Determining Stress State and Sense of Shear Joints and Shear Zones Determining Stress State and Sense of Shear

Twiss and Moores [1992]

Tensional Joints - Mode I The 3 direction is perpendicular to joint face P-waves affect propagation of fracture, producing plumose pattern Tensional Joints need true tension or highly elevated fluid pressures

Shear Joints - Mode II and III The 3 direction is not perpendicular to the joint face Joint face undergoes both tension and shear

Shear Zones and Riedel Shears Shear zone with maximum principal stress oriented 45° to orientation of shear zone If the fault zone is brittle, get Reidel shears forming

R

Veins and Shear Zones Veins can form and then rotate within shear zones Tip of vein points in the direction of maximum principal stress 1

Allmendinger (Cornell)

From Allmendinger (Cornell)

S-C Fabrics S-C fabrics is type of foliation formed within shear zones S is foliation attributed to finite strain C is the shear plane

S-C Fabrics

C S