TETHYS RECONSTRUCTED - PLATES, CONTINENTAL FRAGMENTS AND THEIR BOUNDARIES SINCE 260-MA FROM CENTRAL-AMERICA TO SOUTH-EASTERN ASIA

This paper describes the evolution of the former Tethys ocean in terms of successive positions of the involved plates, their boundaries, and the oceanic and continental elements they bore. Most of the past posi tions for the major continents are obtained from published data on the kinematics of the Atlantic and Indian oceans. As they do not cover th e complete time span and area under study, additional solutions for th e missing parts are proposed on the basis of paleomagnetic and geologi c data. A revised reconstruction of Eastern Gondwana and a reappraisal of the early history of the Indian ocean is presented, as well as a d iscussion of the Pangea megashear concept which applies to the relatio ns between Gondawa and Laurussia during latest Palaeozoic to Mid-Trias sic times. Past positions of continental blocks, basins and associated plate boundaries within the now disappeared Tethys ocean, are determi ned through the kinematic inversion of the Alpine chains interpreted i n terms of break-up, oceanic spreading and drift, subduction and colli sion. A quantitative solution is obtained by associating Eulerian rota tion parameters to any relative displacement, computing their combinat ion, then adjusting until a complete coherency through time and space is obtained. Our method puts emphasis on the former positions of the p late boundaries in the Tethys, and stresses the drastically different organisation of Tethys west and east of a fundamental triple junction located in its Mediterranean part. A symmetric Atlantic part of Tethys , governed by a mid-ocean diverging boundary, was thus linked eastward s to an asymmetric ''Transit'' part of Tethys where southern diverging boundaries and northern converging boundaries defined a Transit plate (or group of plates). Besides several reorganisations through southwa rd shift of these boundaries, the successive (or coeval) transit plate s systematically exhibited the same north-south asymmetry responsible for the northward migration of the various continental blocks they car ried, which were successively detached from Gondwana to become accrete d to Eurasia. The transit process, while an essential element of the T ethyan tectonic pattern, was not restricted to lifetime of Tethys, as it is still active nowadays and was already active during the Palaeozo ic. Nevertheless, its association with the Atlantic Tethys diverging b oundary is typical for the lifetime of Tethys, both constituting a ''T ethys plate pattern'', in opposition to the present-day and to Hercyni an ''Anti-Tethys plate pattern'' where a converging boundary is and wa s associated with the transit plate system. Two pre-eminent tectonic e vents, the Late Triassic switchover from the Anti-Tethys pattern to th e Tethys pattern, and the Late Cretaceous switchover back to the Anti- Tethys pattern, decided birth and death of the Tethys ocean.