1. Undergraduate opportunities (1) Web design: "Structural geology of southwest U.S. and northwest Mexico" preceptorship vs. paid? Contact: Stuart Glogoff Manager, Distributed Learning Projects stuartg@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 stuartg@u.arizona.edubutler@geo.arizona.edu

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

3. TODAY Deformation, Metamorphism, and Time A 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- thermobarometry To 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. amphibolite: hornblende, maybe garnet mod. to high T

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

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

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

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

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

26. 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 beam Isotopic abundances (more often, ratios) are measured with a mass spectrometer

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

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

29. 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".

30. THERMOCHRONOLOGY: determining the time when a rock was at a certain temperature

31. Calculated cooling history for a granite in New Zealand

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

33. Geographic Setting

34. Regional Geologic Setting

35. Geometry

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

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

38. Footwall rocks include blueschists + greenschists and amphibolites.

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

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

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

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

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

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

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

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

47. Tectonic significance