Microwave paleointensities indicate a low paleomagnetic dipole moment at the Permo-Triassic boundary

AbstractThe quantity of igneous material comprising the Siberian Traps provides a uniquely excellent opportunity to constrain Earth’s paleomagnetic field intensity at the Permo-Triassic boundary. There remains however, a contradiction about the strength of the magnetic field that is exacerbated by the limited number of measurement data. To clarify the geomagnetic field behavior during this time period, for the first time, a microwave paleointensity study has been carried out on the Permo-Triassic flood basalts in order to complement existing datasets obtained using conventional thermal techniques. Samples, which have been dated at ∼250Ma, of the Permo-Triassic trap basalts from the northern extrusive (Maymecha-Kotuy region) and the southeastern intrusive (areas of the Sytikanskaya and Yubileinaya kimberlite pipes) localities on the Siberian platform are investigated. These units have already demonstrated reliable paleomagnetic directions consistent with the retention of a primary remanence. Furthermore, Scanning Electron Microscope analysis confirms the presence of iron oxides likely of primary origin. Microwave Thellier-type paleointensity experiments (IZZI protocol with partial thermoremanent magnetization checks) are performed on 50 samples from 11 sites, of which, 28 samples from 7 sites provide satisfactory paleointensity data. The samples display corresponding distinct directional components, positive pTRM checks and little or no zig-zagging of the Arai or Zijderveld plot, providing evidence to support that the samples are not influenced by lab-induced alteration or multi-domain behavior. The accepted microwave paleointensity results from this study are combined with thermal Thellier-type results from previously published studies to obtain overall estimates for different regions of the Siberian Traps. The mean geomagnetic field intensity obtained from the samples of the northern part is 13.4±12.7μT (Maymecha-Kotuy region), whereas from the southeastern part is 17.3±16.5μT (Sytikanskaya kimberlite pipe) and 48.5±7.3μT (Yubileinaya kimberlite pipe), suggesting that the regional discrepancy is probably due to the insufficient sampling of geomagnetic secular variation, and thus, multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period. It demonstrates that the overall mean paleointensity of the Siberian Traps is 19.5±13.0μT which corresponds to a mean virtual dipole moment of 3.2±1.8×1022Am2. Results indicate that the average magnetic field intensity during Permo-Triassic boundary is significantly lower (by approximately 50%) than the present geomagnetic field intensity, and thus, it implies that the Mesozoic dipole low might extend 50Myr further back in time than previously recognized

Microwave paleointensities indicate a low paleomagnetic dipole moment at the Permo-Triassic boundary

© 2016 The AuthorsThe quantity of igneous material comprising the Siberian Traps provides a uniquely excellent opportunity to constrain Earth's paleomagnetic field intensity at the Permo-Triassic boundary. There remains however, a contradiction about the strength of the magnetic field that is exacerbated by the limited number of measurement data. To clarify the geomagnetic field behavior during this time period, for the first time, a microwave paleointensity study has been carried out on the Permo-Triassic flood basalts in order to complement existing datasets obtained using conventional thermal techniques. Samples, which have been dated at ∼250 Ma, of the Permo-Triassic trap basalts from the northern extrusive (Maymecha-Kotuy region) and the southeastern intrusive (areas of the Sytikanskaya and Yubileinaya kimberlite pipes) localities on the Siberian platform are investigated. These units have already demonstrated reliable paleomagnetic directions consistent with the retention of a primary remanence. Furthermore, Scanning Electron Microscope analysis confirms the presence of iron oxides likely of primary origin. Microwave Thellier-type paleointensity experiments (IZZI protocol with partial thermoremanent magnetization checks) are performed on 50 samples from 11 sites, of which, 28 samples from 7 sites provide satisfactory paleointensity data. The samples display corresponding distinct directional components, positive pTRM checks and little or no zig-zagging of the Arai or Zijderveld plot, providing evidence to support that the samples are not influenced by lab-induced alteration or multi-domain behavior. The accepted microwave paleointensity results from this study are combined with thermal Thellier-type results from previously published studies to obtain overall estimates for different regions of the Siberian Traps. The mean geomagnetic field intensity obtained from the samples of the northern part is 13.4 ± 12.7 μT (Maymecha-Kotuy region), whereas from the southeastern part is 17.3 ± 16.5 μT (Sytikanskaya kimberlite pipe) and 48.5 ± 7.3 μT (Yubileinaya kimberlite pipe), suggesting that the regional discrepancy is probably due to the insufficient sampling of geomagnetic secular variation, and thus, multiple localities need to be considered to obtain an accurate paleomagnetic dipole moment for this time period. It demonstrates that the overall mean paleointensity of the Siberian Traps is 19.5 ± 13.0 μT which corresponds to a mean virtual dipole moment of 3.2 ± 1.8 × 1022 Am2. Results indicate that the average magnetic field intensity during Permo-Triassic boundary is significantly lower (by approximately 50%) than the present geomagnetic field intensity, and thus, it implies that the Mesozoic dipole low might extend 50 Myr further back in time than previously recognized

An exceptionally weak Devonian geomagnetic field recorded by the Viluy Traps, Siberia

The detection of anomalous time averaged geomagnetic behaviour is crucial for understanding past magnetospheric shielding and inferring deep Earth evolution. Links have been suggested between geomagnetic field variation over timescales of tens to hundreds of millions of years and processes near the core–mantle boundary (CMB); however, this becomes difficult to establish prior to the Permo-Carboniferous Reversed Superchron (PCRS; 267–319 Ma) due to a lack of reliable data. To improve the record prior to the PCRS, we present multi-method produced paleointensity results from nines dykes and lava flows from the Viluy Traps, Siberia, emplaced during the Upper Devonian between 376.7 ± 1.7 Ma and 364.4 ± 1.7 Ma. These sites have previously been published as part of two paleodirectional studies, one of which produced the accepted 360 Ma pole for Siberia (Q factor 6). All of the sites produced very weak field values ranging from 4.3–14.9 Z A m2, in close agreement with other recent results from Mid-Lower Devonian Siberian samples. QPI criteria have been used to illustrate the reliability of these new, low paleointensities, confirming the period of weak field suggested by other recent Siberian work, and the period of implied increased incidence of solar wind radiation, extended into the Upper Devonian. Along with evidence for moderate-high reversal frequencies and a potentially significant multipolar component during the Devonian, these weak field values also suggest a significantly different pattern of heat flow across the CMB relative to more recent times

New ⁴⁰Ar/³⁹Ar dating of the Antrim Plateau Volcanics, Australia: clarifying an age for the eruptive phase of the Kalkarindji continental flood basalt province

The Kalkarindji flood basalt province of northern Australia erupted in the mid-Cambrian. Today the province consists of scattered volcanic and intrusive suites, the largest being the Antrim Plateau Volcanics (APV) in Northern Territory. Accurate dating of Kalkarindji has proved challenging with previous studies focused on minor volcanics and intrusive dykes in Northern Territory and Western Australia. These previously published data, corrected to the same decay constants, range from 512.8 to 509.6 ± 2.5 Ma [2σ], placing Kalkarindji in apparent synchronicity with the Cambrian Stage 4–5 biotic crisis at 510 ± 1 Ma. This study utilises 40Ar/39Ar dating of basalts from the APV to accurately date the major volcanic eruptions in this province. Results yield an age of 508.0–498.3 ± 5.5 Ma [2σ], indicating the APV is younger than the intrusives. These dates allude to a relative timing discrepancy, where intrusive activity in the North Australian Craton preceded the eruption of the APV as the last magmatic activity in the region. The determination of these largest eruptions to be later than 510 Ma, effectively disassociates Kalkarindji lavas from being a major cause of the 510 Ma biotic crisis, but cannot definitively discount any deleterious effects on the fragile Cambrian ecosystem

Derivation of paleolongitude from the geometric parametrization of apparent polar wander path: Implication for absolute plate motion reconstruction

Obtaining ancient longitude position of continents in the past has always been a challenge for plate tectonic reconstructions. Paleomagnetism has been commonly used to reconstruct paleolatitudes and relative rotations but not paleolongitudes. In this work, we present a synthesized method to derive paleolongitude by geometrically parametrizing apparent polar wander path (APWP). Great and small circle modeling are implemented concurrently to the identified APWP tracks to calculate the paleomagnetic Euler parameters (stage rotation pole and rotation angle). From the Euler parameters of the optimal fitting option, the absolute motion history can be restored for the reference geometries. Using our method as well as the results from relative plate motion studies, we reevaluate the dispersion history of East Gondwana since 140 Ma. To further test the validity of our method, we compare the predictions from four other absolute motion models mainly in paleolatitude movement, longitudinal variation, and great circle distance, which suggest the most similarity with the global hybrid reference frame. Key Points Great and small circle APWP parametrization with paleocolatitude correction Paleolongitude can be derived from apparent polar wander path Reconstruction of East Gondwana since 140 Ma is reevaluated ©2014. American Geophysical Union. All Rights Reserved

Evolution of the Mongol-Okhotsk Ocean as constrained by new palaeomagnetic data from the Mongol-Okhotsk suture zone, Siberia

International audienceThis paper presents new data from palaeomagnetic investigations on the Upper Palaeozoic and Mesozoic geological units from the Siberian platform and the Mongol-Okhotsk suture zone. Within the southern portion of the Mongol-Okhotsk suture we collected palaeomagnetic samples from the Late Permian Belektuy formation (Borzya region; 50.7°N, 116.9°E) and the Middle-Late Jurassic Shadaron formation (Unda-Daya; 51.5°N, 117.5°E). We sampled the Late Permian Alentuy formation (Khilok region; 50.8°N, 107.2°E), the Early to Middle Jurassic Irkutsk sedimentary basin (ISB; 52.0°N, 104.0°E), the Late Jurassic Badin formation (Mogzon region; 51.8°N, 112.0°E), and the Early Cretaceous Gusinoozesk formation (Gusinoe Lake region; 51.2°N, 106.5°E) additionally in the northern region of the Mongol-Okhotsk suture. Apart from the results of the ISB and Gusinoozersk formations, which show very large ellipses of confidence and might be the present-day geomagnetic field overprint, our results allow us to constrain the evolution of the Mongol-Okhotsk Ocean palaeomagnetically from the Late Permian to the Middle-Late Jurassic. They confirm that this large Permian ocean closed during the Jurassic, ending up in the late Jurassic or the beginning of the Cretaceous in the eastern end of the suture zone, as suspected on geological grounds. However, although geological data suggest a Middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data show evidence of a still large palaeolatitude difference between the Amuria and Siberia blocks. This is interpreted as a result of the quite fast closure of the ocean after the Middle Jurassic. Finally, our new palaeomagnetic results exhibit very large tectonic rotations around local vertical axes, which we interpret as probably arising both from collision processes and from a left-lateral shear movement along the suture zone, due to the eastward extrusion of Mongolia under the effect of the collision of India into Asia

PMTec: A new MATLAB toolbox for absolute plate motion reconstructions from paleomagnetism

© 2015 Elsevier Ltd. Established on Euler rotations (rotation poles and angles), quantitative representation of plate motion history has been one of the focus fields in geoscience since the beginning of the age of plate tectonics. Here we present a new MATLAB based toolbox PMTec primarily developed for (1) construction of apparent polar wander paths (APWP, in the form of running means and spherical splines) from paleomagnetic data and (2) absolute plate motion calculations through APWP geometric parameterizations. We choose to build the graphical users interface of PMTec using MATLAB considering its powerful mapping toolbox for geospatial data visualization and its rising popularity in the geoscience community. Theoretical background, functioning modules, and data and file management in PMTec are formulated in this paper. The computational and graphical capabilities of PMTec are demonstrated using published data to provide an overview about its operation procedure and potential applications. The PMTec package and associated tutorials are available for download from the website: http://www.ualberta.ca/~vadim/software.htm. PMTec is a freeware for plate tectonic research and education purposes and allowed to be redistributed among users

Magnetic susceptibility and particle size for the Shilou red clay section on the eastern Chinese Loess Plateau

The Chinese Loess Plateau red clay sequences display a continuous alternation of sedimentary cycles that represent recurrent climatic fluctuations from 2.58 Ma to the Miocene. Deciphering such a record can provide us with vital information on global and Asian climatic variations. Lack of fossils and failure of absolute dating methods made magnetostratigraphy a leading method to build age models for the red clay sequences. Here we test the magnetostratigraphic age model against cyclostratigraphy. For this purpose we investigate the climate cyclicity recorded in magnetic susceptibility and sedimentary grain size in a red clay section previously dated 11Myr old with magnetostratigraphy alone. Magnetostratigraphy dating based on only visual correlation could potentially lead to erroneous age model. In this study the correlation is executed through the iteration procedure until it is supported by cyclostratigraphy; i.e., Milankovitch cycles are resolved in the best possible manner. Our new age model provides an age of 5.2Ma for the Shilou profile. Based on the new age model, wavelet analysis reveals the well-preserved 400 kyr and possible 100 kyr eccentricity cycles on the eastern Chinese Loess Plateau. Further, paleomonsoon evolution during 2.58-5.2Ma is reconstructed and divided into three intervals (2.58-3.6Ma, 3.6-4.5Ma, and 4.5-5.2Ma). The upper part, the youngest stage, is characterized by a relatively intensified summer monsoon, the middle stage reflects an intensification of the winter monsoon and aridification in Asia, and the earliest stage indicates that summer and winter monsoon cycles may have rapidly altered. The use of cyclostratigraphy along withmagnetostratigraphy gives us an effectivemethod of dating red clay sequences, and our results imply that many presently published age models for the red clay deposits should be perhaps re-evaluated