Transects are vertical sections of the Earth’s crust, which reveal the nature of tectonic zones, as well as their spatial relationships through a combined analysis of their geology and geophysics. Transect documents contain a geological map for a strip of land 100 km wide, a geological section of the upper crust, gravity and magnetic maps (and/or corresponding profiles along the transect), and a geophysical profile of the crust, differentiated by seismic velocities, densities and other geophysical properties. These data are used to compose a combined cross-section (the resulting section), which shows a set of rocks typical of various geodynamic conditions (rifts, oceans, collision zones, orogenic basins, continental platforms and magmatic arcs, including Andean island arcs, active continental outskirts, trenches, basins of front and rear arcs). The objective of this project was to build deep sections according to unified legends based on the interpretation of all available geological and geophysical data in order to determine the spatial relationship of terranes and their geodynamic nature in terms of plate tectonics.
A number of terranes have been discriminated in the territory of the southern part of Eastern Siberia and the territory of Mongolia, and their geodynamic nature and space-time relations were analysed. The terranes were found out to be Vendian-Early Paleozoic, Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic island arcs and microcontinents. Moreover, Middle-Late Paleozoic and Late Paleozoic-Early Mesozoic Andean-type active continental margins and Late Paleozoic-Early Mesozoic passive margins and Early Cretaceous rifts were identified and studied. The rock complexes related to the island arcs and Andean-type active continental margins are thrust over the bordering continents and microcontinents, the width of the respective tectonic nappes attaining 150 km. Schematic paleogeodynamic reconstructions for the area of the Mongolia-Okhotsk ocean have been performed, spanning the period from Devonian to Late Jurassic.
“Non-geosyncline” granitoid magmatism finds straightforward and sound explanation in terms of plate tectonics where provinces of Devonian-Carboniferous and Permian-Triassic magmatism correspond to Andean-type active continental margins and Middle-Late Jurassic magmatism is associated with Siberia/Mongolia-China collision. The presence of a subalkaline (mantle) element in collisional magmatism and the great extent of the area it occupies can be explained by suggesting that an oceanic rift (a mantle hotspot) was buried under thick continental lithosphere after closure of the Mongolia-Okhotsk ocean. In the Early Cretaceous, the setting of collision gave way to that of continental rifting.
The existence of an Andean-type active margin over the great extent of the southern border of Siberia is likewise responsible for minor abundance of ophiolites along the Mongolia-Okhotsk suture. When one colliding continent has an Andean-type active margin and the other has a passive margin, the continental crust of the former thrusts over the latter, and no conditions arise for ophiolites to expose. Blocks of dismembered ophiolites, that are remnants of truncated seamounts, can be part of chaotic complexes building accretion-subduction wedges. However, accumulation of such wedges in the Late Permian-Early Jurassic was not typical of the active margin of Siberia because of rapid subduction.
An analysis of geological and geophysical data on transects shows that the Asian continent was formed in the Phanerozoic as a result of accretion of terranes, some of which were microcontinents with a Precambrian foundation. Precambrian blocks are separated by deformed and strongly eroded Phanerozoic igneous arcs of various widths, also classified as specific terranes.
