The factors relevant to the mechanism of normal faulting and brecciation are discussed. Normal faults develop as the consequence of an increase in the magnitude of the differential stress above the critical limit under the prevailing pore water conditions. The accumulation of a body of hydrothermal solution on the fault zones under pressures greater than the pore water pressure, results in the extension of the faults by hydraulic fracturing. The abrupt drop in the pressure of the hydrothermal solution when fracturing occurs, causes the bursting apart of the rock into which the hydrothermal solution has permeated under high pressure, thus forming angular breccias.
The fracture may be extended by hydraulic fracturing even though the differential stress decreases, provided the pressure of the hydrothermal solution on the fracture plane exceeds the pore water pressure by increasing amounts. Under these conditions the dip of each extension of the normal fault increases, and eventually the fracture develops as a vertical breccia zone.