to a strong uplift.

In the Latest Mesozoic, the Western Margin of Sunda plate was subjected to a major E-W compression, accommodated by oblique conjugate strike slip faults, leading to the formation of a large orogenic belt which is nowadays difficult to observe due to a poly-phase rifting that took place in the internal zone of this previously formed belt. The obliquity of this deformation illustrates the importance of inherited Paleozoic Gondwanian basement discontinuities on the subsequent rifting patterns.