ON THE LOW VELOCITY LAYER AND CONRAD SURFACE WITHIN THE CRUST OF SOUTHERN PARTS OF CHINA

Analogies are drawn between the models of crustal structure and velocity distribution on the basis of explosion seismological data of the southern parts of China. Further, the distributions of low velocity layer and Conrad surface (or Conrad discontinuity) within the crust, which are relative to the formation and evolution of tectonics of active continental margins, are discussed. (1) Conrad surface exists extensively in the crust of southern China. The velocity of the longitudinal waves under Conrad surface is about 6.8-7.0km/s. Hence, it make known that medium under Conrad surface consist of basalt, and probably contains a high proportion of ultrabasic rocks. The depth of Conrad surface is 23.4km in Shui county and Huangmei county, Hubei provnce, and becomes gradually intc shallow from the inner parts of Continent to the Coast in Qidony County Jiangsu Province and in Shantou, Guangdong Provinces, it is 14.3km and 22km respectively. (2) The layer of granite with velocities of 6.0-6.3km/s overlaies on Conrad surface. In the bottom of granite layer, there is probably a low velocity layer with thickness of 3-5km. Such as Lower Yangzi area is thus the district in which low velocity layer is possessed. Existence of low velocity layer makes Conrad surface into strong reflective.surface because of higher contrast of wave impedences between low velocity layer and basalt layer. The crust of paleoisland-arc (Wuyi Mountain) expresses to be two layers, nowadays we have not received information about low velocity layer yet. (3) In coast area the average P velocity of crust is about 6.32km/s, which is higher than one of other areas, hence it is considered to be corresponding to big relative thickness of basalt layer or a high proportion of ultrabasic rocks in the crust. This is the first characteristic of transitional crustal structure. The second is the thin crustal thickness, little velocity gradient with depth, low contrast of velocity from layer to layer and lacks low velocity layer generally. For the reasons above, there is the feature which expresses no fullness of crustal evolution and differentiation. Sometimes is difficult to distinguish Conrad surface in these areas. (4) In the area of the eastern parts of Hunan Province, there is the crust of the type with rift valley during Mesozoic-Cenozoic. In which the Conrad surface is clear, and low velocity layer is lack. The depths of Conrad surface and Moho are shallow (16.8km and 28.6km respectively), and the P velocity on the top of mantle is merely 7.86km/s as well.