192 Nature Vol. 293 17 September 1981 ment with the known seismic refraction (Business and Technological Systems, Inc.) pretation of long-wavelength crustal fields and heat flow data. His group believes that magnetization model for the United States, in terms of a geological/geophysical the region is the site of a late Precambrian derived from Magsat scalar data, had much model, promises to contribute significantly rift which was reactivated in the Mesozoic better resolution than that using POGO to our understanding of the Earth's crust. to form an aulacogen and its model data. In the older disciplines of main-field involves thinning of the upper crust and an Although preliminary, the results dis­ modelling and studies of external fields, increase in density in the lower crust. The cussed at the meeting indicate that sep­ there are significant new developments in magnetic low is accounted for by either an aration of the measured field into its core, both analytical techniques and in our isotherm upwarp or, less likely, a litho­ crustal and external 'components' is being understanding of the physics of the field logical variation in the crust. Mayhew's achieved. The newest discipline, inter- sources. 0 The East Asian jigsaw puzzle Pangaea at risk? from Neville Haile UNTIL fairly recently, reconstructions of the world palaeogeography followed Wegener in showing Eurasia, excluding the Indian subcontinent, as a single block, with the Malay Peninsula and part or all of the Indonesian Archipelago depending from it and looking rather vulnerablel ,2. Other world palaeogeo­ graphical maps simply omit South-east Asia and most of China (see the figure)3,4. The suggestion that South-east Asia, or part of it, was initially separate from mainland Asia, to which it later became welded, dates back at least to Wing Easton5 in 1921. More recent variations show South-east Asia (with or without parts of China) in Palaeozoic or early Mesozoic times attached to various parts of Gondwana, or to Laurentia in the same configuration as at present, or detached from both6,7. Argand's8 idea that the rest of mainland East Asia south of the Siberian craton may be an aggregate of a number of blocks, which Reconstruction of the continents into the universal landmass of Pangaea as of the end of the have had a separate history and become Permian, 225 Myr ago - the 'classical' concept of Wegener in a version by Dietz and 3 welded together after colliding, has been Holden • Where were East and South-east Asia? recently revived9,10 on the basis of new The paper by McElhinny et 01. (this apparent polar wander curves for each of and more detailed maps produced by issue of Nature, page 212) is significant in the postulated blocks, and to integrate Chinese geologists, showing belts of calc­ showing that rocks from the same system the results with evidence from structural alkaline rocks and ophiolites interpreted (Permian) on two of the postulated geology, palaeoclimatology, fossils and as fossil subduction zones 1 I • A recent tectonic blocks of China show different (for the late Mesozoic and Cenozoic) Sino-American review l2 asserts: "In a palaeomagnetic vectors, supporting the ocean-floor spreading history. To nutshell, China is the product of the idea that in the Permian they were achieve this would seem to call for a processes which led to gradual accretion separated from each other and from major cooperative effort - an inter­ onto the Siberian craton of a number of other parts of Asia for which there is national decade of palaeogeography? C! island arc systems and the capturing of published data. The authors' geographi­ relatively small continents. This process cal reconstruction for the Permian shows I. Wilson, J.T. Sci. Am. 208(4), 86 (1963). continued with the collision of the Indian four continental blocks drifting 2 . Smith. A.G. & Briden, J.e. Mesozoic and Cenozoic Palaeocontinental Maps (Cambridge Univc:r :)ity Press. subcontinent with Asia". Palaeomag­ independently in a proto-Pacific Ocean, 1977). netic methods are ideally suited to test waiting to collide with and be welded to 3. Dietz, K.S. & Holden, J .C. J.geophys. Res. 75,4939 these hypotheses, delineate the pieces of the mainland (India and Qinhai-Tibet (1970). 4 . Irving, E. Nuture 270, 304 (1977). the jigsaw puzzle and trace their being off-stage, presumably to the south, 5. Wing Easlon. ciled by du Toil. A.l.. in Our Wandering movements through geological time, but forming part of Gondwana). If this is Continents - an Hypothesis o/Continental Drifting. 126 (Oliver and Boyd. Edinburgh, 1937). hitherto palaeomagnetic evidence from correct, then Wegener's percipient idea, 6. Scotese, C.R .. Kambach, K.K .. Barton. C .• vander Voo, China has been sparse and inconclusive. now orthodox theory, that all the con­ R. & Ziegler. A.M. 1. Oeol. 87. 127 (1979). 7. Haile. N.S. Earlhplanet. Sci. Lell. 48, 233 (1980). Moreover, this lack of information from tinents were joined into a single super­ 8. Argand. E. c.r. 13th into geol. Congr. 1922. Brussels, China makes it difficult to interpret and continent of Pangaea in the Permian, is 1.171 (1942). integrate the data available from the peri­ an over-simplification. 9. Burett. c. r. Earth planet. Sci. LeU. 21181 (1974). 10. Terman. M.J. Abstr. Bull. Am. Ass. petrol. Oeol. 62. pheral areas of Siberia, Korea, Japan and The authors' model is only one of 12,2513 (1978). the Indian subcontinent, and South-east several that could satisfy the data. Ideally II. Huang. Chi·Ching (T.K .) Ec/og. geo/. He/v. 71. 611 13 (1978). Asia • the next step should be an attempt to 12. Bally, A.W. et al. U.S. geo/. Survey Open·file Rep. establish palaeomagnetic vectors from all 80-501, Reston. Virginia (1980). 13. Haile, N.S. in Paleoreconslruction oj {he Continents, Neville Haile is at Robertson Research geological systems over wide areas of East 129 (American Geophysical Union Geodynamics International, Llanrhos, UK. and South-east Asia in order to define Series 2, 1981). 0028'{)836/ 81 / 380192'{)2$01.OO C>1981 Macmillan Journals Ltd .