Forearc basins: Types, geometries, and relationships to subduction zone dynamics

Abstract

Forearc basins are one of the fundamental sedimentary basins, and they preserve various geological phenomena arising from dynamic interactions between subducting and overriding plates along convergent plate boundaries. Previous studies have proposed descriptive classification schemes for such basins and performed analog experiments and numerical simulations examining basin formation and evolution; however, the great variability of forearc basins means that a comprehensive understanding of these features remains elusive. This paper proposes a new classification scheme for forearc basins from the viewpoints of material transfer between the two plates (accretionary or nonaccretionary) and the long-term strain field in the basin (compressional or extensional) by using characteristics of 41 modern forearc basins. Geometric characteristics were also analyzed in order to reveal the relationship to subduction zone dynamics, including material transfer at the plate boundary (accretion/erosion), plate convergence configurations and kinematics, and the mechanics of strain and its partitioning. Compressional accretionary-type basins, which are characterized by landward-tilting strata and the landward migration of depocenters, show a constant width/thickness, ratio indicating self-similar growth of the basin as the outer wedge builds up. Extensional accretionary-type basins develop normal faults in the basin, probably due to a gravitational instability of the outer wedge. An increase of sediment flux at the trench may grow the accretionary prism and thicken the subduction channel faster, leading to a reduction of basal and internal shear strengths of the outer wedge. Subsidence caused by crustal thinning characterizes the nonaccretionary-type basins, which have thin trench-fill sediments, large forearc slopes, steady widths of the outer wedge, and variable width/thickness ratios of the basins. From these results, the sediment flux at the trench and the plate boundary can be considered as a dominant factor that determines accretion or erosion of the outer wedge and uplift or subsidence of the outer-arc high, and that is also responsible for the style of deposition in the forearc basin. An increase or decrease of the sediment flux may change the type, geometry, and deformation manner of the forearc basin. The results of this study provide important clues to further our understanding of dynamic interactions between forearc basins and subduction zone processes.