摘要
中国东南部地壳内广泛存在康拉德面,其分布深度由内陆向沿海逐渐变浅.康氏面之上,花岗岩层底部可能存在有厚度为3-5km的低速层.古岛弧区(武夷山)尚未观测到低速层,滨海地区属过渡地壳,一般没有低速层.湖南省东部中新生代裂谷型地壳,康氏面清楚,未发现低速层,康氏面与Moho深度都较小.
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?.3km/s overlaies on Conrad surface. In the bottom of granite layer, there is probably a low velocity layer with thickness of 3?km. 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 l
出处
《石油实验地质》
CAS
CSCD
北大核心
1989年第4期331-340,共10页
Petroleum Geology & Experiment
