GEO 311 Study Guide - Final Guide: Deformation Mechanism, Differential Stress, Cataclasite
How to make an accurate cross section:
1) Line up the edge of a piece of plain paper along a line marked XY
2) Make tick marks where each contour line is met on the XY line. Near each tick mark
write the line of the strike and dip (or contour interval if only doing contours)
3) Line up the edge of the piece of paper with the graph drawn and record the tick marks
on this paper
4) Record the strikes (W or E) and the dip of each of the lines
a) If this is contour then record the elevation and make the tick marks at the
appropriate elevation
5) Fill in the rest of the cross section drawing in the appropriate lines for the strike and dip
then color
Interpreting Stereonets
● This website tells you how to plot stuffs and interpret some plotted stuff
http://www.see.leeds.ac.uk/stepup/pdfs/Introduction%20to%20stereonets%201.pdf
● This lady tells you how to plot fold data
https://www.youtube.com/watch?v=zqUdQkMmzso&index=5&list=PL4C8A54C58C181B
7D
Fractures and Joints
● Know the difference between the Mode 1, 2, and 3 fractures
● Different types of of surface markings on joint faces
○ Hackles diverge from a point or central axis
○
■ Indicate zones where the joint propagate rapidly
■ Radiate from origin and fan outward
■ Often very fine by the origin
○ Origin is the point where the fracture starts
○ Ribs: ramps in orientation of joint surface giving rounded ridges
■ Direct expression of the joint path
find more resources at oneclass.com
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○ Plumose Structure: the structures formed on the joint surface during propagation
and provides information about the joint propagation direction
● Mohr’s Circle
○ http://web.mit.edu/16.unified/www/FALL/materials/documents/HO-M-
6(Mohrs)(08).pdf 5 Page handout explaining Mohr Circles
○ https://www.youtube.com/watch?v
=-bIh3xSZBqE Youtube video
explaining how to draw a Mohr
Circle
● Mohr’s Circle Normal Stress Equation:
○ N= ((1+3)/2) + ((1-3)/2) *
cos(2θ)
■ Normal Stress
○ S= ((1-3)/2) * sin(2θ)
■ Shear Stress
● Normal Stress: Stress that acts
perpendicular to a plane
○ (+) compression
○ (-) tension
● Shear Stress: Stresses that act parallel to material planes
○ (+) Counterclockwise compressions
○ (-) Clockwise compressions
● Coulom
○ SCritical= +/- [So+ tan (0) (N)]
● Von Mises
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find more resources at oneclass.com
○ SCritical= +/- (Constant)
● Byerlees
○ SCritical= tan (0) (N)
Deformation Mechanisms
● Pressure Solution
○ Important at shallow to moderate levels in the crust
○ Involves directed diffusion along grain boundaries that have small amount of fluid
along them
■ Occurs under differential stress
■ Common in most sedimentary and metamorphic rocks
○ Diffusion along “wet” grain boundaries and reprecipitation at low stress points
○ Not doing anything with the crystal
lattice!! Just shape preferred
orientation
■ Just dissolving the original
grains and then reprecipitate
the grains
○ What to look for
■ Dark stripes in the rock
■ (opaque)
● Formed from material that does not dissolve away (residue)
● Micas do not dissolve away so easily so they tend to get
concentrated and then occur in dark stripes in the rock
○ Stylolites occur
● Sharp contacts on the edges of the dark stripes and truncated
mineral grains
● Very strong shape preferred orientation with no lattice preferred
orientation
○ High temperature, High pressure, Low stress?????????
● Cataclasis
○ Brittle behavior occurring at the grain
scale
○ End up with a rock that has lots of
angular fragments in it
■ Increase the spread of grain
sizes (some are big and some
are small)
○ Cataclastic deformed rocks end up with
an increase in volume (decrease in grain
size)
■ Cataclastic rocks can get filled in with veins (we are making room for
these new structures to come in and form and therefore we end up with
more volume)
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
How to make an accurate cross section: line up the edge of a piece of plain paper along a line marked xy, make tick marks where each contour line is met on the xy line. This website tells you how to plot stuffs and interpret some plotted stuff http://www. see. leeds. ac. uk/stepup/pdfs/introduction%20to%20stereonets%201. pdf. This lady tells you how to plot fold data https://www. youtube. com/watch?v=zqudqkmmzso&index=5&list=pl4c8a54c58c181b. Know the difference between the mode 1, 2, and 3 fractures. Different types of of surface markings on joint faces. Hackles diverge from a point or central axis. Origin is the point where the fracture starts. Ribs: ramps in orientation of joint surface giving rounded ridges. Plumose structure: the structures formed on the joint surface during propagation and provides information about the joint propagation direction. =-bih3xszbqe youtube video explaining how to draw a mohr. Normal stress: stress that acts perpendicular to a plane (+) compression (-) tension.
