The evolution of Central Asia - The Himalayas - When continents collide. In the scope of the geologic graduate seminar 2003 to the topic:

Transcription

1 The evolution of Central Asia - The Himalayas - When continents collide In the scope of the geologic graduate seminar 2003 to the topic: THE EVOLUTION OF CENTRAL ASIA THE HIMALAYAS - When continents collide Gurla Mandata 2003 June 24 TU Bergakademie Freiberg

2 The evolution of Central Asia - The Himalayas - When continents collide Content Introduction 3 1. Short chronology of convergence 3 2. Geology and major structures of the Himalayas 4 3. Island arc-continent collision in Western Himalaya A problem of subduction Chemenda s model Continent-continent collision in Central-to-Eastern Himalaya Conclusions References 14

3 Introduction In numerous regions of the Himalaya much fieldwork has been done till now and it is nothing new that the Himalaya is the most obvious result of the collision between India and Eurasia that occurred 50 Ma ago. Considering the detailed research in the fields of structural geology, petrology of rocks and physical modelling of plate convergence scientists have found out a lot of new facts concerning the detailed evolution of this mountain range. It is discussed that the western Himalaya is the result of an island-arc - continent collision. In contrast, the central and eastern regions represent an Andean type of orogenesis. The question is how these settings could evolve. In the following, I would like to give an overview about the geology and the collision scenarios in the Himalayas. 1. Short chronology of the convergence Marine magnetic anomalies of the Atlantic and Indian Ocean and paleomagnetic data in Asia make it possible to trace the northward movement of India relative to Asia since the late Upper Cretaceous. This drift has led to at least 2600±900 km of intra-continental convergence, and ca km of Thetyan oceanic lithosphere with a tremendous volume were subducted. Up to anomaly 22 (~50 Ma) the drift-speed was very fast, ca cm/year. Between anomaly 22 and 21 the behavior of convergence seems to be irregular which is attributed to the initial collision. From anomaly 22 to 18 (42 Ma) the velocity decreased to ca. 4 cm/year. This represents the estimated convergence between India and Eurasia since the collision. At the present time the shortening rate along the Himalaya is estimated to be 1.5 cm/year, representing a little less than the half of the postulated convergence between India and Eurasia. These three episodes of relative plate-convergence are interpreted from a dynamic point of view as follows: I. Subduction of Tethyan oceanic lithosphere with a pronounced slab-pull force on the Indian plate caused the rapid movement. II. Subduction of continental crust, after the continents of Asia and India had come into contact, led to a decreased relative movement. Subduction is hindered because buoyant crust had entered the subduction zone. 3

4 III. Intra-continental imbrication, thrusting and slow rates of lateral extension occur along the Himalayas. Fig. 2: Relative movement of India in respect to Eurasia over the last 70 Ma. The numbers on the northern margin of India show the age in Ma and its corresponding position. The frogs indicate a possible continental connection at 65 Ma. The smallest relative convergence occurred between 61 and 59 Ma, probably the time of initial collision, which is also indicated by a reduction of the driftspeed of India from 16 to 5 cm/a. Note, that the northward drift is coupled with a counterclock wise rotation of ca. 20. After PATRIAT & ACHACHE (1984). 5 cm/year 16 cm/year 2. Geology and major structures of the Himalayas The closing and subduction of the Tethyan Ocean, located between India and Asia during the late Paleozoic and Mesozoic, followed by the collision of these continents which produced the structures we see today in the Himalayas. Consequently, the mountains and surrounding regions are characterized by astounding complexity, represented by a variety of deformed and collisionproduced lithologies and representing several phases of tectonic and deformational events. The Himalayas can be divided into six lithotectonic zones that occur in parallel belts. These zones consist of the Trans-Himalayan batholith, the Indus-Tsangpo suture zone, the Tethyan (Tibetan) Himalaya, the Higher (Greater) Himalaya, the Lesser (Lower) Himalaya, and the Sub-Himalaya (see Fig. 1). The immense collision of plates at 45 million years gave rise to an Andean-type margin in central-to-eastern Himalayan regions (WINDLEY 1995). In contrast, the occurrence of the Kohistan island-arc complex and the Dras 4

5 volcanics are seen as remains of an island-arc-continent collision in the western Himalayas (BURG et al. 1998). Fig. 1: Simplified geologic map showing the main units of Himalayan mountain range. After Dèzes et al. (1999) 1. Trans-Himalayan Batholith The Trans-Himalaya zone is a linear plutonic complex. It is partly covered by fore-arc rocks and molasse. These assemblages are derived from uplift of the magmatic rocks and their subsequent erosion. The igneous complex consists of I- type lithologies, including gabbro, diorite, and granite. Formation of the complex is thought to have occurred in several phases between 110 and 40 million years. Partial melting of the subducting Neo-Tethyan slab beneath the Asian plate is thought to have resulted in these magmas (SORKHABI 1999). This zone varies in a west-east direction. To the west, plutons were emplaced into an area, called the Kohistan-Ladakh region, and represent an island-arc environment. In contrast to that, eastern igneous rocks represent an Andean-type environment (WINDLEY 1995). Western Trans-Himalaya An island-arc formed on the northern side of the Neo-Tethys and became trapped in between Asia and India. This allowed for two stages of deformation. The first consisted of collision of the arc with Eurasia, followed by collision of the amalgamated southern rim of Eurasia with India, while the Neo-Tethyan oceanic lithosphere continued to subduct under Eurasia (WINDLEY 1995). The area is 5

6 dominated by granodiorites and tonalites composed of a quartz+potassiumfeldspar+biotite+hornblende+sphene mineral assemblage. U-Pb in monazite and allunite dated the granitic rocks at 60.7 ± 0.04 million years. Late-stage bodies of pegmatite and leucogranitic dikes are also present. Pre-collision phases, consisting of felsic intrusions, have been dated by U-Pb in zircons and record a date of 101 ± 2 million years (SEARLE 1991). Eastern Trans-Himalaya The Kangdese sub-alkaline batholith extends along the north side of the Indus- Tsangpo Suture and represents an Andean-type continental arc. Pre-intrusive rock types in this zone include slate, phyllite, schist, gneiss, amphibolite, and migmatite of Ordovician to Cretaceous age (WINDLEY 1995). 2. Indus-Tsangpo Suture Zone The Indus-Tsangpo Suture Zone (ITZS) defines the areas of collision between the Indian plate and the Kohistan-Ladakh arc in the western Himalayas and the Tibetan Lhasa block which is the substrate on which the Kangdese (Gangdese) arc is built, in the east (WINDLEY 1995). It also marks the zone along which the Tethys Ocean was subducted. The ITSZ can be traced for more than 2000km (SEARLE 1991) between these regions and host a variety of rock types: Complete successions of ophiolites occur, some containing diamonds, suggesting high pressures during subduction and rapid exhumation along the suture zone. Glaucophane schists occur in narrow belts along the ITSZ in Pakistan. Olistoliths occur in north-western India and consist of reef and continental slope sediments in abyssal tubidite deposits. Mafic to felsic lavas as well as cherts, serpentinites, and dunites are also observed. Limestones and red sandstones are associated with Tethys Ocean sediments and found in the Ladakh region (WINDLEY 1995). The wide variety of rock types along the ITSZ further indicates a complex collision, affecting the intervening terrains. 3. Tethyan (Tibetan) Himalaya The Tethyan Himalayas are located to the south of the ITSZ. They consist of thick, 10-17km, marine sediments that were deposited on the continental shelf and slope of the Indian continent. This occurred as India was drifting northward but still located in the southern hemisphere (VERMA 1997). The sediments are 6

7 largely unmetamorphosed, which led to excellent preservation of fossils, and occur in foreland basins. Some however, have experienced greenschist-facies metamorphism (WINDLEY 1995). The large variety of organism s size and distribution of fauna suggest that life was flourishing in this area before the orogen. Such success in biological diversity is accounted for by the relatively stationary environment in the Tethyan Zone between mid-proterozoic and Eocene time. Episodic formation of land barriers enabled life to grow and diversify (SORKHABI 1999). 4. Higher (Greater) Himalayas The Higher Himalayas are also known as the Main Central Crystalline zone or sheet, are comprised of ductily deformed metamorphic rocks and mark the axis of orogenic uplift. Mica schist, quartzite, paragneiss, migmatite, and leucogranite bodies characterize this uppermost Himalayan zone. They represent a multiply deformed crystalline basement. The first being Barrovian type comprises a normal geothermal gradient. Later, there was a shift to Buchan-type metamorphism with low pressure and high temperature conditions (SORKHABI 1999). Local retrograde events have also been noted. The peak temperatures and pressures were at C and MPa. Deformation seems to have progressed in a north-south direction and is associated with the Main Central Thrust (MCT), which brings the Higher Himalayas on top of the Lesser Himalayas (SORKHABI 1999). Initially, it was thought that approximately 350km of shortening had occurred in the Greater Himalayan sequence of rocks. However, through studies by DeCelles et al. (1998), a major thrust fault within the zone was discovered. As a result, it is now estimated that between 600 and 650km of shortening occurred here. There was also a question concerning the provenance of the Great Himalayan rocks. Previous work suggested that lower Indian crust comprised this area. New interpretations of rocks there indicate that the Higher Himalayas are made of supercrustal rock. It means that upper crustal material of India accreted northward onto the Asian continent and that this crustal material was originally an appendage of India that, itself, was accreted to India during Paleozoic time. This idea implies that India probably had significantly more continental crust than previously thought, much more crust to be shortened in the formation of the Greater Himalayas. 7

8 5. Lesser (Lower) Himalayas The Lesser Himalayan zone is bounded by the Main Central Thrust (MCT) in the north and Main Boundary Thrust (MBT) to the south. Unlike the Higher Himalayas, the Lesser Himalayas experienced up to greenschist-facies metamorphism. It is related to the late Miocene thrusting of high-temperature metamorphic rocks of the Higher Himalaya over the cold footwall units of the Lower Himalaya. The rock types present in the Lesser Himalaya are also different. They are primarily sedimentary rocks from the Indian platform. The rock units show a series of anticlines and synclines which are often sheared. Fossils have been documented in this zone, but they do not occur at the same abundance as within the Tethyan zone. Main Central Fault (MCT) This thrust was first described by Heim and Gansser (1939) when they noted a contact between terrigenous carbonate rocks and thick overlying metamorphic rocks, mostly mica schists and gneiss (SINHA 1987), in the hanging wall. The Main Central Fault marks the boundary between the Higher and Lesser Himalaya. It is a wide-spread thrust fault, but it is in many places marked by a several kilometre thick zone of deformed rocks with varying characteristics in folding and imbrication (SORKHABI 1999). Mylonites or retrograde metamorphic assemblages also occur. The MCT is the actual suture between Gondwanaland (India) and the Proto-Tethys microcontinent to the north (SPIKANTIA 1987). Movement along the fault has brought crystalline rock from the Higher Himalayan zone on top of Lesser Paleozoic sediments; the former are preserved as klippen in synclines (WINDLEY 1995). These klippen are called the Outer Crystallines. These rocks, garnet and kyanite-bearing, were emplaced by slip along the MCT followed by uplift and erosion of 10 km of overlying rock (MOLNAR 1986). 6. Sub-Himalaya The foreland zone consists of clastic sediments that, produced by the uplift and subsequent erosion of the Himalayas, were deposited by rivers. These rocks have been folded and faulted to produce the Siwalik Hills that are at the foot of the main mountain range. Sub-Himalayan rocks have been overthrusted by the Lesser Himalayas along the Main Boundary Thrust. This steeply dipping thrust flattens towards north, developed during the Pliocene and was active throughout the Pleistocene (NI 1984). In turn, the Sub-Himalayas are bounded by a thrust to 8

9 the south and are forced over sediments on the Indian plate. This fault system is called the Himalayan (or Main) Frontal Thrust (MFT) (SORKHABI 1999). Another view of Himalayan zones is the following schematic cross section through the Indus-Tsangpo suture zone (Fig. 2, after Burg, 2002). It not only shows the geological divisions of the mountain belt, but also the main structures and rock types. It is effective in showing relative motion along faults. Fig. 2: Schematic cross section through the Indus Tsangpo Suture Zone showing the complex arrangement of thrust scheets and major fault systems. Modified after Burg, 2002) 3. Island-arc - continent collision in the Western Himalaya The northwestern Himalaya is a mountain range which is the result of a continent - island-arc collision. The Cretaceous Kohistan island-arc was formed on the northern edge of the Neo-Tethys by intraoceanic subduction. In the late Cretaceous, this arc collided along another N-directed subduction zone with Eurasia. The collision between the Indian continent and a Kohistan-arc - Eurasia assemblage occurred in the early Eocene. The western Himalayan mountain-belt represents a foreland-thrust-belt which shows active deformation along its southern end in the Salt Range and Potwar Plateau. 3.1 A problem of subduction Simple isostatic calculations show that a 100 kilometre thick oceanic lithosphere (7 km crust) with an island-arc can be subducted when the arc-crust has not until reached the maximal thickness of 8 km for granitic or 10 km for basaltic 9

10 magmatism (see Fig. 3, below). The question which comes to mind is how long has an arc to be active to increase its thickness, so that buoyancy prevents a subduction. For island-arcs with fast production rates of magma, 10 Ma seem to be enough. The upper limit amounts to 40 Ma for slow production rates. The last estimation seems to indicate the absolute maximum because the calculation does not consider the effect of heating caused by intruding plutons. Because of this effect the lithosphere would have a younger age due to the heat advection. Fig 3: Relationship between age of a volcanic arc, rate of arc-magmatism (per km length) and buoyancy. Modified after Cloos (1993). 3.2 Chemenda s model The structural evolution of the arc-continent collision in the western Himalaya can be discussed in analogy to the evolution of the southern Urals after CHEMENDA (1997) where there was a collision between the Magnitogorsk Islandarc and the East European Plateform. The development of this collision can be divided into five steps (see Fig. 4): a) Early subduction of the Indian continental margin successively increased the compressive stress in the upper plate. The continental subduction occurred in a constant way until the upper plate released along conjugated thrust systems on both sides of the arc. The most important, south-vergent thrust system would correlate with the ITS while the north-vergent system would appear along the northern suture, on which north directed thrust faults had been found. 10

11 b) As a consequence of the ongoing convergence, the north-directed subduction affected both the Indian continental crust and huge parts of the fore-arc region. The subduction of fore-arc lithosphere, representing relatively cold, oceanic lithosphere in addition to that of the cold Tethyan oceanic lithosphere heavily contributes to reach low-temperature gradients in the subduction zone and to generate blueschists. This seems also to be a good explanation for the precollision ages of the blueschists. c) The continental crust is subducted until its break-off is caused by increased buoyancy forces. A thrust is created in front of the suture. Fig. 4: Evolutionary model for the obduction and exhumation of HP/LT rocks in the southern Urals; 1 = continental crust (a, upper strong, brittle layer, and b, lower weak, ductile layer); 2 = lower Paleozoic and Riphean sediments; 3 = Silurian- Devonian flyschs; 4 = mantle layer of the oceanic lithosphere; 5 = oceanic crust; 6 = Magnitogorsk volcanic arc; 7 = thrust (a) and normal (b) faults; 8 = marker corresponding to depth of several kilometres (17-25 kbar) at the stage shown in (c). After CHEMENDA et al. (1997). d) The exhumation of the subducted continental margin happens due to its negative buoyancy. The ascending continental part surfaces further towards the Foreland than the material, subducted along the suture and containing ophiolites, and high-pressure/low-temperature rocks. This would explain the eclogites and blueschists along the ITS. The ascent of the continental part is accomodated by two main fault systems: one normal fault system at the top and one thrust fault system at the base. The upper normal fault system would correspond to the reactivation along the ITS with normal faulting. As a result of early thrust faulting, units of different origin and metamorphic grad lie closely together. The basal thrust system could contain thrust faults in the Indian Foreland (i.e. the Panjal thrust fault), which are contemporary with the extension within the suture 11

12 zone. Therefore, the normal faults along the ITS are features developed during exhumation driven by buoyancy during convergence. On the basis of geochronological data, a maximum age for the beginning of the thrust event can be defined between 73 and 40 Ma. A minimum age is delivered by fission track dating of rocks, which belong to uplifted continental part, and which cooled at 20 Ma. e) The ascending continental part extruded along the border between the upper and lower plate and so produced a wide, antiformal area. In Pakistan, this area would correspond to the large domes between the suture and the Dargai ophiolite sheet. 4. Continent-continent collision in Central-to-Eastern Himalaya The Himalaya-South-Tibet regions may be a young analogue to old continentcontinent collisions. The geology in South Tibet shows that the suture zone consists of different thrust nappes with distinct stratigraphy, deformation status and metamorphic grade. It provides evidence for the existence of an oceanic lithosphere (occurrence of ophiolites) and a passive and active continental margin (see Fig. 5). The angle and direction of subduction can be derived from (1) the emplacement direction of the ophiolites and the adjacent nappes, (2) the dip of the axial planes, foliations and folds (3) the location of the calc-alkaline magmatism along the active continental margin. The occurrence of high-grade metamorphic rocks on the surface is the result of tilting and displacement along the great thrust systems. Intracontinental thrusts create: (1) decreasing deformation to the top, (2) migration of deformation to the foreland and (3) deformation belts of different age showing the same structures if they result from an ongoing convergent compression. In contrary, differences in direction of deformation should be detected, if the geodynamic situation changes. A rapidly thickened crust collapses what leads together with erosion to the exhumation of formerly deeper rocks. During an obduction, the ophiolite klippen will be eroded nearly without leaving any features of the ancient oceanic material in the orogen. There is no evidence that there has been a phase of isobar heating between imbrication of a thrust 12

13 fault system and the beginning of erosion. A break in the deformation can lead to discontinuities, as it can be seen in the flysch, although this must not have been an orogenic break anyway, because the overthrust can migrate in the lower crust and further directed to the foreland. Calc-alkaline magmatic rocks are formed during the subduction phase. In contrast leucogranites were created during the imbrication of the thrust sheets. These rocks are typical for post-collisional crustal shortening and should to be found in all orogenic belts which have experienced a collision and overthrusting similar to the Himalayas. Fig. 5: Cretaceous to Miocene development of the collision zone for the Central- and East Himalaya showing the main thrust systems. After Burg (2002 b) 5. Conclusions 13

14 The Himalaya is the youngest mountain belt formed by collision on earth and therefore it must be our analogue to earlier continental collisions. Similar structures can be expected at many locations along the suture between Eurasia and the continents and mirco-terrains derived from Gondwana. The driving force which is necessary to create and to develop such configurations of overthrusting, gives evidence about a process in crustal scale. Nevertheless, the origin of the referring shear zones can possibly be related with the subduction zone itself. The evolution of the overthrust system leads to a distribution of the inner thrust fault planes caused by the increase of the footwall crystalline sheets. The vertical inner thrust faults and the suture are reactivated as strike-slip faults, as it can be seen in South Tibet. These observations should be considered when interpreting deep underground structures of old and heavily eroded orogens. These structures illustrate that deep parts of the crust have different behaviour than higher ones. References BURG et al. (1998):, Terra Nova 10(2), pp BURG, J.-P. (2002 a): West-Himalaya: Inselbogen-Kontinent Kollision, Vorlesungsscript WS02/03 BURG, J.P. (2002 b): Himalaya-Süd-Tibet: ein typisches Kontinent-Kontinent Kollisions-Orogen, Vorlesungsscript WS02/03 CHEMENDA, A. I., MATTE, P. & SOKOLOV, L. (1997): A model of Paleozoic obduction and exhumation of high-pressure/low-temperature rocks in the Southern Urals, Tectonophysics 276, pp CLOOS, M. (1993): Lithospheric buoyancy and collisional orogenesis: subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts, Geologic Society of America Bulletin, 105, pp DECELLES, P. G., GEHRELS, G. E., QUADE, J., OJHA, T. P., KAPP, P. A., AND UPRETI, B. N. (1998): Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal, Geological Society of America Bulletin 110, pp DÈZES, P. (1999): Tectonic and Metamorphic Evolution of the Central Himalayan Domain in Southeast Zanskar (Kashmir, India), Mémoires de Géologie (Lausanne), No. 32, 160 p. 14

15 HEIM, A., AND GANSSER, A., (1939): The Central Himalayas - Geological observations of the Swiss Expedition of 1936: Memoires de la Societe Helvetique des Sciences Naturalles, v. 73, 245p. HODGES, K. V. 2000: Tectonics of the Himalaya and southern Tibet from two perspectives, Geological Society of America Bulletin 112 (3), pp MOLNAR, P. (1986): The Geologic History and the Structure of the Himalayas, American Scientist 74, pp Mount Everest, digital image, everest.jpg, from NI, J. and BARAZANGI, M. (1984): Seismotectonics of the Himalayan Collision Zone: Geometry of the Underthrusting Indian Plate beneath the Himalaya, JGR, v.89, no.b2, pp PATRIAT, P. & ACHACHE, J. (1984): India-Eurasia collision chronology and its implications for crustal shortening and driving mechanisms of plates, Nature, 311, pp SEARLE (1991): Geology and tectonics of the Karakoram Mountains. Chichester, U.K., John Wiley and Sons, 358 p. SINHA, A.K. (1987): Tectonic zonation of the Central Himalayas and the Crustal evolution of collision and compressional belts, Tectonophysics 134, pp SORKHABI, R.B., and MACFARLANE, A. (1999): Himalaya and Tibet: Mountain roots to mountain tops, in Macfarlane, A., Sorkhabi, R.B, and Quade, J., eds., Himalaya and Tibet: Mountain Roots to Mountain Tops, Geol. Survey of America Special Paper 328, 1-8. SPIKANTIA, S.V. (1987): Himalaya - the collided orogen: a plate tectonic evolution on geological evidences, Tectonophysics 134, pp VERMA, R.K. (1997): Paleomagnetism from Parts of Tethys Himalaya, Indus Suture Zone, Ladakh and South Tibet: Implications for Collision between Indian and Eurasian Plate, Himalayan Geology, v. 18, pp WILSON, L. & WILSON B.: Homepage under WINDLEY, B.F. (1995): The Evolving Continents, 3rd Edition, John Wiley & Sons: Chichester, 526p. 15