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 Source: Martin Bond/Science Photo Library/Photo Researchers, Inc. Folded SandstoneSource: 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 bendingb) Tension causes thinningShearing 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: PencilBrittle failure (it breaks) Demo PencilCauses: 1. subjected to great stress that exceeds the yield point AKA elastic limit, ORSubjected to sudden stress AKA “impact”Plastic deformationDoes not spring back … keeps deformed shapeDemo Chewing gumCause can be high temperature – near meltingor high pressure … squeezed like a ball of clay
11 Relationship 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 stressHeat –Temperature of the RockAmount of Time the Stress is appliedRock Composition
15 Interpreting Deformed Rocks Most apparent in sedimentary rocksImportance of deformationIndicates past plate motionsIndicates other past geological eventsLocates specific natural resourcesMapping 101: Rock orientation: strike and dip
16 Strike and DipStrike is long line, dip is short lineNote the angle of dip given 45oStrike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
17 Folds Folds def: Bends in rock layers Types: synclines and anticlines Syncline (downfold) innermost rocks youngestAnticline (upfold) innermost rocks oldestParts 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 Note highest pointAdjacent Anticline and SynclineSource: Breck P. Kent
19 Folded Rocks (Dorset, England) Center has overturned area OlderOverturnedAreaYoungerLucky we have ways of recognizing right side upWhat are they?Source: Tom BeanYoungerOlder
31 Interpreting FoldsDetermine 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 DipUse nose rules for anticlines and synclines
41 Reverse Fault (called “Thrust Fault” if shallow angle) Typical of convergent margins(Hanging wall Up)Structural Geology is taught by Dr. KrallYoungerWhat 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?Miners pay geologists to find their lost orebodyOne friend earned enough to buy a houseThis poor guy is out of luck
42 Evidence of faults a) Visible displacement of rocks b) Pulverized rock and “Slickensides”c) Key beds cut out by faulting reappear elsewhere.
64 Model for the Evolution of the Southern Appalachians Supercontinent breaks up, rifts apart.riftAnotherRiftOver here somewhereAnother rift starts moving Africa west. The ocean floor breaksand one side subducts, starting a new island arc.
65 Model for the Evolution of the Southern Appalachians (cont’d) The ocean floor breaks again, new subduction adds volcanics to an existing microcontinentWeak riftsNet westward movement pushes the ridge, subduction zone and fragment into N.AmericaRifting restarts to the East
66 Model for the Evolution of the Southern Appalachians (cont’d) Arc and subduction zone collide w/ N.Am., westward subduction startsThe 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 PhotoParadigm shifts: What is wrong with our model? More on this later
69 Fault-block mountains Rift Valleys, Mid Ocean RidgesBasin and Range province ???Normal Fault Blocks as in East AfricaDivergent 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 materialc) Far from plate boundariesd) 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 OilEarly USA petroleum exploration, e.g. Pennsylvania anticlines