The in-situ stress field evolution process

formation cause

formation time and evolution trend of deep fracture is analysed. Combined with deep fractures characteristic，tectonic evolution

valley evolution and near surface action theory

the formation and evolution process are summarized by stress field change and strain energy release with numerical simulation. The results show that: deep fractures in-situ stress field is fundamentally changed after vertical unloading; but deep fractures are not formed in this in-situ stress environment; the primary reason of deep fractures formation is lateral unloading and residual tensile stress caused by difference resilience; the formation time of deep fractures is about after Ⅲ terraces appearance