True Polar Wander Event 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.

Completing the loop of the Late Jurassic–Early Cretaceous true polar wander event

The reorientation of Earth through rotation of its solid shell relative to its spin axis is known as True polar wander (TPW). It is well-documented at present, but the occurrence of TPW in the geologic past remains controversial. This is especially so for Late Jurassic TPW, where the veracity and dynamics of a particularly large shift remain debated. Here, we report three palaeomagnetic poles at 153, 147, and 141 million years (Myr) ago from the North China craton that document an ~ 12° southward shift in palaeolatitude from 155–147 Myr ago (~1.5° Myr−1), immediately followed by an ~ 10° northward displacement between 147–141 Myr ago (~1.6° Myr−1). Our data support a large round-trip TPW oscillation in the past 200 Myr and we suggest that the shifting back-and-forth of the continents may contribute to the biota evolution in East Asia and the global Jurassic–Cretaceous extinction and endemism.

Empirical record, geochronology and theoretical determinates of Mesozoic climate in the Junggar Basin, NW China, in relation to other basins in NE China

Abstract Mesozoic continental basins of northern China, including the Junggar Basin, provide some of the most spectacular and important fossil assemblages in the world, but their climatic and environmental contexts have been shrouded in uncertainty. Here we examine the main factors that determine those contexts: palaeolatitude; the effects of changing atmospheric gases on the radiative balance; and orbitally paced variations in insolation. Empirical evidence of these factors is accumulating rapidly and promises to upend many long-standing paradigms. We focus primarily on the Junggar Basin in Xinjiang, NW China, with the renowned Shishugou Biota, and the basins in Liaoning, Hebei and Inner Mongolia with their famous Jehol and Yanliao biotas. Accurate geochronology is necessary to disentangle these various factors, and we review the Late Triassic to Early Cretaceous U–Pb ages for these areas and supply a new laser ablation inductively coupled plasma mass spectrometry age for the otherwise un-dated Sangonghe Formation of Early Jurassic age. We review climate-sensitive facies patterns in North China and show that the climatic context changed synchronously in northwestern and northeastern China consistent with a previously proposed huge Late Jurassic–earliest Cretaceous true polar wander event, with all the major plates of East Asia docked with Siberia and moving together since at least the Triassic when the North China basins were at Arctic latitudes. We conclude that this true polar wander shift was responsible for the coal beds and ice-rafted debris being produced at high latitudes and the red beds and aeolian strata being deposited at low latitudes within the same basin. The climatic and taphonomic context in which the famous Shishugou, Yanliao and Jehol biotas were preserved was thus a function of true polar wander, as opposed to local tectonics or climate change.

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.

Deformation along the Deseado Massif (Patagonia, Argentina) during the Jurassic Period and its relationship with the Gondwana breakup: paleomagnetic and geochronological constraints

This work presents the analysis of paleomagnetic results obtained from four sampling areas of the Jurassic Bahía Laura Complex in the Deseado Massif and their implications on the regional deformation history during the breakup of Gondwana. Paleomagnetic data show cessation of tectonic block rotations about vertical axes around 160 Ma and a change from a transtensional to a mainly extensional tectonic regime. Two biotite samples yielded 40Ar–39Ar radiometric ages: one from the eastern outcrops of the Chon Aike Formation (plateau age of 184.66 ± 0.55 Ma) and the other from the La Matilde Formation (plateau age of 157.40 ± 0.65 Ma), located at the central part of the Deseado Massif. Finally, an apparent polar wander path (APWP) was calculated for the Jurassic of South America (200 to 140 Ma). This APWP indicates that South America experienced a northward drift between 200 Ma and 170 Ma, a clockwise rotation (~10°) between 170 Ma and 160 Ma, and a westward drift between 160 Ma and 140 Ma. The 170–160 Ma rotation could has been triggered by the combined effects of the uncoupling of the Antarctic Peninsula, the high rates of subduction of the Phoenix plate beneath Patagonia, the opening of the Rocas Verdes basin and the Weddell Sea, and the cessation of the Tethys slab-pull. Furthermore, the calculated APWP for South America does not support a Jurassic massive true polar wander event.

Jurassic fast polar shift rejected by a new high-quality paleomagnetic pole from southwest Greenland

A selective compilation of paleomagnetic data from North America indicates that a vast amount of rapid polar motion occurred in Late Jurassic time. The over 30° polar shift that accumulated during a relatively short time interval (~160–145 Ma) suggests an episode of fast true polar wander (TPW) and was referred to as the Jurassic “monster polar shift” by some workers. However, this rapid TPW event is not supported by paleomagnetic data on a global scale. Here, we scrutinize the Jurassic apparent polar wander path (APWP) by virtue of a new paleomagnetic and 40Ar/39Ar geochronology study of Mesozoic coast-parallel dykes exposed in southwest Greenland. Combined with existing geochronological data, our results show that the dykes were emplaced during a prolonged period centered at 147.6 ± 3.4 Ma (2σ). A primary nature of the characteristic remanent magnetization is supported by multiple positive baked contact tests and a reversal test. The paleomagnetic pole calculated from 40 site-mean paleomagnetic directions is located at Plat = 69.3°S, Plong = 5.0°E (A95 = 4.6°), or at Plat = 73.9°S and Plong = 0.4°E when reconstructed to North America. Our new high-quality paleomagnetic pole and an updated global APWP do not support the fast Jurassic polar shift but instead indicate steady polar motion with moderate rates of about 0.7°/Myr. The new pole effectively reduces the mismatch between the APWPs for Laurentia and Europe. Our critical reassessment of the monster polar shift indicates that it may be an artifact of paleomagnetic and geochronological data that were previously used to argue for its existence.

A pan-latitudinal Rodinia in the Tonian true polar wander frame

A new paleomagnetic and geochronological study was conducted on the Tonian Chéngjiāng Formation from Yúnnán Province, South China, to examine the recent reported true polar wander event (TPW) during 825–750 Ma and clarify the position of South China in the Rodinia supercontinent. Detailed thermal demagnetization revealed two remanent magnetic components. A low-temperature component separated below 300°C is interpreted as a recent viscous remanence. Additionally, a high-temperature component (H1) with unblocking temperature up to 690°C is revealed. Passing both the fold test and reversal test, and recording at least two and half magnetochrons, H1 is suggested to be a primary remanence. The paleomagnetic pole (CJH1, 33.4°N, 56.1°E, dp/dm=7.3°/8.9°) calculated from component H1 has no similarities to the Phanerozoic apparent polar wander path of South China. SHRIMP II U-Pb zircon data from a tuff bed near the paleomagnetic sampling sites suggest that the H1 pole has an age of ca. 800 Ma. Combining the reported Tonian paleomagnetic results from South China and Svalbard, we suggest a ∼63° TPW event during 825–790 Ma. Under this TPW paleomagnetic frame and using the reliable Tonian paleomagnetic results from other continents, we reconstruct the main part of Rodinia. This new configuration of Rodinia reconstructs South China and India at its northern periphery, and within the polar or high latitude zone during ca. 900–750 Ma. The polar location of those cratons, together with the mid-high latitudinal distribution of Australia, Congo and Baltica, suggests that Rodinia was a pan-latitudinal rather than an equatorially distributed supercontinent. The reconstruction also suggests that the breakup of the Rodinia should have been after 750 Ma but before 720 Ma. Despite similarities of their assembly and configuration, Rodinia and Pangea affected the Earth system differently.

Testing the occurrence of Late Jurassic true polar wander using the La Negra volcanics of northern Chile

True polar wander (TPW) is the reorientation of the crust-mantle system driven by the redistribution of masses in the mantle and on the Earth's surface. In the ideal case, characterization of TPW requires paleomagnetic constraints on the motion of all major plates and independent reconstructions of relative plate positions. While such complete datasets are absent for pre-Mesozoic TPW inferences due to the absence of oceanic plates, they are available for the Late Jurassic (165-145 Ma) “monster shift”, a ∼30° amplitude proposed TPW event. Here we perform paleomagnetic sampling and Ar-Ar geochronology on the La Negra volcanics of Northern Chile, producing two new paleomagnetic poles with ages 165.8 ± 1.8 Ma (1σ; 84.3°N 0.9°E; α95=7.6°; N=28) and 152.8 ± 0.8 Ma (84.5°N 256.4°E; α95=10.8°; N=18). By combining these data with other recently published results, we compute a net lithospheric rotation of 25.3° ± 7.3° (1σ) at a mean rate of 1.21° ± 0.35° My−1 between 170 and 145 Ma with a peak rate of 1.46° ± 0.65° My−1 between 160 and 145 Ma. These rates are consistent with inferences from the Pacific Plate, implying true whole lithosphere rotation. Given coherent motion involving the entire lithosphere, we conclude that the Earth underwent rapid TPW between approximately 165 and 145 Ma, potentially driven by the cessation of subduction along the western North American margin.

Ediacaran glaciations of the west African Craton – Evidence from Morocco

The northern margin of the West African Craton preserved in the Anti-Atlas area of southern Morocco, represents an outstanding geological archive covering parts of the Neoproterozoic and the Paleozoic. Despite isolated mentions in the literature, relics of Neoproterozoic glaciations have hitherto not received much attention in this area and tight geochronological constraints had been lacking. This paper documents the sedimentology and stratigraphic context of several occurrences from the eastern and central Anti-Atlas. Based on outcrop-scale sedimentological and microsedimentological investigations, different types of diamictites are distinguished and their origin with reference to modern glacial systems is discussed. The sedimentary record is explained by one single glacial period (albeit interrupted by interglacial phases) affecting both mountainous terrestrial and marine realms. Sedimentary facies and preserved paleotopographies are best accounted for by invoking a fjord-like depositional system with steep-sided valleys being repeatedly drowned by marine water or occupied by valley glaciers. High-precision geochronological constraints suggest that this glacial period must have taken place sometimes between 592 Ma and 579 Ma. Hence it is suggested that this is the first temporally constrained report of the Ediacaran Gaskiers glaciation on the West African Craton (WAC). Existing Ediacaran paleomagnetic data from the WAC are sparse and allow different interpretations regarding paleolatitude of this glaciation. The latter is finally discussed vis-à-vis potential rapid true polar wander events and the possibility that the Gaskiers glaciation was more global in extent than hitherto assumed and might even have brought Earth close to a true pan-glacial state.

Paleomagnetism of a well-dated marine succession in South China: A possible Late Cambrian true polar wander (TPW)

The Cambrian true polar wander (TPW) hypothesis remains controversial largely because of the uncertainties in the quality and/or fidelity of the paleomagnetic data as well as their chronological control. Testing the TPW hypothesis requires high-quality paleomagnetic data of sufficient spatial and temporal resolutions. Here, we present paleomagnetic results of a continuous Cambrian shallow marine succession from South China where available detailed biostratigraphy provides exceptional chronological constraints. Forty-three sites of paleomagnetic samples were collected from this limestone-dominated succession. Stepwise thermal demagnetization generally reveals three-component magnetizations. Low- and intermediate-temperature components can be cleaned by ∼330 °C, and the high-temperature component (HTC) was isolated typically from ∼350 to ∼450 °C. A positive fold test and the presence of reversed polarity in the strata, together with rock magnetic data as well as the scanning electron microscopic (SEM) and transmission electron microscopic (TEM) results, collectively suggest that the HTCs are likely primary. A directional shift of the HTCs occurs between the lower-middle Cambrian and the upper Cambrian strata in the succession and is tentatively interpreted to indicate a ∼57° polar wander from ∼500.5 to 494 Ma. Because the rate of polar wander is too fast to be a tectonic origin, this polar wander is interpreted to represent a Late Cambrian TPW. This TPW appears coeval with the Steptoean positive carbon isotope excursion (SPICE) and the major trilobite mass extinctions, suggesting a potential link between the TPW and the Late Cambrian biotic and climatic changes. Because the proposed TPW event is exceptionally well-dated, it should be testable through examination of other worldwide sections.

Scaling rates of true polar wander in convecting planets and moons

Mass redistribution in the convecting mantle of a planet causes perturbations in its moment of inertia tensor. Conservation of angular momentum dictates that these perturbations change the direction of the rotation vector of the planet, a process known as true polar wander (TPW). Although the existence of TPW on Earth is firmly established, its rate and magnitude over geologic time scales remain controversial. Here we present scaling analyses and numerical simulations of TPW due to mantle convection over a range of parameter space relevant to planetary interiors. For simple rotating convection, we identify a set of dimensionless parameters that fully characterize true polar wander. We use these parameters to define timescales for the growth of moment of inertia perturbations due to convection and for their relaxation due to true polar wander. These timescales, as well as the relative sizes of convective anomalies, control the rate and magnitude of TPW. This analysis also clarifies the nature of so called “inertial interchange” TPW events, and relates them to a broader class of events that enable large and often rapid TPW. We expect these events to have been more frequent in Earth’s past.

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

Abstract We 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 α 95 =12.3°, corresponding to a palaeopole at 42.5 °N, 131.8 °E with A 95 =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 α 95 =14.2, corresponding to a virtual geomagnetic pole (VGP) at 18.2 °N, 31.0 °E with A 95 =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).

Was the Cambrian explosion both an effect and an artifact of true polar wander?

Charles Darwin suspected that the Cambrian “explosion” might be an artifact of fossil preservation. A more recent, initially controversial hypothesis that repeated true polar wander (TPW) triggered the Ediacaran-Cambrian explosion of animal life has been supported by numerous paleomagnetic and geochronologic refinements. These data imply ∼75° of TPW between 535 and 515 million years ago, coinciding with the paleontologically observed rise in metazoan diversity and disparity. We show here that this evolutionary trend is explained simply by the well known ecology-driven increase of diversity in low latitudes, coupled by other ecological effects as well as the enhanced deposition of sedimentary rocks during TPW-driven sea-level transgressions. During the Cambrian TPW event, Laurentia and parts of Gondwanaland moved into the equatorial zone while experiencing local TPW-induced transgressions; these areas dominate the paleontological record of the time. Although diversity might thus be considered partly artifactual, TPW acted on Cambrian biogeography to increase net diversity; and enhanced rates of origination and extinction also could increase disparity, especially if Early Cambrian TPW occurred at a time when genetic regulatory networks were critically poised for expansion and exaptation.

True polar wander and supercontinent cycles: Implications for lithospheric elasticity and the triaxial earth

The amplitude of true polar wander events is shown to occur in cycles out of phase with the formation of supercontinents over the past 3 Gyr. Associated with small-amplitude true polar wander, supercontinents act to stabilize the spin axis. Stabilization can be explained by reduced lithospheric elasticity and/or the triaxial (oblate) figure of the Earth, both of which are legacies of the supercontinent cycle. An excess triaxial ellipticity would only be expected to affect the first transition between supercontinents, whereas decreased lithospheric elasticity would have also influenced formation of the first supercontinent, if sizable enough. My analysis indicates the presence of 4 supercontinents since 3 Ga and proposes that the triaxial Earth originates from the supercontinent cycle.

Mechanisms for oscillatory true polar wander

Palaeomagnetic studies of Palaeoproterozoic to Cretaceous rocks propose a suite of large and relatively rapid (tens of degrees over 10 to 100 million years) excursions of the rotation pole relative to the surface geography, or true polar wander (TPW). These excursions may be linked in an oscillatory, approximately coaxial succession about the centre of the contemporaneous supercontinent. Within the framework of a standard rotational theory, in which a delayed viscous adjustment of the rotational bulge acts to stabilize the rotation axis, geodynamic models for oscillatory TPW generally appeal to consecutive, opposite loading phases of comparable magnitude. Here we extend a nonlinear rotational stability theory to incorporate the stabilizing effect of TPW-induced elastic stresses in the lithosphere. We demonstrate that convectively driven inertia perturbations acting on a nearly prolate, non-hydrostatic Earth with an effective elastic lithospheric thickness of about 10 kilometres yield oscillatory TPW paths consistent with palaeomagnetic inferences. This estimate of elastic thickness can be reduced, even to zero, if the rotation axis is stabilized by long-term excess ellipticity in the plane of the TPW. We speculate that these sources of stabilization, acting on TPW driven by a time-varying mantle flow field, provide a mechanism for linking the distinct, oscillatory TPW events of the past few billion years.

The influence of degree-1 mantle heterogeneity on the past dynamo of Mars

The hemispheric dichotomy in the crustal magnetic field of Mars may indicate that the planet’s past dynamo was influenced by a degree-1 heterogeneity on the outer boundary of its liquid metallic convecting core. Here we use numerical dynamos driven by purely volumetric internal heating with imposed degree-1 heat flux heterogeneities to study mantle control on the past dynamo of Mars. We quantify both south–north and east–west magnetic field dichotomies from time-average properties that are calculated according to two different end member crust formation scenarios. Our results indicate that a moderate heat flux anomaly may have been sufficient for obtaining the observed dichotomy. Because of the excitation of a strong equatorial upwelling in the dynamo, the efficiency of a mantle heterogeneity centered at the geographical pole in producing a south–north dichotomy is much higher than that of an heterogeneity centered at the equator in producing an east–west dichotomy. These results argue against a significant True Polar Wander event with major planet re-orientation after the cessation of the dynamo.

Rotational bulge and one plume convection pattern: Influence on Martian true polar wander

Motion of the entire solid planet with respect to its spin axis have been proposed on Mars. This movement is known as True Polar Wander (TPW). According to the conservation of angular momentum with no external torque, on geological time scales the axis of maximum inertia of a planet is aligned with the rotation axis. Then rearrangement of masses within the mantle disturbs the planet's inertia and induces TPW. The convection pattern on Mars is possibly controlled by a sequence of single plumes originating from the core-mantle boundary. Using a homogeneous model of the martian mantle and modelling the plume as a sphere, we calculate the inertial tensor perturbations caused by the plume mass anomaly. We investigate the stabilizing influence of the remnant rotational bulge due to the lithosphere elasticity on these perturbations. It appears that, during early martian history, the elastic lithosphere was thin enough to allow its fractures under the inertia perturbations induced by a hot plume. Consequently, the lithosphere's behaviour became effectively viscoelastic and the plume could induce large TPW. We conclude that one plume convection pattern should have greatly influenced the rotation pole behaviour during early Mars history: around 4 Gyr ago, Mars already could have experienced two TPW events lasting possibly only a few million years each. We then compare our scenario with others already published in the literature.

Evidence for an ancient martian ocean in the topography of deformed shorelines

A suite of observations suggests that the northern plains of Mars, which cover nearly one third of the planet's surface, may once have contained an ocean. Perhaps the most provocative evidence for an ancient ocean is a set of surface features that ring the plains for thousands of kilometres and that have been interpreted as a series of palaeoshorelines of different age. It has been shown, however, that topographic profiles along the putative shorelines contain long-wavelength trends with amplitudes of up to several kilometres, and these trends have been taken as an argument against the martian shoreline (and ocean) hypothesis. Here we show that the long-wavelength topography of the shorelines is consistent with deformation caused by true polar wander--a change in the orientation of a planet with respect to its rotation pole--and that the inferred pole path has the geometry expected for a true polar wander event that postdates the formation of the massive Tharsis volcanic rise.

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.

Circum-Iapetus paleogeography of the Precambrian–Cambrian transition with a new paleomagnetic constraint from Laurentia

The paleogeography of the Precambrian–Cambrian transition is still poorly known, but is fundamental for understanding Late Neoproterozoic climate extremes and the spatial associations of the rapidly evolving Ediacaran and Cambrian fauna. This period has also been proposed to host unusual geodynamic events such as rapid plate motions and/or the bulk tumbling of the Earth with respect to its spin axis, called true polar wander (TPW). New paleomagnetic and 40Ar– 39Ar geochronologic results have been obtained from a pair of shallowly emplaced syenitic intrusions in western Quebec. The Mont Rigaud and the Chatham-Grenville stocks are related to a failed rift arm of the Iapetus Ocean. Both intrusions bear hornblendes which provide overlapping 40Ar– 39Ar plateau ages of 533.2 ± 1.1 Ma (Mont Rigaud) and 531.4 ± 3.4 Ma (Chatham-Grenville), corresponding to Early Cambrian (Nemakit-Daldynian) time. Characteristic paleomagnetic (reversed) directions from both intrusions are easterly and shallow ( D = 89.3°, I = 30.1°; α 95 = 6.2, k = 38.8; N = 15 sites), and the Mont Rigaud intrusion also carries antipodal normal-polarity directions. The combined result yields a north paleopole at location 11.9°S 184.6°E; d p = 3.8° d m = 6.9°, placing the Montreal region at 16 ± 4°S paleolatitude in the Early Cambrian, in good agreement with Late Ediacaran and Middle to Late Cambrian paleomagnetic results from Laurentia. Laurentia likely resided at low southerly paleolatitudes throughout the Cambrian, ruling out a major inertial-interchange TPW event for Early to Middle Cambrian time. A comparison of Late Ediacaran to Middle Cambrian paleomagnetic results from Laurentia, Baltica, Siberia and elements of West Gondwana implies that by Early Cambrian time a large paleolatitudinal gap existed between Laurentia and its presumed conjugate rift margins, thus placing a wide Iapetus Ocean between them. The position of Baltica within the circum-Iapetus paleogeography is contentious, due to differing interpretations of the few paleomagnetic results. During Ediacaran time, Laurentia underwent large apparent polar wander that is manifested as motion from low latitude at 615 Ma to the south polar region at 590–575 Ma, and then rapidly back to low latitude by 565–550 Ma. The large and perhaps rapid apparent polar wander of Laurentia and other continents in the Ediacaran remains enigmatic and controversial.