Chapter 11

Mountain BuildingMountain Building

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Outline

• Mountains, mountain (orogenic) belts, & building them

• Deformation-Results (translation, rotation, distortion (strain))-Types: Brittle vs. ductile-Cause: stress (3 types)

• Geologic structures-Measurement, joints & faults-Faults: movement, recognition, types, fault systems-Folds: types, identification, formation-Foliation due to compression & shear

• Orogenesis-Uplift, mtn roots, isostasy, erosion, collapse, causes-Case study: history of the Appalachians

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Mountains• Incredible landscapes.• Beautiful, refuge from the grind

• Vivid evidence of tectonic activity.• They embody

• Uplift• Deformation• metamorphism

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Mountain BeltsMountains often occur in long linear beltsBuilt by tectonic plate interactions in a process called orogenesis

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Mountain Building• Mountain building involves…

Deformation

Jointing

Faulting

Partial melting

Foliation

Metamorphism

Glaciation

Erosion

Sedimentation

Constructive processes build mountains, destructive processes tear them down

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Orogenic Belts• Mountains have a finite lifespan.

• Young -> high, steep, uplifting• Middle-aged -> dissected by erosion• Old ->deeply eroded and often buried

• Ancient mtn belts are in continental interiors• Orogenic continental crust is too buoyant to subduct• Hence if little erosion, can be preserved

Young

(Andes)

Old (Appalachians)

Chapter 11

Outline

• Mountains, mountain (orogenic) belts, & building them

• Deformation-Results (translation, rotation, distortion (strain))-Types: Brittle vs. ductile-Cause: stress (3 types)

• Geologic structures-Measurement, joints & faults-Faults: movement, recognition, types, fault systems-Folds: types, identification, formation-Foliation due to compression & shear

• Orogenesis-Uplift, mtn roots, isostasy, erosion, collapse, causes-Case study: history of the Appalachians

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Deformation• Orogenesis causes crustal deformation.

• Consists of…• bending

• breaking

• tilting• squashing• stretching• shearing

• Deformation is a force applied to rock• Change in shape via deformation -> strain• The study of deformation is called structual geology

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Results of Deformation• Deformation results in...

• Translation – change in location• Rotation – change in orientation• Distortion – change in shape• Deformation is often easy to see

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Results of Deformation• STRAIN: shape changes caused by deformation• Stretching m shortening, shear

• Elastic strain – reversible shape change• Permanent strain – irreversible shape change

-> 2 types of permanent strain: brittle & ductile.

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Strain• Deformation creates strain -> geologic structures.

• Joints – fractures without offset• Folds – layers bent by• Faults – • Foliation –

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Undeformed vs. DeformedUndeformed (no strain).

• Horizontal beds• Spherical sand grains• No folds, faults

Deformed (strained).• Tilted bed• Metamorphic alteration• Clay--- slate, schist, gneiss• Folding and

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Deformation Types• 2 major types: brittle & ductile.

1. Brittle – rocks break by fracturing

2. Brittle/ductile transition occurs at -10-15 km

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Deformation Types

2. Ductile deformation – rocks deform by flow and folding

3. Brittle above -10-15km depth, ductile below that

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Brittle vs. Ductile

1. High T & P results in ductile deformation.

Occurs at depth

2. Deformation rate

Sudden change promotes brittle, gradual ductile

3. Other factors like rock type

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Stress: Cause of Deformation • Strain is result of deformation. What causes strain?

• Caused by force acting on rock, called stress

• Stress =force applied over an area• Large stress =much deformation• Small stress =little deformation

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Stress

• Pressure – stress equal on all sides

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1. Compression –

3 Types of Stress

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2. Extension –

3 Types of Stress

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3. Shear – rocks sliding past one another

4. Crust is neither thickened or thinned

3 Types of Stress

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Stress: force over an area

Strain: Amount of deformation an object experiences compared to original shape/size

Note: Rocks at plate boundaries are very stressed and hence deformed

Stress vs. Strain

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Outline

• Mountains, mountain (orogenic) belts, & building them

• Deformation-Results (translation, rotation, distortion (strain))-Types: Brittle vs. ductile-Cause: stress (3 types)

• Geologic structures-Measurement, joints & faults-Faults: movement, recognition, types, fault systems-Folds: types, identification, formation-Foliation due to compression & shear

• Orogenesis-Uplift, mtn roots, isostasy, erosion, collapse, causes-Case study: history of the Appalachians

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Geologic Structures• Geometric features created by deformation.• Folds, faults, joints• Often preserve information about stress field• 3D orientation is described by strike & dip.

• Strike – deformed rock intersection with horizontal• Dip – angle of tilted surface form horzontal

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Measuring Structures• Dip is always…• Perpendicular to strike, measured downslope• Linear structures measure similar properties.

• Strike (bearing) –compass direction• Dip (plunge) – angle down from horizontal• Strike and dip measurements are common

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Joints

• Rock fractures without offset• Systemic joints occur in parallel sets• Minerals can fill joints to form vents• Joints control rock weathering

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Faults• Fractures with movement along them causing offset• Abundant and occur at many scales

• Vary by type of stress and crustal level.

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Faults• Faults may offset large blocks of earth• Offset amount is displacement• San Andreas (below) –displacement of 100s of kms• Recent stream is offset

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Fault Movement• Direction of relative block motion…• Reflects stress type• Defines fault type (normal vs reverse)• All motion is relative.

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Recognizing Faults• Rock layers are displaced across a fault• Faults may juxtapose different rock types• Scarps may form where intersect the surface• Fault friction motion may fold rocks• Fault-zone rocks are broken and easily erode• Minerals can grow on fault surfaces

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? Fault

• Hanging wall moves down relative to footwall• Due to extension (pulling apart) stress

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Reverse & Thrust Faults• Hanging wall moves over footwall• Reverse faults – steep dip

• Thrust faults – shallow dip

• Due to compressional stress.

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Thrust Faults• Old rocks up and over young rocks• Common at leading edge of orogen deformation• Can transport trust sheets 100s of kms • Thickens crust in mountain belts

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Strike-Slip Faults • Motion parallel to fault strike. • Classified by relative motion• Imagine looking across fault• Which way does other block move• Right lateral – opposite block moves right• Left lateral – opposite block moves left

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Fault Systems• Faults commonly co-occur in fault systems • Regional stresses create many similar faults• May converge to a common detachment at depth• Example: Thrust fault systems.• Stacked fault blocks (thrust sheets)• Result: shorten and thicken crust• Result from compression

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Fault Systems• Normal fault systems.

• Fault blocks slide away from one another• Fault dips decrease with depth into detachment• Blocks rotate on faults and create half-graben basins• Result: stretch and thin crust• Result from extention stress

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Folds• Layered rocks deform into curves called folds.• Folds occur in a variety of shapes sizes• Terminology to describe folds:

• Hinge – place of maximum curvature on a fold

• Limb – less curved fold sides

• Axial plane – imaginary surface defined by connecting hinges of nested fold

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Folds

• Folds often• Orogenic settings

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3 Fold Types1. Anticline – arch like, limbs dip away from hinge

2. Syncline – bowl like, limbs dip toward hinge

• Anticlines & synclines alternate in series:

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3 Fold Types

3. Monocline – like a carpet draped over a stairstep.

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Fold Identification

• Folds are described by• Plunging fold –>a tilted hinge• Non-plunging fold –>a horizontal hinge

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Fold Identification

• Folds described by 3D shape. • Dome –> an overturned bowl• Old rock in center, younger rocks outside• Basin –fold shaped like a bowl• Young rocks in center, older outside• Domes/Basins result from vertical crustal motions

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Forming Folds• Folds develop in 2 ways:

1. Flexural folds – rock layers slip as they are bent

2. Analogous to sheer as a deck of cards is bent

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Forming Folds

• Folds develop in 2 ways: 2. Flow folds – form by ductile flow of hot, soft rock

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Why do folds form?

• Horizontal compression causes rocks to buckle • Shear comes rocks to smear out

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Tectonic Foliation• Foliation develops via

• Grains flatten and elongate, clay reorient• Foliation parallels fold axial planes

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Tectonic Foliation• Foliation can result from

• Created as ductile rock is smeared• Shear foliation is not perpendicular to compression• Sheared rocks have distinctive appearance

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Outline

• Mountains, mountain (orogenic) belts, & building them

• Deformation-Results (translation, rotation, distortion (strain))-Types: Brittle vs. ductile-Cause: stress (3 types)

• Geologic structures-Measurement, joints & faults-Faults: movement, recognition, types, fault systems-Folds: types, identification, formation-Foliation due to compression & shear

• Orogenesis-Uplift, mtn roots, isostasy, erosion, collapse, causes-Case study: history of the Appalachians

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Orogenesis & Rock Genesis• Orogenic events create all kinds of rocks.

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Uplift• Mountain building results in substantial uplift• Mt everest (8.85 km above sea level)• Comprised of marine sediments • High mountains are supported by thickened crust

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Crustal Roots• High mountains are supported by thickened lithosphere. • Thickening caused by orogenesis.

• Average continental crust –> 35-40 km thick.• Beneath mtn belts –> 50-80 km thick.

• Thickened crust helps buoy the mountains upward.

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Isostasy• Surface elevation represents a balance between forces:

• Gravity – pushes plate into mantle• Buoyancy – pushes plate back to float higher on mantle

• Isostatic equilibrium describes this balance.• Isostasy is compensated after a disturbance• Adding weight pushes lithosphere down• Removing weight casues isostatic rebound• Compensation is slow, requiring asthenosphere to flow

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Erosion• Mountains are steep and jagged from erosion• Mountains reflect balance between uplift and erosion• Rock structures can affect erosion• Resistant layers form cliffs• Erodible rocks form slopes

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Orogenic Collapse: Limit to Uplift!• Himalayas are the max height possible. Why?• Upper limit to mountain heights

• Erosion accelerates with height• Mountain weight overcomes rock strenght • Deep, hot rocks eventually flow out from beneath mtns• Moutains then collapse by:• Spreading out at depth and by normal faulting at surface

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Causes of OrogenesisCovergent plate boundaries create mountainsSubduction related volcanic arcs grow on overriding plateAccretionary prisms (of scraped sediment) grow upwardThrust fault systems on far side of arc

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Causes of Orogenesis• Continent-continent collision…• Creates a belt of crustal thickening• Due to thrust faulting and folding• Belt center- high grade metamorphic rocks• Fold thrust belts extend outward on either side

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Causes of Orogenesis• Continental rifting.• Continental crust is uplifted in rifts• Thinned crust is less heavy, mantle responds

isostatically• Decompressional melting adds

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Case Study - Appalachians• A complex orogenic belt formed by 3 orogenic events. • The Appalachians today are eroded remnants.

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Case Study - Appalachians• A giant orogenic belt existed before the Appalachians.

• Grenville orogeny (1.1 Ga) formed a supercontinent.• By 600 Ma, much of this orogenic belt had eroded away.

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Case Study - Appalachians• Grenville orogenic belt rifted apart ~600 Ma.

• This formed new ocean (the pre-Atlantic). • Eastern NA developed as a passive margin. • A thick pile of seds accumulated along margin. • An east-dipping subduction zone built up an island arc.

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Case Study - Appalachians• Subduction carried the margin into the island arc. • Collision resulted in the Taconic orogeny ~420 Ma.

• Next 2 subduction zones developed. • Exotic crust blocks were carried in.• Blocks added to margin during Acadian orogeny ~370

Ma.

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• E-dipping subduction continued to close the ocean.

• Alleghenian orogeny (~270 Ma): Africa collided w/ N.A.• Created huge fold & thrust belt• Assembled supercontinent of Pangaea.

Case Study - Appalachians

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Case Study - Appalachians• Pangaea began to rift apart ~180 Ma.

• Faulting & stretching thinned the lithosphere.• Rifting led to a divergent margin.• Sea-floor spreading created the Atlantic Ocean.