The Pangaea Theory Essay
The Pangaea Theory
The Pangea theory describes that all continents were joined together in one enormous land mass millions of years ago. Later on the continents broke apart and start drifting in opposite directions and still continued to make another arrangement. In 1912, Alfred Wegner, a German meteorologist and geologist gave the hypothesis the all the continents were joined together in a single continental land mass surrounded by a single ocean (Panthalassa) Late Paleozoic times. The Wegner used the term Kontinentalverschiebung for the breakup and displacement of crustal blocks. Pangea situated around where Antarctica is now presently.
During Jurassic Period the Pangaea started to break up into smaller units called Laurasia and Gondwanaland. In late Cretaceous period, the continents were further separated and transformed as present day continents (William Lowrie, 2007). Figure-1 shows the reconstruction of Wegener’s continental mass using paleo-climatic data from Carboniferous, Permian, Eocene and to Quarternary.
Figure-1 (a) Pangaea reconstruction by Wegener in Late Carboniferous time (b) continents in Eocene times (c) continents in Early Quaternary, where K, S, W, E refers coal, salt, desert areas, ice sheets respectively (after William Lowrie, 2007).
Evolution of Himalayas
The Himalaya holds very important geological and tectonic history. The arc of Himalayan belt is about 2500 km from northwest to southeast. It comprises of well-known famous peaks like Nanga Parbat, Evereast and Namche Barwa etc. The Himalayan ranges hold a huge concentration of lithospheric mass comprising Precambrian to Recent sediments.
The contnent to continent collision of Indian and Asian plates is considered as the grave reason for the origion and development of Himalaya. This collision occurred in last 100 Ma yielding the uplifting of Himalayan chain of mountains. This lead the closing of Tethyian sea during 60-50 Ma. The over trust sheets and formation of nappe and klipps was generated as a result of crustal shortening.
The uplift resulted huge erosion and deposition phases in the Arabian sea and Bay of Bengal. The subduction process is still under continuation causing earthquakes and tsunamis in the entire region (Anshu Kumar Sinha, August 2008),(An Yin et. al, May 2000) During cretaceous age the indian plate started it journey of collision with Eurasian plate. The subduction of the Indian plate occurred under Eurasia making Tibetan crust with huge thickness (Figure-2). The Tsangpo Indus suture zone was formed in western while MKT/ MMT zones were created. Several regional faults like main continental and boundary thrusts as well as salt range thrusts were generated (Klootwijk et al., 1992). Figure-3 show the distribution of major thrust faults associated with the indianeurasian plate collision Hamalyian orogeny.
Figure-2, The subduction of Indian plate under Eurasian plate (after Klootwijk et al., 1992).
Figure-3, Regional thrust faults distribution due to the collision of Indian plate (after Harald Drewes, 1995)
Tectonics of Sulaiman & Kirthar Range
Sulaiman Range The Sulaiman & Kirthar ranges comprises of about 1250 km long and 75 to 180 km wide zone. The zone is highly complex structurally (Figure-4). There are regional thrust belts in northern and southern side of these ranges. On the eastern and southern side of these belts the successions are highly folded. The intensity of folding diminishes as we move more eastward and southward. A huge foredeep zone containing 10 km thick Jurassic to Recent sediment is present in front of these ranges in the eastern and southern direction (A. H. Kazmi et al., 1997).
Sulaiman & Kirthar ranges comprises of various tectonos tratigraphic zones. The fold belt contains arc shaped zone is east-west direction. There is exposure of Jurassic to recent strata having a regional unconformity at the base of Dungan Limestone of Palaeocene age having Biabi volcanics a protolith.
The folding style in Sulaiman fold belt area are enechelon, parallel/ sub parallel or open which becomes tighter as we move northward. The northern and central part of this belt holds the steep and huge thrust faulting effects including NW and NNW trending strike slip faults originating syntaxis. The lobe shape of Sulaiman fold is believed to be the result of presence of weak basal decollement composed of pilitic rocks or fine carbonates at 14 km deep above the crystalline basement which is supported by tear faults. The northward zone also comprises tight detachment folds as well as forward propagating anticlines with duplex geometry in piggy-back style while in south eastern side broad and dome shaped wider anticlines are present.
The Sibi trough exist between Sulaiman and Kirthar fold belts. The trough contains 15 km thick sequences of Triassic to Recent sedimentary rocks above the basement. The thickness of Siwalik molasses in Sibi trough is 7000 meters (A. H. Kazmi et al., 1997).
Kirthar Range The Kirthar Fold Belt is 330 km long and 50 to 70 km wide. The belt runs between Quetta and Karachi from north-south. In north and western side Bela-Zhobophiolite are present while on eastern side the Sibi trough, Kirthar foredeep and Indus platform are present (Figure-4). Jurassic to Recent sedimentary rocks are present in the Kirthar Fold Belt. Smaller structural units appear in the belt by differences in tectonic style and variations in stratigraphic successions (A. H. Kazmi et al., 1997).
Kalat anticlinorium It lies in northern-most part of this belt contains massive Jurassic carbonates and comprises parallel or en echelon folds. Tallest peaks of more than 3000 meters are present in the area. The doubly plunging broad anticlines with steep limbs and flatter crests are common which are cut by normal and reverse faults. Wider synclinal valleys covered with Cretaceous and Paleogene shales and limestones are present.
Kalat Plateau: The plateau is situated in the south of Kalat anticlinorium. This plateau is a depression or downthrown block that is containing Kirthar Limestone of Eocene age. This plateau is a gently undulating synclinorium and holds small and gentle folds having tear and reverse faults. Khuzdar knot: This area is having irregular geometry of structural feature. The zone has gone through intense deformation that lessens in southward. The massive Jurassic limestone here is forming tightly folded
anticlines that are separated by tight valleys of irregular geometry. The trend of anticlinal axes is dancing in almost every direction. The folds in eastern side are larger & broader.
Khude Range Fold Belt The range consists of Paleogene sediments comprising thrust faults that are dipping away from the range. Narrow elongated and en echelon anticlines are present with Jurassic-Cretaceous exposed in the core.
Nagau-KirtharFold Belt This belt comprises tight, en echelon, subparallel and doubly plunging folds characterized by reverse and tear faults. On the eastern side of the belt a lot of small popup structures are present due to presence of Kirthar Limestones. The passive roof duplex models are observed in this area.
Karachi Embayment Zone This zone is characterized as a synclinorium and contains Miocene to Recent sediments forming SSW trending anticlines.
Sanbakh-Lakhrauplift zone The zone lies in the east of Karachi embayment. Paleogene and Neogene sedimentary rocks have filled this area. The zone holds a lot of unconformities and comprises of broad doubly plunging, gentle northsouth trending folds that are cut through by numerous reverse and normal faults. The unconformities depicts that this zone was a structural hight in the paleotimes.
KakarKhorasanBasin Kakar Khorasan is referred as Flysch basin. This basin is present in the north of Zhob ophiolite and thrust belt. The basin is comprised of flysch sediments. The deltaic and molasse sediments are also present in the basin. It is assumed that the oceanic crust related to Indian plate has subducted beneath Afghan block, this analysis is based on the gravity surveys which depicts that the oceanic crust beneath the Afgnan block is getting thinner causing Afghan block about 57 km thick. Nisai Limestone of Eocene age as well as Khojak Flysch sediments of Oligocene to Miocene age are exposed in the basin which are underlain by Pliocene sediments or younger molasse. Broad to tight anticlines and synclines associated with the reverse faults are present in this area (A. H. Kazmi et al., 1997).
Figure-4, Geological map of a part of Sulaiman-Kirthar Fold Belts (after A. H. Kazmi et al., 1997).
Geology & Stratigraphy of Salt Range
Salt range is composed of very interesting geology and tectonics. Tectonically the salt range is the result of Himalayan orogeny. The salt range thrust is the key tectonic feature that controls the whole configuration of these ranges. Various syntaxes were created due to the movements along vertical axes. Hazarasyntaxis (Jehlum fault) and Indus River syntazxis (Kalabagh fault) are characterized by lateral movements are present in East and west of the salt range respectively (Figure-5) which are the result of adjustment of major thrust plate around the subducting one. The area mainly comprises of thrusted salt cored anticlines and popups where salt is acting as decollement surface (A. H. Kazmi et al., 1997).
Figure-5, Tectonic map for salt range (after Harald Drewes, 1995)
The salt range comprises of Infra-Cambrian evaporates deposits which are under the coverage of EoCambrian package. The absence of Middle Cambrian to Early Permian rocks depicts regional unconformity in salt range.
In Nammal, Chichli & Nilawahan gorges of western salt range the younger while in western side at Choa Sayden Shah the older strata is exposed. The salt range area contains molasse deposits of Miocene to Pliocene age which are the result of Himayalian erosion. These recent sediments cover the thick EoCambrian package that overlies evaporite deposits (A. H. Kazmi et al., 1997).
Mesozoic and earlier rock rocks exposures are exposed also in Surghar and Khisor ranges. The dolomites, shales and sandstones of Jehlum Group are unconformably overlain by a thick succession of carbonates and clastics of Nilawahan and Zaluch groups. Talchir boulder beds are the indications of a regional angular unconformity at the base of Permian. The exposed Eo-Cambrian formations are about 550m thick in Eastern salt range while the Permian sequence is about 700m thick in western Salt range. Figure-6 is a schematic cross section of Salt, Surghar and Khisor range showing the stratigraphic distribution of formations.
The area is devoid of Ordovician to Carboniferous strata depicting a regional unconformity. In western salt range Permian to recent sediments are present uncomfortably over the Cambrian succession. On the contrary in the eastern salt range the Cambrian package is quite preserved and also do not contain Ordovician to Carboniferous successions. There is an abrupt change of stratigraphic successions that just above the Cambrian the Oligocene to recent sediments. An average thickness of continental molasses sediments is about 8000 meters overlying the Eo-Cambrian packages. The Nilawahan group of Lower Permian age acts as a regional unconformity in the entire area (A. H. Kazmi et al., 1997).
Figure-6, Stratigraphic distribution of various successions in Khisor, Surghar and Salt range (after A. H. Kazmi et al., 1997).
Tobra formation’s Talchir Boulder bed is a result of a global glacial event; the Bain Boulder bed of Marwat formation of Pliestocene age in Bhittani and Shinghar ranges is result of a single catastrophic flood event comprising volcanic debris flow.
The salt range area contains highly fossiliferous stratigraphic sequences. Permian sequences contains wide range of fossils, the lower part contains cold while its upper part has water fauna and flora. Lower Eocene’s Kuldana Formation contains vertebrate fauna of whales. The successions in Cretaceous age contain vertebrates like rodents, artiodactyls, bounodonts, anthracobunid and proboscideans. The Siwaliks on the other hand contains mammalian fossils (A. H. Kazmi et al., 1997).