Recall three PT margins Convergent Divergent Transform

Shallow cold rock behaves elastically, deep hot rock behaves plastically

Stress = Pressure = Force/Area

If we push on the rock, with Brittle behavior, it cracks We would like to convert the horizontal and vertical stresses to the stress in the direction of the crack and the direction perpendicular to the crack shown as dashed arrows The vertical stresses, and so the vertical forces, are balanced. The horizontal stresses, and so the horizontal forces, are balanced. Nothing moves. So let’s just look at the lower part, below the crack

2 || lines crossed by another Similar angles 65o 90o - 65o = 25o previous slide = 8.83 So rest is 16.47o

For a divergent margin the calculation is similar

Our results For 15000 psi and 5000 psi, the angle with the greatest stress ratio is 30 degrees. However, in the case where directed pressure (horizontal stress) is great, the most probable break angle is different. This situation occurs in plate tectonic margins. We will revisit this idea when we discuss Listric Faults

Our ultimate goal is to find a graphical method that can predict fracture in any situation