mirrored file at http://SaturnianCosmology.Org/ For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== #[1]next [2]previous [3]up [4]next [5]next [6] up [7] previous _Next:_ [8]A look at the _Up:_ [9]Introduction _Previous:_ [10]Paleomagnetic Poles Apparent Polar Wander - a second look _We are now in a position to compute the location of the paleomagnetic pole assuming that the field has remained dipolar. By hypothesis, the paleomagnetic pole averaged to the geographic North pole when the rock formed. (If measurements are taken from a time period when the field was reversed the paleomagnetic south pole was at the North pole.) By using measurements from a continental block of many different ages, the apparant polar wander path as a function of time can be constructed. The polar wandering path will be the same for all measurements from a single plate and can be used to reconstruct the absolute location of the plate in the past (relative to the geographic poles) though you should note that longitudinal motions of continents will not be detectable because of the assumed axial symmetry of the field._ _Before we begin discussing applications of paleomagnetic poles, we will first consider the ingredients of reliable poles. The generally accepted criteria (see Van der Voo, Rev. Geophys., vol. 28, pp. 167-206, 1990) are based on common sense and the principles outlined earlier in the course. They are as follows:_ $\bullet$ _The age of the formation must be known rather accurately._ $\bullet$ _In order to average out errors in orientation of the samples and scatter caused by secular variation, there must be a sufficient number of individually oriented samples from enough sites. What constitutes ``sufficient" and ``enough" here is somewhat subjective, some suggest a minimum of 24 samples whith $\kappa $ be greater than 10. Some authors also compare $\kappa $ within and between sites in order to assess whether secular variation has been sufficiently sampled._ $\bullet$ _It must be demonstrated that a coherent component has been isolated by the demagnetization procedure._ $\bullet$ _The age of the magnetization relative to the age of the rock should be addressed using field tests such as the fold test or the conglomerate test._ $\bullet$ _There should be agreement in the pole position from similar aged units from a broad region._ $\bullet$ _If a particular pole postion falls on a younger part of the pole path or on the present field, suspicions are aroused._ _ _ _Fig. 6.37: Data compiled by Van der Voo, 1990._ _In the rest of this section, we will consider several uses of paleomagnetic pole data, including constructing APWP for a single continent, contraining motions of tectonic terranes, and testing plate tectonic reconstructions for multiple continents._ _We begin with a look at the APWP of North America. In Fig. 6.37, we have plotted all the poles, regardless of quality, compiled by Van der Voo (1990), for cratonic North America._ _The poles form a smear spreading along arcs from the geographic north pole to south of the equator. This brings up the point that the polarity of an ancient pole can be ambiguous, without a densely sampled track. The oldest poles on the diagram are from the early Cambrian. Given that rock units of increasingly older ages become increasingly rare and the paleomagnetic behavior becomes more difficult with age, it is quite difficult to do paleomagnetism in the Pre-Cambrian._ _ _ _Fig. 6.38: Data compiled by Van der Voo, 1981; 1990._ _Data meeting minimum standards (three or more of the criteria described earlier) were grouped by age. These are plotted as circles in Fig. 6.38. Also plotted are a small selection of so-called ``discordant poles". The discordant poles are plotted as triangles. What is immediately obvious is that the discordant poles do not fall anywhere near the APWP. Most are from western North America and indicate some clockwise rotations (the directions are to the right of the expected directions). When taking into account the age of the formations, many also seem to have directions that are too shallow, indicating northward transport of 1000's of kilometers. The validity and meaning of these discordant directions is still under debate, but it is obvious that most of the Western cordillera is not in situ._ _One of the first uses of paleomagnetic data was as a test of the idea of continental drift. Data from one continent, like those shown in Figure 6.37, could be interpreted as either motion of the continent with respect to a fixed geomagnetic pole, or as motion of the geomagnetic pole with respect to a fixed continent. To test continental drift, then, data from at least two continents are required._ _Below we plot data meeting minimum standards of reliability, for the continents of North America (circles) and Europe (triangles). On the left, the poles are plotted in with respect to present day coordinates and they clearly fall on two separate tracks. This means that either the field was not at all dipolar, or that the two continents have moved not only with respect to the geomagnetic pole but also with respect to each other._ __ _Fig. 6.39: After Van der Voo, 1990._ _Many people who have contemplated the globe have had the desire to fit North and South America against Europe and Africa, closing the Atlantic Ocean. One such attempt, known as the Bullard fit, fits them together using misfit of a particular contour on the continental shelves as a criterion. If we rotate the European poles, using the reconstruction of the Bullard fit, into North American coordinates we get the diagram on the right of Fig. 6.39. Now the curves overlap each other very well, and, if the ages of the poles are also taken into account, they match well too. The agreement is strong support of the continental drift hypothesis and also of the Bullard fit._ _________________________________________________________________ [11]next [12] up [13] previous _Next:_ [14]A look at the _Up:_ [15]Introduction _Previous:_ [16]Paleomagnetic Poles _Lisa Tauxe_ _1998-10-12_ References Visible links 1. file://localhost/www/sat/files/node33.html 2. file://localhost/www/sat/files/node31.html 3. file://localhost/www/sat/files/node1.html 4. file://localhost/www/sat/files/node33.html 5. file://localhost/www/sat/files/node33.html 6. file://localhost/www/sat/files/node1.html 7. file://localhost/www/sat/files/node31.html 8. file://localhost/www/sat/files/node33.html 9. file://localhost/www/sat/files/node1.html 10. file://localhost/www/sat/files/node31.html 11. file://localhost/www/sat/files/node33.html 12. file://localhost/www/sat/files/node1.html 13. file://localhost/www/sat/files/node31.html 14. file://localhost/www/sat/files/node33.html 15. file://localhost/www/sat/files/node1.html 16. file://localhost/www/sat/files/node31.html Hidden links: 17. file://localhost/www/sat/files/node33.html 18. file://localhost/www/sat/files/node1.html 19. file://localhost/www/sat/files/node31.html 20. file://localhost/www/sat/files/node33.html 21. file://localhost/www/sat/files/node1.html 22. file://localhost/www/sat/files/node31.html 23. file://localhost/www/sat/files/node33.html 24. file://localhost/www/sat/files/node1.html 25. file://localhost/www/sat/files/node31.html 26. file://localhost/www/sat/files/node33.html 27. file://localhost/www/sat/files/node1.html 28. file://localhost/www/sat/files/node31.html