Chapter 9 Folds, Faults,and
Mountains
Stress:
Force per unit area
Strain: Change in length/area/volume to original
length/area/volume
Rocks are subjected to great forces- particularly at
plate margins
Compression-
converging plate boundary
Tension- diverging plate boundary
Shearing- transform plate boundary
Strain-
rocks response to stress
Three types of stresses
Types of Deformation

ELASTIC DEFORMATION
 BRITTLE FAILURE
 PLASTIC DEFORMATION
 Elastic deformation
deformation not permanent (minor amount of stress)

Brittle
stress amount exceeds the yield point or elastic limit

Plastic (Ductile) deformation
stress applied gradually to deep warm rocks
Elastic Deformation
Plastic Deformation
Brittle Failure
Folding at Converging Plate Boundaries
Factors affecting rock deformation

Intensity of Applied Stree
 Lithostatic Pressure
 Heat
leads to stretching of rocks at near Earth’s surface
without breaking

at depth- plastic deformation

Time
 Composition
– different minerals have different strength
Deformed rocks in the field

Most apparent in Sedimentary Rocks
 Importance of deformation
– Indicate Past Plate Motions
– Indicates other Past Geological Events
– Locates Specific Natural Resources
– Rock Orientation: Strike and Dip
Interpretation of Rock DeformationFolds

In describing folds, need to know the orientation
of the rock in space- STRIKE and the angle at
which rock is inclined to the horizontal- DIP
 Folds- rocks deform plastically (most occur at
convergent plates)
 Syncline- trough-like
 Anticline- arch-like
 Types of folds- symmetrical, broad, open, overturn,
recumbent

Basin
 Rock deformation that is bowl shaped.

Domes
 Rock deformation that is ova-shaped bulges
Geometry of anticlines and synclines
Artificial valleys and synclinal ridges
Artificial valleys and synclinal ridgescontd.
Various folds – Symmetrical (open)
Various Folds - Assymetrical
Various Folds - Overturned
Various Folds - Recumbent
Various Folds - Plunging
Structural Domes and Basins
Faults

A fracture is a break in a rock (joint or a fault)
 Joints: Fractures with no relative movement
 Fault is when there is relative movement along the break
 Fault Types
 Strike-Slip fault
horizontal movement (transform plate boundary)
 Dip-Slip fault
 Normal (tensional stress)
 Reserve (compressional stress)
 Thrust (low angle reserve fault)
 Oblique (combination of strike-slip & dip-slip)

Evidence of Faults

Visible displacement of rocks
 Pulverized rock
 Slickensides
 Discontinuity of Rock Sequences

Types of Faults:
– Strike-Slip Fault
– Dip-Slip Fault
Fault Planes
Fault Disruption
Map of San Andreas Fault
Horizontal movement along strike-slip fault
Dip-slip Fault
Mid-Atlantic Ridge
Plate Tectonics and Faulting

Normal Faults: Mid-Ocean Ridge &
Continental Rifts
 Reverse and Thrust Faults: Convergent Plate
Boundaries
 Strike-Slip Faults: Transform Boundaries
Geology at a Glance
Oblique Slip
Correlation of different fault types
Accumulation of Oil & Natural Gas
Accumulation of oil along fault planes
A terrace-producing scenario
The mountain ranges of North America
A terrace-producing scenario
Provinces of the Applachian mountain system
Mountainbelt collapsing
Mountainbelt collapsing-contd.