Geology lecture 12

3,907 views

Published on

0 Comments
12 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
3,907
On SlideShare
0
From Embeds
0
Number of Embeds
5
Actions
Shares
0
Downloads
525
Comments
0
Likes
12
Embeds 0
No embeds

No notes for slide
  • Pull apart gives you normal faults Push together gives you reverse faults
  • Thick crust are results from stacking crust on top of each other
  • Beneath mountain belts crust is very thick (what is beneath the surface is much larger than what is above the surface)
  • There has to be a new equilibrium to deal with what happens
  • 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
  • All different colors represent different aged crusts
  • Geology lecture 12

    1. 1. Mountain Building Chapter 11
    2. 2. 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 Chapter 11 11
    3. 3. Mountains• Incredible landscapes. beautiful, refuge from the grind, inspire poetry and art• Vivid evidence of tectonic activity.• They embody • Uplift • Deformation • Metamorphism Chapter 11
    4. 4. Mountain BeltsMountains often occur in long, linear beltsBuilt by tectonic plate interactions in a process calledorogenesis (mountain building; mountain= orogen) Chapter 11
    5. 5. 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
    6. 6. 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
    7. 7. 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 Chapter 11 11
    8. 8. 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
    9. 9. 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
    10. 10. 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
    11. 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
    12. 12. Undeformed vs. DeformedUndeformed (no strain). Deformed (strained). horizontal beds • Tilted beds spherical sand grains • Metamorphic alteration no folds, faults • Clay > slate, schist, gneiss • Folding and faulting Chapter 11
    13. 13. Deformation Types• 2 major types: brittle & ductile. 1. Brittle – rocks break by fracturing 1. Occurs in shallow crust1. Brittle/ductile transition occurs at ~10-15 km depth Chapter 11
    14. 14. Deformation Types2. Ductile deformation – rock deform by flow and folding3. Brittle above ~10-15 km depth, ductile below that Chapter 11
    15. 15. Brittle vs. Ductile1. 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 ductile3. Other factors like rock type Chapter 11
    16. 16. 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
    17. 17. Stress• Pressure – stress equal on all sides Chapter 11
    18. 18. 3 Types of Stress1. Compression – squeeze (stress greater in 1 direction) 1. Tends to thicken material Chapter 11
    19. 19. 3 Types of Stress2. Extension – pull apart (greater stress in 1 direction) 1. Tends to thin material Chapter 11
    20. 20. 3 Types of Stress3. Shear – rock sliding past one another 1. Crust is neither thickened or thinned Chapter 11
    21. 21. Stress vs. StrainStress: force over an areaStrain: Amount of deformation an object experiences compared to original shape/sizeNote: Rocks at plate boundaries are very stressed and hence deformed (strained)! Chapter 11
    22. 22. 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 Chapter 11 11
    23. 23. 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
    24. 24. 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
    25. 25. Joints• Rock fractures without offset• Systematic joints occur in parallel sets• Minerals can fill joints to form veins• Joints control rock weathering Chapter 11
    26. 26. 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
    27. 27. 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
    28. 28. 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
    29. 29. 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
    30. 30. What type of Fault• Hanging wall moves down relative to footwall• Due to extensional (pulling apart) stress Chapter 11
    31. 31. 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
    32. 32. 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
    33. 33. 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
    34. 34. 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
    35. 35. 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
    36. 36. 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
    37. 37. Folds• Folds often occur in series• Orogenic settings produce lots of folded rock Chapter 11
    38. 38. 3 Fold Types1. Anticline – arch-like; limbs dip away from hinge2. Syncline – bowl-like; limbs dip toward hinge • Anticlines & synclines alternate in series: Chapter 11
    39. 39. 3 Fold Types3. 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
    40. 40. Fold Identification• Folds are described by hinge geometry • Plunging fold –> a titled hinge • Non-plunging fold –> a horizontal hinge Chapter 11
    41. 41. 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
    42. 42. 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
    43. 43. Forming Folds• Folds develop in 2 ways: 2. Flow folds – form by ductile flow of hot, soft rock Chapter 11
    44. 44. Why do folds form?• Horizontal compression causes rocks to buckle• Shear causes rocks to smear out Chapter 11
    45. 45. Tectonic Foliation• Foliation develops via compressional deformation • Grains flatten and elongate; clays reorient • Foliation parallels fold axial planes Chapter 11
    46. 46. 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
    47. 47. 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 Chapter 11 11
    48. 48. Orogenesis & Rock Genesis• Orogenic events create all kinds of rocks. Chapter 11
    49. 49. 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
    50. 50. 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
    51. 51. 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
    52. 52. 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
    53. 53. 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
    54. 54. Causes of OrogenesisConvergent 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
    55. 55. 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
    56. 56. 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
    57. 57. Case Study - Appalachians• A complex orogenic belt formed by 3 orogenic events.• The Appalachians today are eroded remnants. Chapter 11
    58. 58. 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
    59. 59. 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
    60. 60. 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
    61. 61. Case Study - Appalachians• 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. Chapter 11
    62. 62. 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. Chapter 11

    ×