Structure of the Makran subduction zone from wide-angle and reflection seismic data

Makran is one of the largest accretionary wedges on the globe, formed by the convergence between the Eurasian and the Arabian Plates. It is characterised by an extremely high sediment input of 7 km and a shallow subduction angle. We present seismic velocity models from four wide-angle seismic lines that image the wedge sediments and the subducted oceanic crust. Three strike-lines show complex but rather one-dimensional structures, where the observed seismic phases have been studied with the reflectivity method. A 160 km long dip line was surveyed coincident with a MCS line collected by Cambridge University in 1986. Prestack depth migration of this data with the new wide-angle velocities shows improved images compared to earlier results. Interpretation of the wide-angle and the MCS data indicates that a decollement is developed along a bright reflector within the turbiditic sequence. More than 3 km of sediment bypasses the first accretionary ridges (underthrusting) and is transported to greater depth. This might help to explain the sparse earthquake activity associated with subduction here. In the strike lines, low-velocity layers occur landward of the deformation front, in zones where active thrusting or underplating takes place. The oceanic crust is characterised by a strong velocity contrast between layer II and layer III. (C) 2000 Elsevier Science B.V. All rights reserved.