THE ACTIVE TECTONICS OF NE ASIA

We describe the active faults and tectonics of central and eastern Mongolia and surrounding parts of NE Asia. Left-lateral motion on east-west faults dominates within central Mongolia. In northern Mongolia this left-lateral slip is transferred, through the Busingol, Darhad, and Hovsgol grabens, to the Baikal rift zone of Siberia. Diffuse normal faulting is also present within the Hangay mountains, the Hentay uplands and also within a broad region of Siberia southeast of the main Baikal rift (the Chita rift zone). East-west left-lateral slip at the northern end of the Baikal rift zone is transferred eastwards to the Sea of Orhotsk through a narrow zone of left-lateral shearing within the Stanovoy mountains. A second zone of left-lateral shear connects the diffuse graben systems of Hentay and Chita to Orhotsk.

Left-lateral faulting in the Gobi-Altay of Southern Mongolia ends on north-south thrust faults close to the northern edge of the Ordos block of China. We suggest that slip on the Gobi-Altay faults may be transferred eastwards, towards the Pacific Ocean, through a right-stepping arrangement of normal faults and east-west left-lateral faults within the mountainous regions north of Beijing.

Right-lateral faults, such as the Tanlu fault of China and the Primorsky fault of Siberia, run parallel to the Pacific coast of China and Siberia. These faults may represent partitioning of lateral and dip-slip components of slip associated with plate motions at the Pacific margin. The deformation of Mongolia and its surroundings appears to be an interplay between forces arising from the India-Eurasia collision to the south and forces caused by plate motions at the Pacific margin to the east. The distribution of active faults within NE Asia is widespread and diffuse including within regions with little in the way of recorded seismicity. Nonetheless, eastern Mongolia and adjacent parts of China appear to be almost devoid of active faults.