Scientific drilling of mass- Transport deposits in the nankai accretionary wedge: First results from iodp expedition 333

Abstract

Integrated Ocean Drilling Program (IODP) Expedition 333 drilled and cored a Pleistocene- To-Holocene succession of a slope-basin in the Nankai accretionary wedge, offshore southwest Japan. The slope-basin represents the depocentre for downslope sediment transport and comprises several mass- Transport deposits (MTDs), including an up to 180 m thick MTD. The recovered section (0-315 m below seafloor) records ∼ 1 Million years of submarine landsliding history in this active tectonic setting. We present D/V Chikyu shipboard results from Site C0018, including litho- bio- magneto- And tephra-stratigraphy and physical properties data. Six MTDs of thickness ranging from 0.5 to 61 m were identified from visual core description and X-ray CT-scans. The thickest MTD is also the oldest (emplaced between 0.85 and 1.05 Ma) and it coincides with a lithological transition between a sandy turbidite sequence below, and ash-bearing hemipelagites comprising several MTDs above. The deformation style within the MTD is heterogeneous: intervals of disturbed sediments are interbedded within intervals inferred to retain original, coherent bedding. In three occurrences the base of the MTD is defined by a shear zone within fine-grained sediments. These observations suggest that slumping and mass- Transport is dominated by localized plastic deformation. Furthermore, mass movements are an active process that is shaping the present-day seafloor, as evidenced by numerous surficial slump scars and the occurrence of a shallowly buried, presumably Holocene MTD at Site C0018. © Springer Science+Business Media B.V. 2012.