Geology of Pakistan
The geology of Pakistan is a frighteningly complex matter, but in a sense geology is the ‘science of the subcontinent’. A basic under –standing of the grand processes of continental formation reveals much about Pakistan’s landscape.
The mountain chain comprised of the, Hindu Kush Pamir Karakoram, and Himalayan Ranges was born in stupendous ultra-slow-motion collision between India and Asia. This collision started about 50 million year ago, and still going on. According to the theory of plate tectonics, the earth’s crust is made up of continent-sized slabs of rock (plates), afloat on a more fluid layer (mantel). As a result of currents and up wellings within the mantle, the plates move around, bump into each other, break up and reform.
About 130 million years ago, when dinosaurs still roamed the earth, the ‘Indian Plate’ broke away from a primordial super- continent that geologists call Gondwanaland (ancestor of Africa, Australia and Antarctica as well) and drifted north at a geologically breathtaking 15 to 20 cm per year toward another landmass called Laurasia, the ‘Asian Plate’.
Between the converging continents lay a wide, shallow ocean called Tethys. Beneath this ocean a separate plate, pushed along ahead of India, slid beneath Asia. Melting as it sank, it generated a front of volcanic activity from Afghanistan into eastern Tibet and formed a chain of volcanic islands (similar to Present-day Indonesia or Japan) off the shore of Laurasia.
Some 80 million years after breaking away from Gondwanaland, India collided with Asia. The small plate supporting the offshore island was flipped on its side and trapped between them, and the Ocean of Tethys disappeared. Today India is still Ploughing north ward, and the ‘wreckage’ in the collision zone keeps piling up, in some cases faster than erosion can wear it down. Pakistan lies right across part of this collision zone.
While dense ‘oceanic’ plates like the one beneath the Tethys Ocean tend to sink in plate encounters, lighter ‘continental’ plates float high. In the India-Asia collision neither gave way and there was much buckling, crumpling and thickening – and the thickening, in a nutshell, is what made the mountains.
While some continental ‘crust’ (eg in Tibet) thickens in a homogeneous way under pressure –imagine pushing two blocks of soft clay together –this collision proceeded in a way that is characteristic of more brittle material, rather like trying to drive two grate stones slowly together. As India pushed northward under Asia, great slabs and slices of crust were sheared off the top of it and stacked atop one another. The finer details of this process, called ‘thrusting’, can shed light on the sometimes peculiar profiles of these mountain chains. A ‘fault’ is a relatively smooth surface along which rock can break under competing force, with material on each moving in opposite directions (vertical ‘strike-slip’ faults are the ones you see in news photos showing how an earthquake displaced one end of a road from the other). A ‘thrust’ is an almost-horizontal fault, but dipping slightly so that along its surface originally-deeper material can be shoved right up and over once-shallower material. This is the kind that has given Pakistan mountain face.
Since the India-Asia collision began, India has continued to move north at about five cm per year. Over 50 million years that works out to a shortening of 2500 horizontal km. Much of this shortening has been accommodated by deformation within Tibet, and some by India having slid beneath Asia. Within the Himalaya the shortening has been by thrusting, with slabs of northern rock up to 10 km thick being bulldozed over on top of southern rock, along several roughly parallel fronts. The fronts correspond roughly wit the ramparts of the Himalaya and the Karakoram.
Figure 1 is a simplified map of the entire collision zone. The so-called-Tsangpo suture is the main boundary between India and Asia rocks –roughly marked to day by the upper Indus in Pakistan and Ladakh and the Tsangpo River in Tibet. The same continental collision has generated other types of ‘squashing’ beyond the Pamirs and in Tibet, with blocks of crust being squeezed out laterally along major strike- slip faults.
Geology of Northern Pakistan
Figure 2 is a cross-section of the mountains north from Islamabad into the Pamirs of Tajikistan, showing the effects of thrusting, especially in the India plate. The Kohistan-Ladakh Islamabad Arc or Kohistan arc is the remnants of the up-ended plate bearing the offshore volcanic island china.
The pre-collision India plats consisted of a ‘basement’ of extremely old (more than1000-million-year-old) rock, overlain by much younger limestones and sandstones. On impact the India plate nudged beneath the Kohistan Arc, and under intense temperature and pressure deep in the earth, some of its material began to melt and recrystallize into new rocks. Thrusting subsequently dragged this so-called ‘metamorphic’ rock back up and stacked it on top of the unmetamorphosed limestones and sandstones, forming the earliest mountains.
Figure 3 is a geological map of Pakistan. The first thrust north of the Punjab plains coincides with the abruptly rising Salt Range . the next major thrust (the Main Boundary Trust in the figure) runs from south of Kohat in the NWFP though the Murree and Margalla Hills north of Islamabad. Between these two – eg along the Grand Trunk Road between Taxila and Jhelum – red rock outcrops indicate 20 million-year-old sandstones and shales, originally eroded from the embryo Himalaya yan China but now being pushed out onto the India Plate ‘basement’.
Other thrusts are associated with steep mountain fronts like those at the Malakand Pass or on the Havelian-Abbottabad road. As you go north, the exposed rocks have been pushed up from deeper and deeper; note the non-metamorphic and then metamorphic India-Plate rocks. The line between India Plate and Kohistan Arc (called the Main Mantle Thrust) is not a distinct chain of hills but does pass through the rugged terrain of southern Swat, the Kaghan Valley and Northern Kashmir.
North of this is the Kohistan arc, with its original ‘underside’ exposed in the south (eg around Pattan and Chilas on the Karakoram Highway, and in south-western Baltistan) and old seabed sediments and volcanic mate-rials to the north (eg in upper Swat, around Gilgit and Skardu). The Shyok Suture Zone is where it joins the Asian Plate.
The steep peaks and deep glacier valleys of the Karakoram are the result of geologically recent (ie within the last five million years) thrusting of Asian Plate rock south ward onto Kohistan.
An unmistakable feature of northern Pakistan is massive Nanga Parbat, 8126 meters high and rising by seven mm every year, faster than almost any other part of the Himalayan chain. Here the sharpest elevation differences found anywhere onearth:6½ vertical km from the summit into the adjacent Indus gorge, and the mountain’s sheer, unbroken 4000-metre south wall. The Nanga Parbat massif represents a mass of Indian-Plate rock domed up from beneath the overlying Kohistan Arc. Its position and growth are still matters of active research, but in effect it’s being squeezed upward in a Pincer movement between the south-moving thrusts of northern Pakistan and the south-west-moving thrusts of Kashmiri India.
Geology of Western Pakistan
The western edge of the India Plate (Baluchistan) didn’t collide with Afghanistan until well after the main crash was underway – probably within the last 10 million years. At about 65 million years ago some oceanic plate material, known as an ophiolite, worked its way into the Indian Plate. In the Afghan –Baluchistan collision this oceanic crust, plus limestones overlaying the old Indian-Plate ‘basement’, plus more recent marine sediments, sandstones and shales-known collectively as ‘flysch’ –were buckled up into what as now the Kirthar Range in eastern Baluchistan and the Suleiman Range of the western NWFP.
Most of Pakistan’s oil, natural gas and coal deposits are located in the Potwar and Kohat Plateaux and east of the Kirthar and Suleiman ranges. They lie in the pre-collisional sandstones and limestones overlying the basement, which are in turn now overlain by sediment eroded off the mountain fronts.
The Arabian Sea floor is also part of the Indian Plate; as it shoves northward under Baluchistan, flysch deposits continue to be scraped off it, accumulating in southern Baluchistan and gradually building the Makran coast outwards and southwards.
Other Evidence of Geological Activity
Insistent reminders of the strains of mountain-building are the earthquakes that constantly jar the mountains. The worst one in recent times struck Indus Kohistan in December 1976, killing many thousands. Sizeable quakes also struck along the northern Afghanistan-Pakistan border in February 1991 and May 1992.
Especially active is the so-called Chaman Fault near Quetta – probably the source of a quake that nearly obliterated that city in May 1935 and killed about 20,000 people. There is also volcanic activity in the Chagai area of Baluchistan. The 2510 –meter Koh-i-Sultan is part of a china of volcanoes reaching into Afghanistan and southern Iran, resulting from melting of the Arabian Sea floor as it is driven under Baluchistan.
A happier consequence of geological activity is the high frequency of hot springs in the mountains. Relatively recent fractures permit easy upward movement of underground water that has been in contact with hot rock deep in the crust.
This section was written with the very essential help of Dr Peter J Treloar of Kingston University, Surrey, UK.