Ch. 17
Crustal Deformation and Mountain Building
Chapter 17 Opening Figure
Rock Deformation
Rock deformation refers to changes in the shape, volume,
or orientation of a rock due to changes in temperature
and pressure over time.
Rock Deformation due to Stress
Stress is strictly defined as
force per unit area, and is
used to describe all forces
that deform rocks
• Compressional stress
(squeezes and shortens)
• Tensional stress
(elongates)
• Shear stress (causes
splaying, e.g. deck of
cards).
Elastic deformation
• We’ve already talked
about elastic
deformation, which is
temporary. The rock
will return to original
shape and form after
stress is removed.
• What happens if the
elastic limit is
surpassed?
Brittle Deformation: Rocks behave like
a brittle solid and fracture
• near surface
conditions
• relatively low
pressures and
temperatures
• result is
faulting
Ductile deformation: solid state flow of rocks
that results in permanent deformation without
fracture
• conditions at
depth
• relatively high
pressures
and
temperatures
• result is
folding
Factors in Rock Deformation
•
•
•
•
Pressure
Temperature
Rock composition
Time rock is exposed to pressure and
temperature
Synclines and Anticlines
• An anticline is a structure in
which the strata (layers) in the
middle are older than those of
the limbs (sides).
• Anticlines are formed by
upfolding or arching of rock
layers.
• A syncline (“sin makes you
smile”) is a structure in which
the strata in the middle are
younger than those of the
limbs.
• Synclines are formed by
downfolding of rock layers.
Synclines and Anticlines
• These two types of folds are most often found
in adjacent to one another.
• Both are examples of ductile deformation.
• Both are commonly due to horizontal
compressional stress.
Anticline
Syncline: See
the smile?
Syncline, anticline, syncline (the right
syncline is cut by a vertical fault)
Overturned Fold
Plunging Folds
Synclines and
anticlines are
“plunging”
when their
axis is no
longer parallel
to the land
surface, due
to a
perpendicular
component of
stress.
Plunging folds vs non-plunging folds
Monocline
• A monocline is a large, steplike fold in
otherwise horizontal sedimentary strata
• Monoclines are associated with the
reactivation of faults of faults in the basement
rock below the sediments.
The San Rafael Swell in Utah, is an example of a
monocline
Brittle Deformation: Faults
Faults- fractures
in rocks along
which there is
(or has been)
displacement.
Dip-Slip Faults
• Movement
primarily along the
inclination (dip) of
fault plane (i.e.
up/down)
• Parts of a dip-slip
fault
– Hanging wall – the
rock above the fault
surface
– Footwall – the rock
below the fault
surface
Normal Dip-Slip Fault
• Hanging wall block moves
down due to gravity (that’s
the normal part
• Associated with fault-block
mountains
• Prevalent at spreading
centers
• Caused by tensional forces.
Fault block mountain range produced by
normal faulting
Horst (high)– uplifted block,
Graben (grave) – downdropped block
• Horst and Graben Topography: results from a series
of normal faults in an extensional environment.
• Dip of normal faults decrease with depth.
• Nearly horizontal detachment fault separates brittle
deformation above with ductile deformation below.
Basin and Range Geographic Province,
(Nevada and parts of California, Utah)
Reverse Dip-slip Faults
and Thrust Fault
• Hanging wall block moves up
(this is the reverse of normal!)
• Caused by strong compressional
stresses
• Reverse fault - dips greater than
45º
• Thrust fault - dips less than 45º
Thrust fault – low angle reverse fault
Thrust Fault in Nevada
What kind of fault, normal or
reverse?
Strike-Slip Faults: Dominant displacement is
horizontal and parallel to the trend, or strike
Strike-slip vs. Transform
• Strike-slip faults
– Dominant
displacement is
horizontal and
parallel to the
trend, or strike
– Often associated
with tranform-fault
boundaries, BUT
– Not all strike-slip
faults are
transform!
Strike-slip vs. Transform
• Transform fault
– Strike-slip fault that
links spreading
centers lithosphere
– Considered a plate
boundary if large
enough
– All transform faults
are strike-slip, as
they move parallel
to strike.
Strike-slip faults are classified as
right-lateral or left lateral. Which
is the San Andreas?
Right-lateral or Left lateral?
San Andreas Fault System
San Andreas Fault System
From E to W, San Andreas, San Jacinto,
Elsinore Faults
Peninsular Ranges Batholith and Transverse Ranges
Extension in the Basin and Range
Figure 17.20
Joints
•
•
Fractures along which no
appreciable displacement has
occurred
Most are formed when rocks in the
outer-most crust are deformed
Figure 17.11
Orogenesis (Mountain Building)
Orogenesis
Processes that collectively produce
a mountain belt
Occurs due to plate movements and
often (not always!) at plate
boundaries.
Orogenesis at Convergent
Boundaries
Island Arcs (Oceanic-oceanic crust
convergence )
• Subduction zone forms
• Volcanic arc forms
• Often associated with deep ocean
trench
Volcanic Island Arc
Orogenesis at Convergent
Boundaries
Andean-type orogenesis (Oceaniccontinental crust convergence )
• Subduction zone forms
• Continental volcanic arc forms
• Accretionary wedge* forms
seaward of arc
*Large mass of sediments scraped from
subducting oceanic plate which attaches to
to the overiding block
Andean-Type Orogenesis
Andean-Type Orogenesis
Andean-Type Orogenesis
Andean-type orogenesis
Active Examples-Volcanic Arcs
Andes (accretionary wedge under water?)
Cascades – active volcanoes, slightly
inland from non-volcanic coastal
mountains, the Olympus Range.
Inactive Example:
Sierra Nevada Range (magma chamber of
volcanic arc) and California's Coast
Ranges (accretionary wedge)
Orogenesis at convergent boundaries
Accretion of Exotic Terranes
• Small crustal fragments collide with and
accrete to continental margins
• Accreted crustal blocks are called
terranes (note spelling). Terrane refers
to any landmass that has a geologic
history distinct from that of an adjoining
landmass
• Much of western North America is
composed of exotic terranes.
Accretion of Exotic Terranes
Accretion of Exotic Terranes
Accretion of Exotic Terranes
Accreted Terranes in the
Western United States
Source material include:
• island arc material
(igneous parent rock)
• submarine deposits
(sedimentary parent rock)
• Ancient ocean floor
(igneous parent rock)
• displaced continental
(igneous parent rock)
•
Orogenesis at Convergent
Boundaries
Collisional
Mountain
Belt: Two
plates with
continental
crust
converge
• Characterized
by shortening
and
thickening of
continental
crust
Formation of Himalayas
Formation of Himalayas
Collisional Mountain Ranges
Active Example
• Himalayan Mountains and the
Tibetan Plateau
Inactive Example
Appalachians (collision of N.
America and Africa)
Figure 17.19a,b
Figure 17.19b,c
Figure 17.19c,d
Landsat Image of Valley and Ridge
Province, with location map on right.
Orogenesis not associated with
Convergence – Fault Block Mountains
• Associated with Normal Faulting
• Basin and Range Province
• Teton Range, Wyoming
Isostasy
The earth’s lithosphere can be thought of as “floating” in
gravitational balance upon the denser, deformable rocks
in the asthenosphere.
To support more weight (i.e mountains) above, means you
need to displace more material below.
Isostasy
Horizontal compressional forces cause shortening
and thickening of crust, in both directions.
Isostatic Adjustment
• As weight is removed
from the top by erosion,
the crust “rebounds”
upward.
• The processes of erosion
and isostatic uplifting will
continue until mountain
block reaches normal
thickness.
• Eroded sediments cause
adjacent areas to
subside.
Normal Fault
FAULT SCARP
Reverse Fault