Constraints on the locations of Asian microcontinents in Palaeo-Tethys during the Late Palaeozoic

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Abstract
Useful constraints for Late Palaeozoic reconstructions of the Palaeo-Tethys and the development of Asia come from tectonic, palaeomagnetic, biogeographic and palaeoclimatic data. Tectonic constraints include the timing of collisions of the microcontinents which traditionally have been regarded as proceeding from north to south, and ranging in time from Late Palaeozoic to the Tertiary. Such a view gives a good account of the palaeobiogeographic connections, but the Mongolo-Okhotsk suture of northern Mongolia continued to close by counterclockwise rotation as late as the Jurassic. The biogeographic provinces are well developed in the Late Palaeozoic due to the relatively steep equator-to-pole gradients, thus as continents rifted from the southern margin of Palaeo-Tethys, they lost their south temperate Gondwanan affinities and acquired sub-tropical to tropical floras and faunas. Eventually, the north temperate Angaran floras and faunas inhabiting the northern margin of Palaeo-Tethys invaded some of the Cathaysian microcontinents by the end of the Palaeozoic. Unfortunately, the tropical Cathaysian floras could have occurred over a considerable latitudinal range (25° N to 25° S) and thus do not provide precise constraints. These floras, however, do contain seed-dispersed plants which implies that most of these separate microcontinents must have been geographically connected while apparently tectonically distinct. The climatically sensitive sediments include tillites and glacio-marine deposits associated with some of the terranes, and with Gondwana, but they are clearly temperate in origin, not polar, and are overlain by carbonates in Gondwana and the terranes. We interpret this as due to a climatic amelioration, rather than to a latitudinal plate motion. Palaeomagnetic data are now available for a number of south Asian microcontinents and enough determinations have now been made in North and South China to show consistency with the tectonic, biogeographic and climatic information. In detail, a southern belt of terranes, from the Helmand block in Iran and Afghanistan, through the Western Qiangtang and Lhasa blocks of Tibet and to the Sibumasu block of Thailand and Malaya, all rifted off the margins of Gondwana in the Permian. Few palaeomagnetic data are available to support this, but the tillites, floras and faunas are shared with the midlatitude portions of Gondwana from Iran, India and Australia. In the Late Permian the Cathaysian flora is known from at least the Helmand and Western Qiangtang blocks, suggesting that they reached lower latitudes, and were in physical contact with other Cathaysian floras, although the nature of this connection is not understood. The major Cathaysian microcontinents of Yangtze, Indochina, Eastern Qiangtang, Sino-Korea and Tarim were tropical throughout the Carboniferous and Permian. Again, a degree of geographic interconnection is implied by their common floras, and this is shared with low latitude portions of Gondwana, including Arabia and North Africa, but the location and nature of the 'land bridges' are unknown. Collision of Tarim with Asia in the Early Permian and of Sino-Korea with the Mongolian arcs in the Late Permian is indicated from tectonic and biogeographic data, but, as stated above, the Mongolo-Okhotsk suture did not close until the Late Jurassic, thus rotation of the combined Mongolia and Sino-Korean block continued until that time. The other Cathaysian microcentinents collided with Asia about the Late Triassic giving rise to the Indosinian Orogeny.