The Shikoku Basin lies in the northwestern Philippine Sea. It is one of the largest marginal basins in the world, and is a topographically and tectonically complex region dominated by subduction of the Pacific plate and the Indo-Australian plate. The Shikoku basin is an inactive back-arc basin with high heat flow and a history of rifting due to seafloor spreading. The main evidence for rifting comes from the analysis of linear magnetic anomaly patterns. The magnetic anomaly patterns of the Shikoku basin have asymmetrical characteristics in its east-west direction, which sustains Shih's Double-Limb Asymmetrical Spreading Model. The lithofacies of the Shikoku basin are typical of marginal or back-arc basins, consisting predominantly of hemipelagic and pyroclastic sediments with minor pelagic components. Thickness relationships and distribution of the principal facies in each of the five time-equivalent intervals (early Miocene through Pleistocene) at the DSDP sites suggest the time-variable source areas for coalescing clastic wedges within the basin and indicate the timing of volcanism in neighboring volcanic arcs. Clastic sediments came principally from northern and eastern sources during the middle Miocene, from northern and northeastern sources during the late Miocene, and from northern, northeastern and northwestern sources during the Pliocene and early Pleistocene.