Three submarine Diamante cross-chain volcanoes in the southern Mariana arc mark a magma-healed zone of along-arc (north–south) extension that allows either mafic mantle-derived basalts or felsic magmas from the middle of thickened arc crust to erupt. The largest volcano is East Diamante, with a well-developed (5×10 km) caldera that formed via violent felsic submarine eruptions beginning nearly 0.5 Ma. One or more of these eruptions also formed a giant submarine dune field extending 30 km to the NW of the volcano. Felsic igneous activity continues at least as recently as c. 20 000 years ago, with emplacement of resurgent dacite domes, some hot enough to power the only black smoker hydrothermal system known in the Mariana arc. In contrast, felsic eruptions do not occur on the two volcanoes to the west, implying that the mid-crustal felsic zone does not underlie the thinner crust of the Mariana Trough back-arc basin. Diamante cross-chain lavas define a medium K suite; mafic lava phenocryst assemblages show arc-like associations of anorthite-rich plagioclase with Fe-rich olivine. Magmatic temperatures for a basaltic andesite and three dacites are c. 1100 °C and c. 800 °C, respectively, typical for cool, wet, subduction-related felsic magmas. Felsic magmas formed under low-P crustal conditions. The Diamante cross-chain is the southernmost of at least seven and perhaps eight Mariana arc volcanoes in a c. 115 km long arc segment characterized by felsic eruptions. This is the ‘Anatahan Felsic Province’, which may have formed above a mid-crustal tonalite body that formed by fractionation or was re-melted when heated by c. 1200 °C mafic, mantle-derived magmas. Across- and along-arc variations suggest that felsic eruptions and dome emplacement occurred when midcrustal tonalite was remobilized by intrusions of mafic magma, while north–south extension facilitated the development of conduits to the surface.Supplementary material:Detailed Hyperdolphin ROV dive tracks, Cook 7 dredge locations, 40Ar/39Ar analytical data, analytical methods, major and selected trace element analyses of whole rock samples, and compositional data for minerals are available at http://www.geolsoc.org.uk/SUP18611

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TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractA 3D seismic survey was conducted in Ogata area, one of Japanese mature oil fields. The survey was aimed to obtain a basic data set, which is useful for studying the effects of irregular data acquisition geometry, and to provide the precise information on the subsurface structure for finding additional reserves. The survey area was previously the second largest lake in Japan and has been changed to rice fields after reclamation. Few difficulties in seismic data acquisition were expected and dense regular data acquisition geometry was planned to use in the survey. However, acquisition work encountered many hazards because a lot of unexpected constraints came from the reclaimed land. Moreover, obtained shot records showed the very complex nature of seismic wave fields. Especially, strong ringing waves masked weak reflection signals. Spatial variation of these ringing waves was investigated and the origin was modeled as critical refraction of multiple reflected waves within a very thin surface layer. Those results can provide valuable knowledge for seismic survey in areas with similar surface conditions. In seismic data processing, these ringing waves were successfully suppressed at the earlier data processing stage by FK filter in common shot and receiver domains followed by deconvolution. After this suppression, remaining data processing became more effective and gave a precise depth image. Various analyses to the obtained processing vintage such as coherency cube and attributes revealed the existence of minor faults precisely, which were not recognized previously. These could give a new interpretation of the area. As a result, the data set can be very useful for the evaluation of irregular data acquisition geometry as well as the exploration in the area.