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Geologic Setting

The Mariana convergent plate margin system is nonaccretionary, and the forearc between the trench and the arc is pervasively faulted. It contains numerous large (30 km diameter and 2 km high) mud volcanoes (Fryer and Fryer, 1987; Fryer, 1992, 1996) (Fig. F2). The mud volcanoes are composed principally of unconsolidated flows of serpentine mud with clasts consisting predominantly of serpentinized mantle peridotite (Fryer, Pearce, Stokking, et al., 1990). Some have also brought up blueschist materials (Maekawa et al., 1995; Fryer and Todd, 1999). Faulting of the forearc to great depth produces fault gouge that when mixed with slab-derived fluids generates a thick gravitationally unstable slurry of mud and rock that rises in conduits along the fault plane to the seafloor (Fig. F3) (Fryer, 1992; Fryer, 1996). These mud volcanoes are our most direct route to the dècollement and, episodically, through protrusion events, open a window that provides a view of processes and conditions at depths as deep as 35 km beneath the forearc.

Prior to Leg 195, only one other active serpentine mud volcano (Conical Seamount) (see Fig. F2) had ever been sampled by drilling (Fryer, Pearce, Stokking, et al., 1990). Little was then known of either the processes that formed such seamounts, their distribution, and their relation to the tectonics of the forearc region or of the potential for understanding the deeper forearc processes they reflect. Recent advances in the understanding of the structure and tectonic evolution of nonaccretionary forearcs, the nature of geochemical cycling within them, and the various active thermal, hydrologic, metamorphic, and biological processes involved in the formation of mud volcanoes permitted the planning of comprehensive studies of the intermediate-depth processes occurring within the "subduction factory." By revisiting descriptions of serpentine melanges and "sedimentary" serpentinite terranes from past literature (Lockwood, 1972), we now realize that serpentine mud volcanism in convergent margin settings is not merely a local curiosity of the Mariana system but occurs worldwide.

Site 1200 is located on a 200-m-high summit knoll on South Chamorro Seamount at 13°47'N, 146°00'E in a water depth of 2910 m, about 125 km east of Guam in the western Pacific Ocean. It lies 85 km from the trench, where the depth to the downgoing slab is ~26.5 km, based on studies by Isacks and Barazangi (1977). Pore fluids collected in gravity cores from this seamount exhibit a strong slab signal. It is the only known site of active blueschist mud volcanism in the world and supports the only documented megafaunal assemblages associated with serpentine/blueschist mud volcanism (Fryer and Mottl, 1997).

Side-scan surveys of this seamount (Fig. F4) show that the southeastern sector of the edifice has collapsed, and debris flows of serpentine material (dredged in 1981 and observed on Shinkai 6500 dives in 1995) blanket the inner slope of the trench from the summit of the seamount to the trench axis. The summit knoll sits at the apex of the sector collapse, and its formation was most likely initiated in response to the collapse. Submersible observations show that the knoll's surface is broken into uplifted slabs of cohesive serpentine mud (Fryer, 1996) separated by meter-deep fissures with crosscutting orientations. Medium blue-green to dark blue serpentine mud and clasts of metamorphosed rocks are exposed. Low-temperature springs in the fissures support a vigorous biological community of mussels, gastropods, worm tubes, and galatheid crabs (Fryer and Mottl, 1997). The mussels are likely of the genus Bathymodiolus, one which contains methylotrophic symbionts in its gills and requires high ambient concentrations of methane in its feeding source (Fryer and Mottl, 1997). The pore fluid composition profiles and the presence of a vigorous biological community at the surface suggest that the summit knoll is a currently active seep region. The interior of the seamount shows little structure on six-channel seismic reflection profiles (Fryer and Mottl, 1997) (Fig. F5). This seamount is likely an active serpentine mud volcano similar to Conical Seamount, drilled during Leg 125 (Sites 778-780), and thus provides an excellent drill target for studies of the active processes of these mud volcanoes. It has the strongest slab signature in pore fluids from among the seamounts sampled in 1997 and is comparable to Conical Seamount in the strength of its slab signal.

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