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
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
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
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
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
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
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
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
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
