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• 1. [Article] Stratigraphic and structural framework of Himalayan foothills, northern Pakistan

  The oldest sedimentary and metasedimentary rocks exposed in the Himalayan foothills of Pakistan record a gradual transition seaward from the evaporites of the Salt Range Formation to pelitic sediments ...

  Citation × Citation Citation Abstract Copy Title: Stratigraphic and structural framework of Himalayan foothills, northern Pakistan Author: Pogue, Kevin R. The oldest sedimentary and metasedimentary rocks exposed in the Himalayan foothills of Pakistan record a gradual transition seaward from the evaporites of the Salt Range Formation to pelitic sediments deposited in deeper water to the north. The Upper Proterozoic Tanawal Formation was derived from erosion of a northern highland produced during the early stages of Late Proterozoic to early Ordovician tectonism. Early Paleozoic tectonism is indicated by an angular unconformity at the base of the Paleozoic section, the intrusion of the Mansehra Granite, and the local removal of Cambrian strata. Paleozoic shallow-marine strata are preserved in half-grabens created during extensional tectonism that began during the Carboniferous and climaxed with rifting during the Permian. Paleozoic rocks were largely or completely eroded from northwest-trending highlands on the landward side of the rift shoulder. Thermal subsidence of the rifted margin resulted in transgression of the highlands and deposition of a Mesozoic section dominated by carbonates. Compressional tectonism related to the impending collision with Asia commenced in the Late Cretaceous. Rocks north of the Panjal-Khairabad fault were deformed and metamorphosed during Eocene subduction of northern India beneath the Kohistan arc terrane. Following their uplift and exhumation, rocks metamorphosed beneath Kohistan were thrust southward over unmetamorphosed rocks along the Panjal and Khairabad faults which are inferred to be connected beneath alluvium of the Haripur basin. Contrasts in stratigraphy and metamorphism on either side of the Panjal-Khairabad fault indicate that shortening on this structure exceeds that of any other fault in the foothills region. The migration of deformation towards the foreland produced south- or southeast-vergent folds and thrust faults in strata south of the Panjal-Khairabad fault and reactivated Late Cretaceous structures such as the Hissartang fault. The Hissartang fault is the westward continuation of the Nathia Gali fault, a major structure that thrusts Proterozoic rocks in the axis of a Late Paleozoic rift highland southward over Mesozoic strata. Fundamental differences in stratigraphy, metamorphism, and relative displacement preclude straightforward correlation of faults and tectonic subdivisions of the central Himalaya of India and Nepal with the northwestern Himalaya of Pakistan. Title: Stratigraphic and structural framework of Himalayan foothills, northern Pakistan Author: Pogue, Kevin R. The oldest sedimentary and metasedimentary rocks exposed in the Himalayan foothills of Pakistan record a gradual transition seaward from the evaporites of the Salt Range Formation to pelitic sediments deposited in deeper water to the north. The Upper Proterozoic Tanawal Formation was derived from erosion of a northern highland produced during the early stages of Late Proterozoic to early Ordovician tectonism. Early Paleozoic tectonism is indicated by an angular unconformity at the base of the Paleozoic section, the intrusion of the Mansehra Granite, and the local removal of Cambrian strata. Paleozoic shallow-marine strata are preserved in half-grabens created during extensional tectonism that began during the Carboniferous and climaxed with rifting during the Permian. Paleozoic rocks were largely or completely eroded from northwest-trending highlands on the landward side of the rift shoulder. Thermal subsidence of the rifted margin resulted in transgression of the highlands and deposition of a Mesozoic section dominated by carbonates. Compressional tectonism related to the impending collision with Asia commenced in the Late Cretaceous. Rocks north of the Panjal-Khairabad fault were deformed and metamorphosed during Eocene subduction of northern India beneath the Kohistan arc terrane. Following their uplift and exhumation, rocks metamorphosed beneath Kohistan were thrust southward over unmetamorphosed rocks along the Panjal and Khairabad faults which are inferred to be connected beneath alluvium of the Haripur basin. Contrasts in stratigraphy and metamorphism on either side of the Panjal-Khairabad fault indicate that shortening on this structure exceeds that of any other fault in the foothills region. The migration of deformation towards the foreland produced south- or southeast-vergent folds and thrust faults in strata south of the Panjal-Khairabad fault and reactivated Late Cretaceous structures such as the Hissartang fault. The Hissartang fault is the westward continuation of the Nathia Gali fault, a major structure that thrusts Proterozoic rocks in the axis of a Late Paleozoic rift highland southward over Mesozoic strata. Fundamental differences in stratigraphy, metamorphism, and relative displacement preclude straightforward correlation of faults and tectonic subdivisions of the central Himalaya of India and Nepal with the northwestern Himalaya of Pakistan. Close

• 2. [Article] Structural interpretation of the eastern Sulaiman foldbelt and foredeep, Pakistan

  The Sulaiman foldbelt is an active and conspicuous tectonic feature on the northwestern margin of the Indo-Pakistani plate. Seismic reflection data have been combined with surface geological mapping and ...

  Citation × Citation Citation Abstract Copy Title: Structural interpretation of the eastern Sulaiman foldbelt and foredeep, Pakistan Author: Humayon, Mansoor The Sulaiman foldbelt is an active and conspicuous tectonic feature on the northwestern margin of the Indo-Pakistani plate. Seismic reflection data have been combined with surface geological mapping and drillhole 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 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 back-thrust 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. Title: Structural interpretation of the eastern Sulaiman foldbelt and foredeep, Pakistan Author: Humayon, Mansoor The Sulaiman foldbelt is an active and conspicuous tectonic feature on the northwestern margin of the Indo-Pakistani plate. Seismic reflection data have been combined with surface geological mapping and drillhole 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 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 back-thrust 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. Close

• 3. [Article] Crustal structure of the northwestern continental margin of the Indian subcontinent from gravity and magnetic data

  The continental margin off the coast of Pakistan between the Murray ridge and the Gulf of Cambay has been studied in this work using gravity, magnetic and bathymetric data. Two dimensional gravity and ...

  Citation × Citation Citation Abstract Copy Title: Crustal structure of the northwestern continental margin of the Indian subcontinent from gravity and magnetic data Author: Soofi, Muhammad Asif The continental margin off the coast of Pakistan between the Murray ridge and the Gulf of Cambay has been studied in this work using gravity, magnetic and bathymetric data. Two dimensional gravity and magnetic models based on free-air gravity and residual magnetic data are developed along a north-south profile off the coast of Karachi. The purpose was to interpret the gross crustal structure of the region. A magnetic map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E. The gravity model extends to a distance of about 1200 km seaward south of Karachi. The seaward end of the gravity model is constrained by seismic refraction data which suggest the presence of typical oceanic crust. The Moho depth at this end of the profile is about 12 km. At the landward end of the profile A-A' the Moho depth is not constrained by seismic data. The gravity model suggests 27 to 17 km as the possible range of the depth of the Moho and a gradual thinning of the crust from land to sea. In addition, the gravity models as interpreted in this study show grabens at the distances of 350 and 450 km along the profile. If the graben-like structures are rift grabens formed during the rifting of India from Africa then transitional crust can be expected to extend to the 500 km mark along the profile A-A'. Two dimensional models for the magnetic data along the profile were also developed. These anomalies can be interpreted as due to oceanic crust or magnetic bodies embedded in transitional crust. The possibility that the observed magnetic anomalies are due to oceanic crust has been studied in detail in this work. The location of the observed magnetic anomalies with respect to marine magnetic anomaly (28) observed earlier on the Indian Ocean floor, were compared to a marine magnetic time scale. To get a reasonable correlation between the observed and theoretical anomalies requires a considerable amount of adjustment in the spreading rate of individual magnetic blocks. Also on the magnetic map the trend of the lineation of these anomalies is perpendicular to the continental margin instead of being parallel to the continental margin as expected for a rifted continental margin. The presence of horst-and-graben structures in the inland region suggests the rifted nature for the continental margin off Karachi than the sheared nature. This indicates that the lineations should be parallel to the margin. But the magnetic lineations are perpendicular to the continental margin and if they are from oceanic crust then they would suggest that the margin is a sheared margin, which contradicts the extensional structures observed inland. This makes it very unlikely that the source of these anomalies is oceanic crust. However, it is quite possible that the magnetic lineations observed in the map were parallel to the continental margin initially but later on the continent rotated clockwise along a fault landward of the magnetic lineation. This rotation is perhaps responsible for making the lineation perpendicular to the continental margin. One objective of this study was to locate the continent-ocean boundary, but with the available amount of data it is not possible to decide on the most appropriate source for the observed magnetic anomalies. Hence it was not possible to decide exactly on the location of continent-ocean boundary. However, on the basis of gravity and magnetic data it can be said that the continent-ocean boundary lies at a distance of 500 km or greater along the profile. Title: Crustal structure of the northwestern continental margin of the Indian subcontinent from gravity and magnetic data Author: Soofi, Muhammad Asif The continental margin off the coast of Pakistan between the Murray ridge and the Gulf of Cambay has been studied in this work using gravity, magnetic and bathymetric data. Two dimensional gravity and magnetic models based on free-air gravity and residual magnetic data are developed along a north-south profile off the coast of Karachi. The purpose was to interpret the gross crustal structure of the region. A magnetic map has also been developed for the region between latitudes 20°N and 27°N and between longitude 60°E and 70°E. The gravity model extends to a distance of about 1200 km seaward south of Karachi. The seaward end of the gravity model is constrained by seismic refraction data which suggest the presence of typical oceanic crust. The Moho depth at this end of the profile is about 12 km. At the landward end of the profile A-A' the Moho depth is not constrained by seismic data. The gravity model suggests 27 to 17 km as the possible range of the depth of the Moho and a gradual thinning of the crust from land to sea. In addition, the gravity models as interpreted in this study show grabens at the distances of 350 and 450 km along the profile. If the graben-like structures are rift grabens formed during the rifting of India from Africa then transitional crust can be expected to extend to the 500 km mark along the profile A-A'. Two dimensional models for the magnetic data along the profile were also developed. These anomalies can be interpreted as due to oceanic crust or magnetic bodies embedded in transitional crust. The possibility that the observed magnetic anomalies are due to oceanic crust has been studied in detail in this work. The location of the observed magnetic anomalies with respect to marine magnetic anomaly (28) observed earlier on the Indian Ocean floor, were compared to a marine magnetic time scale. To get a reasonable correlation between the observed and theoretical anomalies requires a considerable amount of adjustment in the spreading rate of individual magnetic blocks. Also on the magnetic map the trend of the lineation of these anomalies is perpendicular to the continental margin instead of being parallel to the continental margin as expected for a rifted continental margin. The presence of horst-and-graben structures in the inland region suggests the rifted nature for the continental margin off Karachi than the sheared nature. This indicates that the lineations should be parallel to the margin. But the magnetic lineations are perpendicular to the continental margin and if they are from oceanic crust then they would suggest that the margin is a sheared margin, which contradicts the extensional structures observed inland. This makes it very unlikely that the source of these anomalies is oceanic crust. However, it is quite possible that the magnetic lineations observed in the map were parallel to the continental margin initially but later on the continent rotated clockwise along a fault landward of the magnetic lineation. This rotation is perhaps responsible for making the lineation perpendicular to the continental margin. One objective of this study was to locate the continent-ocean boundary, but with the available amount of data it is not possible to decide on the most appropriate source for the observed magnetic anomalies. Hence it was not possible to decide exactly on the location of continent-ocean boundary. However, on the basis of gravity and magnetic data it can be said that the continent-ocean boundary lies at a distance of 500 km or greater along the profile. Close