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doi:10.2204/iodp.proc.324.104.2010 Site U13471Expedition 324 Scientists2Background and objectivesBackgroundIntegrated Ocean Drilling Program Site U1347 (proposed Site SRSH-3B) was the second site completed during Expedition 324, after a quick switch in plans to avoid the track of passing Typhoon Choi-Wan. The site was planned as one of three sites to be drilled on Tamu Massif and was envisioned as the deepest penetration into basement. This site is at the center of a north–south transect across the massif and was intended to sample near the summit of this large volcanic edifice (Fig. F1). Tamu Massif is the largest volcanic construct within Shatsky Rise, with a volume of ~1.8 x 106 km3 (Sager, 2005). It may have formed over a geologically short period of time (<1 m.y.) with a high effusion rate, similar to those of flood basalt eruptions (Sager and Han, 1993; Mahoney et al., 2005). In the context of the plume head hypothesis, Tamu Massif appears to represent the initial plume head eruptions. With this massive eruption, the Pacific-Izanagi-Farallon triple junction jumped eastward by 800 km to the location of Tamu Massif. Subsequently, the melt anomaly that formed the rise appears to have "captured" the triple junction, which should have moved northwestward relative to the Pacific plate according to kinematic analysis, but instead followed the axis of Shatsky Rise to the northeast (Sager et al., 1988). After the voluminous Tamu Massif eruption, volcanism declined as two smaller volcanoes formed to the north, Ori and Shirshov massifs, which were followed in turn by lesser output forming Papanin Ridge on the northeast end of the rise (Sager et al., 1999). Magnetic lineations surrounding Tamu Massif indicate that it formed on lithosphere of latest Jurassic to earliest Cretaceous age. Anomaly M21 brackets the southwest flank, whereas Anomalies M19–M17 cross the northeast flank (Nakanishi et al., 1999) (see Fig. F3 in the "Expedition 324 summary" chapter). The spacing between these lineations is larger than that in adjacent basins, which implies that the lithosphere upon which Tamu Massif erupted was captured from the Farallon plate by a northeastward ridge jump. Because of the complete isostatic compensation (Sandwell and McKenzie, 1989) and the close relationships between the morphology of the rise and past ridge positions shown by magnetic anomalies, Tamu Massif appears to be the same age as the surrounding lithosphere (Sager et al., 1999; Nakanishi et al., 1999). This idea is supported by the age of basalts cored at Ocean Drilling Program (ODP) Leg 198 Site 1213 on the southwest flank of Tamu Massif. These igneous rocks were radiometrically dated at 144.6 ± 0.8 Ma (Mahoney et al., 2005), a date consistent with Anomaly M19 in the geomagnetic polarity timescale (GPTS) (Ogg et al., 2008) and close to the Jurassic/Cretaceous boundary. The sediments directly above these igneous units are Berriasian in biostratigraphic age (Shipboard Scientific Party, 2002b, 2002c), which is also consistent with formation in the earliest Cretaceous. Tamu Massif appears to be a large, central volcano dissected by faulting on its north side (Sager et al., 1999). The southern and western flanks appear to be normal, largely unfaulted volcanic flanks, whereas the north and east sides show linear troughs and ridges that may represent ridge-related faulting. The Tamu Massif summit has a large, rounded dome shape caused mainly by a cap of pelagic sediments as thick as 1.2 km (Sliter and Brown, 1993). Beneath these sediments, the center of Tamu Massif contains a bowl-shaped minibasin bounded by volcanic basement ridges on the west and east sides (Sager et al., 1999). The western ridge ("Toronto Ridge" of Sager et al., 1999) is tallest, protruding 500–1000 m above the rest of the summit (Sager et al., 1999). The eastern ridge is completely buried and has a subdued, rounded shape. Site U1347 is on the east flank of Tamu Massif on the upper slope where the volcanic basement is ~800 m deeper than the top of the buried eastern ridge. This location was selected for several important reasons. Because it is ~30 km downslope from the nearby buried eastern summit ridge, it is thought that the site was far enough from a potential volcanic center that it is less likely to have been inundated by a large thickness of lava during any one eruption. This would potentially allow coring to sample several eruptive sequences. In addition, the seismic signature at the site suggests that layers within volcanic basement can be sampled with reasonably shallow coring (Fig. F2). Over much of Tamu Massif, igneous basement is characterized by a curious "layered" appearance. At Site 1213, igneous material with this signature was cored and found to be sills or sheet flows interbedded with sediment (Shipboard Scientific Party, 2002a). In most places, this layered basement is ~0.1 s two-way traveltime (TWT) thick on seismic reflection profiles (~200 m assuming a seismic velocity of 4 km/s) and thus too thick to drill though in a short time. In contrast, at Site U1347, this layer is only ~0.03 s TWT thick (~60 m) on the seismic reflection profile and yet the layer can be followed upward into the basin on the Tamu Massif summit, where it is much thicker. Thus, at Site U1347 coring through this transition was considered an achievable objective. Another reason for coring at Site U1347 is that the sediments are thin at this location, estimated before drilling at 154 m thick. By targeting such a location, the time spent drilling through sediment to get to igneous basement is minimized. At 154 m thick, the sediments are just thick enough that the bottom-hole assembly (BHA) can be surrounded and supported by sediment before the drill bit makes contact with igneous rock. At Site U1347, the original plan was to drill ~300 m into igneous basement (i.e., ~454 meters below seafloor [mbsf]). This was predicated on an estimate of igneous rock penetration at a rate of 2.5–3.0 m/h, a value based on previous experience with drilling seamounts (Shipboard Scientific Party, 2002a) and oceanic plateaus (Shipboard Scientific Party, 2001). At this rate, it was expected that the total penetration could be achieved with two drill bits and a single drill bit change using a free-fall funnel (FFF). Unfortunately, Shatsky Rise did not read any of these reports and the formation was harder than expected. The overall penetration rate was about half of the planned value with the result that only a little more than half of the planned igneous basement penetration was achieved before time ran out for this operation. Scientific objectivesSampling the summit of Tamu Massif was an important objective because this volcano is the main edifice within Shatsky Rise and Site U1347 is closest to its center. As with most Expedition 324 sites, the operational goal for the site was to drill through the sediment overburden, core the oldest sediment overlying igneous basement, and then core as deeply into the igneous formation as possible with the time allowed. Scientific objectives of Expedition 324 sites are similar (for more details and rationale, see the "Expedition 324 summary" chapter). Coring of igneous rock was planned to determine the age of igneous basement so that the age progression and duration of volcanism at Shatsky Rise can be constrained. A critical objective at Site U1347, and indeed all Expedition 324 sites, was to core enough igneous rock of suitable freshness and composition to allow at least one reliable radiometric date to be established. Igneous rocks were also critical to geochemical and isotopic studies, the goals of which are to establish the elemental compositions, variations in compositions, and isotopic characteristics of the rocks. Such data are crucial for determining the source of magma, inferring its temperature and depth of melting and crystallization, deducing the degree of partial melting, and tracking its evolution over time. Operationally, this meant that at Site U1347 the goal was to core a representative suite of igneous units that was fresh enough to provide reliable geochemical and isotopic measurements. In addition, with the three-hole transect across Tamu Massif, it is hoped that questions of age trends and geochemical and isotopic variations within a single large volcanic edifice can be addressed. Expedition 324 also sought to constrain the evolution of Shatsky Rise by collecting samples for a host of nongeochemical studies focusing on varied aspects of rise geology. Physical volcanologists, structural geologists, and logging geophysicists will use cores and logs to infer the eruption style, igneous products, and physical structure of Shatsky Rise. Given its size and inferred eruption rate, Tamu Massif is likely to be an example of an unusual volcanic construct, the development which is poorly understood. Shatsky Rise core samples will also be used to study the submarine alteration of igneous rock and its effect on other analyses. Studies of sediments overlying igneous basement are planned to better understand the paleontological age of Shatsky Rise sediments and the processes and rates of Cretaceous sedimentation atop the rise volcanoes. Moreover, sediment types and paleontological environment data will indicate the paleodepths of sediment deposition, information that is important for understanding the eruption and subsidence history of the volcanic edifices. Paleomagnetic study of the samples recovered during Expedition 324 seeks to determine the magnetic polarity of basement for comparison with surrounding magnetic lineations and the GPTS as well as the paleolatitude of the rise and its plate tectonic drift. Tamu Massif samples are important in this effort because they were likely formed farthest south and are the oldest, thus, they should show the maximum tectonic drift. Physical properties of Shatsky Rise core samples will be measured to better understand the nature of the rocks that make up the rise and to constrain fundamental physical properties that affect geophysical imaging and remote sensing. Such data will be useful for constraining seismic and gravity studies in particular. | |||
