Geology of the d'Entrecasteaux - New Hebrides arc collision zone : results from a deep submersible survey

International audienceDuring the SuBPSOl cruise, seven submersible dives were conducted between water depths of 5350 and 900 m over the collision zone between the New Hebrides island arc and the d'Entrecasteaux Zone (DEZ). The DEZ, a topographic high on the Australian plate, encompasses the North d'Entrecasteaux Ridge (NDR) and the Bougainville guyot, both of which collide with the island-arc slope. In this report we use diving observations and samples, as well as dredging results, to analyse the geology of the Bougainville guyot and the outer arc slope in the DEZ-arc collision zone, and to decipher the mechanisms of seamount subduction. These data indicate that the Bougainville guyot is a middle Eocene island arc volcanocapped with reef limestones that appear to have been deposited during the Late Oligocene to Early Miocene and in Miocene-Pliocene times. This guyot possibly emerged during the Middle and Late Miocene, and started to sink in the New Hebrides trench after the Pliocene. The rocks of the New Hebrides arc slope, in the collision zone, consist primarily of Pliocene-Recent volcaniclastic rocks derived from the arc, and underlying fractured island-arc volcanic basement, possibly of Late Miocene age. However, highly sheared, Upper Oligocene to Lower Miocene nannofossil ooze and chalk are exposed at the toe of the arc slope against the northern flank of the NDR. Based on a comparison with cores collected at DSDP Site 286, the ooze and chalk can be interpreted as sediments accreted from the downgoing plate. East of the Bougainville guyot an antifonn that developed in the arc slope as a consequence of the collision reveals a 500-m-thick wedge of strongly tectonized rocks, possibly accreted from the guyot or an already subducted seamount. The wedge that is overlain by less deformed volcaniclastic island-arc rocks and sediments includes imbricated layers of Late Oligocene to Early Miocene reef and micritic limestones. This wedge, which develops against the leading flank of the guyot, tends to smooth its high-drag shape. A comparison between the 500-m-thick wedge of limestones that outcrops southeast of the guyot and the absence of such a wedge over the flat top of the guyot, although the top is overthrust by island-arc rocks and sediments, can be interpreted to suggest that the wedge moves in the subduction zone with the guyot and facilitates its subduction by streamlining

Signification géodynamique des calcaires de plate-forme en cours de subduction sous l'arc des Nouvelles-Hébrides (Sud-Ouest de l'océan Pacifique)

Note présentée par Jean DercourtInternational audienceThe analysis of carbonates from New Hébrides Trench shows that three main épisodes of shallow water carbonate déposition occurred during Late Eocene,Late Oligocene-Early Miocène,Mio-Pliocene-Quaternary, controlled by eustatism and tectonic.L'analyse de carbonates issus de la fosse des Nouvelles-Hébrides a permis de reconnaître trois périodes favorables au développement de plates-formes(Éocène supérieur,Oligocène supérieur-Miocène inférieur,Mio-Pliocène-Quaternaire)contrôlé par l'eustatisme et la tectonique

LES ÉTAPES DE LA SÉDIMENTATION SUR LE PLATEAU CONTINENTAL DU ROUSSILLON AU COURS DU QUATERNAIRE RÉCENT

International audienc

LES ÉTAPES DE LA SÉDIMENTATION SUR LE PLATEAU CONTINENTAL DU ROUSSILLON AU COURS DU QUATERNAIRE RÉCENT

International audienc

Plio-Quaternary evolution of the North Banda sea and east Sulawesi margin

A recent cruise of the Indonesian Research Vessel Baruna Jaya III provides news seismic data and dredged samples from the Western part of the Sinta ridge. Seismic data shows a thin and uniform Plio-Quaternary cover overlying both the Late Miocene oceanic crust of the North Banda basin and the Banda ridges (Tukang Basi and Sinta ridges). Dredged samples from the Western part of the Sinta ridge (now rising at about 3000m deep) confirm the existence of a Triassic carbonate platform and thus the continental origin of these Banda ridges. Correlations between the "on land "tectonic events and the seismostratigraphic data, pointed out the opening of the North Banda basin during the Late Miocene and then the local deformation of the Lower Pliocene sediments unconformably covered by upper Pliocene to Pleistocene sedimentary deposits. It is suggested that the Sinta ridge went down, from the surface to its present depth, during the Late Miocene time, when the North Banda oceanic crust was created