MARI-BUGTI POP-UP ZONE IN THE CENTRAL SULAIMAN FOLD BELT, PAKISTAN

The Sulaiman fold-and-thrust belt is an active tectonic feature of the Himalayan mountain system in Pakistan. Seismic reflection profiles, b orehole, surface geology data and Bouguer gravity modeling suggested a 'passive-roof duplex' geometry over a transitional crust related to t he former passive margin of the Indian subcontinent. In the frontal pa rt of the Sulaiman fold belt, a passive-roof sequence of Cretaceous an d younger rocks is structurally uplifted. At the surface, the roof seq uence displays a coherent stratigraphy over the underlying duplex sequ ence of Jurassic and older strata. The folds in the roof sequence refl ect blind faults in the duplex sequence. The duplex style of deformati on persists throughout the central Sulaiman fold belt. However, unlike the frontal Sulaiman fold belt, stratigraphy at the surface in the ce ntral Sulaiman is disrupted by E-W- and NE-trending faults, with appar ent map lengths of tens of kilometers. These foreland- and hinterland- verging high-angle faults juxtapose Cretaceous rocks in the cores of t ight, symmetrical anticlines against Eocene Ghazij Shale and Kirthar L imestone. According to seismic reflection data, they have only minor v ertical offsets of 1-2 km and are mostly restricted to the roof sequen ce. As a result Cretaceous rocks bounded between reverse faults are ex posed at the surface in the cores of tight anticlines as pop-up struct ures. This implies that: (1) the exposed faults in the central Sulaima n 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 decolleme nt (passive-roof thrust).