CE-312

Engineering Geology and Seismology

Geological Structures

Instructor:
Dr. Shahid Ullah
shahid.ullah@uetpeshawar.edu.pk

Earthquake Engineering Center

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 Basics
 Stress: Force applied to an area.
• Differential stress – magnitude of stress is greater in one
direction
 Strain: A rock’s response to stress
• Deformation resulting in change of shape or position
 Rock Strength: Rock’s resistance to deformation
(stress)

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 Behavior of Rocks under Stress
Elastic deformation
 Deformation is reversible, after stress is removed, materials
return to original shape, i.e. A rubber band
Plastic deformation
 Deformation is permanent
 Rock flow in response to stress
 Need high pressures and temperatures, generally deep in
earth’s interior.
Brittle deformation
 Deformation is permanent
 Rock break (fracture) or lose cohesion
 Generally low temperature and low pressure, common in
shallow crust
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 Types of Stresses
 Tensional (Extension) Stress
 Compressional Stress
 Shear Stress

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 Types of Stresses
Tensional or Extensional Stresses
 Strata move away from each
other (pull apart)
 Rock lengthens
 Common in divergent
boundaries
 Ductile deformation – stretching
and thinning of rock
 Brittle deformation – fracturing
and faulting

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 Types of Stresses
Compressional Stresses
 Stress presses (squeezes) rock
together
 Rock shortens
 Common at convergent
boundaries
 Ductile deformation –
shortening and thickening of
rock – rock folds
 Brittle deformation –
fracturing and faulting
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 Types of Stresses
Shear Stresses
 Stress creates a lateral shift in
the rock
 Rock slides past other rock
 Common at transform
boundaries
 Results in faults

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 What are geological structures?
 Geological structures are dynamically-produced patterns or
arrangements of rock or sediment that result from, and give
information about, forces within the Earth. These are produced
as rocks change shape and orientation in response to applied
stress.
• e.g. Are Fold, faults, fractures, unconformities etc.
 Structural Geology is that branch of geology which deals with
the study of rock structures or geological structures in earth’s
crust. It is mainly concerned with shapes, arrangement,
interrelationships of bedrock units & forces that cause them.
 Important to understand
• How mountains form
• How continents evolve
• How plate tectonics works
• Safe siting of public facilities, dams, power, etc.
• Search for energy resources
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 Folds
 The wavy undulations in the rock beds are called folds.
 They mainly occur due to tectonic activity.
 When a strata is subjected to stresses beyond those that their
strength can resist, it can be deforemed permanently either by
buckling or fracturing. The type of deformation depends on the
mechanical properties of the rocks and the nature of the stresses.
 In general, stresses which are applied slowly, either deep within the
Earth where the confining pressure produced by overburden is high,
or to rocks that are not brittle, tend to produce folds by buckling or
plastic flow.

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 Folds
 Folds are caused by pressures within the Earth's crust resulting
from plate-tectonic activity. Rocks are slowly pushed and
compressed together, forming folds. Such deformation usually
occurs in sedimentary layers that are softer and more flexible.
If the force is more sudden, and the rock more brittle, then
another geological structure called “fault” is formed instead of
a fold.
 They consist of arches and troughs in alternate manner.
 The size of folds vary greatly. Width of some folds are
measured in kilometers while those of others in meters or
centimeters
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 Fold Terminology
Anticline: Geologists caIl the arches of strata as Anticline. It
is an up fold where the limbs dip away from the axis. It forms
a ridge.
Syncline: Geologists call the troughs of strata as Syncline. It
is a down fold where the limbs dip towards the axis. It forms
a trough.
Monocline: when a stratum is merely droped in a bend to a
lower level without producing a complete Anticline or
Sncline, this half fold is termed as Monocline.
Limbs: The sloping side of a fold from crest to trough is
called the limb.
Axial Plane: It is an imaginary plane or surface which
divides the fold into equal halves, marks location of
maximum curvature.
Hinge line: The center axis of a fold is called the hinge line.
Axis: the line of intersection of axial plane with the surface of
any of the constituent rock beds. 12
 Types of Folds
 Symmetrical fold is one where the axial plane is vertical and
the two limbs have the same amount of dip. Or a fold with the
sides/limbs showing a mirror image with respect to the axial
plane
 Asymmetrical fold is one where the axial plane is inclined and
the limbs dip at different angles, and in opposite directions. Or
a fold without a mirror image w.r.t. the axial plane
(b) Asymmetrical folds

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 Types of Folds
 An overturned fold, or overfold, has the axial plane inclined
to such an extent that the strata on one limb are overturned.
In this case both the limbs dip in the same direction, and
one of the limb is turned upside down.
 Recumbent fold: when the folding is so intense that the
axial plane becomes almost horizontal, and the lower limb,
which also becomes nearly flat/horizontal, gets overturned.

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 Types of Folds
 Isoclinical fold: The two limbs of a fold
are essentially parallel to each other and
are nearly vertical. The limbs dip at the
same angle and in the same direction.
 Plunging fold: Plunging is the tilting of a
fold towards the front or back end. The
axis of a fold may be horizontal or
inclined. Folds having inclined axes are
called plunging folds.

Isoclinical fold
Plunging fold

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 Types of Folds
 Open folds: In open folds, the folding is open, and the
beds meet at bends at an obtuse angle (larger than 90
and less than 180 degrees).
 Close folds: the folding is so tight that relatively
mobile beds flow plastically towards crest and trough,
thereby causing thinning and thickening of beds.

Open fold
Close fold 16
 Types of folds
 Chevron fold: the folds which are sharp and angular at the
anticlinical and synclinical axes. They are characterized by very
long limbs and narrow hinge zones.
 Dome and Basin: When the strata have been subjected to folding
in two directions at right angles, each anticline is converted into a
dome and each syncline is converted into a basin. In dome the
beds dip away from a central point and in basin it dips towards
the central point.

Chevron fold 17
 Uses of geological structes to age strata
 Superposition of the lavas, lava is youngest of all
 Dyke in the Marl, dyke is youngest.
 Dyke displacement shows, fault is younger.
 Granite is older.
 Thermal aureole shows, granite younger than Schist.

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 Recognition of folds in the field
 The repetition of outcrops of beds suggests the
presence of a fold.
 If folding is open, the reversal of dip direction is
enough to identify folds. In anticlins, the oldest bed
will occupy an axial position, and in synclines the
youngest bed will occur there.

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 Fractures
 Fracture may be defined as the
surfaces along which rocks or
minerals have broken; they are
therefore surfaces across which the
material has lost cohesion.
 The term fracture encompasses
both joints and faults.
 Fractures have two parallel surfaces
that meet at the fracture front.
 These surfaces are approximately
planar.
 The relative displacement of
originally adjacent points across the
fractures is small compared to the
fracture length.
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 Types of Fractures
Three main types of fractures :
 Mode-I fractures (joints) it is the extensional fractures
and formed by opening with no displacement parallel
to the fracture.
 Mode-II and Mode-III are shear fractures. These are
faults like fractures one of them is strike -slip and the
other is dip-slip

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 Joints
 Joints are fractures presents in the rocks along which
there has been no displacement.
 They are formed as a result of contraction due to
cooling or consolidation of rocks. They are also
formed when the rocks are subjected to compression
or tension during earth movements.
 Commonly a large number of joints lie parallel to one
another. These parallel joints together form a “joint
set”.

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 Types of joint
On the basis of origin, joints are classified into two groups.
 Tensional joints: are those which are formed as a result of
tensional forces. They are relatively open, and have rough
and irregular surfaces. e.g. the columnar joints in lava flows
and logitudinal joints in the anticlines, that run parallel to
the axis of fold.
 Shear joints: those which are formed due to shearing
stresses involved in the folding and faulting of rocks. They
are rather clean cut and tightly closed.

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 Types of joints
Based on their geometry, they are classified as:
 Systematic joints: are roughly planar, parallel to each
other, usually regularly spaced
 Nonsystematic joints: are curved and irregular
 Columnar joints: are formed in tabular igneous masses
such as dykes, sills and lava flows. These joints divides
the rocks into hexagonal columns, which are arranged at
right angle to the main cooling surfaces. In lavas and
sills, the columns are vertical , while in dykes they are
more or less horizontal.

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 Faults
 A fault is a fracture along which there has been
relative displacement of beds, which were once
continuous. The fracture surface along which
displacments take place is called “fault plane”.
 Faults may be hundred of meters or a few
centimeters in length.
 Faults may extend from the ground surface to
depths of several tens of kilometers.
 Faults may be observable on ground surface, or
may be buried.
 Presence of faults DO NOT necessarily mean
that earthquakes will occur. Their movement
may be aseismic (very slow movement that
does not cause earthquakes) or the faults may
be inactive (no movement).
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 Fault Terminology
 Hanging wall: The rock mass resting on the
fault plane.
 Footwall: The rock mass beneath the fault
plane.
 Dip: Fault dip is the angle between the fault
and a horizontal plane, 0° to 90°.
 Slip: Describes the movement parallel to the
fault plane.
 Dip slip: Describes the up and down
movement parallel to the dip direction of the
fault.
 Strike slip: Applies where movement is
parallel to strike of the fault plane.
 Oblique slip: Is a combination of strike slip
and dip slip.
 Net slip (true displacement): Is the total
amount of motion measured parallel to the
direction of motion.
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 Fault Terminology
 Throw: The vertical displacement of the fault.
 Heave: The horizontal displacement of strata.

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 Fault Terminology
 Strike: Fault strike is the direction of a line created by the
intersection of a fault plane and a horizontal surface, 0° to
360°, relative to North. Strike is always defined such that a
fault dips to the right side of the trace when moving along the
trace in the strike direction. The hanging-wall block of a fault
is therefore always to the right, and the footwall block on the
left.
 Rake or slip angle: Rake is the direction a hanging wall block
moves during rupture, as measured on the plane of the fault. It
is measured relative to fault strike, ±180°. For an observer
standing on a fault and looking in the strike direction, a rake of
0° means the hanging wall, or the right side of a vertical fault,
moved away from the observer in the strike direction (left
lateral motion). A rake of ±180° means the hanging wall
moved toward the observer (right lateral motion). For any
rake>0, the hanging wall moved up, indicating thrust or
reverse motion on the fault; for any rake<0° the hanging wall
moved down, indicating normal motion on the fault.
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 Fault Terminology
 Strike, Dip and Rake:

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 Fault Terminology
 Scarp: The exposed upward block forms a cliff-like feature
known as a fault scarp. A scarp may range from a few to
hundreds of meters in height and their length may continue
for 300 or more kilometers (around 200 miles).
Scarp
A normal dip-slip fault

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 Classification of Faults
Faults are classified into different types
based on the direction of relative
displacement.
 Normal Fault: a normal fault is one in
which hanging wall appears to have
moved downward relative to foot wall.
It is also called gravity fault.
• Associated with tensile stresses in crust.
• The faulting in Oceanic Ridge earthquakes
is predominantly Normal.
• Horst – raised block of material bounded by
two normal faults.
• Graben – trough (valley) bounded by two
normal faults.

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 Classification of Faults
 Reverse Fault: A reverse fault is one in which the
hanging wall appears to have moved upward relative
to foot wall.
• It is also called a thrust fault.
• Associated with compressive stresses in crust.

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 Classification of Faults
Strike Slip Fault: the displacements occurs
parallel to strike of the fault.
• Associated with shear stresses in crust.
• Associated with shear stresses in crust.
There are two types of strike slip faults:
Right lateral strike-slip fault (dextral): Where
the side opposite the observer moves to the
right.
Left lateral strike-slip fault (sinistral): Where
the side opposite the observer moves to the
left.
Note that the same sense of movement will
also be observed from the other side of the
fault.
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 Classification of Faults
 Oblique Fault: Displacement with both dip-slip and
strike-slip components.

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Blind/Hidden faults

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