A Developing Plate Boundary: Tianshan-Baikal Active Tectonic Belt

Abstract

It is obvious that a great amount of strong earthquakes in central Asia occurred densely along Tianshan-Baikal belt; however, none of huge continuous fault has been revealed on the surface along the belt. It caused academic argument related to their relationship. The authors analyzed its seismic activity in terms of seismicity in time-space domain, zoning feature, stress field, and recurrence period of earthquakes. In addition, we studied the Bouguer gravity and isostatic anomaly, and calculated the distribution of density and shear wave velocity in the upper mantle. It is found that there is a gradient belt of density and velocity along NE direction at the depths of 70–250 km, which is actually a Cenozoic boundary of cold mantle and hot mantle, and has an overpass relation with the structure existing in the upper lithosphere. It is inferred that this gradient belt in the upper mantle played an important role in controlling present tectonic movement and stress field in Eurasia continent. As far as the earthquake nature in this belt is concerned, it is neither an interplate earthquake nor an intraplate one in the traditional sense. Such kind of earthquake caused by overpass structure actually belongs to the structural earthquake, and is of interplate within a continent. In this paper, the overpass relation and mirror relation in time-space domain between deep and shallow structures are discussed briefly. It is also noted that the overpass structures shown in the areas of both north-south seismic belt of China and eastern seismic belt of Iran are related to a special structure in the upper mantle. We conclude that the Tianshan-Baikal active tectonic belt is a developing boundary of plates, a typical tectonic within a continent, which separates Eurasian plate into two parts, i.e., the stable Russian-Siberia subplate to the north and the active China-Southeast Asia subplate to the south.