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Folds Faults and Mountains. Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics.

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Presentation on theme: "Folds Faults and Mountains. Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics."— Presentation transcript:

1 Folds Faults and Mountains

2 Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics.

3 Rock Distortion

4 Convergent Plate Boundaries and Folding Ocean-Ocean collision forms Island Arc: Japan, Aleutians, Cent. Am. Continent-Continent collision forms Folded Mountain Belt:Alps, Himalayans, Appalachians

5 Evidence of Lateral Compression Formerly horizontal layers are twisted, bent, or broken. Some folded rocks are pushed over on their sides, or even upside down.

6 Folded Sandstone Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.

7 Studying Faults and Folds The branch of geology that studies crustal deformation is called Structural Geology. Geologic structures determine ground stability, and where to build cities.

8 Stress Units are Pressure: Force/Area Three types of stress a)Compression causes bending b)Tension causes thinning c)Shearing causes one type of faults

9 Compression, Tension, and Shearing Stress Convergent Divergent Transform

10 Types of deformation Elastic deformation up to elastic limit –Springs back to original shape Demo: Pencil Brittle failure (it breaks) Demo Pencil –Causes: 1. subjected to great stress that exceeds the yield point AKA elastic limit, OR –Subjected to sudden stress AKA “impact” Plastic deformation –Does not spring back … keeps deformed shape –Demo Chewing gum –Cause can be high temperature – near melting or high pressure … squeezed like a ball of clay

11 Relation ship Between Stress and Strain Strain can be a change in shape (a deformation) due to an applied stress

12 Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress

13 Relationship Between Stress and Strain under high Temps or Pressure

14 Factors affecting rock deformation Intensity of applied stress Heat –Temperature of the Rock Amount of Time the Stress is applied Rock Composition

15 Interpreting Deformed Rocks Most apparent in sedimentary rocks Importance of deformation –Indicates past plate motions –Indicates other past geological events –Locates specific natural resources Mapping 101: Rock orientation: strike and dip

16 Strike and Dip Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface Strike is long line, dip is short line Note the angle of dip given 45 o

17 Folds Folds def: Bends in rock layers Types: synclines and anticlines Syncline (downfold) innermost rocks youngest Anticline (upfold) innermost rocks oldest Parts of a fold (limbs, axial plane, axis) Note: Anticlines and synclines are structures in rocks, not surface landforms

18 Folded Rocks, Hwy 23 Newfoundland, New Jersey Source: Breck P. Kent Adjacent Anticline and Syncline Note highest point

19 Folded Rocks (Dorset, England) Center has overturned area Source: Tom Bean Lucky we have ways of recognizing right side up What are they? OlderYounger Overturned Area Older Younger

20 Folded Rock Before Erosion

21 Folded Rock After Erosion Eroded Anticline, older rocks in center. Syncline is opposite.

22 Topography may be opposite of Structure Anticline Before/After Erosion Notice center rock oldest

23 Topography may be opposite of Structure Syncline Before/After Erosion Notice center rock youngest

24 Fold symmetry a)Symmetrical or open folds b)Asymmetrical folds c) Overturned folds d)Recumbent folds e)Plunging folds

25 Various Folds

26 Various Folds (cont'd)

27

28 Not a good drawing, axial plane should be closer to horizontal

29 Plunging Folds Nose of anticline points direction of plunge, syncline nose in opposite direction Up End Down End Demo: Plastic box, water, paper folds

30 Plunging Folds Source: GEOPIC©, Earth Satellite Corporation

31 Interpreting Folds Determine if center rocks are older or younger than flanks: fossils, right side up clues (graded bedding and mudcracks) Are limbs parallel or “Nosed”? Determine limb dips from measurements, stream V’s. Strike and Dip Use nose rules for anticlines and synclines

32 Again: Strike and Dip

33 Domes and basins 1.Domes 2.Basins 3.Occur within plates 4.Result from vertical forces 5.Geographic examples (later)

34 3-D: Dome and Basin

35 Fractures Fractures - Joints: fractures with no relative movement - Faults: fractures with relative movement

36 Joints: Fractures – with no movement Source: Martin G. Miller/Visuals Unlimited

37 Fault Type 1 - Dip-slip faults 1)Terms: Hanging wall and footwall 2)Normal faults (a) Grabens (b) Horsts 3)Reverse faults a) low angle called Thrust faults 4)Oblique-slip faults

38 Dip-Slip Faults

39 Source: John S. Shelton Normal Fault: Hanging Wall Down Key Bed Hanging wall overhangs the fault plane Especially common in divergent margins

40 Normal Fault (Hanging Wall down)

41 Reverse Fault (called “Thrust Fault” if shallow angle) Younger (Hanging wall Up) Miners pay geologists to find their lost orebody One friend earned enough to buy a house This poor guy is out of luck What phase of magma fractionation would result in the placement of this ore body? Which formed first, the ore body or the fault? What common mineral is mostly likely in the ore body? Structural Geology is taught by Dr. Krall Typical of convergent margins

42 Evidence of faults a)Visible displacement of rocks b)Pulverized rock and “Slickensides” c) Key beds cut out by faulting reappear elsewhere.

43 Fracture Zones and Slickensides

44 Types of Faults - 2 Strike-slip faults 1 1)Example: San Andreas Transform fault 2)Distinctive landforms (linear valleys, chains of lakes, sag ponds, topographic saddles) 3)Fresh pulverized rock. Transform fault through granite: Arkose sandstone 4)Evidence of Shear stress

45 San Andreas Fault Source: Georg Gerster/Wingstock/Comstock

46 Horizontal Movement Along Strike-Slip Fault

47 Oblique Slip Also seen in Transform Faults such as San Andreas Both strike slip and dip-slip

48 Types of faults Strike-slip faults 2 1)Example: Mid-Ocean Ridge Transform faults 2)Small offsets in ridge 3)San Andreas is also ridge offset, but on a huge scale with a historical twist

49 Faults & Plate Tectonics Divergence Convergence Transform

50 Plate tectonics and faulting Normal faults: mid-ocean ridges and continental rifts are the same thing. Divergent Margins –Surface rock is pulled apart –Hanging wall drops down

51 Horst and Graben Formation

52

53 Graben in Iceland Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.

54 Plate tectonics and faulting Reverse and thrust faults: convergent plate boundaries Hanging Wall is pushed up.

55 Lewis Thrust Fault

56 Lewis Thrust Fault (cont'd)

57 Source: Breck P. Kent PreCambrian Limestone over Cretaceous Shales

58 Plate tectonics and faulting c)Strike-slip faults: Transform Boundaries

59 San Andreas Fault

60 Types and processes of mountain- building (Orogenesis) 1.Volcanic mountains 2.Fold-and-thrust mountains 3.Fault-block mountains 4.Upwarped mountains

61

62 Types of Mountains 2. Fold-and-thrust mountains –Formed by Continent-Continent Collisions

63 Appalachian Mountain System

64 Model for the Evolution of the Southern Appalachians Supercontinent breaks up, rifts apart. Another rift starts moving Africa west. The ocean floor breaks and one side subducts, starting a new island arc. Another Rift Over here somewhere rift

65 Model for the Evolution of the Southern Appalachians (cont’d) The ocean floor breaks again, new subduction adds volcanics to an existing microcontinent Net westward movement pushes the ridge, subduction zone and fragment into N.America Rifting restarts to the East Weak rifts

66 Model for the Evolution of the Southern Appalachians (cont’d) Arc and subduction zone collide w/ N.Am., westward subduction starts The continents collide

67 Model for the Evolution of the Southern Appalachians (cont’d) Rifting Restarts

68 Collisional Mountains ??? (The Grand Tetons in Wyoming) Source: Peter French/DRK Photo Paradigm shifts: What is wrong with our model? More on this later

69 Fault-block mountains Rift Valleys, Mid Ocean Ridges Basin and Range province ??? Normal Fault Blocks as in East Africa Divergent Margins? Paradigm Shifts

70 Origin of the Basin and Range Southwestern North America Looks different Paradigm Shifts

71 Upwarped mountains a)Gently bent without much deformation b)Ascent of buoyant mantle material c) Far from plate boundaries d)Adirondack Mountains: Uplift of deep PreCambrian Igneous and Metamorphic rocks

72 The Adirondack Mountains of Northern New York Source: Clyde H. Smith/Allstock/Tony Stone Images

73 Anticlines and Oil Early USA petroleum exploration, e.g. Pennsylvania anticlines

74 Faults and Oil

75 75 End of Chapter 9


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