Polymodal faulting in subducting slabs by the interaction of compressive anticracks

Brittle faults in the upper crust are widely held to be composite shear fractures formed through the interaction and coalescence of tensile microcracks. One mechanism for subduction seismicity below 300 km is phase transformation faulting through the interaction and coalescence of compressive anticracks filled with transformed spinel. The geometry of these flaws and their surrounding elastic stress fields exert a fundamental control on the orientation of the final shear planes. The Coulomb-Mohr failure criterion predicts the development of conjugate bimodal shear planes inclined at an acute angle to the maximum compressive principal stress and parallel to the intermediate principal stress. However, Coulomb-Mohr theory is incapable of explaining more complex threedimensional fracture populations that are widely observed in rocks and recorded in subduction zone seismicity in which multiple sets of shear fractures are oriented oblique to the intermediate principal stress direction. Previous models of anticrack interaction have employed some form of simplifying two-dimensional approximation. Our fully three-dimensional model is based on the solution of Eshelby. We show that the elastic stress fields around compressive anticracks in three-dimensions promotes interaction and coalescence to form shear planes oriented oblique to the remote principal stresses, and can therefore account for polymodal fault patterns. The variations in the orientation of focal mechanisms is therefore seen to be of primary significance and reflects the oblique nucleation of shearing under truly triaxial stress conditions in the downgoing slab.