Ishtiaq A. K. Jadoon

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

Abstract 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.

Balanced and retrodeformed geological cross-section from the frontal Sulaiman Lobe, Pakistan: Duplex development in thick strata along the western margin of the Indian Plate

A balanced structural cross-section has been constructed integrating seismic reflection profiles, drillhole, surface geology, and Landsat data across the technically active frontal Sulaiman fold belt in the western Himalayas. Restoration of the section provides information regarding the chronology of structures, structural style, sequence of thrusting, and the amount of shortening. General structural form evidenced by gentle topography and a broad fold belt is similar to that of other mountain belts underlain by a weak detachment. A sequence of about 10 km of dominantly platform (>7 km) and molasse strata thickens tectonically to about 15 km, 129 km north of the southwards verging deformation front. Nearly all of the 10 km thick stratigraphic sequence has been detached at the deformation front. Structural style is that of a foreland-verging duplex separated from the roof sequence by a passive-roof thrust in thick Cretaceous shale. This structure is expressed at the surface by fault-related folds. Toward the northerly hinterland, progressively older rocks are present at the surface in the hinge zones of the anticlines. They have been uplifted by duplexing several kilometres higher than their regional stratigraphic level. The passive-roof thrust has not been cross-cut by backthrusts, and it is present over a distance of 60 km along the line of section. Progressive deformation reveals a series of structural and geometric features including: (1) broad concentric folding at the fault tip; (2) development of a passive roof and duplex sequence by forward propagation of floor and roof thrusts; (3) forward propagation of the duplex as critical taper is achieved; and (4) tear faults and extensional normal faults within the overthrust wedge. A retrodeformed cross-section shows that about 76 km of orogenic contraction in the cover sequence has occurred across the frontal 129 km of the Sulaiman fold belt.

Seismic Data, Geometry, Evolution, and Shortening in the Active Sulaiman Fold-and-Thrust Belt of Pakistan, Southwest of the Himalayas

Surface and subsurface data from the Sulaiman fold-and-thrust belt are integrated to analyze the deep structure, tectonic shortening, and kinematics of the Sulaiman fold-and-thrust belt at the western margin of the Indian subcontinent. Seismic reflection data show that nearly all the 10-km-thick sequence of dominantly platform (>7 km) and molasse strata is detached at the deformation front. These strata thicken tectonically to about 20 km in the hinterland without significant thrust faults in the foreland. A balanced structural cross-section suggests that structural uplift in the Sulaiman fold-and-thrust belt is a result of a thin-skinned, passive-roof duplex style of deformation. The duplex sequence consists of Jurassic and older strata. It is separated from the roof equence by a passive-roof thrust in thick Cretaceous shales that has a backthrust sense of displacement over foreland-propagating duplexes. The roof sequence with incipient out-of- sequence thrusting is intact for about 150 km northwards, where it emerges along a passive backthrust. Sequential restoration of the balanced section reveals a series of structural and geometrical features including: (1) development of low-amplitude, broad concentric (detachment) folds (Sui and Loti) at the tip of the decollement; (2) increase in amplitude of a detachment fold to a critical level for development of ramp and duplex structures; and (3) out-of-sequence thrusting to create required critical taper for an outward translation of the foreland fold-and-thrust belt. A balanced structural cross-section 349 km long from the Sulaiman fold-and-thrust belt restores to an original length of 727 km, suggesting a maximum of 378 km of shortening since 21 Ma in the cover strata of the Indian subcontinent. Calculation of displacement rates over the Sulaiman fold-and-thrust belt (18 mm/yr) added to the resolved rate of the Chaman fault vector for the component parallel to the plate convergence direction (15 mm/yr) are close to the current India-Asia plate convergence rate (37 mm/yr).

Hinterland-Vergent Tectonic Wedge Below the Riwat Thrust, Himalayan Foreland, Pakistan: Implications for Hydrocarbon Exploration

The Riwat thrust, with a surface trace of over 50 km, is one of the major faults in the footwall of the main boundary thrust in the Himalayan foreland of Pakistan. Surface geology shows that the Riwat thrust is a foreland-vergent thrust along which lower to middle Siwalik molasse strata are thrust southward over upper Siwalik strata. Seismic reflection interpretation shows that the Riwat thrust developed as a roof thrust of a hinterland-vergent tectonic wedge (triangle zone) underlain by evaporites. The Riwat thrust propagates upsection from a depth of about 4 km at the base of the Siwalik Group. At this depth, it merges into a hinterland-vergent blind thrust that propagates upsection as a ramp from Eocambrian evaporites covering the basement at a depth of about 6 km. Bounded between this set of conjugate faults, a tectonic wedge of Eocambrian (evaporites) to Neogene strata is thrust toward the hinterland to form a triangle zone. The roof thrusts of triangle zones have been widely mapped as backthrusts in deformed mountain fronts. Hinterland motion of tectonic wedges as in the Riwat thrust triangle zone may be a feature of the fold-and-thrust belts underlain by evaporites acting as an extremely weak decollement layer. Their recognition, with a trap-forming geometry below a thrust, is important for interpreting particular fold belts and for hydrocarbon exploration. These structures could be predicted by the surface geology data where hinterland vergence of a fold below a thrust is apparent; however, seismic reflection data appear to be critical in recognizing these structures.

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 decollement (passive-roof thrust).

Study of natural radioactivity in Mansehra granite, Pakistan: environmental concerns

A part of Mansehra Granite was selected for the assessment of radiological hazards. The average activity concentrations of (226)Ra, (232)Th and (40)K were found to be 27.32, 50.07 and 953.10 Bq kg(-1), respectively. These values are in the median range when compared with the granites around the world. Radiological hazard indices and annual effective doses were estimated. All of these indices were found to be within the criterion limits except outdoor external dose (82.38 nGy h(-1)) and indoor external dose (156.04 nGy h(-1)), which are higher than the worlds average background levels of 51 and 55 nGy h(-1), respectively. These values correspond to an average annual effective dose of 0.867 mSv y(-1), which is less than the criterion limit of 1 mSv y(-1) (ICRP-103). Some localities in the Mansehra city have annual effective dose higher than the limit of 1 mSv y(-1). Overall, the Mansehra Granite does not pose any significant radiological health hazard in the outdoor or indoor.

Subsurface Fracture Analysis in Carbonate Reservoirs: Kohat/Potwar Plateau, North Pakistan

Carbonate reservoirs in Northern Pakistan are characterized by tight limestone. In these reservoirs, fractures are important for production and reservoir modeling. This paper addresses problems related to subsurface fracture analysis based mainly on image Logs.Natural fractures occur as systematic and unsystematic sets of definite and random orientation respectively. The subsurface analysis of fractures uses electrical and acoustic image logs to characterize fractures as either natural or induced features. They are classified as conductive or resistive features, representing possibly open or closed (mineralized) fractures, respectively. Using image logs, natural fractures are interpreted and classified descriptively to be continuous or discontinuous features representing systematic fractures or classified as chicken-wire (micro fractures) fractures representing unsystematic sets. Statistical analysis of fractures is used to classify them into geometrical and genetic sets as longitudinal (extensional), transverse (tensional), and oblique (shear) to the structure. Transverse fractures are known generally to exist as open. They develop parallel to the maximum horizontal in-situ stress and extend deep into the structure. Longitudinal fractures, those parallel to the fold axes, are observed to produce hydrocarbons in several fields in Northern Pakistan. Fracture density impacts production and reserves calculations. However, fracture density is strongly influenced by the lithology and layer thickness. Widely spaced fractures are observed in massive carbonate reservoirs, and closely spaced fractures of narrower aperture are observed in laminated strata. Thus, individual fractures in massive carbonates require to be identified for their impact on production. Fractures are observed to occur as discontinuous features of right- or left-stepping geometry and as en echelon features of significantly wider aperture in shear bands. These features together with vugs and leached features may provide zones of higher porosity, permeability, and storage capacity with isolated distribution in tight carbonates. Therefore, knowledge about fracture occurrence and distribution is important to predict sweet spots for drilling and field development.

Potentially toxic elements in soil of the Khyber Pakhtunkhwa province and Tribal areas, Pakistan: evaluation for human and ecological risk assessment

Potentially toxic elements (PTEs) contaminations in the soil ecosystem are considered as extremely hazardous due to toxicity, persistence and bioaccumulative nature. Therefore, this study was aimed to summarize the results of published PTEs in soil of Khyber Pakhtunkhwa and Tribal areas, Pakistan. Results were evaluated for the pollution quantification factors, including contamination factor (CF), pollution load index (PLI), ecological risk index (ERI) and human health risk assessment. The highest CF (797) and PLI (7.35) values were observed for Fe and ERI (857) values for Cd. Soil PTEs concentrations were used to calculate the human exposure for the risk assessment, including chronic or non-carcinogenic risks such as the hazard quotient (HQ) and carcinogenic or cancer risk (CR). The values of HQ were > 1 for the Cd, Co and Cr in Khyber Pakhtunkhwa and Tribal areas. Tribal areas showed higher values of ERI, HQ, and CR as compared to the Khyber Pakhtunkhwa that were attributed to the mining activities, weathering and erosion of mafic and ultramafic bedrocks hosting ophiolites. This study strongly recommends that best control measures need to be taken for soil PTEs with the intent to alleviate any continuing potential threat to the human health, property and environment, which otherwise could enter ecosystem and ultimately the living beings. Further studies are recommended to combat the soil PTEs concentrations and toxicity in the Tribal areas for a best picture of understanding the element effects on human, and environment can be achieved that will lead to a sustainable ecological harmony.

THE BALOCHISTAN EARTHQUAKE 2013: EMERGENCE OF A NEW ISLAND IN THE ARABIAN SEA

A massive earthquake of 7.7 magnitude struck south-central Pakistan on September 24, 2013. The epicenter of the earthquake was 63 km northnorth-east (NNE) of Awaran in Baluchistan. The earthquake caused the death of 386 people, leaving thousands homeless. The earthquake also affected different cities in Iran, India and Afghanistan. The fundamental cause of the earthquake was displacement along obliquestrike-slip fault at a depth of only 15 km (9.3 miles). The quake has created an island in the Arabian Sea which is an unusual in case of strikeslip faulting and it raises the question of the nature and original mechanism of the earthquake. The formation of the island indicates that there is a significant push of the northward moving IndoPakistani plate along with oblique-strike-slip faulting which was the original motion of the earthquake. These facts were discussed and explained in this paper.