Super Continent Pangea
A large super continent Pangea (Searle, 1991) existed till the end of Proterozoic era (Piper, 1982). The single enormous ocean which surrounded Pangea is known as Panthalassa (Nield, 1998). Before Pangea, the supercontinent, Rodinia, formed about 1.1 billion years ago during the Proterozoic era, and lasted until 700-750 Ma (Dalziel, 1995). Breakup of super continent initiated in Carboniferous to early Permian (Searle, 1991) and it result super continent divide in two parts, Gondwanaland (Suess, 1885) in south and Laurasia in north and a new ocean Neo Tethys between them (Searle, 1991: Powell, 1979). Among the Gondwana continents, India started to move rapidly northwest relative to Australia and Antarctica during Late Cretaceous.
As a consequence, the Neo-Tethys that was located between the Indian continent and Asian plate started shrinking and this continental drift was facilitated by the consumption of Neo-Tethys and opening up of the Indian ocean behind. Eventually India collided with Kohistan Island arc at 67+2 Ma (Chaudhry et al., 1994; Valdiya, 2002) and the Kohistan Island Arc with southern margin of Eurasia (Powell, 1979) at 65 Ma prior to this collision (Searle, 1991). Propelled by the geodynamic forces, the Indian plate traveled 5000 km northward (Powell 1979).
After the collision, Indian continental crust started to subducted beneath Eurasia and northward movement of Indian plate slowdown and continuous up to present (Powell, 1979).The subduction of the northern margin of the Indian plate finally closed the Neo-Tethys and the Indian Ocean assumed its present widespread expanse (Treloar and Izatti, 1993). This collision is characterized by deformation, crustal thickening and surface uplift. The ongoing convergence caused the development of Himalayas along the northwestern margin of Indian Plate (Spencer, 1993).
As a consequence, the Neo-Tethys that was located between the Indian continent and Asian plate started shrinking and this continental drift was facilitated by the consumption of Neo-Tethys and opening up of the Indian ocean behind. Eventually India collided with Kohistan Island arc at 67+2 Ma (Chaudhry et al., 1994; Valdiya, 2002) and the Kohistan Island Arc with southern margin of Eurasia (Powell, 1979) at 65 Ma prior to this collision (Searle, 1991). Propelled by the geodynamic forces, the Indian plate traveled 5000 km northward (Powell 1979).
After the collision, Indian continental crust started to subducted beneath Eurasia and northward movement of Indian plate slowdown and continuous up to present (Powell, 1979).The subduction of the northern margin of the Indian plate finally closed the Neo-Tethys and the Indian Ocean assumed its present widespread expanse (Treloar and Izatti, 1993). This collision is characterized by deformation, crustal thickening and surface uplift. The ongoing convergence caused the development of Himalayas along the northwestern margin of Indian Plate (Spencer, 1993).
The Himalayan ranges of south Asia are a dilemma due to their complex geology in modern plate tectonic theory. The process of sea floor spreading and subduction together leads to drift of lower density buoyant continents and island arcs, which can eventually, collided against each other, leading to thrusting. Thrusting builds up a stack of lower density crustal sheets involving deformation and leading to metamorphism and magmatism. This deformed metamorphosed and intruded stock eventually starts rising to achieve new isostatic equilibrium giving rise to linear collision mountains or orogenic belts. Tectonically the Himalayas are recognized as young collisional orogen and mountain belt formed as result of collision at about 55 Ma or 65 Ma between north moving Indian plate to south and Asian plate to the north (Lillie, 1988).
The tectonic elements of NW Himalayas (Fig. 3.2) include from north to south as Asian Plate (Brad et al., 1976: Ghazanfar et al., 1991), Main Karakoram Thrust, MKT (Ghazanfar, 1993), Kohistan Island Arc Complex (Brad et al., 1976: Ghazanfar et al., 1991; Tahirkheli et al., 1979; Trelor et al., 1989; Chaudhry et al., 1984), Indus suture or MMT (Ashraf et al., 1991; Tahirkheli et al., 1979) and Indian Plate margin (Chaudhry and Ghazanfar, 1987; Ghazanfar, 1993).
Boundary Faults
On the basis of four regional faults systems, Pakistani Himalayas can be divided into five litho-tectonic terrains, which are characterized by distinctive stratigraphy and physiography. From north to south these gross geological belts and the major faults systems separating them are as given and described respectively below.
Main Karakoram Thrust (MKT)
The Main Karakoram Thrust (MKT) zone marks the contact between the Eurasian Plate in the north and the Indian Plate in the south (Coward et al. 1982). The Main Karakoram Thrust (MKT) is major tectonic feature in northern Pakistan formed as a result of the collision between the Karakoram block in the north and the Kohistan Island Arc in the south (Gansser 1980). The MKT was previously named as the Northern Suture (Pudsey et al,. 1986).
Main Mantle Thrust (MMT)
The MMT is marked by the mantle related ultra-mafics, metavolcancis, metagabbros, phylitic sediments, blueschists, and garnet granulites (Jan, 1980, Jan and Howie, 1981). The MMT bounds the Kohistan Island Arc (KIA) to the south and the Indian Plate to the north. It formed as a result of collision and subduction of Indian Plate underneath the KIA during Eocene time. It is the southernmost thrust involving lower-crust crystalline rocks of the Indo-Pakistani shield (Desio, 1974 b).
Main Central Thrust (MCT)
This thrust was initially defined by the Auden (1937) and Heim et al. (1939) as a thrust fault which has brought the high grade crystalline rocks over the low grade meta-sediments (Kazmi, 1997). The main central thrust (MCT) is an intracontinental thrust that separates the Higher and Lesser Himalayas. It was first described by the Heim and Gansser (1939) as a tectonic boundary between the Himalayan autochthonous sedimentary sequence and the overlying crystalline complex. It has been traced from Nepal and up to Southern Kashmir (Gansser, 1964). In NW Himalayas, It is north dipping, generally 5 to 20 Km thick, low to high angle shear zone associated with mylonization, strong stretching lineation and strong foliation. It is also marked by inverse metamorphism and sudden jump in metamorphic grade from greenschist to upper amphibolite facies with different tectonic style on either side of the shear zone. This ductile shear zone develops in the turbidites of Kaghan as well as Malakand area (Ghazanfar and Chaudhry, 1999).
Main Boundary Thrust (MBT)
MBT is hairpin-shaped system of faults that can be followed around the Hazara Kashmir Syntaxis. It truncates the Murree Formation on the east, north and west (Treloar et al., 1989 a, 1990, Chaudhry et al., 1997a). It abuts the Mesozoic and the earlier rocks against the Murree Formation. The Main Boundary Thrust (MBT) represents the southward migration of Himalayan deformation from the site of MMT. From northeast to southwest, it extends along the front of the northern fold and thrust belt around Hazara-Kashmir Syntaxes (Calkins et al., 1975). The MBT zone is comprised of a series of parallel or en-echelon thrust faults dividing the northwest Himalayan sequence into a deformed sedimentary southern zone or foreland zone, and a deformed and metamorphosed northern zone or the hinterland zone (Di Pietro et al., 1996).
Salt Range Thrust (SRT) and Trans Indus Ranges Thrust (TIRT)
Salt Range Thrust (SRT) / Himalayan Frontal Thrust (HFT) or the Main Frontal Thrust (MFT) runs along western margin of Salt Range between Jhelum and Indus River in NW Himalayas of Pakistan. It is low angle fault or sub horizontal fault and throw of the fault is good enough to expose Pre-Cambrian strata (Salt Range Formation). In northern Pakistan most of the youngest thrusting has occurred along frontal thrust system in Salt Range along Salt Range Thrust (SRT) in the east and in Trans-Indus Ranges Thrust (TIRT) in the West (Blisniuk et al., 1998). The frontal thrust system has accommodated about ≥ 20 km of shortening in Salt Range (Lillie et al., 1987) and ~ 10 km in Trans Indus Ranges. Along this thrust front Eo-Cambrian Salt Range Formation in the Salt Range (Gee, 1980); Permian rocks in the Surghar Range and the Cambrian Jhelum Group rocks in the Khisor Range are Thrusted over the Punjab Fore-deep in the South (Kidwai, 1973).
Salt Range Thrust (SRT) and Trans Indus Ranges Thrust (TIRT)
Salt Range Thrust (SRT) / Himalayan Frontal Thrust (HFT) or the Main Frontal Thrust (MFT) runs along western margin of Salt Range between Jhelum and Indus River in NW Himalayas of Pakistan. It is low angle fault or sub horizontal fault and throw of the fault is good enough to expose Pre-Cambrian strata (Salt Range Formation). In northern Pakistan most of the youngest thrusting has occurred along frontal thrust system in Salt Range along Salt Range Thrust (SRT) in the east and in Trans-Indus Ranges Thrust (TIRT) in the West (Blisniuk et al., 1998). The frontal thrust system has accommodated about ≥ 20 km of shortening in Salt Range (Lillie et al., 1987) and ~ 10 km in Trans Indus Ranges. Along this thrust front Eo-Cambrian Salt Range Formation in the Salt Range (Gee, 1980); Permian rocks in the Surghar Range and the Cambrian Jhelum Group rocks in the Khisor Range are Thrusted over the Punjab Fore-deep in the South (Kidwai, 1973).
Subdivision of Himalaya
With the demarcation of basement of Indian plate and boundary faults, now it is possible to divide NW Himalayas into four tectonic units of Gansser (1964), as in Eastern and Central Himalayas. NW Himalayas are enumerated from north to south as Tethyan Himalaya, Higher Himalaya, Lesser Himalaya and Sub Himalaya.
Trans Himalayas
Trans Himalaya or Tibetan-Tethyan Himalaya occur north of Higher Himalaya and consists of Proterozoic to Eocene siliciclastic and carbonate sedimentary rocks inter-bedded with Paleozoic and Mesozoic volcanic rocks resting over Pre-Cambrian higher Himalayan basement with tectonized unconformity (Baud et al., 1984; Garzanti et al., 1986, 1987). Tethyan sequence has developed as continental margin shelf deposit on the northern edge of Indian plate which occurs as low grade meta-sediments (Ghazanfar and Chaudhry, 1999).The Tethyan Himalayas are limited to north by Indus Tsangpo Suture Zone in NW Himalaya and this zone also acts as the northern most boundery of the Indian plate. Indus suture or MMT demarcate boundary between Indian plate and Kohistan island arc and comprised of dismembered ophiolites (Tahirkheli et al., 1979). MMT is the true extension of Indus-Tsangpo Suture Zone which occurs at 65 Ma (Spencer, 1993; Chaudhry et al., 1994; Smith et al., 1994). Until very recently the Tibetan-Tethys zone of Himalaya was considered absent in the Pakistan. Mapping in Kaghan and Swat however, have indicated that the Tibetan-Tethys zone of Himalaya is indeed present in Pakistan, though much attenuated and discontinuous (Chaudhry and Ghazanfar, 1987; Chaudhry et al., 1992, 1993, Ghazanfar and Chaudhry, 1999).
Higher Himalayas
With the delineation of Main Central Thrust or MCT in (Chaudhry et al., 1997b; Ghazanfar and Chaudhry, 1986, 1990; Chaudhry et al., 1994a, 1994b), now it is possible to establish the different tectono-stratigraphic regions of Himalaya, such as Higher Himalaya, Lesser Himalaya and Sub Himalaya in Pakistan. Higher Himalayas are limited by MMT in north and MCT in south. MCT is intracontinental thrust which demarcates the boundery between the Lesser and Higher Himalayas (Brunel, 1986; Valdiya, 1984; Ghazanfar and Chaudhry, 1986).
In Pakistan, the Higher Himalayas includes the Nanga Parba-Haramosh massif (Yeats and Lawerence, 1984).It extends from Bajaur on the Pakistan Afghanistan border in the west to the Neelum valley of Kashmir in the east. Higher Himalaya comprised of Proterozoic to Late Proterozoic basement and Phanerozoic to Mesozoic cover in Pakistan (Ghazanfar, 1993; Chaudhry and Ghazanfar, 1987). Basement rock represents the oldest exposed rocks of Pakistan and composed of lower autochthonous to Para-autochthonous granitoid-migmatite complex, pelite-psamites with turbidites and associated minor quartzite and marble horizons. Basement is overlained by cover sequence which is composed of calc-pelites and marbles with some garnetiferous calc-pelites, amphibolites and pelites (Ghazanfar, 1993; Chaudhry and Ghazanfar, 1987; Ghazanfar and Chaudhry, 1985; Chaudhry et al., 1994a, 1997; Ghazanfar et al., 1983, 1999a).
Basement and cover sequence rocks are grouped in different areas of Higher Himalaya. Kel Group, Purbinar Group and Pacha Group constitutes the basement rock in Neelum Valley, western Kashmir, Upper Kaghan Valley and Swat area respectively and Sharda Group, Burawai Group and Alpuri group represent cover sequence respectively. The rocks of Higher Himalaya are metamorphosed to upper amphibolites facies and kyanite-silliminite grades just north of Batal area in Naran area of Kaghan (Ghazanfar et al., 1983, 1999a; Ghazanfar and Chaudhry, 1985, 1987; Chaudhry et al., 1994a, 1997).
Lesser Himalayas
Lesser Himalaya is confined between MCT and MBT in north and south respectively. MBT formed due to compressional deformation. In Pakistan, Lesser Himalayaextends in Lower Neelum Valley, Lower Kaghan Valley, Hazara area and Swat area across the Indus River. Lesser Himalaya has biotite-garnet grade rocks (Chaudhry et al., 1997a).
The Lesser Himalayas division in NW Himalayas includes Hill Ranges (including Margala, Kala Chitta, Attock, Kohat Hills and Safed Koh), Plio-Pleistocene basins (Peshawar and Campbellpur basin) and the Lower Kohistan (Yeats and Lawerence, 1984). Lesser Himalayas are divisible into two distinct units, Northern metamorphic Zone and Southern Sedimentary Zone, and Panjal fault marks boundary between them (Ghazanfar, 1993; Ghazanfar et al., 1999a).
Northern Metamorphic Zone represents a terrain of meta pelite-psamite sequence which are intruded by Cambrian granites and some younger ones, and overlained by younger rocks including slates, phyllites, meta conglomerates, graphitic schist, limestone, marble, agglomeratic schist and basic volcanics of Kashmir basin and Peshawar basin. This meta pelite-psamite sequence is designated as Kundalshahi Group in Neelum Valley, Kaghan Group in Kaghan Valley and Tanawal Formation in Hazara area (Chaudhry et al., 1992; Ghazanfar and Chaudhry, 1985; Chaudhry and Ghazanfar, 1987; Ghazanfar et al., 1983). Kundalshahi Group is intruded by Jura granite of Cambrian age in southern parts and younger Neelum granite in north (Ghazanfar et al., 1983). Southern Sedimentary zone is characterized by the Pir Panjal ranges in east and Attock Hazara Fold and Thrust Belt, AHFTB, in west of Hazara Kashmir Syntaxis and south of Kashmir basin and Peshawar basin (Ghazanfar et al., 1999a). AHFTB is possible continuation of parautochthonous folded belt (Wadia, 1931) or Panjal Imbricate zone or Pir Panjal Range of Kashmir.
Sub Himalaya
Murree Fault or Main Boundary Thrust (MBT) and Salt Range Thrust (SRT) act as northern and southern boundaries of Sub-Himalaya respectively. The Himalayan foot hills form the Sub-Himalayan zone from the Punjab to Assam. These hills are comprised of a narrow belt of folded Neogene Murree and Siwalik red colored molasse type sediments.
The Sub Himalayas of Pakistan in the longitudnal sense can be subdivided into Azad Kashmir Zone in the east and the Punjab Zone or Punjab foreland in the west. In a transverse sense the Punjab foreland can be subdivided into North Potwar or Rawalpindi Zone (a fold and thrust belt culminating in the Khair-e-Murat structure and intensity of deformation increases northward), the Soan Zone (comprising a broad syncline under the plateau in the east and Kohat Basin in the west), the upthrusted inlier of Salt Range and Trans Indus Range and the only slightly deformed Punjab Plateform further to south (Ghazanfar and Chaudhry, 1999). Simply to say, Sub Himalayas subdivision includes Potwar Plateau, Kohat Plateau, Salt Range and Trans Indus Salt Ranges (Yeats and Lawerence, 1984). The Salt Range Fold and thrust belt gives way northward in the Potwar plateau to a broad syncline, the axis of which is followed by Soan River. This syncline is asymmetric and verges south further north. The intensity of folding increases and north dipping thrust fault appeared (Pinfold, 1980; Gill, 1953), culminating in the faults Kala Chitta and Margalla Hills along the Northern margin of Potwar Plateau.
Fig 4.2: Tectonic map of North Western Himalayas of Pakistan by Chaudhry et al (1992).
West of Potwar plateau, Eocene through Siwaliks strata are involved in complex folding, in which Eocene salt occupies the core of many of the anticlines (DeJong and Qayum, 1981). The Kohat plateau structure differs from that of Potwar plateau largely because of this higher salt detachment horizon in Kohat area (Yeats and Lawerence, 1984).
4.4 Tectonic Settings of Salt Range
Salt Range and Trans-Indus Salt Range represent the surface expression of up thrusting of a decollement in which basement is not involved and decollement exists in Cambrian to Eocambrian evaporites (Seeber and Armbuster, 1981; Crawford, 1974). Strata in the Salt Range generally dip northwards into the Potwar Basin (Sameeni, 1997). Structurally, the Salt Range is the result of tectonic forces imposed during the later phases of the Himalayan Orogeny in Late Cenozoic time, the occurrence of thick, incompetent Salt Range Formation at the base of sedimentary sequence has strongly influenced the structure). After the MBT zone under the Kala Chitta Range to the north was locked some 2 Ma ago compression was transferred to the plateform sequence to the south of MBT, as the result of which Salt Range thrust developed and Salt Range rises to surface (Yeats et al; 1984) .
Salt Range rises as 180 km (roughly ENE-WSW trending) long range at the southern edge of Potwar Plateau, making an abrupt escarpment against the Punjab plains in south. Its width is 150 km (Sameeni, 1997). The Salt Range forms an isolated E-W trending arc across the Indus River. It is a discrete structural zone bounded in north by north-dipping Main Boundary Thrust (Sarwar et al. 1979, Yeats et al. 1984, Coward et al. 1985), in south by the Salt Range Thrust, in west by Kalabagh fault and in East by Jhelum fault (Fig. 3.3). Tectonically, the Salt Range is the Himalayan equivalent of the Jura Mountains in the Alps and the Pine Mountains of the Appalachians (Qayyum, 1991). Salt Range is thin-skinned fold-and-thrust belt (Lillie et al., 1987; Butler et al., 1987 and Baker, 1987).
The allochthonous nature of the Salt Range, with a detachment in the Eocambrian Salt Range Formation, was recognized by many earlier workers (e. g., Wynne, 1878; Wadia, 1945 a, 1945 b, 1957; Gee, 1945, 1947, 1980; Voskresenskiy, 1978).
This range is essentially a complex salt anticlinorium with a series of salt anticlines. The structure along its northern slope is comprised of simple, broad, shallow folds followed by a gentle monocline. Southward the folding becomes tighter and the folds are commonly faulted (Gee, 1989). The Eocambrian evaporites are exposed in some of these overfolded and faulted anticlines (Qayyum, 1991).
Because the Salt Range formation is easily eroded, horsts form deep gorges, in which some of the classic stratigraphic section of Salt Range are found (Khewra, Nilawahan and Warchha gorges) (Yeats and Lawrence, 1984).
The overall structure of the Salt Range, except the easternmost part of the Eastern Salt Range, involves basically a fault bend fold geometry modified due to the presence of the underlying ductile salt. The roof sequence has been folded into sharp, salt cored anticlines and broad, flat synclines, which in its southernmost part frequently have been disrupted by forward and back verging thrusts and almost vertical dipping apparent normal faults (Qayyum, 1991).
Longitudinally, Salt Range is divided from East to West into the Eastern, Central and Western Salt Range. The Salt Range thrust is strongly emergent in its Central and Western parts, whereas in the east it is entirely a buried thrust (terminology of Morley, 1986) and folding predominates (Lillie et al., 1987). Davis & Engelder (1985) suggested that the difference in the structural style between East and West is the result of eastward thinning of the evaporites.
4.4.1 Central Salt Range
The surface geology across the Central Salt Range manifests the gradual exposure of older platform strata toward the South. The basement normal fault has acted as a buttress against which the salt has thickened. It also constitutes the lower part of the northern ramp and brings the older platform sequence to the surface. As the roof sequence rides over the northern ramp due to the basement normal fault, the strata become almost horizontal. The roof sequence is very gently folded into a series of small salt-cored anticlines near the leading edge. These anticlines are generally bounded by minor normal faults. Near the leading edge, there is a back thrust that brings Salt Range Formation over either the Jhelum Group or the Nilawahan group. This back thrust may have developed late in the geological history to build topography in order to maintain the forward motion of the roof sequence. The back thrust results in a total shortening of 27 km in the Central Salt Range (Qayyum, 1991).
Westward near to Sakesar peak, the range widens to some 30 km. In the north, the Eocene formations, capped in places by sandstone and clay-shales of the Early Miocene and overlain unconformably by Late Pliocene-Pleistocene Kalabagh Conglomerates, occur as broad, east and west plunging anticlines and synclines of varying wavelengths. The plunge variation results in large domes and basins e.g. between Dhariala and Kallar Kahar and westward via Vasnal and Khabakki in the approach to Sakesar (Gee, 1989). Several of these folds are faulted.
A concealed duplex structure is newly recognized in central Salt Range. It was evolved along a décollement with two ramps and extends more than 40 km along strike. The first ramp is within the Salt Range Formation. The second ramp crosses the platform sequence and follows shaley horizons of the overlying Murree Formation near the contact. The duplex structure is only present in the Central Salt Range, which implies that the décollement at its base must step down section and join the main décollement both in the west and the east. Further west along the duplex edge the platform sequence is flat and has not yet ramped upward. This implies that the décollement at the base of the duplex structure steps down along a Lateral ramp to join the main décollement (Qayyum, 1991).
In Pakistan, the Higher Himalayas includes the Nanga Parba-Haramosh massif (Yeats and Lawerence, 1984).It extends from Bajaur on the Pakistan Afghanistan border in the west to the Neelum valley of Kashmir in the east. Higher Himalaya comprised of Proterozoic to Late Proterozoic basement and Phanerozoic to Mesozoic cover in Pakistan (Ghazanfar, 1993; Chaudhry and Ghazanfar, 1987). Basement rock represents the oldest exposed rocks of Pakistan and composed of lower autochthonous to Para-autochthonous granitoid-migmatite complex, pelite-psamites with turbidites and associated minor quartzite and marble horizons. Basement is overlained by cover sequence which is composed of calc-pelites and marbles with some garnetiferous calc-pelites, amphibolites and pelites (Ghazanfar, 1993; Chaudhry and Ghazanfar, 1987; Ghazanfar and Chaudhry, 1985; Chaudhry et al., 1994a, 1997; Ghazanfar et al., 1983, 1999a).
Basement and cover sequence rocks are grouped in different areas of Higher Himalaya. Kel Group, Purbinar Group and Pacha Group constitutes the basement rock in Neelum Valley, western Kashmir, Upper Kaghan Valley and Swat area respectively and Sharda Group, Burawai Group and Alpuri group represent cover sequence respectively. The rocks of Higher Himalaya are metamorphosed to upper amphibolites facies and kyanite-silliminite grades just north of Batal area in Naran area of Kaghan (Ghazanfar et al., 1983, 1999a; Ghazanfar and Chaudhry, 1985, 1987; Chaudhry et al., 1994a, 1997).
Lesser Himalayas
Lesser Himalaya is confined between MCT and MBT in north and south respectively. MBT formed due to compressional deformation. In Pakistan, Lesser Himalayaextends in Lower Neelum Valley, Lower Kaghan Valley, Hazara area and Swat area across the Indus River. Lesser Himalaya has biotite-garnet grade rocks (Chaudhry et al., 1997a).
The Lesser Himalayas division in NW Himalayas includes Hill Ranges (including Margala, Kala Chitta, Attock, Kohat Hills and Safed Koh), Plio-Pleistocene basins (Peshawar and Campbellpur basin) and the Lower Kohistan (Yeats and Lawerence, 1984). Lesser Himalayas are divisible into two distinct units, Northern metamorphic Zone and Southern Sedimentary Zone, and Panjal fault marks boundary between them (Ghazanfar, 1993; Ghazanfar et al., 1999a).
Northern Metamorphic Zone represents a terrain of meta pelite-psamite sequence which are intruded by Cambrian granites and some younger ones, and overlained by younger rocks including slates, phyllites, meta conglomerates, graphitic schist, limestone, marble, agglomeratic schist and basic volcanics of Kashmir basin and Peshawar basin. This meta pelite-psamite sequence is designated as Kundalshahi Group in Neelum Valley, Kaghan Group in Kaghan Valley and Tanawal Formation in Hazara area (Chaudhry et al., 1992; Ghazanfar and Chaudhry, 1985; Chaudhry and Ghazanfar, 1987; Ghazanfar et al., 1983). Kundalshahi Group is intruded by Jura granite of Cambrian age in southern parts and younger Neelum granite in north (Ghazanfar et al., 1983). Southern Sedimentary zone is characterized by the Pir Panjal ranges in east and Attock Hazara Fold and Thrust Belt, AHFTB, in west of Hazara Kashmir Syntaxis and south of Kashmir basin and Peshawar basin (Ghazanfar et al., 1999a). AHFTB is possible continuation of parautochthonous folded belt (Wadia, 1931) or Panjal Imbricate zone or Pir Panjal Range of Kashmir.
Sub Himalaya
Murree Fault or Main Boundary Thrust (MBT) and Salt Range Thrust (SRT) act as northern and southern boundaries of Sub-Himalaya respectively. The Himalayan foot hills form the Sub-Himalayan zone from the Punjab to Assam. These hills are comprised of a narrow belt of folded Neogene Murree and Siwalik red colored molasse type sediments.
The Sub Himalayas of Pakistan in the longitudnal sense can be subdivided into Azad Kashmir Zone in the east and the Punjab Zone or Punjab foreland in the west. In a transverse sense the Punjab foreland can be subdivided into North Potwar or Rawalpindi Zone (a fold and thrust belt culminating in the Khair-e-Murat structure and intensity of deformation increases northward), the Soan Zone (comprising a broad syncline under the plateau in the east and Kohat Basin in the west), the upthrusted inlier of Salt Range and Trans Indus Range and the only slightly deformed Punjab Plateform further to south (Ghazanfar and Chaudhry, 1999). Simply to say, Sub Himalayas subdivision includes Potwar Plateau, Kohat Plateau, Salt Range and Trans Indus Salt Ranges (Yeats and Lawerence, 1984). The Salt Range Fold and thrust belt gives way northward in the Potwar plateau to a broad syncline, the axis of which is followed by Soan River. This syncline is asymmetric and verges south further north. The intensity of folding increases and north dipping thrust fault appeared (Pinfold, 1980; Gill, 1953), culminating in the faults Kala Chitta and Margalla Hills along the Northern margin of Potwar Plateau.
Fig 4.2: Tectonic map of North Western Himalayas of Pakistan by Chaudhry et al (1992).
West of Potwar plateau, Eocene through Siwaliks strata are involved in complex folding, in which Eocene salt occupies the core of many of the anticlines (DeJong and Qayum, 1981). The Kohat plateau structure differs from that of Potwar plateau largely because of this higher salt detachment horizon in Kohat area (Yeats and Lawerence, 1984).
4.4 Tectonic Settings of Salt Range
Salt Range and Trans-Indus Salt Range represent the surface expression of up thrusting of a decollement in which basement is not involved and decollement exists in Cambrian to Eocambrian evaporites (Seeber and Armbuster, 1981; Crawford, 1974). Strata in the Salt Range generally dip northwards into the Potwar Basin (Sameeni, 1997). Structurally, the Salt Range is the result of tectonic forces imposed during the later phases of the Himalayan Orogeny in Late Cenozoic time, the occurrence of thick, incompetent Salt Range Formation at the base of sedimentary sequence has strongly influenced the structure). After the MBT zone under the Kala Chitta Range to the north was locked some 2 Ma ago compression was transferred to the plateform sequence to the south of MBT, as the result of which Salt Range thrust developed and Salt Range rises to surface (Yeats et al; 1984) .
Salt Range rises as 180 km (roughly ENE-WSW trending) long range at the southern edge of Potwar Plateau, making an abrupt escarpment against the Punjab plains in south. Its width is 150 km (Sameeni, 1997). The Salt Range forms an isolated E-W trending arc across the Indus River. It is a discrete structural zone bounded in north by north-dipping Main Boundary Thrust (Sarwar et al. 1979, Yeats et al. 1984, Coward et al. 1985), in south by the Salt Range Thrust, in west by Kalabagh fault and in East by Jhelum fault (Fig. 3.3). Tectonically, the Salt Range is the Himalayan equivalent of the Jura Mountains in the Alps and the Pine Mountains of the Appalachians (Qayyum, 1991). Salt Range is thin-skinned fold-and-thrust belt (Lillie et al., 1987; Butler et al., 1987 and Baker, 1987).
The allochthonous nature of the Salt Range, with a detachment in the Eocambrian Salt Range Formation, was recognized by many earlier workers (e. g., Wynne, 1878; Wadia, 1945 a, 1945 b, 1957; Gee, 1945, 1947, 1980; Voskresenskiy, 1978).
This range is essentially a complex salt anticlinorium with a series of salt anticlines. The structure along its northern slope is comprised of simple, broad, shallow folds followed by a gentle monocline. Southward the folding becomes tighter and the folds are commonly faulted (Gee, 1989). The Eocambrian evaporites are exposed in some of these overfolded and faulted anticlines (Qayyum, 1991).
Because the Salt Range formation is easily eroded, horsts form deep gorges, in which some of the classic stratigraphic section of Salt Range are found (Khewra, Nilawahan and Warchha gorges) (Yeats and Lawrence, 1984).
The overall structure of the Salt Range, except the easternmost part of the Eastern Salt Range, involves basically a fault bend fold geometry modified due to the presence of the underlying ductile salt. The roof sequence has been folded into sharp, salt cored anticlines and broad, flat synclines, which in its southernmost part frequently have been disrupted by forward and back verging thrusts and almost vertical dipping apparent normal faults (Qayyum, 1991).
Longitudinally, Salt Range is divided from East to West into the Eastern, Central and Western Salt Range. The Salt Range thrust is strongly emergent in its Central and Western parts, whereas in the east it is entirely a buried thrust (terminology of Morley, 1986) and folding predominates (Lillie et al., 1987). Davis & Engelder (1985) suggested that the difference in the structural style between East and West is the result of eastward thinning of the evaporites.
4.4.1 Central Salt Range
The surface geology across the Central Salt Range manifests the gradual exposure of older platform strata toward the South. The basement normal fault has acted as a buttress against which the salt has thickened. It also constitutes the lower part of the northern ramp and brings the older platform sequence to the surface. As the roof sequence rides over the northern ramp due to the basement normal fault, the strata become almost horizontal. The roof sequence is very gently folded into a series of small salt-cored anticlines near the leading edge. These anticlines are generally bounded by minor normal faults. Near the leading edge, there is a back thrust that brings Salt Range Formation over either the Jhelum Group or the Nilawahan group. This back thrust may have developed late in the geological history to build topography in order to maintain the forward motion of the roof sequence. The back thrust results in a total shortening of 27 km in the Central Salt Range (Qayyum, 1991).
Westward near to Sakesar peak, the range widens to some 30 km. In the north, the Eocene formations, capped in places by sandstone and clay-shales of the Early Miocene and overlain unconformably by Late Pliocene-Pleistocene Kalabagh Conglomerates, occur as broad, east and west plunging anticlines and synclines of varying wavelengths. The plunge variation results in large domes and basins e.g. between Dhariala and Kallar Kahar and westward via Vasnal and Khabakki in the approach to Sakesar (Gee, 1989). Several of these folds are faulted.
A concealed duplex structure is newly recognized in central Salt Range. It was evolved along a décollement with two ramps and extends more than 40 km along strike. The first ramp is within the Salt Range Formation. The second ramp crosses the platform sequence and follows shaley horizons of the overlying Murree Formation near the contact. The duplex structure is only present in the Central Salt Range, which implies that the décollement at its base must step down section and join the main décollement both in the west and the east. Further west along the duplex edge the platform sequence is flat and has not yet ramped upward. This implies that the décollement at the base of the duplex structure steps down along a Lateral ramp to join the main décollement (Qayyum, 1991).
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