Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

International audienceThe variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation

Deep structure of the Armorican Basin (Bay of Biscay): a review of Norgasis seismic reflection and refraction data

International audienceThe Bay of Biscay is bounded to the North by the North Biscay margin, which comprises the Western Approaches and Armorican segments. In the 1970s and 1980s, most researchers considered this margin typical of a non-volcanic passive margin: It is characterized by a striking succession of tilted blocks beneath which occurs the S reflector and the continent/ocean boundary is abrupt. This paper examines the Armorican segment and is based on a study of all early seismic profiles together with new multichannel reflection and refraction seismic data (NORGASIS cruise). An important result is the discovery of a 80-km wide Ocean-Continent Transition zone that coincides with the Armorican Basin (a deep sedimentary basin). It is characterized by a High-Velocity Layer-Crust (7.4-7.5 km/s) overlain by sediments. The other results are: i) the main crustal thinning occurs exclusively under the narrow continental slope. ii) The tilted blocks and the S-reflector are observed only at the base of the continental slope in the narrow domain called "neck area". iii) the North Biscay Ridge is a large oceanic plateau present only off the NW Armorican margin rather than a long ridge elongated off the whole North Biscay Margin

The Sedimentary Sequences of the Lesser Antilles Arc South of Guadeloupe FromWide-Angle and Reflection Seismic Data

The Lesser Antilles Island Arc is a European active subduction zone prone to major earthquakes. Huge sedimentary input by the South American rivers, namely Amazon and Orinoco, has formed one of the largest accretionary complexes in the world. In the framework of the THALES project, several coincident wide-angle and multichannel seismic (MCS) profiles (15.5N and 16.5N) have been collected on the accretionary prism beetween the Barracuda and Tiburon Ridges in the Lesser Antilles. We present the analysis results of these data in order to construct a structural model. Preliminary results of 7 different MCS profiles are discussed. The data consist of 1 strike and 6 cross lines. The sedimentary layers imaged on deep-penetrating MCS data were used as a priori information for the wide angle modelling. A total of 16 OBH/OBS (Ocean Bottom Hydrophone/Seismometer) was deployed on a 130 km long wide-angle seismic profile. Seismic velocity models were obtained by a forward modelling of refracted and reflected phases. The final velocity model shows the geometry of the Antilles subduction zone with a sediment thickness of up to 2.6 km. The shallowest layer has a fill velocity of 1.8-2.2 km/s, whereas an older more compacted sediment layer in the deeper portion shows velocities ranged from 2.5 to 3.5 km/s with sediment thickness till 4.3 km. The sedimentary succession marked topographic irregularity and different directional fault system. These layers overlie oceanic crust having velocities in excess 6 km/s with depth of 14-15 km. From the coincident MCS seismic profiles, we incorporated the well resolved sedimentary portions into our model

Structure sismique de la Faille Nord Anatolienne en Mer de Marmara

La campagne SEISMARMARA-Leg 1 permet une imagerie sismique à l échellecrustale de la faille Nord Anatolienne en Mer de Marmara. Les profils desismique réflexion verticale, certains avec un traitement avancé, et lesobservations de sismique réfraction coïncidentes sur les sismomètres fondde mer permettent une imagerie détaillée du remplissage sédimentaire et del activité des failles jusqu au socle. L'architecture complexe peut êtreperçue comme une structure en fleur négative à l échelle crustale. Un partitionnement de la déformation sur plusieurs failles est observé etsemble être le processus prédominant à travers l ensemble du fossé Nord dela Mer de Marmara, incluant ses marges. La croûte inférieure et le Mohosont identifiés pour la première fois par une pénétration sans précédenten sismique multi-trace et la réfraction à grands déports à terre. Unamincissement crustal est ainsi mis en évidence sous le fossé Nord, encontraste abrupt E-O et plus progressif depuis le Sud. Un réflecteurinstracrustal interprété comme un détachement surmonté de blocs basculésde socle est imagé sur le long de la marge sud-ouest du fossé. Lagéométrie du toit de la croute supérieure qui remonte sous le fossé etcelle du détachement suggèrent un amincissement par omission de matérielcrustal avec transport vers le Sud-Ouest. La sismicité enregistrée par leréseau dense de sismomètres fond de Mer et de stations à terre estreplacée et discutée par rapport à l image structurale obtenuePARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF