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Announcements Bad news: It's not Thanksgiving yet

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Announcements Bad news: It's not Thanksgiving yet

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  1. Undergraduate opportunities(1) Web design: "Structural geology of southwest U.S. and northwest Mexico"preceptorship vs. paid?Contact:Stuart GlogoffManager, Distributed Learning Projectsstuartg@u.arizona.edu (520) 626-5347(2) -Assist in paleomagnetic laboratory, Geosciences- Paid position, start ASAP- Contact Dr. Bob Butler (butler@geo.arizona.edu); 621-2324- second-year student preferred

  2. AnnouncementsBad news: It's not Thanksgiving yet Good news: NO CLASS ON WED.

  3. TODAYDeformation, Metamorphism, and TimeA major goal of structural geologists: to decipher magnitude and timing of deformation- history!How much and when were rocks buried to depth?When were rocks deformed?When were rocks metamorphosed?When were rocks brought up from depth (exhumed)? How fast?How did this all happen?

  4. To get at displacement on BIG structures- need to know depths/temperatures from which rocks were brought up- thermobarometryTo get at timing- need geochronology and thermochronology

  5. Geothermal gradient: T increase with depth

  6. Geothermal gradients in different tectonic regimes

  7. Some rocks get subducted deep into the mantle- ultra-high pressure metamorphism and diamonds

  8. An introduction to metamorphic facies mineral assemblages in rocks vary as a function of pressure, temperature, composition, and fluid comp.

  9. greenschist "low grade": chlorite, epidote, actinolite

  10. mod. to high T amphibolite: hornblende, maybe garnet

  11. granulite: two types of pyroxenes--- very high T

  12. High-P, Low-T blueschist:glaucophane, jadeite, kyanite, lawsonite

  13. eclogite: garnet + pyroxene High T and P

  14. Folds in eclogite: green (pyroxene) and red (garnet) layers

  15. Mineral assemblages can give range of P-T conditions. But we want to do better!! HOW?

  16. Thermobarometry: Quantitative determination of temperature (T) and pressure (P) using equilibrium reactions

  17. Example: kyanite, andalusite, and sillimanite have same composition but different crystal structure- function of T and P

  18. One reaction yields one line. To determine a T and P point, at least one other reaction is needed

  19. Fortunately, there are tons of reactions that are useful for constraining T and P

  20. A real example- with real uncertainties The concept of a P-T path and zoned minerals

  21. P-T paths for deeply buried, then exhumed rocks

  22. Linking Deformation with Metamorphism

  23. So far, we known how to determine P and T and timing of metamorphism relative to deformationWhat about precise timing??Exactly when? How fast or slow?

  24. Isotopes: Elements with different numbers of neutronsRadioactive isotopes: are unstable- they decay with time to another isotope. This decay rate has been constant throughout the history of the Universe.

  25. isotopes can be removed from mineral grain(s) by many methods: dissolved out using acids, burned out in a furnace, blasted out using a laser, or tickled out using an ion beamIsotopic abundances (more often, ratios) are measured with a mass spectrometer

  26. In a mass spectrometer isotopes of different masses are separated using a magnet and collected & counted

  27. With modern technology, it is possible to determine ages for little spots in a single grain. Way cool!!

  28. Also cool, is that different minerals loose daughter products due to diffusion at different temperatures. Some minerals like to keep the daughter products, even at high T. Other minerals loose daughter products, even at low T. Closure temperature: Temperature below which a mineral will not loose daughter products. At higher T, daughter products will "run-away".

  29. THERMOCHRONOLOGY: determining thetime when a rock was at a certain temperature

  30. Calculated cooling history for a granite in New Zealand

  31. An attempt at putting it all together (structure, metamorphism, and time)- an example from Tibet

  32. Geographic Setting

  33. Regional Geologic Setting

  34. Geometry

  35. Fault places low-grade limestones on top of a ledge of cataclasite (fault rock)

  36. Kinematics Structural studies suggest that the fault is a normal fault, where the hanging wall moved to the east relative to the footwall

  37. Footwall rocks include blueschists + greenschists and amphibolites.

  38. But more precisely what P and T are the blueschists?

  39. Yikes! Thermobarometry suggests ~500 C at 14 kbar (50 km!!)Did the normal fault exhume the blueschists from this great depth?

  40. Mylonites in the footwall of the normal fault are amphibolite facies.

  41. Here's what they look like under the microscope

  42. The shear zone was active at ~11 kbar (~40 km)- probably cuts the entire crust!

  43. The fault cuts granites and the shear zone is intruded by undeformed granite

  44. Timing When was the fault active? before 204 Ma and after 220 Ma

  45. Thermochronology suggests rocks were exhumed from >35 km depth in <10 Ma!!!!!!!!!

  46. Tectonic significance

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