The Maximum Effective Moment Criterion （MEMC） and Its Implications in Structural Geology

1 Introduction A high-level generalization of structures in the earth crust has been given by Ramsay (1980): low-angle thrusts in the brittle upper crust and high-angle reverse shear zones in the ductile middle-lower crust are formed in contractional regimes; high-angle normal faults in the brittle upper crust and low-angle normal shear zones in the ductile middle- lower crust are formed in extensional regimes. The formation of low-angle thrusts and high-angle normal faults in brittle domains …

The Maximum Effective Moment Criterion (MEMC) and Its Implications in Structural Geology

Abstract The Mohr‐Coulomb criterion has been widely used to explain formation of fractures. However, it fails to explain large strain deformation that widely occurs in nature. There is presently a new theory, the MEMC, which is mathematically expressed as M_eff = (σ₁ — σ₃) L · sin 2α sin α)/2, where σ₁‐σ₃ represents the yield strength of the related rock, L is a unit length and α is the angle between σ₁ and deformation bands. This criterion demonstrates that the maximum value appears at angles of ±54.7° to σ₁ and there is a slight difference in the moment in the range of 55°±10°. The range covers the whole observations available from nature and experiments. Its major implications include: (1) it can be used to determine the stress state when the related deformation features formed; (2) it provides a new approach to determine the W_k of the related ductile shear zone if only the ratio of the vorticity and strain rate remains fixed; (3) It can be used to explain (a) the obtuse angle in the contraction direction of conjugate kink‐bands and extensional crenulation cleavages, (b) formation of low‐angle normal faults and high‐angle reverse faults, (c) lozenge ductile shear zones in basement terranes, (d) some crocodile structures in seismic profiles and (e) detachment folds in foreland basins.