| Popis: |
Over 95 paleomagnetic pole positions have been determined from North American Precambrian rocks, and they are listed in tabulations by Irving and McElhinny. Using rocks which are isotopically dated, five groups of pole positions can be defined, and these are consistent for rock units from widely separated areas of the North American continent. They are named for convenience (1) Keweenawan trend (age range 1000-1200 my) (2) Mackenzie trend (1200 my) (3) Elsonian trend (1300-1475 my) (4) Animikie-Sudbury trend (1600-2060 my) (5) Matachewan-Stillwater trend (2450-2485 my). An apparent polar wandering path can be constructed between the trends, and this will also incorporate data from rock units of uncertain age e.g. the Belt and Grand Canyon Series. Using an average rate of apparent polar wandering of 1°/5 my during the Phanerozoic for comparison, it suggests that the Precambrian was marked by long quasi static intervals in which the rate of polar shift was much less than 1°/5 my (2485-2140 my, 2060-1600 my, 1475-1200 my) separated by short periods of fairly rapid polar shift of much greater than 1°/5 my (2140-2060 my, 1600-1475 my, 1200-1000 my). |
