Inertial Interchange True Polar Wander Research Articles

New paleomagnetic data from the sedimentary cover of the Tandilia System: Further geodynamic or geomagnetic complexities in the Late Ediacaran

Anomalous paleomagnetic data have been found worldwide during the Ediacaran period, giving rise to several non-actualistic hypothesis. In order to get more information about this period, paleomagnetic, magnetic fabric and rock magnetic studies were carried out in the Avellaneda Formation (∼570–560 Ma) from two drill cores of the Alicia quarry in the Olavarría area of the Tandilia System, in the Río de la Plata craton (Argentina). Anisotropy of magnetic susceptibility studies indicate a pre-tectonic origin for the magnetic fabric of this Formation. Rock magnetic studies suggest the presence of magnetite and hematite in different proportions as the main ferromagnetic minerals carrying the remanence. After stepwise thermal demagnetization, two different characteristic remanence directions were obtained for the same unit, one corresponding to the marls (“b1”) of the lower section of the Avellaneda Formation, and the other from the claystones (“b2”) of the upper section of this unit. These results were combined with the remanence directions obtained by Franceschinis et al. (2022) for the same unit at La Cabañita quarry, located 10 km away. This procedure allows the calculation of two paleomagnetic poles for the Avellaneda Formation. The AV1 pole is located at: 2.0° S, 311.1° E, A95: 5.0°, N: 58 while the AV2 pole is at: 3.3° N, 348.9° E, A95: 11.7°, N: 7. These results confirms that the Rio de la Plata craton also presents anomalous paleomagnetic data during the Ediacaran, implying extremely fast movements in very short periods of time. This can be interpreted as evidence of two inertial interchange true polar wander events during this time, as was already proposed by other authors. An alternative possibility suggests a non-actualistic behaviour of the Earth Magnetic Field, switching from axial to equatorial positions, with an intermediate stable position between them. The likelihood and implications of these two hypotheses is discussed.

The Ediacaran apparent polar wander path of the Río de la Plata craton revisited: Paleogeographic implications

The Ediacaran apparent polar wander path (APWP) for the Rio de la Plata Craton was analyzed and a new alternative path is presented. This revised path was constructed considering an opposite polarity for poles older than ca. 590 Ma. This path is more consistent with that recently proposed for West Africa, whose large oscillations were attributed to two events of inertial interchange true polar wander (IITPW). A compilation and selection of Ediacaran paleomagnetic data from the main cratons were analyzed leading to a set of global paleogeographic reconstructions throughout the Ediacaran. This model assumes that a “Clymene Ocean” existed between Central Gondwana and West Africa – Amazonia along this period. All cratons with reliable paleomagnetic information, share similar motions during two time-intervals. The first one (615–590 Ma) can be described by rotation around an Euler pole located in the equator, while the second one (575–565 Ma) by another around an Euler pole on the tropics. Whether these apparent large and fast displacements can be assigned to IITPW events is discussed along with the consistency of considering a large Clymene Ocean during this time.

New palaeoproterozoic palaeomagnetic data from Central and Northern Finland indicate a long-lived stable position for Fennoscandia

SUMMARYThe Svecofennian gabbro intrusions coincide temporally with the global 2100–1800 Ma orogens related to the amalgamation of the Mesoproterozoic supercontinent Nuna. We provide a new reliable 1891–1875 Ma palaeomagnetic pole for Fennoscandia based on rock magnetic and palaeomagnetic studies on the Svecofennian intrusions in central Finland to fill gaps in the Palaeoproterozoic palaeomagnetic record. By using the new pole together with other global high-quality data, we propose a new palaeogeographic reconstruction at 1885 Ma. This, together with previous data, supports a long-lived relatively stable position of Fennoscandia at low to moderate latitudes at 1890–1790 Ma. Similar stable pole positions have also been obtained for Kalahari at 1880–1830 Ma, Siberia at 1880–1850 Ma, and possibly India at 1980–1775 Ma. A new reconstruction at the beginning of this period indicates the convergence of several cratons at 1885 Ma in the initial stages of the amalgamation of the Nuna supercontinent at low to moderate latitudes. The close proximity of cratons at low to moderate latitudes is further supported by global and regional palaeoclimatic indicators. Stable position of several cratons could indicate a global period of minimal apparent drift at ca. 1880–1830 Ma. Before this period, the global palaeomagnetic record indicates large back-and-forth swings, most prominently seen in the high-resolution 2020–1870 Ma Coronation loops of the Slave craton. These large back-and-forth movements have been explained as resulting from an unstable geomagnetic field or basin- or local-scale vertical-axis rotations. However, the most likely explanation is inertial interchange true polar wander (IITPW) events, which is in line with the suggestion of large amplitude true polar wander events during the formation of the supercontinent.

Timing the break-up of the Baltica and Laurentia connection in Nuna – Rapid plate motion oscillation and plate tectonics in the Mesoproterozoic

Meso– to Neoproterozoic geological and paleomagnetic data support a direct connection between Baltica and Laurentia in both Nuna and Rodinia supercontinents, however in different relative configurations. Previous paleomagnetic data limit the time of break-up of Nuna core configuration and ca. 90° rotation of Baltica relative to Laurentia between 1.27 Ga and 0.99 Ga. Despite the well documented relative motion of continents, the tectonic mode during the Meso– to Neoproterozoic has been questioned and the operation of single lid tectonics at 1.6–1.0 Ga during the Nuna-Rodinia supercontinent cycles has been proposed.In this study, new paleomagnetic and whole rock 40Ar-39Ar geochronological data from a basic dyke in Stugun central Sweden are combined with coeval data to produce a 1.22 Ga (1.244–1.200 Ga) moderate-quality paleomagnetic pole. This is done to better estimate the break-up time of the core of Nuna and explore the plate tectonics at the Mesoproterozoic. The new pole fills part of the 1.247–1.140 Ga gap in the paleomagnetic record of Baltica. By comparing apparent polar wander paths (APWPs) and calculated drift velocities, a break-up of Baltica and Laurentia at 1.12–1.04 Ga is suggested. Plate velocities calculated for Laurentia, Baltica and Siberia for the time of the Nuna supercycle are similar and low to moderately high corresponding with the present-day tectonic speeds. Furthermore, the obtained velocity peaks may be related with onset of the break-up of the Nuna supercontinent, the break-up of the direct Baltica–Laurentia connection in Nuna and nascent Rodinia. We suggest that the velocity peaks and large oscillating shifts in late Mesoproterozoic pole positions for Laurentia and Baltica result from a combination of relative plate motion and inertial interchange true polar wander (IITPW) events. Both IITPW events and relative plate motions argue for an operation of plate tectonics in the Meso– to Neoproterozoic.

Paleomagnetic survey of the Goulburn Supergroup, Kilohigok Basin, Nunavut, Canada: Toward an understanding of the Orosirian apparent polar wander path of the Slave craton

The Orosirian paleopoles from the three circum-Slave basins (i.e., the Great Slave, Coronation, and Kilohigok Basins) of the Slave craton show large (∼110°) and back-and-forth swings at 1.96–1.87 Ga, known as the Coronation loops. The Coronation loops, taken at face value, would imply rapid and substantial spin motions of the Slave craton, which is at odds with modern plate tectonics. Alternatively, the Coronation loops have been interpreted as a product of basin-scale rotations, local-scale vertical-axis rotations, or inertial interchange true polar wander (IITPW). One way to differentiate these models is to take advantage of the well-correlated stratigraphy in three circum-Slave basins and directly compare the time-equivalent paleomagnetic results. In this study, we collected ∼300 samples from nine formations from the Goulburn Supergroup of the Kilohigok Basin, in shallowly dipping autochthonous sections east of the Bathurst Fault. We provide seven new reconnaissance-level paleopoles of the Slave craton, namely from the Kenyon, Hackett, Rifle, Beechey, Link, Kuuvik, and Brown Sound Formations of the Goulburn Supergroup. Our results and the compiled Orosirian paleomagnetic data of the Slave craton suggest that although basin-scale rotation or local vertical-axis rotation in fault zones are able to explain some of the disagreements among time-correlative paleopoles, they could not account for the large declination variation within the homoclinal sections in individual basins. Notably, our results from the ∼1963 Ma Rifle Formation show progressive changes in declination through the stratigraphy, which cannot be explained by either basin-scale or local vertical-axis rotations. Selective remagnetization is also considered unlikely to be the cause. Instead, we suggest that IITPW could potentially be responsible for the Coronation loops, which could also provide an explanation for some discrepant paleomagnetic data observed globally during the Orosirian time.

Constraints on the Ediacaran inertial interchange true polar wander hypothesis: A new paleomagnetic study in Morocco (West African Craton)

We conducted a paleomagnetic study on late Ediacaran and early Cambrian volcanic deposits and lava flows from the Ouarzazate and Taroudannt groups in the Anti-Atlas, Morocco. Four stable magnetic mean directions have been isolated using thermal demagnetization technique. A first component “A” is most probably a remagnetization acquired during the Hercynian related tectonic phases, and yields a paleomagnetic pole (λ=−29.3° N, ϕ=56.6° E, A95=4.1°) indistinguishable from the 330–300Ma segment of the Gondwana apparent polar wander path (APWP). The Cambrian Djebel Boho formation of the Taroudannt group and the Ediacaran Tadoughast and Fajjoud formations of the Ouarzazate group yield two poles of similar directions respectively “B1” (λ=21.9°N, ϕ=31.0°E, A95=15.6°) and “B2” (λ=27.3°N, ϕ=27.1°E, A95=14.9°). In the Adrar-n-Takoucht formation (oldest part of the Ouarzazate group), another component is observed, yielding a paleomagnetic pole “C” (λ=−57.6°N, ϕ=295.6°E, A95=15.7°). The primary nature of these last three magnetizations is supported by paleomagnetic tests and we interpreted them as magnetizations acquired during or shortly after the deposit of volcaniclastics or emplacement of lava flows. Using these new data, we reappraised the West African Craton (WAC) APWP and found two large paleomagnetic shifts of some 90°, the first one between 615 and 571Ma and the second one between 571 and 565Ma. A comparison with already published paleomagnetic data from Laurentia, Baltica and Gondwana, along with a discussion on their paleogeographic relationships, showed that our WAC APWP loop is likely recorded in the APWP’s of these blocks. The superimposition of these APWP’s leads to a geologically consistent paleogeography which may be explained by two rapid inertial interchange true polar wander even if we cannot exclude that magnetic field perturbations may have also occurred during the Ediacaran.

Palaeomagnetism and geochronology of mid-Neoproterozoic Yanbian dykes, South China: implications for ac.820–800 Ma true polar wander event and the reconstruction of Rodinia

AbstractWe present new palaeomagnetic data from two generations of mafic dykes in the Yanbian region of the western South China Block, dated by the zircon U–Pb method at 824±6 and 806±8 Ma, respectively. The primary origin for the characteristic remanent magnetizations is supported by a positive baked contact test, a dyke-tilt test and rock magnetic data. After tilt corrections, 10 dykes from the c. 824 Ma group gave a mean remanent direction of D=230.1°, I=−72.6° with k=16.3 and α95=12.3°, corresponding to a palaeopole at 42.5 °N, 131.8 °E with A95=19.0°. Three dykes from the c. 806 Ma group give a mean direction of D=284.5°, I=42.6° with k=76.5 and α95=14.2, corresponding to a virtual geomagnetic pole (VGP) at 18.2 °N, 31.0 °E with A95=20.4°. After correcting for a 5° vertical-axis rotation of the study region, the two pole positions are at 45.1 °N, 130.4 °E and 14.1 °N, 32.5 °E, respectively. The c. 825–720 Ma palaeopole position from South China, East Svalbard and neighbouring continents fall on great circles on two alternative Rodinia reconstructions, possibly reflecting oscillating inertial interchange true polar wander events (IITPWs).

A stable Ediacaran Earth recorded by single silicate crystals of the ca. 565 Ma Sept-Îles intrusion

The archetypical example of inertial interchange true polar wander (IITPW), the rapid rotation of the entire solid Earth by 90°, is based on two nearly orthogonal directions seen in paleomagnetic studies of the Sept-Iles (ca. 565 Ma) intrusion (Quebec, Canada). This motion has also been proposed as the driving force for the early Cambrian explosion of life. Others have challenged these interpretations, linking the data instead to flips between an axial and equatorial dipole field configuration. We examine this enigma using single silicate crystal paleomagnetic analyses. We conclude that only one of the previously reported magnetic directions is carried by single domain magnetic grains and can be considered primary. Thus, the Ediacaran diversification occurred on a rotationally stable Earth driven by biotic and longer term abiotic forcing. Whether IITPW ever occurred in the past on Earth remains uncertain.

Theoretical constraints on true polar wander

For the present geologic epoch, true polar wander (TPW) is relatively small, but simple theoretical considerations suggest that it could have been larger in other epochs. In this work, we use scaling arguments to assess the qualitative behavior of TPW and a simple Maxwell model to analytically describe how changes in mass anomalies translate into TPW. Unlike previous work, we derive simple analytical estimates of TPW based on the characteristic amplitudes and timescales for changes in the moment of inertia. We find estimates for both the amplitude and speed of TPW as a function of Earth properties. The following four main factors influence how large the maximum TPW can be: the (geological) timescale over which the desired TPW occurs τTPW, the viscosity structure of the mantle which yields a weighted average viscosity η, the characteristic amplitude of the nonhydrostatic changes in the moment of inertia δC, and the added moment of inertia due to the equatorial bulge (C − A). For the nominal values η = 3 × 1022 Pa s and δC/(C − A) = 0.003, the maximum TPW is 61° over 100 Myr and 8° over 10 Myr. The maximum TPW speed is only sensitive to η, δC, and (C − A), and is 2.4° Myr‐1 for the nominal values. TPW is shown to act as a low‐pass filter; rapid changes in moment of inertia produce smaller and delayed TPW. A consequence of this is that inertial interchange TPW does not have a different character than TPW. TPW can have an important contribution to plate motions over relatively long timescales but not over shorter timescales. Our simple approach allows us to assess whether multiple TPW events are possible but the major uncertainty continues to be the mantle viscosity structure.

Stratigraphy and geochemistry of a ca 800 Ma negative carbon isotope interval in northeastern Svalbard

The Neoproterozoic Akademikerbreen Group in northeastern Svalbard comprises 2 km of nearly pure carbonate section. The carbonates are generally highly 13C-enriched ( δ 13C > 5‰), but this trend is interrupted by an ∼ 325 m interval of low δ 13C values (− 4 to 0‰) in the upper Grusdievbreen and lower Svanbergfjellet formations. An abrupt negative isotopic shift at the onset of this low δ 13C interval is reproduced in detail in multiple sections along the length of the outcrop belt (125 km) and everywhere coincides with a prominent sequence boundary and change in lithology. Likewise, the return to positive δ 13C values coincides with a second exposure surface. Correlation of the lower Akademikerbreen Group δ 13C record with a nearly identical isotopic profile in the Bitter Springs Formation of Central Australia suggests an age of ∼ 800 Ma for the low δ 13C interval and confirms that it is a global seawater signal. The coincidence of the negative and positive δ 13C shifts with major stratigraphic perturbations in the otherwise conformable succession suggests that both episodes of transient sea level change were related to global phenomena. 87Sr/ 86Sr ratios rise transiently from an average of 0.7063 to 0.7066 within this interval. Whereas large negative δ 13C anomalies in the Neoproterozoic are commonly associated with episodes of widespread glaciation, the Akademikerbreen low δ 13C interval precedes the oldest (Sturtian) of the known Neoproterozoic glacial events, and no other evidence suggests an ice age at this time. We propose instead that the negative δ 13C interval is related to a pair of inertial interchange true polar wander (TPW) events.

Tales of Wander

GEOLOGY True polar wander describes relative motion between Earth's spin vector and the solid Earth. One class of this phenomenon, inertial interchange true polar wander, occurs when normal advection of mantle density heterogeneities produces changes in the relative magnitudes of the principal inertia axes, causing Earth to rotate quickly by as much as 90°, until the new major rotational axis is aligned with the spin vector. In addition to the paleomagnetic variations that would accompany such a rapid change of Earth's orientation, another observable consequence could be transient sea-level variations resulting from the differential response of the slowly re-equilibrating mantle/lithosphere and the rapidly re-equilibrating world ocean. A third potential but indirect effect, arising from sea-level change, is perturbation of the carbon cycle, as marine biological productivity is affected by water depth variations. In an investigation of possible true polar wander, Maloof et al . present paleomagnetic data from three Middle Neoproterozoic carbonate units in Svalbard, Norway, which show large shifts in paleomagnetic orientation coincident with abrupt changes in δ13C and relative sea level. They conclude that the best explanation for the data is that this area experienced rapid shifts of paleogeography during a pair of true polar wander events. Their hypothesis can be further tested by analyzing sediments of the same age from other basins for predictable related changes. — HJS Geol. Soc. Am. Bull. 118 , 1099 2006).

The Tambien Group, Ethiopia: An early Cryogenian (ca. 800–735 Ma) Neoproterozoic sequence in the Arabian–Nubian Shield

The Tambien Group in Tigrai, Ethiopia, comprises a number of inliers each containing 2–3 km thickness of interbedded carbonate and clastic sediments, capped in one inlier by a glaciogenic diamictite. A range of geochemical indices suggest near-pristine C- and Sr-isotope values are preserved and these, together with lithological variations, allow local correlation between these inliers and correlation with the global Neoproterozoic isotope stratigraphy. A composite section of the Tambien Group shows δ 13C in carbonate of ∼+6‰ at its base, decreasing upwards to twin lows of ∼–4‰ separated by a brief excursion back to positive values, then rises again to a plateau of ∼+6‰ before finally decreasing sharply to −2‰ beneath the Negash diamictite at its top. No glaciogenic sediments are observed associated with the lower twinned negative anomalies. The 87Sr/ 86Sr values in carbonates change from ∼0.7063 in the lower units to ∼0.7067 in the upper units. The δ 13C of organic matter changes little through the sequence (−24.2 ± 1.3‰), with the result that the C-isotope fractionation between carbonate and organic matter decreases from +26 to +21‰ up through the second low in carbonate δ 13C before increasing to ∼+29.5‰ in the rest of the sequence. Together with existing radiometric age constraints, the Sr-isotope data indicate that the Negash diamictite is Sturtian in age, and the lower negative C-isotope anomalies appear to correlate with the non-glaciogenic Bitter Springs Stage recognised in Australia and Svalbard. According to this correlation the Tambien Group was deposited in the interval ca. 800–735 Ma. Our data reinforce the emerging view that not all Neoproterozoic negative C-isotope anomalies are associated with glaciation. The variations in C-isotope fractionations within our sequence contrast with those from Australia and are attributed to differences in local environmental variables, probably temperature. These differences are consistent with the recent proposal that the Bitter Springs Stage anomaly is the result of a pair of Inertial Interchange True Polar Wander events [Halverson, G.P., Maloof, A., Schrag, D., Dudas, F., Hurtgen, M., in press. Stratigraphy and geochemistry of a ca 800 Ma negative carbon isotope stage in northeastern Svalbard. Chem. Geol.] [Maloof, A.C., Halverson, G.P., Kirschvink, J.L., Schrag, D.P., Weiss, B.P., Hoffman, P.F., in press. Combined paleomagnetic, isotopic and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard. GSA Bull.], and further high-resolution dating and palaeomagnetic studies of the Tambien Group should allow testing of this hypothesis.

A palaeomagnetic and palaeobiogeographical perspective on latest Neoproterozoic and early Cambrian tectonic events

During the latest Neoproterozoic to Mid-Cambrian time (580–505 Ma ago), the Earth underwent significant changes in palaeogeography that included rifting of a possible supercontinent and the near simultaneous formation of a second, slightly smaller supercontinent. It is against this tectonic backdrop that the Cambrian radiation occurred. Although the general tectonic setting during this interval is fairly well constrained, models of the exact palaeogeography are controversial because of the lack of reliable palaeomagnetic data from some of the continental blocks. Palaeogeographical models based on palaeomagnetic data range from a high-latitude configuration for most continents, to a low-latitude configuration for most continents, or to rapid oscillations in continental configurations triggered by inertial changes within the planet. Palaeobiogeographical data can also be used to help constrain palaeogeographical models. To this end we use vicariance patterns in olenellid trilobites to determine their compatibility with three end-member palaeogeographical models derived from palaeomagnetic data for the Neoproterozoic and early Cambrian. The most congruent palaeogeographical model with respect to the palaeobiogeographical data described herein is the high-latitude configuration for most continents. Those palaeomagnetic models that predict inertial interchange true polar wander or multiple episodes of true polar wander differ significantly from the results from palaeobiogeography. The low-latitude palaeogeographical models also differ from the results from palaeobiogeography, but this may partly arise because of a lack of palaeomagnetic and palaeobiogeographical data from many parts of present-day South America and Africa.

Return to Black Mountain: palaeomagnetic reassessment of the Chatsworth and Ninmaroo formations, western Queensland, Australia

SUMMARY Palaeomagnetic results from late Middle Cambrian‐Early Ordovician carbonate sequences sampled at Black Mountain (Mt Unbunmaroo), Mt Datson and near Chatsworth Station (southeastern Georgina Basin) are presented. A palaeomagnetic reassessment of these carbonates was designed in an effort to constrain regional magnetization ages as results from an earlier study, conducted at Mt Unbunmaroo, play a pivotal role in a proposed Cambrian inertial interchange true polar wander (IITPW) event. Remanent magnetizations within these carbonates were found to be variably developed with most specimens displaying two of the five isolated components. Component PF, for which goethite is the identified remanence carrier, is thought to reflect a chemical remanent magnetization of recent origin. Component TR, held by haematite, has a palaeomagnetic pole consistent with the Tertiary segment of Australia’s apparent polar wander path (APWP) and most probably was acquired as a consequence of prolonged weathering during this period. The A component has a palaeomagnetic pole at 54.7 ◦ S, 262.3 ◦ E( dp = 2.3 ◦ , dm = 4.5 ◦ ) after unfolding. This direction, constrained by positive fold and reversal test statistics, is consistent with Australia’s Early Devonian APWP, perhaps reflecting a remagnetization event associated with the intracratonic Alice Springs Orogeny. A Late Ordovician‐Early Silurian remanence, component B, is also described; with 100 per cent unfolding the associated palaeopole lies at 8.0 ◦ S, 216.8 ◦ E( dp = 2.6 ◦ , dm = 5.1 ◦ ). A third Palaeozoic, and presumed primary or early diagenetic, component, C, also passes applied fold and reversal tests and has a palaeomagnetic pole at 48.6 ◦ N, 186.0 ◦ E( dp= 2.1 ◦ S, dm = 4.0 ◦ ). This palaeopole is dissimilar from younger magnetizations, is consistent with Cambrian poles from other parts of cratonic Australia and falls within a cluster of Middle‐Late Cambrian (515‐500 Ma) palaeopoles from other Gondwanan continents. The age attributed to the palaeopole associated with the C component, ∼510 Ma, provides a tight constraint on the younger boundary of the proposed Cambrian IITPW event and its agreement with other Gondwanan palaeopoles is incompatible with the IITPW hypothesis. Components A, B and C are analogous to palaeomagnetic results reported in the earlier investigation of this region, and a comparison of results from the two studies, coupled with rigorous statistical analyses of the new findings, is presented.

An equatorial Laurentia at 550 Ma confirmed by Grenvillian inherited zircons dated by LAM ICP-MS in the Skinner Cove volcanics of western Newfoundland: implications for inertial interchange true polar wander

Abstract Volcanics of the Skinner Cove Formation of western Newfoundland carry a primary remanence acquired at 550 Ma at a paleolatitude of 19°S. There has been doubt that this represents the latitude of the Laurentian margin at 550 Ma, because the Skinner Cove Formation is allochthonous. We present new evidence from inherited zircons in the volcanics that should remove this doubt. Zircon crystals extracted from an ankaramite flow and a trachyte flow were dated individually by measuring U–Th–Pb isotopes for ∼ 30 μm ×30 μm areas using a laser ablation microprobe (LAM) linked to an inductively coupled plasma mass spectrometer (ICP-MS). Most of the zircons from the ankaramite are concordant and yield a 550±5 Ma date indistinguishable from the 550+3/−2 Ma date previously reported using multi-grain thermal ionisation mass spectrometry. About half of the zircons from the trachyte are also concordant yielding an overlapping date of 556±5 Ma. The other half clustered at ∼1000 and 1500–1600 Ma, which are characteristic ages of the Grenvillian basement exposed nearby in the Long Range Inlier. These zircon xenocrysts were very likely picked up as the Skinner Cove magma ascended through Grenvillian basement of the Laurentian margin. There can now be little doubt that the ∼19°S Skinner Cove paleolatitude represents Laurentia’s southern margin at 550 Ma. This makes it unlikely that Laurentia changed from south polar at ∼523 Ma to equatorial at ∼508 Ma due to a rapid ∼90° change in the Earth’s rotation axis. Such an inertial interchange true polar wander (IITPW) event has been proposed as a trigger for the Cambrian faunal explosion. An IITPW event may have occurred in the Vendian, mostly between 577 and 565 Ma, judging by paleomagnetic results from Laurentia. However, it is more likely that the paleomagnetic results suggesting high latitude for Laurentia at 577 Ma or low latitude at 565 Ma are flawed. We suggest that Laurentia remained at low latitude throughout the Vendian (from the ∼600 Ma start of near-equatorial glaciations). This is supported by magmatic evidence suggesting that Laurentia’s incipient Iapetus margin was stationary over a mantle plume throughout the Vendian. Laurentia at low latitude throughout the Vendian also helps provide the predominance of low latitude continents needed to account for the Vendian’s near-equatorial glaciations, according to the snowball Earth hypothesis.

A paleomagnetic analysis of Cambrian true polar wander

The latest Neoproterozoic through Cambrian is one of the most remarkable intervals in geologic time. Tectonically, the period from 580 to 490 Ma marks a time of rapid plate reorganization following the final stages of supercontinental breakup and Gondwana assembly. The apparent speed at which this reorganization occurred led some to propose a link between tectonic events, biologic changes and climatic changes. One of the more intriguing proposals is that the tectonic changes were triggered by an episode of inertial interchange true polar wander (IITPW) which resulted in a rapid (6°/m.y.) shift of the spin axis relative to the geographic reference frame. IITPW is a special case of true polar wander (TPW) that makes specific demands on the length of apparent polar wander paths (APWPs) recording the motion. Specifically, each path must allow for ∼90° of synchronous motion during the interval from 523 to 508 Ma. A review of paleomagnetic data for Laurentia, Baltica, Siberia and Gondwana indicates that none of the APWPs approaches the necessary length, each path is of a different length and the apparent motions are non-synchronous. Collectively, these observations negate the premise of a Cambrian IITPW event. Since the IITPW hypothesis was proposed as an alternative to rapid plate motion of Laurentia and Gondwana during the Neoproterozoic–Cambrian interval, any alternative model must account for this rapid motion. I suggest that a reasonable explanation for `anomalously' high rates of plate motion for some continents, possibly on the order of 20–40 cm yr−1, is enhanced plate motion driven by lower-mantle thermal anomalies and possibly true polar wander. In fact, the enhanced plate motions driven by these lower-mantle sources may provide a dynamic feedback triggering true polar wander.

A sea-level test for inertial interchange true polar wander events

SUMMARY The possibility of inertial interchange true polar wander (IITPW) events, in which the rotation pole moves 90° with respect to the solid Earth in a matter of ~10 Myr, has been discussed in the geophysical literature for more than three decades. Recent evidence for an IITPW event in Early Cambrian time has renewed interest in the issue; however, the veracity of supporting palaeomagnetic evidence remains a matter of significant debate. We propose that sea-level variations driven by polar wander provide an important independent test for the occurrence of IITPW events. Our numerical simulations of the response of a viscoelastic planet to an IITPW-induced forcing predict sea-level changes of up to 200 m, depending on the details of the earth model, the location of the site relative to the rotation path and the elapsed time for the reorientation of the pole. A preliminary comparison of our predictions to Early‐Middle Cambrian sea-level records for Australia, Laurentia and Baltica shows qualitative agreement. This comparison suggests that a definitive test for the Cambrian IITPW hypothesis is possible given a suYciently accurate, and globally distributed, database of sea-level histories.

The break-up of Rodinia, birth of Gondwana, true polar wander and the snowball Earth

A major global plate reorganisation occurred between ∼750 and ∼550 Ma. Gondwana was assembled following the dispersal of Rodinia, a supercontinent centred on Laurentia in existence since ∼1050 Ma. The reorganisation began when tectonic elements, later composing East Gondwana, rotated piecemeal away from the Pacific margin of Laurentia. These elements swept across the ancestral Pacific (Mozambique) Ocean that lay between Laurentia and the combined African cratons of Congo and Kalahari, which were loosely joined after ∼820 Ma. Simultaneously, the Adamastor (Brasilide) Ocean closed by subduction bordering the West Gondwana cratons, drawing virtually all of Gondwana together by ∼550 Ma. The final assembly of Gondwana occurred contemporaneously with the separation of Laurentia from West Gondwana. It has been postulated that the imprint of Rodinia's long-lived existence on lower mantleconvection produced a prolate ellipsoidal geoid figure. This could give rise to episodic inertial interchange true polar wander (IITPW), meaning that the entire silicate shell of the Earth (above the core-mantle boundary) rolled through 90° with respect to the diurnal spin axis in ∼15 Ma (equivalent to an apparent polar wander velocity of ∼66 cm a −1. Although empirical arguments for IITPW of Cambrian age appear to be flawed, evidence for an ultra-fast ( > 40 cm a −1) meridional component of apparent polar wander for Laurentia between 564 and 550 Ma suggests that IITPW might have occurred at that time. The break-up of Rodinia increased the continental margin area and preferential organic C burial globally, which is reflected by high δ 13C values in seawater proxies. The consequent drawdown of CO 2 is implicated in a succession of runaway ice-albedo catastrophes between ∼750 and ∼570 Ma, during each of which the oceans completely froze over. Each “snowball” Earth event must have lasted for millions of years because their terminations depended on extreme CO 2 levels, built up by subaerial volcanic outgassing in the absence of sinks for C. A succession of ice-albedo catastrophes, each terminated under ultra-greenhouse conditions, must have imposed an intense environmental filter on the evolution of life. They may have triggered the radiation of Ediacaran fauna in the aftermath of the final snowball event. It is increasingly recognised that the Late Neoproterozoic was one of the most remarkable periods in Earth history, and it appears to exemplify the interplay of tectonics, the environment and biology in deep time.

Evidence for a Large-Scale Reorganization of Early Cambrian Continental Masses by Inertial Interchange True Polar Wander

Analysis of Vendian to Cambrian paleomagnetic data shows anomalously fast rotations and latitudinal drift for all of the major continents. These motions are consistent with an Early to Middle Cambrian inertial interchange true polar wander event, during which Earth's lithosphere and mantle rotated about 90 degrees in response to an unstable distribution of the planet's moment of inertia. The proposed event produces a longitudinally constrained Cambrian paleogeography and accounts for rapid rates of continental motion during that time.