Sediment dewatering in the Makran accretionary prism

Abstract

Sediments within the Makran accretionary prism are considerably more compacted than the material entering the subduction zone due to the additional tectonic stresses resulting from plate convergence and accretionary processes. Tectonic compaction of the offscraped sediments has been documented by deriving in-situ porosities in the upper 4 km from their seismic velocities. Wide-angle seismic reflection-refraction profiles were analysed from a transect crossing both the Makran accretionary prism and the undeformed abyssal plain sediments to the south. The 6–7 km thick abyssal plain sediments exhibit normal compaction with depth. The sediments incorporated into the accretionary prism have higher seismic velocities at all depths, corresponding to reductions in porosity of 30–40%. Mapping the porosity-depth gradient across the prism shows that there is a marked change in porosity at the seaward toe of the prism, the frontal fold. There is then little further change until further folding begins to occur 70 km landward of the toe. The dewatering and consolidation of the sediments is an important control on deformation occurring within the sedimentary prism.