华北地区的地壳低速层

LOW VELOCITY LAYERS OF THE CRUST IN NORTH CHINA

虽然华北地区属于克拉通类型的地质单元,然而在其地壳内部,尤其是东部地区的地壳内,低速层相当发育。这些低速层多位于上地壳底部15～25 km的深度范围内。同活动造山带内的低速层相比,华北地区的壳内低速层表现出明显不同的特点。壳内低速层水平延伸尺度不大,互相之间不太连接。同时,低速层内外速度差较小,大都小于5%。考虑到华北盆地岩石圈底部的热侵蚀作用和地壳的热状态,认为部分熔融是产生壳内低速层的一个重要因素。

Although North China belongs to Craton type in geology,there exist quite a lot of low velocity layers（LVL） in its crust,especially in its eastern region.Most of these LVLs locate at the bottom of the upper crust with depth of 15-25 km.Compared with LVLs in active orogens,the LVLs in North China possess greatly different characteristics.Usually,these LVLs are not large in horizontal scale and are not connected each other.In the same time,the velocity drops are small,usually less than 5%.Considering the thermal erosion action at the bottom of the lithosphere and the geothermal state,it is deduced that partial melt may be one important factor to generate LVLs in the crust.