The Role of Heritage in the Architecture of the Andaman Rift: Oblique and Diachronous Reactivation of Basement Faults

Abstract

Abstract

The Andaman sea basin is part of various pull-apart basins all formed in a strike slip environment within a back arc setting. This strike slip tectonic regime results from a strain-partitioning process due to the oblique subduction of the Indian plate below the Sunda plate. The Andaman oceanic spreading started 5Ma ago. Considering the Early Miocene age of the rifted-apart volcanic provinces (Alcock and Sewell rises), the rifting period has been considered started in Lower to Middle Miocene in the Andaman area.

The interpretation of 2D and 3D seismic data located at various locations along the Northern margin of the basin, revealed a 3 phased rifting history. The stratigraphic sequence is composed of a volcanic basement, including volcanoclastic deposits, overlain unconformably by Oligo-Miocene carbonates (locally platforms and reefs) intruded by some volcanic dykes. A Miocene to present siliciclastic sequence indicates the onset of the Irrawady Delta. This last sequence is itself intruded by volcanic sills and dykes indicative of a persistent volcanic activity.

The volcanic basement is mainly framed by EW normal faults associated with NS transfer zones. This structural framework controls the volcanoclastic deposits. The architecture of that initial rift is significantly different from the Mio-Pliocene one, framed by NE-SW normal faults and NS strike slip faults: the first one is typical of an orthogonal rift system whereas the second is typical of an oblique rift system in a strike slip regime. A second Oligo-Miocene rifting phase, highly controlled by basement heritage, was also identified. It has controlled the Oligo-Miocene platforms and basinal carbonates repartition. During that tectonic phase, normal faults with azimuths ranging from N40°E up to N130°E are identified. Finally, the classical Mio-Pliocene phase has generated NE-SW normal faults partly decoupled from the deep structures by a regional shale interval. During that phase, the initial EW normal faults were locally reactivated.

The overall northern margin of the Andaman sea is therefore interpreted as a series of offset initial rifts with a diachronous and oblique reactivation. The identified heritage, framed by NS transforms and EW normal faults, has yet to be understood. Various scenarios could be proposed including the accretion of an Indian oceanic crust, actually framed by a very similar structural pattern.

AAPG Datapages/Search and Discovery Article #90260 © 2016 AAPG/SEG International Conference & Exhibition, Cancun, Mexico, September 6-9, 2016