Relative plate motions and collisional history

The Himalayan-Tibetan orogen was created mainly by the collision between India and Eurasia over the past 70-50 Myr (Yin & Harrison, 2000). The orogen is part of the greater Himalayan-Alpine system, which extends from the Mediterranean Sea in the west to the Sumatra arc of Indonesia in the east over a distance of >7000 km. This composite belt has evolved since the Paleozoic as the Tethyan oceans (e.g. Fig. 11.27) closed between two great converging landmasses: Laurasia in the north and Gondwana in the south (^engor & NataTin, 1996). Tethys may have been only a few hundred kilometers wide in the west but opened to the east to form an ocean that was at least several thousands of kilometers wide.

The India-Eurasia collision was brought about by the rifting of India from Africa and East Antarctica during the Mesozoic (Section 11.5.5) and by its migration northward as the intervening oceanic lithosphere was subducted beneath the Eurasian Plate. Magnetic

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Figure 10.13 Shaded relief map showing major faults and topographic features of the Himalayan-Tibetan orogen. Fault traces are from Hodges (2000), Yin & Harrison (2000), Tapponnier et al. (2001). WS, Western Himalayan Syntaxis; ES, Eastern Himalayan Syntaxis; MMT, Main Mantle Thrust;AKMS, Ayimaqin-Kunlun-Mutztagh suture; JS, Jinsha suture; BNS, Bangong-Nujiang suture; IZS, Indus-Zangbo suture. Map was constructed using the same topographic data and methods as in Fig. 7.1.

Figure 10.13 Shaded relief map showing major faults and topographic features of the Himalayan-Tibetan orogen. Fault traces are from Hodges (2000), Yin & Harrison (2000), Tapponnier et al. (2001). WS, Western Himalayan Syntaxis; ES, Eastern Himalayan Syntaxis; MMT, Main Mantle Thrust;AKMS, Ayimaqin-Kunlun-Mutztagh suture; JS, Jinsha suture; BNS, Bangong-Nujiang suture; IZS, Indus-Zangbo suture. Map was constructed using the same topographic data and methods as in Fig. 7.1.

anomalies in the Indian Ocean and paleomagnetic measurements from the Ninety-East Ridge and the Indian subcontinent record the northerly drift of the Indian plate and allow the reconstruction of its paleo-latitude (Fig. 10.14). The data show a rapid decrease in the relative velocity between the Indian and Eurasian plates at 55-50 Ma. This time interval commonly is interpreted to indicate the beginning of the India-Eurasia collision. However, it is uncertain whether the decrease resulted from an increase in the resistance to continued motion of the India plate as it collided with Eurasia or if it simply reflects a sudden decrease in spreading rate along the mid-oceanic ridge south of India. This latter possibility allows the age of the initial contact between India and Eurasia to be older than 55-50 Ma.

Stratigraphic and sedimentological data provide additional information on the age and progressive evo lution of the India-Eurasia collision. Gaetani & Gar-zanti (1991) showed that marine sedimentation stopped and terrestrial deposition along the southern margin of Asia commenced at 55-50 Ma, which is in accord with interpretations of the age of the initial collision derived from magnetic anomalies. However, this observation in fact only constrains the youngest possible age of the onset of the collision because as much as 500-1000 km of the Indian passive continental margin has been underthrust beneath Asia, potentially eliminating the early record of the collision (Yin & Harrison, 2000). Beck et al. (1995) showed that trench and forearc material along the southern margin of the Eurasian plate near Pakistan was thrust onto the northern edge of India after 66 Ma and before 55 Ma. Willems et al. (1996) found changes in sedimentary facies and depositional patterns in south-central Tibet that suggest initial contact between some parts of India and Asia could

Figure 10.14 Northward drift of India with respect to Asia from 71 Ma to the present, determined from magnetic lineations in the Indian and Atlantic oceans (redrawn from Molnar & Tapponnier, 1975, Science 189, 419-26, with permission from the AAAS).

Figure 10.14 Northward drift of India with respect to Asia from 71 Ma to the present, determined from magnetic lineations in the Indian and Atlantic oceans (redrawn from Molnar & Tapponnier, 1975, Science 189, 419-26, with permission from the AAAS).

have occurred as early as 70 Ma. These relationships suggest that the initial collision may have begun as early as the Late Cretaceous. In general, most authors agree that all Tethyan oceanic lithosphere had disappeared by 45 Ma, and at ~36 Ma there was a decrease in the veloc ity of India's northward drift from over 100 mm a-1 to about 50 mm a-1 or less. This latter time may mark the final stage of true continent-continent collision (Yin & Harrison, 2000).

Geologic observations in Tibet and China add important details to the sequence of events leading up to the India-Eurasia collision. The geology indicates that the main collision between India and Eurasia was preceded by the collision of several microcontinents, flysch complexes, and island arcs during Paleozoic and Mesozoic time. The collision and accretion of these terranes is marked by a series of suture zones (Fig. 10.13), some of which preserve ophiolites and blocks of high-pressure metamorphic rocks (Section 9.9). Some of these sutures expose relics of ultra-high-pressure (UHP) minerals such as coesite and microdiamond, commonly as inclusions in unreactive phases of zircon and garnet. The presence of these minerals, and the high pressures (2.5-4.0 GPa) under which they form, can reflect situations where a section of continental crust enters the subduction zone and descends to depths of 60-140 km before decoupling from the downgoing plate (Ernst, 2003; Harley, 2004). The mechanisms by which UHP and other high pressure metamorphic rocks are exhumed to the surface may involve contrac-tional, extensional and/or strike-slip deformation accompanying the evolution of the plate boundary zone. Hacker et al. (2004) describe processes associated with the exhumation of UHP terranes in South China.

The Songpan-Ganzi terrane exposes thick Triassic flysch sequences that rest on top of Paleozoic marine sediments belonging to the passive margin of North China. These sequences were deposited, uplifted, and deformed during the Triassic collision between the North and South China blocks, forming the Ayimaqin-Kunlun-Mutztagh suture (Yin & Harrison, 2000). By the end of the Triassic (Fig. 10.15a), the Lhasa and Qiangtang terranes had rifted from Gondwana and began their journey toward Eurasia (Fig. 10.15b). The Qiangtang terrane collided with the Songpan-Ganzi by 140 Ma, forming the Jinsha suture. Continued convergence brought the Lhasa terrane into juxtaposition with Qiangtang and eventually welded the two fragments together, forming the Bangong-Nujiang suture. The formation of a new subduction zone beneath Lhasa (Fig. 10.15c) created an Andean-type orogen (Fig. 10.15d) and eventually resulted in the collision between India and Eurasia (Fig. 10.16e), forming the Indus-Zangbo suture. Continued convergence (Fig. 10.15f) resulted in intra-plate shortening and uplift, and is asso-

(a) Permian-Triassic rifting

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^ Eurasia

(b) Late Triassic-Early Jurassic rifting

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(c) Late Jurassic-Early Cretaceous possible backarc extension

Qntg SG Eurasia

Qntg SG Eurasia

Collision and distributed shortening

BNS

f Lhasa

Qntg^JSG| Eurasia

(d) Late Cretaceous

Gond

(e) Early Cenozoic

Shortening and some uplift

Shortening and some uplift

Lhas^^Qntg^SGl Eurasia I

Collision

Lhas^^Qntg^SGl Eurasia I

-f IndiaY Lhas^Qnt^^SG Eurasia I

(f) Late Cenozoic continued convergence, uplift and deformation continued convergence, uplift and deformation

Figure 10.15 Possible sequence of events in the evolution of the Himalayan-Tibetan orogen (modified from Haines et al., 2003, by permission of the American Geophysical Union. Copyright © 2003 American Geophysical Union). Interpretation incorporates relationships developed by Allègre et al. (1984) and Yin & Harrison (2000). BNS, Bangong-Nujiang suture; SG, Songpan-Ganzi terrane; Gond, Gondwana; Qntg, Qiantang terrane; IZS, Indus-Zangbo suture.

Collision

ciated with a new plate boundary that is beginning to form in the Indian Ocean (Van Orman et al., 1995). The Indus-Zangbo suture now forms the southern boundary of the Tibetan Plateau (Fig. 10.13), which lies more than 5000 m above mean sea level and covers an area of more than a million square kilometers.

This history shows that the Himalayan-Tibetan orogen is built upon a collage of exotic material that became welded to the Eurasian Plate before the main India-Eurasia collision (Sjengor & Natal'in, 1996; Yin & Harrison, 2000). This type of sequential amalgamation of microcontinents and other material during prolonged subduction is characteristic of accretionary orogens (Section 10.6.2) and represents one of the most efficient mechanisms of forming supercontinents (Section 11.5). This history also resulted in a hot, weak Eurasia continental plate prior to its collision with India.

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