Ahmad Hussain

Timing of structural events in the Himalayan foothills of northwestern Pakistan

The Attock-Cherat Range forms the southern boundary of the Peshawar basin and includes rocks transitional between metasediments of the Lesser Himalaya and foreland-basin strata to the south. The Attock-Cherat Range comprises three fault-bounded structural blocks which are, from north to south, (1) Precambrian metaclastic strata overlain by unfossiliferous limestone which is itself apparently overlain by Paleozoic strata with the contact not exposed; (2) unfossiliferous flysch of Precambrian(?) age overlain by Cretaceous and Paleogene marine strata and Murree red beds at least in part of early Miocene age; and (3) unfossiliferous limestone, argillite, and quartzite correlated in part to Paleozoic strata in the Peshawar basin, overlain by a Tertiary sequence generally similar to that in block 2. Farther south, in the Kala Chitta Range, strata of Triassic to Eocene age occur in south-verging folds and thin thrust sheets. The similarity of the Tertiary sequences in the Kala Chitta Range and in blocks 2 and 3 demonstrates that the pre-Tertiary sequences were juxtaposed by faults prior to deposition of the Paleocene Lockhart Limestone. This may coincide with initial contact of the west-northwest–facing passive margin of India with Eurasia or nearby microplates. Major late Tertiary imbricate thrusting and folding took place prior to uplift of the Attock-Cherat Range and to deposition of Peshawar intermontane basin fill of Pliocene-Pleistocene age. The Peshawar basin formed as the Kala Chitta Range was faulted south on the Main Boundary thrust (MBT), forcing Siwalik foreland basins still farther south. Late Quaternary deformation in the southern Peshawar basin occurred along a seismically active zone of en echelon , stepped-left faulted pressure ridges that may reflect a subsurface ramp on the older MBT.

Paleozoic and Mesozoic stratigraphy of the Peshawar basin, Pakistan: Correlations and implications

The most complete Paleozoic sequence described from Pakistan is exposed in bedrock inliers and in ranges fringing the eastern Peshawar basin. Interbedded quartzite and argillite of the Precambrian and Cambrian Tanawal Formation is overlain unconformably by the Cambrian(?) Ambar Formation. The Misri Banda Quartzite unconformably overlies the Ambar and contains Ordovician Cruziana ichnofossils. New conodont discoveries restrict the ages of overlying formations as follows: Panjpir Formation, Llandoverian to Pridolian; Nowshera Formation, Lochkovian to Frasnian; and Jafar Kandao Formation, Kinderhookian to Westphalian. The Karapa Greenschist, consisting of metamorphosed lava flows, separates the Jafar Kandao from Upper Triassic (Carnian) marbles of the Kashala Formation. The Upper Triassic and Jurassic(?) Nikanai Ghar Formation forms the top of the section. Correlatives to the Peshawar basin stratigraphy are present locally in the Sherwan synclinorium of Hazara and in the Khyber Pass region. The sequence contrasts markedly with the Paleozoic and Mesozoic section exposed south of the Khairabad thrust in the Attock-Cherat Range. This thrust and its northeastern continuation in Hazara north of Abbottabad thus form the boundary in Pakistan between the Lesser Himalayan and Tethyan Himalayan sections, a function performed by the Main Central thrust (MCT) in the central Himalaya of India and Nepal. The newly dated Carboniferous to Triassic horizons provide the first firm age constraints on the protoliths of the high-grade Swat metasediments. The dating of the metasediments has, in turn, provided age constraints on pre-Himalayan tectonism and associated intrusions. Two major tectonic episodes during the Late(?) Cambrian and Carboniferous produced positive areas north of the Peshawar basin that provided coarse detritus to the Misri Banda Quartzite and Jafar Kandao Formation.

Stratigraphy south of the Main Mantle Thrust, Lower Swat, Pakistan

Abstract The metamorphic sequence in Lower Swat is described and placed into the geological framework of Pakistan. The stratigraphic sequence consists of the Precambrian-Cambrian(?) Manglaur formation unconformably overlain by the Alpurai group which is subdivided into the Carboniferous or younger Marghazar formation, and the Triassic or younger Kashala, Saidu, and Nikanai Ghar formations. A third unit, the Jobra formation of unknown age, is present as discontinuous lenses unconformably below the Alpurai group. Type sections are indicated for each rock unit. Comparison with the stratigraphy in the Peshawar Basin indicates that the Lower Swat area existed as a highland with active normal faulting during and perhaps before deposition of the Marghazar formation. The Marghazar formation appears to be a rift facies that correlates with the Panjal Traps of western India. The Kashala and Nikanai Ghar formations represent a transition to a stable shelf environment. These units may correlate, in part, with the Zanskar Supergroup of western India. The Saidu formation may represent drowning of the shelf as it was pulled down and overridden by the Main Mantle Thrust suture melange. It may correlate with the Lamayuru Formation of western India. The Alpurai group thus records a depositional history from Late Paleozoic breakup of Gondwana, to development of a passive Mesozoic shelf, to drowning of the shelf at the onset of Himalayan orogeny.

The Main Mantle Thrust in Pakistan: its character and extent

Abstract The Main Mantle Thrust (MMT) represents the tectonic boundary between metamorphic shield and platform rock of the Indian plate hinterland, and dominantly mafic and ultramafic rock of the Kohistan-Ladakh arc complex in Pakistan. In some areas, this boundary is a sharp planar fault with development of mylonite; in other areas, it is a brittle-ductile imbricate zone; in still other areas, it contains large, discontinuous, slices of internally sheared and deformed ophiolitic mélange. The character of the MMT along its entire trace is discussed and it is concluded that there is no single continuous fault which marks the contact between the Indian plate and the Kohistan-Ladakh arc. On this basis, we propose a revised definition for the MMT that is consistent with both the original definition and with the usage of the term in literature. We suggest that the MMT fault contact be defined as the series of faults, of different age and tectonic history, that collectively define the northern margin of the Indian plate in Pakistan. On this basis, faults that define the MMT vary in age from Quaternary to possibly as old as Late Cretaceous. Discontinuous lenses of ophiolitic mélange that overlie the MMT fault contact, and which intervene between the Indian plate and the Kohistan-Ladakh arc, are considered to be part of an MMT zone that is equivalent with the Indus Suture Zone.

Cenozoic kinematic history of the Kohistan fault in the Pakistan Himalaya

Detailed mapping suggests a new scenario for the kinematic history of the Late Cretaceous Kohistan arc complex and for the cause of Cenozoic metamorphism and exhumation along the Indian continental margin in Pakistan. The earliest deformational phases resulted in early Cenozoic amphibolite-facies metamorphism, superposed tight to isoclinal F1/F2 folds, and development of strong penetrative rock fabric within the Indian plate in response to southwestward, and then southeastward, overthrusting of ophi-olitic melange in the Indus suture zone and West Pakistan fold belt, respectively. This deformation included the development of the Banna and Malakand thrust faults. Prograde metamorphism ended following this deformational phase, and the early structures were deformed along N-trending, open, upright, F3 folds and domes. This was followed by ESE- to E-directed thrusting of the Kohistan arc complex onto the Indian continent along the dominantly brittle Kohistan fault. Thrusting ended in the late Oligocene–early Miocene when the Kohistan arc assumed its present position along the northern margin of India. South-verging structures began to form on the Indian plate for the first time in the late Oligocene–early Miocene. Deformation had expanded to the foreland by the middle to late Miocene with development of the Panjal-Khairabad thrust. Neogene structures in the metamorphic zone include N-trending, east-side-up, high-angle fault zones that deform or reactivate the Kohistan fault, and NW-trending, east-side-up reverse faults, both of which are seismically active. In the tectonic scenario outlined here, shortening along the Kohistan fault occurred subsequent to metamorphism, and was directed toward the ESE-E throughout its history. The southern termination of the fault is considered to be a right-slip sidewall ramp such that the Kohistan arc never advanced much father south on the Indian plate than its present position. Broad flexure, folding, and erosion in front of and south of the actively advancing Kohistan arc complex contributed to exhumation of the Indian plate. This differs from interpretations that state that Kohistan was thrust in a southward direction; that underthrusting beneath Kohistan was the primary cause of early Cenozoic metamorphism on the Indian plate; and that top-side-north extensional reactivation of the Kohistan fault contributed to exhumation.

The Main Boundary Thrust and propagation of deformation into the foreland fold-and-thrust belt in northern Pakistan near the Indus River

Abstract The Main Boundary Thrust in northwestern Pakistan is a floor thrust along which a thrust system incorporating Precambrian and Phanerozoic rocks of the Kala Chitta and Attock-Cherat Ranges was emplaced over Cenozoic strata of the northern Kohat and Potwar Plateaus. The MBT and successive thrusts toward the foreland are interpreted as low angle to flat decollement thrusts at 8–10 km depths that bound thrust sheets with large lateral dimensions. Admissible cross sections indicate over 60 km of shortening along the largely pre-Palaeocene Khairabad, Cherat, and Hissartang thrusts within the MBT allochthon. In the toe of the MBT allochthon, contraction is manifest at the surface as tight folds in largely Mesozoic and lower Tertiary strata, decoupled in the upper third of the allochthon. Displacement of the MBT is estimated at over 40 km and south of the MBT, shortening is interpreted along blind thrusts accompanied by shallow (0–3 km deep) backthrusting. The MBT allochthon was deformed prior to its low angle displacement over the northern Kohat and Potwar Plateaus, and folded and eroded during or after its emplacement. The MBT thrust front may participate in its own later deformation, but not as an emergent low angle thrust through the Quaternary.