Folds, Faults, and Mountain BuildingFolds, Faults, and Mountain Building

Chapter 9Chapter 9

Stress!!!

• Stress—a force exerted on an object• Rocks react to stress by – Deforming plastically– Deforming elastically– Breaking by brittle fracture

• Question of the hour (#1)– How will a particular rock react to a particular stress?

It’s all in the Nature…

• Rock composition– Ex: Hitting a chunk of

concrete with a sledgehammer VS. hitting a giant rubber tire with a sledgehammer

I Can’t Take the Pressure!!

• Pressure and Temp. increase with depth– Temp vs. pressure– Burial promotes plastic

deformation

If you can’t stand the heat…

• Higher temperatures favor plastic deformation– Ex. Bending a steel nail

• Need to be super strong• Or just heat the thing up…

It’s About Time…• Stress applied slowly favors plastic behavior• Stress applied suddenly favors brittle behavior

May the Tectonic Force be With You….

• Plate tectonic forces– Tensional• Stretch and pull formations apart

– Compressive• Squeeze and shorten rock formations

– Shearing• Two sides of a formation are pushed in opposite

directions (sliding past one another)

Geologic Structures

• Plate tectonics—primary stressor• 3 main types of structures– Folds—ductile deformation (primarily)– Faults—brittle deformation– Joints—brittle deformation

Folds

• A fold is a bend in the rock– Plastic deformation

• Three characteristics– Usually results from compression– Folding always shortens the horizontal distances

in rock– A fold usually occurs as part of a group of folds

Anticline Vs. Syncline

Plunging Folds

Overturned and Asymmetric

Circular Features

• Domes– Circular anticlinal

structure (both pics)– Beds dip away from

center

• Basin– Circular synclinal

structure– Beds dip towards center

All My Faults are Stress Related!!!• Brittle deformation• Faults

– Cracks in rock along which motion has occurred

– Movement may be gradual or sudden (earthquake)

– Usually associated with other faults in a fault zone

• Question of the hour (#2): Why do rocks move repeatedly along faults and fault zones?

Nature is Lazy

• Rock moves repeatedly along faults and fault zones because– Tectonic forces often

affect one location for a long time

– It’s easier

• Three types of faults– Depend on tectonic

forces

Normal Faulting

Reverse Fault (part 1)

Reverse Fault (part 2)

Strike-slip Fault

Ah, How My Joints Ache…

• A joint is a fracture in rock along which no motion has occurred– Planes of weakness—like

mineral cleavage– Good for mining

Styles of Continental Deformation

Relating Small Structures to Big Picture

Question of the Hour (#3)

• Where do mountains come from?– In a subducting zone• Magmas/lavas—stall/erupt• High temp—rocks expand when heated

– Continent/continent collision—underthrusting• Crust is shoved under other crust• Compression squeezes crust—thickness increases

Tensional Tectonics

• Plates can split apart– Rift valleys – long narrow troughs formed by a

block that has dropped down between two big normal faults

Rifting and Normal Faulting

Compressive Tectonics• Fold and thrust belts:– When two continental plates collide, crust

compressed– Crust thickened by underthrusting—thicken crust

2x

Transverse Margins