Earth's Spin Axis Research Articles

Embracing Uncertainty to Resolve Polar Wander: A Case Study of Cenozoic North America

AbstractOur understanding of Earth's paleogeography relies heavily on paleomagnetic apparent polar wander paths (APWPs), which represent the time‐dependent position of Earth's spin axis relative to a given block of lithosphere. However, conventional approaches to APWP construction have significant limitations. First, the paleomagnetic record contains substantial noise that is not integrated into APWPs. Second, parametric assumptions are adopted to represent spatial and temporal uncertainties even where the underlying data do not conform to the assumed distributions. The consequences of these limitations remain largely unknown. Here, we address these challenges with a bottom‐up Monte Carlo uncertainty propagation scheme that operates on site‐level paleomagnetic data. To demonstrate our methodology, we present an extensive compilation of site‐level Cenozoic paleomagnetic data from North America, which we use to generate a high‐resolution APWP. Our results demonstrate that even in the presence of substantial noise, polar wandering can be assessed with unprecedented temporal and spatial resolution.

Absolute Paleolatitude of Northern Zealandia From the Middle Eocene to the Early Miocene

AbstractThe absolute position during the Cenozoic of northern Zealandia, a continent that lies more than 90% submerged in the southwest Pacific Ocean, is inferred from global plate motion models, because local paleomagnetic constraints are virtually absent. We present new paleolatitude constraints using paleomagnetic data from International Ocean Discovery Program Site U1507 on northern Zealandia and Site U1511 drilled in the adjacent Tasman Sea Basin. After correcting for inclination shallowing, five paleolatitude estimates provide a trajectory of northern Zealandia past position from the middle Eocene to the early Miocene, spanning geomagnetic polarity chrons C21n to C5Er (∼48–18 Ma). The paleolatitude estimates support previous works on global absolute plate motion where northern Zealandia migrated 6° northward between the early Oligocene and early Miocene, but with lower absolute paleolatitudes, particularly in the Bartonian and Priabonian (C18n–C13r). True polar wander (solid Earth rotation with respect to the spin axis), which only can be resolved using paleomagnetic data, may explain the discrepancy. This new paleomagnetic information anchors past latitudes of Zealandia to Earth's spin axis, with implications not only for global geodynamics, but also for addressing paleoceanographic and paleoclimate problems, which generally require precise paleolatitude placement of proxy data.

Jurassic true polar wander recorded by the Lhasa terrane on its northward journey from Gondwana to Eurasia

Paleomagnetic data constrain paleogeographic motion of rocks relative to the Earth's spin axis, which is a sum of plate motion relative to the mantle and true polar wander. Discerning between these effects is challenging for studies aiming to reconstruct paleo-plate motions from deformed orogenic terranes. Here, we study the paleolatitudinal drift history of the Lhasa terrane of southern Tibet that migrated from the northern Gondwana to the southern Eurasian margin between late Triassic and early Cretaceous time. Previous work identified a 180 Ma near-equatorial Lhasa latitude and assumed near-constant paleolatitudinal drift. Large-scale true polar wander at this time, however, which has been argued for in previous work, requires highly variable Lhasa plate motion rates relative to Gondwana. Here, we test whether the alternative interpretation of constant plate motion rates provides a better prediction of paleomagnetic data. To this end, we present a new paleomagnetic pole from ∼155 Ma volcanics (here dated by U/Pb zircon) of the Lhasa terrane. Our pole comprises site mean directions from 46 lavas, passes a fold test, and is supported by an extensive rock magnetic and microscopic analysis that reveals no evidence of remagnetization. Our results give an average direction of D±ΔDx=337.1°±2.6°, I±ΔIx=−13.8°±4.9°, and a corresponding paleopole position at λp=45.3°N, φp=295.3°E with K=69.8 and A95=2.5°, predicting a similar near-equatorial, paleolatitude as at 180 Ma. This paleolatitudinal standstill is consistent with predicted paleolatitudes from global APWPs that account for Jurassic TPW and assume a constant Lhasa-Gondwana (India) oceanic spreading rate of ∼8 cm/a during the 215-130 Ma opening of the Neotethys and closure of the Mesotethys oceans. Contemporaneous arc magmatism on the Lhasa terrane was previously interpreted to indicate that the Lhasa terrane was located above a subduction zone during its northward journey to Tibet: our results show that if our reconstruction is accurate, then trench migration rates also must have been ∼8 cm/a. Our results are consistent with the range of rates of global long-lived trench migration that have been reconstructed in other studies, but they are much higher than measured in most modern systems. Our results more completely document the Lhasa terrane's separation from Gondwana and provide a straightforward methodology of using paleomagnetic data to test plate kinematic scenarios, overcoming the problem of the unknown effect of true polar wander on paleomagnetic data from deformed terranes.

What drives 20th century polar motion?

Astrometric and geodetic measurements show that the mean position of Earth's spin axis drifted through the solid crust toward Labrador, Canada at an average speed of 10.5±0.9 cm/yr during the 20th century. Understanding the origins of this secular polar motion (SPM) has significance for modeling the global climate, as it provides a link to ice mass balance and sea-level rise. A perplexing issue, however, is that while glacial isostatic adjustment (GIA) models satisfactorily explain the direction of SPM, the associated prediction of the amplitude is insufficient. Our Bayesian GIA analysis, with constraints from relative sea-level and vertical land motion data, reveals that this process only accounts for 33±18% of the observed SPM amplitude. This shortfall motivates a more broadly scoped reassessment of SPM drivers. To address this, we assemble a complete reconstruction of Earth's surface mass transport derived from recent advancements in modeling the global 20th century cryospheric, hydrologic, oceanic, and seismogenic mass exchange. The summed signals, nonetheless, cannot fully reconcile the observed SPM, even when considering the error statistics of each driver. We investigate an additional excitation source: changes in Earth's inertia tensor caused by mantle convection. Sophisticated models have recently been advanced in tectonic plate reconstructions, in conjunction with geoid and seismic tomographic models. Here we use these models to compute new estimates of SPM. While the convection-driven SPM has considerable uncertainty, the average direction of 283 recent models aligns with the residual SPM (within 2.7∘±14.8∘), significantly reducing the gap between observation and prediction. We assert that one key mechanism for driving 20th century SPM is long-term mass movement due to mantle convection.

A new solar wind‐driven global dynamic plasmapause model: 1. Database and statistics

AbstractA large database, possibly the largest plasmapause location database, with 49,119 plasmapause crossing events from the in situ observations and 3957 plasmapause profiles (corresponding to 48,899 plasmapause locations in 1 h magnetic local time (MLT) intervals) from optical remote sensing from 1977 to 2015 by 18 satellites is compiled. The responses of the global plasmapause to solar wind and geomagnetic changes and the diurnal, seasonal, solar cycle variations of the plasmapause are investigated based on this database. It is found that the plasmapause shrinks toward the Earth globally and a clear bulge appears in the afternoon to premidnight MLT sector as the solar wind or geomagnetic conditions change from quiet to disturbed. The bulge is clearer during storm times or southward interplanetary magnetic field. The diurnal variations of the plasmapause are most probably the result of the difference between the magnetic dipole tilt and the Earth's spin axis. The seasonal variations of the plasmapause are characterized by equinox valleys and solstice peaks. It is also found that the plasmapause approaches the Earth during high solar activity and expands outward during low solar activity. This database will help us study and understand the evolution properties of the plasmapause shape and the interaction processes of the plasmasphere, the ring current, and the radiation belts in the magnetosphere.

The Impact of the Earth’s Movement through the Space on Measuring the Velocity of Light (Part Two)

In this paper is presented an experiment that has a goal to measure the speed of light in one direction, using one clock and one mirror. To achieve this, we have to make the measurements during the period of one year (nine months at least), taking into account earth’s movement through the space, earth’s rotation around its axis and the fact that earth spin axis is tilted with respect to the plane of its orbit of the sun.

PRE-LATE HEAVY BOMBARDMENT EVOLUTION OF THE EARTH's OBLIQUITY

The Earth's obliquity is stabilized by the Moon, which facilitates a rapid precession of the Earth's spin-axis, de-tuning the system away from resonance with orbital modulation. It is however, likely that the architecture of the Solar System underwent a dynamical instability-driven transformation, where the primordial configuration was more compact. Hence, the characteristic frequencies associated with orbital perturbations were likely faster in the past, potentially allowing for secular resonant encounters. In this work we examine if at any point in the Earth's evolutionary history, the obliquity varied significantly. Our calculations suggest that even though the orbital perturbations were different, the system nevertheless avoided resonant encounters throughout its evolution. This indicates that the Earth obtained its current obliquity during the formation of the Moon.

Why is the areoid like the residual geoid?

The equipotential figures of Earth (residual geoid), and of Mars (areoid) are characterized by pairs of elevated and pairs of depressed antipodal equatorial regions. Elevated regions of the residual geoid lie vertically above the two Large Low Shear Wave Velocity Provinces (LLSVPs) on the Core Mantle Boundary (CMB). There is evidence that the LLSVPs are dense, physicochemically distinct, and have been stable in their positions with respect to each other and to the Earth's spin axis for at least ∼550 My. The final event in the planetary‐scale development of the Earth's structure was the moon‐forming event and the comparable event on Mars is thought by many to have been the Borealis basin‐forming impact. We attribute the formation of the LLSVPs and postulated comparable masses underlying the elevated regions of the areoid on the CMB of Mars, to the immediate after‐effects of those two giant impacts.

Constraining models of postglacial rebound using space geodesy: a detailed assessment of model ICE-5G (VM2) and its relatives

Using global positioning system, very long baseline interferometry, satellite laser ranging and Doppler Orbitography and Radiopositioning Integrated by Satellite observations, including the Canadian Base Network and Fennoscandian BIFROST array, we constrain, in models of postglacial rebound, the thickness of the ice sheets as a function of position and time and the viscosity of the mantle as a function of depth. We test model ICE-5G VM2 T90 Rot, which well fits many hundred Holocene relative sea level histories in North America, Europe and worldwide. ICE-5G is the deglaciation history having more ice in western Canada than ICE-4G; VM2 is the mantle viscosity profile having a mean upper mantle viscosity of 0.5 × 1021 Pa s and a mean uppermost-lower mantle viscosity of 1.6 × 1021 Pa s; T90 is an elastic lithosphere thickness of 90 km; and Rot designates that the model includes (rotational feedback) Earth's response to the wander of the North Pole of Earth's spin axis towards Canada at a speed of ≈1° Myr−1. The vertical observations in North America show that, relative to ICE-5G, the Laurentide ice sheet at last glacial maximum (LGM) at ≈26 ka was (1) much thinner in southern Manitoba, (2) thinner near Yellowknife (Northwest Territories), (3) thicker in eastern and southern Quebec and (4) thicker along the northern British Columbia–Alberta border, or that ice was unloaded from these areas later (thicker) or earlier (thinner) than in ICE-5G. The data indicate that the western Laurentide ice sheet was intermediate in mass between ICE-5G and ICE-4G. The vertical observations and GRACE gravity data together suggest that the western Laurentide ice sheet was nearly as massive as that in ICE-5G but distributed more broadly across northwestern Canada. VM2 poorly fits the horizontal observations in North America, predicting places along the margins of the Laurentide ice sheet to be moving laterally away from the ice centre at 2 mm yr−1 in ICE-4G and 3 mm yr−1 in ICE-5G, in disagreement with the observation that the interior of the North American Plate is deforming more slowly than 1 mm yr−1. Substituting VM5a T60 for VM2 T90, that is, introducing into the lithosphere at its base a layer with a high viscosity of 10 × 1021 Pa s, greatly improves the fit of the horizontal observations in North America. ICE-4G VM5a T60 Rot predicts most of the North American Plate to be moving horizontally more slowly than ≈1 mm yr−1, in agreement with the data. ICE-5G VM5a T60 Rot well fits both the vertical and horizontal observations in Europe. The space geodetic data cannot distinguish between models with and without rotational feedback, in the vertical because the velocity of Earth' centre is uncertain, and in the horizontal because the areas of the plate interiors having geodetic sites is not large enough to detect the small differences in the predictions of rotational feedback going across the plate interiors.

Absolute plate motions and true polar wander in the absence of hotspot tracks

The motion of continents relative to the Earth's spin axis may be due either to rotation of the entire Earth relative to its spin axis--true polar wander--or to the motion of individual plates. In order to distinguish between these over the past 320 Myr (since the formation of the Pangaea supercontinent), we present here computations of the global average of continental motion and rotation through time in a palaeomagnetic reference frame. Two components are identified: a steady northward motion and, during certain time intervals, clockwise and anticlockwise rotations, interpreted as evidence for true polar wander. We find approximately 18 degrees anticlockwise rotation about 250-220 Myr ago and the same amount of clockwise rotation about 195-145 Myr ago. In both cases the rotation axis is located at about 10-20 degrees W, 0 degrees N, near the site that became the North American-South American-African triple junction at the break-up of Pangaea. This was followed by approximately 10 degrees clockwise rotation about 145-135 Myr ago, followed again by the same amount of anticlockwise rotation about 110-100 Myr ago, with a rotation axis in both cases approximately 25-50 degrees E in the reconstructed area of North Africa and Arabia. These rotation axes mark the maxima of the degree-two non-hydrostatic geoid during those time intervals, and the fact that the overall net rotation since 320 Myr ago is nearly zero is an indication of long-term stability of the degree-two geoid and related mantle structure. We propose a new reference frame, based on palaeomagnetism, but corrected for the true polar wander identified in this study, appropriate for relating surface to deep mantle processes from 320 Myr ago until hotspot tracks can be used (about 130 Myr ago).

Elastic Anisotropy of Earth's Inner Core

Earth's solid-iron inner core is elastically anisotropic. Sound waves propagate faster along Earth's spin axis than in the equatorial plane. This anisotropy has previously been explained by a preferred orientation of the iron alloy hexagonal crystals. However, hexagonal iron becomes increasingly isotropic on increasing temperature at pressures of the inner core and is therefore unlikely to cause the anisotropy. An alternative explanation, supported by diamond anvil cell experiments, is that iron adopts a body-centered cubic form in the inner core. We show, by molecular dynamics simulations, that the body-centered cubic iron phase is extremely anisotropic to sound waves despite its high symmetry. Direct simulations of seismic wave propagation reveal an anisotropy of 12%, a value adequate to explain the anisotropy of the inner core.

Palaeomagnetism of Cretaceous Pacific Seamounts*

Summary. The palaeomagnetism of Cretaceous Pacific seamounts, as deduced from their magnetic anomalies, is reviewed. Results from 35 seamounts are reported here for the first time. Of these 35 results, 15 are of high enough quality to be useful. This has resulted in a considerable increase in the available information concerning the palaeomagnetism of the Pacific plate. Using only the most reliable results, we calculate a palaeomagnetic pole for the upper Cretaceous at 58° North, 350° East, with a 95 per cent cone of confidence of only 5°. This pole is considerably more accurate than the one previously reported and is farther to the west. The previously reported difference between the mean pole position from seamounts close to Japan and the mean pole position from seamounts close to Hawaii is not real, although one group of seamounts close to Japan gives an anomalous pole position. Several mechanisms have been suggested whereby errors might be introduced in the determinations of pole positions from seamounts. No error is introduced in the pole position because of the restricted extent of surveys and the consequent inaccuracy in the determination of the regional field. The demagnetization effect, which can produce differences between the observed direction of magnetization and the direction of the magnetizing field in an elongated body, is unimportant for seamounts. The new pole position is sufficiently accurate for use in many types of calculations in which it is desired to locate the Pacific plate with respect to the Earth's spin axis.

Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway

Research Article| September 01, 2006 Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway Adam C. Maloof; Adam C. Maloof 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Galen P. Halverson; Galen P. Halverson 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Joseph L. Kirschvink; Joseph L. Kirschvink 2Division of Geological and Planetary Sciences, California Institute of Technology 170-25, 1200 East California Boulevard, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Daniel P. Schrag; Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Benjamin P. Weiss; Benjamin P. Weiss 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Paul F. Hoffman Paul F. Hoffman 5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Adam C. Maloof 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Galen P. Halverson 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Joseph L. Kirschvink 2Division of Geological and Planetary Sciences, California Institute of Technology 170-25, 1200 East California Boulevard, Pasadena, California 91125, USA Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Benjamin P. Weiss 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA Paul F. Hoffman 5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Publisher: Geological Society of America Received: 28 Aug 2005 Revision Received: 27 Apr 2006 Accepted: 30 Apr 2006 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (9-10): 1099–1124. https://doi.org/10.1130/B25892.1 Article history Received: 28 Aug 2005 Revision Received: 27 Apr 2006 Accepted: 30 Apr 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Adam C. Maloof, Galen P. Halverson, Joseph L. Kirschvink, Daniel P. Schrag, Benjamin P. Weiss, Paul F. Hoffman; Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway. GSA Bulletin 2006;; 118 (9-10): 1099–1124. doi: https://doi.org/10.1130/B25892.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5–6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in δ13C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway

Research Article| September 01, 2006 Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway Adam C. Maloof; Adam C. Maloof 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Galen P. Halverson; Galen P. Halverson 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Joseph L. Kirschvink; Joseph L. Kirschvink 2Division of Geological and Planetary Sciences, California Institute of Technology 170-25, 1200 East California Boulevard, Pasadena, California 91125, USA Search for other works by this author on: GSW Google Scholar Daniel P. Schrag; Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Benjamin P. Weiss; Benjamin P. Weiss 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Paul F. Hoffman Paul F. Hoffman 5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Adam C. Maloof 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Galen P. Halverson 1Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Joseph L. Kirschvink 2Division of Geological and Planetary Sciences, California Institute of Technology 170-25, 1200 East California Boulevard, Pasadena, California 91125, USA Daniel P. Schrag 3Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Benjamin P. Weiss 4Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, USA Paul F. Hoffman 5Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Massachusetts 02138, USA Publisher: Geological Society of America Received: 28 Aug 2005 Revision Received: 27 Apr 2006 Accepted: 30 Apr 2006 First Online: 08 Mar 2017 Online Issn: 1943-2674 Print Issn: 0016-7606 Geological Society of America GSA Bulletin (2006) 118 (9-10): 1099–1124. https://doi.org/10.1130/B25892.1 Article history Received: 28 Aug 2005 Revision Received: 27 Apr 2006 Accepted: 30 Apr 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Adam C. Maloof, Galen P. Halverson, Joseph L. Kirschvink, Daniel P. Schrag, Benjamin P. Weiss, Paul F. Hoffman; Combined paleomagnetic, isotopic, and stratigraphic evidence for true polar wander from the Neoproterozoic Akademikerbreen Group, Svalbard, Norway. GSA Bulletin 2006;; 118 (9-10): 1099–1124. doi: https://doi.org/10.1130/B25892.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract We present new paleomagnetic data from three Middle Neoproterozoic carbonate units of East Svalbard, Norway. The paleomagnetic record is gleaned from 50 to 650 m of continuous, platformal carbonate sediment, is reproduced at three locations distributed over >100 km on a single craton, and scores a 5–6 (out of 7) on the Van der Voo (1990) reliability scale. Two >50° shifts in paleomagnetic direction are coincident with equally abrupt shifts in δ13C and transient changes in relative sea level. We explore four possible explanations for these coincidental changes: rapid plate tectonic rotation during depositional hiatus, magnetic excursions, nongeocentric axial-dipole fields, and true polar wander. We conclude that the observations are explained most readily by rapid shifts in paleogeography associated with a pair of true polar wander events. Future work in sediments of equivalent age from other basins can test directly the true polar wander hypothesis because this type of event would affect every continent in a predictable manner, depending on the continent's changing position relative to Earth's spin axis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Directional statistics for realistic weakly interacting massive particle direct detection experiments. II. 2D readout

The direction dependence of the WIMP direct detection rate provides a powerful tool for distinguishing a WIMP signal from possible backgrounds. We study the number of events required to discriminate a WIMP signal from an isotropic background for a detector with 2-d readout using nonparametric circular statistics. We also examine the number of events needed to (i) detect a deviation from rotational symmetry, due to flattening of the Milky Way halo and (ii) detect a deviation in the mean direction due to a tidal stream. If the senses of the recoils are measured then of order 20---70 events (depending on the plane of the 2-d readout and the detector location) will be sufficient to reject isotropy of the raw recoil angles at $90%$ confidence. If the senses can not be measured these number increase by roughly 2 orders of magnitude (compared with an increase of 1 order of magnitude for the case of full 3-d readout). The distributions of the reduced angles, with the (time-dependent) direction of solar motion subtracted, are far more anisotropic, however, and if the isotropy tests are applied to these angles then the numbers of events required are similar to the case of 3-d readout. A deviation from rotational symmetry will only be detectable if the Milky Way halo is significantly flattened. The deviation in the mean direction due to a tidal stream is potentially detectable, however, depending on the density and direction of the stream. The meridian plane (which contains the Earth's spin axis) is, for all detector locations, the optimum readout plane for rejecting isotropy. However readout in this plane can not be used for detecting flattening of the Milky Way halo or a stream with direction perpendicular to the galactic plane. In these cases the optimum readout plane depends on the detector location.

Free core nutation: direct observations and resonance effects

The motion of Earth's spin axis in space is monitored by Very Long-Baseline Interferometry (VLBI), and since 1994 also its rate is measured by Global Positioning System (GPS). The method of “combined smoothing”, developed recently at the Astronomical Institute in Prague enables one to combine both series. The solution, based on observations in 1979.6–2004.7, is compared with the recently adopted IAU2000 model of precession-nutation. The analysis reveals substantial residual offsets from the model. The dominant peak in frequency domain can be identified with Retrograde Free Core Nutation (RFCN), and we find also statistically significant deviations of several forced nutation terms. From the direct analysis of VLBI observations of celestial pole offsets from IAU2000 model of precession-nutation, it follows that the dominant period apparently grew from the average 435 days (VLBI observations in 1983.7–2004.7) to 460 days (VLBI/GPS combination in 1994.3–2004.6). A study of indirect determination of the RFCN period from the observed forced nutation terms through the resonance effects is presented, and the difference from the value found by direct analysis is discussed. It is demonstrated that the resonance approach does not confirm the apparent change of the period obtained from the direct analysis of observed celestial pole offsets; the resonance value is close to 430 solar days, and it seems to be very stable in time. The different value found from the direct analysis of observations can be probably ascribed to a small additional excitation by the atmosphere and/or ocean with retrograde terrestrial period around 23h 53min mean solar time.

Paleomagnetic and geochronological constraints on the post-collisional northward convergence of the southwest Tian Shan, NW China

A combined paleomagnetic and geochronological study is reported of Paleogene basalt lavas and an intercalated red bed succession, comprising a minimum of 14 basalt flows and 10 red bed horizons in the Tuoyun Basin of the southwest Tian Shan Range, China. Two basalt matrix samples yield 40Ar / 39Ar isochron ages of 58.5 ± 1.3 Ma (2 σ, MSWD = 0.9) and 60.4 ± 1.3 Ma (2 σ, MSWD = 1.7). These compare well with a previously published K–Ar dilution age of 61.7 ± 2.3 Ma for comparable Paleogene basalts and confirm that the younger pulse of magmatism in this basin is represented by both intrusive and extrusive activity. Demagnetization and component analysis identify a stable characteristic remanence (ChRM) with predominantly reversed polarity following removal of secondary remanence by peak demagnetization steps below 250–350 °C or 5 mT. Rock magnetic analysis identifies pseudo-single domain magnetite or titanomagnetite as carriers. The stable ChRM passes a fold test; it was probably acquired at the time of lava emplacement. Results from the bulk of the collection imply that paleomagnetic data from the upper and lower (∼ 115 Ma) basalt series in the Tuoyun Basin are not distinguishable at the 95% significance level and indicate that this tectonic domain remained essentially stationary with respect to the Earth's spin axis for ∼ 50 Ma prior to onset of the India/Asia collision in early Eocene times. It is therefore probable that no paleomagnetically detectable crustal shortening occurred in the southwest Tian Shan prior to collision. Paleomagnetic data sets from the Tuoyun Basin also show that little or no paleolatitude difference is present between the Tian Shan and the reference latitude of Eurasia at ∼ 60 Ma. This supports previous evidence suggesting that central Asian blocks in the vicinity of the Tian Shan are unlikely to have experienced appreciable northward convergence relative to Eurasia since onset of the India/Asia collision and initiation of the Himalaya.

Derivation of Large Igneous Provinces of the past 200 million years from long-term heterogeneities in the deep mantle

Large Igneous Provinces (LIPs) result from local catastrophically rapid dissipation of great quantities of internal heat. LIPs are overwhelmingly of basaltic affinity representing partial melting of the mantle at shallow depths but whether any of the heat or material involved in the generation of LIP rocks comes from great depth has remained controversial. To address this fundamental issue we restored 25 LIPs of the past 200 My to their eruption sites using a new global palaeomagnetic reference model. Ninety percent of the LIPs, when erupted, lay above low-velocity seismic-shear-wave regions of the Dʺ zone just above the core–mantle boundary (CMB) and ∼50% overlay CMB low-velocity regions with δ V s≤−1%. Considering the modifying effects of plume advection, palaeolongitudinal uncertainty and plate circuit errors, the majority of the restored LIPs may in fact overlie regions with δ V s≤−1%. Because those low velocity regions occupy only 27% of the Dʺ zone, the concentration of LIPs above them indicates that the low velocity (hotter?) regions are the sources of the mantle plumes that generated the LIPs. We demonstrate that most LIPs of the past 200 My owe their origin to plumes that rose from low-velocity regions of the lower mantle, and that this long-term association indicates that the low-velocity regions have been relatively stationary with respect to the Earth's spin-axis and the core since the Early Jurassic, and perhaps since the Permo-Triassic boundary.

Heterogeneity and anisotropy of seismic attenuation in the inner core

We evaluate seismic attenuation in the inner core from the spectral ratio between the seismic phase PKP(DF) and either PKP(AB) or PKP(BC) at the epicentral distances from 150° to 170°. The global data set is divided into two groups according to the position of the turning point of PKP(DF) either in the eastern (from 50°E to 120°W) or the western hemisphere. In the eastern hemisphere the parameter t★ shows strong dependence on the epicentral distance and no dependence on the angle between the Earth's spin axis and PKP(DF) ray at the turning point. In the western hemisphere t★ is weakly dependent on the distance, and varies between approximately 0.7 s and 0.1 s for the angles between 0° and 90°, respectively. Our results allow us to reconcile seemingly incompatible results of many previous studies. The anisotropy of attenuation is correlated with the anisotropy of P velocity.

Seismic anisotropy in the top 400 km of the inner core beneath the “eastern” hemisphere

Depth variation in seismic anisotropy at the top 400 km of the inner core beneath the “eastern” hemisphere is investigated by comparing the differential travel times of PKP phases recorded at distances from 127° to 160°. At distances less than about 150°, there is no noticeable difference in differential travel times for PKP phases sampling both the polar and the equatorial paths. At greater distances, however, PKIKP phases arrive systematically earlier (about 0 ∼ 0.5 second) in the polar path than in the equatorial path. These observations suggest that, in the “eastern” hemisphere, seismic anisotropy exists only in depths greater than about 200 km below the inner core boundary (ICB) with seismic velocities along the polar paths (an average of 27.6° from the earth's spin axis) being 0.4% faster than those along the equatorial paths.