TECTONICS OF PAKISTAN

Tectonics play an important, rather a basic role in the configuration of basins.As an example this can particularly be seen in the case of Pakistan, where the end product of the tectonics play produced diastrophism of unprecedented order and unique in style showing basins of alien origin converge and illustrated with text book tectonic structures; past life of the far flung provinces tends to one point,showing spectacular crustal shortening of hundreds of kilometres of the lithosphere.
          The geological setting of Pakistan, in the framework of the modem concept of Plate Tectonics, is rare and matchless in the world. It is in the sense that within an area of about 800000 ㎢ critical tectonic junctions of different interacting plates and micro-plates are present in a unique environment, where field exposures are excellent and the accessibility is easy. Two types of active plate boundaries are distinguished (Abul Farah written communication 1985).
Convergent Boundaries:Characterized by continent-continent collision, obduction and thrusting in the Himalayan region and by oceanic crust subduction with a volcanic arc and wide trench gap and thrusting in the southern region of Chagai arc and Makran margin. About 6 km thick sequence on the floor of the Arabian Sea is largely carried along with the down going plate when it enters the Makran trench and only about the upper third part is beingscrapedoff to form an accretionary prism. Transform boundary of Chaman Transform Zone characterized by mega strike-slip and obduction. The Chaman Transform Zone connects the Makran convergence zone, where oceanic lithosphere is being subducted beneath the Lut and Afghan microplates, and the Himalayan convergence zone, where the Indian lithosphere is under-thrusting Eurasia.

The available geological and geophysical evidence of the development and demeanour of these plate boundaries lead to the conclusion that in Pakistan, Atlantic type (Spreading) plate boundaries have been present since the Early Mesozoic until the Late Cretaceous and Indian type (subduction, collision, obduction, convergence) boundaries are present with the Transform zone (Early Eocene-Oligocene?)connecting the Makran and Himalayan zones. Many models elucidating the evolution of the tectonic framework with enormous details are available in the published literature but to summarize or even to outline the tectonics of Pakistan is neither possible nonconvenient. In fact, dealing with the details of tectonics of Pakistan is beyond the scope of this book. Only the gist as applied to the emergence of sedimentary basins of Pakistan is
presented here. Beginning with Late Palaeozoic and throughout Mesozoic, the north-western part of the Indianplate,.continued to be occupied by the great Tethys Sea, as a result, this region had been the site
of geosynclines/ epicontinental sea where sediments accumulation took place since Permian to Late Cretaceous and the rest of Indian Plate, probably, remained an island throughout its northwards drift. On the withdrawal of the bulk of Tethyan Sea, the area represents the site of epicontinental remnant seas during Paleooene and Eocene times. The continued northward movement of the Indo-Pakistan plate ultimately resulted in collision with Eurasian plate in Late Palcocene-Eocene times.

The drift in the Gondwana fragments is believed to have begun in Jurassic; during which fast northward movement of the Indian plate during Cretaceous followed by its counter clockwise rotation, formed a part of collision process. Transform Zone (Chaman rift) resulted following renewed spreading in Indian Ocean during post collisional northward movement of the Indian plate.
The resultant conpressional forces produced some of the syntaxes, arcs and oroelines throughout Pakistan, probably in Early Oligocene.The structure, so produced, had probably been earlier influenced by pre-collisional tectonicelements in the miogeosyncline and the shield areas of the Indian plate. The pre collisional tectonic elements include the development of welt like structures probably within geanticlinal areas in the western part of Indian plate, which was probably initiated in Jurassic and were fully emerged in
Cretaceous. These narrow, but sometimes, sharp ridges and belts along with their associated intrusives and volcanic emplacements have been compared to island arcs by Hunting Survey Corporation (1961) and with the worldwide oceanic ridges in their origin(Zuberi and Dubois, 1963). The most significant among these strips is named as the Axial Belt, which divided the geosynclines and initiated the establishmentof the two great basinsofPakistan.The belt developed marginal to the western part of the Indian shield and now to its east lies the Indus Basin and to its west the
Balochistan Basin and to the north great tectonic zone of the Tectonostratigraphic Basins and
Tectonostratigraphic Ranges, where each range or chain of mountains and associated depression
indicate a unique tectonostratigraphy(Fig.1).

In the light of the above mentioned brief tectonic history and events, the earlier basin map, Shah (1977), under which the stratigraphy of Pakistan was described, lacked the tectonic details of the northern areas of Pakistan; with the advancement in the geological knowledge of the northern areas of Pakistan, the old basin map has now been revised (Fig. 1.) It may be added; that the new map of sedimentary basins presented here is broadly compatible with the “regional map of tectonic framework” of Shah and Quennell (1980) (Fig. 2).

The revised map of basins presenting the architectonic of Pakistan has twofold usefulness. It provides convenience for the description of the stratigraphy of Pakistan and it is suitable for correlation of the formations with the regional countries.

A. Southern Stratigraphic Basins and Belt:
I. IndusBasin
Kohat-Potwar Province
Sulaiman-Kirthar Province

II. Axial Belt
Southern Sector(Arabian Sea to KurramAgency)
b. Northern Sector(KurramAgency to HazaraArea)
III. Balochistan Basin
a.Chagai-Raskoh Province
b.MakranAccretionary Province

B. Northern Tectonostratigraphic Basin and Range Terrene’.
IV. HimalayanTcctonostratigraphic Basin
HigherHimalaya
b. Lesser Himalaya
V. Kohistan IslandArc
VI. Karakoram-Hindukush Tcctonostratigraphic Basin
NorthernSedimentary Province
b. KarakoramAxial Batholith Belt
Southern Metamorphic Province