UNIVERSITY OF SOUTH ALABAMA
GY 111: Physical Geology
Lecture 22: Folds
Instructor: Dr. Douglas W. Haywick
Last time (before the exam)
Rock Deformation
A) Confining pressure and rock deformation
B) Elastic versus permanent deformation
C) Types of deformation
Web notes 20
Lab Manual Chapter 5
Rock Deformation
Rock deformation: any change in the volume, shape or orientation of a
rock body.
Rock Deformation
The strength of materials (including rocks) can be tested in labs using hydraulic
presses. All materials behave elastically up to a point.
Once past the elastic limit, deformation becomes permanent.
Stress & Strain
Stress: a force applied to an area (Example: tire pressure in psi)
Strain: the amount of deformation produced by stress
Elastic strain: analogous to a steel spring or rubber band
Plastic strain: analogous to deforming mud or putty
Brittle strain: analogous to breaking glass
Stress vs. Strain Diagrams
• Illustrate the mechanical behavior of rock materials
• Ductile deformation requires a significant component of
plastic mechanical behavior
Ductile Deformation
plastic
Elastic Limit
Stress
Distortion below the elastic limit is
100% recoverable
Strain %
Stress vs. Strain Diagrams
• Illustrate the mechanical behavior of rock materials
• Brittle: rocks near the surface of the Earth behave as brittle
materials
Brittle Deformation
Rupture
Stress
Distortion below the elastic limit is
100% recoverable
Strain %
Mechanical Behavior of Rocks
Near-surface rocks that are under low T-P conditions behave
as brittle material:
– Fault fracture (slippage)
– Joint fracture (no slippage)
Deep rocks under elevated T-P conditions behave as ductile
material:
– Folding
Today’s Agenda
A) Types of folds
B) Anatomy of a fold (terminology)
Last Call for Mac and Cheese Bonus
Web notes 22
Lab Manual Chapter 6
Folds
http://www.geosci.unc.edu/faculty/glazner/Images/Structure/Folds.html
Folds
http://www.parstimes.com/spaceimages/zagros_anticlines.jpg
Folds
http://www.geology.wisc.edu/~maher/air/130-27v.jpg
Folds
Anticlines: Open downwards
Synclines: Open upwards
Folds
Folds
Please note: Folds are 3-dimensional structures
Folds
Folds are simply bent planes and as such, can
be described using strike and dip connotation
Folds
Fold axes on maps are indicated with the following symbols:
Folds are simply bent planes and
as such, can
Anticline
be described using strike and dip connotation
Syncline
Folds
A simple geological map of
an anticline
Folds
At the point(s) of maximum curvature, the dip of the fold is 0º
Folds
Folds can be horizontal or plunging
Plunging Folds
Asymmetrical Folds
Folds on Maps
Some of you will interpret these in an upcoming lab exercise
Anticline
Oblique View
Geological Map (no scale implied)
Folds on Maps
Some of you will interpret these in an upcoming lab exercise
Syncline
Oblique View
Geological Map (no scale implied)
Folds on Maps
As well as more complex structures
Today’s Homework
1.
Study! Why wait for the day before the final?
2. Assignment 4 next Thursday
3. Thursday lab students: sed rock exam
4. Go up to a total stranger and give them a hug
(then run away very fast)
Next Time
Faults
GY 111: Physical Geology
Lecture 22: Folds
Instructor: Dr. Doug Haywick
dhaywick@southalabama.edu
This is a free open access lecture, but not for commercial purposes.
For personal use only.