Final Exam 442_2023_FINAL.pdf

1 Geology 442 Fall, 2023 Name_____________ Structural Geology Final Examination (1) Mohr Stress Circle and Fracture (18 points) Laboratory testing of the Solenhofen Limestone has allowed the following failure envelope to be constructed (units are in MPa) The laboratory notebook with the rock deformation results has subsequently been lost. a) Using the failure envelope, label on the failure envelope and list below the values of the following information: i) Cohesion ii) Angle of internal friction iii) Tensile Strength b) Write the equation for the Mohr-Coulomb failure criterion (which represents mode II failure) using the appropriate values from above. c) If we put the rock under low confining pressure (2 MPa = minimum compressive stress) and low differential stress (4 Ma), how much pore fluid pressure would be required to cause fracturing? Show your work using the Mohr Stress Circle. What type of fracture would be produced? σ s σ n 10 15 -5 5 -5

2 Geology 442 Fall, 2023 Name_____________ (2) Deformation. (16 points) What are the 4 basic types of movements (components of deformation) in kinematic analysis? Provide an example of each. (3) Fault Plane Solutions (7 points) Mark the relative displacement across the transform fault between plates A and B. Construct hypothetical fault plane solutions for the mid-ocean ridge and transform segments of the plate boundary shown below, with P and T axes. Extra Credit: If this was a fast spreading ridge system, would the transform or the ridge be more prone to seismicity, and why? Ridge Ridge Transform A B

3 Geology 442 Fall, 2023 Name_____________ (4) Folding. (25 points) In the folding of interlayered competent and incompetent layers (think of interlayered sandstones and shales), a neutral surface will be developed in the competent layers, whereas a neutral surface will not be present in the incompetent layer. In the diagram below showing just two such layers: (a) use the shapes of the folded layers to determine and label which is the competent and which is the incompetent layer: (b) indicate the approximate locality of the neutral surface. (c) indicate the approximate shape and orientation of the strain ellipses that define the neutral surface in the hinge region and on the limbs of the fold. (d) what types of small scale structures can accommodate the strain in the competent layer? (sketch their orientation and position on the fold) (f) draw on the incompetent layer the correct shape and orientation of parasitic folds that might develop. (e) given the points of equal dip (i.e., all points of the same number are of equal dip), draw dip isogons and then indicate what geometric classification you would give layer A (e.g., Ramsay’s Fold Class Classification). (f) what geometric classification you would give layer B? (g) what mechanism of folding would you attribute to layer A? 1 1 1 2 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4 4 4 Layer A Layer B

4 Geology 442 Fall, 2023 Name_____________ (5). Lineations. (15 points). Some lineations in deformed rocks can be useful to determine the direction of movement within rocks (kinematic information), whereas others are useful in helping to determine the axis of folds (geometric information). Identify whether the following lineations are useful for kinematics or fold orientation. (a) Stretching lineation (b) Bedding-cleavage intersection lineation (c) Pencil structure (d) Mullion (e) Slickenside (6). Planar structures (12 points) (a) Outline the basic processes of foliation formation. (b) Which processes are most important for lower greenschist facies slates? (c) Which processes are most important for upper amphibolite facies quartzo-feldspathic gneisses?

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