LIQUEFACTION-INDUCED MASS TRANSPORT DEPOSIT: STRUCTURAL CHARACTERISTICS AND THEIR SEQUENCE OF INTERMITTENT EVENTS (Invited Presentation)

Liquefaction of sandy material is quite a common geologic event in plate subduction margins. Once liquefaction occurred, fluid pressure abruptly increase because the architecture of coarse sediments destroyed and pore fluid has to support the overburden pressure. The liquefied sandy sediments lost the shear strength associated with collapse of the architecture, thus liquefaction is major trigger to make submarine mass transport deposits (MTDs).

In this presentation, we are going to present geometry, inner fabric, and their lateral distribution of liquefaction-induced MTDs developed in the late Miocene accretionary prism and the Plio–Pleistocene trench–slope basin sediments in the Miura and Boso Peninsula, central Japan. The MTDs in this area are originally composed of sand-pebble matrices and the large-sized clasts consisting mudstone or alternating bed of sand and mudstone. The sandy matrix presents no preferred orientation or foliation and commonly intrudes into the large blocks. These occurrences are major difference with the rip-up-clasts in sandy turbidite deposits, and are indicative of that the liquefied mass, sand/pebble grains with high fluid pressure, behaved like Newtonian fluid. Based on the lateral tracing of a key tephra bed, we identified an evidence of lateral age variation of the MTDs, i.e. younging to the west in the slope-basin deposits. The MTDs may be a result of lateral migration of intermittent submarine slope failures.

A similar systematic pattern can also be identified in sandbox experiments, that slope failures occurred more or less continuously at the same or adjacent locations and sequence of slope failures migrated to the lateral-ward. The recognition of this failure pattern adds to our understanding of the mechanisms of submarine slope failures and will assist in the prediction of hazardous failures, particularly regarding the occurrence of series of intermittent failures on submarine slopes.