High-resolution seismic imaging in deep sea from a joint deep-towed/OBH reflection experiment : application to a Mass Transport Complex offshore Nigeria

International audienceWe assess the feasibility of high-resolution seismic depth imaging in deep water based on a new geophysical approach involving the joint use of a deep-towed seismic device (SYSIF) and ocean bottom hydrophones (OBHs). Source signature measurement enables signature deconvolution to be used to improve the vertical resolution and signal-to-noise ratio. The source signature was also used to precisely determine direct traveltimes that were inverted to relocate source and receiver positions. The very high accuracy of the positioning that was obtained enabled depth imaging and a stack of the OBH data to be performed. The determination of the P-wave velocity distribution was realized by the adaptation of an iterative focusing approach to the specific acquisition geometry. This innovative experiment combined with advanced processing succeeded in reaching lateral and vertical resolution (2.5 and 1 m) in accordance with the objectives of imaging fine scale structures and correlation with in situ measurements. To illustrate the technological and processing advances of the approach, we present a first application performed during the ERIG3D cruise offshore Nigeria with the seismic data acquired over NG1, a buried Mass Transport Complex (MTC) interpreted as a debris flow by conventional data. Evidence for a slide nature of a part of the MTC was provided by the high resolution of the OBH depth images. Rigid behaviour may be inferred from movement of coherent material inside the MTC and thrust structures at the base of the MTC. Furthermore, a silt layer that was disrupted during emplacement but has maintained its stratigraphic position supports a short transport distance

Integrated stratigraphic reconstruction for the last 80 kyr in a deep sector of the Sardinia Channel (Western Mediterranean)

A quantitative analysis of planktonic foraminifera, coupled with petrophysical and paleomagnetic measurements and 14C AMS calibrations, was carried out on a deep core recovered in the Sardinia Channel (Western Mediterranean Sea), during the CIESM Sub2 survey, providing an integrated stratigraphic time-framework over the last 80 kyr. Significant changes in the quantitative distribution of planktonic foraminifera allowed the identification of several eco-bioevents useful to accurately mark the boundaries of the eco-biozones widely recognised in the Western Mediterranean records and used for large scale correlations. Namely, 10 eco-biozones were identified based on the relative abundance of selected climate sensitive planktonic foraminiferal species. Sixteen codified eco-bioevents were correlated with the Alboran Sea planktonic foraminiferal data and four climatic global events (Sapropel S1, Younger Dryas, Greenland Isotope Interstadial 1, Greenland Isotope Stadial 2, Heinrich event H1-H6) were recognized. The eco-bioevents together with the 14C AMS calibrations allowed us to define an accurate age model, spanning between 2 and 83 kyr. The reliability of the age model was confirmed by comparing the colour reflectance (550 nm%) data of the studied record with the astronomically tuned record from the Ionian sea (ODP-Site 964). A mean sedimentation rate of about 7 cm/kyr included three turbidite event beds that were chronologically constrained within the relative low stand and lowering sea level phases of the MIS 4 and 3. The deep-sea sedimentary record includes a distinct tephra occurring at the base of the core which dates 78 ka cal. BP. The paleomagnetic data provide a well-defined record of the characteristic remanent magnetization that may be used to reconstruct the geomagnetic paleosecular variation for the Mediterranean back to 83 kyr

Integrated stratigraphic reconstruction for the last 80 kyr in a deep sector of the Sardinia Channel (Western Mediterranean)

A quantitative analysis of planktonic foraminifera, coupled with petrophysical and paleomagnetic measurements and 14C AMS calibrations, was carried out on a deep core recovered in the Sardinia Channel (Western Mediterranean Sea), during the CIESM Sub2 survey, providing an integrated stratigraphic time-framework over the last 80 kyr. Significant changes in the quantitative distribution of planktonic foraminifera allowed the identification of several eco-bioevents useful to accurately mark the boundaries of the eco-biozones widely recognised in the Western Mediterranean records and used for large scale correlations. Namely, 10 eco-biozones were identified based on the relative abundance of selected climate sensitive planktonic foraminiferal species. Sixteen codified eco-bioevents were correlated with the Alboran Sea planktonic foraminiferal data and four climatic global events (Sapropel S1, Younger Dryas, Greenland Isotope Interstadial 1, Greenland Isotope Stadial 2, Heinrich event H1-H6) were recognized. The eco-bioevents together with the 14C AMS calibrations concurred to define an accurate age model, spanning between 2 and 83 kyr cal. BP. The reliability of the age model was confirmed by comparing the colour reflectance (550 nm%) data of the studied record with the astronomically tuned one of the Ionian sea (ODP-Site 964). A mean sedimentation rate of about 7 cm/kyr was evaluated including three turbidite event beds that were chronologically constrained within the relative low stand and lowering sea level phases of the MIS 4 and MIS 3. The deep sea sedimentary record includes a distinct tephra occurring at the base of the core which dates 79 ka. The paleomagnetic data provide a well-defined record of the characteristic remanent magnetization that may be used to reconstruct the geomagnetic paleosecular variation for the Mediterranean back to 83 kyr cal. BP

Mass-transport deposits on the Rosetta Province (NW Nile deep-sea turbidite system, Egyptian Margin): characteristics, distribution, and potential causal processes

Marine Geology, v. 250, n. 3-4, p. 180-198, 2008. http://dx.doi.org/10.1016/j.margeo.2008.01.016International audienc

Les glissements de terrain sous-marins sous haute surveillance

National audienceLa mer ligure : une région de montagnes sous-marines Le relief sous-marin de la région niçoise est particulièrement pentu, creusés par des canyons, et certains évènements, tels que des séismes, peuvent le déstabiliser jusqu'à provoquer des glissements de terrain sous-marins. Glissement de terrain meurtrier et tsunami au large de Nice Le 16 octobre 1979, un glissement de terrain se produit sur le chantier de l'agrandissement de l'aéroport de Nice et provoque la mort de 10 ouvriers ainsi qu'une vague de 2 m qui inonde le quartier de la Salis à Antibes et provoque le décès d'une promeneuse

Les glissements de terrain sous-marins sous haute surveillance

National audienceLa mer ligure : une région de montagnes sous-marines Le relief sous-marin de la région niçoise est particulièrement pentu, creusés par des canyons, et certains évènements, tels que des séismes, peuvent le déstabiliser jusqu'à provoquer des glissements de terrain sous-marins. Glissement de terrain meurtrier et tsunami au large de Nice Le 16 octobre 1979, un glissement de terrain se produit sur le chantier de l'agrandissement de l'aéroport de Nice et provoque la mort de 10 ouvriers ainsi qu'une vague de 2 m qui inonde le quartier de la Salis à Antibes et provoque le décès d'une promeneuse

Triggering factors and tsunamogenic potential of a large submarine mass failure on the western Nile margin (Rosetta area, Egypt)

A large-scale mass-transport deposit (MTD) called Sl6 was recognized on the upper slope of the western Nile margin, downslope from of a 30 km-long scarp located along the outer shelf. Regional mapping indicated that this MTD extends on nearly 505 km2 and involved about 14 km3 of Pleistocene-Holocene sediment. Sl6 was triggered between 10 and 9 kyr BP, during the Holocene sea-level rise and coeval pluvial period (increased river flow). The consequent enhanced sediment supply on the upper slope and the outer shelf area caused local overburdening. This factor combined with the potential accumulation of gas in the sediment and earthquake activity is thought to have been the main factor triggering the Sl6 MTD. From the estimated volume of the MTD, a potential slide-generated tsunami was numerically simulated using the GEOWAVE software. The results indicate that the ~80 km wide Egyptian continental shelf protects the main part of the coastline from a slide-induced tsunami coming from the Rosetta area. An exception is the part of the coastline around Alexandria because focussing and shoaling processes can be simulated very close to the coast

La memoria de los j\uf3venes y las luchas por la emancipati\uf3n

la questione della memoria sociale e della sua trasmissione intergenerazionale nel caso italiano; alcuni esempi di aspetti occultati; una questione di rappresentazione politic

Transient Groundwater Flow Through a Coastal Confined Aquifer and Its Impact on Nearshore Submarine Slope Instability

International audienceRare in situ pore pressure and temperature data sets collected over more than one decade in a submarine landslide-prone area off Nice (Southeast France) have been used to characterize a transient groundwater exchange between a coastal confined aquifer and the nearshore shallow-water submarine shelf. The near-coast study zone is highly exposed to geohazards and remains sadly famous for the 1979 catastrophic tsunamigenic submarine landslide with several casualties and substantial material damage. Measured pore pressure buildup does not appear to be synchronized with either the discharge of the nearby Var River or with precipitation in the Nice area. Unexpectedly, pore pressure fluctuations synchronizes well with seabed temperature trends indicating that the nearshore submarine shelf is mainly impacted by long smooth seasonal variations. We used the pore pressure and temperature data to describe the general transport mechanisms of pore waters and show that pore pressure diffusion is predominantly responsible for the near-seabed pore pressure buildup. Based on our results and previously published geotechnical data, slope stability calculations reveal that groundwater exchange has a major impact on slope instability and shear zone formation in a prone-to-failure area. Minor earthquake-induced ground accelerations seem to contribute to the nearshore slope instability by decreasing its factor of safety