Active Faults of Northern Central Mongolia, Their Correlation with Neotectonics and Deep Structure of the Region

Abstract

The active tectonics of northern Central Mongolia is studied between two largest W–E-trending left lateral fault zones: the Khangai Fault and the Tunka–Mondy. These strike-slip zones are part of a single ensemble of active faults in the Mongol–Baikal region, formed under conditions of maximum northeastern compression and maximum northwestern extension. Their ENE-trending Erzin–Agardag and Tsetserleg faults with a dominant sinistral component extend between these zones. A series of the N-trending graben basins (Busiyngol, Darkhat, and Khubsugul) are located between the eastern end of the Erzin–Agardag strike-slip fault and the western part of the Tunka–Mondy strike-slip zone. The basins form a sinistral deformation zone, which is kinematically similar with the strike-slip faults, which follow the latter. In contrast to the largest boundary strike-slip faults, this structural paragenesis formed under conditions of N–S-trending relative compression and N–S-trending extension. A change in the orientation of the axes of the principal normal stress may be caused by the rotation of the block between the boundary faults. The area of graben-shaped basins is located above the top of a vast volume of low-velocity mantle, which we have identified as the Khangai plume. The lithospheric mantle above this rise is reduced; the remaining part of the lithosphere is heated and softened. The large active strike-slip faults are located above areas of subsidence of the low-velocity top of the mantle. Our trenching of the active faults showed that strong earthquakes repeated in the area of graben-shaped basins more often than in the large strike-slip zones, but they were characterized by lower magnitudes.