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doi:10.2204/iodp.pr.316.2008

Scientific objectives

The first target of Expedition 316 was the shallow portion of the megasplay fault system, just seaward of the break in slope marking the boundary between the inner and outer accretionary wedge (Fig. F2). The scientific objectives of drilling the shallow portion of the megasplay fault system are

• To clarify the character and behavior of the shallow portion of the megasplay:

• Is the megasplay an active blind thrust or an inactive fault?

• Is there evidence for past seismogenic slip, supporting contention that the megasplay is the primary candidate for the source of great earthquakes and tsunamis?

• To clarify the slip and deformation mechanisms in the region above the (inferred) unstable seismogenic fault;

• To clarify the relationship between fluid behavior, slip, and deformation along the megathrust; and

• To clarify the evolutionary development of the splay fault.

Two sites were drilled to investigate the shallow region of the megasplay. Site C0004 was selected during Expedition 314 (LWD) as an alternate to Site C0003 (proposed Site NT2-01B), where the target could not be reached (Kinoshita et al., 2008). Site C0004 was projected to cross the splay fault at a depth of ~300 mbsf; total depth (TD) was 400 mbsf in order to sample the underthrust material. Site C0008 was originally proposed as a contingency site (proposed Site NT2-10). This site was selected as the last site of Expedition 316 because it represented the best balance between science objectives and time available (Fig. F3). Cores from Site C0008 will allow evaluation of the material eroded from thrust blocks following uplift to the surface via faulting on the megasplay system. Results will help to assess the timing and relative age of past fault motions via identification of provenance and age of deposited material and the age across any disconformities between newly deposited sediments and older uplifted fault blocks. Numerous postcruise studies of fault and wall rock frictional properties will test ideas about fault zone strength and sliding stability. The permeability and geotechnical properties of this material will also help interpretation of the fluid flow and consolidation response to splay fault movement as the slope basin sediments are progressively underthrust.

The second target of Expedition 316 was the main frontal thrust at the seaward edge of the accretionary wedge (Fig. F4). Based on seismic data and submersible studies, this thrust is thought to have placed moderately consolidated ~2 Ma clastic rocks over weak and unlithified late Quaternary trench section clastic sediments (Ashi et al., 2002). Two sites were drilled in this area to achieve the following objectives:

• Clarify the function of the frontal thrust with respect to large earthquakes:

• Do great earthquakes trigger slip along this fault plane, and if so, are they tsunamigenic?

• Does the frontal thrust generate low-frequency events or does it creep during the interseismic period?

• Evaluate the relationship between fluid behavior and slip and deformation.

• Assess the evolution of the frontal thrust from its birth to death.

Core samples collected during Expedition 316 allow description of lithology and structure, provide age control through paleontological and paleomagnetic analysis, and provide samples for interstitial water, microbiology, and shipboard and postcruise physical property and geotechnical studies. In situ temperature measurements provide an important control for thermal models of the subduction zone. The shipboard and postcruise results, when integrated with other NanTroSEIZE expeditions, will characterize physical properties, strength, frictional properties and behavior, composition, and structure of the slope sediments, hanging wall, and footwall of the splay fault and frontal thrust. Development of the fault zones at shallow depths will also be compared to drilling of the splay fault at greater depths at Sites C0001 and C0002 in later NanTroSEIZE expeditions.

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