1. Recall three PT margins
Convergent
Divergent
Transform

2. Shallow cold rock behaves elastically, deep hot rock behaves plastically

3. Stress = Pressure = Force/Area

4. 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

7. 2 || lines crossed by another
Similar angles 65o
90o - 65o = 25o
previous slide
= 8.83
So rest is 16.47o

10. For a divergent margin the calculation is similar

11. Our results
For 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

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