Effets de la collision "ride des Loyauté - arc des Nouvelles-Hébrides" sur la terminaison sud de l'ensemble "Nouvelle Calédonie - Loyauté"

Les données de bathymétrie multifaisceaux et de géophysique de la campagne ZoNéCo 1 révèlent, au niveau du prolongement sud des rides de Nouvelle-Calédonie et des Loyauté, la présence de trois directions structurales et de deux phases volcaniques majeures. La direction WSW-ENE se retrouve sur l'ensemble de la zone "ride de Norfolk-bassin et ride des Loyauté" sous la forme de décrochements sénestres. La direction NNW-SSE est caractérisée par un réseau de failles normales. La direction SSW-NNE est présente au Sud de 24°40'S le long de la ride de Norfolk et s'exprime, sur la ride des Loyauté, par des failles normales et des décrochements dextres localisés. La première phase volcanique, ayant généré les guyots qui forment le substratum des deux rides, est vraisemblablement d'âge Oligocène-Miocène inférieur. La seconde phase, qui s'exprime par des volcans non érodés, pourrait être postérieure à la surrection Miocène supérieur-Pliocène. Les décrochements sénestres WSW-ENE qui découpent la plaque australienne sont interprétés comme résultant de la collision active "Loyauté - Nouvelles-Hébrides", l'arc des Nouvelles-Hébrides se comportant comme un poinçon rigide. (Résumé d'auteur

Post-Eocene extensional tectonics in Southern New Caledonia (SW Pacific): insights from onshore fault analysis and offshore seismic data

Ductile to brittle extensional deformation following thrusting of the peridotites nappe during the Upper Eocene has been shown to play a major role in the Tertiary tectonic evolution of the northern part of the main island of New Caledonia and its eastern and western margins. In this study, we provide new tectonic data from southern New Caledonia that allow to better constrain the tectonic evolution of the southern part of the main island. We present a kinematic analysis of faults and striations obtained mainly from exposures of sedimentary rocks in the region of Noumea with complements from measurements made farther north at Nepoui within post-obduction Middle-Miocene deposits. We also present additional results of an interpretation of seismic lines from the lagoon south of the Noumea Peninsula which provide constraints on the current tectonic regime of southern New Caledonia. Extensional faults in the Noumea region have been studied within terranes of various ages including pre- and syn-obduction deposits and ophiolites. Hence, we demonstrate that important extensional events have affected the southern part of the New Caledonia block after the obduction of the peridotite nappe. The direction of maximum extension is variable at the scale of the region. Both high angle and low angle normal faults are present and block rotation is observed at some localities. This suggests that detachments accommodating significant displacements are cutting through the sedimentary pile. The average final strain pattern of the region can be regarded as the results of a multidirectional flattening, a hypothesis consistent with vertical uplift associated with regional extension. These results are in good agreement with conclusions of earlier workers showing late extensional evolution of the ophiolites alon

Extrémités nord du bassin Nord-Fidjien et des fossés arrière-arc des Nouvelles-Hébrides : morphostructure et signature magnétique

Les données obtenues lors de la campagne Santa Cruz (novembre - décembre 1991, No Le Norois), couplées aux données antérieures, permettent de présenter des cartes bathymétrique, magnétique et structurale complètes des terminaisons nord du bassin Nord-Fidjien et des fossés arrière-arc des Nouvelles-Hébrides. Aucun arc volcanique ne borde au Sud la paléo-fosse du Vitiaz. Deux systèmes d'accrétion, orientés NW-SE puis E-W et reconnus respectivement jusqu'à 8°30'S et 9°S, sont à l'origine de la formation du coin N-W du bassin. La ride de Tikopia pourrait correspondre ou se surimposer à l'axe sub-E-W du second système d'ouverture. Plus au Nord, cet axe serait décalé par la ride West Tikopia et se situerait au niveau de la ride de 9°30'S. L'expression topographique du domaine des fossés résulte essentiellement de la construction de la ride volcanique de Duff, continue sur 400 km. La partie orientale de ce domaine n'est finalement que la bordure ouest du bassin Nord-Fidjien. La partie occidentale du domaine des fossés est installée sur le bord de l'arc; sa terminaison nord correspond à un graben intra-arc sub E-W, le fossé de Santa Cruz. (Résumé d'auteur

The Sissano, Papua New Guinea tsunami of July 1998 — offshore evidence on the source mechanism

The source of the local tsunami of 17th July 1998 that struck the north shore of Papua New Guinea remains controversial, and has been postulated as due either to seabed dislocation (fault) or sediment slump. Alternative source mechanisms of the tsunami were addressed by offshore investigation using multibeam bathymetry, sub-bottom profiling, sediment sampling and observation from the JAMSTEC Dolphin 3 K Remotely Operated Vehicle and Shinkai 2000 Manned Submersible. The area offshore of Sissano is a complex active convergent margin with subduction taking place along the New Guinea Trench. Dominant transpressional convergence results in diachronous collision of the highstanding North Bismarck Sea Plate in a westerly direction. The result is a morphological variation along the Inner Trench Slope, with the boundary between eastern and western segments located offshore Sissano in an area of on- and offshore subsidence. This subsidence, together with nearshore bathymetric focusing, is considered to increase the tsunamigenic potential of the Sissano area. The offshore data allow discrimination between tsunami generating mechanisms with the most probable source mechanism of the local tsunami as a sediment slump located offshore of Sissano Lagoon. The approximately 5–10 km3 slump is located in an arcuate, amphitheatre-shaped structure in cohesive sediments that failed through rotational faulting. In the area of the amphitheatre there is evidence of recent seabed movement in the form of fissures, brecciated angular sediment blocks, vertical slopes, talus deposits and active fluid expulsion that maintains a chemosynthetic vent fauna. Dating of the slump event may be approximated by the age of the chemosynthetic faunas as well as by a seismic signal from the failing sediment mass. Faults in the area offshore Sissano are mainly dip–slip to the north with recent movement only along planes of limited lateral extent. A possible thrust fault is of limited extent and with minimal (cm) reverse movement. Further numerical modelling of the tsunami also supports the slump as source over fault displacements

Evidence for sinistral strike-slip deformation in the Solomon Island arc

6 pages, 4 figuresDuring the SOPACMAPS 2 crusie carried out by IFREMER (Institut Français de Recherche pour l'Exploitation de la Mer) and ORSTOM (Institut Français de Recherche Scientifique pour le dévelopement en Coopération) on theR/V L'Atalante, in the Central Solomon Arc area, multibeam bathymetric and imagery data and single-channel seismic reflection profiles were collected from an area of about 3500 km2, to evaluate regional tectonics. Structural data geophysical profiles interpretation provide evidence for left-lateral transtensional tectonics on the southern edge of the Central Solomon Trough. This transtensional deformation is represented by faulting, block tilting, and rhombohedral deformation. The regional geology and the analysis of the sedimentary cover allow us to demonstrate that this tectonic occurred in two different phases during Oligocene to Miocene and Pliocene to Pleistocene timesPeer reviewe