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Chapter 11
Mountain BuildingMountain Building
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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Chapter 11
Mountains• Incredible landscapes.
beautiful, refuge from the grind, inspire poetry and art
• 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 (mountain building; mountain= orogen)
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Mountain Building• Mountain building involves…
deformation
Jointing
Faulting/folding
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, and uplifting (Andes, Himalayas)• Middle-aged -> dissected by erosion (Rockies)• Old -> deeply eroded and often buried (Appalachians)
• 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
Chapter 11
Chapter 11
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 -> called strain• The study of deformation is called structural geology
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Results of Deformation• Deformation results in...
• Translation – change in location• Rotation – change in orientation• Distortion – change in shape (strain)
Deformation is often easy to see
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Results of Deformation• STRAIN: shape changes caused by deformation
• Stretching, 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 plastic flow• Faults – fractures with offset• Foliation – planar metamorphic fabric
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Undeformed vs. DeformedUndeformed (no strain).
horizontal beds
spherical sand grains
no folds, faults
Deformed (strained).• Tilted beds• Metamorphic alteration• Clay > slate, schist, gneiss• Folding and faulting
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Deformation Types• 2 major types: brittle & ductile.
1. Brittle – rocks break by fracturing1. Occurs in shallow crust
1. Brittle/ductile transition occurs at ~10-15 km depth
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Deformation Types
2. Ductile deformation – rock deform by flow and folding
3. Brittle above ~10-15 km depth, ductile below that
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Brittle vs. Ductile
1. High T & P results in ductile deformation.1. Occurs at depth (because T and P increase with depth)
2. Deformation rate2. 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 – squeeze (stress greater in 1 direction)1. Tends to thicken material
3 Types of Stress
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2. Extension – pull apart (greater stress in 1 direction)2. Tends to thin material
3 Types of Stress
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3. Shear – rock sliding past one another3. 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 (strained)!
Stress vs. Strain
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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Chapter 11
Geologic Structures• Geometric features created by deformation.
• Folds, faults, joints, etc• Often preserve information about stress field
• 3D orientation is described by strike & dip.• Strike – deformed rock intersection with horizontal• Dip – angle of tilted surface from horizontal
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Measuring Structures• Dip is always…
• Perpendicular to strike, measured downslope
• Linear structures measure similar properties.• Strike (bearing) – compass direction i.e. N,S,E,W• Dip (plunge) – angle down from horizontal
• Strike and dip measurements are common
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Joints
• Rock fractures without offset• Systematic joints occur in parallel sets• Minerals can fill joints to form veins• Joints control rock weathering
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Faults• Fractures with movement along them causing offset
• Abundant and occur at many scales• May be active or inactive• Sudden movements along faults cause EQs
• 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 ~100m
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Fault Movement• Direction of relative block motion…
• Reflects stress type• Defines fault type (normal vs. reverse/thrust vs. strike-slip)
• 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 faults 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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What type of Fault
• Hanging wall moves down relative to footwall• Due to extensional (pulling apart) stress
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Reverse & Thrust Faults• Hanging wall moves over footwall• Reverse faults – steep dip (>~35 degrees)
• Thrust faults – shallow dip (<~35 degrees)
• Due to compressional stress.
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Thrust Faults• Place old rocks up and over young rocks• Common at leading edge of orogen deformation• Can transport thrust 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 a 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 falut systems
• Regional stresses create many similar faults• May converge to a common detachment at depth
• Example: Thrust fault systems.• Stacked fault blocks (thrust sheets0
• 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 extensional (pull-apart) stress
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Folds• Layered rocks deform into curves called folds.• Folds occur in a variety of shapes, sizes, geometries• 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 folds
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Folds
• Folds often occur in series• Orogenic settings produce lots of folded rock
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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.3. Fold with only 1 steep limb- “a ½ fold”
4. Due to “blind” faults in subsurface rock
5. Displacement folds overlying rocks
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Fold Identification
• Folds are described by hinge geometry• Plunging fold –> a titled hinge• Non-plunging fold –> a horizontal hinge
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Fold Identification
• Folds described by 3D shape. • Dome –> an overturned bowl
• Old rocks in center: younger ricks 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
-Analogous to shear 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 causes rocks to smear out
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Tectonic Foliation• Foliation develops via compressional deformation
• Grains flatten and elongate; clays reorient• Foliation parallels fold axial planes
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Tectonic Foliation• Foliation can result from shearing
• 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
Chapter 11
Chapter 11
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 (formed below sea level)
• 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 causes 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 strength
• Deep, hot rocks eventually flow out from beneath mountains
• Mountains then collapse by:• Spreading out at depth and by normal faulting at surface
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Causes of Orogenesis
Convergent plate boundaries create mountainssubduction-related volcanic arcs grow on overriding plateaccretionary prisms (off-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 magma
• High heat flow form magma expands and uplifts rocks
• Rifting creates linear fault block mountains and basins
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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.
