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Understanding Faults: Types, Mechanics, and Homework Assignments

This document discusses various types of geological faults, including strike-slip, normal, and thrust faults. It details their characteristics such as displacement and rock layering, providing examples of how younger rocks can be found on top of older ones in different fault types. The text also emphasizes the mechanics behind faults, like tensile loading and the effects of shear fractures. Additionally, it includes homework assignments related to these concepts and important field trip information to Cottonwood Canyon.

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Understanding Faults: Types, Mechanics, and Homework Assignments

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  1. What is it?

  2. Faults • Homework due today - • New homework assignment handout today

  3. Joints: Geometry: planar; plumose structure; joint “sets” Kinematics: walls move apart very slightly in direction of “pull” Mechanics: form by tensile loading; cooling; *removal of overlying rock Other: abundant in uppermost crust

  4. Under compression, rocks fail along shear fractures (sliding motion rather than pulling apart motion) Introduction to Faults (p. 269-279; 286-296)

  5. Fault: Shear fractures along which there is visible offset; generally planar or curviplanar

  6. Strike-slip faults: Accommodate horizontal slip between adjacent blocks; steep (~vertical) faults left lateral (sinistral) right lateral (dextral) left lateral vs. right lateral: sense-of-slip relative to a chosen block

  7. What is it? left lateral vs. right lateral

  8. For faults that are not vertical…. Hanging wall: The block above the fault. Footwall: The block below the fault.

  9. Normal fault: hanging wall moves down with respect to footwall

  10. What is it? Normal faults generally place younger and/or lower-grade rocks (in HW) on top of older and/or higher-grade rocks (in FW) HW young low-grade “dirt” FW old high-grade gneiss

  11. Normal faults generally place younger and/or lower-grade rocks (in HW) on top of older and/or higher-grade rocks (in FW) What is it? Cambrian limestone FW HW Tertiary conglomerate

  12. Purchase a time scale card from SESS – only $1!

  13. Thrust fault (reverse fault): hanging wall moves upward relative to footwall What is it? HW FW

  14. Thrust faults generally place older and/or higher-grade rocks (in HW) on top of younger and/or lower-grade rocks (in FW)

  15. Thrust faults generally place older and/or higher-grade rocks (in HW) on top of younger and/or lower-grade rocks (in FW)

  16. Thrust faults generally place older and/or higher-grade rocks (in HW) on top of younger and/or lower-grade rocks (in FW) What is it? HW sandstone low grade blueschist FW high grade

  17. Normal faults accommodate lengthening

  18. Thrust faults accommodate shortening

  19. http://earth.leeds.ac.uk/learnstructure/index.htm

  20. Slip vs. Separation Slip: ACTUAL relative displacement Separation: APPARENT relative displacement

  21. A natural example demonstrating that apparent separation can be much different than true slip!

  22. The key to describing slip along a fault lies in measuring (1) Direction of displacement (2) Sense of displacement (3) Magnitude of displacement

  23. Listric: curved faults that flatten with depth listric normal fault

  24. Map Symbols: a start • Strike-slip fault • Normal fault • Thrust fault

  25. Next lecture: Intro to folds Read pgs. 372-413

  26. Announcements: Field trip to Cottonwood Canyon this Saturday - meet at 7:45 am at the loading dock! Bring pencils, protractor Will it rain? Rain gear Will it be hot? Sunscreen lotion Hiking boots, water, lunches Will be back around 4 pm.

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