ON THE DYNAMICS OF EXTENSIONAL BASIN

Geological and geophysical data from the North China-Bohai Basin and ' 'Basin and Range'' Province were examined and compared. They are simil ar to each other in many respects. Surficial geological structures are characterized by a series of half-grabens with their one flank consti tuted by normal fault. Those extensional structures usually extend to a depth of 6-8km. Therefore, the stress condition in the upper 8 km ca n be written as sigma(x) > sigma(z) > sigma(y). where x, y denote the directions of maximum compression and maximum tension on the horizonta l plane, while z signifies the vertical direction. Some people think t hat this kind of stress condition exists through the entire crust in t he extensional basin. However, the focal mechanisms of the earthquakes in the extensional basins with focal depths usually at 12-20 km are d ominated by strike-slip faults. The stress condition in the focal regi ons can be expressed by omega(x) > omega(z) > omega(y). Geodetic measu rements Geodetic measurements conducted before and after the Tangshan earthquake in 1976 and the Xingtai earthquake in 1966 showed that both horizontal and vertical surficial deformations with magnitudes of a s imilar order occurred during the earthquakes. The surficial deformatio ns during the earthquakes can be explained by a summation of the motio ns produced by both the stress fields in the upper crust and the middl e crust. Dynamical processes other than the homogeneous horizontal reg ional tectonic field are required to explain the vertical variation of the stress condition in the upper and middle crusts. Evidence from th e seismic refractions, reflections and the three-dimensional seismic t omography from both local earthquakes and teleseismic events provide c onvincing evidence that magmatic intrusions from the uppermost mantle to the middle crust occur near the hypocenters of both the Tangshan an d Xingtai earthquakes. The variation from the extensional stress regim e at the upper crust to the compressional stress regime in the middle and lower crusts is considered to be the common feature in extensional basins. And the magmatic intrusions from the upper mantle to the midd le crust observed in the extensional basin is suggested to be its gene tic cause. Numerical simulations of magmatic intrusion from the upperm ost mantle to the middle crust were studied. Both the intruded compres sion and the thermal stress due to magmatic intrusion were considered, also the viscoelasticity of the middle and lower crusts were assumed. The results successfully explain the vertical variation of the stress condition in the crust and the process producing an extensional basin .