Chapter-4

Structural Geology

and

Tectonics

CONTENTS

4.1. Introduction to Deformation, Stress and Strain

4.2. Primary Structures

4.3. Folds 4.3.1. Introduction

4.3.2. Classification and nomenclature of fold

4.3.3. Types of Fold

4.4. Foliations

4.5. Lineations

4.6. Joints 4.6.1. Terminology of Joints

4.6.2. Genetic classification of Joints

4.6.3. Relation of joints to other structures

4.7. Faults 4.7.1. Fault terminology

4.7.2. Nature of movement along faults

4.7.3. Normal faults

4.7.4. Thrust faults

4.7.5. Strike-slip faults

4.7.6. Shear zones

4.8. Introduction to Plate Tectonics

Objectives and Scope

This chapter deals with ways to recognize and characterize major and minor structures in the earth's crust and ways to gain insight into how these structures form.

The chapter develops skills in three-dimensional thinking that are essential for understanding crustal structures. It also explores techniques for determining the sequence in which structures form.

The chapter will also focus on macroscopic structures but will also introduce trainees to some of the fascinating structures that form at the microscopic scale.

Our ability to understand geologic structures depends in large part on how we perceive them. Few geologic structures form by trivially simple processes, but depending on how we view geologic structures, they can appear horribly complicated or amenable to understanding; perspective is critically important.

One key thread throughout the presentation will be ways of viewing the geometry, mathematics, and physics of geologic structures.

Structural geology and Tectonics is the branch of geology

that deals with:

Form, arrangement and internal architecture of rocks

Description, representation, and analysis of structures from

the small to moderate scale

Reconstruction of the motions of rocks

Structural geology provides information about the

conditions during regional deformation using structures

Both are concerned with the reconstruction of the motions

that shape the outer layers of earth

Both deal with motion and deformation in the Earths crust

and upper mantle

Tectonic events at all scales produce deformation structures

These two disciplines are closely related and interdependent

Tectonics: Study of the origin and geologic evolution (history of motion and deformation) of large areas (regional to global) of the Earths lithosphere (e.g., origin of continents; building of mountain belts; formation of ocean floor) Structural Geology: Study of deformation in rocks at scales ranging from submicroscopic to regional (micro-, meso-, and macro-scale). It describes a geometric feature in a rock whose shape, form, and distribution, which can be described as:

Microstructure: The small-scale arrangement of geometric and mineralogical elements within a rock. Texture: Preferred orientation of crystallographic axes in the sample. Microfabric: Comprises the microstructure and the texture of a material.

Fundamental Structures are: Contacts Primary Structures Secondary structures Fractures (Joints and Faults) Vein Fold

Structural Analyses consist of:-

Descriptive: Recognize, describe structures by measuring their

locations, geometries and orientations Break a structure into structural elements - physical &

geometric Kinematic:

Interprets deformational movements that formed the structures

Translation, Rotation, Distortion, Dilation Dynamic:

Interprets forces and stresses from interpreted deformational movements of structures

4.1. Introduction to Deformation, Stress and Strain

Deformation: changes in shape, position, and/or orientation of a body. It includes all changes in the original location, orientation or form of a crustal rock body. Homogeneous deformation: the displacement gradient is a constant throughout the deformed body. For a homogeneous deformation, initially straight lines remain straight, circles become ellipses and parallel lines remain parallel after deformation. Inhomogeneous deformation: the displacement gradient is not a constant throughout the deformed body. For an inhomogeneous deformation, initially straight lines not remain straight after deformation.

Stress - Force applied to a given area. Determines the concentration of force. Therefore, stress (force per unit area) has the dimensions MLT-2L-2 = ML-1T-2. Units commonly used in geology are Bar, Kbar, Dynes cm-2, Atmosphere, Newton meter-2, Pascal, Gigapascal (GPa).

Force Mass x acceleration (F = ma)

The action that puts stationary objects in motion or Changes the motion of moving objects. Differential Stress Unequal in different directions. Hydrostatic stress- uniform in all

directions

3 major types of differential stress Compressional stress Tensional stress Shear stress

Stress act normal to a cube face, are known as normal stresses, and which act parallel to a cube face, and are known as shear stresses. The normal stresses across the principal planes are the principal stresses, often denoted as 1, 2 and 3 with the convention that 1>2> 3.

Stress

Push-together stress.

Shortens and thickens crust.

Associated with orogenesis (mountain building).

Compressional Stress

Pull-apart stress.

Thins and stretches crust.

Associated with rifting.

Tensional Stress

Stephen Marshak

Slippage of one rock mass past another.

In shallow crust, shear is often accommodated by bedding planes.

Shear Stress

Changes in the shape or size of a rock body caused by stress.

Strain occurs when stresses exceed rock strength.

Strained rocks deform by folding, flowing, or fracturing.

Strain

Strain Ellipsoid

Zones Lines Structures formed

Zone 1a Lines that have been elongated only Boudinaged

Zone

1b Lines that underwent early

shortening followed by

elongation (net lengthening)

Remnants of disrupted folds and

isolated fold hinges

Zone 2 Lines that underwent early shortening followed by

elongation (net shortening)

Folds that are becoming

unfolded and boudinaged

Zone 3 Lines that have been shortened only Folds with large amplitudes and short wavelengths

Is the visualization of state of finite strain at a point. Principal axes are lines that remain perpendicular

before and after strain. Their lengths define the major, intermediate, and minor semi-axes of the strain

ellipse Axes x>y>z (1>2>3). Strain Ellipsoid is visualization of strain tensor (2nd rank). A final state

of "finite" strain may be reached by a variety of strain paths. Finite strain is final state; "incremental

strains" represent steps along path or strain increments that result in final finite state of strain.

LoLoL

Lo

L

1 a Extension (or elongation) dimensionless!

S = L1 = Lo L1Lo =1

Lo Lo Lo dimensionless! b Stretch

= L 1 2 = S2 = (1)2

L o

C Quadratic elongation dimensionless!

Change in linear dimension

Change in right angles (change in angle between originally orthogonal

lines):

= tan Note: for small angular changes, tan

Measures of Strain

Elastic deformation The rock returns to original size and shape when stress removed.

When the (strength) of a rock is surpassed, it either flows (ductile deformation) or fractures (brittle deformation).

Brittle behavior occurs in the shallow crust; ductile in the deeper crust.

How Rocks Deform

Stephen Marshak

Factors controlling rock strength and deformation style.

Temperature and confining pressure Low T and P = brittle deformation High T and P = ductile deformation

Rock type Mineral composition controls strength

Time Stress applied for a long time generates change

(contd)

Reflection! 1. Show that the stress (force per unit area) has the

dimensions (ML-1T-2) and that strain (fractional

change of length or angle) is a dimensionless quantity.

2. Are there any units for the measurements strain?

What are the units for the measurement of strain?

3. What is the percentage shortening if a line initially 10

cm long is shortened progressively to 5 cm?

4. What are the units for the measurement of strain-

rate?

5. Based on shortening percent of Q. 3 calculate the

strain rate if shortening takes place in one year.

Answers 1. Force (mass X acceleration) has the dimensions (MLT-2). Area has

the dimensions (L2). Therefore, stress (force per unit area) has the

dimensions

(MLT-2L-2) = (ML-1T-2)

2. Since strain is the ratio of a change of length to an initial length has

no dimension. Hence there are no units for strain. A strain

generally expressed as a percentage change or as a fractional

change

3. If a line initially 10 cm long is shortened until it becomes 5 cm the

change in length is 5 cm and the strain (shortening) is 0.5 (no units)

or 50 percent (no units).

4. The dimensions of strain-rate are (T-1) so the that a strain rate

might be written as 10-5 sec-1,

5. The shortening of 50 percent (3 above) takes place in one year

(3.1536 x 107 sec) then the strain rate is

0.5/(3.1536 x 107 sec) = 15.8 x 10-9 sec-1

4.2. Primary Structures

Structures of rocks that are present before the onset of deformation are called primary structures. They are original features of sedimentary or igneous rocks,

resulting from deposition or emplacement. Structures reflecting subsequent

deformation or metamorphism, which are the subject of most of this chapter,

are secondary structures.

Primary structures play an important role in the interpretation of the structure of deformed areas. Less common, but of considerable value in areas where

they occur, are primary features that can be used to analyze the strain of the

deformed rock. These include pebbles and fossils in sedimentary rocks; and

vesicles, lapilli, and crystals in rocks of igneous origin.

As well as acting as markers, certain primary structures can also provide very valuable additional information. These structures indicate the direction in

which the surrounding rocks get younger or, as it is generally expressed, the

younging direction of the sequence.

Some of the more reliable and commonly occurring structures, from the point of view of younging criteria are discussed below;

Cross Bedding Cross bedding is defined as a structure confined to a single sedimentation unit and characterized by internal bedding or

lamination, called foreset bedding, inclined to the principal surface

of accumulation. The type of cross bedding used to determine the

younging direction is the one, in which the angle between the

foreset and the bed boundary is asymmetrical.

Ripple Marks Ripple is the name given to a group of wavelike depositional structures that may form in

water or in air. Structures of this type vary in amplitude from a few millimeters to mega

ripples, such as sand dunes, which have amplitude measureable in meters or tens of meters.

They can be divided into two groups: Oscillation ripples and current ripples. Oscillation ripples, in profile, are commonly seen to comprise angular ridges separated by

arcuate troughs. This difference between the shape of the ridge and that of trough, often

makes it possible to tell the direction of younging of oscillation ripple-marked sediments.

It is sometimes more difficult to determine the direction of younging from current ripples. They are asymmetrical in profile but both the ridges and the troughs have the same shape, so

that when the structure is inverted their appearance is unchanged.

Thus the direction of younging cannot be determined from the shape of the structure alone. However, in many cases, heavy minerals or organic matter accumulates in the ripple troughs

so that the latter can be distinguished from the crests of the ridges, and the direction of

younging can therefore be determined.

A

B

Ripple marks: (a) Oscillation type; (b) Current type.

Graded Bedding In many clastic rocks there is a systematic variation in grain size within a bed, such that the sediment at one side of the bed is coarse and becomes progressively finer toward the other side. Such a bed is said to be graded and generally, although not invariably, the coarse material is at the base or oldest side of the bed. Sole Marking The name sole marking, is given to certain irregularities in the interface between a pelite and the coarser material (conglomerate, sandstone, or limestone) stratigraphy overlying it. The structure is referred to as sole mark because it is generally observed on the original lower surface of the sandstone after the pelite has disintegrated and fallen away. These structures are commonly preserved in deformed rocks and recognition of them, on the underside of the sandstone, gives the direction of younging. Dessication Cracks Dessication cracks are a fairly common feature of sediments that have been deposited on land. They are very commonly associated with the ephemeral lakes of arid regions. They form when the water that has deposited the sediment drains away or evaporate, leaving the sediment to dry out subaerially. Examination of this structure in present-day sediments reveals that the cracks that develop during drying have polygonal form in plan and that, in section, the individual polygons of sediment become turned up at the edges, so that they have a concave upper surface. This form is commonly preserved in the rock and the upward concavity indicates the direction of younging.

Rocks are bent by crustal deformation into a series of wave-like undulations called folds.

Most folds result from compressional stresses which shorten and thicken the crust.

Stephen Marshak

4.3. Folds

4.3.1. Introduction

Folds form from curving, buckling, and bending of originally planar rock layers (e.g., beds, foliation) through ductile deformation.

Practically, folds are defined by the attitude of their axis and/or hinge line, axial plane.

Folds occur in any geologic layer such as bedding, lava flow layers, and foliation. Folds range in size from mm to km, and are manifestations of ductile deformation

(i.e., form at depth where T, P are high and fracturing does not occur).

Parts of a fold Limbs The two sides

of a fold.

Fold axis or hinge line A line connecting points of maximum curvature along a fold.

Axial plane An imaginary surface that divides a fold symmetrically.

4. 3.2 Classification & Nomenclature of folds

Anticline Upfolded or arched rock layers.

Syncline Downfolds or rock troughs. (Think sink)

Depending on their orientation, anticlines and synclines can be described as

Symmetrical Asymmetrical Plunging

4.3.3 Common Types of Folds

Anticline

Syncline

Anticlines and Synclines are common in fold

and thrust belts related to mountain belts.

Monoclines Large, step-like folds in otherwise horizontal sedimentary strata.

Domes -Upwarped circular or slightly elongated structure. Oldest rocks in center, younger rocks outside.

Basins Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.

Polyclinal Fold Folds with more than two axial plane (rare)

Conjugate fold: Has converging paired axial surfaces

Axial planes intersect along the axis (if cylindrical)

Axial plane may displace another axial plane

Box Fold : Conjugate folds with round hinge zones

Kink Fold: Conjugate fold with sharp hinge zones

Isoclinal fold: limbs are parallel to the axial plane.

Recumbent fold: fold with horizontal axial plane. Commonly isoclinal.

4.4. Foliation

Foliation: Any type of planar fabric in rock, including bedding, cleavage, schistosity. Foliations are penetrative (occur throughout) in samples at 10's of cm scale. Thus faults are not foliations, nor are fractures and joints because the latter are simply fractures and not related to internal structure of rock.

Cleavage: Secondary fabric element (not bedding) formed under low grade metamorphic conditions (or less) that allows the rock to split along planes.

Foliations commonly developed in plane of maximum flattening of strain ellipsoid or perpendicular to direction of maximum shortening: Strain Ellipsoid Axes: X>Y>Z, so foliation commonly in X-Y plane and perpendicular to Z.

There are three common types of foliation. These are:-

Axial plane foliations

Shear (Mylonitic) foliations

Transposed foliations

Types of foliation

Axial plane foliations are referred to as the surface generally parallel to the axial plane of the fold in the hinge area. However, it is important to realize that axial plane foliations commonly are not strictly parallel to the axial planes of folds.

They are divided into;

(i) continuous,

(ii) spaced or disjunctive, and

(iii) crenulation.

Axial plane Foliations

Note that since the axial planes are oriented

perpendicular to the maximum compressional

stress direction, slatey cleavage or foliation should

also develop along these directions. Thus, slatey

cleavage or foliation is often seen to be parallel to

the axial planes of folds, and is sometimes

referred to axial plane cleavage or foliation.

Crenulation cleavage

Fractured/Spaced cleavage

Shear Foliation

Transposed Foliation

4.5. Lineations

Lineations: A fabric element that can be represented

by a line.

Type of Lineations:

Fold hinges,

Mullions: cusps and bulges between contrasting lithologies due to mechanical incompatibilities

Rods: preserved fold hinges

Boudins: lineations formed by stretching and necking of a layer.

Type of lineations: (a) Simple linear fabric defined by preferred orientation of linear bodies. (b) Combined lineation and foliation defined by preferred orientation of elongate tabular bodies. (c) Linear fabric defined by common axis of variably oriented, tabular bodies. (d) Linear fabric defined by penetrative folding. (e) Lineation defined by intersection of two foliations.

Joints are a very common

rock structure.

They are fractures with no

offset.

Result from tectonic

stresses on rock mass.

Occur in parallel groups.

4.6. Joints

Contd

Usually planar

Usually form sets

Two or more sets are a system

Variable size

Spacing more or less consistent

Curved, irregular joints not part of a set are nonsystematic joints

Importance of studying joints:

To understand the nature and sequence of deformation in an area.

To find out relationship between joints and faults and or folds.

Help to find out the brittle deformation in an area of construction (dams, bridges, and power plants).

In mineral exploration to find out the trend and type of fractures and joints that host mineralization which will help in exploration. Joints and fractures

serve as the plumping system for ground water flow in many area and they

are the only routes by which ground water can move through igneous and

metamorphic rocks.

Joints and fractures porosity and permeability is very important for water supplies and hydrocarbon reservoirs.

Joints orientations in road cuts greatly affect both construction and maintenance. Those oriented parallel to or dip into a highway cut become

hazardous during construction and later because they provide potential

movement surfaces.

Chemical weathering tends to be concentrated along joints

Origin of Joints

Joints can be caused by a number of processes that create

tensional effective stress in rock:

Uplift and erosion

Residual stress

Tectonic deformation

Natural hydraulic fracturing

4.6.1. Terminology of Joints

Conjugate joints Conjugate fractures are paired fracture systems, formed in the same time, and

produced by tension or shear.

Curved joints Occur frequently and may be caused by the textural and compositional differences

within a thick bed or large rock mass or they may a result of changes in stress direction

or analysis.

Tectonic joints Form at depth in response to abnormal fluid pressure and involve hydro fracturing.

They form mainly by tectonic stress and the horizontal compaction of sediment at depth

less than 3 km, where the escape of fluid is hindered by low permeability and

abnormally high pore pressure is created.

Hydraulic joints Form as tectonic fractures by the pore pressure created due to the confined pressed

fluid during burial and vertical compaction of sediment at depth greater than 5 km.

Filled veins in low metamorphic rocks are one of the best of examples of hydraulic

fractures.

Unloading joints

Form near surface as erosion removes overburden and thermal elastic contraction occurs. They form when more than half of the original overburden has been removed.

The present stress and tectonic activity may serve to orient these joints. Vertical

unloading fractures occur during cooling and elastic contraction of rock mass and may

occur at depths of 200 to 500 m.

Systematic joints: have a subparallel orientation and regular spacing. Joint set: joints that share a similar orientation in same area. Joint system: two or more joints sets in the same area Nonsystematic joints: joints that do not share a common orientation and those highly curved and irregular fracture surfaces. They occur in most area but are not easily related to a recognizable stress.

Examples of joint arrays

Example of Systematic joints

Release joints Similar to unloading fractures but they form by release of stress. Orientation of release

joints is controlled by the rock fabric. Released joints form late in the history of an area and is oriented perpendicular to the original tectonic compression that formed the dominant fabric in the rock.

Release joints may also develop parallel to the fold axes when erosion begins and rock mass that was under burial depth and lithification begins to cool and contract, these joints start to propagate parallel to an existing tectonic fabric.

Sheared fractures may be straight or curved but usually can't be traced for long distance.

Nontectonic joints Sheeting joints: Those joints form subparallel to the surface topography. These joints

may be more observed in igneous rocks. Pacing within these fractures increases

downward. These fractures thought that they form by unloading overlong time when

erosion removes large quantities of the overburden rocks.

Columnar joints and Mud Cracks: Columnar joints form in flows, dikes, sills and volcanic necks in response to cooling and

shrinking of the magma.

4.6.2. Genetic classification of Joints

From the point of view of fracture mechanics, crack tips have been related to three modes of displacement, namely extensional or Mode I displacement, and shear fractures of Modes II and III.

Mode I fracture (joints): it is the extensional fractures and formed by opening with no displacement parallel to the fracture surface. In extensional fractures the fracture plane is oriented parallel to 1 and 2 and perpendicular to 3.

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. Same fracture can exhibit both mode II and mode III in different parts of the region.

4.6.3. Relation of joints to other structures

Joints may form during brittle folding in a position related to the fold axis and axial surface as follows:

parallel normal oblique

Joints in fold-thrust belts (orogens) seem to form at depth under high pore pressure. Many form parallel to 1 and perpendicular to folds and strike, which is 3. Joints are also formed adjacent to brittle faults, and movement along faults usually produces a series of systematic joints. Fractures form in pluton in response to cooling and later tectonic stress. Many of these joints are filled with hydrothermal minerals as late stage products. Different types of joints are present with pluton (i.e. longitudinal, and cross joints)

Position of joints related to the fold axis and axial surface

Veins

Veins are mineralized fractures. Because fractures channel fluids,

minerals are commonly deposited forming veins. Terminology for

veins is similar to joints, especially if the veins originated from

joint fractures.

Veins

Two common occurrences are:

en echelon veins (right) and

stockwork veins (below).

4.7. Faults & Shear zones

Breaks in rock that exhibit offset.

Exist at a variety of scales.

Sudden movements along faults are the cause of most earthquakes.

Classified by movement Horizontal Vertical Oblique

Faults grind rocks to create fault gouge.

Walls of a fault bear evidence of this grinding as slickensides.

Slicks reveal

fault direction.

Criteria for faulting:

Repetition or omission of stratigraphic units asymmetrical repetition

Displacement of recognizable marker such as (fossils, color, composition, texture .etc.).

Truncation of structures, beds or rock units. Occurrence of fault rocks (mylonite or cataclastic or both) Abundant veins, silicification or other mineralization along

fracture may indicate faulting.

Drag Units appear to be pulled into a fault during movement (usually within the drag fold and the result is thrust fault)

Reverse drag occurs along listric normal faults. Slickensides along a fault surface Topographic characteristics such as drainages that are

controlled by faults and fault scarps.

A. Thrust fault resulting in repeated section in a vertical drill hole. B. Normal fault resulting in missing section in a vertical drill hole.

Change in fault character with depth for a steeply dipping fault. Note the change in fault zone width and types of structures with depth.

4.7.1. Fault terminology

Fault plane: Surface that the movement has taken place within the fault.

Hanging wall: The rock mass resting on the fault plane.

Footwall: The rock mass beneath the fault plane. Slip: Describes the movement parallel to the fault

plane (fault displacement).

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

Separation: The amount op apparent offset of a faulted surface, measured in specified direction. There are strike separation, dip separation, and net separation.

Heave: The horizontal component of dip separation measured perpendicular to strike of the fault.

Throw: The vertical component measured in vertical plane containing the dip.

Dip slip

Net slip

Heave Throw

Footwall (rock mass

below the fault)

Hanging wall (rock mass

above the fault)

Fault blocks classified as

Dip-slip faults Motion is parallel to fault dip.

Strike-slip faults Motion is parallel to fault strike.

Oblique-slip faults Motion is both parallel to fault strike and dip.

4.7.2. Nature of movement along faults

May produce long, low cliffs called fault scarps.

Dip Slip Faults

Two dominant types

Normal fault Reverse Fault Thrust (a low angle reverse fault)

Andersons classification of faults

Anderson 1942 defined three fundamental possibilities of stress regimes and stress orientation that produce the three types of faults (Normal, thrust, and strike-slip). Note that 1> 2> 3.

Anderson's theory of Fault Mechanics: (a) high-angle normal faults, (b) low-angle reverse (thrust) faults, (c) Strike-slip faults

Hanging wall moves down relative to the footwall. Accommodate lengthening or extension of the crust. Exhibit a variety of scales.

Larger scale normal faults are associated with fault-block mountains (Basin and Range of

Nevada).

Normal fault bounded valleys are called grabens (Dobi graben, Afar).

Normal fault bounded ridges are called horsts.

4.7.3. Normal faults

Fig. 11.17b

W. W. Norton

Detachment Fault Accommodating large amounts of extension in the upper crust.

Prior to the recognition that low angle normal faults are widespread features of the extended crust, they were often mapped as thrusts (which makes no sense) or unconformities.

Where low angle faults are common, a "stratigraphic section will show many apparent gaps.

Two models for accommodating large amounts of extension in the upper crust. From Block and Royden

(1984).

Hanging wall block moves up relative to the footwall block

Reverse faults have dips greater than 45o and thrust faults have dips less then 45o

Accommodate shortening of the crust Strong compressional forces

4.7.4. Thrust faults

Thrust faults - A special case of reverse fault.

Hanging wall block moves up relative to the footwall block

Thrust faults are characterized by a low dip angle (less then 45o).

Accommodate shortening of the crust Strong compressional forces

Fig. 11.17a

Dominant displacement is horizontal and parallel to the strike of the fault

Types of strike-slip faults

Right-lateral as you face the fault, the block on the opposite side of the fault moves to the right

Left-lateral as you face the fault, the block on the opposite side of the fault moves to the left

Dextral Sinistral

4.7.5. Strike-slip faults

Strike-slip fault

Transform fault Large strike-slip fault that cuts through the lithosphere

Accommodates motion between two large crustal plates

Types of transform faults are:

Ridge-Ridge

Ridge-Arc

Arc-Arc

Transcurrent fault types of strike-slip faults, which are confined to the crust. These are:-

Indent-linked faults Tear faults Transfer faults

Features of Strike-Slip Faults

Restraining or compressional bends-- folds and thrusts

Releasing or extensional relays--- depression Pull-apart basins

Strain ellipsoid

4.7.6. Shear Zones Shear zones are produced by both homogeneous and inhomogeneous simple shear or

oblique motion and are thought of as zones of ductile shear

Shear zones on all scales are zones of weakness. Associate with the formation of mylonite. Presence of sheath folds. Shear zones may act both as closed and open geochemical systems with respect to

fluids and elements.

Shear zones generally have parallel sides. Displacement profiles along any cross section through shear zone should be identical

Shear zones are classified by Ramsay (1980) as:

1) brittle 2) brittle-ductile 3) ductile

1.rotation of a pre-existing or generated foliation;

2. rotation of deformed markers;

3. asymmetry of intrafolial folds;

4. normal kink-bands (microshears) in the margin or

central fabric of the shear zone;

5. asymmetry of sheared porphyroclasts;

6. rotation of fragments owing to shear fractures;

7. rotation of fragments owing to tensile fractures;

8. asymmetry of trails growing around rotating clasts;

9. asymmetry of trails growing around non-rotating clasts;

10. asymmetry of elongated recrystallized quartz grains;

11. asymmetry of dragged-out mica porphyroclasts;

12. asymmetry of quartz c-axis fabrics; and

13. The relationship between S-C angle.

Shear sense indicators in mylonitic shear zones (after White et al., 1986)

Riedel Shears- subsidiary strike slip shear fractures

set of conjugate shear fractures (R,R) that develop in strike-slip fault

systems

R- synthetic faults

R- antithetic faults

P-through cutting; link R,R

Shear sense indicators

Sibson, 1977

4.8. Introduction to Plate Tectonics

Plate tectonics is a unifying theory that attempts to explain natural phenomena such as earthquakes and volcanoes. The earth's surface had been mapped into a series of plates.

The seven major plates are: Eurasian, Pacific, Australian, North American, South American, African and Antarctic - all comprise both oceanic and continental crust. For

example, the North America Plate includes most of North America plus half of the northern

part of the Atlantic Ocean. (The Pacific Plate is almost entirely oceanic, but it does include the

part of California which lies to the west of the Sand Andreas Fault.)

There are also numerous small plates (e.g., Jaun de Fuca, Nazca, Scotia, Philippine, Caribbean, Arabian).

Boundaries between these plates are of three types: divergent (i.e., spreading), convergent, and transform.

Why Study Plate Tectonics?

Geographic distribution of geologic hazards such as earthquakes and volcanic eruptions controlled by plate

tectonics,

Many global and regional political and economic problems stem from uneven distribution of geologic resources such as oil and

metal ores.

Formation of geologic resources is controlled by plate movement.

THE THEORY OF PLATE TECTONICS

The theory of Plate Tectonics is based around the idea that the crust is broken up into a series of large crustal plates which "float" on the

asthenosphere below.

Motions in the asthenosphere, called convection currents, cause plates to move away from each other at the rising limb of a convection current, forming

a constructive plate boundary where new oceanic crust is formed.

As plates continue to move outwards, eventually the oceanic plate may be subducted at a destructive plate boundary.

Supporting evidence for Plate Tectonics Theory:

1. Discovery of the Mid-Atlantic Ridge - Ocean floor mapping led to the discovery of a global mid-oceanic ridge mountain chain zigzagging around the continents.

2. Magnetic Variations on the Ocean Floor (Palaeomagnetism) - during cooling, minerals in the Basaltic rock, align themselves along the Earth's magnetic filed - forming a permanent record of magnetic field in the rocks. Periodic variations in the earth's magnetic field have produced almost symmetrical magnetic patterns in the rocks either side of the Mid-Atlantic ridge (alternating stripes of magnetically different rocks).

Supporting evidence for Plate Tectonics Theory:

3. Theory of Sea-Floor Spreading Hess, put forward an idea that mid-ocean ridges are a structurally weak point where magma is able to rise to the surface and where due to the upwelling and eruption of this material, new crust is created. This helps, to support the continental drift theory as it helps to explain how the continents may be moving, as they are carried on the 'spreading' ocean floor. Hess's theory was supported by the fact that the youngest rocks are nearest to the ridge (showing the present day magnetic polarity in their mineral alignment) and the oldest rocks (showing reversed polarity) are further away from the ridge.

CONTINENTAL DRIFT AND PLATE TECTONICS

Alfred Wegener proposed the theory of continental drift back in 1912. The theory suggests that there has been large-scale movement of continents across the globe and that during the Permian period, 225 million years ago, all

the continents were joined as one super continent Pangaea.

Around 200 million years ago, Pangaea split into Laurasia and Gondwanaland.

The continents have continued to move and today's configuration of continents represents the most recent stage in their movement.

Continental Drift theory is based on the following evidence THE JIGSAW FIT OF THE CONTINENTS PLANT / ANIMAL FOSSILS

THE RULES OF PLATE TECTONICS

1. Continental crust is less dense, or lighter, than Oceanic crust so it

doesn't sink. It is never destroyed and is considered permanent.

2. Oceanic crust is heavier so it can sink below Continental crust. It is constantly being formed and destroyed at ocean ridges and trenches.

3. Continental crust can carry on beyond the edges of the land and finally end far below the sea. This explains why the edges of all the continents

don't have deep trenches right up against their coastlines.

4. Plates can never overlap. This means that they must either collide and both be pushed up to form mountains, or one of the plates must be

pushed down into the mantle and be destroyed.

5. There can never be gaps between plates, so if two plates move apart, as in the middle of the Atlantic, new rock will be formed to fill the space.

6. We know the Earth isn't getting bigger or smaller, so the amount of new crust being formed must be the same as the amount being destroyed.

7. Plate movement is very slow. This is partly why Wegener's original ideas were ignored. Nobody could 'see' the continents moving. When the

plates make a sudden movement we call it an Earthquake, and it's the

only time we are directly aware of the plates moving.

Supercontinent Cycle Plate movements led to assembly of

Pangaea by the Late Paleozoic Era.

Fragmentation of Pangaea began in

the Triassic Period.

Continued plate movement has led to

the present configuration

The supercontinent cycle of Tuzo

Wilson proposed that super-continents

have formed and fragmented repeatedly

throughout Earths history on a cycle of

500 million years.

The Wilson cycle Evidence from continental geology supports two and possibly as many as five complete opening and closings of all ocean

basins

Spreading rates suggest that crust forms at 2.8km2/year

Therefore 310km2 of ocean crust could have formed in 110 billion years

Over the past two billion years as many as 20 ocean basins could have been created or destroyed

The Wilson Cycle Developed by J. Tuzo Wilson (a Canadian!)

Embryonic - Rift valleys of East Africa

Youthful - Red Sea, Gulf of California

Mature - Atlantic Ocean (growing)

Declining - Pacific Ocean (shrinking)

Terminal - Mediterranean (closing)

PLATE BOUNDARIES

It is important to recognize that plates are not just pieces of continental or oceanic crust, but that, along with the crustal rock, they include a considerable

thickness of the rigid upper part of the mantle.

Together, the crust and the rigid part of the mantle make up the lithosphere, which has a total thickness of approximately 100 km.

At spreading centers, the lithospheric mantle may be very thin because the upward convective motion of hot mantle material generates temperatures that

are too high for the existence of a significant thickness of rigid lithosphere.

The fact that the plates include both crustal material and lithospheric mantle material makes it possible for a single plate to be comprised of both

oceanic and continental crust. For example, the North American Plate includes

most of North America, plus half of the northern Atlantic Ocean. Similarly the

South American plate extends across the western part of the southern Atlantic

Ocean, while the European and African plates each comprise part the eastern

Atlantic Ocean.

Immediately beneath the base of the lithosphere lies the partial melting zone (the low velocity zone) of the upper mantle - which is part of the asthenosphere.

It is thought that the relative lack of strength and rigidity of the partial melting zone facilitates the sliding of the lithospheric plates.

Divergent Boundaries

Divergent boundaries are spreading boundaries, where new oceanic crust is created from molten mantle material. Most are associated with the oceanic-ridges, and the crustal material created at a spreading boundary is always oceanic in character.

Spreading is caused by the convective movement within the mantle, which has the effect of pulling the plates apart.

Magma from the mantle pushes up to fill the voids left by spreading. A variety of volcanic rocks (all of similar composition) are created in the upper part, including pillow lavas which are formed where magma is pushed

out into sea-water.

Beneath that are vertical dykes intruded into cracks resulting from the spreading. The base of the oceanic crust is comprised of gabbro (i.e., mafic

intrusive rock).

By oceanic we mean that it is mafic igneous rock (e.g., basalt or gabbro, rich in ferro-magnesian minerals) as opposed to the felsic igneous rocks (such as

granite, which is dominated by quartz and feldspar) which are typical of

continental areas.

Another term for mafic igneous rock is SIMA (silicon and magnesium rich), and another term for felsic igneous rock is SIAL (silica and aluminum rich).

Spreading rates vary quite considerable, from 2 to 4 cm/y in the Atlantic, to between 6 and 18 cm/y in the Pacific.

Divergent boundary of two continental plates. Creates a rift valley (Example: East African Rift).

Convergent Boundaries

Convergent boundaries, where two plates move towards

each other, are of three types depending on what type of

crust is present on either side of the boundary (i.e.,

ocean-ocean, ocean-continent or continent-continent).

Transform Boundaries

Transform boundaries exist where one plate slides past another, without production or destruction of crustal material.

Most transform faults connect segments of mid-ocean ridges and are thus ocean-ocean boundaries.

Some transform faults connect continental parts of plates. An example is the San Andreas Fault, which connects the Juan de Fuca ridge with the Gulf of

California ridge.

Transform-fault boundary where the North American and

Pacific plates are moving past each other (Example: San Andreas Fault in California).

Summary

How Are Plate Movement and

Motion Determined?

Magnetic anomalies

Matching crustal features and anomalies

Direct measurement

Hot spots

Magnetic anomalies

Average rate of plate movement can be determined by dividing the age of a magnetic anomaly in oceanic crust by

the distance between that anomaly and the present mid-

ocean ridge.

The motion of one continent relative to another can be assessed by moving matching anomalies on either side of

the present ocean ridge back together along the present

ocean ridge.

Hot spots A fixed reference point is required to determine absolute motion of a plate. A hot spot such as lies beneath the island of Hawaii is a stationary plume of rising mantle material.

Drift of the Pacific plate across the hot spot produced the Hawaiian Islands and Emperor Seamounts.

What Is the Driving Mechanism?

The uneven distribution of heat in Earth ultimately drives plate tectonics through the process of convection.

Two models, both of which entail rotating thermal convection cells, have been proposed.

In one model the convection cells are restricted to the asthenosphere

In the other model the convection cells involve the entire mantle.