西藏羊八井—聂拉木地区的重力工作及其初步解释

作为中法青藏地质研究队1982年科学考察内容的一部份,中法两国重力工作者在西藏的羊八井一聂拉本地区沿公路开展了1:1000万的剖面重力测量工作。在总长度近950公里的剖面上共布置了70个测站。本文就是利用这个资料对该地区的地质构造以及莫氏界面的深度作了一些定性与定量的解释,得到了以下一些主要成果:(1)羊八井一聂拉木地区莫氏界面平均深度大约为70公里。其地壳特点为:羊八井地区较深,大致平均为73公里左右,而工区的中间部份;定日～日喀则一带较平缓,深度大致为70公里,喜马拉雅山地区莫氏界面较浅并呈现由南向北大致以8°倾角逐渐变深的趋势。(2)喜马拉雅山地区目前仍处在重力的不均衡上升阶段,均衡补偿作用仍是该地区地壳活动的主要形式之一,地壳厚度要比从均衡补偿状态预计的薄。另外,根据重力异常的均衡信息,可把该区分为两种类型,即:“高喜山”地区与“低喜山”地区。“高喜山”地区的均衡程度要比“低喜山”的低。相对来讲,“低喜山”地区相对较为稳定。(3)喜马拉雅山地区一系列断裂带,其主要形式是地壳断裂,对莫氏界面影响不大。(4)在雅鲁藏布江附近存在一个地壳上地幔变异带,这个变异带的存在可能是印度与西藏陆块碰撞的缝合线以及较大断层的显示。

THE PRELIMINARY INTERPRETATION OF THE GRAVITY PROFILE OF WELL YANG NO.8-NIELAMU,TIBET

As a part of the scientific investigatiion project of the Chinese-French Qinghai-Tibet Geological Research Group, the Chinese and French geo-physicists carried out the gravity profile measurements (1:l,000,000) along the highway in well Yang No.8-Nielamu area, Tibet.70 stations were laid out along the profile which was about 950 km in length. Based on the measurements, certain work on the geological structure of the area and the depth of Mohorovieie discontinuity wore interpreted, qualitatively and quantitatively.Here are some positive results gaintd; 1) The average depth of the Mohorovieie discontinuity of NIelamu-wellYang No.8 is about 70 km. On the whole, the Mohorovieic oliscontinuity in well Yang No.8 area is deeper, about 37 km on the average, and in the middle of the area under investigation Dingri-Rikaze, it is smoother,about 70 km. In addition, it is suggested that in the Himalayas, the Mohorovieic discontinuity inclines from south to north in about 8?and tends to going deeper by-and by. 2)The Himalayas, at present, seems to be not in gravity equilibrium and the isostatic compensation is still one of the principal forms of the crustal activities in the area. The crust underneath the Himalayas is thinner than that expected from isostatic compcnsation. It is suggested that the Himalayas are currently under the influence of upward tectonic forces. According to the isostatic situation, the Himalayas may be divided into two parts- the higher and the lower Himalayas. The isostatic level of the lower Him alayas is better than that of the higher so that the crust of the former is relatively stable. 3) In the Himalayas, there arc a series of fault zones which are mainly crustal ones. Thus there is no significant influence on the Mohorovieie discontinuity. 4)In the vicinity of Yaluzangbujiang area, there is an anomalous crust-mantle variation zone which is probably the indication of deeper faults and the suture line caused by clashing against each ether between the Indian and Tibet plates.