[1] A range of ages have been proposed for the timing of India-Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6-52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U-Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India-Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.

Faunal and palaeomagnetic evidence suggests the existence of a c. 3000 km-wide Tornquist Sea between Gondwana and Baltica in Early Ordovician times, which narrowed to <1000 km by the Late Ordovician. The inferred suture zone between sequences with 'Baltic' faunas in Poland and the 'Gondwana' faunas in Czechoslovakia is characterized by a collage of six terranes which have distinct Cambrian to Carboniferous histories. They have geochemical characteristics and sedimentary associations of volcanic rocks in marginal basins, island arcs, ophiolites and volcanic arc granites, give Tremadoc and Arenig protolith U-Pb zircon ages, and have been regionally metamorphosed. A syn-metamorphic granite is dated as pre-Lower Ashgill ~t461 ÷5°_z Ma). An unmetamorphosed ophiolite gives a 420+_2°Ma age, whilst in two other terranes amphibolite-facies island-arc lavas have fossiliferous Lower Ludlow (424-415 Ma) sequences: all are unconformably overlain by unmetamorphosed Upper Devonian conglomerates. A sinistral transpressive regime is observed in regionally 8 +2 extensive mylonite zones. Titanite and zircon ages (33 -3 and 339 + 4 Ma) record Variscan magmatism. The data suggest considerable narrowing of the Tornquist Sea during the Ordovician, continuing ocean floor and island arc activity in the Silurian, and final sinistral transpressive closure by the Mid-Devonian.

Three principal granite provinces are defined across SE Asia, as follows. (1) The Western Thailand–Myanmar/Burma province consists of hornblende–biotite I-type granodiorite–granites and felsic biotite–K-feldspar (± garnet ± tourmaline) granites associated with abundant tin mineralization in greisen-type veins. New ion microprobe U–Pb dating results from Phuket Island show zircon core ages of 212 ± 2 and 214 ± 2 Ma and a thermal overprint with rims of 81.2 ± 1.2 and 85–75 Ma. (2) The North Thailand–West Malaya Main Range province has mainly S-type biotite granites and abundant tin mineralization resulting from crustal thickening following collision of the Sibumasu plate with Indochina during the Mid-Triassic. Biotite granites around Kuala Lumpur contain extremely U-rich zircons (up to 38000 ppm) that yield ages of 215 ± 7 and 210 ± 7 Ma. (3) The East Malaya province consists of dominantly Permian–Triassic I-type hornblende–biotite granites but with subordinate S-type plutons and A-type syenite–gabbros. Biotite–K-feldspar granites from Tioman Island off the east coast of Malaysia also yield a zircon age of 80 ± 1 Ma, showing Cretaceous magmatism in common with province 1. Geological and U–Pb geochronological data suggest that two east-dipping (in present-day coordinates) subduction zones are required during the Triassic, one along the Bentong–Raub Palaeo-Tethyan suture, and the other west of the Phuket–Burma province 1 belt. Supplementary material: A full description of U–Pb analytical methods used and data tables are available at www.geolsoc.org.uk/SUP18523 .

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Abstract.In this paper we tackle some of the outstanding problems of the Himalaya, in particular the external zone in the Kathmandu Complex, using an integrated approach involving field mapping, microstructure, thermobarometry, and geochronology. The result is a new model showing the evolution of one major Main Central Thrust: therefore we refute suggestions that the Kathmandu Complex is a klippe or separate thrust sheet. Compared to the Main Central Thrust sheet in the High Himalaya, the Kathmandu Complex shows differences in deformational and metamorphic features and timing of metamorphism that are consistent with its position some tOO km south of the High Himalaya, fairly near the leading edge.Unless there was substantial volume loss between the time of peak metamorphism and the beginning of thrusting then our geobarometry results indicate that the Main Central Thrust wedge was-40 km thick on the northern side of the Kathmandu Complex and <20 km thick on the southern margin. Initiation of the Main Central Thrust occurred at-22 Ma, possibly during the closing stages of peak amphibolite facies metamorphism; slip at elevated temperature (500 ø-600øC) continued until -14 Ma. This is a little longer than has previously been proposed. In marked contrast to the famous inverted metamorphism on the Main Central Thrust in the High Himalaya, the metamorphic zonal scheme in the Kathmandu Complex is right way up with the exception of a thin zone of greenschist facies thrust related dynamically metamorphosed rocks at the base. These mylonites postdate the high-grade regional amphibolite metamorphism and give an illusion of inverted metamorphism. A likely reason for the contrast is that the Main Central Thrust cut up section toward the foreland and therefore at Kathmandu, carries high levels in the metamorphic structure. Our model involves reactivation of the Main Central Thrust at 7-8 Ma as inferred from published monazite and mica ages, but because the Kathmandu rocks show no evidence for high-temperature reactivation at this time, we presume that the late reactivation involved only the