Horizontal shortening versus vertical loading in accretionary prisms

Abstract

The analysis of two‐phased deformation of subducted turbidites within the Shimanto Belt on Kyushu (Japan) shows strongly contrasted kinematics between an earlier, noncoxial “fold‐and‐thrust”‐type stage and a later, metamorphic, coaxial vertical shortening stage. The deep deformation is characterized by the development of a subhorizontal muscovite‐chlorite metamorphic foliation, associated with pure shear deformation, which overprinted the structures inherited from the earlier stage. We associate the metamorphic deformation stage with the lower levels of accretionary prism landward segment (inner wedge), which was therefore deformed by vertical loading and horizontal extension, much in contrast with prism trenchward segment (outer wedge), which often shows fold‐and‐thrust structures indicative of horizontal shortening. A possible explanation, based on Wang and Hu's (2006) model, for these longitudinal variations in deformation kinematics of the prism may lie in the contrasted rheology of its basal surface, strain weakening in the seismic portion below the inner wedge while strain strengthening in the aseismic portion below the outer wedge. The vertical shortening active within the inner wedge revealed by this study possibly represents a large contribution to the exhumation of the deeper levels of accretionary prisms.