1985 Volume 33 Issue 3 Pages 157-171
In order to study the layered structure and characteristics of the velocity distributions in the crust and upper mantle of the northern part of the Himalayas, we have made detonations in Puma Lake, Peikü Lake, and the Dinggye region. Four seismic record sections were obtained along a 475 km long profile in a nearly E-W direction from Puma Lake to Peikü Lake.
According to data processing and inversion, 6 groups, t1, t2, t3, t4, t5, t6 of reflected phases through the crust and upper mantle in the area are obtained. They 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. This indicates that the cause of geothermal distribution and its activity in the 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, 6.2-7.3 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.
