Transcript Chapter 7 PowerPoint

Eric H Christiansen
Main Idea
• Deformed rocks are a record of
Earth’s tectonic system and reveal
how it works.
• Joints
• Faults
• Folds
Rock Deformation
• Stress is the pressure or force applied to
rocks that cause deformation to occur
• Uniform (confining) stress is equal in all
directions
– Rocks are confined by the rock around them
• Differential stress is not equal in all
directions
– This is what deforms rocks
Rock Deformation
• Three types of differential stress
– Tensional - pulling apart
– Compressional - squeezing together
– Shear - slipping, twisting, or wrenching
Rock Deformation = Strain
Definition: Strain is the change in the shape or volume of
a rock that results from stress.
Deformation?
Deformation?
Brittle deformation
Fracture
• Stress exceeds the
brittle strength
• Irreversible break
Ductile deformation
Irreversible change in
size and/or shape
Volume and density
may change
Geometry of Rock Structures
• Structures may be defined by
the orientation of planes
– Dip – the angle of inclination
downward from a horizontal plane
– Strike – the compass bearing of a
horizontal line where the inclined
plane intersects an imaginary
horizontal plane
Strike and Dip
Joints
• Fractures created in brittle
rocks
– No shear or displacement has
occurred
– Form as overburden is removed,
confining stress reduced
– Form by cooling of igneous rocks
– Often occur in sets
Joints
• Fractures created
by in brittle rocks
– No shear or
displacement has
occurred
– Any joints here?
Why are
joints
important?
•
•
•
•
•
Fluid flow
Ground water
Oil
Ore deposits
Weathering and
erosion
Why are
joints
important?
•
•
•
•
•
Fluid flow
Ground water
Oil
Ore deposits
Weathering and
erosion
Faults
•
Fractures along which
displacement has occurred
• Blocks on either side
have moved
• Most faults are inclined
at some angle measured
from horizontal
• The dip angle of the fault
• Two blocks are defined,
one on either side of the
fault
Faults
• Fault geometry
– Imagine a horizontal tunnel cutting
through a fault in a cross-section
Horizontal
Surface
Dip angle
Foot Wall
Hanging Wall
Fault plane
Three Types of Faults
• Normal faults
• Hanging wall moves down relative to foot wall
• Block slides down the dip angle
• Reverse faults
• Hanging wall moves up the dip angle
• Hanging wall moves up relative to foot wall
• Reverse to what seems normal
• Strike slip faults
• Displacement along fault is horizontal
• Parallel to the strike of the fault plan
Which fault is “normal”?
A
B
C
Which fault is “reverse”?
A
B
C
Types of Faults
Normal
Reverse or thrust
Strike Slip
Fault Types
1. Normal faults
• Hanging wall
moves down
relative to foot
wall
• Block slides
down the dip
angle
Hanging
Wall
Foot
Wall
•Normal faults are created by tension
–Rifts are created by normal faults
Fig. 7.8b. Normal faults produce grabens & horsts
Normal Faults
• Normal faults are created by tensional
forces
• Rifts are created by parallel normal
faults dipping toward each other
• The block in the center which drops down is a
graben
• The Rio Grande valley in New Mexico is a rift
graben
Reverse Faults
• Compressional stress usually
causes reverse faults to form
– Reverse faults are common at
convergent plate boundaries
– Reverse faults cause a thickening
of the crust as rocks are piled up
– Older rocks may be found above
younger rocks
Reverse Faults
• Thrust faults are a special kind
of reverse fault
– Shallow dip angle, > 45o
– Common in large mountain ranges
– Horizontal displacement may be
many tens of kilometers
– Evidence of thrust faults in
sedimentary rocks is seen when a
sequence of the same rocks are
repeated
2. Reverse faults
• Hanging wall moves up relative to foot wall
• Block moves in the reverse direction to what
seems normal
• created by compression
Hanging
Wall
Foot
Wall
Reverse Faults
• Thrust faults are a special kind of reverse fault
– Shallow dip angle, > 45o
– Common in large mountain ranges
Strike-Slip Faults
• Strike-Slip faults
– Principle movement is horizontal
• Left or Right Lateral
• Little or no vertical movement
– Caused by shear stress
– Indicated by abrupt changes in
drainage patterns
Fig. 7.8e. Strike-slip faults offset drainage
Strike-Slip Faults
3. Strike-Slip faults
• Principle movement is horizontal
• Left or right
• Caused by shear stress
Movement Along Faults
• Rarely exceeds a
few meters in a
single event
• Small movements,
cm scale, may
occur on a regular
basis
• Total
displacement may
be km, but does
not occur in a
single event
Wasatch Fault: Our Fault
• What type of fault is
it?
• How much
displacement at each
event?
• What is this event
called?
• How many events if
altitude is 2,500 m?
• Is that an accurate
assessment of the
total amount of
displacement?
Fault Breccia
Fig. 7.8a. Easily recognized displacement
Folds
• Warps in rock layers due to ductile
deformation
– Generally indicate horizontal compression
– Multiple generations of folding may exist
– Folds are
described by:
• strike of their
hinge line
• The angle of
dip of their limbs
Folds
• Folds are described by:
– The strike of their hinge line
• The hinge line is the intersection of
the hinge plane with the folded layer
• Hinge lines may be inclined in a
plunging fold
– The angle of dip of their limbs
Fig. 7.12. Fold geometry
Folds
• Three simple fold forms exist
– Synclines warp downward
– Anticlines warp upward
– Monoclines dip in one
direction
Fig. 7.11. Types of folds
Anticlines & Synclines
• The sequence of ages of strata
indicate the geologic structure in
folds
– Anticlines have the oldest layers
exposed at the center of the fold
along the axial plane
– Synclines have the youngest strata
exposed along the axial plane
Fig. 7.15a. A series of anticlines & synclines
– Anticlines have the oldest layers exposed at
the center of the fold along the axial plane
– Synclines have the youngest strata exposed
along the axial plane
Youngest
rock
Anticline
(a)
(b)
Syncline
Monocline
Oldest rock
Fold Belts (folded mountains)
• Orogenic belts
are a long linear
series of folds
– Fold geometry is
not overly
complex
– Pattern of
outcrops may
appear complex
– Complex folds:
• Re-folded
• Cut by thrust
faults
Orogenic belt with complex folding
Complex Folds
• Folds may be
very complex
– Application of
shear stress
– Multiple folding
events
– Complex forms
are created
Complex Folds
• Folds may be very complex
– Application of shear stress
– Multiple folding events
– Complex forms are created
Complex Folds
• Plunging folds occur when the folds axis is
dipping or plunging
• Limbs of some folds are not the same, one
dips more steeply than the other
• Some folding is so extreme that beds are
turned upside-down
Fig. 7.15d. A plunging anticline
Domes & Basins
– Generally occur in continental
interiors
– Broadly warped regions
– Roughly circular pattern of
outcrops
Fig. 7.15b. A small dome
Complex Folds
• Diapirs
– Less dense salt layers may rise up
– Some overlying strata may be
pierced
– Salt diapir has an inverted teardrop
shape
– Strata above diapir are domed
upward
Unconformities
Stress and Structure
• Geode II 818
• 847
• 819
Structures and Plate Tectonics
End of Chapter 7