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doi:10.2204/iodp.proc.333.101.2012

NanTroSLIDE Ancillary Project Letter

Scientific objectives

Expedition 333 drilled and sampled the slope basin seaward of the megasplay that is characterized in 3-D seismic data by stacked mass transport deposits (MTDs) (Strasser et al., 2011) (Fig. F3). The aim of the coring was to establish the submarine landslide history and reconstruct transport dynamics. Core from IODP Site C0018 (proposed Site NTS-1A) was to be integrated with 3-D seismic interpretation and data from nearby NanTroSEIZE sites to determine the relation of submarine landslides to tectonic evolution. By establishing a better physical understanding of tectonic processes and slope failures, we planned to also gain a general understanding of failure-related sedimentation patterns and the significance of episodic mass transport events. Ultimately, this could help us assess the tsunamigenic potential of tectonic landslides. The primary goals of drilling Site C0018 were

1. To establish a well-dated Quaternary mass-movement event stratigraphy and

2. To sample the distal part of an exceptionally thick MTD for analyzing its rheological behavior to constrain sliding dynamics and tsunamigenic potential.

These aim at providing answers to the following questions:

1. What is the frequency of submarine landslides?

2. What is the source materials of the MTDs?

3. What is the importance of accretionary wedge remobilization versus surficial processes?

4. What controls type, size, and magnitude of turbitides and MTDs and how do they change through time?

5. How do large MTDs relate to the timing of splay fault activity as inferred from NanTroSEIZE Stage 1 drilling (Strasser et al., 2009)?

6. What are the dynamics of large submarine landslides and can we infer their tsunamigenic potential?

By addressing these questions, we aimed to isolate tectonic processes influencing magnitude and occurrence of submarine landslides along active subduction zone margins and to understand their potential for triggering catastrophic events in terms of both hazards (tsunamigenic landslides) and sediment mass transfer within the context of margin evolution.

Drilling strategy

Site C0018 (water depth = 3100 m) is located on a margin-perpendicular transect 4.5 km southwest of the NanTroSEIZE Stage 1 drilling transect (Fig. F1). It is located 5 km south-southwest of Site C0008, which was drilled into a small slope basin seaward of the megasplay fault (Kinoshita, Tobin, Ashi, Kimura, Lallemant, Screaton, Curewitz, Masago, Moe, and the Expedition 314/315/316 Scientists, 2009). Site C0008 results show the utility of using the ages of MTDs to reconstruct slope failure activity related to megasplay fault movements (Strasser et al., 2009, 2011). Apart from the deepest section, Site C0008 lacks clear evidence for MTDs because of a significant hiatus in its upper part, suggesting erosion related to a prominent slope collapse structure seaward of the megasplay fault (Strasser et al., 2011; Conin et al., 2011). On the basis of new 3-D seismic data interpretation, the Ancillary Project Letter proponents have identified a lower slope basin that (1) better represents the depocenter for downslope mass transport, (2) is clearly characterized by stacked MTDs as seismically imaged by acoustically transparent to chaotic bodies with ponded geometries (Fig. F3), and (3) includes a large, as thick as 182 m, MTD (Strasser et al., 2011). Expedition 333 drilled at a location where the MTD bodies wedge out and where basal erosion is minimal. Continuous coring with the hydraulic piston coring system (HPCS) and the extended shoe coring system (ESCS)/extended punch coring system (EPCS) to ~315 mbsf allowed sampling of the MTDs across the most complete and longest stratigraphic succession.

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