Dynamics of normal faulting

Based on fracture mechanics, the authors examine the formation of normal faults by the propagation of a mode II crack. According to engineering fracture mechanics a mode II (shear) crack is unstable and branches to form a mode I (tension) crack. This unstable growth is due to large tensile stresses near the crack tip. However for a normal fault at depth in the crust, lithostatic stress at the fault tip exceeds the rock yield strength. To examine the quasistatic growth of a normal fault, the authors have carried out boundary element calculations of a propagating fault in an elastic layer with an imposed horizontal tensile stress. Gravity causes the hanging wall to slide down the fault surface resulting in both mode I and mode II stresses at the fault tip. Analysis of stresses in the plastic zone at the fault tip shows that the orientation of a plane radial to the tip along which the fault extends by Mohr-Coulomb failure depends on the ratio of mode I and II stress intensity factors. This ratio depends on the applied horizontal tensional stress, the lithostatic stress at the fault tip, and the frictional sliding stress on the fault surface. Failure along amore » plane with shallower dip than the original fault plane may explain the formation of listric normal faults.« less