Polar Wander and the Cambrian

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Abstract
Joseph L. Kirschvink et al. ([1][1]) propose that true polar wander (TPW) occurred during the Cambrian (520 to 535 Ma). This conclusion is based on (i) apparently high drift rates from Gondwana-Laurentia, (ii) an anomalous pole reading from Siberia, and (iii) reinterpretation of polarity from a Vendian pole reading from Baltica. Their intrepretation of Laurentian TPW ([1][1]) relies exclusively on the contentious Sept Iles Complex result ([2][2]), which yielded two different poles and seven imprecise Rb-Sr ages (586 to 475 Ma). There are no Cambrian data from Baltica ([3][3]); a polarity switch of a Vendian pole ([1][1]) would increase apparent polar wander (APW) path length, but this exercise would have no bearing on Cambrian TPW. Kirschvink et al. ([1][1]) state that the Siberian data are problematic, but this conundrum stems from a single anomalous result ([4][4]). Excluding this one result, the Siberian data define a gentle APW path (Fig. [1][5]A) from Vendian through Ordovician times ([5][6]). ![Figure 1][7] Figure 1 Palaeomagnetic poles from south Siberia ( A ) plotted with confidence ovals along with a spherical spline (APW) path ([5][6]). Numbers are mean poles ages (Ma) using the time scale of Tucker and McKerrow (8). Geological boundaries are marked with bold numbers. An anomalous pole is evident ([4][4]). ( B through E ) Calculated APW rates (centimeters per year), based on APW paths for Laurentia, Baltica, Siberia, and Gondwana. Data sources include ([3][3]) and ([5-7][6]), with Lower Paleozoic ages adjusted to the time scale of Tudur and McKerrow ([8][8]). Inadequate or no data-coverage indicated by wide APW bars and may be entirely interpolated (for example, that of Baltica between 580 and 480 Ma). APW bars within the proposed TPW interval (520 to 535 Ma), which have been said to bracket peak generic diversity ([1][1]), are shaded. A reliable analysis of continental drift-rates or APW rates (Fig. [1][5], B through D) requires a robust mathematical analysis, but Kirschvink et al. ([1][1]) only quote drift-rates between selected poles or group of poles before stating their conclusions. In arguing that spherical spline analysis ([3][3]) masks rapid shifts in pole positions, they incorrectly state that the method averages poles over 15- to 20-Ma intervals; the TPW model ([1][1]) requires that data from all continents must show the same amount of APW between 520 and 535 Ma. Phanerozoic APW rates for Laurentia average 5 cm/year, with peaks in Vendian, Siluro-Devonian, and Jurassic times (Fig. [1][5]B). The Cambrian data show a local minimum. The gap in the Baltic record (580 to 480 Ma; Fig. [1][5]C) requires interpolation beyond reasonable limits and does not shed light on Cambrian TPW. The Siberia data show a local Cambrian maximum (10 cm/year), but considerable higher rates are observed in the Ordovician (Fig. [1][5]D). Analysis of a more complete Gondwana data set [([6][9]) and including all the data of ([1][1])] shows a high Cambrian APW, but the highest peak occurs in the late Cambrian, outside the proposed TPW period (Fig. [1][5]E). None of the examples show convincing evidence for Cambrian TPW, and rather than “stretching” geodynamic mechanisms, one can fit the data better with conventional plate tectonic systematics ([3][3], [7][10]). Thus, the TPW idea ([1][1]), while intriguing, is not supported when the most unreliable data are removed from the database. 1. [↵][11]1. J. L. Kirschvink, 2. R. L. Ripperdan, 3. D. A. Evans , Science 277, 541 (1997). [OpenUrl][12][Abstract/FREE Full Text][13] 2. [↵][14]1. E. I. Tanczyk, 2. P. Lapointe, 3. W. A. Morris, 4. P. W. Schmidt , Can. J. Earth Sci. 24, 1431 (1987); [OpenUrl][15][Abstract][16] ; M. D. Higgins and R. Doig, ibid. 18 , 561 1981). 3. [↵][17]1. T. H. Torsvik 2. et al. , Earth Sci. Rev. 40, 229 (1996). [OpenUrl][18] 4. [↵][19]J. L. Kirschvink and A. Yu. Rozanov, Geol. Mag. 121 , 189 (1984). 5. [↵][20]M. A. Smethurst et al. , Earth Sci. Rev. , in press. Siberian palaeomagnetic data north and south of the Viljuy basin confirm a mid-Palaeozoic counterclockwise rotation (20 degrees) of north Siberia relative to south Siberia, and thus pre-Devonian APWP paths from north and south Siberia (Fig. [1][5]A) must be analyzed independently. From south Siberia, there are many Vendian through Ordovician (590 to 443 Ma) palaeomagnetic poles; most sources are old and often undocumented Russian data, but more recent data on the whole confirm the earlier, and the sometimes mistrusted, old Russian data-set. Stratigraphic sections from Siberia bracket the inferred, TPW period (520 to 535 Ma) with no within-section sign of rapid APW. The Lena River pole data from a study by Kirschvink and Rozanov ([4][4]) remains an outstanding anomaly. One of us (T.H.T.) resampled part of the Cambrian Lena section (1994), but the data were of such poor quality that they were not submitted for publication. 6. [↵][21]J. G. Meert and R. Van der Voo, J. Geodynam. 23 , 223 (1997). 7. [↵][22]1. M. Gurnis, 2. T. H. Torsvik , Geology 22, 1023 (1994); [OpenUrl][23][Abstract/FREE Full Text][24] ; in the Vendian data, we link high APW rates (Laurentia-Baltica) with final breakup of the Rodinia Supercontinent, driving Laurentia toward equatorial latitudes. During Cambrian and Ordovician times, Laurentia was flooded and stationary at equator; thus, there was little APW (Fig. [1][5]B). Conversely, rapid rotation of Baltica combined with a northerly drift-component toward Laurentia is reflected by high Ordovician APW (Fig. [1][5]C). 8. [↵][25]1. R. D. Tucker, 2. W. S. McKerrow , Can. J. Earth Sci. 32, 368 (1995). [OpenUrl][26][Abstract][27] # {#article-title-2} Response: Torsvik et al. question the reliability of the paleomagnetic data used in our report ([1][28]). In discussing the Sept Iles B pole ([2][29]), which they and other workers have previously treated as one of the most reliable Vendian pole...