Brittle Deformation Remember that is the angle between 3 and a plane Definitions Differential Stress: Difference between largest and smallest stresses Deviatoric Stress: Difference between total stress and mean stress. Magnitude of Normal and Shear Stresses 11 33 Normal Shear What does this mean for fractures? Fractures will form when n is low but s is high. is usually 60 when fractures form Coulomb Fracture Criterion predicts failure in rock: s = n tan is the angle of internal friction tan is the coefficient of internal friction Rocks: Avg. is 0.6 C is the point at which n = 0 T is the point at which s = 0 = 90 -2 Shaded area is stable (No brittle failure) No failure Failure Exercise Sketch two cross sections: In one, 1 is horizontal, 3 vertical What is the dip of the fault predicted by this model? In the other 3 is horizontal, 1 vertical. What is the dip of the fault? Remember that is the angle between 3 and the plane. See also fig Once a fracture exists deformation continues by frictional sliding or cataclastic flow CCW Rotation Formation of Shear Fractures 33 11 2 faults form 1 fault stays active Crack is Parallel to 1 These two are not faults. Shallow crust Deep crust Non-frictional sliding on Fault surface: Calcite fibers from fluids in fault zone