A Late Cretaceous true polar wander oscillation

True polar wander (TPW), or planetary reorientation, is well documented for other planets and moons and for Earth at present day with satellites, but testing its prevalence in Earth’s past is complicated by simultaneous motions due to plate tectonics. Debate has surrounded the existence of Late Cretaceous TPW ca. 84 million years ago (Ma). Classic palaeomagnetic data from the Scaglia Rossa limestone of Italy are the primary argument against the existence of ca. 84 Ma TPW. Here we present a new high-resolution palaeomagnetic record from two overlapping stratigraphic sections in Italy that provides evidence for a ~12° TPW oscillation from 86 to 78 Ma. This observation represents the most recent large-scale TPW documented and challenges the notion that the spin axis has been largely stable over the past 100 million years.

The paleoinclination of the ancient lunar magnetic field from an Apollo 17 basalt

Paleomagnetic studies of Apollo samples indicate that the Moon generated a core dynamo lasting for at least 2 billion years. However, the geometry of the lunar magnetic field is still largely unknown because the original orientations of essentially all Apollo samples have not been well-constrained. Determining the direction of the lunar magnetic field over time could elucidate the mechanism by which the lunar dynamo was powered and whether the Moon experienced true polar wander. Here we present measurements of the lunar magnetic field 3.7 billion years (Ga) ago as recorded by Apollo 17 mare basalts 75035 and 75055. These samples formed as part of basalt flows in the Taurus-Littrow valley that make up wall-rock within Camelot crater, now exposed at the rim of the crater. Using apparent layering in the parent boulder for 75055, we inferred its original paleohorizontal orientation on the lunar surface at the time of magnetization. We find that 75035 and 75055 record a mean paleointensity of ~50 µT. Furthermore, 75055 records a paleoinclination of 34 ± 11°. This inclination is consistent with, but does not require, a selenocentric axial dipole field geometry (i.e., a dipole in the center of the Moon and aligned along the spin axis). Additionally, although true polar wander is also not required by our data, true polar wander paths inferred from some independent studies of lunar hydrogen deposits and crustal magnetic anomalies are consistent with our measured paleoinclination.

New age constraints on the Ouarzazate Group (Morocco): implications on the hypothesis of True Polar Wander during the Ediacaran

<p>The Ediacaran (635-541 Ma) is the last geological period of the Precambrian during which major changes occurred in the superficial layers of the Earth (biosphere, cryosphere, oceans, atmosphere). The paleomagnetic data from the main continents of this epoch display very fast polar wander excursions, which seemed to occur simultaneously on several continents. Two main competing hypotheses have been proposed in the literature to explain these data: (1) very fast True Polar Wander episodes (TPW), which represent the global movement of the mantle and the crust with respect to the Earth's spin axis, or (2) perturbations of the Earth’s magnetic field. On geological timescales, the TPW is speed-limited to some degrees per million years while magnetic field changes could be much faster (degrees per kyrs). The velocity of the polar wander excursions of the Ediacaran is therefore a critical parameter to distinguish these two families of solutions. The volcanic rocks of the Ouarzazate group (575-545 Ma) in the Anti-Atlas belt recorded a large polar wander excursion from ~571 to ~565 Ma, which is also observed in Laurentia and Baltica at about the same time. Because the age uncertainties are too high, the existing SHRIMP U-Pb ages obtained on zircons are not precise enough to distinguish these two hypotheses. In this study, we bring new high-precision CA ID-TIMS ages on zircons from seven tuff layers that recorded the rapid paleomagnetic variations. Our results show, in most of the samples, a large spread in age, indicating either the presence of inherited zircons or strong Pb loss in some of the zircons. Four of the samples display a good consistency in the zircon ages, and could represent the age of the tuff emplacement. In this presentation, we will discuss the two hypotheses based on these new geochronological constraints.</p>

Skewness Pole from Magnetic Anomaly C21n Implies Rapid Early Eocene True Polar Wander

<p>Skewness analysis of marine magnetic anomalies is the most misunderstood methodology in paleomagnetism. Such analysis has several advantages. First, marine magnetic anomalies innately average secular variation. Second, paleomagnetic poles determined by analysis of their skewness are not biased by overprints. Third, skewness analysis can determine high precision paleomagnetic poles. Specifically, skewness analysis of magnetic anomalies recording Late Cretaceous and early to mid-Cenozoic seafloor spreading between the Pacific and Farallon plates, because of their geometry with respect to the paleo-spin axis, results in high-precision paleomagnetic poles. These anomalies in many cases span ~140° of effective remanent inclination over a span of ~40° of latitude, reducing uncertainty by a factor of ~0.3 when mapping from direction space to pole space (Zheng et al. 2018).</p><p>Paleomagnetic poles have been previously determined from skewness analysis for six Pacific plate anomalies: C32n (74-71 Ma), C31n-C27r (60-63 Ma), C26r (62-59 Ma), C25r (59-58 Ma), C24r (57-54 Ma), C20r (46-43 Ma), and C12r (33-31 Ma). The younger group, C20r and C12r, together with independent paleo-spin axis estimates from the paleo-distribution of sediment accumulation rates from 12-46 Ma, define an approximately stationary paleo-spin axis location relative to the Pacific hotspots but offset from the current spin axis by 3°. The older group, 74-54 Ma, also shows that the Pacific hotspots remained approximately stationary relative to an additional paleo-spin axis location separated by 8° from the 12-46-Ma paleo-spin axis, implying an episode of reorientation of the entire solid earth – i.e., true polar wander (TPW) – of ~8° over at most 8 Ma between 54 and 46 Ma, or a rate of TPW of ~1°/Ma or more.</p><p>To constrain the timing and rate of reorientation, we analyze anomaly C21n (47-46 Ma), the youngest anomaly inside the 54-46-Ma interval. We incorporate 33 total-intensity ship- and 11 vector aero-magnetic track lines and find a well-constrained paleomagnetic pole near 77N, 23E in the fixed-Pacific plate reference frame.</p><p>Our new paleomagnetic pole is consistent with a prior, more uncertain, 48-Ma paleo-spin axis location from the paleo-distribution of sediment accumulation rates. When reconstructed into the Pacific hotspot reference frame, our new paleomagnetic pole lies close to the younger 46 to 12-Ma TPW stillstand location, indicating that true polar wander was completed by 47 Ma, if not earlier. Thus the ~8° shift occurred in, at most, 6.0 Ma at a rate of at least ~1.3°/Ma, and potentially even faster. The lower bound of ~1.3°/Ma of TPW indicate that Early Eocene TPW is comparable to the rate of present-day TPW (~1.1°/Ma extrapolated from geodetic data (Argus and Gross, 2004)). This new pole bounds the Early Eocene TPW episode between approximately the old and young ends of the Early Eocene Climatic Optimum (EECO; 53.2-49.1 Ma (Westerhold et al. 2018)). Thus, there may be a link between Early Eocene TPW and important climate events, such as the frequency of hyperthermals and the onset of Eocene cooling. In addition, TPW was likely complete before the 47.4-Ma age of the bends in Pacific plate hotspot chains (Gaastra & Gordon, this meeting).</p>

The Neoproterozoic geomagnetic field: new insights from a high-resolution paleomagnetic study in South China

<p>The paleomagnetic record during the middle Neoproterozoic (~825-780 Ma) displays rapid apparent polar wander variations leading to large discrepancies in paleogeographic reconstructions. Some authors propose that these data may represent true polar wander events, which correspond to independent motion of the mantle and lithosphere with respect to Earth’s rotation axis. An alternative explanation might be a perturbation of the geomagnetic field, such as a deviation from a predominantly dipole field or a hyper-reversing field. To test these hypotheses, we sampled 1200 oriented cores over a stratigraphic height of 100 metres in sedimentary rocks of the 820-810 Ma Laoshanya Formation in South China. We will present preliminary paleomagnetic and rock magnetic analyses together with results of petrologic and geochemical experiments to better understand the origin of the paleomagnetic signal.</p>

Inverted South China: A novel configuration for Rodinia and its breakup

Disentangling records of Rodinia fragmentation and true polar wander remains a challenge for understanding late Tonian plate tectonics. The ca. 760 Ma lower member of the Liántuó Formation, South China, yields a primary paleomagnetic remanence that passes both the fold and reversal tests. This new result and recently reported ca. 800 Ma data from elsewhere in South China suggest a new interpretation of its apparent polar wander path, whereby pre–770 Ma poles have inverted absolute polarity relative to traditional interpretations. Based on this inversion, and an interpretation of several oscillations of true polar wander documented by global data during 810–760 Ma, we propose a novel reconstruction for Rodinia and its breakup. Our reconstruction places the South China, India, and Kalahari cratons to the southwest of Laurentia, with connections that might have been established as early as ca. 1000 Ma. Our model also suggests that initial rifting of Rodinia occurred at ca. 800 Ma via fast northward motion of the India craton and South China.