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Geologylecture12 130408195843-phpapp01

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Cleophas Rwemera

Mountain building, or orogenesis, involves tectonic processes that cause deformation, uplift, and erosion of the Earth's crust. Orogenesis results in geologic structures like faults, folds, and foliation through brittle and ductile deformation processes driven by stress. The Appalachian Mountains provide a case study of repeated and complex orogenesis over hundreds of millions of years, forming through three main orogenic events as various tectonic plates collided and rifted apart.

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Chapter 11 Mountain 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 Chapter 11
Chapter 11 Mountains • Incredible landscapes. beautiful, refuge from the grind, inspire poetry and art • Vivid evidence of tectonic activity. • They embody • Uplift • Deformation • Metamorphism
Chapter 11 Mountain Belts Mountains often occur in long, linear belts Built by tectonic plate interactions in a process called orogenesis (mountain building; mountain= orogen)
Chapter 11 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
Chapter 11 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
Chapter 11 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
Chapter 11 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.
Chapter 11 Strain • Deformation creates strain -> geologic structures. • Joints – fractures without offset • Folds – layers bent by plastic flow • Faults – fractures with offset • Foliation – planar metamorphic fabric
Chapter 11 Undeformed vs. Deformed Undeformed (no strain). horizontal beds spherical sand grains no folds, faults Deformed (strained). • Tilted beds • Metamorphic alteration • Clay > slate, schist, gneiss • Folding and faulting
Chapter 11 Deformation Types • 2 major types: brittle & ductile. 1. Brittle – rocks break by fracturing 1. Occurs in shallow crust 1. Brittle/ductile transition occurs at ~10-15 km depth
Chapter 11 Deformation Types 2. Ductile deformation – rock deform by flow and folding 3. Brittle above ~10-15 km depth, ductile below that
Chapter 11 Brittle vs. Ductile 1. High T & P results in ductile deformation. 1. Occurs at depth (because T and P increase with depth) 2. Deformation rate 1. Sudden change promotes brittle, gradual ductile 3. Other factors like rock type
Chapter 11 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
Chapter 11 Stress • Pressure – stress equal on all sides
Chapter 11 1. Compression – squeeze (stress greater in 1 direction) 1. Tends to thicken material 3 Types of Stress
Chapter 11 2. Extension – pull apart (greater stress in 1 direction) 1. Tends to thin material 3 Types of Stress
Chapter 11 3. Shear – rock sliding past one another 1. Crust is neither thickened or thinned 3 Types of Stress
Chapter 11 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 Chapter 11
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
Chapter 11 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
Chapter 11 Joints • Rock fractures without offset • Systematic joints occur in parallel sets • Minerals can fill joints to form veins • Joints control rock weathering
Chapter 11 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.
Chapter 11 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
Chapter 11 Fault Movement • Direction of relative block motion… • Reflects stress type • Defines fault type (normal vs. reverse/thrust vs. strike-slip) • All motion is relative.
Chapter 11 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
Chapter 11 What type of Fault • Hanging wall moves down relative to footwall • Due to extensional (pulling apart) stress
Chapter 11 Reverse & Thrust Faults • Hanging wall moves over footwall • Reverse faults – steep dip (>~35 degrees) • Thrust faults – shallow dip (<~35 degrees) • Due to compressional stress.
Chapter 11 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
Chapter 11 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
Chapter 11 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
Chapter 11 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
Chapter 11 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
Chapter 11 Folds • Folds often occur in series • Orogenic settings produce lots of folded rock
Chapter 11 3 Fold Types 1. Anticline – arch-like; limbs dip away from hinge 2. Syncline – bowl-like; limbs dip toward hinge • Anticlines & synclines alternate in series:
Chapter 11 3 Fold Types 3. Monocline – like a carpet draped over a stairstep. 1. Fold with only 1 steep limb- “a ½ fold” 2. Due to “blind” faults in subsurface rock 3. Displacement folds overlying rocks
Chapter 11 Fold Identification • Folds are described by hinge geometry • Plunging fold –> a titled hinge • Non-plunging fold –> a horizontal hinge
Chapter 11 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
Chapter 11 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
Chapter 11 Forming Folds • Folds develop in 2 ways: 2. Flow folds – form by ductile flow of hot, soft rock
Chapter 11 Why do folds form? • Horizontal compression causes rocks to buckle • Shear causes rocks to smear out
Chapter 11 Tectonic Foliation • Foliation develops via compressional deformation • Grains flatten and elongate; clays reorient • Foliation parallels fold axial planes
Chapter 11 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
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 Orogenesis & Rock Genesis • Orogenic events create all kinds of rocks.
Chapter 11 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
Chapter 11 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.
Chapter 11 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
Chapter 11 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
Chapter 11 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
Chapter 11 Causes of Orogenesis Convergent plate boundaries create mountains subduction-related volcanic arcs grow on overriding plate accretionary prisms (off-scraped sediment) grow upward thrust fault systems on far side of arc
Chapter 11 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
Chapter 11 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
Chapter 11 Case Study - Appalachians • A complex orogenic belt formed by 3 orogenic events. • The Appalachians today are eroded remnants.
Chapter 11 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.
Chapter 11 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.
Chapter 11 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.
Chapter 11 • 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
Chapter 11 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.

More Related Content

Geologylecture12 130408195843-phpapp01

  • 2. 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
  • 3. Chapter 11 Mountains • Incredible landscapes. beautiful, refuge from the grind, inspire poetry and art • Vivid evidence of tectonic activity. • They embody • Uplift • Deformation • Metamorphism
  • 4. Chapter 11 Mountain Belts Mountains often occur in long, linear belts Built by tectonic plate interactions in a process called orogenesis (mountain building; mountain= orogen)
  • 5. Chapter 11 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
  • 6. Chapter 11 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)
  • 7. 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
  • 8. 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
  • 9. Chapter 11 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
  • 10. Chapter 11 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.
  • 11. Chapter 11 Strain • Deformation creates strain -> geologic structures. • Joints – fractures without offset • Folds – layers bent by plastic flow • Faults – fractures with offset • Foliation – planar metamorphic fabric
  • 12. Chapter 11 Undeformed vs. Deformed Undeformed (no strain). horizontal beds spherical sand grains no folds, faults Deformed (strained). • Tilted beds • Metamorphic alteration • Clay > slate, schist, gneiss • Folding and faulting
  • 13. Chapter 11 Deformation Types • 2 major types: brittle & ductile. 1. Brittle – rocks break by fracturing 1. Occurs in shallow crust 1. Brittle/ductile transition occurs at ~10-15 km depth
  • 14. Chapter 11 Deformation Types 2. Ductile deformation – rock deform by flow and folding 3. Brittle above ~10-15 km depth, ductile below that
  • 15. Chapter 11 Brittle vs. Ductile 1. High T & P results in ductile deformation. 1. Occurs at depth (because T and P increase with depth) 2. Deformation rate 1. Sudden change promotes brittle, gradual ductile 3. Other factors like rock type
  • 16. Chapter 11 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
  • 17. Chapter 11 Stress • Pressure – stress equal on all sides
  • 18. Chapter 11 1. Compression – squeeze (stress greater in 1 direction) 1. Tends to thicken material 3 Types of Stress
  • 19. Chapter 11 2. Extension – pull apart (greater stress in 1 direction) 1. Tends to thin material 3 Types of Stress
  • 20. Chapter 11 3. Shear – rock sliding past one another 1. Crust is neither thickened or thinned 3 Types of Stress
  • 21. Chapter 11 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
  • 22. 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
  • 23. 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
  • 24. Chapter 11 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
  • 25. Chapter 11 Joints • Rock fractures without offset • Systematic joints occur in parallel sets • Minerals can fill joints to form veins • Joints control rock weathering
  • 26. Chapter 11 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.
  • 27. Chapter 11 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
  • 28. Chapter 11 Fault Movement • Direction of relative block motion… • Reflects stress type • Defines fault type (normal vs. reverse/thrust vs. strike-slip) • All motion is relative.
  • 29. Chapter 11 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
  • 30. Chapter 11 What type of Fault • Hanging wall moves down relative to footwall • Due to extensional (pulling apart) stress
  • 31. Chapter 11 Reverse & Thrust Faults • Hanging wall moves over footwall • Reverse faults – steep dip (>~35 degrees) • Thrust faults – shallow dip (<~35 degrees) • Due to compressional stress.
  • 32. Chapter 11 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
  • 33. Chapter 11 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
  • 34. Chapter 11 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
  • 35. Chapter 11 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
  • 36. Chapter 11 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
  • 37. Chapter 11 Folds • Folds often occur in series • Orogenic settings produce lots of folded rock
  • 38. Chapter 11 3 Fold Types 1. Anticline – arch-like; limbs dip away from hinge 2. Syncline – bowl-like; limbs dip toward hinge • Anticlines & synclines alternate in series:
  • 39. Chapter 11 3 Fold Types 3. Monocline – like a carpet draped over a stairstep. 1. Fold with only 1 steep limb- “a ½ fold” 2. Due to “blind” faults in subsurface rock 3. Displacement folds overlying rocks
  • 40. Chapter 11 Fold Identification • Folds are described by hinge geometry • Plunging fold –> a titled hinge • Non-plunging fold –> a horizontal hinge
  • 41. Chapter 11 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
  • 42. Chapter 11 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
  • 43. Chapter 11 Forming Folds • Folds develop in 2 ways: 2. Flow folds – form by ductile flow of hot, soft rock
  • 44. Chapter 11 Why do folds form? • Horizontal compression causes rocks to buckle • Shear causes rocks to smear out
  • 45. Chapter 11 Tectonic Foliation • Foliation develops via compressional deformation • Grains flatten and elongate; clays reorient • Foliation parallels fold axial planes
  • 46. Chapter 11 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
  • 47. 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
  • 48. Chapter 11 Orogenesis & Rock Genesis • Orogenic events create all kinds of rocks.
  • 49. Chapter 11 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
  • 50. Chapter 11 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.
  • 51. Chapter 11 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
  • 52. Chapter 11 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
  • 53. Chapter 11 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
  • 54. Chapter 11 Causes of Orogenesis Convergent plate boundaries create mountains subduction-related volcanic arcs grow on overriding plate accretionary prisms (off-scraped sediment) grow upward thrust fault systems on far side of arc
  • 55. Chapter 11 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
  • 56. Chapter 11 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
  • 57. Chapter 11 Case Study - Appalachians • A complex orogenic belt formed by 3 orogenic events. • The Appalachians today are eroded remnants.
  • 58. Chapter 11 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.
  • 59. Chapter 11 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.
  • 60. Chapter 11 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.
  • 61. Chapter 11 • 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
  • 62. Chapter 11 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.

Editor's Notes

  1. Pull apart gives you normal faults Push together gives you reverse faults
  2. Thick crust are results from stacking crust on top of each other
  3. Beneath mountain belts crust is very thick (what is beneath the surface is much larger than what is above the surface)
  4. There has to be a new equilibrium to deal with what happens
  5. Pattern in the topography- there are a bunch of ridges that are parallel to one another The mountain belts have mostly eroded because they are so old- took place over three different stages
  6. All different colors represent different aged crusts