喜马拉雅山北部地区的地壳结构模型和速度分布特征

本文根据1981年西藏南部喜马拉雅地区的人工地震测深资料进行了震相对比,分辨出t₁、t₂、t₃、t₄、t₅和t₆六组地壳中和莫霍界面的反射波,并用理论走时曲线、绘制速度曲线图、射线跟踪和综合地震图等方法得到了主测线（PP）上各地段的地壳结构模型。初步结果表明,该地区地壳西段较薄（约73公里）,东段稍厚（约77公里）,平均总厚度约为75公里。地壳的平均P波速度约为6.2-6.3公里/秒。 地壳为高低速相间的多层结构。在中上部有一低速层,其厚度为数公里,速度为5.6-5.7公里/秒,与上层速度差为0.5-0.6公里/秒。低速层在测线东段比较肯定,在西段则不甚明显。结合藏南定日、岗巴一线有强烈水热活动的事实,低速层的存在可能意味着地壳中存在部分熔融的高温物质。下部地壳的速度为6.7-6.8公里/秒,且比较均匀。从莫霍面反射波的特征来看,在紧靠其上方可能有一个速度反转带,其厚度亦为数公里。上部地壳的结构在横向上有较大的差异,这说明在地质历史上,西藏特提斯带曾经历过强烈的地壳变动。

STRUCTURE OF THE CRUST AND UPPER MANTLE PATTERN AND VELOCITY DISTRIBUTIONAL CHARACTERISTICS AT NORTHERN REGION OF THE HIMALAYAN MOUNTAINS

In order to study the layered structure and characteristics of the velocity distributions in the crust and upper mantle of the northern part of Himalayas we have made detonations in the lakes of Pumayum Co, Peigu Co and in the Dinggye region Four seismic record sections were obtained along a profile in a nearly E-W direction and 475 km long from Pumayum Co to Peigu Co.According to data processing and inversion, six groups t1, t2, t3, t4, t5, t6 of reflected phases through the crust and upper mantle in the area are obtained which show different kinematic and dynamic properties The results of data analysis are as follows:1. The crust is multilayered and there exists a low velocity layer in the crust. The thickness of the low velocity layer is a few kilometers, and with the layer velocity 5.6 ~ 5.7 km/s. It indicates that the cause of geothermal distribution and its activity in Xizang plateau is due to the high temperature in the crustal medium and the existence of melting or partial melting matter in the crust.2. Structure and velocity of thick crust are horizontally inhomogeneous. The crustal thickness from the north of the Himalayas is 73 ~ 77 km and its velocity, 6.2 ~ 6.3 km/s. Crustal deformation is very strong in the Tethys Himalaya region.3. On the basis of results from ray tracing, theoretical seismogram and phases of reflection waves a preliminary model of crust and upper mantle in the northern part of Himalayas is put forward.The extremely thick crust was caused by the results of the collision of the Indian plate against the Eurasian plate, and during the process of continuous pressing the horizontal shortening took place on a large scale in the crust.