Seismic evidence of continental margin influence on the NinetyEast Ridge in the Bay of Bengal

International audienceThe 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

Seismic images of the sliver strike-slip fault and back thrust in the Andaman-Nicobar region

International audiencesliver 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

Seismic images of structural variations along the deformation front of the Andaman-Sumatra subduction zone: Implications for rupture propagation and tsunamigenesis

International audienceSeven deep seismic reflection profiles cover the 3000 km-long subduction system from Andaman to Southern Sumatra, including zones that ruptured in 2004, 2007, and 2010. We find that (1) the frontal zone is characterized by a series of thrusts bounding folded blocks of sediments with preserved layering, showing a northward transition from dominantly seaward vergence of the frontal thrusts to dominantly landward vergence of the frontal thrusts, (2) the accretionary wedge is characterized by poor reflection of the seismic energy likely to be due to a high degree of faulting and compaction of the sediments, and (3) the oceanic crust is highly disturbed by faults and topographic reliefs along most of the margin. Landward vergence at the deformation front is associated with a thick incoming sediment section. The segment of the subduction zone where landward vergence is observed corresponds to an area with high near-trench slip during the December 2004 earthquake, the main tsunami source, and lies just west of the hypocenters of several intraplate events (Mw > 7) in the years following the 2004 event

Influence of the Mediterranean Outflow Water on benthic ecosystems: answers and questions after a decade of observations

Deep-water Circulation: Processes & Products. International Congress 16-18 june 2010, Baiona, Pontevedra, Spain.-- 2 pagesCold-water coral (CWC) ecosystems are closely tied to oceanography; they necessitate a careful balance of bottom current strength in order to survive. The discovery of the large CWC mounds in the Porcupine Seabight gave insight to the strong association with the dynamics of a very distal Mediterranean Outflow Water (MOW), both in the present-day as in the geological past. Although the MOW is thoroughly mixed north of Iberia, its physical characteristics enable local enhancing through internal tides. Observations of CWC reefs along the Bay of Biscay and the Cantabrian margins confirm this water mass is a real driver of benthic habitats (corals and oysters), controlling both physical (morphological) as oceanographic factors. They also give hints regarding the possiblenucleation and growth patterns of the large CWC mounds as well as their (palaeo)environmental constraints. However, the discovery of «dead» CWC reefs on mounds along the Moroccan margin has left many questions on the influence of intermediate water masses on these ecosystems and calls for a thorough investigation of the local palaeoceanography; are these glacially thriving reefs influenced by MOW or meddies