Greater India
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Earth-Science Reviews 72 (2005) 169–188 www.elsevier.com/locate/earscirev Greater India Jason R. Ali *, Jonathan C. Aitchison Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China Received 8 January 2005; accepted 7 July 2005 Abstract bGreater IndiaQ is an 80-yr-old concept that has been used by geoscientists in plate tectonic models of the India–Asia collision system. Numerous authors working on the orogen and/or plate models of the broader region have added various sized chunks of continental lithosphere to the now northern edge of their reconstructed Indian plate. Prior to plate tectonic theory, Emile Argand (1924) [Argand, E., 1924. La tectonique de l’ Asie. Proc. 13th Int. Geol. Cong. 7 (1924), 171–372.] and Arthur Holmes (1965) [Holmes, A., 1965. Principles of Physical Geology, Second Edition. The Ronald Press Company, New York, 1128.] thought that the Himalayan Mountains and Tibetan Plateau had been raised due to the northern edge of the Indian craton under-thrusting the entire region. Since the advent of plate tectonic theory, Greater India proposals have been based principally on three lines of logic. One group of workers has added various amounts of continental lithosphere to India as part of their Mesozoic Gondwana models. A second form of reconstruction is based on Himalayan crustal-shortening estimates. A third body of researchers has used India continent extensions as means of allowing initial contact between the block and the Eurasian backstop plate in southern Tibet to take place at various times between the Late Cretaceous and late Eocene in what we call bfill-the-gapQ solutions. The Indian craton and the southern edge of Eurasia were almost invariably some distance from one another when the collision was supposed to have started; extensions to the sub-continent were used to circumvent the problem. Occasionally, Greater India extensions have been based on a combination of fill-the-gap and shortening estimate arguments. In this paper, we exhume and re-examine the key Greater India proposals. From our analysis, it is clear that many proponents have ignored key information regarding the sub-continent’s pre break-up position within Gondwana and the bathymetry of the Indian Ocean west of Australia, in particular the Wallaby–Zenith Plateau Ridge and the Wallaby–Zenith Fracture Zone. We suggest that the Indian continent probably extended no more than 950 km in the central portion of the Main Boundary Thrust, up to the Wallaby–Zenith Fracture Zone. At the Western Syntaxis, the extension was about 600 km. These estimates are broadly compatible with some of the geophysically-derived models depicting subducted Indian litho- sphere beneath Tibet, as well as estimates of Himalayan shortening. Models requiring sub-continent extensions N98 ahead of the craton are probably wrong. We also suggest that northern India did not have a thinned rifted passive margin due to the earlier rifting of blocks away from it when it formed part of Gondwana. Instead, the boundary developed as a transform fault * Corresponding author. Tel.: +852 2857 8248; fax: +852 2517 6912. E-mail address: [email protected] (J.R. Ali). 0012-8252/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.earscirev.2005.07.005 170 J.R. Ali, J.C. Aitchison / Earth-Science Reviews 72 (2005) 169–188 and probably had a very narrow ocean–continent transition zone (5–10 km wide), similar to the Romanche Fracture Zone offshore of Ghana, West Africa. D 2005 Elsevier B.V. All rights reserved. Keywords: Gondwana; Indian Ocean; Tethys; Wallaby Plateau; Palaeogeography 1. Introduction after 15 yr of researching different aspects of the evolution of the eastern Asia–western Pacific, and 8 The most important advances in the earth yr of looking specifically at the geology of Tibet, sciences came about following the widespread we were utterly confused with the vast body of acceptance of plate tectonic theory in the late opinions and ideas regarding the size of India 1960s. The paradigm best explains why the Hima- prior to its collision with Asia, as well as where layan chain to the north of the Indian craton has and when the process started. Indeed, it is our Earth’s greatest collection of high peaks, and the contention that future historians of science will Tibetan Plateau immediately to the north forms the view bGreater IndiaQ as one of the geoscience com- planet’s largest–highest elevated surface. The Indian munity’s most fascinating and flexible concepts. We sub-continent slammed into Eurasia sometime in the hope this review will (1) explain how the concept last 70 million yr (c.f. Yin and Harrison, 2000, who of bGreater IndiaQ has developed, (2) present some suggest collision started possibly as early as ~70 of the important reconstructions and (3) provide Ma with Aitchison and Davis, 2004, who argue that some constraints on how big India was in the the event started in the late Oligocene–early Mio- Cretaceous, prior to its collision with Asia. It should cene) and has since continued indenting in the thus be useful both to those modeling this key backstop plate thus creating this gigantic topo- continent–continent collision system, and to those graphic feature. studying the Cenozoic evolution of the broader Since the 1970s, a large number of geologists have region. proposed bGreater Indias,Q that is, the Indian sub- continent plus a postulated northern extension. One type of proposal has been based on the need for the sub-continent’s collision with Asia to take place at the right time and/or place (Fig. 1). Other forms of model have been based on reconstructions of Gondwana in the Mesozoic, or estimates of shortening in the Hima- layas, between the Indian craton and the Yarlung Tsangpo suture zone. It is important to note, however, that bGreater IndiaQ is much older than plate tectonic theory. Emile Argand’s groundbreaking (literally and figuratively) work in the mid-1920s argued for what we would now call continental plate extending north Fig. 1. Schematic diagram summarizing the rationale behind bfill- from the Indian craton beneath the entire Tibetan the-gapQ Greater India proposals. The image is clipped from an orthogonal projection in which the latitudes and longitudes are Plateau region, in order to raise the huge tract of spaced at 108 intervals and is viewed from 208N. An extension to land such that its average elevation is ~5 km (see the Indian plate fills the void between the northern edge of the below). Indian craton and the southern edge of Eurasia at the time when the Reviews of Greater India have been carried out two continents are believed to have made initial contact. As the previously, e.g., Powell and Conaghan (1975) Har- 100–0 Ma motion history of India has been well-defined for the past two decades (e.g., Besse and Courtillot, 1988; Acton, 1999; see also rison et al. (1992) Le Pichon et al. (1992) Packham Fig. 3), early collision events have tended to result in modellers (1996); Matte et al. (1997) DeCelles et al. (2002). proposing large Greater India extensions. Conversely, models The work presented herein stems from the fact that assuming late collision result in small Greater Indias. J.R. Ali, J.C. Aitchison / Earth-Science Reviews 72 (2005) 169–188 171 2. Present-day Indian plate dam, which are separated respectively by the Bang- gong and Jinsha sutures (e.g., DeCelles et al., 2002). Indian plate rocks can be divided into two types: those currently attached to the craton, i.e., south of the Main Boundary Thrust, and those north of the fault 3. Motion history of the Indian plate since the Late and south of the Indus River–Yarlung Tsangpo suture Cretaceous zone (Fig. 2). The latter occupy a band ~38 S–N which is defined by the Himalayas. North of the Apart from its key role in creating Earth’s most Yarlung Tsangpo suture are three major crustal blocks spectacular orogen, the Indian continent is famous for forming Tibet: Lhasa, Qiangtang and Songpan–Qai- the speed it attained during the Late Cretaceous–early Fig. 2. Simplified tectonic map of the northern Indian Ocean and southern Asia. The Indian craton terminates at the Main Boundary Thrust (MBT). Indian plate-derived rocks are exposed between the thrust and the Indus River–Yarlung Tsangpo suture (YSTZ) where they form the Himalayas. BSZ is the Banggong Suture, between Lhasa (S) and Qiangtang (N) blocks; JSZ is the Jinsha suture, separating the Qiangtang (S) and Qaidim–Songpan Ganze (N) terrains; KF and JRF are the Karakoam and Jiali-Red River Faults respectively. The base map has been drawn over an image generated using the GEBCO Digital Atlas (2003). Details of the bathymetry in the Carlsberg Ridge area, SW of India, are not shown. 172 J.R. Ali, J.C. Aitchison / Earth-Science Reviews 72 (2005) 169–188 Fig. 3. Indian craton’s motion history since 75 Ma (a) based on Acton (1999). The stencil for the Indian craton using Acton’s 55 Ma pole is also shown (b) and is drawn using the GMAP computer program (Torsvik and Smethurst, 1999). The latter is used in all of the Greater India redrafts (Figs. 5b, 8, 10, 11). Palaeogene when it was traveling, relative to the spin graphic records of eastern India and western Australia axis, at the almost phenomenal rate of 16–20 cm/yr to align the two margins against one another (Fig. 4). (Patriat and Achache, 1984; Besse and Courtillot, This contrasted with a large body of authors who 1988; Klootwijk et al., 1992; Lee and Lawver, favoured positioning the southeast-facing coast of 1995).