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Joints and Shear Fractures (D & R; p. 205-226)

Joints and Shear Fractures (D & R; p. 205-226). Figures not from D & R are from: Earth Structure: An Introduction to Structural Geology and Tectonics, by Ben van der Pluijm and Stephen Marshak. Copy available in TA office, Rm. 314.

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Joints and Shear Fractures (D & R; p. 205-226)

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  1. Joints and Shear Fractures (D & R; p. 205-226) Figures not from D & R are from: Earth Structure: An Introduction to Structural Geology and Tectonics, by Ben van der Pluijm and Stephen Marshak. Copy available in TA office, Rm. 314.

  2. Joint: A natural fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops

  3. Joints/Fractures: Geometry Planar and often smooth; no appreciable displacement. Most abundant structural element in crust. What do the surfaces look like? Moscow Kremlin - Bell Tower of Ivan the Great. Fractured in 1737 due to uneven cooling

  4. Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather. Due to inhomogeneity of rock.

  5. Joints: commonly elliptical

  6. Close-up views of hackles in plumose structure. Plumose structure is more prominent away from origin due to stress concentrations at crack tips

  7. Joints/Fractures: Kinematics ribs are arrest lines- opening is not instantaneous, but rhythmic, like splitting wood

  8. Griffith cracks: preexisting microcracks and flaws in a rock The largest properly oriented Griffith crack propagates to form a through-going crack

  9. Joint arrays

  10. Three competing mechanisms that contribute to joint formation during uplift and erosion: (1) Contraction during cooling (2) Poisson effect- e.g., rock expands in vertical direction and contracts in horizontal direction during unloading (3) Membrane effect- expansion due to increase in curvature of layer

  11. Cooling joints: form by thermal contraction

  12. Exfoliation joints: Form by unloading of bedrock through erosion. They form parallel to topography

  13. Exfoliation joints: Form by unloading of bedrock through erosion. They form parallel to topography

  14. Tectonic joints: Form by tectonic stresses as opposed to stresses induced by topography.

  15. Joint analysis Significance: determine orientation of tectonic stresses

  16. Significance for Engineering Planes of weakness!

  17. Significance: Geologic Hazards

  18. Joints and Geomorphology

  19. Shear fracture: A fracture that grows in association with a component of shear

  20. Shear fractures en echelon tension gashes -form ~45 degrees from plane of max. shear stress -preexisting vein material rotates while new vein material grows

  21. What is it? What are these structures?What is the sense-of-shear?Describe how the veins grew. en echelon tension gashesright lateral or top-to-the-rightfrom center to tips during rotation

  22. What is it?

  23. Determining the sense of shear

  24. Vein filling during crack opening

  25. Significance: Economic Geology Alteration/Mineralization along fractures; Veins preserve dilational separation

  26. Joints/Fractures: “no appreciable displacement”Next: Geometry and Kinematics: Faults (Read D&R, p. 269-279; 286-296)

  27. Important terminology/concepts Joints- what are they? Joint ornamentation- plumose structure Joint kinematics: opening, sliding, scissoring Griffith cracks and tensile crack formation Tectonic joints Exfoliation/unloading joints Cooling joints Joint arrays and joint analysis Shear fracture formation - en echelon tension gashes - sense-of-shear indicators Significance - tectonics - engineering - economic geology - hazards

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