GY 111: Physical Geology

UNIVERSITY OF SOUTH ALABAMA

Lecture 22: Folds

Instructor: Dr. Douglas W. Haywick

Last time (before the exam)

A) Confining pressure and rock deformation B) Elastic versus permanent deformation C) Types of deformation

Rock 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

Strain %

Stress Elastic Limit

plastic Ductile Deformation

Distortion below the elastic limit is 100% recoverable

Stress vs. Strain Diagrams • Illustrate the mechanical behavior of rock materials • Brittle: rocks near the surface of the Earth behave as brittle

materials

Strain %

Stress

Distortion below the elastic limit is 100% recoverable

Rupture

Brittle Deformation

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

Web notes 22 Lab Manual Chapter 6

A) Types of folds B) Anatomy of a fold (terminology) Last Call for Mac and Cheese Bonus

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

Folds are simply bent planes and as such, can be described using strike and dip connotation

Fold axes on maps are indicated with the following symbols:

Anticline 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

Oblique View Geological Map (no scale implied)

Anticline

Folds on Maps

Oblique View Geological Map (no scale implied)

Syncline

Some of you will interpret these in an upcoming lab exercise

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.