structural geology i

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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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Isoclinal Folds


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Overturned folds


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Overturned Folds


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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

structural geology i

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