Geodynamic nature of the Baikal Rift Zone and its sedimentary filling in the Cretaceous–Cenozoic: the effect of the far-range impact of the Mongolo-Okhotsk and Indo-Eurasian collisions

Abstract

The problem of the Late Meso-Cenozoic tectonics and geodynamics of the southern framing of the Siberian Platform is discussed. This area abounds in Late Meso-Cenozoic structures composed of sedimentary deposits. Analysis of voluminous geologo-stratigraphic, geophysical, and geochronological (track dating) data showed that these structures, including the Baikal Rift Zone, resulted from the far-range impact of two collisions related to the prolonged convergence of the North China and Indian continents with Eurasia, which took place in the Late Jurassic–Paleocene and Cenozoic, respectively. The synchronous occurrence of two great tectonic events led to the formation of a complex structure and a sedimentary basin, which are of different geodynamic nature and cannot be united into the same sequence of Late Cretaceous–Cenozoic formation of the Baikal Rift Zone. The Baikal system of shears and conjugate rifts formed in the Pliocene–Quaternary as a result of the far-range impact of the Indo-Eurasian collision. The latter led to the accumulation of the upper layered undeformed seismostratigraphic complex in all three Baikal basins. The sediments in Central Baikal are up to 3 km thick, and in the Selenga River valley they reach 5–6 km in thickness. The active intracontinental rift structures are characterized by zonal sedimentation. The middle layered deformed seismostratigraphic complex 1–1.5 km thick is recognized in all three Baikal basins and is similar to the Upper Oligocene–Lower Pliocene sediments of lacustrine-littoral, deltaic, and lacustrine facies widespread throughout the Baikal and Altai–Sayan regions and in Mongolia, where they formed in the environments of large lake systems. As a result of the deformations caused by the Indo-Eurasian collision, the Upper Oligocene–Lower Pliocene sediments were involved in the Pliocene–Quaternary ramp and unilateral-ramp structures in intermontane basins of the Altai–Sayan region and in Mongolia. The lower seismically transparent seismostratigraphic complex occurs only in South Baikal and Central Baikal. It is 1 km thick in the east and up to 45 km thick in the west. The complex is a fragment of the Late Cretaceous–Paleogene lacustrine-river sediments of the large fore-Baikal piedmont basin, which formed at the final stage of evolution of the vast Mongolo-Okhotsk orogen.