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doi:10.2204/iodp.proc.322.104.2010

Site C0012¹

Expedition 322 Scientists²

Background and objectives

Integrated Ocean Drilling Program (IODP) Site C0012 (proposed Site NT1-01) was the contingency site for IODP Expedition 322 (Saito et al., 2009). The site is located in the Shikoku Basin on the crest of a prominent basement high (Kashinosaki Knoll) that was constructed on the subducting Philippine Sea plate (Fig. F1). Because of premature destruction of the drill bit in IODP Hole C0011B, we failed to reach the total depth (TD) target, and the decision was made shortly thereafter to collect a full suite of cores and wireline logs at the contingency site. The primary purpose of drilling at this new location was to recover the entire succession of sedimentary strata and uppermost igneous basement, thereby characterizing the subduction inputs to the Nankai Trough. Analysis of seismic reflection data just prior to drilling indicated a depth to basement of ~515 meters below seafloor (mbsf), which is considerably less than the value used in the Scientific Prospectus (600 mbsf). The adjustment to TD at Site C0012 was made after refining the acoustic velocity model following successful acquisition of logging-while-drilling data in IODP Hole C0011A during the final days of IODP Expedition 319.

By recovering a complete suite of cores and wireline logs from Site C0012, the Expedition 322 scientists expected to demonstrate conclusively how depositional history varied between the flank and the crest of the subducting basement high. This spatial comparison between two reference sites (expanded section versus condensed section) was designed to help provide answers to the following questions:

1. How does the physical hydrogeology of the Shikoku Basin respond to variations in primary lithologic architecture and basement structure?

2. How do fluids in the igneous basement affect subduction processes?

3. How have system-wide patterns of sediment dispersal affected composition within the Shikoku Basin, particularly on the northeast side of the fossil spreading ridge (Kinan seamount chain)?

4. Which factor(s) inherited from the Shikoku Basin control(s) the décollement's position near the toe of the Nankai accretionary prism, as well as the location of ramps and flats and mechanical behavior throughout?

5. Does the plate boundary fault, near its updip limit of seismicity, shift its position from a sediment/sediment interface (stable sliding) to the sediment/basalt interface (stick-slip)? If so, what are the causes?

Answers to these questions will require data from multiple drilling sites and expeditions and will contribute to the success of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) in many important ways. The importance of the lower stratigraphic units with the Shikoku Basin cannot be overstated; those intervals yield information about initial conditions within presubduction equivalents of the seismogenic zone.

¹Expedition 322 Scientists, 2010. Site C0012. In Saito, S., Underwood, M.B., Kubo, Y., and the Expedition 322 Scientists, Proc. IODP, 322: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/iodp.proc.322.104.2010

²Expedition 322 Scientists' addresses.

Publication: 10 October 2010
MS 322-104

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