Tectonic deformation

and geologic structures

Mountain building

• Anatomy of a convergent mountain belt

Valley & ridge

Blue ridge

Mountain building

Continental rift; opening of an ocean basin?

• Extensional horsts and grabens

HorstGraben

Deformation

Deformation:1) translation2) rotation3) distortion (shape change)

Deformation results from stress,

Stress = force per unit area

Deformation - types of stress

Compressivestress

Tensionalstress

Lateral shearstress

Uniform pressure from all directions

Deformation - tectonic

Compression in a principal direction

Extension in a principal direction

Lateral shear

• Brittle: shallow crust; with increased stress little change until failure (breaks)

• Ductile: deeper crust; with increased stress gradual change (squishes)

Rock deformation: When does it fold and when does it fault?

depends on rock type, crustal depth, rate

Products of deformation

Faults

Folds

• Anticline: opens downward;

oldest rocks in center

• Syncline: opens upward;

youngest rocks in center

Types of folds

Strike and dip measurements along the limbs of a fold

Deformation - folds

dome basin

Types of folds

• fold appears to die out on ground surface

Plunging folds

http://www.fault-analysis-group.ucd.ie/papermodels/models/syncline_anticline_lowres.jpg

Types of faults

Thrust faults:• Compression• HW up• shallow dip

Normal faults:• Extension• HW down• steep dip

Types of faults

Strike-slip faults:• lateral shear• no vertical change• vertical dip

(map view)

Thrust, strike-slip or normal faults?

• Southern Appalachians - fold and thrust belt

• Cross-section through western Virginia

Interrelated Folds & faults

• imbricate thrusts

Wes

t V

irgin

ia

Virg

inia

Nor

th M

tn f

ault

Sta

unto

n fa

ult

Way

nesb

oro

Blu

e R

idge

Roc

kfis

h V

alle

y

Cha

rlott

esvi

lle

Gum

Spr

ing

10 km

Mountain building

Richmond Basin = continental rift; related to opening of the Atlantic ocean

• Extensional horsts and grabensR

ichm

ond Richmond Basin

Yor

ktow

n

Che

sape

ake

Bay

Gum

Spr

ing

Cap

e C

harle

s

10 km • Cross-section through eastern Virginia