Palaeostress reconstructions and geodynamics of the Baikal region, Central Asia, Part I. Palaeozoic and Mesozoic pre-rift evolution

Abstract

This paper presents the first palaeostress results obtained for the basement of the Baikal rift system, in southern Siberia (Russia). Large-scale structural analysis and palaeostress reconstructions show that the Palaeozoic-Mesozoic kinematic history, precursor of the Baikal Cenozoic rifting, is characterized by the succession of six regional palaeostress stages. Stress inversion of fault-slip data and earthquake focal mechanisms is performed using an improved right-dieder method, followed by rotational optimization (D. Delvaux, TENSOR program). The results are interpreted in the light of recent developments in the investigation of regional intraplate stress field, and used as additional constraints for palaeogeodynamic reconstruction of Central Asia. After the final Palaeozoic closure of the Palaeo-Asian ocean on the southern margin of the Siberian platform, the marginal suture with the Sayan-Baikal Caledonian belt was repeatedly and preferentially reactivated during the subsequent Palaeozoic and Mesozoic history. This suture zone also controlled the opening of the Baikal rift system in the Cenozoic. The progressive closure of the Palaeo-Asian and Mongol-Okhotsk oceans generated successive continental collisions, which were recorded in the Baikal area by brittle-ductile and brittle deformations. The first two palaeostress stages correspond to the successive collage of Precambrian microcontinents and Caledonian terranes along the southern margin of the Siberian platform: (1) Late Cambrian-Early Ordovician N-S compression; and (2) Late Silurian-Early Devonian NW-SE compression. The next two stages are related to the remote effects of the complex evolution of the western Palaeo-Asian ocean, southwest of the Siberian continent: (3) Late Devonian-Early Carboniferous N-S compression, recorded only in the Altai region; and (4) Late Carboniferous-Early Permian E-W compression, recorded both in the Altai and Baikal regions. The last stages 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 Jurassic 10–15-km-wide molassic basins in Trans-Baikal and large foredeeps along the southern margin of the Siberian platform in probable extensional context, but for which no palaeostress data are available; and (7) final closure of the Mongol-Okhotsk ocean in the Cretaceous. This last event results from the collision between the Mongol-China and the Siberian plates and is evidenced by the inversion of the Middle Mesozoic basins and by Late Jurassic-Early Cretaceous coal-bearing sedimentation in Trans-Baikal. This long tectonic history yielded a highly heterogeneous basement in the Baikal area, precursor of the Cenozoic rifting.