st and mantle
movement. The decoupling can be thought as result of exist of the weak lower crust in this region
(Royden et al, 1997). With high temperature and low strength, the middle and lower crust in the
Sichuan-Yunnan region are more ductile and easier to flow than the upper crust. This character
results in the shear force in south-east direction between the upper crust and lower crust (ZHU and
SHI, 2004).
In order to study the influence of lower crustal flow on the deformation of the Sichuan-
Yunnan region, we construct a three-dimensional finite element model for the Sichuan-
Yunnan region, which is constrained by the active tectonic blocks, active fault zones, and
three-dimensional seismic velocity data. Linear Maxwell visco-elastic constitutive is applied. The
dynamic mechanism for crustal deformation and motion in the Sichuan-Yunnan region is also discussed
in this paper.
1 Three-dimensional finite element model
1.1 Three-dimensional finite element model
The tectonic activity in the Sichuan-Yunnan region is intense. The Sichuan-Yunnan region
mainly consists of the Sichuan-Yunnan block. The block is surrounded by many fault zones, such
as the Jinsha River fault, Lancang River fault, and Nujiang fault in the northwewst,
Xianshuihe-Anninghe-Xiaojiang fault system in the northeast and east and Red River fault in the
southwest (ZHANG et al, 2003). Figure 1 is the sketch tectonic map of the Sichuan-Yunnan region.
Considering the spatial distribution of active faults, the surface ruptures of historical earthquakes,
and the major earthquakes MS≥6.0 which occurred in the 20th century, the Sichuan-Yunnan region
can be divided into Barkam block, Sichuan-Yunnan block, Baoshan-Puer block and Mizhina-
Ximeng block (XU et al, 2003). The Sichuan-Yunnan block is splited by the Lijiang-
Xiaojinhe fault, and is divided into two sub-blocks. They are namely the northwest Sichuan
sub-block and the central Yunnan sub-block. In this paper, we simplify the active faults in the region
and only consider the main fault zones, such as Garzê-Yushu, Xianshuihe, Longmenshan,
Anninghe-Zemuhe, Xiaojiang, Red River, and Lijiang-Xiaojinhe fault zones. Because the Longmenshan
fault is a fault striking northeast and trending northeast to the depth of the low crust, we
define its dip angle as 30° in our model. To simplify the model, the trends of other faults are considered
as vertical.
Based on the simplified geological model described above, we construct a three-dimensional
finite element model. The study area spans from 97°E to 106°E, from 24°N to 33°N (Figure 2).
No.6 WANG Hui et al: THE INFLUENCE OF LOWER-CRUSTAL FLOW ON DEFORMATION 619
The finite element model includes the Sichuan-
Yunnan block, Barkam sub-block,
southwest Yunnan block, eastern Tibetan Plateau
block, and part of south China. The
northern boundary of the model is located in
the Barkam block, the eastern boundary is
located in the south China, and the southern
boundary is to the south of Red River fault
zone. The total area is about 1.1×106 km2. The
active fault zones in the model are described
as weak zone with width less than 10 km.
Depth of the three-dimensional model is
140 km. It contains four layers with different
viscosity. The four layers are upper crust,
lower crust, lithosphere mantle and asthenosphere
mantle respectively. Grids of the finite
element model are divided into 21 layers in
depth. The 1~6 layers represent the upper
crust, the 7~12 layers represent the lower crust,
the 13~18 layers represent the lithosphere
mantle, and the 19~21
