General geology of the Himalaya and Tibetan Plateau

The Himalaya are composed of three large, imbricated thrust slices and related folds separated by four major fault systems (Figs 10.19, 10.20). These imbricated thrusts, which occupy a section about 250-350 km wide, appear to accommodate approximately one-third to one-half of the ~2000 or more kilometers of post-col-lisional shortening between India and Eurasia (Besse & Courtillot, 1988; DeCelles et al., 1998). At the base of the stack the mostly buried Main Frontal Thrust lies along the topographic front of the mountain range (Wesnousky et al. 1999). This fault is the youngest and most active fault in the mountain range and carries rock of the Himalaya southward into a flexural foredeep (Section 10.3.2) called the Ganga foreland basin. The

Ganga basin contains over 5 km of Miocene-Pliocene terrigenous sedimentary sequences overlain by late Pleistocene alluvium (DeCelles et al., 2001). The northern part of this basin, which forms the Himalayan foothills, defines a 10- to 25-km-wide physiographic province commonly referred to as the Sub-Himalaya.

Above and to the north of the Main Frontal Thrust is the Main Boundary Thrust (Fig. 10.19). This latter fault system dips gently to the north and appears to have been active mostly during the Pleistocene, although slip on it may have initiated during the Late Miocene-Pliocene (Hodges, 2000). The fault carries Precam-brian-Mesozoic low-grade schist and unmetamorphosed sedimentary rock of the Lesser (or Lower) Himalaya southward over the Sub-Himalaya. The Lesser Himalaya form a zone at elevations between about 1500 and 3000 m. Above the Lesser Himalaya, high-grade gneisses and granitic rocks of the Greater (or Higher) Himalaya are carried southward along the Main Central Thrust (DeCelles et al., 2001). This latter thrust accommodated significant shortening during the Early Miocene and Pliocene, and appears inactive in most places today (Hodges, 2000).

The Greater Himalaya, which reach altitudes of over 8000 m, consist of Precambrian gneiss overlain by

Indian Ocean

Depression ~40m

Flexural forebulge 450m

Himalaya

TIBET

Indian Ocean

Depression ~40m

Flexural forebulge 450m

Himalaya

TIBET

reverse faulting (compressional) earthquakes normal faulting (tensile) earthquakes

Figure 10.18 (a) Earthquake focal mechanism solutions in India and southern Tibet (modified from Jackson et al., 2004, with permission from the Geological Society of America). Numbers represent depths. Black solutions are from events that occur within the Indian craton, light gray solutions are at depths of 10-15 km. Depths highlighted by a box are Moho depths from receiver function studies. Ellipse in Tibet is the high velocity anomaly imaged by Tilmann et al. (2003) and shown in Plate 9.4 (bottom) (between pages 244 and 245). (b) Schematic cross-section showing flexure of Indian lithosphere as it is underthrust to the north beneath Tibet (modified from Bilham, 2004). At the crest of the flexural bulge the surface of the Indian plate is in tension (T) and its base is in compression (R).

reverse faulting (compressional) earthquakes normal faulting (tensile) earthquakes

Figure 10.18 (a) Earthquake focal mechanism solutions in India and southern Tibet (modified from Jackson et al., 2004, with permission from the Geological Society of America). Numbers represent depths. Black solutions are from events that occur within the Indian craton, light gray solutions are at depths of 10-15 km. Depths highlighted by a box are Moho depths from receiver function studies. Ellipse in Tibet is the high velocity anomaly imaged by Tilmann et al. (2003) and shown in Plate 9.4 (bottom) (between pages 244 and 245). (b) Schematic cross-section showing flexure of Indian lithosphere as it is underthrust to the north beneath Tibet (modified from Bilham, 2004). At the crest of the flexural bulge the surface of the Indian plate is in tension (T) and its base is in compression (R).

Paleozoic and Mesozoic sedimentary rock of Tethyan origin. These rocks have been thrust southward for a distance exceeding 100 km. The unit includes migmatite and amphibolite grade metamorphic rocks intruded by light-colored granitic bodies of Miocene age called leu-cogranite (Hodges et al., 1996; Searle et al., 1999). The migmatite and leucogranite have originated by the partial melting of the lower crust beneath Tibet (Le Fort et al., 1987) and are absent north of the Greater Himalaya.

The progressive decrease in the age of thrusting from north to south within the Himalaya defines a fore land-propagating fold-thrust system. At depth, each of three main thrusts of the system merges downward into a common décollement called the Main Himalayan Thrust (Fig. 10.20). Seismic reflection and velocity profiles (Zho et al., 1991; Nelson et al., 1996) show that the décollement continues beneath the Greater Himalaya where it disappears beneath southern Tibet amid a zone of weak reflectivity thought to represent a zone of partially molten rock (Section 10.4.5).

Bounding the top of the thrust stack at the surface is a system of normal faults that form the South Tibetan Detachment System (Burchfiel et al., 1992). The basal

Main Boundary Thrust Himalaya
Figure 10.19 Geologic map of the Himalaya (modified from Hodges, 2000, with permission from the Geological Society of America). BNS, Bangong-Nujiang suture; IZS, Indus-Zangbo suture; MBT, Main Boundary Thrust; MCT, Main Central Thrust; MMT, Main Mantle Thrust; STDS, South Tibetan Detachment System.

detachment dips gently-moderately to the north and separates the high-grade gneisses of the Greater Himalaya from low-grade Cambrian-Eocene rocks of the Tethyan zone (Fig. 10.19). These latter rocks were deposited on the passive margin of northern India prior to its collision with Eurasia. The basal detachment records Miocene and, possibly, Pliocene north-directed normal displacements of at least 35-40 km that occurred contemporaneously with south-directed motion on the Main Central Thrust (Hodges, 2000). In its hanging wall, Tethyan rocks are dissected by complex arrays of splay faults (Fig. 10.19) whose cumulative displacement probably approaches that of the basal detachment (Searle, 1999).

Throughout the Tethyan zone are a discontinuous series of metamorphic culminations called gneiss domes. The most extensively studied of these is the Kangmar gneiss dome, which forms part of an antiform cored by Precambrian metamorphic basement surrounded by a mantle of less metamorphosed Carbonif-erous-Triassic rocks (Burg et al., 1984). A few of the largest gneiss domes preserve eclogite-facies metamor-phic assemblages that are overprinted by amphibolite-facies assemblages (Guillot et al., 1997). The domes are dissected by normal faults and bear some resemblance to the extensional metamorphic core complexes in the western USA and elsewhere (Section 7.3). However, their origin is not well understood and several different mechanisms have been proposed to explain them, including thrust faulting and folding in addition to normal faulting and lower crustal flow.

At the northern end of the Tethyan Zone the Indus-Zangbo suture separates rocks that once formed part of the Indian Plate from Paleozoic-Mesozoic rocks of the Lhasa terrane (Section 10.4.2). The suture is defined by a deformed mixture of components derived from

Figure 10.20 (a) Map showing location of INDEPTH profiles (modified from Xie et al., 2001, by permission of the American Geophysical Union. Copyright © 2001 American Geophysical Union). Right-way-up triangles, INDEPTH I, II; upside-down triangles, INDEPTH III. (b) Composite of INDEPTH seismic information, including S-wave velocity models derived from waveform modeling of broad-band earthquake data (error bars shown around central profiles) and wide-angle reflection data beneath and north of the Indus-Zangbo suture (modified from Nelson et al., 1996, Science 274,1684-8, with permission from the AAAS). LVZ, midcrustal low velocity zone; STDS, South Tibetan Detachment; i, reflections interpreted to represent fluids at 15-20 km depth; ii, steep reflection in the lower crust interpreted to represent thrusting; iii, Moho at 75 km depth; iv, fault that accommodates underthrusting of India beneath Tibet. (c) Interpretive cross-section of the central Himalaya and southern Tibet (section provided by C. Beaumont and modified from the compilation of Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Section incorporates observations from Nelson et al. (1996), Hauck et al. (1998), and DeCelles et al. (2002). Roman numerals are explained in Section 10.4.6. MFT, Main Frontal Thrust; MHT, Main Himalayan Thrust. Other abbreviations as in Fig. 10.19.

Figure 10.20 (a) Map showing location of INDEPTH profiles (modified from Xie et al., 2001, by permission of the American Geophysical Union. Copyright © 2001 American Geophysical Union). Right-way-up triangles, INDEPTH I, II; upside-down triangles, INDEPTH III. (b) Composite of INDEPTH seismic information, including S-wave velocity models derived from waveform modeling of broad-band earthquake data (error bars shown around central profiles) and wide-angle reflection data beneath and north of the Indus-Zangbo suture (modified from Nelson et al., 1996, Science 274,1684-8, with permission from the AAAS). LVZ, midcrustal low velocity zone; STDS, South Tibetan Detachment; i, reflections interpreted to represent fluids at 15-20 km depth; ii, steep reflection in the lower crust interpreted to represent thrusting; iii, Moho at 75 km depth; iv, fault that accommodates underthrusting of India beneath Tibet. (c) Interpretive cross-section of the central Himalaya and southern Tibet (section provided by C. Beaumont and modified from the compilation of Beaumont et al., 2004, by permission of the American Geophysical Union. Copyright © 2004 American Geophysical Union). Section incorporates observations from Nelson et al. (1996), Hauck et al. (1998), and DeCelles et al. (2002). Roman numerals are explained in Section 10.4.6. MFT, Main Frontal Thrust; MHT, Main Himalayan Thrust. Other abbreviations as in Fig. 10.19.

both the Indian and Eurasia plates, as well as Tethyan ophiolites and blueschist (Section 9.9). The ophiolites are not continuous, and in places are replaced by sediment deposited in a forearc environment. South-dipping thrusts and strike-slip faults deform these rock units. North of the suture, the Paleozoic-Mesozoic sedimentary rocks that form most of southern Tibet are intruded by the Cretaceous-Eocene Gangdese batholith of the Transhimalayan zone (Fig. 10.19). This batholith formed along an ocean-continent convergent plate margin in response to northwards underthrusting of Tethyan oceanic lithosphere prior to the India-Eurasia collision (Fig. 10.15c,d). In the western Himalaya the equivalent unit is an island arc that formed within the Tethys

Ocean in mid-Cretaceous times. The Bangong-Nujiang suture separates this unit from the Karakorum granite batholith on its northern side (Fig. 10.19).

North of the Indus-Zangbo suture, active normal faulting and east-west extension are dominant. This style of deformation has formed a series of young rift basins that trend approximately north-south. At most these basins record extension of a few tens of kilometers. Most are filled with Pliocene and younger conglomerates and appear to have formed since the Miocene. Some are associated with major strike-slip faults, such as the Jiali Fault, and may represent pullapart basins (Section 8.2). These observations and geo-chronologic data suggest that the east-west extension is either younger than or outlasted the north-south extension recorded by the South Tibetan Detachment System (Harrison et al., 1995). Late Cenozoic intrusive and extrusive activity also occurs in southern and central Tibet (Chung et al., 2005).

Between the Bangong-Nujiang suture and the Qaidam Basin (Fig. 10.13) are three major mid-late Cenozoic fold-thrust belts. All three are associated with the development of a foreland basin (Yin & Harrison, 2000). The cumulative amount of shortening accommodated by these belts is poorly constrained but may reach several hundreds of kilometers. At the northern margin of Tibet deformation is partitioned between active folding and thrusting and several major active strike-slip faults, including the Altyn Tagh and Kunlun faults. Along the former fault, left-lateral strike-slip motion is transferred to as much as 270 km of northeast-southwest shortening in the Qilian Shan. Farther east and southeast of the Qilian Shan, the shortening direction turns east-west where motion on east-striking strike-slip faults is transferred onto active north-striking thrust faults in the Longmen Shan (Burchfiel, 2004). This latter mountain range also records Mesozoic shortening and rises more than 6 km above the rigid, virtually undeformed Sichuan Basin, forming one of the steepest fronts along the Tibetan Plateau (Clark & Royden, 2000). To the south of the basin, many of the major strike-slip faults, including the Jiali and Xianshuihe faults, are curved. These faults rotate clockwise around the eastern Himalayan syntaxis relative to South China (Wang et al., 1998).

North of the plateau active shortening also occurs in the Tien Shan and the Altai ranges of northern China and Mongolia. The deformation in these regions appears to be controlled mostly by pre-existing strength heterogeneities in the Eurasian lithosphere.

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  • Maxima Proudfoot
    Which fault separates himalayafrom tibbet?
    1 year ago

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