Structural Geology

Lecture – 1

The meaning of the Geologic Structures. Basic Branches of the Science.

Scale of Structural Geology.

Importance of Structural Geology.

Strike and dip

Structural Geology

What are geological structures? They are all the features on the surface of the Earth which are by definition within the Crust; Crust is the uppermost layer of the Lithosphere - Composed of rocks. Litho = rocks + Sphere i.e. a sphere of rocks Surrounding the Earth

The word structure is derived from the Latin word

, to build. The closely related word tectonicsstruere

which has a ,tektoscomes from the Greek word

similar, but often broader, meaning.

Both terms relate to the building or structure of the

Earth’s crust, and to the continuous movements that

change and shape the outer layers of our planet.

Structural geology is the study of those features from the

architectural point of view;

Features are deformed because of tectonic activities, these

Structures are called Secondary Structures.

Deformation is the changes in location, orientation, shape and

volume as a result of stresses that exceed rock strength.

After deformation

Architecture = form, symmetry, geometry and the elegance of the components of the Earth's crust on all scales.

Basic Branches of the Science The study of structural geology can be divided into three parts which are strongly integrated and interlinked: 1- The concepts of stress, strain and rheology of the lithosphere. (Rheology is the study of the flow of matter, primarily in a liquid state, but also as "soft solids" or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force). 2- Description of structures from the grain scale over the outcrop scale to the mountain and tectonic scale. 3- Interpretation of geological maps and identifying the structures on them such as folds and faults.

Scale of structural geology

1- The microscopic scale relates to sizes 1 cm or smaller. They are commonly studied with the aid of a microscope. Samples are taken to be studied in the labs.

Scanning Electron Microphotograph of a mm-thin zone of grain deformation (deformation band)

2- The mesoscopic scale ranges from 1 cm to 100 meters, which is the scale of the road-cut-photo (observed in the field visually).

3- The macroscopic scale is greater than 100 m. Studies on this scale are aimed at determining regional geology, such as a whole mountain (eg. Safen anticline)

4- The megascopic scale ranges hundreds or even thousands of kilometer, such as studying great ranges of Zagros Mountain of global level.

Importance of Structural Geology

The study of structural geology has a primary importance in: 1- Economic geology, both petroleum geology and mining geology; in order to know areas that contain folds and faults. These structures can form traps in which the accumulation and concentration of fluids such as oil and natural gas occur. 2- Environmental and hydrogeology; because structures are sites of groundwater flow and penetration which may have an effect on leakage of toxic materials from waste dumps or leakage of salty water into aquifers.

3- Understanding geological structures, helps us to determine the kinds of stresses that have existed within Earth in the past. This type of information is critical to our understanding of plate tectonics, earthquakes, the formation of mountains, metamorphism, and Earth resources. Some of the types of geological structures that are important to study include fractures, faults, and folds. Structural geologists make careful observations of the orientations of these structures and the amount and direction of offset along faults. 4- Engineering geology needs to know in details the structural geology of a certain area for engineering projects.

Strike and Dip

- We use the terms strike and dip to express the orientation of bedding planes. They can be gathered in field using a simple geological compass. Bedding planes are supposed to be horizontal but because of several forces acting in nature, they are usually inclined. - Strike is the directional orientation of a rock bed in degrees from zero (due North) to 360o. Also can be in the form of: N51oE, S23oW, ……(quadratic system-it is classic)

- Dip is angle the rock beds make with the horizontal. Strike and dip are perpendicular to each other. - The magnetic needle of the compass measures strike, while a clinometer (within the same compass) measures dip. A clinometer measures angles relative to flat surfaces.

These two words are of great importance for all geologists, without them geological maps will be not complete. They have a well known symbol The long line indicated the strike direction while the short one indicates the dip direction and the number shows the dip angle amount in degrees (out of 90o ) - We have true dip (perpendicular to strike direction) and apparent dip ( any other one).

32o

90o horizontal

True dip angle

Dip and Strike of a certain layer are together called “attitude”

E-W strike is noted as 090

Strike direction examples In the field dip and strike are written in the notebook in a special system, as follows: 040, 30 225, 50

Strike direction N40E

Dip angle

Strike direction S45W

Azimuth and Quadrant systems

For linear structures, a similar method is used, the strike or bearing is the compass direction and angle the line makes with a horizontal surface is called the plunge angle. For planes strike is denoted before dip 225,50 040,30 as in the previous page While for line the plunge is before bearing (direction)

040, 30 225, 50

Lecture -2

Primary Geological Structures

They are structures formed at the time of sedimentation

(during the formation of the rock body), i.e. present before

the activity of deformation.

They are also called None Tectonic structures;

1- Stratification (bedding)

Bedding is a planner arrangement of suspended particles in liquids such as sea or river waters or wind, deposited and settled down at the bottom. The boundaries between beddings (layers) are called contacts. They range in thickness between few millimeters to tens or hundreds of meters. When they crop out on the surface they are called exposures or outcrops. They are mostly deposited horizontally before deformation. Lower layers are older.

Examples for primary structures:

2- Mud cracks

Form when a water rich in mud dries out on the air and shrink.

ancient mud cracks from rocks that are over 1 billion years old, see that the cracks are filled later by newer materials

Recently formed mud cracks

Up down

3- Cross bedding

Smaller layering within one or more beds in a series of rock strata that does not run parallel to the plane of stratification. Also called: false bedding

Up down

4- Ripple marks

are produced by flowing water or wave action, analogous to cross-bedding. There are two types; symmetric (oscillation ripple marks) or asymmetric (current ripple marks).

All these structures are mainly studied by

Sedimentologists not by structural geologists but

they are of great benefits Structural geologist.

From examining of the bottoms and upsides of

these structures, it can be decided whether the

tilted layers are overturned or not especially when

the case of study is a fold.

WHAT IS THE JOB OF STRUCTURAL

GEOLOGISTS :

(1) Measure rock geometries.

(2) Reconstruct their deformational histories.

(3) Calculate the stress field that resulted in that

deformation.

GEOLOGIC STRUCTURES:

Firstly, let us define what we mean by geologic structure:

It is a geometric feature in a rock whose shape, form and

distribution can be described.

Examples of geologic structures are: folds, faults , joints,

veins, cleavage, foliation and lineations.

Consequently, there are many schemes for classification

of these structures.

Classification of Geological

Structures

I. Classification based on geometry (shape and form of a particular structure):

a. planer surface

b. linear surface

c. curviplaner surface

This classification may be the most important because it includes: folds, faults , joints, veins, cleavage, foliation and lineations.

II. Classification based on

geological significance:

a. primary: ripple mark, cross bedding, mud cracks.

b. local gravity driven: slumping.

c. local density –inversion driven: salt dome (form due to variation in rock density).

d. fluid-pressure driven: injection of unconsolidation material due to sudden release of pressure.

e. tectonic: due to interaction between lithospheric plates.

First four usually primary and nontectonic structures while the fifth is the main aspect of structural geology.

III. Classification based on

timing of formation :

a. synformational: structure forms with initial

deposition of rock.

b. penecontemporaneous: structure forms

before full lithification, but after initial

deposition.

c. postformational: structure forms after the

rock has fully lithifide.

IV. Classification based on Process

of formation

(the deformation mechanism) • Fracturing: related to cracks in rocks.

• Frictional sliding: related to slip of one body of rock past another.

• Plasticity: deformation by internal flow of crystals without loss of cohesion.

• Diffusion: material transport in either solid-state or assisted by a fluid (dissolution). Stylolotes

• Combination: combinations of deformation mechanisms contributing to the overall strain.

V. Classification based on

Mesoscopic cohesiveness during

deformation

• Brittle: structure forms by loss of cohesion.

• Ductile: structure forms without loss of cohesion.

• Brittle/ Ductile: deformation with both brittle and

ductile aspects.

VI. Classification based on Strain

significance, in which a reference

frame must defined (usually earth

surface or the deformed layer):

• Contractional: shortening of a region (convergence).

• Extensional: stretching of a region (divergence).

• Strike-slip: movement without either shorting or

stretching (lateral slip).

VII. Classification based on

Distribution of deformation in a

volume of rock

• Continuous: occurs at the rock body at all

scales.

• Penetrative: occurs throughout the rock body at

observation scale.

• Localized: structure in continuous or penetrative

only within a definable region.

• Discrete: structure occurs as an isolated feature.

Finally, most crustal structures are a

consequence of plate tectonics

activities that include; convergence,

divergence and transform (lateral

slip) movements.

Lecture 3

Bubble to show horizontal For strike measurement

Bubble to show horizontal For dip measurement

Magnetic needle

Clinometer For dip angle measurement

Direction scale

GEO

LOG

ICA

L C

OM

PASS

(B

run

ton

)

GEOLOGICAL COMPASS (Silva)

Field situation of compass using Measurements with a Brunton (a) strike; (b) dip.

B

Dip and Slopes: Dip is a particular expression related to the inclination of geologic layer, while slope is the inclination of the surface of the earth in general. Dip slope is a land surface inclined in the same direction and at often in the same angle as the dip of the underlying rocks.

Dip slope surface

Unti-Dip slope surface

Lines To describe a linear feature, student should know: 1- Trend: a general term to show the direction of a line in general projected on a horizontal plane. It is also called azimuth. It is a horizontal angle (between 0 and 360°) measured clockwise from true north, which has an azimuth of 000. As a couple of examples, E (East) has an azimuth of 090o while S45W (South 45o West) has an azimuth of 235. 2- Plunge (inclination): The vertical angle, measured downward, between the horizontal and a line.

N

45o

E

Down

Inclination angle

Horizontal plane azimuth

For this line the attitude of this line Is noted as: 45, 045

Plunge or inclination

Trend

In brief

Structural geology involves the description of the structure of the crust at various scales and seeks to understand how any given structure or set of structures formed. Our methods are field observations, laboratory experiments And numerical modeling. All of these methods have advantages and challenges. Field examples portray the final results of deformation processes, while the actual deformation history may be unknown. Histories that may span millions of years in nature must be performed in hours, days or weeks in the laboratory. In addition, numerical modeling is approached by simplifications necessary for the models to be suitable with today’s codes and computers. However, by combining different approaches we are able to obtain realistic models of how structures form and what they mean. Field studies will always be the key to success. Any modeling, numerical or physical, must be based directly or indirectly on accurate and objective field observations and descriptions. Objectivity during field work is often a challenge.

Structural Study of any Area: Structural study concerning any area may goes through the following analysis steps or (stages):

1: Descriptive analysis: The characterization of the shape and appearance of geologic structures that permits one geologist to create an image of a structure that any other geologist can understand. As well as describing the orientation of a structures in three dimensional shape.

2: Kinematic analysis : determination of the movement paths that rocks or parts of rocks have taken during transformation from the un-deformed to the deformed state.

3: Dynamic analysis: The development of an understanding of the stress and its relation to deformation ( Relation between deformation and stresses which are acting on the subject).

4: Mechanism analysis: The study of processes on the atomic scale that allow structures to develop.

5: Tectonic analysis: The study of the relationship between structures and global tectonic processes

Types of Structural Study: 1- Observation: Observation of natural structure, or deformed features in rock, this observation can take at many different scales, from submicroscopic to the global observation usually involves the description of the geometry and orientations of structures and their relations to other structures.

2- Experimental: an attempt to reproduce under controlled laboratory conditions various features similar to those in naturally deformed rocks. The aim of experimental work is to gain insight into the stress systems and processes that produced the deformation. Two major drawbacks: (1) in the real earth, we seldom know all of the possible factors effecting the deformation (P, T, t, fluids, etc.); (2) More important, real earth processes occur at rates which are far slower than one can possibly reproduce in the laboratory (Natural rates: 10 -12to 10 -18sec-1; in lab, the slowest rates: 10-6–10-8sec-1)

3- Theoretical: application of various physical laws of mechanics and thermo-dynamics, through analytical or numerical methods, to relatively simple structural models. The objective of this modeling is to duplicate, theoretically, the geometries or strain distributions of various natural features. Main problem is the complexity of natural systems.

Thickness and Depth: Thickness: the perpendicular distance between the parallel planes bounding a tabular body, as displayed on any section perpendicular to these planes; also called the true or stratigraphic thickness. Apparent thickness: the distance between the bounding planes measured in some other direction, for example, the perpendicular distance between the traces of the bounding planes on an oblique section, or in some other specified direction, as in a drill hole. It is always greater than true thickness.

Outcrop width: the strike-normal distance between the traces of the parallel bounding planes measured at the earth’s surface. It may be measured horizontally or on an incline. Depth: the vertical distance from a specified level (commonly the earth’s surface) downward to a point, line or plane.

True thickness t,

apparent thickness t’, outcrop

w width w and depth d.

Problem: If the true thickness t = 50 m and the dip δ = 30◦, what will be the apparent vertical thickness ta? ta is also called drilling thickness.

δ = 30◦

ta = ?

Rock Deformation It is the changes in volume or shape or both of a rock, it is almost the same as strain. Deformation can be either: 1- non-permanent, in which exists only while the stress is applied and the material returns to its un-deformed state upon removal of the stress ( i.e. elastic deformation). 2- Permanent deformation, in which the deformation is forever (plastic deformation).

Deformation occurs in nature because of natural or artificial forces Force is a push or pull acting upon an object that causes a body to accelerate. F (force)=m (mass) x a (accelaration) - this is the second law of motion. F is measured in Newton (N) 1N is the force required to accelerate 1kg by 1m/s2 Force is a vector quantity, it has magnitude and direction

Stress is„ the force that acts on a rock unit to change its shape and/or its volume and„ causes strain or deformation So stress is: force/ area (i.e. newton/ m 2

When stress acts by the same amount from all directions,

it is called confining stress, while If stress is not equal

from all directions then we say that the stress is a

differential stress.

There are three kinds of differential stress occur. 1. Tensional stress (or extensional stress), which stretches rock. 2. Compressional stress, which squeezes rock; 3. Shear stress, which result in slippage and translation.

When strain is occurred in length, ex. L/∆L or in volume V/∆V Hence strain has no units.

K shows how much stress is needed to produce a given strain In studying geological structures we have strain and need to know stresses

There are three types of strain: 1- Dilation: change in size only. 2- Distortion: change in shape only. 3- Deformation: is the change in size and shape.

Stages of Deformation When a rock is subjected to an increasing stress it passes through 3 successive stages of deformation. 1- Elastic Deformation -- wherein the strain is reversible. 2- Ductile (or plastic) Deformation -- wherein the strain is irreversible. 3- Fracture - irreversible strain wherein the material breaks.

Elastic Limit

Fracture or Rupture

The law which relates stress to strain is called (Hook’s Law)

Materials are divided into two classes that depend on their relative behavior under stress. 1- Brittle materials have a small or large region of elastic behavior but only a small region of ductile behavior before they fracture. 2- Ductile (plastic) materials have a small region of elastic behavior and a large region of ductile behavior before they fracture.

Fracture of Brittle Rocks Faults and Joints

brittle rocks tend to fracture when placed under a high enough stress. Such fracturing, while it does produce irregular cracks in the rock, sometimes produces planar features that provide evidence of the stresses acting at the time of formation of the cracks. Two major types of more or less planar fractures can occur: Faults and joints.

Joints Joints are fracture surfaces along which rocks or minerals have broken, thus generating two free surfaces where none existed before. They appear at all rock exposures. - They are without any displacement along the fracture surface. - Joints are either straight or curved in shape. - Some of them are open others are closed. - Joints are present in nature as sets or systems.

Importance of joints

1- Fractures provide information on what kind of stress caused them (i.e. history of deformation).

2- Because they alter the characteristics of the rocks in which they occur: for instance a fracture would weaken a rock (and we need to know that if we build a dam, or a tunnel).

3- Joints allow fluids to move through it (and we need to know that if we are looking for oil or gas, or if we are dealing with groundwater).

Joint set is a number of joints which have same strike and dip. Joint system is the case when two sets of joint cross or cut each other in a certain system. Some joints are non-systematic. System is of two types: 1- Conjugate system, when two joint sets are cross cutting at acute angle between them (30-60o). 2- Orthogonal system, when two sets are perpendicular to each other.

An example of orthogonal system

An example of conjugate system

Because joints are planes (such as bedding plane ) in structural geology so it has : 1- strike 2- dip ( i.e. dip direction and dip amount ). Relative with bedding planes; joint planes may be: 1):Strike joint when the joint strike is parallel to the strike of the bed. 2):dip joint when strike of the joint is parallel to the dip direction of the bed. 3):bedding joint when the strike and dip of the bed coincide with the strike and dip of the joint. 4): diagonal joint when there are no coherency neither in strike nor dip between bed and joint.

The four types of joints relative to a bedding plane

When a lot of information dealing with

attitudes of the joint sets were collected in the

field; a structural geologist can give valuable

information for the tectonic history of the

structures. Those information can be used by

Engineers in constructing projects and even for

a petroleum geologist to evaluate structural

units of an oil field (reservoir).

Faults

Faults occur when brittle rocks fracture, and

there is an offset along the fracture. When the

deformation of offset is small, the

displacement can be easily measured, but

sometimes the displacement is so large that it

is difficult to measure.

A fault contains fault plane and fault trace; the

fault plane is a plane along which the

displacement is happened, but fault trace is the

intersection between fault plane and the

topographic surface ( it may be straight or/and

curved line).

A fault plane has attitudes like a bedding plane; so it has strike, dip angle and dip direction. The faulted block on both sides of the fault plane has many parts: Hanging wall: is a rock mass which located above the fault plane. Foot wall : is a rock mass which located below the fault plane. Dip angle: is the angle between fault plane and horizontal plane, measured perpendicular to strike. Hade angle: is complementary angle for dip ( i.e. dip +hade= 90 ) That was the case if the f-plane is not vertical

Dip angle

Dip angle

Hade

Horizontal line

Vertical line

Upthrown side

downthrown side

Classification of faults Based on slip direction

1- Dip slip: the slip is along the dip of the fault, either up or down, there are two types:

In this case the rake angle of the striation on fault plane = 90 degree

Dip-slip 1 Dip slip A

Important: Difference between reverse and thrust faults is the angle of plane dip

Reverse

Dip slip B

Dip slip C

2- Strike-slip: the slip is parallel to strike, there are two types a- right-lateral ( dextral ) b-left-lateral ( sinistral)

In this case the rake angle of the striation on fault plane = zero

3- Oblique slip: the net slip is composed from strike slip + dip slip four main types can be recognized

In this case the rake angle of the striation on fault plane > zero & < 90 degree

4- scissor type:

Fault combinations: Horst - two normal faults dipping away from each other Graben - two normal faults dipping toward each other Synthetic fault – dips in same direction as the main fault Antithetic fault - dips towards the main fault

A fault separates two rock bodies from each other by sliding. The separation distance may be horizontal or vertical

B

B

C

AB: Horizontal separation (Heave) BC: Vertical separation (Throw) AC: Dip separation

A

A

AB: Horizontal (strike separation)

Folds

Folds, faults and joints are the main types of structural

deformation in the Earth’s crustal rocks. Among them,

folds often create the most spectacular geological

scenes.

Petroleum exploration has historically been associated

with folds more than with any other geological

structure.

‘anticlinal theory of oil accumulation.’

Under differential compressional stress, formations

that are brittle will undergo faulting; usually normal

faults while formations that are ductile will most likely

undergo folding.

Often we observed combination of both ductile and

brittle deformation in our lithological or rock record

and these deformations almost always occur under

multiple episodes.

So folds are flexing or bending of strata under

the action of stress. All types of rocks,

sedimentary, Igneous or metamorphic may bend

to form a fold.

A layer (bedding plane ) subjected to a compressional stress

Anticline syncline anticline

Fold size is measured by millimeters to tens of

kilometers;

- microscopic folds------- mm’s

- mesoscopic folds--------m’s

- macroscopic folds------- regional type, km’s

- Dealing with shape, there are two types of folds:

1) Anticline : The core containing older rocks, convexing

upward (Hat like).

2) Syncline : The core containing younger rocks,

concaving upward.

Core of the fold Older rocks in anticline and younger in syncline

Description of folds

Any fold can be described according to some definitions

Related to its geometry.

1- Hinge: Maximum curvature within folded surface.

2- Hinge line: line of greatest curvature in a folded surface.

It is approximately the axis of the fold.

3- Axial plane: The Plane containing the hinge lines.

Description of folds

4- Trough: greatest depressions in folded surface Hinge

(i.e. axis of a syncline)

5- Crest: greatest height in folded surface

(i.e. anticlinal axis)

6- Limb: The flank area on both sides of axis.

Folds may be tight or wide

Inter-limb angle

Plunging fold: Anticlines or synclines should be ended at some place, at this place the fold called plunged. When the fold is plunged the strike of the beds are changed, for example if the trend of the fold is NW-SE then the strike is changed from NW-SE to N-S NNE-SSW then NW-SE for another time. Fold at middle part is not plunged while at ends are plunged

A structure that plunges in all directions to form a

circular or elongate structure is a dome. Domes are

generally formed from one main deformation event, or

via diapirism from underlying magmatic intrusions or

movement of upwardly mobile, mechanically ductile

material such as rock salt (salt dome) and shale (shale

diapir).

A DOME is a doubly plunging anticline while A BASIN is

a doubly plunging syncline.

i.e. Domes and basins are structures with

approximately circular or slightly elongate, closed

outcrop patterns. Domes are convex upward; basins

are concave upward.

Dome

Basin

Folds might be: a- Symmetrical, means that their axial planes are vertical and both limbs are almost equal. b- Asymmetrical, when beds in one limb dip more steeply than those in the others. c- Both limbs dip in in the same direction but one limb has been tilted beyond vertical.

The Formation A mappable rock unit.

Describing and mapping the orientation of a

geologic structure or fault surface involves

determining …

Strike (trend)

Dip (inclination)

Mapping Geologic Structures

Strike (trend)

The compass direction of the line produced by

the intersection of an inclined rock layer or fault

with a horizontal plane.

Generally expressed an angle relative to north.

N37°E

N12°W

Mapping Geologic Structures

Dip (inclination)

The angle of inclination of the surface of a rock

unit or fault measured from a horizontal plane.

Includes both an angle of inclination and a

direction toward which the rock is inclined.

82°SE

17°SW

Mapping Geologic Structures