PHYSICAL CONSTRAINTS OF FAULTING IN A GOPRESSURED SEDIMENTARY BASIN*

On the basis of the triaxial stress concept, the tectonic stress required for faulting and sliding in a sedimentary basin can be estimated. The horizontal tectonicstress necessary for thrust faulting, for example, increases with depth from the surface to about 9,000 ft (2.7 km), and decreases below 9,000 ft (2.7 km), primarily because of a changing Poisson's ratio. Changing the vertical effective stress caused by generation of abnormal fluid pressure would also change the tectonic stress requirements for faulting. The internal friction of sedimentary rocks in the subsurface must be exceeded by the tectonic stress applied to the basin for faulting or sliding to take place. Estimates of the internal friction suggest that there is a high probability of faulting at depth if there are thick undercompacted and geopressured sections. Diapirs may also occur in such a physical environment. If the sedimentary rocks are normally compacted and the fluid pressure is hydrostatic from the surface to depth, on the other hand, the rocks are generally too rigid to cause any significant sliding. There would be very little chance of faulting and diapirism in such a case. The concepts developed in this paper may have a practical application in the under-standing of the physical conditions and constraints of faulting and diapirism in a relatively-young sedimentary basin with significant geopressuring at depth.