Abstract:
From late 2007 to early 2008 the first stage of the IODP Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) began drilling and coring along the Nankai Trough convergent margin. The NanTroSEIZE project is a multi-stage IODP drilling program focused on understanding the mechanics of seismogenesis, rupture propagation, and tsunamigenesis through direct sampling, in situ measurements, and long-term monitoring. Stage 1A included three coordinated riserless drilling expeditions to visit several sites across the continental slope and rise offshore the Kii Peninsula, SW Japan. Whereas the primary goals were to investigate the shallow portion of the plate boundary and splay fault, these drilling expeditions also provide valuable information to study submarine mass movements in an active continental setting. First results reveal several lines of evidence for slope instability observed in five drill sites, both in the upper (site C0001), intermediate (site C0004) and lower slope region (sites C0006, -7), as well as within an intermediate slope basin seaward of a topographic rise that originates by fault movement and uplift along a prominent splay fault (site C0008). At all these drill sites, slumping, mass-flow deposits, syn-sedimentary breccias and mud-clast conglomerates occur at different stratigraphic levels. They record information about upper Pliocene-to-Recent sediment remobilization by past slope failures, which may be related to splay fault slip, tectonic uplift and oversteepening, or earthquake tremor. Most prominently, C0008C targeted coring a normal fault identified in 3D seismic site survey data along which surface material appears to have slid downward. A 5 cm-thick, gently dipping shear zone within an interval characterized by high concentration of small core-scale normal faults was observed at nearly equal 40 mbsf. Shipboard initial data reveal lateral fluid flow from areas with thicker to thinner overburden along sand layers that are truncated by this slope collapse structure. This suggests that apart from seismicity and tectonic over-steepening also pore pressure may be an important mechanism for reducing slope stability in the study area. Also, sigmoidal fissures in the slope apron (e.g site C0001) sediments attest hydrofracturing, possibly coinciding with coseismic slip along an underlying fault. All these lines of evidence document that the slope along this active continental margin is potentially unstable and easily collapses when triggered by tectonic movement or earthquakes. Depending on size and location, such slope failures may have a considerable tsunamigenic potential. Post-cruise examination of sedimentary structures, petrography and hydrological and geotechnical properties will help to reconstruct mass wasting history and to assess the interaction between fault movements, uplift, slope steepening, fluid flow and seismic shaking as well as its role in slope failure initiation and tsunamigenesis.