Morphotectonics and mechanics of the central Makran accretionary wedge off Pakistan

Swath bathymetry images and 3.5 kHz data acquired during RV Sonne cruise 123 in autumn 1997 are the base for analysing the morphology and deformation styles of the huge Makran accretionary wedge. The continental slope off Pakistan is characterised by three morphological provinces: (a) The lower slope is built of a sequence of imbricate thrust slices which form long and narrow accretionary ridges with Ranks locally as steep as 20 degrees and between 10 degrees and 20 degrees on average, while the regional slope is only 1-2 degrees, (b) the nearly flat mid-slope terrace is present in the central east part of the margin, narrows significantly towards the east, and is absent east of 63 degrees 45' and (c) the upper slope up to the shelf has a regionally uniform slope as steep as about 8 degrees and comprises a very rough topography with many gullies and canyons and evidence for mass wasting. Two meandering canyons crossing the entire margin down to the abyssal plain at about 63 degrees 15'-63 degrees 30', can be correlated with onshore rivers. They have been formed by episodic erosion through turbidity Rows and are characterised by erosional and depositional portions. Sinuosity of the canyons is exclusively caused by the accretionary ridges functioning as obstacles for flow to be directly downhill. Application of Coulomb rheology to the frontal part of the Makran Wedge implies that the mid-level decollement is intrinsically extraordinary weak with a great strength contrast to the overlying sediments. Comparison with Nankai, Cascadia, and the Western Mediterranean Ridge reveals new insights in the parameters controlling the shape of an accretionary wedge and the role of a mid-level decollement. (C) 2001 Elsevier Science Ltd All rights reserved.