adapted to HTML from lecture notes of Prof. Stephen A. Nelson Tulane
Within the Earth rocks are continually being subjected to forces that tend
to bend them, twist them, or fracture them. When rocks bend, twist or
fracture we say that they deform (change shape or size). The forces that
cause deformation of rock are referred to as stresses (Force/unit area).
So, to understand rock deformation we must first explore these forces or
Stress is a force applied over an area. One type of stress that we are all
used to is a uniform stress, called pressure. A uniform stress is a stress
wherein the forces act equally from all directions. In the Earth the
pressure due to the weight of overlying rocks is a uniform stress, and is
sometimes referred to as confining stress.
If stress is not equal from all directions then we say that the stress is
a differential stress. Three kinds of differential stress occur.
Tensional stress (or extensional stress), which stretches rock;
Compressional stress, which squeezes rock; and
Shear stress, which result in slippage and translation.
When rocks deform they are said to strain. A strain is a change in size,
shape, or volume of a material.
When a rock is subjected to increasing stress it passes through 3
successive stages of deformation.
Elastic Deformation -- wherein the strain is reversible.
Ductile Deformation -- wherein the strain is irreversible.
Fracture - irreversible strain wherein the material breaks.
We can divide materials into two classes that depend on their relative
behavior under stress.
Brittle materials have a small or large region of elastic behavior
but only a small region of ductile behavior before they fracture.
Ductile materials have a small region of elastic behavior and a
large region of ductile behavior before they fracture.
How a material behaves will depend on several factors.
Among them are:
Temperature - At high temperature molecules and their bonds can
stretch and move, thus materials will behave in more ductile manner.
At low Temperature, materials are brittle.
Confining Pressure - At high confining pressure materials are less
likely to fracture because the pressure of the surroundings tends to
hinder the formation of fractures. At low confining stress, material
will be brittle and tend to fracture sooner.
Strain rate -- At high strain rates material tends to fracture. At
low strain rates more time is available for individual atoms to move
and therefore ductile behavior is favored.
Composition -- Some minerals, like quartz, olivine, and feldspars
are very brittle.
Others, like clay minerals, micas, and calcite are more ductile This
is due to the chemical bond types that hold them together.
Thus, the mineralogical composition of the rock will be a factor in
determining the deformational behavior of the rock. Another aspect is
presence or absence of water.
Water appears to weaken the chemical bonds and forms films around
mineral grains along which slippage can take place. Thus wet rock
tends to behave in ductile manner, while dry rocks tend to behave in
Properties of the Lithosphere
We all know that rocks near the surface of the Earth behave in a brittle
manner. Crustal rocks are composed of minerals like quartz and feldspar
which have high strength, particularly at low pressure and temperature. As
we go deeper in the Earth the strength of these rocks initially increases.
At a depth of about 15 km we reach a point called the brittle-ductile
transition zone. Below this point rock strength decreases because
fractures become closed and the temperature is higher, making the rocks
behave in a ductile manner. At the base of the crust the rock type changes
to peridotite which is rich in olivine. Olivine is stronger than the
minerals that make up most crustal rocks, so the upper part of the mantle
is again strong. But, just as in the crust, increasing temperature
eventually predominates and at a depth of about 40 km the brittle-ductile
transition zone in the mantle occurs. Below this point rocks behave in an
increasingly ductile manner
Deformation in Progress
Only in a few cases does deformation of rocks occur at a rate that is
observable on human time scales. Abrupt deformation along faults, usually
associated with earthquakes caused by the fracture of rocks occurs on a
time scale of minutes or seconds. Gradual deformation along faults or in
areas of uplift or subsidence can be measured over periods of months to
years with sensitive measuring instruments.
Evidence of deformation that has occurred in the past is very evident in
crustal rocks. For example, sedimentary strata and lava flows generally
follow the law of original horizontality. Thus, when we see such strata
inclined instead of horizontal, evidence of an episode of deformation is
present. In order to uniquely define the orientation of a planar feature
we first need to define two terms - strike and dip
For an inclined plane the strike is the compass direction of any
horizontal line on the plane. The dip is the angle between a horizontal
plane and the inclined plane, measured perpendicular to the direction of
In recording strike and dip measurements on a geologic map, a symbol is
used that has a long line oriented parallel to the compass direction of
the strike. A short tick mark is placed in the center of the line on the
side to which the inclined plane dips, and the angle of dip is recorded
next to the strike and dip symbol as shown above. For beds with a
900 dip (vertical) the short line crosses the strike line, and for beds
with no dip (horizontal) a circle with a cross inside is used as shown
Fracture of Brittle Rocks
Faults - Faults occur when brittle rocks fracture and there is an
offset along the fracture. When the offset is small, the displacement
can be easily measured, but sometimes the displacement is so large
that it is difficult to measure. Types
of Faults Faults can be divided into several different types
depending on the direction of relative displacement. Since faults are
planar features, the concept of strike and dip also applies, and thus
the strike and dip of a fault plane can be measured. One division of
faults is between dip-slip faults, where the displacement is measured
along the dip direction of the fault, and strike-slip faults where the
displacement is horizontal, parallel to the strike of the fault.
Dip Slip Faults -
Dip slip faults are faults that have an inclined fault plane and along
which the relative displacement or offset has occurred along the dip
direction. Note that in looking at the displacement on any fault we
don't know which side actually moved or if both sides moved, all we
can determine is the relative sense of motion.
For any inclined fault plane we define the block above the fault as
the hanging wall block and the block below the fault as the footwall
Normal Faults -
Are faults that result from horizontal tensional stresses in brittle rocks
and where the hanging-wall block has moved down relative to the footwall
Horsts and Gabens -
Due to the tensional stress responsible for normal faults, they
often occur in a series, with adjacent faults dipping in opposite
directions. In such a case the down-dropped blocks form grabens and the
uplifted blocks form horsts. In areas where tensional stress has recently
affected the crust, the grabens may form rift valleys and the uplifted
horst blocks may form linear mountain ranges. The East African Rift Valley
is an example of an area where continental extension has created such a
rift. The basin and range province of the western U.S. (Nevada, Utah, and
Idaho) is also an area that has recently undergone crustal extension. In
the basin and range, the basins are elongated grabens that now form
valleys, and the ranges are uplifted horst blocks
A normal fault that has a curved fault plane with the dip decreasing with
depth can cause the down-dropped block to rotate. In such a case a
half-graben is produced, called such because it is bounded by only one
fault instead of the two that form a normal graben.
Reverse Faults -
are faults that result from horizontal compressional stresses in brittle
rocks, where the hanging-wall block has moved up relative the footwall
Thrust Fault -
is a special case of a reverse fault where the dip of the fault is less
than 15o. Thrust faults can have considerable displacement, measuring
hundreds of kilometers, and can result in older strata overlying younger
are faults where the relative motion on the fault has taken place along a
horizontal direction. Such faults result from shear stresses acting in the
crust. Strike slip faults can be of two varieties, depending on the sense
of displacement. To an observer standing on one side of the fault and
looking across the fault, if the block on the other side has moved to the
left, we say that the fault is a left-lateral strike-slip fault. If the
block on the other side has moved to the right, we say that the fault is a
right-lateral strike-slip fault. The famous San Andreas Fault in
California is an example of a right-lateral strike-slip fault.
Displacements on the San Andreas fault are estimated at over 600 km
are a special class of strike-slip faults. These are plate boundaries
along which two plates slide past one another in a horizontal manner. The
most common type of transform faults occur where oceanic ridges are
offset. Note that the transform fault only occurs between the two segments
of the ridge. Outside of this area there is no relative movement because
blocks are moving in the same direction. These areas are called fracture
zones. The San Andreas fault in California is also a transform fault.
Evidence of Movement on
Slikensides are scratch marks that are left on the fault plane as
one block moves relative to the other. Slickensides can be used to
determine the direction and sense of motion on a fault.
Fault Breccias are crumbled up rocks consisting of angular fragments
that were formed as a result of grinding and crushing movement along a
Folding of Ductile Rocks
When rocks deform in a ductile manner, instead of fracturing to form
faults, they may bend or fold, and the resulting structures are called
folds. Folds result from compressional stresses acting over considerable
time. Because the strain rate is low, rocks that we normally consider
brittle can behave in a ductile manner resulting in such folds. We
kinds of folds.
are the simplest types of folds. Monoclines occur when horizontal strata
are bent upward so that the two limbs of the fold are still horizontal
Anticlines are folds where the originally horizontal strata has
been folded upward, and the two limbs of the fold dip away from the hinge
of the fold
Synclines are folds where the originally horizontal strata have been
folded downward, and the two limbs of the fold dip inward toward the hinge
of the fold. Synclines and anticlines usually occur together such that the
limb of a syncline is also the limb of an anticline.
Geometry of Folds
Geometry of Folds - Folds are described by their form and orientation. The
sides of a fold are called limbs. The limbs intersect at the tightest part
of the fold, called the hinge. A line connecting all points on the hinge
is called the fold axis. In the diagrams above, the fold axes are
horizontal, but if the fold axis is not horizontal the fold is called a
plunging fold and the angle that the fold axis makes with a horizontal
line is called the plunge of the fold. An imaginary plane that includes
the fold axis and divides the fold as symmetrically as possible is called
the axial plane of the fold
Note that if a plunging fold intersects a horizontal surface, we will see
the pattern of the fold on the surface.
Classification of Folds Folds can
be classified based on their appearance.
If the two limbs of the fold dip away from the axis with the same
angle, the fold is said to be a symmetrical fold.
If the limbs dip at different angles, the folds are said to be
If the compressional stresses that cause the folding are intense,
the fold can close up and have limbs that are parallel to each other.
Such a fold is called an isoclinal fold (iso means same, and cline
means angle, so isoclinal means the limbs have the same angle. Note
the isoclinal fold depicted in the diagram below is also a symmetrical
If the folding is so intense that the strata on one limb of the fold
becomes nearly upside down, the fold is called an overturned fold.
An overturned fold with an axial plane that is nearly horizontal is
called a recumbant fold.
A fold that has no curvature in its hinge and straight-sided limbs
that form a zigzag pattern is called a chevronfold
Relationship Between Folding and Faulting
Because different rocks behave differently under stress, we expect that
some rocks when subjected to the same stress will fracture or fault, while
others will fold. When such contrasting rocks occur in the same area, such
as ductile rocks overlying brittle rocks, the brittle rocks may fault and
the ductile rocks may bend or fold over the fault
Also since even ductile rocks can eventually fracture under high stress,
rocks may fold up to a certain point then fracture to form a fault.
Folds and Topography
Since different rocks have different resistance to erosion and weathering,
erosion of folded areas can lead to a topography that reflects the
folding. Resistant strata would form ridges that have the same form as the
folds, while less resistant strata will form valleys
Mountain Ranges - The Result of Deformation of the Crust
One of the most spectacular results of deformation acting within the crust
of the Earth is the formation of mountain ranges. Mountains originate by
three processes, two of which are directly related to deformation. Thus,
there are three types of mountains:
Fault Block Mountains - As the name implies, fault block mountains
originate by faulting. As discussed previously, both normal and
reverse faults can cause the uplift of blocks of crustal rocks. The
Sierra Nevada mountains of California, and the mountains in the Basin
and Range province of the western U.S., were formed by faulting
processes and are thus fault block mountains.
Fold & Thrust Mountains - Large compressional stresses can be
generated in the crust by tectonic forces that cause continental
crustal areas to collide. When this occurs the rocks between the two
continental blocks become folded and faulted under compressional
stresses and are pushed upward to form fold and thrust mountains. The
Himalayan Mountains (currently the highest on Earth) are mountains of
this type and were formed as a result of the Indian Plate colliding
with the Eurasian plate. Similarly the Appalachian Mountains of North
America and the Alps of Europe were formed by such processes.
Volcanic Mountains - The third type of mountains, volcanic
mountains, are not formed by deformational processes, but instead by
the outpouring of magma onto the surface of the Earth. The Cascade
Mountains of the western U.S., and of course the mountains of the
Hawaiian Islands and Iceland are volcanic mountains