CENTRIFUGE MODELLING STUDY OF CONTRASTING STRUCTURAL STYLES IN THE SALT RANGE AND THE POTWAR PLATEAU, PAKISTAN

Abstract

The ENE-trending Himalayan fold-thrust belt in Pakistan exhibits contrasting deformation styles both along and across the strike. The centrifuge modelling technique has been used to investigate these variations in structural style. For the purpose of modelling, the Salt Range and Potwar Plateau (SR/PP) stratigraphy has been grouped into four mechanical units. From bottom to top these are the Salt Range Formation, carapace unit (Cambrian-Eocene platform sequences), Rawalpindi Group, and Siwalik Group. These stratigraphic units of alternating competence, composed of thin layers of plasticine modelling clay and silicone putty, rest on a rigid base plate that represents the crystalline basement of the Indian plate. The models are built at a linear scale ratio of ~10-6 (1mm=1km) and deformed in a centrifuge at 4000g. The models are subjected to horizontal shortening by collapse and lateral spreading of a “hinterland wedge” which simulates overriding by the Himalayan orogen (above the Main Boundary Thrust). The models of the central SR/PP show that the accretionary wedge develops a prominent culmination structure with fault-bend fold geometry over the frontal ramp, while the eastern SR/PP is more internally deformed by detachment folds, fault-propagation folds and pop-up and pop-down structures. Model results show that the transition from fault-bend fold to detachment-fold and fault-propagation-fold geometry in the prototype may take place in a transfer zone marked by an S-bend structure (Chambal Ridge and Jogi Tilla) at the surface and the lateral ramp in the subsurface. Moreover, the models suggest that an oblique ramp below the Kalabagh strike-slip connecting the two frontal ramps below the Surghar Range and the central Salt Range developed similar structure that can be observed in the prototype. The model results also show that the Northern Potwar Deformed Zone may have been developed over ductile substrata due to the close similarity between the models and the prototype structures. The deformation style in the models illustrates the importance of mechanical stratigraphic and basement ramp systems in the evolution and the structural styles of the SR/PP.