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doi:10.2204/iodp.proc.324.108.2010

Results

During Expedition 324, bathymetry data and marine magnetic data were collected for 8559 km (Fig. F5). The transits of the expedition were divided into six legs and named sequentially (Transits EXP324-L1T– EXP324-L6T) for convenient identification of each transit (Table T1).

Bathymetry

Shatsky Rise contains three large volcanic massifs (Tamu, Ori, and Shirshov), and the flanks of these massifs typically have gentle slopes of ~1.5° (Sager et al., 1999) (Fig. F6). During the first transit (EXP324-L1T) from Yokohama to Site U1346, the ship passed over the Japan Trench, a deep abyssal plain, and the northern flank of the Ori Massif and ended at the summit of Shirshov Massif (Figs. F6, F7). The deepest part of the Japan Trench is deeper than 7000 meters below sea level (mbsl), with a ~2.6° slope angle, whereas the water depth of the abyssal plain ranges from 5900 to 5600 mbsl, except over a seamount (36°48.7970′N, 154°04.8012′E), which shallows up to 5200 mbsl. The northern flank of the Ori Massif has a gentle surface with some high peaks and shallows toward the Shirshov Massif. The seafloor bathymetry between Sites U1346 and U1347 (Transit EXP324-L2T) is characterized by several seamounts that shoal to between 3900 and 3500 mbsl, jutting from a flat plain at ~5000 mbsl ("Sliter Basin") (Fig. F8). The third transit (EXP324-L3T), from Sites U1347 to U1348, crossed the northeast flank of Tamu Massif, where water depth varies between 3000 and 3500 mbsl (Fig. F9). Bathymetry data were collected over the summit of "Cooperation Seamount" while in transit between Sites U1348 and U1349 (Transit EXP324-L4T). The seamount has a steep flank and twin peaks with the shallowest depth at 2700 mbsl, as reported by Sager et al. (1999) (Fig. F10). The summit of Ori Massif is flat, and the flanks have a gentle slope angle of ~1.2° on Transit EXP324-L5T (Fig. F11). After drilling at Site U1350, the ship moved to the southwest to pass over a high feature (4400 m) within Helios Basin (Transit EXP324-L6T). The ship then turned to the south over the western summit of Tamu Massif (Fig. F12A). The summit has a broad, dome-shaped feature, and the southern flank of the massif has a ~2° slope toward the south. The depth of the ocean floor surrounding the Shatsky Rise is deeper than 5500 mbsl, and some sea knolls rise a few hundred meters from the floor. The ship continued to move toward the south and crossed the Marcus-Wake and Magellan seamount chains, which developed around the Jurassic Quiet Zone (Fig. F5). The Marcus-Wake Seamounts show a west–northwest trend, and numerous seamounts are scattered over ~400 km. Most of them have pointed peaks, but some have flat tops (guyots) (Fig. F12B). Likewise, the Magellan Seamounts show a trend of north–northwest to south–southeast, and seamounts rise above the surrounding ocean floor at ~6000 mbsl. The ship passed through the Caroline Islands located on the southern margin of the Mariana Basin and over the western flank of Ontong Java Plateau, where water depth ranges from 4000 to 2000 mbsl (Figs. F5, F12C). Between the Ontong Java Plateau and the Solomon Islands, the Kilinailau Trench lies with an ~30 km width. The Soloman Island arc is separated from Solomon Basin by the New Britain Trench, the deepest point of which reaches below 7500 mbsl. Bathymetry of Solomon Basin generally shoals to the south up to 4000 mbsl. Another deep submarine basin, Woodlark Basin, is located between Woodlark and Pocklington ridges.

Magnetic anomalies

Magnetic anomalies over the abyssal plain west of Shatsky Rise vary from –250 to 250 nT and show mostly positive anomalies except in a location over a seamount (Fig. F7). However, near the northern part of Ori Massif, the magnetic anomalies are again negative with low values of about –380 nT. The summit of Shirshov Massif shows anomalies of –50 to 100 nT, and between the southwestern flank of the Shirshov Massif and the northern flank of Tamu Massif, anomalies vary from –250 to 250 nT (Fig. F8). At the northern flank margin of Tamu Massif, anomalies fluctuate highly (between –400 and 400 nT) and show mainly positive values toward the western slope of the massif (Site U1347). Magnetic anomalies in transit from Site U1347 to Site U1348 show positive values up to 250 nT (Fig. F9). In the confines of Helios Basin, magnetic data show high positive anomalies up to 450 nT, whereas over Cooperation Seamount, magnetic data show weakly negative anomalies (Fig. F10). On the summit of Ori Massif, magnetic anomalies vary between –150 and 150 nT (Fig. F11). Anomalies between Ori and Tamu massifs fluctuate from –250 to 250 nT, but show mainly positive anomalies in the summit area of Tamu Massif (Fig. F12A). On the southern slope of Tamu Massif, magnetic anomalies show a deep trough of –300 nT and fluctuate with mostly positive values toward the abyssal plain where the Hawaiian magnetic lineations are developed (Fig. F12A). However, from ~1100 km along the transit from Site U1350 to Townsville over the Hawaiian lineations, magnetic anomalies have very low amplitudes, except in the regions of seamounts (Marcus-Wake and Magellan seamount chains) (Fig. F12A, F12B). The area where magnetic anomalies show low-amplitude features in the western part of the mid-Pacific plate is known as the Jurassic Quiet Zone (Tominaga et al., 2008) (Fig. F5). These low-amplitude anomalies continued southward to the Caroline Islands north of the Ontong Java Plateau. At the Caroline Islands, magnetic data show an anomalous high peak up to 800 nT, whereas in the vicinity of the western flank of Ontong Java Plateau, magnetic anomalies vary between –150 and 150 nT (Fig. F12C). Deep troughs of negative magnetic anomalies less than –350 nT are shown in the southwestern margin of Ontong Java Plateau near the Kilinailau Trench and the Soloman Island arc. Magnetic anomalies in the Solomon and Woodlark basins also show mostly negative values, but some high peaks of positive anomalies (~250 nT) are observed around the Pocklington Ridge (Fig. F12C).