Mari-Bugti pop-up zone in the central Sulaiman fold belt, Pakistan

Abstract

The Sulaiman fold-and-thrust belt is an active tectonic feature of the Himalayan mountain system in Pakistan. Seismic reflection profiles, borehole, surface geology data and Bouguer gravity modeling suggested a ‘passive-roof duplex’ geometry over a transitional crust related to the former passive margin of the Indian subcontinent. In the frontal part of the Sulaiman fold belt, a passive-roof sequence of Cretaceous and younger rocks is structurally uplifted. At the surface, the roof sequence displays a coherent stratigraphy over the underlying duplex sequence of Jurassic and older strata. The folds in the roof sequence reflect blind faults in the duplex sequence. The duplex style of deformation persists throughout the central Sulaiman fold belt. However, unlike the frontal Sulaiman fold belt, stratigraphy at the surface in the central Sulaiman is disrupted by E-W- and NE-trending faults, with apparent map lengths of tens of kilometers. These foreland- and hinterland-verging high-angle faults juxtapose Cretaceous rocks in the cores of tight, symmetrical anticlines against Eocene Ghazij Shale and Kirthar Limestone. According to seismic reflection data, they have only minor vertical offsets of 1–2 km and are mostly restricted to the roof sequence. As a result Cretaceous rocks bounded between reverse faults are exposed at the surface in the cores of tight anticlines as pop-up structures. This implies that: (1) the exposed faults in the central Sulaiman fold belt are not primary structures with major shortening; and (2) recognition of these faults in the roof sequence may reflect an early stage of development of overstep back thrusts from the upper décollement (passive-roof thrust).