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

Pervasive compression and deep structure of the Hellenic subduction forearc, west of Crete, revealed by penetrative long offset multichannel seismic data

International audienceThe Hellenic subduction system exhibits a fairly atypical structure, resulting from the intense radial extension undergone by the Aegean domain from Eocene to Miocene. It features a very wide fore-arc, that itself includes an external non-volcanic arc between southeastern Peloponnesus and southwestern Anatolia, with Crete occupying a frontal position midway along it. South of it, separated from the Aegean domain by a large escarpment, the Hellenic trenches marking the most advanced position of the forearc Alpine nappes and the wide abutting accretionary wedge are found. The Hellenic subduction is widely regarded as poorly coupled from historical seismicity. Yet, a large tsunamigenic destructive event is reported in 365 CE just west of Crete, for which various rupture processes are proposed. In order to gain better insight into the structures of this major event, some deep penetrating seismic acquisition was performed in 2015 during the SISMED project, using the leading-edge equipment of R/V Marcus Langseth along a 210 km profile crossing through the fore-arc; an 8 km-long streamer and a voluminous source were used. Pre-stack depth imaging was applied to the recorded data set, with a strong effort to build a reliable velocity model using common image focusing analysis, both in the time and depth domains. Good imaging conditions thus were reached down to a maximum depth of 25 km. From south to north, our results reveal that: (1) a 6-7 km-thick oceanic crust with hints of compressive deformation enters the subduction, carrying no sediment past the backstop; (2) it crosses the continental Moho as shallow as 13 km depth; (3) above, a set of reactivated antithetic reverse faults controlling the Matapan forearc basin is clearly imaged; (4) no evidence is found supporting a speculated inverse splay fault outcropping at the toe of the Hellenic scarp and tentatively related to the 365 CE event; (5) some steep faulting is observed at mid-escarpment, whose downward continuation coincides with a jump in the forearc Moho’s depth, and that likely accommodates some of the required dextral strike slip partitioning motion; (6) possible clues are found for some compressive reactivation of the Maleas basin, north of the external arc

Heat flow in the Sea of Marmara Central Basin: Possible implications for the tectonic evolution of the North Anatolian fault

The Central Basin in the Sea of Marmara is a syntectonic basin related to the evolution of the North Anatolian fault. A well-dated (ca. 15.5-16 ka) homogenite sediment can be used as a marker in three-dimensional depth model calculations, allowing a precise determination of the seafloor subsidence rates during the Holocene. A steady-state model based on the propagation of the rates downward through the basin fill provides a good correlation with the deeper seismic reflection imagery for the past 250 ka but indicates variation of subsidence pattern for older ages. Heat flow measured at the seafloor is affected by sedimentation blanketing effects. Heat flow and subsidence data can only be reconciled if the Central Basin depocenter migrated northward with time. According to that scenario, subsidence and deposition started earlier (ca. 5-3.5 Ma) in the southern subbasin, and an acceleration of subsidence in the northern subbasin occurred at ca. 2.5-1.5 Ma. These results allow us to propose that a southern fault system distinct from the Main Marmara fault is responsible for the southern onset of the subsidence. Changes in the fault network and slip rates are implied during the last 2.5-1.5 Ma despite no apparent change since 250 ka

Along-arc segmentation and interaction of subducting ridges with the Lesser Antilles Subduction forearc crust revealed by MCS imaging

We present the results from a new grid of deep penetration multichannel seismic (MCS) profiles over the 280-km-long north-central segment of the Lesser Antilles subduction zone. The 14 dip-lines and 7 strike-lines image the topographical variations of (i) the subduction interplate décollement, (ii) the top of the arcward subducting Atlantic oceanic crust (TOC) under the huge accretionary wedge up to 7 km thick, and (iii) the trenchward dipping basement of the deeply buried forearc backstop of the Caribbean upper plate. The four northernmost long dip-lines of this new MCS grid reveal several-kilometre-high topographic variations of the TOC beneath the accretionary wedge offshore Guadeloupe and Antigua islands. They are located in the prolongation of those mapped on the Atlantic seafloor entering subduction, such as the Barracuda Ridge. This MCS grid also provides evidences on unexpected huge along-strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments. Their mapping clearly indicates two principal areas of active deformation in the prolongation of the major Barracuda and Tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the Atlantic seafloor. Although different in detail, the two main deforming forearc domains share similarities in style. The imaged deformation of the sedimentary stratification reveals a time- and space-dependent faulting by successive warping and unwarping, which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely-oriented elongated and localized topographical ridges. The induced faulting producing vertical scarps in this transport does not require a regional arc-parallel extensional regime as proposed for the inner forearc domain, and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver. A contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone. Its mapping documents along-arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench. With the exception of the southernmost part, the newly mapped updip limit reaches 25 km closer to the trench, thus indicating a possible wider seismogenic zone over almost the whole length of the study area

Along-arc segmentation and interaction of subducting ridges with the Lesser Antilles Subduction forearc crust revealed by MCS imaging

Highlights • We image the deep structure of the Lesser Antilles Subduction Zone by MCS profiles. • The complex deformation of the outer forearc crust is induced by subducting ridges. • We discuss also the effect of the subducting compressive NAM–SAM Plate-boundary. • Along-strike variations of the seaward edge of the outer forearc crust are discovered. • The updip limit proxy of the seismogenic part reaches 20 km trenchwards than believed. Abstract We present the results from a new grid of deep penetration multichannel seismic (MCS) profiles over the 280-km-long north-central segment of the Lesser Antilles subduction zone. The 14 dip-lines and 7 strike-lines image the topographical variations of (i) the subduction interplate décollement, (ii) the top of the arcward subducting Atlantic oceanic crust (TOC) under the huge accretionary wedge up to 7 km thick, and (iii) the trenchward dipping basement of the deeply buried forearc backstop of the Caribbean upper plate. The four northernmost long dip-lines of this new MCS grid reveal several-kilometre-high topographic variations of the TOC beneath the accretionary wedge offshore Guadeloupe and Antigua islands. They are located in the prolongation of those mapped on the Atlantic seafloor entering subduction, such as the Barracuda Ridge. This MCS grid also provides evidences on unexpected huge along-strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments. Their mapping clearly indicates two principal areas of active deformation in the prolongation of the major Barracuda and Tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the Atlantic seafloor. Although different in detail, the two main deforming forearc domains share similarities in style. The imaged deformation of the sedimentary stratification reveals a time- and space-dependent faulting by successive warping and unwarping, which deformation can be readily attributed to the forearc backstop sweeping over the two obliquely-oriented elongated and localized topographical ridges. The induced faulting producing vertical scarps in this transport does not require a regional arc-parallel extensional regime as proposed for the inner forearc domain, and may support a partitioned tectonic deformation such as in the case of an outer forearc sliver. A contrasted reflectivity of the sedimentary layering at the transition between the outer forearc and accretionary domains was resolved and used to define the seaward edge of the outer forearc basement interpreted as being possibly a proxy to the updip limit of the interplate seismogenic zone. Its mapping documents along-arc variations of some tens of kilometres of the subduction backstop with respect to the negative gravity anomaly commonly taken as marking the subduction trench. With the exception of the southernmost part, the newly mapped updip limit reaches 25 km closer to the trench, thus indicating a possible wider seismogenic zone over almost the whole length of the study area

The Eastern North American Margin Community Seismic Experiment: An Amphibious Active- and Passive-Source Dataset

Abstract The eastern North American margin community seismic experiment (ENAM‐CSE) was conceived to target the ENAM Geodynamic Processes at Rifting and Subducting Margins (GeoPRISMS) primary site with a suite of both active‐ and passive‐source seismic data that would shed light on the processes associated with rift initiation and evolution. To fully understand the ENAM, it was necessary to acquire a seismic dataset that was both amphibious, spanning the passive margin from the continental interior onto the oceanic portion of the North American plate, and multiresolution, enabling imaging of the sediments, crust, and mantle lithosphere. The ENAM‐CSE datasets were collected on‐ and offshore of North Carolina and Virginia over a series of cruises and land‐based deployments between April 2014 and June 2015. The passive‐source component of the ENAM‐CSE included 30 broadband ocean‐bottom seismometers (OBSs) and 3 onshore broadband instruments. The broadband stations were deployed contemporaneously with those of the easternmost EarthScope Transportable Array creating a trans‐margin amphibious seismic dataset. The active‐source portion of the ENAM‐CSE included several components: (1) two onshore wide‐angle seismic profiles where explosive shots were recorded on closely spaced geophones; (2) four major offshore wide‐angle seismic profiles acquired with an airgun source and short‐period OBSs (SPOBSs), two of which were extended onland by deployments of short‐period seismometers; (3) marine multichannel seismic (MCS) data acquired along the four lines of SPOBSs and a series of other profiles along and across the margin. During the cruises, magnetic, gravity, and bathymetric data were also collected along all MCS profiles. All of the ENAM‐CSE products were made publicly available shortly after acquisition, ensuring unfettered community access to this unique dataset