Systematic Development of Submarine Slope Failures at Subduction Margins: Fossil Record of Accretion-Related Slope Failure in the Miocene Hota Accretionary Complex, Central Japan

摘要

A comparative study of fossil sliding masses and the results of analog experiments indicates the systematic accumulation of submarine sliding masses at subduction margins. An analysis of sliding masses in the Middle Miocene Hota accretionary complex, Central Japan, combined with the results of analog experiments that simulated an accretionary wedge, revealed two types of slope failures at the toe of the accretionary wedge or at the head of an out-of-sequence thrust: relatively small but frequently developed failures on the lower parts of the slope (Type I failures), and relatively large but less frequent failures that affect the entire slope (Type II failures). The Type I slides are precursors of the larger Type II failures. These successive failure processes are recorded in the Hota accretionary complex as the following depositional sequence: three thin conglomerate layers containing small clasts (type I failure deposits) are overlain by a thick slope-failure deposit containing larger clasts (type II). In such an environment, carbonate-cemented and brecciated sandstones, which contain web structures, generally form at depth and behave as competent layers during failure. Clasts of such sandstones are indicative of large slope failures and are found only in type II failure deposits. Type II failures affect the sediment to a depth of ～2-10 m, whereas type I failures are surficial (<1 m depth), as inferred from the results of analog experiments and field observations. This systematic pattern of slope failure is important in terms of understanding the formation mechanism of submarine slope failures and in predicting such failures.