OCEAN PLATE STRATIGRAPHY (OPS): THE KEY CONCEPT FOR UNDERSTANDING ACCRETIONARY (MIYASHIRO-TYPE) OROGENS

Subduction processes usually destroy ocean floors, whereas smaller pieces of ancient ocean floors are occasionally preserved within accretionary complexes (ACs) along active continental margins. ACs are mostly composed of coarse-grained terrigenous clastics with minor oceanic material, such as MORB greenstones, deep-sea chert, seamount-derived OIB/atoll carbonates etc. Owing to the tectonic accretion under horizontal compression regime, ACs are characterized by structurally repeated, out-of-sequence sedimentary rocks, sometimes with chaotically-mixed features (mélange/olistostrome). Ocean Plate Stratigraphy (OPS) represents a pre-accretion primary stratigraphy of AC components that consists of MORB, deep-sea pelagic-hemipeagic sediments, and trench-fill turbidites, in ascending order. OPS records a travel log of an oceanic plate from its birth at MOR to its demise at trench, and an age of subducted plate that formed an ancient AC. In other words, an OPS provides an identity code for each ancient AC. OPS analysis is critical in reconstructing tectonic history of ancient subduction-related (Miyashiro-type) orogens because it is the only information source for ancient ridge-trench systems that disappeared. As demonstrated in SW Japan, OPS analysis can clarify tectonic history and internal structures of accretionary orogenic belts; 1) onset time of suduction, 2) discrimination of AC belts, 3) subhorizontal nappe pile, and 4) episodic subduction of MOR. Additional fruit of OPS study is the first detection of the mid-oceanic deep-sea Permo-Triassic boundary that recorded the biggest mass extinction in the Phanerozoic. The basic concept of OPS came earlier with plate tectonics in the early 1970s; however, the newly developed high-resolution microfossil (conodont, radiolaria) stratigraphy enabled the practical reconstruction of OPS for on-land exposed ACs in SW Japan for the first time in the world in the late 1970s (cf. Isozaki et al., 1990; Matsuda & Isozaki, 1991).