erth2404 l13 structural upload

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ERTH2404

Lecture 13: Structural Geology

Dr. Jason Mah

Folding of Rocks Due to Compression Along the

San Andreas Fault near Palmdale, California


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Lab Midterm

Mineral/rock ID mixed with short answer

Lab manual only

No loose leafs, no text book, no photos

Notes written in lab manual are OK

Remember to attend your lab section

Contact me or Ray immediately

[email protected]

Attend TA office hours to study

Good luck

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mailto:[email protected]:[email protected]


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Lab Midterm

Recall from the Course Outline:

A passing grade must be achieved in the lab to

complete the course.

Good luck

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Updated Course Schedule

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Date Lectures Laboratory

March 4 13. Structures LAB EXAM Even

(Labs 1, 2, 3)

March 6 14. Rock mechanics

March 11 15. Mass movement LAB EXAM ODD

(Labs 1, 2, 3)

March 13 16. Weathering & Erosion

March 18 17. Groundwater Lab 4 Even

Structures & Geological

MapsMarch 20 18. Rivers

March 25 19. Glacial processes Lab 4 ODD

Structures & Geological

MapsMarch 27 20. Resources

April 1 21. Geophysics Labs completed

April 3 22. Geomagnetic hazards

April 8 Review


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Reading assignment

Please read Kehews book to complement the

material presented in this lecture:

Chap. 8 p. 251-269;

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Lecture contents

Deformation

Mapping planar features

Fractures Faults

Joints

Folds

Engineering considerations

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Deformation

Deformation: alteration of size and/or shape

Structural geology: Earth science disciplinestudying

The processes responsible for the deformation ofthe Earths crust

The geological structures produced bydeformation

Faults

Joints

Folds

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Stress and strain

Stress(): Force applied per unit area [N/m2]

= force/area

Normal stress: component of stress perpendicular

to a given plane

Compressional: to shorten a body

Tensional: to pull apart a body

Shear: component of stress applied

parallel to a given plane

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Stress and strain

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Stress and strain

Strain ():Change in the shape and/or size of a

body as a result of stress [dimensionless]

= L/L

Elastic and Plastic deformation

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Stress and strain

Elastic deformation: returns to original shape

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Slope = Modulus of Elasticity (E)

Strain ( )

Elastic deformation

Yield stress

Plastic deformation


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Stress and strain

Rocks typically behave as combination of ideal

materials

Some rocks have high modulus (strong) while

others have a low modulus (weak)

Some rocks will exhibit elastic deformation if

the stress is small or over a short time period

Some rocks deform plastically AFTER

observing other types of deformation

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Stress and strain

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(yield stress)


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Stress and strain

Strength of the different rock types

Igneous rocks generally strong

Especially plutonic rocks due to large, interlocking crystals

Sedimentary rocks vary Salt, mudstones weak

Quartz-rich sandstones strong

Metamorphic rocks vary

Quartizites strong

Schists weak due thin layering

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Stress and strain

At shallow depth (low pressure)

Rocks behave elastically to elastic limit before brittle

failure

Forces primarily vertical, weight of overlying materials

Middle to lower crust (higher pressure)

Rocks first behave elastically Forces/Temperature from different directions

Ductile failure

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Stress and strain

Above the elastic limit, two scenarios:

Brittle rocks fail abruptly producing fractures

Ductile rocks undergo plastic deformation

producing undulations called folds

Remember that layers are always deposited

horizontally

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Requires coordinate system

With respect to North

Planar features are expressed by Strikeand Dip

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Earths surface

Depth

North

East


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Strike: intersection of planar structure with a

horizontal plane

Expressed as compass angle from North

(clockwise)

0 strike 360

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Dip: inclination of planar structure, measured

90 from strike line

Specify angle and direction

Water will flow in direction of dip

Dip always measured perpendicular to strike

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When measuring Strike and Dip, we apply the

right hand rule

Thumb in direction of Strike

Fingers in direction of Dip

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Mapping planar features: Strike & Dip

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What about inverted planes?

Overhanging features?

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Strike = Polar opposite

= add 180

Dip = With respect

to horizontal

North

East


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Fractures

Brittle rocks produce fractures

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Devils Post Pile, CA

Columnar joints


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Fractures

Fractures are the most common geologicalstructure

Fracturing occurs in all rock types

Fracturing occurs at several scales Meters to hundreds of kilometers

Factors controlling the brittleness ofa rock:

Rock composition and texture Temperature and pressure

Presence of fluids

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Fractures

Two types of fractures, scale dependent

Faults: major fractures, showing appreciable

movement between rock blocks

Joints: minor fractures, showing little or no

movement between rock blocks

Both faults and joints have significant engineeringimplications

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Faults

Stresses building up in the Earth's crust are

relieved by relative motion between rock

blocks

Fault: fracture in the Earth's crust resulting

from the displacement of one rock block with

respect to the other

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Faults

Hangingwall: rock block above the fault

Footwall: rock block below the fault

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Ref.:Abbott,P.L.2004.NaturalDisasters.

4thE

dition.Fig.3.8.S

hownwithpermission

.


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Faults

Sudden movement along active faults are the

cause of most earthquakes

Many faults are inactive

Evidence of past deformation

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Faults

Faults are classified according to the relative

movement between blocks

Dip-slip fault: movement in the direction of dip

Normal fault

Reverse fault

Strike-slip fault: lateral movement along strike

Several faults display a combination ofdip-slip and strike-slip movement

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Faults

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Source:http://earthsci.org/


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Faults

Naming convection based on direction ofmovement

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USGS


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Normal fault on campus

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Normal fault on campus

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Photo: C. Samson

Hanging wall

Footwall

The Rideau rapids

Expression of theGloucester normalfault


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Strike-Slip fault (1992 Landers earthquake M7.3)

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Thrust faults

Thrust fault: low-angle reverse fault

Moves older rocks (hanging wall) over younger

rocks (foot wall)

Associated with plate collision and mountainbuilding

Large displacements (up to 100s km)

Typical dip < 20

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Thrust faults

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Source:Natura

lResourcesCanada

Rockies, Alberta


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Thrust faults

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Photo: C. Samson

Rockies, Alberta


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Mini-Thrust faults

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Engineering Considerations: Faults

Faults can introduce a number of conditions

that can have a negative impact on

engineering projects

Differing rock types on either side of the fault

Presence of weaker rock material

Faults provide access to water

Movement between rock blocks

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Narrow zone of intense deformation

Rocks within the zone might be weaker

Fault breccia:

pieces of broken rocks Fault gouge: clay material

resulting from rock pulverizedduring movement

Surrounding rock is intact and strong

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Source:http://earthsc

i.org/


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The obvious: Earthquakes!!!

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Joints

Joints are a concern for road cuts, slope

stability, tunneling, mining operations

Joint: a fracture with little or no movement

between rock blocks

Joint set: a group of parallel joints

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Joints

Joints are fractures

Frequently form parallel to pre-existing zones ofweakness:

Bedding planes Bedding joints

Foliations Foliation joints

Slaty cleavage Cleavage joints

Joint frequency is not necessarily constant

throughout a rock mass In sedimentary rock, regular joints

In granite, irregular joints

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Joints: the cause

Joints result from internal stresses

Stresses transmitted into continents by plate

tectonics

Expansive joints: loading (burial) and unloading(removal of overlying rocks by erosion)

Cooling joints: thermal contraction/expansion in

relation to igneous processes

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Joints

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Expansive joints in sandstone

Cooling joints: Giants Causeway, Ireland


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Joints

Systematic joints

parallel, regularly-spaced fractures

Created by a regional uniform stress

Non-systematic joints

randomly orientated fractures with irregular or

curved joint faces

Created by local non-uniform stresses

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Joints

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Source: http://earthsci.org/


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Joints: measurement

Stereonetplotsstrike and dip

Stereographicprojection

Points closer to thecircumferencerepresent verticalfaces

Points closer to thecenter representhorizontal faces

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Significance of joints

Impact on the strength (quality) of the rock

Water flow: increased permeability and fluid

movement along joints

In soluble rocks, dissolution occurs preferentially

along joints

Concentration of chemical/mechanical weathering

along joints Favors circulation of mineral-rich hydrothermal

fluids

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Engineering Considerations: Joints

Orientation

Orientation of joints are a major concern for slope

stability

Take advantage of planes of weakness duringquarrying

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Anisotropy: characteristic of a property having

a different value when measured

in different directions

Rock masses with non-systematic joints have lessanisotropy than masses with systematic joints

Rock masses with systematic joints might have

significantly weaker properties in a specificdirection

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Spacing

Closely-spaced joints tend to cause numerous rock

falls

More widely-spaced joints tend to cause massiverock failures

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Folds

55USGS


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Fractures and Folds

Ductile rocks produce folds

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Folds

Fold: geological structure formed when rocks

are bent or curved as a result of plastic

deformation

Folds are produced by lateral compression of thecrust

There might be multiple phases of deformation

Folds can be re-folded by a later event

Folding occurs at several scales

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Folds Composition of a fold

Hinge: point of maximumcurvature

Limbs: parts of fold that are notcurved; interlimb angle

Axial plane: imaginary planeequidistant from each limb,bisects angle between limbs

Axis: intersection of hinge andaxial plane

Plunge: angle betweenhorizontal and hinge

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hinge


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Anticline and syncline

Anticline: arched fold in which the central part

contains the oldest rock layer

Convex upwards

Syncline: arched fold in which the central part

contains the youngest rock layer

Convex downwards

Neutral: axial plane horizontal

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dip

dip

dip

AnticlineMap symbol

Neutral

dip

Axial Plane

NeutralMap symbol

SynclineMap symbol


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Ref.: Kehew, A.E. 1995. Geology for Engineers & Environmental

Scientists. 2nd Edition. Fig. 7-18. Shown with permission.

Oldest rocks Youngest rocks


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Folds can be

complex when

considering all

theparameters

Note double

anticlineforms a dome

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Attitude of axial plane

Four types of folds based on dip of axial plane

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(upright) (inclined)


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Symmetry about axial plane

Symmetric: lengths of limbs L1 and L2 equal

Asymmetric or Overturned: limb lengths not equal,L1 > L2

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Axial

Plane

AxialPlane


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Progression of Folding

As stress and strain increase,intensity of folding increases

Minor horizontal shortening--> symmetric, upright, openfolds

Increased shortening -->asymmetric folds, inclinedaxial planes, close folds

Greater shortening -->overturned, highly inclinedaxial planes, tight torecumbent

Even greater shortening -->failure, fault

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Map Example

Note dip arrows stillpoint away fromplunging antiforms, into synforms

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Engineering Considerations: Folds

Unequal stresses can be present in a folded rockmass

Event within the same rock unit

Stresses are a function of: Position in the fold

Style of the fold

Variations in bedding, foliation, etc.

Civil engineering operations may meet withunexpected results when the stresses arereleased

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Case Study: Sudbury structure

The Sudbury structure formed by meteoritic

impact (1.85 Ga)

Over the time, the structure has been deformed

by compressional forces from a circular to an ovalshape

Major mineral deposits (Ni, Cu)

Renew interest in the economic potential of otherimpact craters

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Sudbury structure

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50 km

Airborne radar and magnetic data

Ref.:Canadian

CentreforRemote

Sensing,NRCan.

Shownwithp

ermission.


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Sudbury structure

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At time of impact

At present

Reconstructed gravity data Observed gravity data

Ref.:NR

Can.Shownwithpermission.


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Sudbury: INCO R&D project

Joints are mapped to estimate the quality (structuralintegrity) of the rock

It is difficult to map joints underground Harsh environment

Poor lighting conditions

Manual, requires compass measurements

Business drivers

Quantitative structural analysis of joint orientation and

block size for planning support Data archiving

Money

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Sudbury: INCO project, mapping joints

3D Laser imagingapplied to map jointsdigitally Quartzite road cut,

45km east ofKingston, ON

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3D Laserimaging 6 images

merged to

form a single3D model

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Sudbury: INCO project, mapping joints Stereographic projection (equal area, lower hemisphere)

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3D Pole density contouring method

165 462 measurements

Average angular difference = 9.9

Manual measurements

160 measurements


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Next: Rock Mechanics

Photo:B.Eade

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