Mountain building involves tectonic processes that cause deformation and create geologic structures within the Earth's crust. Deformation results from stress and strain, and can be brittle or ductile depending on temperature and pressure conditions. It results in translation, rotation, and distortion of rocks. Geologic structures like folds, faults, joints and foliation form due to the strain caused by deformation. Orogenesis, or mountain building, occurs through a cycle of uplift, erosion and collapse over a mountain belt's lifespan and leaves evidence of tectonic activity in incredible landscapes.

1. Mountain Building
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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
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3. Mountains
• Incredible landscapes.
• Beautiful, refuge from the grind
• Vivid evidence of tectonic activity.
• They embody
• Uplift
• Deformation
• metamorphism
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4. Mountain Belts
Mountains often occur in long linear belts
Built by tectonic plate interactions in a process
called orogenesis
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5. Mountain Building
• Mountain building involves…
Deformation
Jointing
Faulting
Partial melting
Foliation
Metamorphism
Glaciation
Erosion
Sedimentation
Constructive processes build mountains, destructive
processes tear them down
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6. Orogenic Belts
• Mountains have a finite lifespan.
• Young -> high, steep, uplifting
• Middle-aged -> dissected by erosion
• Old ->deeply eroded and often buried
• 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)
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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
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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 -> strain
• The study of deformation is called structual geology
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9. Results of Deformation
• Deformation results in...
• Translation – change in location
• Rotation – change in orientation
• Distortion – change in shape
• Deformation is often easy to see
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10. Results of Deformation
• STRAIN: shape changes caused by deformation
• Stretching m shortening, shear
• Elastic strain – reversible shape change
• Permanent strain – irreversible shape change
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11. Strain
• Deformation creates strain -> geologic structures.
• Joints – fractures without offset
• Folds – layers bent by
• Faults –
• Foliation –
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12. Undeformed vs. Deformed
Undeformed (no strain). Deformed (strained).
• Tilted bed
• Horizontal beds • Metamorphic alteration
• Spherical sand grains • Clay--- slate, schist, gneiss
• No folds, faults • Folding and
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13. Deformation Types
• 2 major types: brittle & ductile.
1. Brittle – rocks break by fracturing
2. Brittle/ductile transition occurs at -10-15 km
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14. Deformation Types
2. Ductile deformation – rocks deform by flow and folding
3. Brittle above -10-15km depth, ductile below that
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15. Brittle vs. Ductile
1. High T & P results in ductile deformation.
Occurs at depth
2. Deformation rate
Sudden change promotes brittle, gradual ductile
3. Other factors like rock type
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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
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17. Stress
• Pressure – stress equal on all sides
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18. 3 Types of Stress
1. Compression –
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19. 3 Types of Stress
2. Extension –
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20. 3 Types of Stress
3. Shear – rocks sliding past one another
4. Crust is neither thickened or thinned
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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
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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
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23. Geologic Structures
• Geometric features created by deformation.
• Folds, faults, joints
• Often preserve information about stress field
• 3D orientation is described by strike & dip.
• Strike – deformed rock intersection with horizontal
• Dip – angle of tilted surface form horzontal
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24. Measuring Structures
• Dip is always…
• Perpendicular to strike, measured downslope
• Linear structures measure similar properties.
• Strike (bearing) –compass direction
• Dip (plunge) – angle down from horizontal
• Strike and dip measurements are common
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25. Joints
• Rock fractures without offset
• Systemic joints occur in parallel sets
• Minerals can fill joints to form vents
• Joints control rock weathering
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26. Faults
• Fractures with movement along them causing offset
• Abundant and occur at many scales
• Vary by type of stress and crustal level.
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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
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28. Fault Movement
• Direction of relative block motion…
• Reflects stress type
• Defines fault type (normal vs reverse)
• All motion is relative.
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29. Recognizing Faults
• Rock layers are displaced across a fault
• Faults may juxtapose different rock types
• Scarps may form where 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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30. ? Fault
• Hanging wall moves down relative to footwall
• Due to extension (pulling apart) stress
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31. Reverse & Thrust Faults
• Hanging wall moves over footwall
• Reverse faults – steep dip
• Thrust faults – shallow dip
• Due to compressional stress.
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32. Thrust Faults
• Old rocks up and over young rocks
• Common at leading edge of orogen deformation
• Can transport trust sheets 100s of kms
• Thickens crust in mountain belts
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33. Strike-Slip Faults
• Motion parallel to fault strike.
• Classified by relative motion
• Imagine looking across fault
• Which way does other block move
• Right lateral – opposite block moves right
• Left lateral – opposite block moves left
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34. Fault Systems
• Faults commonly co-occur in fault systems
• Regional stresses create many similar faults
• May converge to a common detachment at depth
• Example: Thrust fault systems.
• Stacked fault blocks (thrust sheets)
• Result: shorten and thicken crust
• Result from compression
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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 extention stress
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36. Folds
• Layered rocks deform into curves called folds.
• Folds occur in a variety of shapes sizes
• 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 fold
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37. Folds
• Folds often
• Orogenic settings
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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:
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39. 3 Fold Types
3. Monocline – like a carpet draped over a stairstep.
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40. Fold Identification
• Folds are described by
• Plunging fold –>a tilted hinge
• Non-plunging fold –>a horizontal hinge
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41. Fold Identification
• Folds described by 3D shape.
• Dome –> an overturned bowl
• Old rock in center, younger rocks outside
• Basin –fold shaped like a bowl
• Young rocks in center, older outside
• Domes/Basins result from vertical crustal motions
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42. Forming Folds
• Folds develop in 2 ways:
1. Flexural folds – rock layers slip as they are bent
2. Analogous to sheer as a deck of cards is bent
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43. Forming Folds
• Folds develop in 2 ways:
2. Flow folds – form by ductile flow of hot, soft rock
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44. Why do folds form?
• Horizontal compression causes rocks to buckle
• Shear comes rocks to smear out
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45. Tectonic Foliation
• Foliation develops via
• Grains flatten and elongate, clay reorient
• Foliation parallels fold axial planes
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46. Tectonic Foliation
• Foliation can result from
• Created as ductile rock is smeared
• Shear foliation is not perpendicular to compression
• Sheared rocks have distinctive appearance
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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
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48. Orogenesis & Rock Genesis
• Orogenic events create all kinds of rocks.
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49. Uplift
• Mountain building results in substantial uplift
• Mt everest (8.85 km above sea level)
• Comprised of marine sediments
• High mountains are supported by thickened crust
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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.
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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 casues isostatic rebound
• Compensation is slow, requiring asthenosphere to flow
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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
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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 strenght
• Deep, hot rocks eventually flow out from beneath mtns
• Moutains then collapse by:
• Spreading out at depth and by normal faulting at surface
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54. Causes of Orogenesis
Covergent plate boundaries create mountains
Subduction related volcanic arcs grow on overriding plate
Accretionary prisms (of scraped sediment) grow upward
Thrust fault systems on far side of arc
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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
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56. Causes of Orogenesis
• Continental rifting.
• Continental crust is uplifted in rifts
• Thinned crust is less heavy, mantle responds
isostatically
• Decompressional melting adds
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57. Case Study - Appalachians
• A complex orogenic belt formed by 3 orogenic events.
• The Appalachians today are eroded remnants.
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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.
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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.
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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.
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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.
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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.
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