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Joints and shear fractures

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  1. Joints and shear fractures

  2. Announcements • Need two HOV drivers for Sat field trip; • Extra credit • Talk to Al Pullen asap

  3. Structural Analysis: Approach • Geometry: mapping, measurements of orientations • Kinematics: motions related to deformation • Translation: change in position • Rotation: change in orientation • Distortion: change in shape • Dilation: change in volume Dynamics/Mechanics: relating deformation to stresses

  4. faults joints drag folds joints What is it?? Marker bed

  5. Structures from shallow to deep in the crust, beginning with…. Joints (D & R; p. 205-226) Next lecture: Faults (D&R; p.269-279; 286-296)

  6. Types of joints • Opening (mode 1) • Shearing (mode 2) • Scissoring (mode 3) Can be filled with veins materials (calcite, quartz), Form networks that is preferentially oriented in the regional stress field

  7. Joint: A fracture that forms by tensile loading • walls of fracture move apart slightly; no appreciable displacement • forms perpendicular to tensile direction • abundant structural element

  8. Joint: A fracture that forms by tensile loading- walls of fracture move apart slightly as joint develops; form perpendicular to tensile direction; abundant structural element

  9. Joint arrays

  10. Joints in 3-D: commonly elliptical

  11. Joint surfaces Planar and often smooth, but not that smooth... Some texture Moscow Kremlin - Bell Tower of Ivan the Great. Fractured in 1737 due to uneven cooling

  12. Origin: analogous to the focus of an earthquake Plumose structure: A subtle roughness on surface of some joints; resembles imprint of a feather.

  13. hackles in plumose structure.

  14. Joints: Kinematics ribs are arrest lines- opening is not instantaneous, but rhythmic, like splitting wood (ripping paper analogy)

  15. GEOMETRY- planar, plumose structure KINEMATICS- very small “pull-apart” movements (dilation) ON TO MECHANICS...

  16. Griffith crack theory: pre-existing microcracks in a rock- act as stress "concentrators" The largest properly oriented Griffith cracks (perpendicular to tensile direction) propagate to form a through-going crack

  17. But what produces the tensile stress?

  18. Thermal contraction can produce joints

  19. REMOVAL OF OVERLYING ROCKS (unloading) PRODUCES JOINTS! Unloading joints: due to removal of overburden-- rock expands in vertical direction and contracts in horizontal direction ("Poisson effect") (marshmallow analogy) - forms near vertical and horizontal joint arrays

  20. Jointing in response to unloading and "residual stresses"

  21. Exfoliation joints: Are parallel to topography; form by unloading of "residual" stress

  22. WHY ARE JOINTS IMPORTANT? Joints related to regional stress

  23. Can also tell us about orientation of tectonic stress Can strongly influence the landscape – weathering is localized along joint surfaces-- hoodoos

  24. Significance for Engineering Planes of weakness! Rock bursts in mines…. Significance for petroleum: pathways for hydrocarbons; pump water into ground to artificially produce joints! (hydrofracturing)

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

  26. Geologic Hazards Rockbursts in mines

  27. Joints form/occur mainly in the uppermost crust (upper few kilometers) WHY? Stresses become more compressive with depth to the point where rocks can’t “pull-apart”

  28. Shear fracture: A fracture with a component of “sliding” motion due to compression

  29. Determining the sense of shear

  30. Shear fractures with appreciable displacements = FAULTS

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

  32. Next: Geometry and Kinematics: Faults (Read D&R, p. 269-279; 286-296)