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

Abstract

The Khangai plume is situated under Central and Eastern Mongolia and is a mantle volume with significantly reduced longitudinal (P) wave velocities. The plume has been identified as a result of the analysis of the MITP08 volumetric model of P-wave velocity variations, representing the deviations of P-wave velocities from the average values (δV_p), given as a percentage. The lithospheric mantle is thinned to ca. 50 km above the plume. Especially low velocities (δV_p ≤ –0.6%) are found in the sublithospheric mantle up to a depth of 400 km. The main body of the plume is located under the Khangai Highland and extends northward to the edge of Southern Siberia. The Khentei branch of the plume that is located SE of the Khentei Highland is connected with the main plume body at depths of 800–1000 km. Branches of the plume and its Khentei branch extend into Transbaikalia. The area of the plume decreases with depth, and its deepest part (1250–1300 km) is located under the southern Khangai Highland. The main body of the Khangai plume is expressed on the land surface by the Cenozoic uplift reaching 3500–4000 m in the southern Khangai Highland. From the SE, the Khangai plume and its Khentei branch territory are limited by Late Cenozoic troughs stretching along the southeastern border of Mongolia. From other sides, the Khangai uplift is bounded by a C-shaped belt of basins. The belt includes the southwestern part of the Baikal Rift Zone, the Tunka and Tuva Basins in the north, the Ubsu-Nur Basin and the Basin of Big Lakes in the west, and the Valley of Lakes in the south. The basins are filled with lacustrine and fluvial deposits of the Late Oligocene to Pliocene. In the Quaternary, the South and Central Baikal Basins, which existed as early as the Early Paleogene, became a part of the Baikal Rift, and the other basins were involved in the general uplift of the region. The structural paragenesis of the Khangai uplift and the surrounding basins is caused by the influence of the Khangai plume. On the territory above the plume, including its Khentei and Transbaikalia branches, the Cenozoic basaltic plume volcanism occurred, inheriting the Cretaceous volcanic manifestations in some places. The structural paragenesis associated with the Khangai plume is combined with the structural paragenesis produced by lithospheric plate interaction. The latter is expressed the best of all by active faults, but developed synchronously to the plume paragenesis. The active fault kinematics shows that the eastern and central parts of the region developed in the transpression conditions and the north-eastern part developed in conditions of extension and transtension. The Khangai plume is connected at depth with the Tibetan plume, which is situated under the central and eastern Tibetan Plateau north of the Lhasa block. The Tibetan plume has the shape of a funnel rising from depths of 1400–1600 km and is accompanied by thinning of the lithosphere and uplift of the land surface. The Khangai and Tibetan plumes represent a specific category of plumes. They rise from the upper Lower Mantle and differ by this from the Upper Mantle plumes and the African and Pacific superplumes rising from the core-mantle boundary. Data are presented on the possible connection of the Khangai and Tibetan plumes with the superplume branches, but independent origin of the plumes is also admitted.