Khangai Intramantle Plume (Mongolia): 3D Model, Impact on Cenozoic Tectonics and Comparative Analysis

The Khangai plume is located beneath Central and Eastern Mongolia and corresponds to the mantle volume with significantly reduced longitudinal wave (P) velocities. The plume was identified as a result of the analysis of the MITP08 volumetric model of variations in P wave velocities, expressed as deviations of these velocities from the mean values for the corresponding depths in percent. Above the plume, the lithospheric mantle is thinned to ~50 km. Particularly low velocities (up to –6%) were found in the sublithospheric mantle down to a depth of 400 km. The main body of the plume is located under the Khangai Highland and spreads north to the edge of the Siberian Platform. The Khentei branch of the plume is identified southeast of the Khentei Highlands. It is connected to the main body of the plume at depths of 800–1000 km. Branches of the plume and its Khentei branch spread to Transbaikalia. The size of the plume decreases with depth, and its deepest part (1250–1300 km) is located under the southern part of the Khangai Highland. On the Earth’s surface, the main body of the Khangai plume corresponds to a Cenozoic uplift up to 3500–4000 m high in the south of the Khangai Highland. From the southeast, the territory of the Khangai plume and its Khentei branch is limited by the Late Cenozoic troughs stretching along the southeastern border of Mongolia. On other sides, the Khangai uplift is limited by a C-shaped belt of depressions, consisting of the southeastern part of the Baikal rift zone, the Tunka and Tuva basins in the north, the Ubsunur Basin and the Great Lakes Basin in the west and the Valley of Lakes in the south. The depressions are filled with lacustrine and fluvial sediments from the Late Oligocene to the Pliocene. In the Quaternary, the Southern and Central basins of Baikal, formed no later than the Early Paleogene, became part of the Baikal rift, and other depressions were involved in the general uplift of the region. The structural paragenesis of the Khangai uplift and surrounding basins is due to the impact of the Khangai plume. Above the plume with its Khentei and Transbaikalian branches, the Cenozoic basaltic volcanism of the plume type occurred, in some places inheriting Cretaceous volcanic manifestations. Plume structural paragenesis is combined with structural paragenesis, derived from the interaction of plates and lithosphere blocks, which is expressed by active faults, but developed synchronously with plume paragenesis. The kinematics of active faults shows that the western and central parts of the region develop under conditions of transpression, and the northeastern part ‒ under conditions of extension and transtension. The Khangai plume is connected at depth with the Tibetan plume, located under the central and eastern parts of Tibet north of the Lhasa block. The Tibetan plume rises from depths of 1400–1600 km and is accompanied by thinning of the lithosphere and rise of the earth’s surface. The Khangai and Tibetan plumes represent a special category of plumes that rise from the upper part of the lower mantle and this differs from the upper mantle plumes and the African and Pacific superplumes, rising from the core-mantle boundary. A connection between the Khangai and Tibet plumes with branches of superplumes is possible, but their independent origin is also admitted.