Rifts of the Baikal mountain region

Abstract

Large and small graben comprising the Baikal rift zone are connected with an extensive Pliocene-Pleistocene uplift with amplitude 3,000–4,000 m above sea level. These grabens inherit the strike of the Early Neogene subsidences and they cannot be considered as the result of fracturing or sinking of the crust in the arch of the uplift. The long and discontinued basalt volcanism, which preceded and accompanied the rift formation, as well as active mountain formation and the character of fracture tectonics point to the leading role of stretching of the Earth's crust during the formation of this zone; the zone has no evident connection with the world rift system; moreover, it has a specific strain field in which, according to seismological data, near-horizontal stretching forces are dominant. They are directed across the major tectonic and orographie lines. A reverse strain pattern in earthquakes foci in the western region of the zone (near-horizontal compression), in the presence of the outer structure-morphological monotony of the latter, can testify to the growing influence on this region of conditions which are inherent in the neighbouring region of the central-Asiatic depressions (Mongolia) where earthquake foci have similar strains and seismic activity is much higher. Numerous signs of increase of rift depressions in width and depth, the presence of embryonal but active rift structures within the Stanovoi Mountains indicate the continuation of the development of the zone in the modern epoch. Its rift structures are only partially connected with the system of the boundary seam of the Siberian Platform. They also utilize the inner ancient faults in the Baikalides and Early Caledonides belt. Contrary to the opinions of some authors, the horizontal displacement component in the structure of the Baikal Rift has a clearly subordinate role. Among the existing interpretations of the gravimetrical data related to the Baikal zone the hypothesis suggested by Borisov(1965) should be particularly noted. The hypothesis proposes “a crust—mantle mixture” comprising the root of the general structure of the Baikal Rift, while the cause of considerable mobility of the crust in this zone can be found in highly heated upper layers of the upper mantle. This is supported by recent researches in Pribaikalie carried out by the magneto-telluric sounding.