GHASS2 – Traitement des données de sismique multitrace. Analyse de vitesse RMS et imagerie avec migration après sommation

Les acquisitions de sismique réflexion réalisées pendant la campagne GHASS2 (2021, N/O Pourquoi Pas ?) avaient pour but de densifier et d’étendre la zone couverte par les précédentes acquisitions de sismique Haute Résolution (HR) lors de la campagne GHASS en mer Noire sur la marge roumaine (2015, N/O Pourquoi Pas ?). La campagne GHASS2 a permis l’acquisition de 1000 km de sismique HR long offset (source mi-GI 24/24 ci et flûte de 3000 m) et 470 km de sismique BF (source SIS1 480 ci + flûte de 6000 m). Ce rapport présente le traitement exhaustif appliqué à l’ensemble des profils sismiques multitrace acquis pendant GHASS2, incluant une première caractérisation des vitesses de propagation. Après un traitement complet et définitif des capteurs de navigation, l’ensemble des positions sources et récepteurs est dans un premier temps estimé avec une précision meilleure que la distance inter-trace de 6.25 mètres. La source HR est également corrigée du délai de déclenchement des canons. L’ensemble des traces acquises (67 millions) est analysé en terme de niveau de bruit et d’anomalies pour éditer les traces les plus bruitées. Après le tri et le reformatage des données, des panneaux de semblance sont calculés tous les 250 à 500 mètres et servent au pointé manuel des lois de vitesse RMS le long des profils (2330 lois pointées en sismique HR, 930 lois pointées en sismique BF). Ces pointés de vitesse permettent de produire des sections de vitesses d’intervalle. Les vitesses RMS sont ensuite utilisées pour sommer les traces (NMO+stack) qui sont ensuite migrées en temps pour obtenir les sections sismiques traitées. Pour le cas des données SIS1 480 ci qui visent une pénétration maximale, les arrivées multiples du fond de l’eau doivent être atténuées. Une séquence de traitement de type SRME (Surface-Related Multiple Elimination) a été appliquée à l’aide du logiciel Promax sur tous ces profils. Le volume important de ces données SIS1 480 ci (960 traces) a nécessité au préalable de les ré-échantillonner en temps et en trace avant de procéder à l’atténuation des multiples

Upslope migrating sand dunes in the upper slope of the Mozambican margin (SW Indian Ocean)

The upper slopes of continental margins are very energetic areas where nepheloid layers are often observed. Multibeam bathymetry, sub-bottom profiler and multi-channel highresolution seismic reflection data acquired during the PAMELA-MOZ04 survey in the Mozambique Channel revealed the presence of sand dunes on the upper slope at 120-250 m water depth. The dunes migrate upslope and their crests are oblique to the contours. They are medium to large dunes, with wavelengths between 20 and 150 m and heights between 0.15 and 1.50 m, and their size decreases upslope. Seismic reflection data of the water column show internal solitary waves travelling offshore in the depth range of the dune field. The formation of the dune field could be related to the interaction of the barotropic tide with the upper slope that results in the generation of internal tides

The pianosa contourite depositional system (northern Tyrrhenian sea): drift morphology and plio-quaternary stratigraphic evolution

The Pianosa Contourite Depositional System (CDS) is located in the Corsica Trough (Northern Tyrrhenian Sea), a confined basin dominated by mass transport and contour currents in the eastern flank and by turbidity currents in the western flank. The morphologic and stratigraphic characterisation of the Pianosa CDS is based on multibeam bathymetry, seismic reflection data (multi-channel high resolution mini GI gun, single-channel sparker and CHIRP), sediment cores and ADCP data. The Pianosa CDS is located at shallow to intermediate water depths (170 to 850 m water depth) and is formed under the influence of the Levantine Intermediate Water (LIW). It is 120 km long, has a maximum width of 10 km and is composed of different types of muddy sediment drifts: plastered drift, separated mounded drift, sigmoid drift and multicrested drift. The reduced tectonic activity in the Corsica Trough since the early Pliocene permits to recover a sedimentary record of the contourite depositional system that is only influenced by climate fluctuations. Contourites started to develop in the Middle-Late Pliocene, but their growth was enhanced since the Middle Pleistocene Transition (0.7–0.9 Ma). Although the general circulation of the LIW, flowing northwards in the Corsica Trough, remained active all along the history of the system, contourite drift formation changed, controlled by sediment influx and bottom current velocity. During periods of sea level fall, fast bottom currents often eroded the drift crest in the middle and upper slope. At that time the proximity of the coast to the shelf edge favoured the formation of bioclastic sand deposits winnowed by bottom currents. Higher sediment accumulation of mud in the drifts occurred during periods of fast bottom currents and high sediment availability (i.e. high activity of turbidity currents), coincident with periods of sea level low-stands. Condensed sections were formed during sea level high-stands, when bottom currents were more sluggish and the turbidite system was disconnected, resulting in a lower sediment influx

Active Fault System across the Oceanic Lithosphere of the Mozambique Channel: Implications for the Southern Part of the East African Rift System

International audienceSeismic reflection and multibeam echosounder data acquired in the Mozambique Channel during the PAMELA project (PAssive Margin Exploration Laboratories, collaboration between IFREMER-TOTAL-IFPEN-CNRS-Univ. Brest, Rennes, Paris) revealed that an active fault system is deforming the oceanic lithosphere of the Mozambique Basin which has developed during Jurassic-Cretaceous times. The correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the East African Rift System which corresponds to the seismically active graben system bounding the northern part of the Davie ridge. The fault zone extends southwestward of the Mozambique Ridge along the same trend as the Agulhas-Falkland transform fault zone. The general organization of the fault zone shows the characteristics of an extensional system north of the Mozambique Channel (north of the Europa Island) and a right-lateral transtensional system with coeval normal faults and strike-slip faults south-west of Europa. This tectonic activity is associated with volcanic activity since at least Late Miocene times. Our findings emphasize that the eastern branch of East African Rift System is extending largely toward the south, not only in continental margin domains but also through the oceanic lithosphere of the Mozambique Basin. This fault zone is participating to the complex plate boundary between the main African continent (Nubia Plate) and the Somalia Plate

Active fault system across the oceanic lithosphere of the Mozambique Channel: Implications for the Nubia–Somalia southern plate boundary

International audienceSeismic reflection and multibeam echosounder data were acquired in the Mozambique Channel in 2014 and 2015 during the PTOLEMEE, PAMELA-MOZ02 and -MOZ04 marine surveys aboard the RV Atalante and Pourquoi Pas? These data revealed that an active fault system is deforming the oceanic lithosphere of the Mozambique Basin which has developed during Jurassic-Cretaceous times. The correlation between the fault system and the arrangement of earthquake epicenters suggests that this tectonically active zone directly connects northward with the southern part of the eastern branch of the East African Rift System which corresponds to the seismically active graben system bounding the northern part of the Davie ridge. The fault zone extends southwestward of the Mozambique Ridge along the same trend as the Agulhas-Falkland transform fault zone. The general organization of the fault zone shows the characteristics of an extensional system north of the Mozambique Channel (north of the Europa Island) and a right-lateral transtensional system with coeval normal faults and strike-slip faults south-west of Europa. This tectonic activity is associated with volcanic activity since at least Late Miocene times. Our findings emphasize that the eastern branch of East African Rift System is extending largely toward the south, not only in continental domains but also through the oceanic lithosphere of the Mozambique basin. This fault zone is participating to the complex plate boundary between the main African continent (Nubia Plate) and Madagascar (Somalia Plate)

The impact of internal waves on upper continental slopes: insights from the Mozambican margin (SW Indian Ocean)

Evidences of sedimentation affected by oceanic circulation, such as nepheloid layers and contourites are often observed along continental slopes. However, the oceanographic processes controlling sedimentation along continental margins remain poorly understood. Multibeam bathymetry and high‐resolution seismic reflection data revealed a contourite depositional system in the Mozambican upper continental slope composed of a contourite terrace (a surface with a gentle seaward slope dominated by erosion) and a plastered drift (a convex‐shape sedimentary deposit). A continuous alongslope channel and a field of sand dunes (mainly migrating upslope), formed during Holocene, were identified in the contourite terrace at the present seafloor. Seismic reflection data of the water column show internal waves and boluses propagating in the pycnocline near the upper slope. The channel and the dunes are probably the result of the interaction of the observed internal waves with the seafloor under two different conditions. The alongslope channel is located in a zone where intense barotropic tidal currents may arrest internal solitary waves, generating a hydraulic jump and focused erosion. On the other hand, upslope migrating dunes may be formed by bottom currents induced by internal solitary waves of elevation propagating landwards in the pycnocline. These small‐scale sedimentary features generated by internal waves are superimposed on large‐scale contouritic deposits, such as plastered drifts and contourite terraces, which are related to geostrophic currents. These findings provide new insights into the oceanographic processes that control sedimentation along continental margins that will help interpretation of palaeoceanographic conditions from the sedimentary record