[1] We present the interpretation of newly acquired high-quality industry-standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the Mentawai Fault Zone (MFZ). The MFZ lies along the boundary between the accretionary wedge and the proposed continental backstop. This zone exhibits arcuate ridges on the seafloor, convex toward the east. Beneath these ridges the structures developed as landward-vergent imbricated backthrusts in the inner part of the accretionary wedge and higher-angle backthrusts that deformed the forearc basin sediments. In the forearc high, anticlines were developed due to the seaward-vergent forearc high thrusts originating in the accretionary wedge. The imbricated backthrusts may have initiated during the Early-Middle Miocene contemporaneously with the slide and back-rotation of forearc high thrusts. In the Late Miocene, the higher-angle backthrusts were initiated. Continuous contraction induced the frontal higher-angle backthrusts and formed a fold-thrust belt toward the east during the Pliocene. The folds and thrusts were disturbed by diapirs and mud volcanoes. Backthrusting and fold-thrust belts developed in the MFZ may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern Sumatra margin. The backthrusts along the MFZ are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small.

The sliver strike‐slip Great Sumatra Fault (GSF) traverses mainland Sumatra from the Sunda Strait in the southeast to Banda Aceh in the northwest, and defines the present day plate boundary between the Sunda Plate in the north and the Burmese Sliver Plate in the south. It has been well studied on mainland Sumatra but poorly north of Banda Aceh in the Andaman Sea. Here we present deep seismic reflection images along the northward extension of the GSF over 700 km until it joins the Andaman Sea Spreading Centre, and we interpret these images in the light of earthquake, gravity, and bathymetry data. We find that the GSF has two strands between Banda Aceh and Nicobar Island: a transpression in the south and a deep narrow active rift system in the north, dotted with volcanoes in the center, suggesting that the volcanic arc is coincident with rifting. Farther north of Nicobar Island, an active strike‐slip fault, the Andaman‐Nicobar Fault, cuts through a rifted deep basin until its intersection with the Andaman Sea Spreading Centre. The volcanic arc lies just east of the rift basin. The western margin of this basin seems to be a rifted continental margin, tilted westward, and flooring the Andaman‐Nicobar fore‐arc basin. The Andaman‐Nicobar fore‐arc basin is bounded in the west by back thrusts similar to the West Andaman and Mentawai faults. The cluster of seismicity after the 2004 great Andaman‐Sumatra earthquake just north of Nicobar Island coincides with the intersection of two strike‐slip fault systems.

The Andaman-Nicobar region is the northernmost segment of the Sumatra-Andaman subduction zone and marks the western boundary of the Andaman Sea, which is a complex active back-arc extensional basin. We present the interpretation of a new set of deep seismic reflection data acquired across the Andaman-Nicobar fore-arc basin, from 8°N to 11°N, in order to better understand its structure and evolution, focusing on (1) how obliquity of convergence affects deformation in the fore arc, (2) the nature and role of the Diligent Fault (DF), and (3) the Eastern Margin Fault (EMF). Despite the obliquity of convergence, back thrusting and compression seem to dominate the Andaman-Nicobar fore-arc basin deformation. The DF is primarily a back thrust and corresponds to the Mentawai and West Andaman Fault systems farther in the south, along Sumatra. The DF is expressed in the fore-arc basin 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 fore-arc basin; it is associated with subsidence and is expressed as a deep piggyback basin, containing recent Pliocene to Pleistocene sediments. The eastern edge of the fore-arc basin is the Invisible Bank (IB), which is thought to be tilted and uplifted continental crust. Subsidence along the EMF and uplift and tilting of the IB seem to be related to different opening phases in the Andaman Sea.

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28We report the northernmost and deepest known occurrence of deep-water pycnodontine 29 oysters, based on two surveys along the French Atlantic continental margin to the La 30Chapelle continental slope (2006)

The Andaman–Nicobar subduction system is the northwestern segment of the Sunda subduction system, where the Indian Plate subducts beneath the Sunda Plate in a nearly arc-parallel direction. The entire segment ruptured during the 2004 great Andaman–Sumatra earthquake (Mw=9.3). Using recently acquired high-resolution seismic reflection data, we characterize the shallow structure of the whole Andaman–Nicobar subduction system from west to east, starting from the nature of the subducting plate in the Bay of Bengal to back-arc spreading in the Andaman Sea. We find that the Ninety-East Ridge is overlain by thick continental margin sediments beneath the recent Bengal Fan sediments. The boundary between these two sedimentary units defines the plate interface. We observe evidence of re-activation of fracture zones on the subducting plate beneath the forearc, influencing the morphology of the upper plate. The forearc region, which includes the accretionary wedge, the forearc high and the forearc basin, is exceptionally wide (250 km). We observe an unusually large bathymetric depression within the forearc high. The forearc high is bounded in the east by a normal fault, whereas the forearc basin contains an active backthrust. The forearc basin is floored by the continental crust of Malayan Peninsula origin. The active sliver strike-slip fault lies in a deep basin, created during the rifting of the forearc continental crust and the Malayan Peninsula. The sliver fault connects with the Great Sumatra Fault in the south and with the Sagaing Fault in the north, via the Andaman Sea spreading centre and a large transform fault in the Andaman Sea.

The NinetyEast Ridge (NER), one of the most enigmatic features in the Indian Ocean, is covered by thick Bengal Fan sediment north of 9°N. We present seismic reflection data on the eastern flank of the NER, at 10°N, that show the presence of 4-5 km thick sediments beneath the Bengal Fan sediments. These sediments can be imaged up to 60 km beneath the Andaman fore-arc accretionary wedge, suggesting that the décollement lies above these sediments. The presence of thick sediments above the northernmost segment of the NER suggests that this segment was close to a continental margin during its emplacement. We propose that these sediments were deposited soon after the breakup of India and Antarctica, between 130 and 100 Ma, and might act as source rocks for oil and gas generation beneath recent Bengal Fan sediments. Furthermore, subducting thick sediments can significantly change the seismogenic behavior of the Andaman subduction zone.

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