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DEFORMATION
Modifcation of Rocks by
Folding and Fracturing
Chapter 7
Folded
Tilted
Faulted
Mangled!
Strike and Dip
N
W
S
Dip
45°
angle
E
N
W
S
45°
Dip angle
E
A special compass is used by geologists for the purpose of
recording the orientation and dip of planar rock structures like
folds, beds, faults, and metamorphic laminations. These
measurements are reported on 2-D geological maps to help
geologists see how the structure is are oriented relative to the
north pole, and the surface of the earth.
N
W
S
Dip
45°
angle
E
N
W
S
45°
Dip angle
Water trickles down
slope parallel to dip. The strike
is oriented 90 degrees to the dip.
E
Rocks strain when subjected to stress. The strain
is released through folding (plastic deformation)
or faulting (brittle deformation).
An undeformed sample
Under conditions
representative of
the shallow crust,
the marble is brittle.
Under conditions
representative of
the deeper crust,
marble is ductile.
An undeformed sample
Normal fault
Fault plane
Index Fossils and Biostratigraphy
• The shorter the time-stratigraphic range of
the fossil, the more precise are the
correlations that can be made
• Fossils representing free swimming or drifting
organisms can be found world wide and
deposited in a wide variety of rock type
(different depositional environments). These
are the most useful for global correlation of
the stratigraphic records. They are called
Index Fossils.
Reverse fault
Thrust fault
Left-lateral strike-slip fault
Right-lateral strike-slip fault
Oblique-slip faulting is caused by a
combination of forces.
Tectonic Forces Determine the Style of
Faulting
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Youngest
rock
Oldest
rock
Horizontal fold
Fold axis
Plunging fold
Symmetrical folds
Asymmetrical folds
Tensional tectonics
Ductile lower crust
Sinai
Egypt
RED SEA
Continental
crust
Oceanic
crust
Asthenosphere
Saudi Arabia
Rift valley
Downfaulted
blocks
The African Plate and the Arabian
Plate are drifting apart.
Sinai
Egypt
RED SEA
Continental
crust
Oceanic
crust
Asthenosphere
Saudi Arabia
Rift valley
Downfaulted
blocks
The African Plate and the Arabian
Plate are drifting apart.
Sinai
Egypt
RED SEA
Continental
crust
Oceanic
crust
Saudi Arabia
Rift valley
Downfaulted
blocks
Asthenosphere
Tensional forces have created a rift valley.
Compressive tectonics
Shearing tectonics
A left bend in the fault
results in local compression.
A right bend in the fault
results in local extension.
San Francisco
Lavas offset
from volcano
Northwest
Offset of 315 km
Volcano
Los
Angeles
Stream
Offset
of 130 m
Southeast
Stream
Pacific
Plate
North
American
Plate
Unravelling
Geological
History
C D
B
A
C D
B
A
1 Compressive forces
create a fault.
C D
B
A
1 Compressive forces
create a fault.
D
B
A
C
C
D
B
C D
B
C D
B
A
1 Compressive forces
create a fault.
2 Old layers now
overlie younger layers.
D
B
A
C
C
D
B
C D
B
C D
B
A
1 Compressive forces
create a fault.
2 Old layers now
overlie younger layers.
D
B
C
B
C
C D
B
A
D
C
B
A
D
B
C D
B
C D
B
A
1 Compressive forces
create a fault.
2 Old layers now
overlie younger layers.
D
B
C
B
C
C D
B
A
3 Erosion reveals the
view we see today.
D
C
B
A
D
B
C D
B
Keystone thrust fault, southern Nevada
B
D
TIME 1
Sediments are deposited
on the seafloor.
TIME 2
Compressive forces cause
folding and faulting.
Compressive
forces
Faults
TIME 3
Uplift is followed by erosion.
TIME 4
Volcanic eruptions flood the new
surface with lava sheets.
Lava flows
TIME 5
Tensional forces cause normal faults,
creating downfaulted blocks.
Tensional forces
Normal faults