Shatsky Rise is a large (450 km long, 1650 km wide) oceanic plateau located in the northwestern Pacific 1600 km east of Japan (Fig. F1). Magnetic lineations (Fig. F1 inset) show that the Pacific Plate near Shatsky Rise was formed during the Jurassic and Early Cretaceous at two spreading ridges that met at a triple junction (Larson and Chase, 1972; Hilde et al., 1976; Handschumacher et al., 1988; Sager et al., 1988; Nakanishi et al., 1989, 1999). The northeast-trending Japanese lineation set formed at the spreading ridge separating the Pacific from the Izanagi plate to the northwest. The southeast-trending Hawaiian lineations formed at the ridge separating the Pacific from the Farallon plate to the northeast (Woods and Davies, 1982). Tectonic reconstructions indicate that this part of the Pacific Plate formed near the equator (e.g., Larson and Chase, 1972).
The regional seafloor surrounding Shatsky Rise is deep (5.5-6.0 km) and sediment cover is thin, typically a few hundred meters (Houtz and Ludwig, 1979; Ludwig and Houtz, 1979). Sediments on the top of Shatsky Rise are much thicker, up to 1.2 km (Ewing et al., 1966; Zdorovenin et al., 1972; Neprohnov et al., 1984; Karp and Prokudin, 1985; Khankishieva, 1989; Sliter and Brown, 1993; Klaus et al., 1995) and are composed of Cretaceous pelagic carbonates deposited above the calcite compensation depth when the rise was younger and nearer the equator (e.g., Sliter and Brown, 1993).
Shatsky Rise is a thick igneous construct with a seismic velocity structure similar to oceanic crust, but thickened by several times under the highest parts (Den et al., 1969; Gettrust et al., 1980; Kogan, 1981). This suggests that the rise has a composition similar to oceanic crust and that dredges have recovered basalts from basement outcrops on the rise (Kashintsev and Suzymov, 1981; Sager et al., 1999). The huge size of the igneous pile, however, indicates that it was not formed by typical seafloor spreading.
A relationship between Shatsky Rise and the Pacific-Izanagi-Farallon triple junction has long been suspected because the Japanese and Hawaiian magnetic lineations converge at the rise axis (Larson and Chase, 1972; Hilde et al., 1976). Magnetic bights occur in many places within the lower parts of the rise, implying that the triple junction was composed of spreading ridges (Fig. F1) (Nakanishi et al., 1999). Because its size implies a large amount of volcanism, several authors have suggested that the rise formed as the result of a mantle plume (Sager et al., 1988; Nakanishi et al., 1989; Sager and Han, 1993; Sager et al., 1999). A predominantly reversed magnetic polarity over the southwestern part of the South High suggests a short formation period and an extremely high magma eruption rate similar to those of flood basalts and postulated for plume heads (Sager and Han, 1993).
There are few age dates for Shatsky Rise because basalts dredged from it are typically too altered for radiometric dating. Despite several attempts at drilling basement prior to Leg 198, no borehole has penetrated basement to obtain fresher rocks. The oldest well-dated sediments recovered from drilling the rise, which are at DSDP Site 306 on the southwest side of the South High (Fig. F2), are Berriasian (earliest Cretaceous; 137-144 Ma [timescale of Gradstein et al., 1994]) and were recovered ~80 m above a reflector thought to represent basement (Larson, Moberly, et al., 1975). Late Jurassic magnetic lineations M21 (148 Ma) to M19 (145 Ma) bracket the South High, so this massif must have formed near the end of the Jurassic or beginning of the Cretaceous. The Central High, North High, and Papanin Ridge must be younger because the magnetic lineations indicate that the lithosphere beneath is younger than early Berriasian (Fig. F1 inset). Shatsky Rise gravity data indicate Airy-type isostatic compensation typical for features emplaced near spreading ridges (Watts et al., 1980; Sandwell and MacKenzie, 1989), suggesting an age for the rise near that of the underlying lithosphere. These observations imply that the age of the rise becomes younger to the northeast (Sager and Han, 1993).
Volcanism occurred on Shatsky Rise after the main shield-building volcanic phases. Both the South and Central Highs have late-stage volcanic ridges on their top; the one on the South High rises ~1 km above the level at which shallow-water fossils were dredged. Assuming that this ridge formed below sea level (it shows no obvious subaerial erosion), the high subsided as normal lithosphere (Johnson and Carlson, 1992), and the fossils are autochthonous, the depth difference implies a time gap of ~8 to 9 m.y. In addition, basalts dredged from Toronto Ridge on top of the South High are trachytes and trachyandesites (Tejada et al., 1995), which are typical of fractionated, late-stage eruptions (Macdonald and Abbott, 1970).