STRUCTURE OF THE CRUST AND UPPER MANTLE PATTERN AND VELOCITY DISTRIBUTIONAL CHARACTERISTICS AT NORTHERN REGION OF THE HIMALAYAN MOUNTAINS
TENG JI-WEN XIONG SHAO-BAI YIN ZHOU-XUN (Institute of Geophysics, Academia Sinica)Xu ZHONG-XIN WANG XIANG-JING(Changchun Geological Institute of China)Lu DE-YUAN(Geological Academia Sinica of Geological Ministry of China)GEORGES JOBEET ALFRED HIRN(Geophysical Institute of University Paris 6, France)
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.