Maps Structure

8/13/2019 Maps Structure

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Folds, Faults, and other

Records of Rock Deformation


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Deformation of Rocks

Folding and faulting are the mostcommon forms of deformation inthe sedimentary, igneous and

metamorphic rocks thatcompose the Earths crust

Structural Geology is the studyof the deformation of rocks andits effects.


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Orientation of Deformed Rocks

We need some way to describe thegeometry of geologic structures. Sowe use the terms strike and dip.

Strike: compass direction of a rocklayer as it intersects with ahorizontal surface.

Dip: acute angle between the rocklayer and the horizontal surface,measured perpendicular to strike.


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Strike and Dip on a Rooftop


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Strike and Dip in a Rock Structure


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Displaying Strike and Dip

on a Map

35O

N

S

Direction

of strike

E

Direction of

down dip

W


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Creating a Geological

Map and Cross Section

A geological map is a convenient

tool which is commonly used bygeologists for recording suchinformation and the location of

outcrops, the nature of theirrocks, and the dips and strikes oninclined layers


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What determines if a rockFolds (bends)

or faults (breaks)?

Type of force applied

Pressure

Temperature

Rock (mineral) composition


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Strength

Ability of an object to resist

deformation

In lab, marble was tested

Fractured Deformed


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Three Major Types

of Directed Stress

Compression Extension

Shear


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Compression

Action of oppositely directed forcesacting towards each other at thesame time


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Tension

Action of coinciding and oppositelydirected forces acting away from eachother


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Shear

Action of coinciding and oppositelydirected forces acting parallel to eachother across a surface


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Deformation Textures

As rocks along a fault plane shear

past each other, they grind andmechanically fragment solid rockand promote recrystallization

under the high pressures


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Fault breccias:

Along the fault,the rocksfragment and

then are lithifiedinto brecciascomposed ofangularfragments ofvarious sizes


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Mylonites

When the deformation along afault occurs, if the temperatureand/or pressure is high enough,

you can form metamorphic rockscalled mylonites

A mylonite rock can display

minerals that have beentexturally changed by physicalmovement


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Texture deformation in mylonite


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Types of Deformation

Elastic

Ductile (plastic)

Brittle (rupture)


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Elastic Deformation

Temporary change in shape or

size that is recovered when the

deforming force is removed

(Like squeezing a piece of

rubber)


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Ductile (Plastic) Deformation

Permanent changein shape or size

Under goes

smooth andcontinuous plasticdeformation under

stress

Does NOT recover

original shape


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Brittle Deformation (Rupture)

As the appliedforce is increased,the rockundergoes littlechange until itsuddenly breaks

Usually along azone of weakness


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Brittle Versus Ductile

Some rocks are naturally brittle

Some rocks are naturally ductile

A rock that is brittle at shallow

depths can be ductile at greaterdepths


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Factors that Affect Deformation

Temperature

Pressure

Strain rate

Rock type

The variation of these factors determineswhether a rock will fault or fold


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Effects of Rock Type on Deformation

Some rocks arestronger than others.

Competent: rocks

that deform onlyunder great stresses

Incompetent: rocks

that deform undermoderate to lowstresses


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How Rocks Fracture

Joints

Faults


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Definition of Fault

Fractures in rockscreated by

tectonic processes

(Movement of the rock on

both sides of the fault)


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Faults

Regardless of size, all faults

are classified by the directionof relative movement (which iscalled slip)


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Types of Faults

Dip-slip faults

Normal

Reverse Strike-slip faults

Right-lateral

Left-lateral

Oblique-slip faults


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Hanging Wall: Term used byminers. They could hang their

light on this side of the faultbecause it was above them.

Footwall: Also from the

miners, this side of the wall uponwhich they could stand belowthe hanging wall.

Faults


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Dip-slip Faults

Motion of the fault blocks is parallelto the dip direction

Two types:

Normalmovement is down dipReversemovement is up dip


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Hanging wall

Foot wall


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Normal Fault

footwall

hanging wall

cross section

hanging wall

footwall

Before After


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Hanging wall

Foot wall


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R F lt


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Reverse Fault

footwall

hanging wall

cross section

Rift Valley


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Rift Valley

Rift Valley


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Rift Valley


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Thrust Fault

footwall

hanging wall

cross section

Thrust faults are low-angle reverse faults


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Strike-slip Faults

Motion of the fault blocks isparallel to the strike direction

There are 2 types

Right-lateral

Left-lateral


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Left-lateral


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Left-lateral Strike Slip Fault

aerial (map) view


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Right-lateral Strike Slip Fault

aerial (map) view

San Andreas Fault (right-lateral)


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( g )


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Displacementboth verticallyand

horizontally


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Folding of Rocks

Produced by tectonic forces

Very common form ofdeformation that is viable inlayered rock

Scale can be from centimetersto hundreds of kilometers

Fold Terminology


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Fold Terminology

Axial plane: the plane of mirror symmetrydividing the fold into two limbs

Axis: the line formed by the intersection of

the axial plane and a bedding plane

Horizontal fold: fold where the axis ishorizontal

Plunging fold: fold where the axis is nothorizontal


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More Fold Terminology


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More Fold Terminology

Syncline:a sequence of folded rockswith the youngest rocks on the inside

of the fold

Anticline:a sequence of folded rocks

with the oldest rocks on the inside ofthe fold


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S M F ld T i l


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Some More Fold Terminology

Symmetrical folds: have bedsdipping symmetrically on eachside of axial plane

Asymmetrical folds: have thebed on one side of fold dippingmore steeply than other

Overturned folds: one or bothsides of fold dipping beyondvertical


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Asymmetrical folds


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Overturned folds


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Overturned folds


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And Even More Fold Terminology


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And Even More Fold Terminology

Dome: a sequence of folded rocks inwhich all the beds dip away from a

central point

Basin: a sequence of folded rocks in

which all the beds dip towards acentral point


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Syncline exposed in a quarry near Middletown, Virginia


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y p q y , g

Folds are Clues to a Regions


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Folds are Clues to a Region s

Tectonic History

Valley and Ridge Province Near


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Valley and Ridge Province NearAppalachian Mountains: Remnants

of a Folded Mountain Belt


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Dating the Order of Deformation

Use geometry:

Inclusions

Cross-cutting relationships

Combine with fossils andradiometric dating


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Maps Structure

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