BAIKAL RIFT BASEMENT - STRUCTURE AND TECTONIC EVOLUTION

Baikal rift zone in East Siberia has, due to its very long history, st arting in the Early Precambrian and continuing into the Cenozoic, a he terogeneous structure. Two major structural elements are distinguished : the Siberian craton and the Sayan-Baikal fold system, which is part of the Central Asian Fold Belt. Within the craton, the following units are recognized: the Early Precambrian metamorphic rocks of the baseme nt, the Vendian-Paleozoic sedimentary cover with superimposed Paleozoi c and Mesozoic basins and a reworked margin which is transitional betw een the craton and the fold belt. The Sayan-Baikal fold system compris es the Barguzin, Tuva-Mongolian and Dzhida terranes. The Barguzin and Tuva-Mongolian terranes are composite, consisting of separate Early Pr ecambrian massifs, volcanic-sedimentary and carbonate Phanerozoic terr anes, impregnated by granites. They are thus super-terranes. The oldes t units are ophiolites, dated around 1.1-1.3 Gy in the Barguzin terran e and 0.9-1.1 Gy in the Tuva-Mongolian terrane. They associate with co mplexes of immature island-arc accretionary wedges of mainly terrigeno us or terrigeno-volcanic composition with occasional slices of ophioli te and glaucophane schists. Two major periods are distinguished: the E arly Precambrian period which comprises several phases of deformation and metamorphism with diverse magmatic activity, and the Riphean-Phane rozoic period which includes: a) the opening of the paleo-Asian ocean, b) the formation, during the Riphean, of peri-oceanic structural syst ems such as island arcs, fore-arc and inter-arc basins and accretionar y wedges, c) the amalgamation of these systems, during the Late Precam brian, to form the Barguzin and Tuva-Mongolian super-terranes. At the same lime, a system of island arcs was formed which became the Dzhida terrane, d) the collision of all three terranes with the Siberian crat on, in the Early Paleozoic, with accompanying high-grade metamorphism and the formation of the huge Angara-Vitim batholith, e) a post-collis ion phase, marked by orogenesis and continental extension which, in th e Middle Paleozoic, resulted in the tectonic exposure of the metamorph ic core complexes, and f) the development of an active continental mar gin during the Late Paleozoic.