IODP Proceedings    Volume contents     Search


Site U13501

Expedition 324 Scientists2

Background and objectives


Integrated Ocean Drilling Program Site U1350 (proposed Site SRCH-4) on Ori Massif was the fifth and last site completed during Expedition 324. This site is on the lower east flank of Ori Massif (Fig. F1), where it was intended as a comparison site to the summit of Ori Massif (Site U1349) and sites to the south on Tamu Massif (Ocean Drilling Program [ODP] Site 1213 and Sites U1348 and U1349) and to the north on Shirshov Massif (Site U1346). Only one hole, U1350A, was cored at the site, reaching a total depth of 315.8 meters below seafloor (mbsf). According to the initial expedition plan, Site U1350 was an alternate site and proposed Site SRSH-8 was planned as the last place to drill during Expedition 324. Site U1350 was substituted because the scientific party decided that it was more important to recover cores from the flank of Ori Massif than to drill another site on Tamu Massif close to Site 1213, where igneous rocks were cored during Leg 198 (Shipboard Scientific Party, 2002). It was felt that the lower flank location of Site U1350 would be a good spot to core fresh igneous rocks in contrast to the highly altered rocks cored at Site U1349 on the Ori Massif summit. Because it was the last site to be explored during Expedition 324, Site U1350 was cored until time expired. Nevertheless, a better than average rate of penetration allowed the deepest penetration into igneous basement (172.7 m) of any Expedition 324 site.

Ori Massif is the second largest volcanic construct within Shatsky Rise, with a volume of ~0.7 x 106 km3 (Sager et al., 1999). Like Tamu Massif, it may have formed over a geologically short period of time (<1 m.y.) with a high effusion rate, but the actual age and duration are unknown. Because Ori Massif is separated from Tamu Massif by a narrow, rectangular, faulted basin ("Helios Basin;" see Fig. F2 in the "Expedition 324 summary" chapter), speculation that Ori and Tamu massifs formed together and later rifted (Sager et al., 1999; Nakanishi et al., 1999) has been raised. In the context of the plume head hypothesis, Ori Massif appears to represent the eruptions during a transition in volume from plume head (Tamu Massif) to plume tail (Papanin Ridge).

Magnetic lineations surrounding Ori Massif imply that it formed on lithosphere of earliest Cretaceous age. Anomaly M16 brackets the southeast flank, whereas Anomaly M14 crosses the northwest flank (Nakanishi et al., 1999) (Fig. F3 in the "Expedition 324 summary" chapter). According to the geomagnetic polarity timescale (Ogg et al., 2008), the age of the lithosphere at Ori Massif is ~142–140 Ma, which may also be the age for Ori Massif if it formed nearly synchronously with the lithosphere, as implied by isostatic compensation (Sandwell and McKenzie, 1989). The northeast and southwest flanks of Ori Massif appear to be bounded by fracture zones (Nakanishi et al., 1999) and this bracketing by spreading ridges and fracture zones, which are orthogonal, may be the reason that the massif appears subcircular or nearly square in plan view (Sager et al., 1999).

Like Tamu Massif, Ori Massif has the appearance of a large central volcano with low flank slopes (Sager et al., 1999). Too few bathymetry data exist to say which flanks of this edifice are smooth and which are faulted, although the shape of the southern margin implies rifting, whereas the smooth nature of the eastern flank suggests a volcanic slope (Sager et al., 1999; Klaus and Sager, 2002). As with other Shatsky Rise massifs, the rise top is covered with pelagic sediment reaching a maximum thicknesses of 600–700 m on existing seismic profiles (Sager et al., 1999; Klaus and Sager, 2002). Ori Massif has volcanic summit ridges, just like Tamu Massif, but all are buried or mostly so and thus are not as prominent as the large ridge (Toronto Ridge) (Sager et al., 1999) atop Tamu Massif. The origin of these ridges is not known, although they appear to be volcanic constructs and not fault-bounded ridges (i.e., horsts). Site U1349 was drilled on one of these ridges, recovering igneous rocks that appear to have formed in a subaerial or shallow-water environment and perhaps represent a late stage of Ori Massif formation. In contrast, the smooth, gently sloping eastern flank acoustic basement, where Site U1350 is located, probably represents lava flows erupted during the main shield-building phase of Ori Massif eruptions. From predrilling interpretation of the site survey seismic reflection profile (Fig. F2), we expected to encounter 157 m of pelagic sediments overlying igneous basement consisting of basaltic lava flows.

Scientific objectives

Sampling the flank of Ori Massif was considered an important objective because this volcano is a major edifice within Shatsky Rise. Furthermore, Site U1350 is near the center of a transect of sites along the axis of Shatsky Rise planned to yield age and geochemical trends within the plateau, so coring relatively fresh basalt from Ori Massif was a high priority. 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.

Scientific objectives of all 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 could be constrained. A critical objective at Site U1350, 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 are critical to geochemical and isotopic studies whose goals are to establish the elemental compositions, variations in compositions, and the isotopic characteristics of the rocks. 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. Operationally, this meant that the goal at Site U1350 was to core a representative suite of igneous units that was fresh enough to provide reliable geochemical and isotopic measurements.

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 to infer the eruption style, igneous products, and physical structure of Shatsky Rise. Given its size and inferred eruption rate, Ori Massif is likely to be an example of an unusual volcanic construct whose development 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. Although few sediment samples were cored from Site U1350, sedimentologists will carefully examine the sparse sediments that were cored. Sediment types and paleontological environment data will hopefully 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 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 rise and to constrain fundamental physical properties that affect geophysical imaging and remote sensing. Because Site U1350 igneous rocks are relatively fresh, physical property measurements will be important for determining rock properties that can be used for geophysical data interpretation.

1Expedition 324 Scientists, 2010. Site U1350. In Sager, W.W., Sano, T., Geldmacher, J., and the Expedition 324 Scientists, Proc. IODP, 324: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.324.107.2010

2Expedition 324 Scientists' addresses.

Publication: 3 November 2010
MS 324-107