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Chapter 11 – Crustal Deformation and Mountain Building
Mountains form because of subduction at convergent plate boundaries, rifting,
continental collisons, and motion on continental transform faults.
Mountain Belts
Orogenic belts, or orogens generally appear in linear ranges. There are about
12 major mountain belts worldwide.
 Some smaller orogens are contained with a larger ranges. Example, the Sierra
Nevada and Rocky Mountains lie within the North American Cordillera mountain
range.
 Orogeny is an ongoing process that can last for tens of millions of years. When
uplift ceases, erosive forces planes the mountains back to the land surface in < 50
million years.
 The processes that produce mountain belts is called orogenesis. These
processes can include:

Folding

Anticlines

Synclines

Thrust faulting

Metamorphism

Igneous activity
There are different types of mountains:

Volcanic mountains

Folded and thrust-faulted mountains

Fault-block mountain

Mountains formed as a result of erosion of less resistant surrounding rocks
(such as a buried pluton).

Mountain belts are typically formed by plate tectonic activity, most often
continental collision (compression). During extensional (pulling apart)
tectonic forces, valleys can form.

The largest mountain range on Earth is the mid-ocean ridge system which is
about 65,000 km (~40,000 miles) long and are 500 to 5000 km wide. They
cover more than 20% o the Earth’s surface.
Orogeny causes deformation: bending, breaking, squashing, stretching and
shearing.
Top figure shows a diagram of a
road cut in a non-orogenic
region. Beds of sandstone and
shale alternate. The undeformed
sandstone (see inset) has
spherical grains.
This figure shows an orogenic
outcrop showing folded layers
of quartzite and slate and a fault.
These beds have been deformed.
The inset shows that grains of
sand in the quartzite have
become flattened and are aligned
parallel to one another. The slate
shows slaty cleavage.
Components of deformation: a block of rock can change (1) location,
(2) orientation, and (3) shape.
Folds and faults represent deformation because they involve changes in
location, orientation, and shape.
Horizontal layers of undisturbed sedimentary rock in the Badlands, South Dakota.
Tilted beds of strata in Arizona. An example of deformation.
Folded layers of quartzite and schist in Australia, another example of deformation.
Strain is when
objects change
shape when subject
to deformation.
Normal shape
Stretched shape
Shortened
Sheared
Shear strain
Kinds of deformation
An example of brittle
deformation
Ductile deformation
Deformation depends on:




temperature: warm rocks > ductile; cold rocks > brittle
pressure: hi pressure > ductile
deformation rate: sudden change > brittle; slow change > ductile
rock composition: softer rocks > ductile; harder rocks > brittle
The brittle-ductile transition is at 10-15 km depth. Above = brittle; below =
ductile. Continental crust earthquakes occur above this transition zone.Both
kinds of deformation can occur in the same outcrop depending on whether
the deformation circumstances were fast or slow.
Force = mass x acceleration. Tectonic movement applies force to rocks.
This force deforms rocks by changing their position, reorientation, and
applying strain.
Stress = force/area
Different kinds of stresses




Pressure
Compression
Tension
Shear
Joints are natural cracks in rocks.
Systematic joints are long planar
cracks that occur fairly regularly
throughout a rock body. A joint
set is a group of systematic joints
and are typically vertical planes
as in picture at right.
Nonsystematic joints are short
cracks that vary in orientation and
are randomly spaced. Mineralenriched groundwater can seep
through joints and precipitate out
and fill veins.
Veins of white quartz in gray shale,
an example of non-systematic
jointing.
The terms strike and dip are used
to describe orientation of a
geologic structure. The strike and
dip are perpendicular to each
other. The strike is the compass
angle between the strike line (an
imaginary horizontal line on the
plane) and true north.The dip is
the angle between the strike line
and the dip line (an imaginary line
parallel to the steepest slope on
the plane, as measured in a
vertical plane perpendicular to the
strike). The line segment
represents the strike direction and
the tick mark the dip direction.
The plunge is the angle between
the line and an imaginary
horizontal plane. The bearing is
the compass orientation of the
line. A vertical line has a plunge
of 90o, a horizontal line, 0o.
Faults
Faults are curved layers of rock. A fault is a fracture on which sliding has
occurred, creating shear strain.
Faults are planar structures so geologists use strike and dip to describe them. The
hanging wall block is the block above the fault plane. The footwall block is the
rock below the fault plane. There are different types of faults.
On normal faults, the hanging wall block moves down the slope of the fault. In
reverse and thrust faults, the block moves up the slope of the fault.
Dip > 35o
Dip < 35o
Sliding parallels the strike line.
Sliding takes place diagonally along the surface.
A fault can be recognized by its displacement or offset (rock layers not continuous
on either side of the fault, see below). The figure below illustrates a thrust fault.
Rock units on either side of a fault may be different. Thrust or reverse faults cutting
sedimentary beds place older beds on younger ones while normal faults place
younger beds on older ones. Faults that intersect the surface may displace streams,
fences, rows of trees, etc.
A fault scarp formed after an earthquake.
Faulting under brittle conditions may crush or break adjacent rock. If crushed rock
consists of angular fragments, its called fault breccia.
Polished fault surfaces are called slickensides. Slip lineations are linear grooves
on fault surfaces. Orientation of slip lineations given by plunge and bearing.
Mylonite formed in a shear zone under metamorphic recrystallization that
subdivided large grains into smaller ones. This is a shear zone (movement occurs
ductilely).
A diagram of a thrust fault. Triangles point to the hanging wall block.
Thrust faults merge at depth with a detachment fault. The layer of rock above the
detachment is shortened.
A graben is a fault-bounded basin. A horst is a high block between 2 graben.
Horst and graben in Brazil.
Folds
Folds
 Flexing (layered stack bends and slips between layers)
 Flow folds – different parts of a rock move at different rates
 Buckle folds – a response to compression
 Shear folds – shear stress moving one layer up and over a second layer
 Folds forming as a result of a fault – layers move up and over bends of the
fault
 Folds can form when basement rock moves and bends the overlying
sedimentary layers
A fold is a curving rock layer, a type of ductile deformation. The hinge is the
portion of the fold where curvature is greatest and limbs are the sides of the fold
with less curvature. The axial plane is an imaginary surface that includes the
hinges of successive layers.
Small angle between limbs
Large angle between limbs
Tilted hinge
Horizontal hinge
Open anticline with stripes representing sedimentary beds positioned
symmetrically around hinge. Oldest rock layers near center.
An open syncline. Youngest rock layers near center, oldest outside.
A tight fold
A train of folds
After erosion, folds may dictate where valleys and ridges are seen on the surface.
Valleys can form over synclines and in the hinge of anticlines.
On a map view of a nonplunging anticline, the same
rock units appear on either side
of the hinge.
Dome
On a map view of a
plunging anticline,
the units curve
around the hinge.
Basin
A fold that developed by flexing
A fold that developed
by flowing. The
boundaries moved at
different rates
creating the fold.
How folds develop
Tectonic foliation: the layering created by the alignment of deformed and/or
reoriented grains.
Slaty cleavage oriented
parallel to the axial plane of a
fold and perpendicular to the
direction of shortening.
A stream cut showing axial-planar cleavage.
Schistosity oriented in the direction of shear. Note large grains are parallel to
each other as a result of shear rotation.
Rock formation during orogeny
Sediment eroded from mtns forms a basin. Rocks buried in the orogen become
squeezed, sheared, and heated to form metamorphic rocks. Igneous rocks intrude
from below.
Orogeny (how mountains form)
Igneous activity
 Convergent boundaries
 Collision zones
 Rifting
 Metamorphism
 Contact
 Regional
 Thrust faulting > burial of rock at depth > heat

Metamorphic recrystallization and foliation
Crustal roots
Collision
Convergence
Crust very thick
Buoyant so rises higher
Deeper roots
Erosion
Rock type
ice/water main erosive force
Glaciers > steep-sided valleys and peaks
Rivers in warmer climates
Desert mountains
Orogenic collapse
 Collapse and lateral spread
 Exhumation
 exposure of metamorphic and
plutonic rocks
 If erosion rate > uplift rate, mountains
erode
Isostatic equilbrium
 Buoyancy force –
gravitational force
 Equilibrium maintained
by crustal rocks rising or
falling in response to
geological events
Convergent mountain building
> 200 my
Exotic terranes > accreted terranes > new subduction zone + new volcanic arc >
continent grows larger
Accretionary orogens (ex: Cordillera)
Orogenic collision
 fold-thrust belts > very large ranges on orogen margins (ex: Himalayas,
ancient Appalachians)
Mountain building by collision of two
continental plates (India and Asia)
Mountain building in rift zones
 Crust thinned and stretched
 Significant uplift because heating by the asthenosphere makes the lithosphere less
dense and more buoyant
 to maintain isostatic equilibrium – crust rises
 Rift heating creates fault-block mountains (basins + tilted rock range)
Craton
 > 1 billion years old
 Shields (ex: the Canadian shield)
 Precambrian metamorphic
and igneous rocks at surface
 Cratonic platform – thin sediment
layer on top of Precambrian rocks
End of Chapter 11