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Structural Geology

Appalachian MountainsPennsylvania

It has long been evident to geologists that great forces operate in Earth’s crust

These forces apparently involve bending, breaking, and overturning rock layers

Differential t

Stress is a force acting upon a solid.Strain is a change in shape or volumeof a solid as a result of stress

stress

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Solid rock may respond to stress in two ways:

Ductile Behavior (bending, stretching)( g g)

Brittle Behavior (breaking)

Behavior determined by several factors:

Rate of strain, confining pressure, rock composition, temperature, presence of water

Joints

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Faulting

H i ll d f t llHanging-wall and footwall

Types of Faults

Normal Fault Reverse Fault

Strike-Slip Fault

Normal Fault

Strike-Slip Fault

Reverse Fault

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Horst and Graben Da Basics• Tensional stress = normal fault• Compressive stress = reverse faultp• Shear stress = strike slip fault

Geometry of a foldGeometry of a fold

AnticlineAnticline

SynclineSyncline

Plunging AnticlinePlunging Anticline

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Open (symmetrical)

I li lIsoclinal

Asymmetrical

Overturned

Recumbent

Evolution of a fold into a

reverse fault

Age relationships of folded beds

An eroded anticline will have older beds in the middleAn eroded anticline will have older beds in the middleAn eroded syncline will have younger beds in middle

Strike and Dip

How do geologistsmap complex 3-Dstructures with only surface data?

Strike and DipMaking a Geologic Map

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Measuringorientation ofrock outcrops

What are the age relationshipsof these rocks?

Erosion removes portions of a fold

Strike and Dip symbols on maps

Plunging folds revealed by topography

“Law of V’s” - Plunging anticline points in direction of plungePlunging syncline opens in direction of plunge

GeologicMap

Pay attention to age relationships andstrike and dip symbols

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Describe the structures and stress

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Draw the geologic map Three General Types of Mountains

• Fold and Thrust Belts• Fault Block Mountains

• Volcanic Mountains

Fold and Thrust Mountains

Fault Block

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Volcanic Mountains

So…how would you figure out the Appalachians? A Low-Angle Reverse Fault - Thrust Fault

Fig. 15.16

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Keystone Thrust west of Las Vegas

Fig. 15.17B

What is the geologic structure of Michigan?

Geologic Map of U.S.

Ordovician Late Carboniferous

U.S. Orogenies Western U.S. Eastern U.S.Permian

Cretaceous

TaconicOrdovician

AcadianDevonian

AlleghenianPennsylvanian

LaramideTertiary

SevierCretaceous

NevadanJurassic

GrenvilleProterozoic

SonomanTriassic

AntlerLate Devonian

KenoranArchean

time

PangeaBreakup

RodiniaAssembly

RodiniaBreakup

PangeaAssembly

Precambrian

Paleozoic

Mesozoic

Cenozoic

HudsonianProterozoic

AppalachiansIsostatic rise

Rocky Mts, CO. Plat.,Sierra Mts. – Tectonic Rise

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Mountain BuildingIsostasy - equilibrium Appalachian Fold and Thrust Belt