最大有效力矩准则的理论与实践

Theory and Practice of the Maximum Effective Moment Criterion(MEMC)

共轭断裂面对σ1一侧为锐角(一般小于60°),而共轭变形带面对σ1一侧为钝角(一般为110°)。锐角可用力学的摩尔-库仑准则解释,钝角则可用最近提出的最大有效力矩准则予以说明。最大有效力矩准则的数学表达式为Meff=0.5(σ1-σ3)Lsin2αsinα。式中,σ1-σ3代表变形岩石的屈服强度,L为单位长度,α为σ1与变形带间的角度。该准则证明最大有效力矩出现在σ1轴左右54.7°方向。55°±10°区间力矩无显著变化,天然和实验的观测值全部在该区间内。该准则在实践中可解释膝褶带、伸展褶劈理、膏盐层中的屈服带、低角正断层、高角逆断层、结晶基底中的菱网状剪切带、地震反射剖面中的鳄鱼嘴构造和前陆盆地中的拆离褶皱等地质构造的形成,可藉以确定有关构造形成时的应力状态和运动学涡度,并说明推覆构造与伸展构造间的运动学和动力学关系以及深俯冲超高压岩石的折返-出露机制。

Conjugate fractures have an acute angle to σ1 , while conjugate deformation zones with an obtuse angle to it. The acute angle can be explained with the Mohr-Coulomb criterion, whereas the obtuse one can be elucidated with the Maximum Effective Moment Criterion （MEMC）, which is recently proposed. The MEMC is mathematically expressed as Meff=0.5（σ1-σ3）L·sin2α sinα, where σ1 - σ3 represents the yield strength of the related rock, L is a unit length and a 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 little difference in the moment in the range of 55° ± 10°. The range covers all the observations available from nature and experiments. In practice, it can be used for elucidate many geological structures such as kink-bands and extensional crenulation cleavages, yield zones in evaporates, low-angle normal faults and high-angle reverse faults, lozenge ductile shear zones in basement terranes, some crocodile structures in seismic profiles and detachment folds in foreland basins. It may be used to determine the stress state when the related deformation features formed and provides a new approach to determine the Wk of the related ductile shear zone. It can also be extended to explain the relationship between orogenic and late-orogenlc extension events, and uplift and exhumation of ultrahigh pressure rocks that experienced deep subduction.