Neotectonics of the Altai-Sayan Mountains and reactivation of regional faults controlling seismicity

The formation of mountain system and neotectonic structure of the Altai-Sayan region is regarded to be a result of intercontinental deformations, related to a distant effects of tectonic stress from the Indo-Eurasian collision. Within this tectonic model we carried out the joint analysis of the geology, seismicity data and topographic materials enable to assume that the maximum changes in the relief and seismic activity in the northern part of Central Asia mountain belt are confined to the zones of intersections of the Late Paleozoic regional faults. The intersections and junctions of faults should be considered as one of the most important structural factors that increase the fragmentation of the substrate, affect changes in the local stress field and predetermine the localization of large earthquake foci with a magnitude M≥5. Some regularities were revealed, based on the example of helium and travertine manifestations in the junction zone of the Charysh-Terekta and Kurai regional faults, which can be used as precursors of earthquakes

Deep mechanisms in the Kyrgyz Tien Shan orogen (<i>from results of seismic tomography</i>)

AbstractSeismic-tomography studies were conducted in the Kyrgyz Tien Shan using two different observation schemes. The first was based on the arrival times of P and S waves from regional earthquakes recorded with local seismological networks (local scheme). Nonlinear tomographic inversion based on the LOTOS algorithm was used to construct the 3D distributions of P and S wave velocities in the crust beneath the Kyrgyz Tien Shan and to refine the earthquake locations. The second scheme was used to study the upper-mantle structure based on data from global earthquake catalogs (regional scheme). All the data on waves which at least partly travel within the volume studied were used here, including (1) those from regional earthquakes recorded at world seismic stations and (2) teleseisms recorded at the local stations. This approach was earlier applied to calculate the upper-mantle structure beneath Asia. We used a fragment of this structure beneath the Tien Shan and adjacent areas. A series of synthetic tests was performed to estimate the resolution provided by both schemes. The tomography shows traces of the delamination of the Tarim mantle lithosphere from south to north. Also, the local and regional schemes reveal evidence for cold-matter descent from north to south in the northern Tien Shan but on a much smaller scale. Low velocities in the upper mantle beneath the Tien Shan might indicate lithospheric thinning. These data suggest that mantle-lithosphere delamination is taking place underneath both the northern and the southern margins of the Tien Shan collision belt. Lack of the mantle lithosphere beneath the Tien Shan leads to lithospheric weakening and active deformation, thus causing intense orogeny.</jats:p

Eclogite-Like Metagabbro of the Olkhon Terrane, West Baikal Area

© 2020, Pleiades Publishing, Ltd. Abstract: Eclogite-like rocks of the Olkhon terrane correspond to two types according to the mode of occurrence and mineral compositions, although both types developed after the same type of gabbro of a single complex. The rocks of the first type are composed of garnet, clinopyroxene, pargasite (± scapolite, ± anorthite, ± spinel) and occur as thin veins or patches in the marginal parts of several small gabbro massifs of the Tonta Zone. The garnet is of almandine–pyrope–grossular composition and contains up to 40% of the pyrope end member. The rocks of the second type have been found in the Zamogoi Island in the Maloe More Strait. The island is composed of metasomatically altered gabbro cut by numerous granite and syenite veins. Carbonate and carbonate–silicate rocks compose dismembered veins and veinlets, as well as matrix embedding variably sized fragments of metasomatized gabbro and eclogite-like rocks in the southern part of the island. Based on the mode of occurrence and the presence of calcite–anorthite symplectite in some of the rocks, we suggest that the gabbro was penetrated by carbonate fluid or fluid–melt in a tectonic zone. The zone contains veins of garnet–clinopyroxene composition (varying from monomineralic garnetite to monomineralic pyroxenite) with minor pargasite, zoisite, anorthite, scapolite, and titanite. The garnet of Zamogoi Island has a grossular–almandine composition with a minor andradite concentration (about 10%). The compositions of minerals of the Zamogoi massif differ from the compositions of minerals in Tonta Zone, and the former are much more similar to skarn minerals