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Brittle fault rocks and kinematic indicators (D&R: 280-286; 297-300)

Announcements Midterm Monday! Bring calculator, protractor, ruler, stereonet and tracing paper Talk This Thurs. 4 pm, Rm. Haury Bldg. Rm 216, "The role of orogen-parallel extension during the India-Asia collision", write 1 paragraph summary (+1% extra credit).

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Brittle fault rocks and kinematic indicators (D&R: 280-286; 297-300)

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  1. AnnouncementsMidterm Monday!Bring calculator, protractor, ruler, stereonet and tracing paperTalk This Thurs. 4 pm, Rm. Haury Bldg. Rm 216, "The role of orogen-parallel extension during the India-Asia collision", write 1 paragraph summary (+1% extra credit)

  2. Brittle fault rocks and kinematic indicators(D&R: 280-286; 297-300) 1. Descriptive terminology2. Determining direction of slip3. Determine sense-of-shear

  3. Breccias: fault rocks composed of angular fragments of wall rock set in a finer-grained matrix of crushed wall rock; classified based on clast size megabreccia! (well, almost)

  4. breccia

  5. gouge: very fine-grained; clayey

  6. cataclasite: very fine-grained strongly indurated fault rock (white in photo) concept of fault zone

  7. cataclasite: commonly forms "ledges"

  8. pseudotachylite: dark, very fine-grained and generally glassy fault rock; thought to represent rapidly chilled rock melt, with melting of rock due to shear heating during an earthquake event- WOW! the glassy surface is the "generation" surface and the black veins are "injection veins"

  9. Determining kinematics from fault fabrics1st step: figure out direction of slip2nd step: figure out sense-of-shear- this requires looking at fabrics that are oriented perpendicular to slip direction

  10. slickensides: polished shiny fault surfaces due to abrasive actionslickenlines/striations: scratches

  11. Grooves

  12. Larger Grooves

  13. onto sense-of-shear... chatter marks: step-like features oriented perpendicular to striations

  14. What is it? top-to-the right Crystal fiber lineations: produced by preferred directional growth of minerals during faulting in direction of movement

  15. Drag folds

  16. conjugate Riedel shears R: synthetic Riedel shearR': antithetic Riedel shearP: synthetic shear, subordinate to R and R' or absent

  17. What is it? R' R' R R main fault left lateral Where are R and R' shears?What is the sense-of-shear?Explain why R' shears are associated with veins

  18. photo courtesy of Brent Morris

  19. A summary What is it? breccia cataclasite Riedel shears gash fractures: form perpendicular to s3 normal fault

  20. Important terminology/concepts Breccias megabreccia breccia gouge cataclasite fault zone pseudotachylite slickensides slickenlines/striations grooves chatter marks crystal fiber lineations drag folds Riedel shears gash fractures

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