The Manus backarc basin is set between opposed fossil (Manus Trench) and active (New Britain Trench) subduction zones. A chain of young arc volcanoes has formed above the north-dipping Wadati-Benioff Zone associated with the New Britain Trench, along the concave northern side of New Britain (Bismarck or New Britain arc) (Johnson, 1976). According to Taylor (1979), basement underlying the central Manus Basin may be predominantly basaltic crust formed in an earlier phase of backarc spreading that commenced ~5 m.y. ago. A series of neovolcanic edifices extends en echelon across the Eastern Manus Basin, wherein seismic data are consistent with rifting over the last ~1 m.y.

The PACMANUS hydrothermal field targeted by Leg 193 is located on the crest of Pual Ridge, a prominent felsic neovolcanic edifice with negligible sediment cover. Mapping by submersible and camera tows indicates the ridge is built of stacked, subhorizontal 5- to 30-m-thick lava flows (Waters et al., 1996).

Isolated hydrothermal deposits were photographed along 13 km of the main crest of Pual Ridge (Binns and Scott, 1993; Binns et al., 1995, 1996a, 1997a, 1997b). The most active hydrothermal areas occur in the center of this zone between two low knolls on the ridge crest. Extensive bottom-tow photography and manned submersible observation identified four principal areas of hydrothermal activity, including sulfide chimneys and areas of diffuse venting.

ODP sites occupied four targets on Pual Ridge (Fig. F2). Site 1188 (Snowcap) is an area of diffuse venting where the deepest penetration (~400 m) into the ridge was achieved. Roman Ruins (Site 1189) is characterized by closely packed simple and complex columnar chimneys as high as 20 m. Site 1190 is a reference site in an area devoid of visible hydrothermal activity. Satanic Mills (Site 1191) is an area of active black smoker chimneys.

At Snowcap (Site 1188), there is a 35-m-thick layer of fresh, moderate porphyritic dacite (sampled in Hole 1188A) overlying intensely altered aphyric dacite. This lava flow occupies a local knoll on the crest of Pual Ridge reaching 1640 meters below sea level (mbsl). At Roman Ruins, fresh aphyric dacite is restricted to <10 meters below seafloor (mbsf) in Hole 1189A, which was drilled beside a chimney on the edge of the hydrothermal discharge area. Hole 1189B is at the base of an active black smoker chimney and was cased to a depth of 30 mbsf. Below a sequence of strongly altered dacite, including a hydrothermal stockwork zone, there are less altered dacite lavas and breccia units at >120 mbsf. Here, a thick, vesicular, coherent dacite was recovered and petrographic (abundant, subvertically aligned vesicles) (Shipboard Scientific Party, 2002c) and geophysical evidence (steeply inclined top and base contacts apparent in electrical resistivity images) (Bartetzko et al., 2003) indicate a dikelike discordant geometry of this body. A sample of this fresh dacite is included in Table T1 (Sample 193-1189B-11R-3, 3 cm; 129.72 mbsf).

The ODP sites to the northwest of Snowcap (Satanic Mills and the reference site) are located at 1680–1690 mbsl (Fig. F2), and the holes penetrated aphyric or sparsely porphyritic dacite. Fresh samples of these dacite types were recovered at Sites 1190 and 1191, where holes reached <20 mbsf.