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Geology lecture 12
Mountain building, or orogenesis, involves the deformation, uplift, and erosion of mountain belts. It results from tectonic plate interactions that cause crustal thickening, folding, faulting, and metamorphism. Key aspects of mountain building discussed in the document include the results of deformation like translation, rotation, and strain; different types of geologic structures such as faults, folds, and foliation that form during deformation; and the processes of orogenesis such as uplift, erosion, and isostatic rebound that shape mountain belts over time. The document uses the Appalachian Mountains as a case study of an ancient orogenic belt formed through multiple tectonic events over one billion years.
Geology lecture 12
- 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 oftenoccur in long, linear belts Built by tectonic plate interactions in a process called orogenesis (mountain building; mountain= orogen) Chapter 11
- 5. Mountain Building • Mountainbuilding 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 • Mountainshave 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 causescrustal 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 createsstrain -> 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 • 2major 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. Ductiledeformation – 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 ofDeformation • 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 ofStress 1. Compression – squeeze (stress greater in 1 direction) 1. Tends to thicken material Chapter 11
- 19. 3 Types ofStress 2. Extension – pull apart (greater stress in 1 direction) 1. Tends to thin material Chapter 11
- 20. 3 Types ofStress 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 • Geometricfeatures 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 • Dipis 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 fractureswithout offset • Systematic joints occur in parallel sets • Minerals can fill joints to form veins • Joints control rock weathering Chapter 11
- 26. Faults • Fractures withmovement 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 mayoffset 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 • Directionof 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 • Rocklayers 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 ofFault • Hanging wall moves down relative to footwall • Due to extensional (pulling apart) stress Chapter 11
- 31. Reverse & ThrustFaults • 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 • Placeold 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 • Motionparallel 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 • Faultscommonly 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 • Normalfault 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 rocksdeform 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 oftenoccur 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 • Foldsare described by hinge geometry • Plunging fold –> a titled hinge • Non-plunging fold –> a horizontal hinge Chapter 11
- 41. Fold Identification • Foldsdescribed 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 • Foldsdevelop 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 • Foldsdevelop in 2 ways: 2. Flow folds – form by ductile flow of hot, soft rock Chapter 11
- 44. Why do foldsform? • Horizontal compression causes rocks to buckle • Shear causes rocks to smear out Chapter 11
- 45. Tectonic Foliation • Foliationdevelops via compressional deformation • Grains flatten and elongate; clays reorient • Foliation parallels fold axial planes Chapter 11
- 46. Tectonic Foliation • Foliationcan 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 & RockGenesis • Orogenic events create all kinds of rocks. Chapter 11
- 49. Uplift • Mountain buildingresults 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 • Highmountains 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 elevationrepresents 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 aresteep 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: Limitto • 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 Convergentplate 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
Editor's Notes
- #31 Pull apart gives you normal faults Push together gives you reverse faults
- #50 Thick crust are results from stacking crust on top of each other
- #51 Beneath mountain belts crust is very thick (what is beneath the surface is much larger than what is above the surface)
- #52 There has to be a new equilibrium to deal with what happens
- #58 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
- #59 All different colors represent different aged crusts