Abstract: Hydropower project is usually built on high mountain canyoned river reaches, then this terrain condition causes deep factures. And the deep fracture means the sheeted fracture or fracture zone in fresh rock mass. There are a lot of deep factures in the left bank of the proposed hydropower station dam site. This paper analyzes distribution characteristics and deformation behaviors. It is concluded that the deep fracture is the product of epigenetic reformation of structural plane of original constitution in rock mass which is caused by high ground stress , lithology and quick valley cutting, then strain energy in rock loosens strongly and high ground stress releases, and specific rock characteristics have different restoring origins.

Key words: deep fracture, unloading , formation mechanism, epigenetic reformation