Saad Saud Burhan-Ul Haq

Oblique convergence and the lobate mountain belts of western Pakistan

The thin-skinned structures of the Pakistani convergent margin have formed as a consequence of the relative motion between India and Eurasia. Most of the resultant motion is being accommodated along or near the current edge of the Eurasian plate: the southwest-northeast striking Chaman fault zone. It has been observed at oblique margins that the total plate motion is resolved into a component parallel to the margin, accommodated through strike-slip faulting, and a component normal to the margin taken up as contraction. However, the orientations of structures along the Pakistani convergent margin in and around the Sulaiman lobe and Sulaiman Range cannot be explained simply by resolving the plate motion vector into components normal and parallel to the plate boundary. Our modeling suggests that the complex juxtaposition of strike-slip faults with thrust faults of various orientations can be explained by the presence of a block centered upon the Katawaz basin that translates along the southwest-northeast structural barrier of the Chaman fault zone, moving with respect to both Eurasia and India. As this relatively rigid block moves northeastward relative to Asia, it causes deformation of the sedimentary cover and is responsible for much of the structural complexity in the Pakistani foreland. Our simple model explains several first-order features of this oblique margin, such as the eastward-facing Sulaiman Range, the strike-slip Kingri fault (located between the Sulaiman lobe and Sulaiman Range), and the reentrant at Sibi. This leads us to conclude that very complex structural and geometric relationships at oblique convergent plate boundaries can result from the accommodation of strain with simple initial geometric constraints.

Extension during active collision in thin-skinned wedges: Insights from laboratory experiments

The occurrence of strike-normal extension in mountain belts undergoing active contraction is not predicted by analytical solutions for the mechanics of purely frictional orogens. To test the idea that ductile rocks at depth might allow extensional deformation to occur in otherwise frictional and contractional orogens, we have conducted a series of analog experiments with purely frictional and layered frictional-ductile rheologies. By precisely measuring the horizontal deformation field, we have quantitatively confirmed the qualitative observation that analog frictional wedges do not extend. This is consistent with published numerical models, and is in contrast to purely viscous models that commonly display coeval extension with contraction. Our modeling also demonstrates that layered frictional-ductile models can, during active convergence, result in discrete extensional normal faulting even when only a relatively thin ductile layer is present. The resulting extension, though relatively small in magnitude compared to the active contraction, can greatly modify the final profile of a convergent orogen and can lead to the formation of a broad plateau between the pro-wedge and the retro-wedge.