Thin-skinned tectonics on continent/ocean transitional crust, Sulaiman Range, Pakistan Public Deposited

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  • Surface and subsurface data from the Sulaiman thrust belt show that nearly all the 10 km thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to a minimum of 20 km in the hinterland of the Sulaiman fold belt without significant thrust faults at the surface. The balanced structural cross-:section suggests that the tectonic uplift in the Sulaiman fold belt is a result of thin-skinned, passive-roof duplex style of deformation. The duplex sequence of Jurassic and older rocks is separated from the roof sequence by a passive-back thrust in thick Cretaceous shales. The passive-roof sequence remains intact for about 150 km and becomes emergent along a passive-back thrust in the hinterland. The structures are expressed at the surface by fault-related folds in the foreland and out-of-sequence structures (secondary faults and related pop-ups) in the interior. The duplex structure varies from fault-bend folds to anticlinal stacks, and hinterland dipping duplexes. Progressive deformation reveals a series of structural and geometrical features including: (1) broad concentric folding at the fault tip; (2) development of a passive-roof and duplex sequence; (2) forward propagation of the duplex as critical taper is achieved; (4) tear faults and extensional normal faults within the overthrust wedge; and (5) out of sequence (secondary) thrusting. The 349 km long balanced cross-section from the Sulaiman fold belt restores to an original length of 727 km that provides 378 km of shortening in the cover strata of the Indian subcontinent. Minimum estimate of shortening is 328 km. Modelling of the Bouguer gravity profile from the Sulaiman foredeep across the Indian/ Afghan collision zone suggests the depth to the Moho at the Sulaiman deformation front is about 36 km. Depth to Moho increases northward with a gentle gradient of 1.1° (20 m/km) for 280 km to the hinterland where the depth to the Moho is about 42 km. About 150 km north across the Khojak flysch the Moho gradient steepens abruptly to about 7.8° (136 m/km) to attain an average depth of about 57 km in eastern Afghanistan. This suggests that the Sulaiman fold belt is underlain by transitional crust associated with the western passive margin of the Indian subcontinent.
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