Mud volcanoes at the front of the Makran accretionary complex, Pakistan

Conical mounds, 1-1.5 km in diameter, and up to 65 m high were mapped at th e foot of the active Makran continental margin. The mounds developed seawar d of the accretionary front in a relatively planar zone where the beginning of build-up of tectonic pressure initiates deformation. Based on shallow h igh-resolution 4 kHz sediment echosounding, the sedimentary sequence in thi s area is generally well stratified, as indicated by closely spaced horizon tal reflections. However, in the vicinity of the mounds the sediment is cha racterised by many acoustically transparent zones, which are 100-300 m in d iameter and cut near-vertically through the horizontal reflectors. Two sediment cores from the top of the largest cone and a neighbouring acou stically transparent zone reveal small-scale post-depositional deformation in a stratified sequence and methane concentrations up to 40,000 ng/g. This deformation and disruption of potential reflectors provides a clue to expl ain the acoustic transparency; we interpret it as caused by the rise of cha rged fluids and mud, leading initially to the (slight) disturbance of the g enerally good acoustic reflectors and eventually to the formation of conica l mud mounds (mud volcanoes). MCS data, showing a buried mound in an analog ous structural position, support the idea of tectonically induced mud/fluid expulsion seaward of the accretionary front. (c) 2001 Elsevier Science B.V . All rights reserved.