Crustal deformation
Deformation: ‘change in shape’
Rocks become deformed because
of stress (stress=force/area)
Stress may be applied equally in
all directions-uniform (confining) stress
or
Stress can be larger in one directiondifferential stress
Structural geology studies three-dimensional
distribution of rock bodies
www.geo.cornell.edu/geology/indepth/MainMenu/Photo/Highlights.html
http://www.geo.cornell.edu/geology/classes/geol326/photos.html
Types of stresses
Uniform (confining) stress
Differential stress (tensional,
compressional, shearing)
http://earthsci.org/education/teacher/basicgeol/deform
Strain: change is a body of rock caused by stress
Elastic: rock (or other material) returns to its
original shape after stress is removed
Plastic: rock (or other material) deforms permanently
by bending without being broken
Brittle: rock (or other material) deforms permanently
by fracturing and breaking down
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At lower temperatures,
lower confining pressures
and high strain rates rocks
tend to be brittle
At higher temperatures,
higher confining pressures
and lower strain rates
rocks tend to exhibit
ductile behavior
Rock type is also important: some rocks such as salt, gypsum, shale are likely
to behave in a ductile manner. On the other hand, quartzite, granite and basalt
are brittle. The behavior depends on rock texture and mineral composition
http://earthsci.org/education/teacher/basicgeol/deform/deform.html#NormalFaults
Rock response to stress: folding and faulting
Folds: wavelike bends in
rocks that are mostly the result
of compressional stress.
Faults: fractures in rocks along
which the displacement took
place. Faults form in response
to compressional, tensional
and shearing stress.
fault
folds
folds
Folded structure, Appalachian Mountains
http://www.geo.cornell.edu/geology/classes/geol326/photos.htmlans
Anticline: upfold
Syncline: downfold
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lim
Axial plane separates fold as symmetrically
As possible
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lim
b
lim
Fold axis: intersection of axial plane with
Surface of the fold
Fold classes based on their
appearance:
- Symmetrical
- Asymmetrical
- Isoclinal (limbs are parallel)
- Overturned
- Recumbant
- Chevron
homepage.usask.ca
/earthsci.org/education/teacher/basicgeol/deform
Plunging folds: axis is not horizontal
Plunging fold (anticline), California
Photo by M. Collier
homepage.usask.ca
www.earthscienceworld.org/images
Turbuck & Lutgens, 2002
Domes and Basins
Very large circular or elliptical areas of upwarped (dome) or downwarped
(basin) rocks where rocks are gently sloping away from the area of maximum
uplift (dome) or toward the area of maximum subsidence (basin)
Basins: Illinois and Michigan
Domes: Black Hills of South Dakota
eps.mcgill.ca
Faults
Fault types:
- dip-slip: vertical movement
- strike-slip: horizontal
movement
- oblique-slip: both
vertical and horizontal
1994 W. H. Freeman and Company textbook,
Understanding Earth, by Press and Siever
Stress type and faulting
Dip-slip faults
Have footwall and hanging wall
Subdivided into:
- normal (footwall moves up)
- reverse (footwall moves down)
- thrust: shallow-angle reverse fault
(less than 450 dip)
Normal faults (tensional stress)
Geologic structures
resulting from normal
faulting: half-grabens,
horsts and grabens
Landscapes caused by normal faulting
earthsci.org/education/teacher/basicgeol/deform/deform.html#NormalFaults
Tarbuck and Lutgens, 2002
Reverse and thrust faults (compressional stress)
Strike-slip faults
Left-lateral: ground on the other side
of the fault moves to the left
Right –lateral: ground on the other side
of the fault moves to the right
Monoclines: ‘blind’ fault deforms rocks above
Tarbuck & Lutgens, 2002
Evidence of movements along the faults
- landscape features (e.g. mountains and valleys,
offsets or sharp bends in streams)
- smooth fault planes at the surface
- position of rock layers shifted along
the fault
- changes in rocks around the fault,
e.g. slickensides and fault breccia
Rocks become broken
because of movement
along the fault (fault breccia)
Slickensides: parallel striations on the rock
surface indicating movement
www.geos.ed.ac.uk
www.earthscienceworld.org/images
Joints: fractures with no displacement
Caused by:
-Weathering (e.g. when igneous rocks become exposed)
-Stress
Occur in parallel sets
Make rocks weaker
Mineral deposits are often emplaced along joints
Affect groundwater movement
http://www.pitt.edu/~cejones/Geoimages/7Structures.html
Reverse faults
Normal fault
Thrust fault
San Andreas Fault
strike-slip fault (California)
www.pitt.edu/~cejones/Geoimages/7Structures.html
pubs.usgs.gov
www.geo.cornell.edu/geology/classes/geol326/photos.html
Mapping geological structures: Brunton compass, strike and dip
Orientation of
rock layers and
faults is mapped
using strike
and dip
Geologic map and cross section showing
strike and dip
Strike: compass direction of a line produced
by intersection of rock (or fault) surface and horizontal plane
Dip: angle and compass direction of inclination of rock (or fault)
wardsci.com
homepage.usask.ca
Geologic maps
Show rock units, their ages and geologic structures
http://wrgis.wr.usgs.gov/docs/usgsnps/gmap/gmap3.html#strike
Geophysics and Crustal Deformation
Geophysics: study of the Earth by
physical methods, e.g. seismic, radar,
electromagnetic and radioactivity
methods
Radar remote sensing
Hydro-seismic surveys
Seismic surveys on land
Subsurface structure based on
seismic reflection data
http://geopubs.wr.usgs.gov/open-file/of02-368/,
http://pangea.stanford.edu/GP/research/crustaldeform.html
Geologic structures and man-made structures
Vaiont Dam disaster,
(NE Italy, 1963):
A block of rock (270 m m3)
slid into the lake
along the bedding
plane resulting in
devastating flood (2500 dead)
Damage to the dam as a result of
vertical movement along the fault
(Chi-Chi earthquake, Taiwan, 1999)
Subsidence in Anchorage (~11 ft) as a result of landslide
following 1964 earthquake
www.geocities.com/geogsoc2000/Vaiont1.htm
www.earthscienceworld.org