EVIDENCE FOR A WEAK AND OVERPRESSURED DECOLLEMENT BENEATH SEDIMENT-DOMINATED ACCRETIONARY PRISMS

Presents structural data from several underthrust sequences in southwest Alaska that suggest that the deformation regime beneath accretionary prisms is characterized by a subvertical σ1 and regionally extensive layer-parallel shearing. These results contrast markedly with the results obtained from the more shallow parts of accretionary prisms and fold and thrust belts where σ1 plunges gently seaward or toward the deformation front. The results emphasize the importance of dewatering and fluid flow in controlling the position of the decollement, which in turn determines how the accretionary prism grows. In essence, it is the development of a weak, overpressured decollement within a thick sediment pile that allows plate convergence to be partitioned into offscraping and underthrusting regimes. -from Authors