PALEOSTRESS RECONSTRUCTIONS AND GEODYNAMICS OF THE BAIKAL REGION, CENTRAL-ASIA .1. PALEOZOIC AND MESOZOIC PRE-RIFT EVOLUTION

This paper presents the first palaeostress results obtained for the ba sement of the Baikal rift system, in southern Siberia (Russia). Large- scale structural analysis and palaeostress reconstructions show that t he Palaeozoic-Mesozoic kinematic history, precursor of the Baikal Ceno zoic rifting, is characterized by the succession of six regional palae ostress stages, Stress inversion of fault-slip data and earthquake foc al mechanisms is performed using an improved right-dieder method, foll owed by rotational optimization (D, Delvaux, TENSOR program). The resu lts are interpreted in the light of recent developments in the investi gation of regional intraplate stress field, and used as additional con straints for palaeogeodynamic reconstruction of Central Asia. After th e final Palaeozoic closure of the Palaeo-Asian ocean on the southern m argin of the Siberian platform, the marginal suture with the Sayan-Bai kal Caledonian belt was repeatedly and preferentially reactivated duri ng the subsequent Palaeozoic and Mesozoic history. This suture zone al so controlled the opening of the Baikal rift system in the Cenozoic. T he progressive closure of the Palaeo-Asian and Mongol-Okhotsk oceans g enerated successive continental collisions, which were recorded in the Baikal area by brittle-ductile and brittle deformations. The first tw o palaeostress stages correspond to the successive collage of Precambr ian microcontinents and Caledonian terranes along the southern margin of the Siberian platform: (1) Late Cambrian-Early Ordovician N-S compr ession; and (2) Late Silurian-Early Devonian NW-SE compression. The ne xt two stages are related to the remote effects of the complex evoluti on of the western Palaeo-Asian ocean, southwest of the Siberian contin ent: (3) Late Devonian-Early Carboniferous N-S compression, recorded o nly in the Altai region; and (4) Late Carboniferous-Early Permian E-W compression, recorded both in the Altai and Baikal regions. The last s tages are the consequences of the Mongol-Okhotsk oceanic closure: (5) Late Permian-Triassic NW-SE extension with development of Cordilleran- type metamorphic core complexes and volcanism along the active margin of the Mongol-Okhotsk ocean; (6) initial development of Early-Middle J urassic 10-15-km-wide molassic basins in Trans-Baikal and large forede eps along the southern margin of the Siberian platform in probable ext ensional context, but for which no palaeostress data are available; an d (7) final closure of the Mongol-Okhotsk ocean in the Cretaceous. Thi s last event results from the collision between the Mongol-China and t he Siberian plates and is evidenced by the inversion of the Middle Mes ozoic basins and by Late Jurassic-Early Cretaceous coal-bearing sedime ntation in Trans-Baikal. This long tectonic history yielded a highly h eterogeneous basement in the Baikal area, precursor oi the Cenozoic ri fting.