tailieunhanh - The Ecology of the Cambrian Radiation - Andrey Zhuravlev - Chapter 2

CHAPTER TWO Recent revisions to the early Paleozoic time scale have been used to recalibrate ages assigned to stratigraphically dated paleomagnetic poles of that era. In particular, a value of 545 Ma has been used for the base of the Cambrian. Selected poles have then been used to derive apparent polar. | 02-C1099 8 10 00 2 02 PM Page 9 PART I The Environment 02-C1099 8 10 00 2 02 PM Page 10 02-C1099 8 10 00 2 02 PM Page 11 CHAPTER TWO Alan G. Smith Paleomagnetically and Tectonically Based Global Maps for Vendian to Mid-Ordovician Time Recent revisions to the early Paleozoic time scale have been used to recalibrate ages assigned to stratigraphically dated paleomagnetic poles of that era. In particular a value of 545 Ma has been used for the base of the Cambrian. Selected poles have then been used to derive apparent polar wander paths APWPs for the major continents Laurentia Baltica Siberia and Gondwana for late Precambrian to Late Ordovician time. The scatter of the paleomagnetic data is high for this interval and the number of suitable Precambrian poles is very low with confidence limits expressed as a95 commonly 20 and occasionally 40 . The scatter is attributed to noisy paleomagnetic data rather than to any non-uniformitarian effects such as large-scale true polar wander significant departures from the geocentric axisym-metric dipole field model very rapid plate motions and the like. There is a clear need for many more isotopically dated poles of late Precambrian to Cambrian age from all the major continents. The data from Laurentia are considered the most reliable. Maps have been made for 620 460 Ma at 40 . intervals. For the 460 Ma map the orientation and position of all the major continents have been determined by paleomagnetic data the longitude separation has been estimated from tectonic considerations. The 500 Ma map has been similarly constructed except that Balticas position has been interpolated between a mean pole at 477 Ma and its position on a visually determined reassembly at 580 Ma Pannotia . The 540 Ma map is interpolated between the positions of Gondwana Baltica and Siberia at 533 Ma 477 Ma and 519 Ma respectively and their position in Pannotia. There is a significant difference between the paleomagnetically estimated latitude of Morocco at .