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Structural GeologySpring 2003Structural GeologyStructural geologists are concerned with why parts of the Earth have been bent into folds and others have been broken by faults.Mapping of these structures provides important information to land managers and mineral exploration.Understanding of these features help us understand the dynamic Earth.Plate Tectonics

Tectonic StructuresMost structures are driven by the forces of Plate TectonicsThe kinds of structures are determined by:Temperature and pressureCompositionLayeringAnisotropy or Isotropy of the layersAmount of fluids presentTectonic StructuresTime (or rate of change) is very importanceA rock may behave in a ductile or brittle fashion depending upon how quickly it is deformedTectonic StructuresDuctile deformation produces:FoldsDuctile FaultsCleavagesFoliationTectonic StructuresBrittle DeformationCertain types of foldsBrittle FaultsJointsNontectonic StructuresNontectonic structures can mimic tectonic structuresMeteor impactsLandslidesStructures produce by gravitational forces3-Dimensional ObjectsVisualization of 3-Dimensional Objects

Structural GeologySubdisciplines of Structural GeologyField RelationsMake accurate geologic mapsMeasure orientations of small structures to inform us of the shape of larger structuresStudy the sequence of development and superposition of different kinds of structuresRock Mechanics the application of physics to the study of rock materials.Tectonic and Regional Structural Geology Study of mountain ranges, parts of entire continents, trenches and island arcs, oceanic ridgesApplications of Structural GeologyEngineering IssuesBridgesDamsPower PlantsHighway CutsLarge Buildings AirportsApplications of Structural GeologyEnvironmental IssuesEarthquake hazardLocation of landfill sitesContamination cleanupDistribution of groundwaterMineral explorationScale in Structural GeologyMicroscopic Need magnificationFoliation, Micro foldsMesoscopic Hand specimens and outcropsFoliation, Folds, FaultsMacroscopic Mountainside to map levelsBasins, domes, Metamorphic Core ComplexesScale in Structural GeologyNon-penetrative structures not present on all scalesFaultsIsolated foldsPenetrative structures found on any scale that we chose to studySlaty cleavageFoliationSome foldsScale and Folds

Figure 1-6Fundamental ConceptsDoctrine of UniformitarianismLaw of SuperpositionLaw of Original HorizontalityLaw of Cross-Cutting RelationshipsLaw of Faunal SuccessionMultiple Working HypothesesOutrageous HypothesisFundamental ConceptsPumpellys Rule Small structures are a key to and mimic the styles and orientations of larger structures of the same generation within a particular area.Plate TectonicsDriving MechanismsConvectionPush-Pull TheoryPlate BoundariesDivergentConvergentTransformGeochronologyAbsolute Age DatingReview of atomic structureMost useful isotope decay processes

Using radioactivity in datingReviewing basic atomic structureAtomic numberAn elements identifying numberEqual to the number of protons in the atoms nucleusMass numberSum of the number of protons and neutrons in an atoms nucleus

Using radioactivity in datingReviewing basic atomic structureIsotopeVariant of the same parent atomDiffers in the number of neutronsResults in a different mass number than the parent atom

Using radioactivity in datingRadioactivitySpontaneous changes (decay) in the structure of atomic nucleiTypes of radioactive decayAlpha emissionEmission of 2 protons and 2 neutrons (an alpha particle)Mass number is reduced by 4 and the atomic number is lowered by 2

Using radioactivity in datingTypes of radioactive decayBeta emissionAn electron (beta particle) is ejected from the nucleusMass number remains unchanged and the atomic number increases by 1Using radioactivity in datingTypes of radioactive decayElectron captureAn electron is captured by the nucleusThe electron combines with a proton to form a neutronMass number remains unchanged and the atomic number decreases by 1

Common Types of Radioactive Decay

Using radioactivity in datingParent an unstable radioactive isotopeDaughter product the isotopes resulting from the decay of a parentHalf-life the time required for one-half of the radioactive nuclei in a sample to decay

A radioactive decay curve

Using radioactivity in datingRadiometric datingPrinciple of radioactive datingThe percentage of radioactive atoms that decay during one half-life is always the same (50 percent)However, the actual number of atoms that decay continually decreasesComparing the ratio of parent to daughter yields the age of the sample

Using radioactivity in datingRadiometric datingSources of errorA closed system is requiredTo avoid potential problems, only fresh, unweathered rock samples should be usedBlocking Temperature The temperature below which a crystal lattice traps radioactive daughter products.

GeochronologyMineralSystemDaughterBlocking T CZirconU-Pb207, 206Pb>800GarnetU-Pb207, 206Pb700-725RutileU-Pb207, 206Pb550-650MuscoviteRb-Sr87SrK-sparRb-Sr87SrBiotiteRb-Sr87Sr300HornblendeK-Ar40Ar480BiotiteK-Ar40Ar300MuscoviteK-Ar40Ar350GeochronologyUranium-Lead Method (U-Pb) Most reliable technique for rocks Ages exceed 10 million yearsUse of Zircons for dating

238U 206Pb (half-life = 4.5x109yrs)235U 207Pb (half-life = 0.7x109yrs)232Th208Pb (half-life = 1.4x109yrs)

Uranium-Lead Method

Uranium-Lead Method

GeochronologyRobidium-Strontium (Rb-Sr)Most applicable in rocks over 100 million years oldWhole-rock ages are more reliable in Rb-SrNo gaseous daughter elementsPrinciple source of error is later metamorphism and hydrothermal alteration.

87Rb87Sr + (half-life = 48.8x109yrs)GeochronologyPotassium-Argon (K-Ar) Used for rocks around 1 million years oldAr is a gas and can be easily released from most rocksBiotite, muscovite, hornblende retain argon better than other minerals Low blocking temperatures (300C - 480 C)

40Ca + 40K (half-life = 1.2x109yrs)40Ar

GeochronologyArgon-Argon (40Ar-39Ar) Samples must be irradiated to convert 39K to 39ArCan determine the cooling history of the rocksUseful for determining the time of uplift, metamorphism, or emplacement of structures

GeochronologySamarium - Neodynium (Sm-Nd)Used mainly for dating ocean floor basalts because sea water is abundant in Sr but depleted in NdTherefore, can be used to determine contamination by sea water and hydrothermal alteration

147Sm143Nd (half-life = 106x109yrs)Rock Cycle