Abstract
Abstract. The Kirthar Fold Belt is part of the transpressive transfer zone in Pakistan linking the Makran accretionary wedge with the Himalaya orogeny. The region is deforming very obliquely, nearly parallel to the regional S–N plate motion vector, indicating strong strain partitioning. In the central Kirthar Fold Belt, folds trend roughly N–S and their structural control is poorly understood. In this study, we use newly acquired 2-D seismic data with pre-stack depth migration, published focal mechanisms, surface and subsurface geological data, and structural modelling with restoration and balancing to constrain the structural architecture and kinematics of the Kirthar Fold Belt. The central Kirthar Fold Belt is controlled by Pliocene to recent linked thick-skinned to thin-skinned deformation. The thick-skinned faults are most likely partially inverting rift-related normal faults. Focal mechanisms indicate dip-slip faulting on roughly N–S-trending faults with some dip angles exceeding 40∘, which are considered too steep for newly initiated thrust faults. The hinterland of the study area is primarily dominated by strike-slip faulting. The inverting faults do not break straight through the thick sedimentary column of the post-rift and flexural foreland; rather, the inversion movements link with a series of detachment horizons in the sedimentary cover. Large-scale folding and layer-parallel shortening has been observed in the northern study area. In the southern study area progressive imbrication of the former footwall of the normal fault is inferred. Due to the presence of a thick incompetent upper unit (Eocene Ghazij shales) these imbricates develop as passive roof duplexes. In both sectors the youngest footwall shortcut links with a major detachment and the deformation propagates to the deformation front, forming a large fault-propagation fold. Shortening within the studied sections is calculated to be 18 %–20 %. The central Kirthar Fold Belt is a genuine example of a hybrid thick- and thin-skinned system in which the paleogeography controls the deformation. The locations and sizes of the former rift faults control the location and orientation of the major folds. The complex tectonostratigraphy (rift, post-rift, flexural foreland) and strong E–W gradients define the mechanical stratigraphy, which in turn controls the complex thin-skinned deformation.
Highlights
The external regions of fold–thrust belts are typically interpreted using templates from classical thin-skinned thrustrelated deformation
Our model of inversion with linked thin-skinned deformation for the central Kirthar Fold Belt is in line with the observations and the model proposed by Smewing et al (2002a) and Fowler et al (2004) for the southern Kirthar Fold Belt
It is important to briefly discuss some potential reasons why the central Kirthar Fold Belt is dominated by inversion with thin-skinned deformation instead of following a classical thin-skinned fold–thrust belt model
Summary
The external regions of fold–thrust belts are typically interpreted using templates from classical thin-skinned thrustrelated deformation. As an outcome of structural investigations for hydrocarbon exploration we are able to report a well-constrained example of linked thin-skinned to thick-skinned deformation at the deformation front of the central Kirthar Fold Belt in Pakistan. This example illustrates the kinematic linkage between inverting deep-reaching faults and the associated thin-. R. Hinsch et al.: Inversion in the central Kirthar Fold Belt plate structural models to interpret in areas of poor seismic image resolution. Based on the resulting balanced cross sections through the area, we conclude that the central Kirthar Fold Belt is driven by thick-skinned inversion linked with thinskinned deformation further toward the foreland. Constraining the deformation style is fundamentally important for the exploration of resources, as seismic interpreters usually use tem-
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
