Transcript topic #8 - rock deformation

DEFORMATION OF ROCKS
[L17 P. 363-373 /IP-B]
Deformation of rocks
• Folds and faults are geologic
structures.
• Structural geology - the study of
the forces that deform rocks
(stress applied) and the effects
of this force (strain).
Small-scale Folds
Phil Dombrowski
Fig. 10.1
Small-scale Faults
Tom Bean
Fig. 10.2
Stress
(force per unit area)
Types of directed (or differential )
stresses include:
• Compression - shorten
• Extension - elongate
• Shear - distort shape
Differential
stress
Strength
• Ability of an object to resist deformation
(low pressure vs. high pressure)
Strain
is any change in original shape or
size of an object in response to
stress acting on the object
Types of deformation
• Elastic
• Ductile (plastic)
• Brittle (rupture)
Elastic deformation
Temporary change in shape or
size that is recovered when
the deforming force is
removed
Think “rubber band”
Ductile (plastic) deformation
• Permanent change in
shape or size that is not
recovered when the stress
is removed
• Occurs by the slippage of
atoms or small groups of
atoms past each other in
the deforming material,
without loss of cohesion
• Think “deck of cards”
Brittle deformation (rupture)
• Loss of cohesion of a
body under the
influence of deforming
stress
• Sucker breaks!!!
• Usually occurs along
sub-planar surfaces
that separate zones of
coherent material
Typical
stress and
strain curve
Factors that affect deformation
• Temperature
• Pressure
• Strain rate
• Rock type
The variation of these factors
determines if a rock will fault or fold.
Effects of rock type on deformation
Some rocks are
stronger than
others.
competent: rocks that
deform only under
great stresses
incompetent: rocks
that deform under
moderate to low
stresses
Effects of deformation on rock type
Experimental Deformation of Marble
Brittle Deformation
(Under low pressure)
Ductile
Deformation
(Under high pressure)
Fig. 10.7
M.S. Patterson
Orientation of deformed rocks
- way to describe the
orientation of geologic structures.
ATTITUDE
Strike: (compass) bearing of a line
defined by the intersection of the
plane in question and the horizontal
Dip: acute angle between the plane
and the horizontal, measured
perpendicular to strike.
Fig. 10.4
Fig. 10.4
Dipping Sedimentary Beds
Chris Pellant
Fig. 10.3
Cockscomb Ridge, S. Utah
P.L.
Kresan
P.L.
Kresan
Cockscomb Ridge, S. Utah
Strike
Dip
P.L. Kresan
Tectonic Forces and Resulting
Deformation
Fig. 10.6
BRITTLE DEFORMATION
A. Abrupt movement breaks or cracks
strata
B. 2 kinds of breaks:
– 1. JOINTS - NO movement of blocks
– 2. FAULTS - YES movement of blocks
1. Movement along STRIKE
2. Movement along DIP
Columns
Formed by
Jointcontrolled
Weathering
Terry Englander
Fig. 10.20
Joint-controlled Landscape, S.E. Utah
Faults
Fractures in rocks created by
earthquakes that have moved
A. Dip-slip faults
normal
reverse
thrust
B. Strike-slip faults
right lateral or left lateral
Hanging-wall and footwall
Dip-slip faults
Motion of the fault blocks,
parallel to the dip direction.
Classification of Faults
hanging wall
footwall
cross section
Normal Fault
hanging wall
footwall
cross section
Normal Dip-slip Fault
Reverse Fault
hanging wall
footwall
cross section
Reverse Dip-slip Fault
Drape Fold over Reverse Fault, WY
George Davis
Thrust Fault
Thrust faults are low-angle reverse faults.
hanging wall
footwall
cross section
Thrust fault
Keystone Thrust Fault, S. Nevada
Cambrian Limestone
Jurassic
Sandstone
John S..Shelton
Lewis
Thrust,
Sawtooth
Range,
Wyoming
Kurt N. Coonstenius
French Thrust, Wyoming
Mississippian
Limestone
Cretaceous Shale
Kurt N. Coonstenius
Strike-slip faults
Motion of the fault blocks is
parallel to the strike direction.
To determine the direction of
strike, put toes on the fault
line & look at the direction
the opposite block moved.
Left-lateral Strike Slip Fault
map view
Right-lateral Strike Slip Fault
map view
Strikeslip
Fault
Gudmundar E. Sigvaldason
Rift Valley Formed by Extension
Horst
Graben
Horst
Wildrose Graben, Southern California
NASA/TSADO/Tom Stack
1872 Fault Scarp, Southern California
1988 Armenian Earthquake Fault Scarp
Armando Cisternas
1992
Landers
Earthquake
Fault Scarp
PLASTIC DEFORMATION
FOLDS
anticline: older rocks on the inside
syncline: older rocks on the outside
(scale - from mm to tens of km)
Fold Terminology
Fig. 10.10
Symmetrical
Isoclinal
Asymmetrical
Overturned
Recumbent
Fig. 15.22
Anticline
Axial plane
Bill Evarts
Fig. 10.11
Asymmetric Folds
Breck Kent
Overturned Folds
Phil Dombrowski
Fig. 10.1
Overturned
Syncline,
Israel
Geological Survey of Israel
Fig. 10.13
Valley and
Ridge
Province of
the
Appalachian
Mountains
Fig. 10.19
Valley and Ridge Province
P. L. Kresan
Plunging Folds in the
Valley and Ridge
J. Shelton, Geology illustrated
Fig. 10.15
Raplee Anticline, S.E. Utah
Raplee Anticline on the San Juan River, Utah
Domes and Basins
Fig. 10.16
Sinclair Dome, Wyoming
John S. Shelton
Fig. 10.17
Anticlines and Synclines
Fig. 10.9
Geologic
Map and
Cross
Section
Fig. 10.5
Outcrops of Syncline
Fig. 10.18
All pau