Rock
Deformation
Horizontal contact between rock units with no
break in deposition or erosional gaps
 no significant gaps in geologic time

Strike and dip of a rock layer
Dipping Sedimentary Beds
Chris Pellant
Strike and Dip of a Dyke- Southern Sinai
The Earth is a dynamic body (it moves over
time)
Forces that result from plate tectonic activity
and other geological processes are commonly
called “tectonic forces”
Tectonic forces deform rocks mainly by
faulting, folding, stretching, or shearing.
folding
faulting
Whether a rock deforms by faulting,
folding, shearing, or stretching depends
on whether the rock is brittle or ductile.
When tectonic forces are applied to
rocks:
Brittle rocks do NOT flow easily, so they
break (faulting)
Note: the lithosphere is strong and brittle. For the
most part this is true, however, under some conditions,
rocks in the lithosphere become ductile.
Ductile rocks flow more easily, so they fold,
shear, or stretch.
There are 3 main types of tectonic forces that
act on rocks
Compressive forces: forces that squeeze and
shorten a rock
Tensional forces: forces that stretch and pull apart
a rock
Shearing forces: forces that push two sides of a
rock in opposite directions
In ductile rocks
Compressive forces causes folding
Tensional forces causes stretching
Shearing forces causes shearing
compressive
tensional
shearing
In brittle rocks
Compressive forces causes reverse faulting and thrust
faulting
Tensional forces causes normal faulting
Shearing forces causes strike-slip faulting
compressive
tensional
shearing
Geologic Structures

A geologic structure is any feature produced
by rock deformation. Tectonic forces create
three types of geologic structures: folds,
faults, and joints.
FOLDS
 A fold is a bend in rock. If you hold a sheet of
clay between your hands and exert
compressive stress, the clay deforms into a
sequence of folds.
Clay deforms into a sequence of folds
when compressed.
FOLDS
Folding usually results from
compressive stress.
Folding always shortens the
horizontal distances in rock.
Folds usually occur as a
repeating pattern of many folds
as in the illustration using clay.
Fold Terminology
 a fold arching upward is called an
anticline and one arching downward is a
syncline.
 1 The sides of a fold are called the limbs.
Notice that a single limb is shared by an
anticline–syncline pair.
 A line dividing the two limbs of a fold and
running along the crest of an anticline or the
trough of a syncline is the fold axis.
Anticline - oldest rock formation in the middle.
Syncline - youngest rock formation in the
middle.
Anticline
Symmetrical
Non-plunging
Open
Syncline
Symmetrical
Non-plunging
Open
Fold Terminology
 The axial plane is an imaginary plane that runs
through the axis and divides a fold as
symmetrically as possible into two halves.
 In many folds, the axis is horizontal. In other folds,
the axis is inclined or tipped at an angle called the
plunge.

Fold with a plunging axis is called a plunging
fold.
 A special type of fold with only one limb is a
monocline.
Monocline
Anticline
Symmetrical
Non-plunging
Anticline
Symmetrical
plunging
Plunge
Measured from
a horizontal
line.
axial
plane
limbs
“hinge”
Hinge
Plunging Folds
Plunging Folds
Plunging Folds
Fold Terminology
A circular or elliptical anticlinal
structure is called a dome.
Domes resemble inverted bowls.
Sedimentary layering dips away
from the center of a dome in all
directions
. A similarly shaped syncline is
called a basin.
Domes
Oldest
rock in
center
Basins
Youngest
rock in
center
Domes &
Basins
Chernicoff and Whitney
Think of an Egg Carton!
Fold Terminology
 Folds are described as
symmetrical when the limbs on
either side of the axial plane diverge
at the same angle and
asymmetrical when they do not.
Folds are described in terms of the
orientation of their axial plane:
Fold Terminology
 An
upright fold is one whose axial
surface is vertical or nearly so.
 An
inclined fold has an axial surface
that dips significantly; one limb may be
upside down or overturned.
A
Recumbent fold has a nearly
horizontal axial surface. One limb is upside
down.
Fold Geometry
Symmetrical
Asymmetrical
Overturned
Overturned
Recumbent
Oil and Gas Concentrate in Domes
Chernicoff and Whitney
Fold Traps
Folds are important economically.
They trap buoyant hydrocarbons,
and may contain ores.
Folds
Oil & gas can be trapped in the earth in ways other than
reefs. Other traps are found near Calgary.
Structural Trap
Calgary area
Reef trap
Edmonton
area
Stratigraphic
Trap
Calgary area
FAULTS
A fault is a fracture along which rock on one
side has moved relative to rock on the other side.
Slip is the distance that rocks on opposite sides
of a fault have moved.
 Some faults are a single fracture in rock; others
consist of numerous closely spaced fractures
called a fault zone . Rock may slide hundreds of
meters or many kilometers along a large fault zone.
Dykes are Discordant Bodies and Faults are Cross –
cutting (Discordant) Surfaces- Wadi Firan Sinai
Faults are Cross –cutting (Discordant) Surfaces
Fault Zone
(a) Movement along a single fracture surface characterizes
faults with relatively small slip. (b) Movement along
numerous closely spaced faults in a fault zone is typical of
faults with large slip.
FAULTS
 Hydrothermal solutions often precipitate
in faults to form rich ore veins. Miners
then dig shafts and tunnels along veins to
get the ore. Many faults are not vertical
but dip into the Earth at an angle.
Therefore, man y veins have an upper side
and a lower side. Miners referred to the
side that hung over their heads as the
hanging wall and the side they walked
on as the footwall.
Hanging Wall vs. Foot Wall
Fault Geometries
Types of Faults
Classified by relative motion
Dip - Slip
 Strike - Slip
 Oblique - Slip

Dip – Slip Faults
1. Normal Fault
2. Reverse Fault
high angle
 low angle – (Thrust
Fault)

FAULTS
 A fault in which the hanging wall has moved down
relative to the footwall is called a normal fault.
 Notice that the horizontal distance between points on
opposite sides of the fault is greater after normal
faulting occurs. Hence, a normal fault forms where
tectonic tension stretches the Earth’s crust, pulling it
apart.
 The Figure shows a wedge-shaped block of rock called a
graben dropped downward between a pair of normal
faults.
 If tectonic forces stretch the crust over a large area,
man y normal faults may develop, allowing numerous
grabens to settle downward between the faults. The
blocks of rock between the down dropped grabens then
appear to have moved upward relative to the grabens;
they are called horsts.
Types of faults
Tension forces cause Normal faults.
Normal faults lead to the region being „extended‟
Normal faults occur most often along divergent plate
boundaries
5
Normal
Faulting
Horsts and Grabens
FAULTS
 Normal faults, grabens, and horsts are common
 where the crust is rifting at a spreading center,
such as the mid-oceanic ridge and the East African
rift zone.
 A reverse fault is another structure that
accommodates shortening. In a reverse fault, the
hanging wall has moved up relative to the footwall.
The distance between points is shortened by the
faulting.
 A thrust fault is a special type of reverse fault
that is nearly horizontal. In some thrust faults, the
rocks of the hanging wall have moved many
kilometers over the footwall.
Reverse
Faulting
Reverse & Thrust
Faults
Fault systems
Graben: when one block of the rock subsides
relative to the adjacent blocks.
Horst: when one block of a rock rises relative to
the adjacent blocks.
FAULTS
 A strike–slip fault is one in which the
fracture is vertical, or nearly so, and rocks
on opposite sides of the fracture move
horizontally past each other .
A
transform plate boundary is a
strike–slip fault. As explained previously,
the famous San Andreas fault zone
is a zone of strike–slip faults that form the
border between the Pacific plate and the
North American plate.
Strike-slip fault: in which
the dominant
displacement is horizontal
and parallel to the strike of
the fault surface. The two
blocks of rock simply slide
past each other with no
compression or extension.
If the motion is to the right, it
is called a Right-lateral
fault.
If the motion is to the left, it
is called a left-lateral
fault.
Strike-slip
Faulting
Left – lateral
Right - lateral
Strike Slip Fault
Oblique - Slip
Dip - Slip and Strike - Slip movement
Fault
and
Other
Traps
JOINTS
A joint is a fracture in rock and is therefore
similar to a fault, except that in a joint rocks
on either side of the fracture have not moved.
We have already discussed columnar joints
in basalt.
 Tectonic forces also fracture rock to form
joints. Most rocks near the Earth’s surface are
jointed, but joints become less abundant with
depth because rocks become more plastic at
deeper levels in the crust.
Joints in limestone of Wadi Sidr Sinai, Egypt
Joints in chalk, Bahariyaoasis, Western Desert of Egypt
Maximum
Principal
Stress
Horizontal
Joint Sets
Vertical
Columnar
Joints

Extension fractures characteristic of
tabular extrusive igneous rocks
i.e., form in lava flow, sill,
dike
Joints
Not all fractures in
rocks are faults.
Some are joints.
Joints are fractures in
rock in which there have
been NO appreciable
movement.
Faults are fractures in
rock in which movement
has occurred.
joints