Late Cretaceous True Polar Wander: Not So Fast

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Abstract
Using recalculated paleopoles from seamount anomaly modeling (SAM), Sager and Koppers ([1][1]) proposed an episode of rapid Late Cretaceous true polar wander (TPW). A critical review of the data used, however, suggests that Sager and Koppers have underestimated the effects that errors and data selection have on inferences of TPW and that they may not have adequately taken into account alternative explanations consistent with error sources, modeling uncertainties, and the geology of the Pacific Ocean basin. Further, their TPW hypothesis fails a test based on paleomagnetic data from a well-studied, highly regarded pelagic sedimentary section of the same age. In framing their TPW hypothesis, Sager and Koppers ([1][1]) implicitly challenged a basic tenet of paleomagnetic research: to be considered reliable indicators of the ancient field, magnetizations from rocks of indistinguishable age should yield well-grouped directions. They considerably revised previously published SAM-based pole solutions ([2][2]) and selected data to construct an apparent polar wander path (APWP). The result was two new subsets of data from 84 million years ago (Ma)—which Sager and Koppers ([1][1]) labeled 84E and 84W—that were indistinguishable in age, yet showed greatly different mean directions ([Fig. 1][3]). Sager and Koppers interpreted this pattern not as a reflection of varying data quality but as recording a TPW event. Previous TPW studies ([3][4]) have relied on a fixed-hotspot reference frame that has since been seriously questioned ([4–6][5]). Tarduno and Gee ([4][5]), for example, previously showed that the cumulative 15° to 20° of TPW quoted by Sager and Koppers ([1][1]) is an overestimate; TPW may not have exceeded 5° for the last 200 million years (My). Nevertheless, the TPW hypothesis of Sager and Koppers differs from previous studies, in that the proposed event is so fast (3° to 10° per My) that it cannot be resolved with40Ar/39Ar isotopic age data. This requires that the seamounts that record the TPW inferred by Sager and Koppers formed much more rapidly than the 5 to 7 My that other studies ([7][6],[8][7]) have concluded is typical. ![Figure 1][8] Figure 1 Late Cretaceous paleopoles (triangles) based on SAM. Sidebars indicate seamounts used. Blank denotes a previously identified paleopole deleted in a subsequent calculation; dagger (†) denotes a pole position modified from a previous analysis. N: normal polarity; R: reversed polarity. ( A ) 81 ± 2 Ma pole of Gordon ([11][9]) with 95% confidence interval. ( B ) 82.4 ± 0.7 Ma pole of Sager and Pringle ([2][2]) with 95% confidence interval. ( C ) 84E (83.4 ± 1.6) and 84W (84.0 ± 1.8) poles of Sager and Koppers ([1][1]) with 95% confidence intervals. Arrow indicates change in pole position calculated by Sager and Koppers ([1][1]). Also shown are data from paleomagnetic analyses of basalt samples recovered by ocean drilling: 81.2 ± 1.3 Ma colatitude value from Detroit seamount (solid line) with 99% confidence interval (dotted lines) ([5][10]). The work of Sager and Koppers ([1][1]) was motivated by discrepancies between previously published SAM poles and paleomagnetic data derived from basalts recovered through ocean drilling, especially those from Detroit Seamount ([4–5][5]). The latter are standard paleomagnetic analyses, in which induced magnetizations are excluded and secondary magnetizations are removed through exhaustive laboratory work. SAM paleopole data, in contrast, include induced and secondary magnetizations ([9][11]) and large model uncertainties ([10][12]). The potential importance of these considerations emerges from a close analysis of key data. For example, Sager and Koppers ([1][1]) revised a prior Late Cretaceous (82 Ma) SAM pole ([2][2]) to create the new 84E pole, the 95% confidence interval of which does not overlap with that of the previously published pole ([2][2],[11][9]). If, as Sager and Koppers claim, large biases in the data are unlikely, then we should expect only small, random changes in individual SAM paleopoles. Instead, the changes are mainly to the north, closer to the paleocolatitude constraint from Detroit Seamount ([5][10]) ([Fig. 1][3]C). That tendency suggests either that the modeling in ([1][1]) was parameterized to favor solutions close to those provided by the ocean basalt core paleomagnetic analyses (in which case the SAM paleopoles are not independent measures of Pacific apparent polar wander), or that the SAM data themselves contain systematic errors that bias the final paleopoles. Another example of potential reliability problems in SAM paleopoles lies in three such paleopoles used in prior analyses that were not used in ([1][1]). Sager and Koppers excluded these poles not because of questions about their reliability, but because no new40Ar/39Ar data were available. The reversed polarity of these paleopoles, however, suggests that they should fall on the post–84 Ma APWP, which they do not. These SAM paleopoles probably deviate so greatly from the oceanic-core paleomagnetic data because their reversed magnetizations are heavily contaminated by viscous and induced magnetizations [see, e.g., ([9][11])]. To derive their new 84W paleopole, Sager and Koppers used four seamounts, in two complex areas of crust, for which no new age information is reported. The authors reported that the difference between the 84W and 84E SAM poles had been attributed in previous studies “to microplate rotations in the Musicians and South Hawaiian seamounts” ([1][1]). But the proposed microplates ([12][13]) do not contain the seamounts used to define the new 84W paleopole; instead, these seamount data had been excluded from previous studies because they were thought to be unreliable ([2][2]) or to differ in age. In our view, both possibilities remain viable. One seamount (Kapsitotwa) is in the Line Islands, the archetypal Pacific...