1. Mountain Building
Chapter
11

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. 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. 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

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. 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. 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. 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. 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. 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. 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. Undeformed vs. Deformed
Undeformed (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. 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

14. Deformation Types
2. Ductile deformation – rock deform by flow and folding
3. Brittle above ~10-15 km depth, ductile below that
Chapter
11

15. 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

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. Stress
• Pressure – stress equal on all sides
Chapter
11

18. 3 Types of Stress
1. Compression – squeeze (stress greater in 1 direction)
1. Tends to thicken material
Chapter
11

19. 3 Types of Stress
2. Extension – pull apart (greater stress in 1 direction)
1. Tends to thin material
Chapter
11

20. 3 Types of Stress
3. Shear – rock sliding past one another
1. Crust is neither thickened or thinned
Chapter
11

21. Stress vs. Strain
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)!
Chapter
11

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. 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. 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. 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. 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. 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. 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. 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. What type of Fault
• Hanging wall moves down relative to footwall
• Due to extensional (pulling apart) stress
Chapter
11

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. 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. 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. 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. 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. 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. Folds
• Folds often occur in series
• Orogenic settings produce lots of folded rock
Chapter
11

38. 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

39. 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

40. Fold Identification
• Folds are described by hinge geometry
• Plunging fold –> a titled hinge
• Non-plunging fold –> a horizontal hinge
Chapter
11

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. 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. Forming Folds
• Folds develop in 2 ways:
2. Flow folds – form by ductile flow of hot, soft rock
Chapter
11

44. Why do folds form?
• Horizontal compression causes rocks to buckle
• Shear causes rocks to smear out
Chapter
11

45. Tectonic Foliation
• Foliation develops via compressional deformation
• Grains flatten and elongate; clays reorient
• Foliation parallels fold axial planes
Chapter
11

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. 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. Orogenesis & Rock Genesis
• Orogenic events create all kinds of rocks.
Chapter
11

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. 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. 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. 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. 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. 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

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. 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. Case Study - Appalachians
• A complex orogenic belt formed by 3 orogenic events.
• The Appalachians today are eroded remnants.
Chapter
11

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. 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. 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. 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. 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