Paleomagnetically and Tectonically
Based Global Maps for Vendian
to Mid-Ordovician Time
Alan G. Smith
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 α95) 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 axisymmetric 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 m.y. 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 Baltica's 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 this time and the latitudes implied by archaeocyaths there. This discrepancy is tentatively attributed to incorrect age assignments to poles of this age, rather than to a period of rapid true polar wander or some such effect. The 580 Ma map represents the time when Pannotia—a late Precambrian Pangea—is considered to have just started to break up. Laurentia's position, interpolated between mean poles at 520 Ma and