Today’s lecture

• Application exercise

• Announcements

• Information about the research assignment

• Faults and stress

Announcements

• No field trip on this Thursday, but yes on October 21!

• Note some schedule changes on the web site.

Information about research assignment

• This assignment, due on December 2, will be 3-5 pages of double-spaced typed text (12 pt font like Times Roman). The assignment is to make some structural observations and measurements, or even a small geologic map, for any nearby geologic locality (of your choosing) and explain the geometry and deformation and interpret it in terms of geologic history and geologic processes. The paper must include at least 2 figures (they don't count towards the length) illustrating some aspect of your research. These figures can be anything (cartoon, sketch, simplified geologic cross-section, geologic map), but should be something you illustrate yourself. It should be neat, perhaps colored, but need not be drafted. You will also prepare and present (5 minutes) a poster, consisting of several figures, a summary, and bullet lists outlining key points about your research project.

Research assignment review

• Two classmates must review your written paper before turning it in. Your should give them a typed, double-spaced draft of your paper and drafts of the figures and they will write corrections and suggestions on the draft, and then sign the bottom. You should incorporate their suggestions (at least those that are valid) into your final paper, and turn in the copies of your classmate's reviews along with your final paper. One of the best ways to write is edit someone else's writing. You will lose points if you do not get two meaningful reviews on a typed draft (not the final draft) of your paper--someone simply signing your final paper but making no comments or saying "Good job" is not a sufficient review!

Faults and stress: traction and stress

Mohr CircleGraphical construction that lets us visualize the relationship between the principal stresses and tractions on a boundary (like a fault).

measured positive clockwise from 1 direction clockwise to plane of interest2 measure positive clockwise from –n direction on Mohr circle

Faults and stress: traction and stress

Equations of the Mohr Circle (also “Cauchy’s equations”)

90

Faults and stress: traction and stress

Faults and stress: traction and stress

Faults and stress: traction and stress

Faults and stress: traction and stress

35

Faults and stress: traction and stress

Faults and stress: traction and stress

= s-Davis and Reynolds

-Engelder, 1993, Stress regimes in the lithosphere, Princeton Univ. Press

Faults and stress: traction and stress

Coulomb Law of Failure

Coulomb equation c = c + tan n

Wherec = critical shear stress required for faulting (shear strength)c = cohesive strengthtan = coeffieicnet of internal friction =

Faults and stress: traction and stress

-D+R