The India–Asia collision zone
As the Alpine–Himalayan mountain belt is followed eastwards from Afghanistan into Pakistan and Tajikistan, there is a marked increase in scale compared to the ranges further west. The belt narrows at the eastern end of the Hindu Kush, as the ranges of the Chaman Fault Zone and the Sulaiman belt merge with it (see Figs 7.1, 7.8). Here the mountain belt is less than 350km across but the central ridge is over 6000m high and culminates in the peak of Tirich Mir, at 7690m. Further east, the belt divides again into a southern branch, consisting of the Karakoram and Himalayan ranges, which curve around the northern fringe of the Indian sub-continent, and a northern branch, which strikes north-eastwards as the Tian Shan, forming the border between Kyrgyzstan to the north and Xinjiang Province of China to the south (Figs 8.1, 8.2). Between the two is the vast expanse of the Tibetan Plateau and the Tarim Basin.
The Karakoram–Himalayan Belt
The Karakoram–Himalayan Belt is usually considered to be the ‘type example’ of a collisional orogenic belt, but in reality it is unique, as none of the other contemporary orogenic belts are as complete: all the others involve subduction along some or all of their lengths, whereas in the case of the India–Asia collision zone, the continental plates are in contact along the whole length of the belt, all the oceanic plate that formerly intervened having been subducted. The Himalayan sector itself is over 2500km long and is draped around the northern perimeter of the Indian sub-continent, which projects into it at its western and eastern ends. It contains several of the world’s highest mountains, including Mount Everest, and is bordered to the north by the high Tibetan Plateau (Fig. 8.3). The western (Pakistan) sector is known as the Karakoram.
Figure 8.1 Asia from Space. The curved line of the Himalayas is outlined by the green forested zone and is bounded to the north by the Tibetan Plateau. Note the oval shape of the Tarim Basin and the curve of the mountain ranges around the northeast corner of India, extending down through Burma to the Malaysian Peninsula. © Shutterstock, by VanHart.
Figure 8.2 Main tectonic features of the Himalayan Belt. Main faults: ATF, Altyn Tagh Fault; KF, Karakoram Fault; RRF, Red River Fault; TFF, Talas–Fergana Fault. Sutures: BNS, Bangong–Nujiang Suture; JS, Jinsha Suture; ITS, Indus–Tsangpo Suture. Tectonic zones: CPB, Chin–Paktai Belt; CT, Chanthaburi Terrane; ICT, Indo-China Terrane; SCT, S. China Terrane; ST, Sibumasu terrane. States, etc: Ban, Bangladesh; J&K, Jammu & Kashmir; Kaz, Kazakhstan; Kyrgyz, Kyrgyzstan; Taj, Tajikistan; Uz, Uzbekistan, Cities: Bish, Bishkek; Dh, Dhaka; K, Kathmandu; Ka, Kabul; La, Lanzhou; Tas, Tashkent. Mountains: Kong, Kongur Shan; MtE, Mt. Everest; NP, Nanga Parbat; Po, Pobeda Peak; TM, Tirich Mir. After Molnar & Tapponnier, 1975; and Searle et al., 2006.
Figure 8.3 The Himalayas. Aerial view looking south from the brown, arid Tibetan Plateau. Mount Everest is in the centre of the view, the black north face prominent. NASA image.
The northern margin of the Indian Plate is shaped like a southerly convex arc, with two prongs at either end, jutting into Asia. The more westerly prong has pushed into northern Pakistan and Tajikistan, forcing the uplift of the mountains of Jammu and Kashmir and the arc of the Pamir Range. The belt here is 750km across from north to south and includes many peaks over 6000m in height, including Nanga Parbat at 8126m. From here, the Karakoram Range extends south-eastwards, at the western end of the Himalayas. The Karakoram Range includes the famous mountain known as K2 which, at 8611m, is the world’s second highest peak. Further east, in the Nepal Himalaya, is Everest itself (8848m). The mountain belt here (Fig. 8.3) is narrow, only about 200km wide, and forms the southern fringe of the Tibetan Plateau, which has a mean elevation of over 5000m.
At the eastern end of the Himalayan Range, in the northeast corner of India, the mountain belt turns abruptly through 180°, around the valley of the Brahmaputra River, and divides into several branches, lower and less impressive than the Himalayas, which run roughly southwards through Burma. The pattern of mountain ranges here clearly reflects the shape of the northeastern prong of the Indian Plate.
Plate-tectonic context
The shape of the orogenic belt is dictated largely by the outline of the northern margin of the continental part of the Indian Plate, which is thought of as a relatively rigid ‘indenter’ moulding the southern margin of the less rigid, more deformable, Asian continent. This model was proposed in 1975 in an influential paper by Peter Molnar and Paul Tapponnier, who interpreted a number of recent or active structures within the Asian continent, many of them located far from the plate boundary, as the result of the collision. The basic idea is reminiscent of the mobilistic model put forward more than sixty years previously by F.B. Taylor and Alfred Wegener (see chapter 2), and departs from the notion, inherent in the original plate-tectonic theory, of the continental crust behaving as an undeformable entity, all parts of which move in the same direction with the same angular velocity.
The southern part of Central Asia with which the Indian continent came in contact is an amalgamation of several continental blocks making up the Tibetan Plateau: the Lhasa, Qiangtang and Songpan-Ganzi Terranes, together with the Tarim Basin to the north, all of which joined the Siberian core of Asia during the Mesozoic (Fig. 8.2). The sutures between these blocks represent lines of weakness within the Eurasian continent that were exploited during the India–Asian collision, and along which renewed activity in the form of thrusting or strike-slip faulting took place.
Our knowledge of the history of the Himalayan belt is assisted by detailed information from the Indian Ocean magnetic-stripe data (Fig. 8.4), from which it is deduced that India became detached from the Gondwana supercontinent during the early Cretaceous. Around 50Ma ago the convergence rate slowed from between 100 and 180mm/a to nearer 50mm/a, suggesting that the initial contact between the two continents may have occurred then. Prior to this event, subduction of the oceanic part of the Indian plate had been taking place beneath Asia from Cretaceous times. The climax of the collision event, resulting in the uplift of the Himalayan range itself, occurred in the Miocene around 20Ma ago, although uplift and thrusting continue today.
Figure 8.4 Movement of India from the late Cretaceous to the Present. Successive positions based on ocean-floor magnetic anomaly data, with respect to the present position of Asia. Note (1) that the southern margin of Eurasia would have been much further south at the time of initial contact; and (2) that India has experienced a 36° anti-clockwise rotation during its northward journey. After Molnar & Tapponnier, 1975.
Although the exact shape of the Asian margin prior to the final collision is unknown, it is clear from Figure 8.4 that the first point of contact would have been at the western prong of India. This probably accounts for the anti-clockwise rotation of the sub-continent subsequent to the initial contact, and may at least partly explain the greater degree of deformation and uplift at the western end of the belt. The convergence direction along the northern sector of the belt was approximately at right angles to it, whereas along both sides, it is highly oblique: the relative movement along both the western and eastern sides of the sub-continent was transpressional, with a large strike-slip component.
The central Himalayan sector
The central sector of the Himalayan belt (Figs 8.2, 8.5) forms an arc, 1750km long and 250km across, extending from northwestern India, through Nepal, to Bhutan, and includes southernmost Tibet. It is bounded in the north by the Indus–Tsangpo Suture, which marks the junction between the Indian and Eurasian plates. In the central sector, the suture is a south-directed thrust that is offset by the north-dipping Renbu-Zedong Thrust. The southern margin of the belt is defined by the Himalayan Frontal Thrust, which marks the edge of the fold-thrust belt on the Indian foreland. The central Himalayan belt consists of four separate tectonic zones: these are, traversing from south to north, the Foreland-Thrust Belt, the Lesser Himalayan Schists, the Greater Himalayan Crystalline Complex, and the Tethyan Shelf (Fig. 8.5).
Figure 8.5 The Himalayan belt. Map (A) and cross-section X-Y (B) of the central (Nepal) sector between the Tibetan Plateau and the Indian foreland showing the main tectonic features. HFT, Himalayan Frontal Thrust; ITS, Indus–Tsangpo Suture; MBT, Main Boundary Thrust; MCT, Main Central Thrust; RZT, Renbu–Zedong Thrust; STD, South Tibetan Detachment. After Harrison, 2006.
The Foreland Thrust Belt
This unit consists of south-directed fold-thrust sheets involving the Siwalik Formation, which is composed of unmetamorphosed clastic sediments formed in the foredeep basin and derived from the rising Himalayan Mountains.
The Lesser Himalayan Schists
This unit consists of slates and schists derived from clastic sediments of Proterozoic age, originally laid down on the Indian passive margin and deformed into south-directed fold-thrust packages. The southern boundary is marked by the Main Boundary Thrust.
The Greater Himalayan Crystalline Complex
This zone also consists of metamorphosed Proterozoic clastic sediments from the Indian passive margin, but here they have been transformed into high-grade schists and gneisses. The metamorphic grade is inverted, and increases upwards. Near the upper margin is a zone of granite intrusions with a generally lensoid shape. The southern, or lower, boundary of the crystalline complex is formed by the Main Central Thrust, which is actually a ductile shear zone several kilometres wide.
The Tethyan Shelf
This zone consists of largely unmetamorphosed Cambrian to Eocene marine strata, mainly carbonates and shales, originally laid down on the Indian continental shelf. The lower boundary of this zone is a north-dipping normal fault known as the South Tibetan Detachment (STD).
The upper boundary of the Tethyan Shelf, corresponding to the northern margin of the central Himalayan sector, is the south-dipping Indus–Tsangpo Suture zone, which contains an ophiolite complex of Cretaceous to early Cenozoic age. Immediately north of the suture in the central sector is an elongate granite intrusion, the Gangdese batholith (see Fig. 8.9), which represents part of a volcanic arc resulting from the subduction of Indian oceanic lithosphere in the early Cenozoic.
Tectonic history
The three northern tectonic zones of the central Himalayan belt all represent packages of material scraped off the top of the continental Indian Plate as it descended beneath the Asian Plate after the initial collision; these were emplaced as thrust sheets, as indicated in Figure 8.5B. It is thought that the thrust sheets developed by propagating forwards in the manner of other typical thin-skinned foreland fold-thrust belts already described. As each sheet ramped up, the orogen would shorten and thicken, although erosion would continuously remove material from the roof of the structure.
Two alternative explanations have been put forward to explain the juxtaposition of the high-grade gneisses of the Greater Himalayan Complex and the Tethyan Shelf sediments above them, which are separated by the South Tibetan Detachment. It was formerly thought that successive forward-propagating thrusts would cause the Greater Himalayan Complex and its Tethyan sedimentary cover to be arched up and subjected to erosion. Gravitational spreading then caused the Tethyan cover to slide down to the north, exposing the higher-grade rocks of the crystalline complex. A more recent explanation relies on a mechanism termed ‘channel-flow’. In this model, the metamorphic rocks of the Greater Himalayan Complex are regarded as a hot, partially molten piece of Indian crust that has flowed under gravitational pressure, from its original position beneath the Asian plate, upwards to the surface between the Tethyan Shelf unit and the Lesser Himalayan Schists beneath. It is possible that both mechanisms may have contributed to the present structure.
The Karakoram and the Pamirs
The Karakoram
The Karakoram Range is a relatively short, arcuate mountain belt at the extreme north-westernmost corner of the Indian Plate and includes several very high peaks, including Nanga Parbat (8126m). In geological terms, the southern part of the range lies within the Himalayan fold/thrust belt, situated on the Indian Plate, between the Himalayan Frontal Thrust and the Indus–Tsangpo Suture (or Main Mantle Thrust), which here is a wide, north-dipping shear zone forming the southern boundary of a volcanic arc terrane known as the Kohistan Arc (Fig. 8.6). Kohistan is the mountainous district in the northwestern corner of Pakistan, bordering Kashmir to the east and Afghanistan to the west and north.
Figure 8.6 The Kohistan Arc and the Sulaiman Fold Belt. HFT, Himalayan Frontal thrust; ITS, Indus–Tsangpo Suture; MMT, Main Mantle Thrust (=ITS); NKS, North Karakoram Suture. NWFP, Northwest Frontier Province. Mountains: K2, (Godwin Austen); NP, Nanga Parbat; TM, Tirich Mir. Cities: Q, Quetta. After Butler & Prior, 1988.
The Kohistan arc terrane is only exposed over a length of about 500km at the northwestern apex of the belt and has not been traced further to the east or west. The arc itself is bounded on its north side by the Northern Kohistan Suture, which marks the southern boundary of the Asian Plate. The Kohistan Arc is believed to have originated as a volcanic island arc, of late Jurassic to early Cretaceous age, which collided with the Asian continent in the mid-Cretaceous (Fig. 8.7). The subsequent collision with the Indian Plate around 50Ma ago, and the subsequent anti-clockwise rotation of India, resulted in a concentration of compressional stress and rapid uplift, both of the Kohistan Arc and of the basement of the Indian Plate, of about 10km over the last 10Ma – a much faster rate than that of the rest of the Himalayan Belt.
Figure 8.7 Cartoon sections across the Kohistan–Pamir region. Possible arrangement of the main tectonic crustal units from the late Cretaceous to the final collision in the Eocene. ITS, Indus–Tsangpo Suture; MBT, Main Boundary Thrust; MCT, Main Central Thrust; MKT, Main Karakoram Thrust; MPT, Main Pamir Thrust; NKS, North Kohistan Suture; STD, South Tibetan Detachment.
The Pamirs
The region known as the Pamirs is a high mountainous plateau crossed by several separate mountain ranges. The mean elevation is even higher than the adjacent Tibetan Plateau, and there are many peaks over 6000m high with large permanent ice-fields. The highest peak is Kongur, at 7649m, near the eastern margin of the plateau, just across the Chinese border (see Fig. 8.2). The Pamir region lies mostly in Tajikistan and eastern Afghanistan, and is situated at the confluence of the northern and southern branches of the Alpine–Himalayan Belt. The Hindu Kush and Chaman Fault Zone join it from the west (see Fig. 7.8), and on its eastern side, the Tian Shan extends north-eastwards and the Karakoram south-eastwards, around the north and west sides, respectively, of the Tibetan Plateau.
The Pamir region is situated entirely within the Eurasian Plate, but has experienced the effects of the Himalayan collision event, in terms partly of thrust-related deformation and partly of uplift. On its southern side it is bounded by the arcuate, north-dipping, North Kohistan Suture, and in the north by a similarly arcshaped south-dipping thrust – the Main Pamir Thrust (Fig. 8.7). A second arcuate south-dipping thrust occurs in the middle of the block. The effect of the Himalayan compression was to elevate the whole block, exposing Eurasian basement in the southern sector. Overlying this basement is a sedimentary platform cover ranging from Cambrian to Recent in age.
The Sulaiman Fold Belt
The western side of the Indian Plate is bounded by the Ornach Nal-Ghazaband-Chaman strike-slip fault system (see Fig. 7.8). East of these faults, which effectively constitute a transform fault zone, is a fold-thrust belt that lies entirely within the Indian Plate, and involves the sedimentary cover of the Indian platform. This belt follows two separate mountain ranges: the more southerly, the Kirthar Range, extends for 550km through Baluchistan from Karachi in the south to the city of Quetta; the more northerly forms a great southeast-facing arc, known as the Sulaiman Mountains, which run for about 400km from Quetta northwards to the Northwest Frontier Province, where the fold belt turns eastwards (Fig. 8.6). The mountain ranges are generally over 2000m but decrease in height southwards; the highest point is Takht-e Sulaiman (Throne of Solomon) at 3487m, just east of Quetta.
The Sulaiman Fold Belt is interesting in that the arcuate mountain ridges curve through about 60° from E–W in the south to NNE–SSW in the north, following the trend of the major folds, so that only in the southern part of the range are the folds at right angles to the India–Asia convergence direction (i.e. NNW–SSE); those in the northern sector are highly oblique. However, data from recent seismic activity indicate that the active deformation was accomplished at least partly by movements on south-dipping reverse faults, which must therefore not be directly connected to the surface features in the northern sector of the belt.
The marked bend in the fold belt at Quetta coincides with a bend in the Indian Plate boundary, which at this point diverges from the line of the Chaman Fault to strike north-eastwards, suggesting that the shape of the Sulaiman belt may have been controlled by the original margin of the Indian continent.
The Tibetan Plateau and the Tarim Basin
The vast Tibetan Plateau extends for over 2400km from the edge of the Pamir Mountains in the west to near the city of Lanzhou in Gansu Province in the east. It is 1300km across at its widest, but narrows to the west. It is everywhere over 4000m in height and includes several impressive mountain ranges containing peaks over 7000m high. The southern margin of the plateau is defined by the Himalayan and Karakoram Ranges, while the northern boundary is the Altyn Tagh Fault, which separates the plateau from the Tarim Basin (see Fig. 8.1). The northeastern boundary is defined by the Qilian Shan (or Nan Shan) Range at the southern edge of the Gobi Desert. There are also several less prominent ranges within the Plateau; these correspond to re-activated faults or sutures.
The Tarim Basin lies immediately north of the Tibetan Plateau in the Chinese Province of Xinjiang. In abrupt contrast to the plateau, the mean elevation of the basin is below 1000m, although it rises in height towards the west. The basin has a roughly oval shape and is bounded in the north by the Tian Shan (or Tien Shan) Range, which here marks the northern limit of Himalayan compressive deformation.
Tectonic summary
The geology of the region north of the Indus–Tsangpo suture is less well known than that of the Himalayan belt. This part of the Eurasian plate has been less obviously affected by the Himalayan compressional deformation, part of which is concentrated along the suture zones between the several separate terranes or microplates that had previously accreted to Central Asia and form part of the large Cimmerian ‘super-terrane’ discussed in the previous chapters. These are, from south to north, the Lhasa, Qiangtang, Songpan–Ganzi and Tarim Terranes, together with the Qaidam block in the northeast.
The various terranes and crustal blocks originated by splitting off from Gondwana at different times and travelled separately to join Eurasia: Tarim and Qaidam left in the Devonian and joined Eurasia in the early Permian; the Qiangtang (part of Cimmeria) left in the Permian and joined in the Triassic; and the Lhasa block left in the Triassic and joined in the early Jurassic. The Songpan–Ganzi Terrane is an accretionary complex sandwiched between these continental blocks and the Qiangtang Terrane.
Himalayan deformation has also been accommodated by movements along a network of strike-slip (wrench) faults. These faults form a conjugate set: a sinistral set varying in orientation from NE–SW to ENE–WSW, and a dextral set varying from N–S to WSW–ESE. Towards the east, the faults of both sets curve round into a more N–S orientation as the belt of deformation turns southwards into Burma and Indo-China (see Fig. 8.1). Numerous north–south oriented graben systems also indicate significant E–W extension and this, coupled with the movements on the numerous minor conjugate wrench faults, has been interpreted to indicate that much of the north–south convergence between India and Asia has been accommodated by the sideways extrusion of Asian crust.
Repeated precise GPS measurements have enabled accurate movement vectors across the orogen to be calculated (Fig. 8.8); these range generally between 5 and 15mm/a, confirm the lateral extrusion model, and also indicate a gradual diminution of flow velocity northwards; there is thus no clearly defined northern margin to the deformation resulting from the collision, as there is in the south. Data on slip rates derived from GPS and satellite radar measurements along the wrench faults are typically in the range 5–15mm/a. Interestingly, the slip rates on the very large wrench faults such as the Altyn Tagh and Karakoram Faults, formerly believed to have accommodated the bulk of the north–south compression, are little different from the others.
Figure 8.8 Active deformation of the southern part of the Eurasian Plate. Velocity vectors (red arrows) showing direction and relative amount of movement in the deformed part of the Eurasian Plate relative to stable Eurasia. The pattern shows that the flow is to the north, gradually decreasing northwards in the western sector, while directed north-eastwards, then eastwards and ultimately south-eastwards in the eastern sector. After Searle et al., 2011.
This pattern of distributed deformation, achieved mainly by faulting, applies only to the upper (seismogenic) crust. Beneath this, the middle crust, being warmer and more ductile (see below), will have deformed in a more continuous manner, employing shear zones rather than discrete faults.
The origin of the Tibetan Plateau
There has been a vigorous debate that has lasted for many decades about the reason for the high elevation of the plateau. Orthodox plate-tectonic theory originally suggested that the explanation was the under-thrusting of the Indian Plate, causing a doubling up of the Tibetan crust. However, this would imply a very shallow-dipping subducting slab extending for over 1000km, which mechanically seemed unlikely. Seismic data show that the present base of the Indian crust descends from about 40km depth at the orogenic front to 70km beneath the Indus–Tsangpo Suture and remains around that level for about 200km across South Tibet before it disappears, to be replaced by the quite different structure of the Tibetan Plateau, as described below. As the subducting slab now extends for only a relatively short distance beneath the southern part of the Plateau, as indicated in Figure 8.9, it cannot be responsible for the plateau uplift.
More recent intensive research employing various indirect geophysical techniques has revealed that the crust of the Tibetan Plateau is considerably warmer, and therefore much weaker, than the Indian equivalent, and that its thickness is over double that of the Indian, varying from 70km in the south to around 60km in the north, with a marked change across the Bangong–Nujiang Suture (Fig. 8.9). Moreover, as a result of the different physical properties in the lower part of the crust, the brittle, fault-dominated tectonics of the upper crust appears to be replaced in the lower crust by a pattern of general eastwards flow.
The behaviour of the lithospheric mantle beneath Tibet is more difficult to determine. Because of the marked increase in strength between the lower crustal and uppermost mantle material, the latter would not be expected to behave in the same ductile fashion. Nevertheless, the seismic properties of the Tibetan mantle contrast with those of the Indian lithospheric mantle and are consistent with a model of mantle flow similar to that inferred for the crust, at least for the material below the topmost part. The ‘missing’ Tibetan mantle lithosphere caused by the under-thrusting of the Indian lithosphere, evident from Figure 8.9, could thus be explained in the same way as the Tibetan lower crust, with the exception that the lower part may have been absorbed into the asthenosphere, leaving the total thickness more or less unchanged.
Figure 8.9 Simplified cross-section through the central sector of the Himalayan Belt. Note contrasting arrangement of crust and mantle lithosphere in the Indian and Eurasian Plates. GB, Gangdese Batholith; other abbreviations as in Figure 8.5. Note that the Asian crust is much thicker than the Indian, and that a considerable amount of Asian lithospheric mantle is apparently missing. After Searle et al., 2011.
The conclusion drawn from these observations is that much of the compressive stress generated by the Indian collision has been accommodated by thickening and shortening of both the crust and lithospheric mantle of the Tibetan Plateau, accompanied by sideways transfer of material, mostly to the east and southeast. The height of the plateau can be explained by its warmth and thickness, and its relative flatness by the fact that general isostatic equilibrium can be achieved by the ease of flow in the lower crust. The high heat flow is attributed to large quantities of igneous material within the crust, mostly arising from the pre-collision subduction process.
In contrast to the Tibetan example just described, the Indian lithosphere is believed from seismic evidence to be relatively cool and strong, providing a broad, semi-rigid slab which has underthrust the Asian plate for a distance of over 200km north of the Indus–Tsangpo Suture. The northward movement of this slab is the main driver for the deformation of the orogen. The current convergence rate between the Indian and Eurasian Plates is about 50mm/a towards the north. Of this, approximately half is taken up by shortening across the Himalayan fold-thrust belt and half by the shortening and eastwards extrusion of the Tibetan lithosphere – perhaps 80% by shortening and thickening, and 20% by lateral extrusion. Much of the shortening across the Himalayas is accomplished by crustal thickening, a considerable proportion of which has been removed by erosion.
The northern mountain belts
The northern branch of the Alpine–Himalayan mountain belt contains two separate mountain ranges, the Tian Shan and the Qilian Shan, or Nan Shan, which together extend from the edge of the Pamir Plateau in the west to near the City of Lanzhou in the east – a combined distance of around 2600km. The Tian Shan–Nan Shan line defines the northern limit of the main Himalayan deformation, but movements also occurred north of the Nan Shan in the Altai Range of southwest Mongolia (see chapter 14 and Fig. 14.13), where some thrusts and dextral strike-slip faults remain active.
The Tian Shan
This mountain range, alternatively spelled ‘Tien Shan’, stretches for 1800km in a great arc from the eastern edge of the Pamir Plateau towards the Mongolian border, forming the northern rim of the Tarim Basin (see Fig. 8.2). It is a substantial mountain range, rivalling the Hindu Kush and the Pamirs in scale: the main ridge in the west lies along the boundary between China and Kyrgyzstan, and is everywhere well over 4000m in height, reaching 7439m at the summit of its highest peak, Pik Pobedy (Jangish Choksu).
The Tian Shan is an example of an intra-plate orogenic belt – the Himalayan deformation exploited weak zones within the Eurasian Plate that had resulted from two late Palaeozoic subduction-collision episodes: the older created a north-dipping suture along the southern margin of the belt, and the younger a south-dipping suture on the opposite side of the belt. The Himalayan collision re-activated the former suture zones into steep thrusts that elevated the wedge-shaped block between to form the mountain belt. Most of the deformation is concentrated along the boundary fault zones and the interior of the block experienced minimal effects. In addition to the thrusts, which are the predominant expression of the Himalayan contraction, there are also several dextral strike-slip faults that cut the thrusts – the most important being the NW–SE Talas–Fergana Fault, which forms the western boundary of the Tian Shan. The belt is strongly active seismically, having experienced many recent earthquakes: it has been estimated that the convergence rate across the belt is around 20mm/a, mainly over the last 10Ma, and has resulted in up to 20km of crustal shortening parallel to the convergence direction across the Himalayas.
The Nan Shan (or Qilian Shan)
This range extends for 800km along the northeast side of the Qaidam Block (Fig. 8.2), ending near Lanzhou in the valley of the Yellow River (Huang He). The latter two-thirds of the range follows the south side of the Great Wall. This range is smaller in scale than the Tian Shan: although it contains several peaks over 6,000m in height, it is more accurately described as the northern margin of the high Tibetan Plateau, from which it differs only slightly in elevation. However, the southwest-dipping thrust along the northeastern margin of the Nan Shan is strongly active: a magnitude 7.6 earthquake in 1927 is thought to have been responsible for nearly 41,000 deaths, as well as widespread structural damage.
The Eastern Sector
East of Tibet, the Alpine–Himalayan mountain belt divides into two main branches as it turns southwards through Burma (Myanmar) into Indochina (Fig. 8.2). The more westerly, relatively minor, branch follows the eastern margin of the Indian Plate forming the Patkai,
Chaga and Chin ranges. The more easterly branch descends southwards through eastern Burma and southwestern China as a 1000km-wide belt containing many roughly parallel mountain ranges. In the northern sector of this belt, the individual mountain ranges curve around the northeastern corner of the Indian Plate from the heights of the Tibetan Plateau, separated by the valleys of the Irrawaddy and Salween rivers and the upper reaches of the Yangtze. One of these ranges, the Daxue Shan, contains the impressive peak of Gongga Shan, at 7556m the highest peak east of the Himalayas.
The Chin–Naga–Patkai Fold Belt
This belt of mountains extends for about 950km from Cape Negrais in southwest Burma (see Fig. 9.2) to the northeastern corner of India, where it meets the Himalayas. The belt, which is up to 150km wide, includes four separate ranges, from south to north: the Arakhan, Chin, Naga and Patkai Hills. The northern part of this mountain belt, from the Chin to the Patkai Ranges, forms the eastern boundary of the Indian Plate, and consists of a number of separate parallel ranges which together form an arc facing west towards the interior of the Indian Plate. The highest point in the belt is Saramati, at 3826m, in the Naga Range. The southern extension of the belt, known as the Arakhan, or Rakhine Mountains, lies wholly within Burma and is discussed in the following chapter.
The origin of the fold belt lies in the anti-clockwise rotation of the Indian Plate after its initial contact with Eurasia. This rotation took place about a hinge at the northwest corner of the sub-continent and resulted in eastwards convergence across the western border of Burma. The structure is that of a typical thin-skinned fold-thrust belt involving a Palaeocene to Recent platform cover more than 6km thick overlying Indian Pre-cambrian basement (Fig. 8.10). The belt is made up of an outer zone of thrust slices, the Schuppen Zone, bounded on its western side by the Naga Thrust. This boundary thrust separates the fold belt from the undeformed Assam Shelf, in the northeastern part of the Indian Plate. Several other major thrusts occur east of the Schuppen Zone, bringing successively deeper parts of the Cenozoic sedimentary sequence to the surface.
On the eastern side of the fold-thrust belt lies the Central Burma Basin, which occupies the southern extension of the Lhasa Terrane, squeezed eastwards and southwards by the effects of the Himalayan collision (see Fig. 8.2).
Figure 8.10 The Naga fold-thrust belt of NE India: simplified section. AS, Assam Shelf; DT, Disang Thrust; NT, Naga Thrust. After Oil and Natural Gas Corporation, India, via Wikimedia Commons.
East of the Central Burma Basin is the wide mountain belt of Eastern Burma and the Sichuan Province of SW China. This north–south belt crosses a further four distinct tectonic zones. From west to east, these are known locally as: the Sibumasu Terrane (the equivalent of the Qiangtang of Tibet), the Chanthaburi Arc Terrane (the Songpan–Ganzi of Tibet), the Indochina Terrane and the South China Terrane. These zones are described in the following chapter. The major sutures separating these terranes each experienced renewed compressional effects as a result of the Himalayan convergence.
Tectonic summary
The effect of the Indian collision during Eocene to Oligocene time was to force the various Gondwanan terranes of Tibet eastwards and south-eastwards, wrapping them around the northeastern corner of the Indian Plate, and squeezing them against the more stable blocks to their east (see Figs 8.2, 8.8). The large mountainous region that embraces eastern Burma, northern Laos and southwest China is, at least partly, the result of compression and uplift responding to the north-eastwards motion of the continental part of the Indian Plate.
