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doi:10.2204/iodp.proc.324.105.2010 Site U13481Expedition 324 Scientists2Background and objectivesBackgroundIntegrated Ocean Drilling Program Site U1348 (proposed Site SRSH-6) was the third site completed during Expedition 324. It was planned as the northernmost of three sites to be drilled on Tamu Massif (Fig. F1). Like other holes planned for Expedition 324, the main goal for the site was to core lava flows, so the fact that Site U1348 produced mainly sedimentary and volcaniclastic rocks came as a surprise. As outlined in other chapters (see the "Expedition 324 summary" and "Site U1347" chapters), Tamu Massif is the largest volcanic edifice within Shatsky Rise, and it is thought to represent the initial eruptions, perhaps caused by a plume head (Sager et al., 1999). The location for Site U1348, on the north flank of Tamu Massif, was chosen with hopes of establishing a transect of ages across this large edifice and documenting geochemical variations within this feature. The site is located near an anomaly between M18 and M19 (Nakanishi et al., 1999), implying a basement age of 144–145 Ma (Ogg et al., 2008). Even though Site U1348 is located ~400 km farther north than Ocean Drilling Program (ODP) Leg 198 Site 1213 (Fig. F1 in the "Expedition 324 summary" chapter), the age estimate for Site U1348 is nearly the same as the radiometric date measured for Site 1213 igneous rock (144.6 ± 0.8 Ma) (Mahoney et al., 2005; Shipboard Scientific Party, 2002b). Thus, results from Site U1348 were to help determine the magnitude of the age difference between the north and south flanks of Tamu Massif as well as whether its formation was as rapid as proposed (e.g., Sager and Han, 1993; Sager, 2005). In addition, the tectonic setting of Site U1348 was recognized as very different from other Tamu Massif sites. Whereas Sites 1213 and U1347 are located on the eastern and southern flanks of the massif, where the igneous basement appears to be normal volcanic slope, Site U1348 is located on the north flank, where magnetic lineations and bathymetry imply that spreading ridge–related rifting occurred (Sager et al., 1999; Nakanishi et al., 1999). Moreover, Site U1348 is located south of "Helios Basin," a rectangular basin between Tamu and Ori massifs (see Sager et al., 1999; Fig. F2 in the "Expedition 324 summary" chapter) that is thought to have formed by a short period of rifting and was preserved as a failed rift by a jump of the Pacific-Izanagi ridge farther northeast (Nakanishi et al., 1999). The magnetic lineations that traverse the northern flank of Tamu Massif bend into Helios Basin. Furthermore, the bathymetry of northern Tamu Massif consists of chevron-shaped ridges and troughs, interpreted to be related to seafloor spreading and associated rifting (Sager et al., 1999; Nakanishi et al., 1999). Thus, the igneous rock cored at Site U1348 was predicted to be significantly different from that of the rest of Tamu Massif. Site U1348 is located near the peak of a basement high on site survey seismic Line 13 from Cruise TN037 (Fig. F2). The top of this high was picked for drilling because sediments are thick in the surrounding area and the top of the buried hill is the only shallow basement in the area. Furthermore, there is only one site survey seismic line on northern Tamu Massif, providing few other choices. Although there was concern that drilling on a basement high might recover volcaniclastics rather than lava flows, the basement high at Site U1348 is large (>30 km wide). Therefore, this structure was assumed to be too large for a volcaniclastic cone. At the site, acoustic basement is a relatively strong reflecting horizon beneath which there is little or no coherent return of seismic energy. This is, in part, a result of the small single air gun source used for this seismic line, which gave little penetration into the top of the volcano under the sediments, and the relatively fast survey speed, which also helped to limit the signal to noise ratio (see Klaus and Sager, 2002). As can be seen upon closer examination (Fig. F3), sediments from the north onlap against the buried hill's northern flank and only Cenozoic ooze, Late Cretaceous chalk, and chert were expected in the sediment column at the site. Although that portion of the seismic interpretation was correct, acoustic basement turned out to be the top of volcaniclastic material at Site U1348, not lava flows as encountered at Sites U1346 and U1347. Scientific objectivesSampling the Tamu Massif was an important objective because this volcano is the main edifice within Shatsky Rise and Site U1348 was part of a transect along the axis of the massif. 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 core as deeply into the igneous formation as possible during the time allowed. Although initial objectives focused mainly on understanding the volcanic succession, geochemistry, and radiometric dating, those objectives are now in a state of flux because of the nature of the recovered material. It is presumed that the original objectives remain valid, in as much as they can be satisfied by the recovered cores, but the objectives will change as the science party studies the complex cored section. It appears that the cores may contain considerable information about sedimentation, paleodepth, and paleoenvironment, so those objectives may rise to greater importance. The initial scientific objectives of coring at Site U1348 are similar to those at other sites (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 Tamu Massif can be constrained. A critical objective at Site U1348, and indeed all Expedition 324 sites, was to core enough igneous rock of suitable freshness and composition to establish at least one reliable radiometric date. Igneous rocks were also critical to geochemical and isotopic studies with the goals of establishing the elemental compositions of the rocks, variations in compositions, and isotopic characteristics. Such data are crucial for determining the source of magma, inferring its temperature and depth of melting and crystallization, and deducing the degree of partial melting, as well as tracking its evolution with time. Because the Site U1348 cores appear to contain volcaniclastic material that is highly altered, an important objective will be to examine the metamorphosis of the cored material to help understand not only the initial composition but the processes that have changed the rock over time. Expedition 324 also sought to constrain the evolution of Shatsky Rise by collecting samples for several nongeochemical studies focusing on varied aspects of rise geology. For Site U1348, it appears that sedimentary and paleontological studies will be particularly important because the cores recovered sedimentary material with apparent paleoenvironmental and paleodepth implications. Thus, deciphering the ages of the sediments and their paleoenvironment through studies of biostratigraphy, fossil content, and sedimentary structures will be an important task. In addition, physical volcanologists, structural geologists, and logging geophysicists will use cores and logs to infer the eruption style, igneous products, and physical structure of the cored section. Paleomagnetic study of the samples recovered at Site U1348 will seek to determine the magnetic polarity of basement for comparison with surrounding magnetic lineations and the geomagnetic polarity timescale as well as the paleolatitude of the rise and its plate tectonic drift. Physical properties of Shatsky Rise core samples will be measured to better understand the nature of the rocks that make up the sedimentary section 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. | |||
