DEFORMATION OF OF ROCKSROCKS

[L17 P. 363-373 /IP-B][L17 P. 363-373 /IP-B]

Deformation of Deformation of rocksrocks

• Folds and faults are geologic structures.

• Structural geology - the study of the forces that deform rocks (stress applied) and the effects of this force (strain).

Phil Dombrowski Fig. 10.1

Small-scale Folds

Small-scale Faults

Tom Bean Fig. 10.2

StressStress(force per unit area)(force per unit area)

Types of directed (or differential ) stresses include:

• Compression - shorten

• Extension - elongate

• Shear - distort shape

Differential stress

StrengtStrengthh• Ability of an object to resist deformation

(low pressure vs. high pressure)

StraiStrainnis any change in original shape or

size of an object in response to stress acting on the object

Types of deformationTypes of deformation

• Elastic

• Ductile (plastic)

• Brittle (rupture)

Elastic deformationElastic deformation

Temporary change in shape or size that is recovered when the deforming force is removed

Think “rubber band”

Ductile (plastic) deformationDuctile (plastic) deformation• Permanent change in

shape or size that is not recovered when the stress is removed

• Occurs by the slippage of atoms or small groups of atoms past each other in the deforming material, without loss of cohesion

• Think “deck of cards”

Brittle deformation Brittle deformation (rupture(rupture))

• Loss of cohesion of a body under the influence of deforming stress

• Sucker breaks!!!

• Usually occurs along sub-planar surfaces that separate zones of coherent material

Typical stress and

strain curve

Factors that affect deformationFactors that affect deformation

• Temperature

• Pressure

• Strain rate

• Rock type

The variation of these factors determines if a rock will fault or fold.

Effects of rock type on deformationEffects of rock type on deformation

Some rocks are stronger than others.

competentcompetent:: rocks that deform only under great stresses

incompetentincompetent:: rocks that deform under moderate to low stresses

Effects of deformation on rock type Effects of deformation on rock type Experimental Deformation of Marble

M.S. Patterson

Fig. 10.7

Brittle Deformation (Under low pressure) Ductile Deformation

(Under high pressure)

Orientation of deformed rocks Orientation of deformed rocks

ATTITUDEATTITUDE - way to describe the orientation of geologic structures.

StrikeStrike: (compass) bearing of a line defined by the intersection of the plane in question and the horizontal

DipDip: acute angle between the plane and the horizontal, measured perpendicular to strike.

Fig. 10.4

Fig. 10.4

Dipping Sedimentary Beds

Chris Pellant Fig. 10.3

P.L. KresanP.L. Kresan

Cockscomb Ridge, S. Utah

Cockscomb Ridge, S. Utah

Dip

Strike

P.L. Kresan

Tectonic Forces and Resulting Tectonic Forces and Resulting DeformationDeformation

Fig. 10.6

BRITTLE DEFORMATION

A. Abrupt movement breaks or cracks strata

B. 2 kinds of breaks:

– 1. JOINTS - NO movement of blocks

– 2. FAULTS - YES movement of blocks

1. Movement along STRIKE

2. Movement along DIP

Columns Formed by

Joint-controlled

Weathering

Terry Englander Fig. 10.20

Joint-controlled Landscape, S.E. Utah

FaultsFaultsFractures in rocks created byearthquakes that have moved

A. Dip-slip faultsnormal

reverse

thrust

B. Strike-slip faults

right lateral or left lateral

Hanging-wall and footwall

Dip-slip faultsDip-slip faults

Motion of the fault blocks, parallel to the dip direction.

Classification of FaultsClassification of Faults

hanging wall

footwall

cross section

Normal Normal FaultFault

footwall

hanging wall

cross section

Normal Dip-slip Fault

Reverse FaultReverse Fault

footwall

hanging wall

cross section

Reverse Dip-slip Fault

Drape Fold over Reverse Fault, WY

George Davis

Thrust FaultThrust Fault

footwall

hanging wall

cross section

Thrust faults are low-angle reverse faults.

Thrust fault

Keystone Thrust Fault, S. Nevada

John S..Shelton

Cambrian Limestone

Jurassic Sandstone

Lewis Thrust,

Sawtooth Range,

Wyoming

Kurt N. Coonstenius

French Thrust, Wyoming

Cretaceous Shale

MississippianLimestone

Kurt N. Coonstenius

Strike-slip faultsStrike-slip faults

Motion of the fault blocks is parallel to the strike direction.

To determine the direction of strike, put toes on the fault line & look at the direction the opposite block moved.

Left-lateral Strike Slip FaultLeft-lateral Strike Slip Fault

map view

Right-lateral Strike Slip Right-lateral Strike Slip FaultFault

map view

Gudmundar E. Sigvaldason

Strike-slip Fault

Rift Valley Formed by ExtensionRift Valley Formed by Extension

Graben

Horst

Horst

Wildrose Graben, Southern California

NASA/TSADO/Tom Stack

1872 Fault Scarp, Southern California

1988 Armenian Earthquake Fault Scarp

Armando Cisternas

1992 Landers

Earthquake Fault Scarp

PLASTIC DEFORMATIONFOLDSFOLDS

anticlineanticline: older rocks on the inside

synclinesyncline: older rocks on the outside

(scale(scale - from mm to tens of km)

Fold TerminologyFold Terminology

Fig. 10.10

Symmetrical

Isoclinal

Asymmetrical

Overturned

RecumbentFig. 15.22

Bill Evarts

Axial plane

AnticlineAnticline

Fig. 10.11

Breck Kent

Asymmetric Folds

Phil Dombrowski Fig. 10.1

Overturned Folds

Overturned Syncline,

Israel

Geological Survey of Israel Fig. 10.13

Valley and Valley and Ridge Ridge

Province of Province of the the

Appalachian Appalachian MountainsMountains

Fig. 10.19

Valley and Ridge Province

P. L. Kresan

J. Shelton, Geology illustratedJ. Shelton, Geology illustrated Fig. 10.15

Plunging Folds in the Plunging Folds in the Valley and RidgeValley and Ridge

Raplee Anticline, S.E. Utah

Raplee Anticline on the San Juan River, Utah

Domes and BasinsDomes and Basins

Fig. 10.16

John S. Shelton Fig. 10.17

Sinclair Dome, Wyoming

Anticlines and SynclinesAnticlines and Synclines

Fig. 10.9

Geologic Geologic Map and Map and

Cross Cross SectionSection

Fig. 10.5

Outcrops of SynclineOutcrops of Syncline

Fig. 10.18

All pau