Sulaiman Fold Belt Research Articles

Gravity and tectonic model across the Sulaiman fold belt and the Chaman fault zone in western Pakistan and eastern Afghanistan

Gravity data from the western boundary of the Indian subcontinent have been analysed to infer the gross crustal structure across the Indian/Afghan collision zone. Seismic reflection profiles reveal the gross structural (duplex) geometry of the Sulaiman fold belt. These data show that the wedge, which is 10 km thick at the deformation front, thickens northwestward to attain a tectonic thickness (thickness due to deformation) of about 20 km in the hinterland. Gravity modeling depicts the depth of the Moho between 33 and 35 km at the deformation front of the Sulaiman fold belt. The Moho depth shows an upward convexity along an E-W profile. It decreases northward with a gentle gradient of 1.1° (20 m/km) below the Sulaiman fold belt, and then deepens abruptly with a gradient of about 7.8° (136 m/km) across the Chaman fault zone, attaining a depth of about 57 km in eastern Afghanistan. The model suggests that the Sulaiman fold belt is underlain by transitional crust (15–27 km thick), in contrast to the continental crust (38 km thick) underneath the fold belt of the Himalayan collision zone in northern Pakistan. The about 57-km-thick crystalline crust in eastern Afghanistan may be due to: (1) underplating by crust of the Indian subcontinent; and (2) structural thickening within the Afghan block.

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, 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).

Structural interpretation of the Eastern Sulaiman Foldbelt and Foredeep, Pakistan

The Sulaiman foldbelt is an active tectonic feature on the northwestern margin of the Indian plate. Seismic reflection data have been combined with surface geological mapping and drill hole data to interpret the structural style and tectonic shortening of the eastern Sulaiman foldbelt and its adjacent foredeep. The data show that the basement is more than 8 km deep near the deformation front and that it deepens towards the west. The foredeep adjacent to the eastern Sulaiman foldbelt is a broad synclinal structure with a steep western limb; the more gentle eastern limb has monoclinal dips from near zero to 2.5°. Several salt‐cored anticlines are observed on the eastern part of the monocline, but the salt structures are lacking in the western foredeep and in the frontal part of the Sulaiman foldbelt. The basal decollement under the Sulaiman lobe lies either in ductile Eocambrian salt or within a deep zone of other material that undergoes ductile behavior. Near the deformation front, ramps from the basal detachment become flats in Lower Triassic and Lower Cretaceous shales. A steep and highly elevated zone immediately to the west of the frontal folds is interpreted to be underlain by a passive‐roof duplex. The culmination wall of this oblique duplex is separated from the overlying roof sequence by a passive‐roof thrust in Lower Cretaceous shales. The roof‐thrust has a backthrust sense of movement, relative to the forelandward propagating duplex, and may extend obliquely 100 km into the interior of the foldbelt. Fault‐propagation and fault‐bend folds are present in the frontal portion of the eastern Sulaiman foldbelt. Palinspastic restoration of a balanced cross section shows 108 km of shortening in the eastern Sulaiman foldbelt, east of the Kingri strike‐slip fault. The presence of more than 11 km of pre‐Neogene strata on top of basement suggests that the Mesozoic rifted margin of the Indian subcontinent may lie beneath the Sulaiman foldbelt. This interpretation is also favored by the presence of ophiolites and flysch deposits overthrusted on Late Cretaceous shelf strata.

Changes in paleocurrents during the development of an obliquely convergent plate boundary (Sulaiman fold-belt, southwestern Himalayas, west-central Pakistan)

Pre-orogenic Cretaceous shelf sandstones at the northern and southern ends of the Sulaiman Range in west-central Pakistan show basically westward dispersal, which is off the craton. However, Paleocene marine sandstones in the south spread east-southeast, and early Eocene deltaic sandstones in the north spread southeast and north. These are mostly onto the craton, and indicate slope reversal of the Cretaceous shelf. Post-Oligocene fluvial molasse shows mostly southward drainage. A 3852 m section of molasse in the northern Sulaiman Range comprises a lower 2378 m of claystones and upwardly fining sandstones that represent meandering rivers and an upper 1474 m that are dominated by conglomerates that represent gravelly braided rivers. Paleocurrents in the lower, meandering, part of the section were generally southward, but very variable, whereas those in the upper, braided, part were more uniform and show eastward flow. A 3146 m section of molasse in the southern Sulaiman Range comprises (1) a basal 1160 m sequence of claystones and sandstones (meandering streams with very diverse flow directions, overall to the southwest), (2) a middle 1603 m sequence dominated by pebbly sandstone with large bedforms (big sandy braided rivers with less diverse flow to the southwest), and (3) an upper 383 m conglomeratic sequence (gravelly braided streams that still flow uniformly southwest). Rivers in the early molasse paralleled the modern Sulaiman Range in the north and flowed obliquely into it in the south, which suggests no uplift of the modern front of the range until the onset of modern transverse (eastward) drainage and the associated influx of conglomerates. In present and admittedly uncertain lithostratigraphic terms, uplift of the modern front of the range apparently began about 700 m lower in the north than in the south. The northern and southern molasse sections otherwise show broadly similar histories of river development.