Maximum effective moment criterion and the origin of low-angle normal faults

The origin of the low-angle normal faults of metamorphic core complexes has been debated for over two decades. Proponents of Andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults. A new theory, named the maximum effective moment criterion, is proposed here for their origin. Using an effective moment approach formulated as Meff=FH, where F is the force tangentially acting on the unit shear boundaries and H represents its arm. The maximum value appears at angles of ±54.7° with the σ1. Since extensional crenulation cleavages (eccs) and the contractional crenulation cleavages (cccs) occur in conjugate pairs with ∼110° angle between them, it is suggested that they tend to be oriented in the directions of maximum effective moment. The differential stress for formation of the ecc or ccc are less than that for fracturing. The orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components. The synthetic ecc set is much better developed than the antithetic set due to anisotropy. Upward propagation of the synthetic ecc set from mid-crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle-normal faults of metamorphic core complexes.