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Figure 1. Major active hydrothermal sites (stars) at convergent margins of the western Pacific Ocean. Large arrows indicate general plate motion directions.

Figure 2. Regional tectonic setting of the PACMANUS area to be drilled during Leg 193. The Manus Basin occupies a backarc position relative to present-day subduction on the New Britain Trench to its south. Creation of new oceanic crust occurs at the Manus spreading center and at smaller segments to its west. Major transform faults are somewhat oblique to the spreading segments. The eastern Manus rift zone is a pull-apart structure between two of the major transform faults. It is underlain by thinned lower Tertiary arc crust, equivalent to exposures on New Ireland to the north and New Britain to the south. This older crust was generated during subduction on the now inactive Manus Trench. Active volcanoes of the Bismarck arc, above the New Britain subduction-Benioff zone are indicated by serrated-edged circles. Submarine volcanism in the eastern Manus rift lies well off the trend of this chain. Known hydrothermal sites include Conical Seamount, SuSu, Franklin Seamount, Vienna Woods, and Williaumez Rise. Plate motions are denoted by large arrowheads on thin lines annotated with rates. Curved thin arrows denote the sense of rotation on microplates as defined by GPS geodesy (Tregoning et al., 1998) or by opening and westward propagation of the Woodlark Basin (Taylor et al., 1995).

Figure 3. Tectonic model for the Manus Basin, following Martinez and Taylor (1996). About 80 km of extension by low-angle normal faulting and crustal thinning has occurred in the eastern Manus Basin between the Weitin and Djaul transform faults. The same amount of movement occurred on the Willaumez transform fault, where a slight obliquity between extension direction and fault strike allowed volcanism in the Extensional transform zone. Between the Willaumez and Djaul transforms, equivalent movement was accommodated by wedge-shaped opening of the Manus spreading center and compensating counter-clockwise rotation of the Manus microplate. MORB-type basaltic volcanism dominates the Manus spreading center, the Extensional transform zone, the east Sherburne volcanic zone, which overlies a sediment basin, and limited activity in the Southern rifts. By contrast, the eastern Manus Basin is dominated by arc-type volcanism.

Figure 4. Bathymetry of the eastern Manus Basin, from multibeam data compiled by Institut francàis de rescherche pour l'exploitation de la mer (IFREMER). The southeast-trending Djaul transform is conspicuous. The northeast trending deep on the western side is a failed spreading segment. PACMANUS lies at the crest of a northeast trending ridge of dacite (Pual Ridge).

Figure 5. Seafloor geology of the eastern Manus Basin. Edifices of the Eastern Manus volcanic zone, which extends between the active ends of the Djaul and Weitin transform faults, range from picritic basalt to rhyodacite in composition. Filled circles denote known hydrothermal sites, including the three main active sites of PACMANUS, DESMOS, and Susu Knolls. Gray lines indicate extensional fault scarps.

Figure 6. Geology of the PACMANUS hydrothermal field as derived from bottom-tow photography and manned submersible dives. Tracks shown are from the PACMANUS cruises (Frankin, 1991, 1993, 1996, and 1997), EDISON-I cruise (Sonne, 1994), ManusFlux cruise (Yokosuka, 1995), BIOACCESS cruises (Natsushima, 1996, 1998), and KODOS'99 cruise (Onnuri, 1999).

Figure 7. Primary and alternate drill sites (PCM-xA) along the crest of Pual Ridge. Active hydrothermal areas containing sulfide chimneys are identified. Proposed Site PCM-4A lies at the foot of the southeastern flank of Pual Ridge. Alternate Site PCM-5A is ~600 m northeast of primary Site PCM-1A, in a flat area farther from hydrothermal discharge.

Figure 8. A True-scale cross section and (B) longitudinal section of Pual Ridge (see transect lines on Fig. 7), showing inferred subsurface geology and presumed mixing between magmatic fluids and circulating seawater—models to be tested during Leg 193. Zones of progressively higher temperature alteration are expected to be telescoped under the focused vent site with massive sulfide chimneys relative to the situation under the diffuse venting zone at Snowcap. Leg 193 is not expected to intersect the main intrusive body but might cut some smaller apophyses. mbsl = meters below sea level. T = temperature.

Figure 9. A High-resolution single channel seismic profile across Pual Ridge at the position of the PACMANUS hydrothermal field and (B) interpretation. Proposed Sites PCM-2A and PCM-4A lie on the section, whereas proposed Sites PCM-1A and PCM-3A have been projected along the crest of Pual Ridge, which is seismically opaque. A fault is interpreted under Site PCM-4A, where 150 m of basaltic andesite is interpreted to overlie earlier Tertiary basement in the hanging wall.

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