FOREARC BASINS: TYPES, GEOMETRIES, AND RELATIONSHIPS TO SUBDUCTION ZONE DYNAMICS

Forearc basins are one of the fundamental types of sedimentary basin that form along convergent plate boundaries. Previous studies have proposed descriptive classification schemes for such basins, and performed analog experiments and numerical simulations in order to examine the basin formation and evolution; however, the great variability of forearc basins means that a comprehensive understanding of these features remains elusive. In this study, 37 modern forearc basins were analyzed in terms of geometry, style of deposition, and relationship to the outer wedge (accretionary or non-accretionary wedge) at the subduction zone, in order to reveal the geometrical characteristics of forearc basins and to discuss the factors controlling basin evolution. On this basis, modern forearc basins worldwide were classified into accretionary (AC) and non-AC types; the former include compressional and extensional sub-types, whereas the latter are all extensional. Each basin type has a typical geometry and characteristic relationships with the geometry of the outer wedge. Compressional AC type basins, which are characterized by landward-tilting strata and the landward migration of depocenters, show a constant width/thickness ratio (avg. 20) indicating self-similar growth of the basin. Non-AC type basins, which commonly lack trench-fill sediments and show seaward-tilting of the basin floor, are characterized by a high width/thickness ratio (17--130). Some basins with high obliquity of subduction angle have similar width/thickness ratios to AC type basins, but their widths are smaller than those of typical AC type basins for a given size of outer wedge. The geometry of forearc basins is closely related to the size, shape, and accretionary/erosive style of the outer wedge, which are considered to be dominated mainly by sediment flux at the trench and at the plate contact. It suggests sediment flux is an important factor controlling the evolution of forearc basins and can result in changes of basin type and style during their development.