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Geology for Engineers
VAB1033
Geological Structures I
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Structural Geology - Introduction
Structural Geology is the study of the
architecture of the earths crust, its
deformational features and their mutual
relations and origin. Structural Geology can be defined as a
branch of geology concerned with the
shapes, arrangements, and inter-
relationships of bedrock units and the forcesthat cause them.
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Geologic Sructures - Introduction
Main Objective:
To recognise certain geologic structures,
understand the forces that caused them, and
thus determine the geologic history of anarea.
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Geologic Sructures - Introduction
Why an understanding and knowledge of StructuralGeology is important?
To understand earthquake for example, one must know aboutfaults.
Appreciating how major mountain belts and the continent have
evolved calls for a comprehension of faulting and folding. Understanding plate-tectonic theory as a whole also requires a
knowledge of structural geology
In areas of active tectonics, the location of geologic structure isvery important in selection of suitable sites for buildings, dams,highway, bridge, tunnels, nuclear power plants, etc.
Understanding structural geology can help us more fullyappreciate the problem of finding more of the earths naturalresources, such as metal ores, petroleum & gas, rock aggregates,etc.
The knowledge of structural geology is also very important ingeohazards (landslide, earthqukae, tsunami, subsidence,
erosions, etc) mitigation and control measures.
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Distribution of Earthquakes Epicenters around South East Asia
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Alaskan Earthquake
P. Nias Earthquake, Indonesia
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Tunelling works require special skills in geologic structural mapping.
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Stress & Strain
Stress - a force per unit
area at a particular point.
Strain - the change insize (volume) or shape,
or both, while an object
is undergoing stress.
The effects of compressional & tensional stress on silly putty. A)
Compressing silly putty results in shortening either by folding or
flattening, B) Pulling (tensional stress) silly putty causes
stretching or extension; if pulled (strained) too fast, or chilled, the
silly putty will break after first stretching.
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Stresses
Compressive Stress
pushed together or squeezed from opposite directions.
common along convergent plate boundaries; typically results inrocks being deformed by a sho rtening strain;
Tensional Stress Forces pulling away from one another in opposite directions;
results in a stretching orextensio nal strain
Quite rare in the earth crust
Shear Stress
Due to movement parallel but in opposite directions along a faultor other boundary
Results in a shear strainparallel to the direction of the stresses.
Notable along transform plate boundaries and along other activelymoving faults.
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Behaviour of Rocks to Stress &
Strain Rocks behave as elastic, ductile, or brittle materials depending on the
amount and rate of stress applied, the type of rock, and the
temperature and pressure under which the rock is strained.
Elasticif a deformed body recovers its original shape after the stress isreduced or removed (e.g. rubber). Rocks can behave in an elastic way at very
low stresses, however once the stress exceeds the elast ic lim itthe rock willdeform permanently.
Ductilea rock that behaves in a ductile or plastic manner will bend whileunder stress and does not return to its original shape after relaxation of the
stress. Under high pressure & temperature (e.g. during regional metamorphism)
rocks behave in a ductile manner. Ductile behaviour results in folding or bending
or rock layers.
Brittle a rock exhibiting brittle behaviour will break or fracture at stress higher
that its elastic limit. Rock typically exhibit brittle behaviour at or near the earths
surface where pressure & temperatures are low. Faults and joints are examples
of structures that form by brittle behaviour of the crust.
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Behaviour of Rocks to Stress & Strain
Behaviour of rocks with increasing stress and strain.
Elastic behaviour occurs along the straight line portions (blue)
At stresses greater than the elastic limit (red points) the rock will
either deform as a ductile material or break, as shown in the
deformed rock cylinders.
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Brittle Deformation
Joints
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Brittle Deformation
Faults
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Ductile Deformation
Folds
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Measuring Attitude of Rocks
Dip- Angle of bed with
the horizontal
Str ike- Bearing
(compass direction) of
line of intersection
between horizontal planeand the inclined bed.
Dip Direct ion is the
compass direction in
which the angle of dip is
measured.
Attitude of planar structures (bedding, faults, joints, foliations, etc.) is often
depicted by the reading ofstrike and dip, ordip direction and dip.
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Map Symbols of
Geological Structures
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Geologic Map
An example of simple geological map.
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Geologic Cross Section
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Geologic Cross-Section
For engineering purposes..
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Geologic Cross-Section
For engineering purposes..
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Folds
Folds are bends or wave-like featuresin layered rocks.
Formed by plastic (ductile)
deformation under compressional
stress.
Folding took place when the rock was
buried at depth where high confining
pressure & temperature favour plastic
behaviour.
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Basic Geometry of Folds
Syncline and Anticline
Terms
Anticline
Syncline
Limb
Axial plane
Hinge Lines/Foldaxes
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Can you spot the sync l ine and anticl ine?
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Block Diagram of Folded Rocks
Folded Rock
Note: Plan view geological map
Side view geologic cross sections
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Plunging Folds
Plunging Folds
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Geometry of Folds
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Structural Domes and Structural
Basins
Structura l Domeis a
structure in which the beds
dip away from a central
point. In cross section, a
dome resembles an
anticline
Structural Basin the
beds dip towards a central
point. In cross section its is
comparable to a syncline
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Dome near Casper Wyoming. The ridges are sedimentary layers that are
resistant to erosion. Beds dip away from the center of the dome (Photo by
D.A. Rahm, WWU)
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Types of Folds
Folds occur in many varieties and sizes.
A number of fold classification schemes can
be applied to describe folds (refer to any Structural
Geology text books).A simple types of folds are given in the
following slides
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Folds Created by Movements of the Earths Crust
Open Folds
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Folds Created by Movements of the Earths Crust
Isoclinal Folds
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Folds Created by Movements of the Earths Crust
Overturned folds
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Folds Created by Movements of the Earths Crust
Overturned Folds
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Folds Created by Movements of the Earths Crust
Recumbent Folds
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Refolded Folds
Crenulation Folds
More Complex Types of Folds
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Hook Folds
Refolded Folds
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Disharmonic Folds
Chevron Folds
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The Importance of Folds
Folds are good
traps for oil & gas
deposits.
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END OF PART I
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Fractures in Rocks
If a rock is brittle, or if the strain rate is too
great for deformation to be accomodated by
plastic behaviour, the rock fractures.
Types of rock fractures: Joints
Faults
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Types of Rock Fractures
Fractures - narrow planar openings in rock
Joints - Fractures w/ no movement parallel to
fracture surface. Often occur in sets
Shear Zones - fractures along which a smallamount (cms) of movement has occurred
Faults - fractures along which large amounts
(m - kms) of movement has occurred.
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Types of Faults
Terminology
Footwall
Hanging Wall
Strike and Dip
Normal Fault
Reverse Fault
Strike Slip Fault
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Normal Faults
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Faults and Stresses
Maximum principal stress (s1)
Normal faults - vertical
Reverse faults - horizontal
Faults form at 30 - 60 deg. from the maximumprincipal stress
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Other structures
Boudinage
Pinch & Swell
Veins
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Summary