Forearc Basin Structure in the Andaman-Nicobar Segment of the Sumatra-Andaman Subduction Zone: Insight from High-Resolution Seismic Reflection Data

Abstract:

The Andaman-Nicobar subduction is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex backarc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar forearc basin, from 8°N to 11°N, to understand the structure and evolution of the forearc basin, focusing on how the obliquity of convergence affects deformation in the forearc, as well as on the Diligent (DF) and Eastern Margin Faults (EMF). Constraining the evolution of this basin, which is strongly related to the collision of India and Eurasia, can help shed light onto present-day deformation processes along this segment of the subduction zone, where convergence is highly oblique and little data is available. We find that he DF is a backthrust and corresponds to the Mentawai (MFZ) and West Andaman Fault (WAF) systems further south, offshore Sumatra. The DF is expressed as a series of mostly landward verging folds and faults, deforming the early to late Miocene sediments. The DF seems to root from the boundary between the accretionary complex and the continental backstop, where it meets the EMF. The EMF marks the western boundary of the forearc basin; it is associated with subsidence and is expressed as a deep piggyback basin, associated with recent Pliocene to Pleistocene subsidence at the western edge of the forearc basin. The eastern edge of the forearc basin is marked by the Invisible Bank (IB), which is thought to be tilted and uplifted continental crustal block. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea. The sliver Andaman-Nicobar Fault (ANF), which is the active northward extension of the Great Sumatra sliver Fault (GSF), lies to the east of the IB, and marks the boundary between continental crust underlying the forearc basin and crust accreted at the Andaman Sea Spreading Center.