SECONDARY STRUCTURES I. SECONDARY STRUCTURES - structures formed after deposition or intrusion A. Brittle structures - formed due to brittle failure, i.e., breaking of bonds 1. Deals with fracturing or granulation 2. After brittle deformation, pieces ideally can be put back together because the pieces do not change shape (not internally deformed) 3. Produces angular fragments, breccia 4. Faults and joints B. Plastic structures - formed mainly due to the distortion of bonds, deals with material flow. 1. Ductile shear zones and folds II. BRITTLE FAILURE (new heading) A. Types of brittle fractures 1. Shear fracture - pieces slide past each other, movement is parallel to the fracture 2. Tensile fracture - pieces pull apart, movement is either perpendicular or at a high angle to the fracture, leaves a hole B. Brittle failure of unconsolidated material 1. Coulomb's sand box experiment (200 years ago) When he plotted the results, he found a linear relationship between s and t required for failure. tc = "critical stress" critical value of t which causes fracture (all points along the line) s = normal stress along the plane of failure f = the angle that the Coulomb failure envelope makes with the s axis 2. Coulomb's work was important, but remember that it dealt only with loose sands, which differ from rocks in that sands lack cohesion; that is, they cannot absorb tensile stresses (s<0). C. Brittle failure in rocks 1. Studied in laboratories using rock deformation machines III. TYPES OF ROCK EXPERIMENTS A. Uniaxial tests 1. Remember s1 > s2 = s3 = 0 2. Uniaxial test apparatus = vise 3. Samples must be tiny (>1cm3), smaller means higher stresses can be applied, otherwise the machine will fail 4. How the mantle is studied, using a diamond press 5. High heals = 2,000 lbs/in2 B. Triaxial tests 1. Remember s1 > s2 > s3 ¹ 0 2. Triaxial stress apparatus = 2 perpendicular presses within a fluid filled cylinder 3. Rrequires a cube shaped sample (hard to make) 4. Could mimic a lot of stress states, but it is extremely difficult to do 5. Boundary problems 6. Focusing presses to meet at the center of the cube 7. Oonly a few tests have been successfully carried out C. Axial tests - most common type of test 1. Remember s1 > s2 = s3 2. Axial test apparatus - similar to uniaxial test rig, but done in a fluid filled cylinder to provide s2 = s3 3. Fluid usually is kerosene, but deformable solids that cannot support deviatoric stresses are also used (e.g., talc, NaCl and alsimag) for higher pressure runs 4. Can reach upper mantle pressures (~25 kb) IV. RUNNING A ROCK DEFORMATION EXPERIMENT UNDER AXIAL STRESS CONDITIONS A. Steps: 1. Increase the difference between s1 and s2 = s3 until the rock fractures (set s2 = s3 and increase s1); you will know when it passes through the "zone of acoustic emissions" and catastrophically ruptures 2. Monitor the applied stresses and note the value of s1 when rupture occurs 3. Remove the rock and measure the angle q, which here is defined as the angle between s1 and the normal to the plane of fracture 4. Plot the results on a Mohr circle diagram 5. Re-run at a new value of s2 = s 3 B. Diagram of a single run: C. Diagram of multiple runs: 1. Linear relation 2. Stress states along the line are where fracture occurs 3. Called Mohr fracture envelope D. Mohr Coulomb law tc = ms + t0 1. The strength of a rock to resists faulting is derived from two sources: a. t0 = "cohesive strength" or "cohesive shear strength", which is the bonding that holds the rock together, an intrinsic property of the rock. * a threshold value of t where s = 0 that must be exceeded to shear a cohesive material b. Internal frictional resistance to faulting - once the rock is fractured frictional resistance must be overcome to allow movement (i.e., sliding) along the fracture; not an intrinsic property, same from rock to rock * A function of both: m = coefficient of internal friction (tan f) s = normal stress across the fracture 2. Diagram showing modification of the Coulomb failure envelope: a. Opens to the right b. Rocks have more strength in compression than in tension c. Higher states of stress require larger deviatoric stresses