Puzzles and the maximum-effective-moment (MEM) criterion in structural geology

Abstract

The essential difference in the formation between conjugate brittle shear fractures and ductile shear zones is that the intersection angle of conjugate faults in the contractional quadrants in the former is acute (usually ∼60°) and obtuse (usually 110°) in the latter. The Mohr-Coulomb failure criterion is an experimentally validated empirical relationship, which structural geologists use to interpret the stress directions based on the orientation of the brittle shear fractures. However, a simple application of this criterion assuming that the principal stresses are vertical or horizontal throughout the crust fails to explain crustal scale low-angle normal faults, high-angle reverse faults and certain types of conjugate strike-slip faults that have intersection angles in the compressional quadrants greater than 90°. Although the Maximum-Effective-Moment (MEM) criterion has been used to analyze structures in several natural cases and obtained some new evidence in nature and experiments, it is not yet commonly used to interpret non-Mohr-Coulomb features widely distributed in the crust. It is important to review the MEM criterion and its implications for explaining the formation mechanism of naturally occurring faults and shear zones. Behavior of a rock depends on its mechanical properties; the latter in turn depends on pressure and temperature conditions related to crustal depth and strain rates. Conjugate fractures with obtuse angles, ca. 110°, in the contractional direction at different scales and at different crustal levels are consistent with the prediction of the MEM criterion and suggest deformation at low strain rates. Thus, strain-rate clearly plays an important role in the style of crustal deformation and the many observed departures from the Mohr-Coulomb failure criterion.