Geology 355 Week 9 notes
Geology 355 Week 9 notes Geology 355
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This 9 page Class Notes was uploaded by Brandon Cook on Tuesday October 18, 2016. The Class Notes belongs to Geology 355 at University of South Carolina - Columbia taught by Dr. J Knapp in Fall 2016. Since its upload, it has received 3 views. For similar materials see Structural Geology in Geology (GEOL) at University of South Carolina - Columbia.
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Date Created: 10/18/16
Geology 355 Notes October 11, 2016 Geochronology PowerPoint on Appalachians will be on Blackboard Susie will be available from 2 to 4:30 Fall Break!!! Deformation Metamorphism and Time (chapter 13) Metamorphic Facies Predictable set of mineral comp. From P and T Aluminosilicate triple jct. Polymorphs but same exact composition. Al is highest in clay minerals rocks started as siltstone or claystone Garnet is one of the most important mineral: large Stability range rep. In many conditions. Look at comp. From P and Temp difference -Metamorphic Fabrics Pre-tectonic (pre-kinematic) Syn-tect. (syn-kinematic) Post-tect. (post-kinematic) I-Clicker Question If we find a shale (pelite) metamorphosed blueschist facies conditions, we should expect to find blue minerals such as glaucophane. True False (not formed in shales but in basalts) Petrologic Terminology Porphryoclast-relict large crystal in a metamorphic rock. Large crystal predated meta. Porphyroblast-large crystal grown during metamorphism. (Garnets) rotating during metamorphism. Poilkiloblast-overgrowth of a mineral around small inclusions of other minerals Schistosity- layering developed during metamorphism Foliation planar fabric strike and dip Measure (foliation of Appl. mtns NE strike and SE dip) Geochronology (time of earth study--study of time on earth) Tell time relative time A < B< C a before B Absolute time (talk about how to put geologic age and number to an event) Not until radiometric dating/ radioactivity, before then, the use of fossils Isotopic dating Radioactive elements (parents) decay to stable (daughters) non-radioactive elements The rate at which this decay occurs is constant and known If we know the rate of decay and the amount present of parent and daughter, calculate how long this reaction had been occurring. Compare to decay constant chemical mineral put in a certain point Atoms of elements w/ the same # of p and varying # of neutrons (n) Z = atomic number # of protons N= neutron number # of neutrons A= mass number= sum of protons and neutrons Is Z+N=A Examples: 235U, 238U, 87Sr, 86Sr, 14C, 12C ( C is known as carbon-dating) Types of Radioactive Decay (know this!) -Alpha Decay: loss of a 4He (2n, 2p) -e.g. 147Sm decays to 143Nd (dating materials extracted from Earth’s mantle) -Beta Decay: n to p -e.g. 87Rb decays to 87Sr (not electron in outer shell) shoot electron out of nucleus Electron capture: p to n - 40K decays to 40Ar (no change in atomic mass but changing the element) -important radioactive decay for Geology as K is in alot of different minerals Half Life The time half of the parent to decay to daughter (is constant) Half-lives for different isotopes have a wide range 1 half-life is ½ 2 half-lives is ¼, to ⅛ so on 5 half-lives is 1/32 U238 U235 has long half-lives decay to Lead U235 half-life is 4.5 by U235 half-life is .7 by (Zircons and Apatites) (Zircons crystalizes in magma tough don’t weather away- limited amount) K40 to Ar40 is 1.3 billion yrs half-life (electron capture) C14 to N14 5730 year half-life (used in dating biological, water from dissolved carbon dioxide, and artifacts <70,000 years) Rb87 to Sr87 half-life is 47 billion years Closure Temperature Above a certain temperature (Tc), elemental species will diffuse out of the mineral structure If parent and /or daughter atoms are lost, the radiometric clock will not be accurate. Low closure temp calculated since last above the last closure temperature--- key in metamorphicism function of T. We calculate the time since the mineral was last above Tc for the particular element on interest Rb-Sr System Decay to Sr from Very long half-live 48 to 49 billion years Usually measured several samples of the whole rock and plagioclase I-Clicker Question 2 The decay of 87Rb to 87Sr , involving the change …. Alpha Decar Beta Decay Gamma Decay Electron Capture U-Ph System Typically date individual U-bearing minerals: -Zircons Analyze different pop. of these mineral, which record different amounts o K-Ar System Modified in the 1970s to the Ar-Ar system to date moon rocks 40Ar 39Ar System Measure the ratio of these two isotopes in stepwise increments Analyze separate mineral fractions from same rock Hornblende Biotite Muscovite K-Feldspar Each mineral has a diff. closure temp which respect to Ar Fission Track dating When radioactive decay takes place, damage occurs to the crystal lattice of minerals With time, these fission tracks will heal or anneal By counting the # of tracks in a mineral, we can place quantitative estimates on the time at which mineral has been below the annealing temperature Different minerals heal at different temperatures and over different timescales I Clicker question #3 If the horizontal lines in the diagram at the right represent schistosity, the metamorphism that….. Pre-kinematic Syn-kinematic Post-kinematic All of the above Lecture 16 Brittle Deformation October 18, 2016 Faults and Faulting iClicker Question #1 Within the Appalachian orogen, rocks of igneous origin are found primarily within The Piedmont B) Blue Ridge C The Valley and Ridge D The Appalachian Plateaus DEcompression Melting - Mid Ocean Ridges high to low pressure melting Add volitles (fluid) melting occur- Subduction zones water forced hydrates to overlaying wedge and melts iClicker Question #2 The geometry of faults within the fold and thrust belt of a collisional orogen are influenced by: Lateral continuity of the stratigraphy Discontinuities within the foreland stratigraphy Regional stress fields All of the above Faults and Faulting-Chapter 8 VDP&M pg 166-201 Different Kinds of Faults Reverse, Normal, Strike-Slip Discontinuous Isotopes Different mass numbers (p + n) of the same elements Radioactive decay mechanisms Alpha decay (He expulsion) fission tracks Beta Decay (n converted to p) e- out of nucleus Electron capture (p to n) Geochronologic systems Rb-Sr U-Pb K-Ar, Ar-Ar iClicker Question #3 Which of the following isotopic system would be best suited for dating an undeformed granitic dike which intrudes a deformed sequence of Paleozoic sedimentary rocks? 14C Rb-Sr Ar-Ar U-Pb Rock Rheology Elastic behavior-recoverable strain Plactic behavior - progressive breaking Brittle Deformation Permanent change in solid material due to growth of fractures and/ or sliding on those fractures Includes fractures, joints, faults, shear zones, veins Rock is breaking is brittle deformation Faults Basic Terminology For non-vertical faults two description blocks on either side of fault (strike and dip) Hanging wall - hangs over the head of a miner Footwall- the block on which the miners feet were located Draw fault- never use a ruler, faults are not perfect planes; irregular surfaces with main faults and splays Ductile shearing in the middle crust with higher temperature and pressure Planar-a flat planar surface Listric- shovel shaped fault dips becomes shallower with depth (listron) Deeper you go more confining pressure and higher temperature; change in temp and pressure of the crust deeper down. Steepening downward- or convex up Subduction zones (not terribly common) Anastomosing-numerous branching irregular traces (braiding pattern) 3-D Fault Geometry In 3 D, faults are irregular surfaces. All faults have a point at which Displacement falls to 0 They are truncated by /intersect with another fault They intersect with Earth’s surface iClicker Question #4 Faults are usually well-behaved planar features in 3-D, and appear as straight lines True False Tip line- where fault displacement goes to 0 Branch Line- the line along which one fault intersects with or branches off of another fault Surface trace - the line of intersection between fault surface and the land surface (mapping faults you see on surfaces) If not appearing on surfaces known as “blind fault” only appears in subsurfaces Net slip have to have some geologically naturally point object split in half- some line line trough offset to restore a line (where two planes meet) dike with intersection of bedding and could determine the direction and mag. of net slop Apparent and Real Displacement The displacement of one block relative to another is known as slip vector (very rare) This vector connects two points which were originally adjacent We can get similar info from a linear feature that is offset by fault “piercing point” Intersection of dike and bed Fold axis Intersection of bed w/ unconformity Fault Separation Infinite number of possible sips produce a given separation (images on Blackboard) ( on maps, ornaments are on hanging wall) Three Types of Faults: Dip Slip Normalhanging wall moves down with respect to footwall horizontal extension. Juxtaposes younger rocks over older rocks Reversehanging wall moves up with respect to footwall. Results in horizontal shortening. Juxtaposes older rocks over younger rocks. Low angle 45 degrees high angle for both normal and reverse StrikeSlip Rotational
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