Rock magnetizations that are most useful for paleomagnetism Detrital Remanent Magnetization (DRM)

1. Rock magnetizations that are most useful for paleomagnetism • Detrital Remanent Magnetization (DRM) • formed during or soon after deposition of sediments • locked in by compaction and lithification to sedimentary rock • relatively weak • Thermo-remanent Magnetization (TRM) • formed in basic igneous rocks (e.g., basalt) upon cooling through Curie temperature • locked in upon further cooling • very strong

2. “ferromagnetic” minerals: Ternary diagram of the iron-titanium oxide solid solution magnetic minerals

3. Paleomagnetic measurements • Collect many suitable samples (e.g. basalt) with in situ orientation determined

4. Paleomagnetic measurements • For each sample measure stable component of magnetization intensity and direction (declination, D, and inclination, I) in laboratory (“cleaning” samples by removing unstable superimposed magnetizations. • alternating current (AC) demagnetization • thermal demagnetization

5. Paleomagnetic measurements • Determine VGP locations, average VGP for specified time in past, and error

6. Fisher statistics for VGP’s • Consider each VGP as the intersection of a unit vector at the center of the earth with the earth’s surface. The latitude and longitude of the VGP can be transformed into a cartesian coordinate system at the earth’s center, with the x axis pointing to 0N, 0W, the y axis pointing to 0N, 90E, and the z axis pointing to the North Pole. • For a set of N VGP’s., one can add up the unit vectors and find the resultant direction. The intersection of this resultant vector with the earth’s surface is the best estimate of the average VGP for the N data. • The magnitude of the resultant vector, say, is L. If all the data pointed in the same direction (same VGP location), then L = N. Otherwise, L is less than N, and is the key measure of the clustering of the data about the average. • Precision parameter K = (N-1)/(N-L) • angular standard deviation S = 81°/sqrt(K) • 95% cone of confidence about the mean vector is a small circle centered on the mean vector with angle (measured from the mean vector) given approximately by • a95= 140°/sqrt(KN)

7. Paleomagnetic measurements • Plot time sequence of VGP locations on equal area projection of earth

8. Apparent polar wander paths for North America and Europe

9. N Rotation on a sphere W E S This is a projection of the earth’s surface, looking from the outside downwards, with the axis of rotation intersecting the surface at the pole of rotation shown by the green circle. Again, the great and small circles are NOT geographic coordinates, but are arrayed relative to the pole of rotation.

10. N W E S Examples of two abitrary great circles emanating from the pole of rotation. These two great circles represent a rotation of 20 degrees about the pole of rotation. For positive rotation, the left hand great circle 1 rotates to the right hand great circle 2. 1 2

11. N W E S Examples of two abitrary small circles (perpendicular to the great circles) are shown in blue. For a rotation about the pole, all small circes concentric to the pole give possible paths for rotation about the pole.

12. N W E S Point, P1, moves to position shown by P2 as a result of the 20 degree rotation about the pole of rotation. P1 P2

13. N W E S Pole of rotation Now we show the situation in a geographic framework, with the pole of rotation located somewhere at mid-latitudes in the northern hemisphere. Again, we are looking down on the earth, with the grid now representing latitudes and longitudes. P2 P1

14. Re-assembling continental fragments T0 to T4

15. Re-assembling continental fragments T0 to T4

16. Re-assembling continental fragments T0 to T4 T4 to T8

17. Re-assembling continental fragments T0 to T4 T4 to T8 Note that the VGP’s are rigidly connected to the continental fragments, and are rotated along with the fragments A

18. Re-assembling continental fragments T0 to T4 T4 to T8 Note that the VGP’s are rigidly connected to the continental fragments, and are rotated along with the fragments A

19. Closing the North Atlantic Ordovician to Jurassic (500-200 Ma) “Apparent polar wander” (APW) paths for North America and Europe Europe rotated by 38 degrees about rotation pole at 88.5N, 27.7E

20. 65 My 600 My 4600 My Geologic time scale http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

21. 600 My Geologic time scale http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

22. Closing the North Atlantic Ordovician to Jurassic (500-200 Ma) “Apparent polar wander” (APW) paths for North America and Europe Europe rotated by 38 degrees about rotation pole at 88.5N, 27.7E

23. Errors (a95cones of confidence)

24. 65 My 600 My 4600 My Geologic time scale http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

25. seafloor tape recorder

26. 65 My 600 My 4600 My Geologic time scale http://www.geo.ucalgary.ca/~macrae/timescale/timescale.html

27. Paleozoic Continental Drift • Paleomagnetic VGP’s and APW’s • Geological structures • Paleo-environments and paleo climatic zones • See continental reconstructionsfrom PLATES project of the University of Texas