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Makran of Iran and Pakistan |
Bullard Laboratories, Department of Earth Sciences, Cambridge University, Madingley Rise, Madingley Road, Cambridge CB3 0EZ, England
The offshore Makran of Pakistan and Iran in the Gulf of Oman forms the seaward margin of a folded and faulted accretionary sediment wedge which extends several hundred kilometres inland across the onshore Makran. Sediment is being scraped off the oceanic Arabian plate as it subducts shallowly northward beneath the Eurasian plate. The submerged front of the accretionary prism has been traced along the entire width of the Makran continental margin, from the Strait of Hormuz in the west to the boundary with the Indian plate in the east. The structure of a 70 by 90 km portion of the deformed sediment belt was mapped in detail by a grid of continuous seismic reflection profiles. This survey demonstrates that the folds are laterally continuous and are well lineated, that the deformed belt appears to be migrating southward at a rate of about 10 km every million years and that folding of the initially undisturbed abyssal plain sediments occurs in the southernmost, or ‘frontal’ fold. Mapping of the three-dimensional structure of the frontal fold shows that it systematically increases then decreases in height along strike over a distance of about 80 km. Folding is initially restricted to the uppermost 2
km of the sediment pile, and there is local evidence of small-scale shale diapirism. The frontal fold is then incorporated into the accretionary prism by uplift along a thrust fault which flattens out within the sedimentary section, possibly in overpressured shales. There is little subsequent deformation of the uplifted material or of the sediment which infills the basins between the fold ridges until some 70 km north of the frontal fold when further rapid uplift occurs, eventually elevating the sediment above sea-level. I attribute this second phase of uplift to major imbricate thrust faulting extending down to the oceanic basement; most of the shortening and thickening of the sediment wedge occurs in this region. The relatively simple pattern of uplifted folds and intervening basins in the frontal 70 km of the accretionary wedge results from a balance of compressive and normal stresses on the thick sediment section. This simple deformational style of the seaward part of the accretionary prism can be traced eastwards until it becomes chaotically disrupted by the consumption of a basement ridge which protrudes to just beneath the seafloor.
