Brittle-ductile transition in rocks

The deformational characteristics of two limestones, one gabbro, and one dunite have been investigated as a function of confining pressure. It was found that friction of these rocks and friction of granite and serpentinite studied elsewhere are nearly identical and that the brittle-ductile transition pressure is simply the pressure at which the stress required to form a fault is equal to the stress required to cause sliding on the fault. The transition pressure is higher in extension than it is in compression. This difference occurs because the frictional shear stress required to cause sliding is determined not by confining pressure but by the principal stresses and the angle of the fault. For the same frictional shear stress on a fault surface, the confining pressure is much higher in extension than it is in compression.