magnetic anomaly number

1. magnetic anomaly number Seafloor ages from deep sea drilling versus geomagnetic reversal chronology paleontological age data for Atlantic ocean; similar data from older oceans permit reversal chronology to be calibrated back to 180 Ma age (Ma) from geomagnetic reversal chronology extrapolated in South Atlantic assuming constant rate of spreading

2. Chronology of geomagnetic field reversals recorded on ocean floor Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program)

3. Chronology of geomagnetic field reversals recorded on ocean floor Ocean floor age, millions of years (Ma), determined largely from deep sea drilling (ODP program) magnetic anomaly “number” is a convenient identifier of specific features of the magnetic anomaly profiles that have proven useful for correlation between different profiles.

4. Age of the ocean floor Spatial correlation of magnetic anomalies produced by the seafloor spreading “tape recorder” of reversals of the geomagnetic field” Chronology of reversals of geomagnetic field Map pattern of magnetic anomalies and age of ocean floor

5. Map pattern of magnetic anomaly number ODP drilling site

6. Map pattern of magnetic anomaly number: North Atlantic

8. Map pattern of ocean floor age M10N anomaly no. 5 6 13 18 21 25 31 34 M0 M4 M16 M21 M25 120.4 154.3 180 10.9 From Muller, et al., 1997

11. Continental rifting continental shelf

12. Mid-ocean ridge: rift valley

13. Fracture zones: transform faults

14. -6500 -5500 -4500 -3500 -1500 -2500 7000 1500 3500 -500 500 0 South Atlantic Mid-Ocean Ridge

15. -6500 -5500 -4500 -3500 -1500 -2500 7000 1500 3500 -500 500 0 South Atlantic Mid-Ocean Ridge inactive fracture zone active ridge crest active transform fault

16. Bathymetry of world’s oceans: close correlation with ocean floor age shown in next slide Global topography and bathymetry

17. Ocean floor age from seafloor spreading tape recorder seafloor age, Ma

18. Close correlation with ocean floor age and bathymetry Seafloor bathymetry seafloor age, Ma

19. spreading ridge sea water heat flow by conduction heat flow by conduction depth Heat flow by convection • Simple thermal model for spreading ridge showing • conductive thermal “boundary layer” (= lithosphere) • convecting mantle beneath. • (oceanic crust is not shown)

20. spreading ridge sea water heat flow by conduction heat flow by conduction depth profile A Heat flow by convection temperature The temperature profile B (temperature versus depth) at the ridge crest has a very rapid increase from sea bottom temperatures near 0 C to temperatures of 1300 C near the top of the hot, convecting mantle profile A depth

21. spreading ridge v v sea water heat flow by conduction heat flow by conduction depth x Heat flow by convection profile B temperature At some distance, x, away from the ridge crest, the seafloor has an age = x/v (where v is half the spreading rate). During the time since the plate formed at the spreading ridge, the plate has cooled to form a conductive “boundary layer”. The thickness of the conductive boundary layer increases with time. profile B depth

22. spreading ridge sea water heat flow by conduction heat flow by conduction depth profile A Heat flow by convection profile B temperature temperature • The decrease in temperature due to cooling of the plate as it moves away from the ridge crest leads to • thermal contraction and • consequent deepening of the ocean bottom. • Thus the depth of the ocean increases with the age of the sea floor. profile A profile B depth depth

23. See thermal.pdffor derivation of these curves ocean depth vrs age of ocean floor

24. Thickness of conductive boundary layer (lithosphere) vrs age of ocean floor

25. Continental Margin subduction zone What happened to the sub-oceanic plates older than 200 Ma? Subduction! trench Subducted oceanic lithosphere volcanic arc trench Oceanic crust “island arc” subduction zone Subducted oceanic lithosphere

26. ocean age from seafloor spreading tape recorder seafloor age, Ma

27. 0 Ma EUR NAM AFR SAM

28. 20 Ma

29. 40 Ma

30. 60 Ma

31. 80 Ma

32. 100 Ma

33. 120 Ma

34. 140 Ma

35. 160 Ma

36. 180 Ma The reconstructions are from C.R. Scotese’s PALEOMAP Project

37. Plate motions: the movie Click on the above to play the the “movie”, a file called SFS.MOV. This is a large file (38+ mb); at first it will be slow as the file is downloading, but then you can run it back and forth quite rapidly. It will play faster if you first right-click on the link above, download the file, and then run it from your own machine. Alternatively, you can find the same file (SFS.MOV) on our class folder in the computer lab. The CD that comes with the class text (Stanley) has a lower resolution version of the same reconstruction. The file shows accurate reconstructions of plate positions based on fitting the map patterns of ocean floor ages decoded from the map patterns of magnetic reversals recorded by the seafloor spreading “tape recorder”. The reconstructions are from the PALEOMAP Project (C.R. Scotese) at the University of Texas in Austin.