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:
Folds Faults and Mountains
Fold and Thrust Mountains Enormous mountain ranges form when plates converge. Contorted rocks show the power of plate tectonics.
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
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.
Folded Sandstone Source: Martin Bond/Science Photo Library/Photo Researchers, Inc.
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.
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
Compression, Tension, and Shearing Stress Convergent Divergent Transform
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
Relation ship Between Stress and Strain Strain can be a change in shape (a deformation) due to an applied stress
Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress
Relationship Between Stress and Strain under high Temps or Pressure
Factors affecting rock deformation Intensity of applied stress Heat –Temperature of the Rock Amount of Time the Stress is applied Rock Composition
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
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
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
Folded Rocks, Hwy 23 Newfoundland, New Jersey Source: Breck P. Kent Adjacent Anticline and Syncline Note highest point
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
Folded Rock Before Erosion
Folded Rock After Erosion Eroded Anticline, older rocks in center. Syncline is opposite.
Topography may be opposite of Structure Anticline Before/After Erosion Notice center rock oldest
Topography may be opposite of Structure Syncline Before/After Erosion Notice center rock youngest
Fold symmetry a)Symmetrical or open folds b)Asymmetrical folds c) Overturned folds d)Recumbent folds e)Plunging folds
Various Folds (cont'd)
Not a good drawing, axial plane should be closer to horizontal
Plunging Folds Nose of anticline points direction of plunge, syncline nose in opposite direction Up End Down End Demo: Plastic box, water, paper folds
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
Again: Strike and Dip
Domes and basins 1.Domes 2.Basins 3.Occur within plates 4.Result from vertical forces 5.Geographic examples (later)
3-D: Dome and Basin
Fractures Fractures - Joints: fractures with no relative movement - Faults: fractures with relative movement
Joints: Fractures – with no movement Source: Martin G. Miller/Visuals Unlimited
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
Source: John S. Shelton Normal Fault: Hanging Wall Down Key Bed Hanging wall overhangs the fault plane Especially common in divergent margins
Normal Fault (Hanging Wall down)
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
Evidence of faults a)Visible displacement of rocks b)Pulverized rock and “Slickensides” c) Key beds cut out by faulting reappear elsewhere.
Fracture Zones and Slickensides
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
San Andreas Fault Source: Georg Gerster/Wingstock/Comstock
Horizontal Movement Along Strike-Slip Fault
Oblique Slip Also seen in Transform Faults such as San Andreas Both strike slip and dip-slip
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
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
Horst and Graben Formation
Graben in Iceland Source: Simon Fraser/Science Photo Library/Photo Researchers, Inc.
Plate tectonics and faulting Reverse and thrust faults: convergent plate boundaries Hanging Wall is pushed up.
Lewis Thrust Fault
Lewis Thrust Fault (cont'd)
Source: Breck P. Kent PreCambrian Limestone over Cretaceous Shales
Plate tectonics and faulting c)Strike-slip faults: Transform Boundaries
San Andreas Fault
Types and processes of mountain- building (Orogenesis) 1.Volcanic mountains 2.Fold-and-thrust mountains 3.Fault-block mountains 4.Upwarped mountains
Types of Mountains 2. Fold-and-thrust mountains –Formed by Continent-Continent Collisions
Appalachian Mountain System
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
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
Model for the Evolution of the Southern Appalachians (cont’d) Arc and subduction zone collide w/ N.Am., westward subduction starts The continents collide
Model for the Evolution of the Southern Appalachians (cont’d) Rifting Restarts
Collisional Mountains ??? (The Grand Tetons in Wyoming) Source: Peter French/DRK Photo Paradigm shifts: What is wrong with our model? More on this later
Fault-block mountains Rift Valleys, Mid Ocean Ridges Basin and Range province ??? Normal Fault Blocks as in East Africa Divergent Margins? Paradigm Shifts
Origin of the Basin and Range Southwestern North America Looks different Paradigm Shifts
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
The Adirondack Mountains of Northern New York Source: Clyde H. Smith/Allstock/Tony Stone Images
Anticlines and Oil Early USA petroleum exploration, e.g. Pennsylvania anticlines