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Site U13461

Expedition 324 Scientists2

Background and objectives


Integrated Ocean Drilling Program Site U1346 (proposed Site SRNH-2) on Shirshov Massif was the first and northernmost site drilled during Expedition 324 (see Fig. F1 in the "Expedition 324 summary" chapter). The expedition began with this site because of the necessity of working from north to south to minimize transit time. The site was proposed as the northernmost site in a transect of five holes across the large volcanic edifices of Shatsky Rise. As with all sites during Expedition 324, the goal was to core through the lowermost sediments and into igneous rocks, recovering as much of the latter as possible in the allotted time.

Sampling Shirshov Massif was considered important because of the location of the volcano within Shatsky Rise. Magnetic lineations imply that Shatsky Rise formed along the trace of a triple junction (Sager et al., 1988; 1999; Nakanishi et al., 1999) and volcanism appears to have followed the track of the triple junction (or vice versa), but the amount of volcanism appears to have diminished over time. Shirshov Massif appears to be one of two intermediate volcanic edifices between the initial eruptions of Tamu Massif, on the southwest end of the rise, and the waning volcanic trail of Papanin Ridge on the northeast end of the rise. Thus, Shirshov Massif is important in the study of Shatsky Rise because it represents the transitional, waning magmatic output of the rise volcanism. In the parlance of the plume head hypothesis, Shirshov Massif is in the transition between plume head and tail.

Shirshov Massif is the northernmost of the large seamounts within Shatsky Rise. It is a subcircular edifice, ~100 km in diameter with a minimum depth slightly less than 3 km (Fig. F1). Seismic sections show thin or absent sedimentary cover on the volcano flanks but a thick volcanic cap atop the summit with the thickest part at the center of the edifice (Sager et al., 1999). Seismic data also show moderately strong acoustic basement across the edifice, with the basement being mostly flat except for some buried ridges that may define the boundaries of a buried caldera or summit depression.

Site U1346 is on the north rim of the summit of Shirshov Massif (Fig. F1) and is the only site drilled on Shirshov Massif during Expedition 324. Ocean Drilling Program (ODP) Leg 198 Site 1207 (Fig. F1), 26 km southeast of Site U1346 near the center of the sediment cap, is the only other site drilled on Shirshov Massif. It was cored for paleoceanographic objectives, and drilling reached only through the Upper Cretaceous sediments (Shipboard Scientific Party, 2002a). Site U1346 was situated along the Cruise TN037 site survey seismic Line 5A (at 2130Z on 9 August 1994; see Klaus and Sager, 2002) at a point where sediments thin toward the edge of the summit platform (Fig. F2). At this location, the igneous basement interface was interpreted to be flat and at ~177 meters below seafloor (mbsf). The rationale for choosing this location was that the igneous basement seemed to be free of structures that might indicate tectonic disturbance or late-stage volcanism and that the sediments were thick enough to support the bottom-hole assembly (BHA) but thin enough that minimal time and bit wear would need to be expended in reaching igneous basement.

Magnetic lineations mapped around Shatsky Rise bracket Shirshov Massif, with M13 on the north side of the edifice and M15 on the south side (see Fig. F3 in the "Expedition 324 summary" chapter; Nakanishi et al., 1999). Site U1346 is near the location of Anomaly M14, but the exact location of the anomaly is poorly known because magnetic tracks are sparse over Shirshov Massif. If the basement of the volcanic edifice is the same age as the anomaly, its age is ~140 Ma and corresponds to a time near the boundary between the Valanginian and Berriasian stages of the fossil timescale (Ogg et al., 2008).

Scientific objectives

Sampling Shirshov Massif was an important objective because this volcano is the farthest north of the large volcanoes within Shatsky Rise. As with most Expedition 324 sites, the operational goal for Site U1346 was to drill through the sediment overburden, core the oldest sediment overlying igneous basement, and core as deeply into the igneous formation as possible with a single rotary core barrel (RCB) bit. Precruise plans anticipated coring ~100 m into the igneous basement.

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 U1346 was to core enough igneous rock of suitable freshness and composition to allow a reliable radiometric date to be established. Igneous rocks are also critical to geochemical and isotopic studies whose goals are to establish the elemental compositions of the rocks, variations in compositions, and the isotopic characteristics. Such data are crucial to determine the source of magma, to infer its temperature and depth of melting and crystallization, and to deduce the degree of partial melt, as well as to track its evolution with time. Operationally, this meant that at Site U1346 the goal was to core a representative suite of igneous units that were also only lightly altered so as to be suitable for 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 and logs to infer the eruption style, igneous products, and physical structure of Shatsky Rise. Given its size and inferred eruption rate, Shatsky Rise is likely to be an example of an unusual volcanic construct for which development is poorly understood. Shirshov Massif, with its intermediate size, is important for understanding the development of large igneous province (LIP) volcanoes that are bigger than normal seamounts but smaller than large plateaus. 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 trends will indicate the paleodepths of sediment deposition, information that is important to 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. Shirshov Massif samples are important in this effort because they were likely formed farthest from the paleoequator (i.e., higher paleoinclination) and thus are less likely to be confounded by uncertainty about whether the massif formed north or south of the Equator. 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.

1Expedition 324 Scientists, 2010. Site U1346. 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.324.103.2010

2Expedition 324 Scientists' addresses.

Publication: 3 November 2010
MS 324-103