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Comment on “Restoration of Cenozoic deformation in Asia and the size of Greater India” by D. J. J. van Hinsbergen et al.

2012; Wiley; Volume: 31; Issue: 4 Linguagem: Inglês

10.1029/2011tc003091

1944-9194

Jason R. Ali, Jonathan C. Aitchison,

High-pressure geophysics and materials

[1] Despite decades of investigation and literally thousands of research papers, there remain strongly divergent opinions on a number of key features/phenomena associated with the India-Asia collision system and the Himalayan Mountain-Tibetan Plateau orogen that resulted. One particularly contentious issue concerns the timing of contact between the two continents; although many favor an early Eocene/early–middle Eocene age, recent proposals have ranged from ∼65 Ma [e.g., Cai et al., 2011] to ∼35 Ma [Aitchison et al., 2007]. [2] In order to deduce the collision timing, we argue that detailed understanding is required first of the elements associated with the suture zone, and second, the broader geodynamic configuration before and during the contact. Regarding the former, the following are critical: (i) age of the youngest marine sediments; (ii) age of the youngest subduction-generated calc-alkaline volcanic rocks; (iii) identification and full interpretation of any trapped terranes; (iv) age and clast-assemblage lithologies of any molasse formations; (v) ages of the youngest entrained blocks in any mélange deposits plus that of the matrix. From a plate-modeling perspective, we need to know (i) past position of stable Eurasia; (ii) extent of deformation (shortening, extrusion etc) Eurasia's leading edge experienced after the contact; (iii) motion path of the Indian block; (iv) size of the Indian sub-continent prior to the collision, which is commonly referred to as "Greater India." [3] Recently, Van Hinsbergen et al. [2011a, 2011b] have proposed a new collision scenario. In their model, contact is interpreted to have taken place at 50 Ma, and following a structural restoration of Asia's southern margin, a sub-continent extension is proposed that bridges the gap between the Indian craton (to the south of the Himalayan frontal thrust) and the Lhasa Block; ∼1350 km in the west, ∼2600 km in the east. By corollary, a huge amount of lithosphere must therefore have been subducted/underthrust beneath the Asian margin in Tibet. The model has provoked our interest, not only because the hypothesized Greater India is the largest so far presented [see Ali and Aitchison, 2005], but we consider it to suffer from two "unintended-consequence" flaws. [4] Although the extension proposal of van Hinsbergen et al. [2011a], based around a 50 Ma contact can in theory explain the collision (to avoid potential misunderstanding, though, we reiterate our long-held view that the India-Asia collision occurred ∼15 my later), a problem arises when it comes to positioning their Greater India at its former site in Gondwana (before the Early Cretaceous, ∼130 Ma). Since the classic "tight-fit" reconstruction of Smith and Hallam [1970], there has been widespread consensus over the manner in which India (eastern), Australia (western) and Antarctica (eastern) are considered to have once abutted [e.g., Schettino and Scotese, 2005; Cocks and Torsvik, 2006] (see Ricou [1994] for a radical alternative). The issue with the van Hinsbergen et al. [2011a] model is that not only does the eastern side of the extension overlap western Australia (Figure 1a), but it fails to accommodate various rafts of continental crust that are present offshore, specifically the Exmouth, Wallaby and Zenith plateaus plus the Carnarvon Terrace. It was the second and third features [see also Colwell et al., 1994; Symonds et al., 1998] that prompted us [Ali and Aitchison, 2005] to propose a Greater India whose northern edge was delimited, at least in the eastern and central parts, as including the area today occupied by the Perth Basin, southwest of the Wallaby-Zenith Fracture zone (Figure 1a). The recent paleontologically focused publications of Quilty [2011] and Stilwell et al. [2012] add considerable weight to this proposal. They indicate that in the Late Jurassic (Oxfordian-Kimmeridgian), around 12 my prior to the India's rifting from the Australian margin, the Wallaby Plateau occupied a shallow marine site. Thus, the widely held view is that the edifice is "continental" and is capped by sea-ward dipping reflectors; critically it is not an Early Cretaceous submarine large igneous province resulting from excessive magmatism as the oceanic crust off western Australia formed. We therefore argue that the various plateaus west of Australia act to eliminate large tracts of lithosphere from the van Hinsbergen et al. [2011a] Greater India. [5] Concerning Greater India's northwestern extremities, although we were unable to use similar elements to define the block's boundaries, based on the uniform width and broadly similar nature of the deformed Indian upper-crustal sequences along the entire strike-length of the Himalayas (∼2500 km), we suggested that it followed a similar line before wrapping back to connect with western edge of the craton. [6] A major episode in Gondwana's history involved the Early Permian (Artinskian stage, ∼275 Ma) detachment of the Cimmerian super terrane. Dwarfing in length any of the modern sliver blocks (e.g., Lord Howe Rise, Tasman Sea; Palawan block, offshore SE Asia), remnants of Cimmeria form an ostensibly continuous belt stretching ∼13,800 km, from central Italy to east of Sumatra (Indonesia), via Greece, Turkey, Iran, Afghanistan, Tibet, southwest China, Myanmar and Thailand [Stampfli et al., 2001; Metcalfe, 2011]. From a plate modeling perspective, one of the intriguing aspects concerns how specific portions of the terrane fitted back to their original sites; it is in this context, we argue that the Greater India of Van Hinsbergen et al. is problematic. A specific issue is the Baoshan sub-block of western Yunnan province, SW China; the paleomagnetic investigation of Huang and Opdyke [1991] indicates that a series of rift-related basalts (Woniusi Formation) were erupted at ∼42°S. Although the paleolatitude determination was based upon directions from just three localities (19 sites), our recent work, as yet unpublished (five localities, 31 discrete cooling units), confirms the earlier findings. Applying the statistics of McFadden and Reid [1982] to the old and new data sets results in a tilt-corrected mean inclination of 61.0°, where α95 = 7.6° and k = 40.8 (N = 8), implying that the eruptions took place at 42.1°S – with errors, the permissible range is 34.0°S–51.9°S. However, if the Greater India of Van Hinsbergen et al. is incorporated into Gondwana (using the appropriate rotation poles listed in Torsvik et al. [2008], and ignoring the problems alluded to in the preceding section), it is difficult to position the Baoshan sub-block against the supercontinent at a latitude that can accommodate the paleomagnetic data (Figure 1b). The edge of the likely matching margin of Gondwana (northern Greater India-northern Australia) sat ∼30°S; the ground directly adjacent to the Greater India's northeastern corner is unavailable; the ocean floor here dates from the latest Jurassic (Hole 261) [Veevers et al., 1974] /earliest Cretaceous (Hole 765) [Ludden et al., 1990], and is thought to be the original site of the Argo and Banda blocks that today form terranes in central southern Indonesia [Hall, 2011]. [7] For the two reasons outlined above, we contend that the "Greater India" of van Hinsbergen et al. [2011a] is untenable when (i) key geotectonic features of the eastern Indian Ocean-western Australia region and (ii) accommodation of Cimmerian-terrane blocks are considered. Any model attempting to delimit the Indian sub-continent's pre-Asia-collision size needs to be compatible with tectonic configurations that are applicable for earlier times in the block's history. [8] Douwe van Hinsbergen kindly supplied rotation data that enabled us to replicate the van Hinsbergen et al. [2011a] tectonic model. Formal critiques by an anonymous reviewer and Robert Hall were very useful. Our research on the India-Asia collision has almost exclusively been supported by grant awards from the Hong Kong Research Grants Council; recent paleomagnetic studies of the Baoshan block were funded by an award to JRA (HKU7001/10).