Triassic trachytic volcanism in the Bangong–Nujiang Ocean: geochemical and geochronological constraints on a continental rifting event

The Bangong–Nujiang suture zone (BNSZ), which separates the Gondwana-derived Qiangtang and Lhasa terranes, preserves limited geological records of the Bangong–Nujiang Ocean (BNO). The timing of opening of this ocean has been hotly debated due to the rare and complicated rock records in the suture zones, which span over 100 Ma from Carboniferous–Permian to Early Jurassic time, based on geological, palaeontological and palaeomagnetic data. A combination of geochemical, geochronological and isotopic data are reported for the Riasairi trachytes, central BNSZ, northern Tibet, to constrain its petrogenesis and tectonic settings. Zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields mean ages of 236 Ma. Geochemically, these rocks are high-K calc-alkaline with moderate SiO2 (59.1–67.5 wt%) and high K2O + Na2O (8.1–11.6 wt%) contents. They are enriched in light rare earth elements with negative Eu anomalies, and show enrichments in high-field-strength elements with positive ‘Nb, Ta’ anomalies, similar to the intra-continental rift setting-related felsic lavas from the African Rift System. The high positive zircon ϵHf(t) and bulk ϵNd(t) values, as well as high initial Pb isotopes, imply a heterogeneous source involving both asthenospheric and subcontinental lithospheric mantle. The field and geochemical data jointly suggest that the Riasairi trachytes within the Mugagangri Group were formed in a continental rift setting. We interpret that the continental-rift-related Riaisairi trachytic lavas as derived from the southern margin of the Qiangtang terrane, implying that the BNO would have opened by Middle Triassic time, well after the commonly interpreted break-up of the Qiangtang terrane from Gondwana.

Discovery of Deccan Inclination Anomaly and its possible geodynamic implications over the Indian Plate

The rapid northward drift of the Indian plate during Deccan volcanism assumes a gradual shallowing of paleomagnetic inclinations in subsequent lava flow formations. A comparison of palaeomagnetic data produced during the last six decades reveals an inclination anomaly during Chron C29r (66.398–65.688 Ma) along with brief clockwise-counter-clockwise rotations during and after the main phase Deccan eruption. This interval temporally coincides with i) an accelerated Indian ocean spreading rates, ii) brief incursion of an inland ‘seaway’ and iii) a major drop in the sea level at the southern tip of the Indian Peninsula. Furthermore, the restoration of tilt later during C29n agrees with the withdrawal of the inland seaway and the development of a regional southward dip of the Deccan lava flow formations. Here, we produce an evolutionary model to postulate the interaction of the Réunion plume with the Indian lithospheric plate with coincident geological evidences demanding further exploration.

The Age and Paleomagnetism of Jurassic dykes, western Dronning Maud Land: implications for Gondwana breakup

New 40Ar/39Ar data from dykes intruded into Sverdrupfjella and Ahlmanryggen, Dronning Maud Land, Antarctica, indicate that dyke emplacement commenced at ca. 207 Ma and lasted until ca. 178 Ma. Whereas the ages ascribed to the Karoo-age magmatism contributing to Gondwana breakup are typically inferred as being ca. 182 Ma, the data indicate that ages older than ca.192 Ma in the broader Karoo Province are restricted to western Dronning Maud Land, Antarctica, indicating the locality where break-up was initiated. Limited palaeomagnetic data from ca. 178-185 Ma dykes combined with published palaeomagnetic data from similar aged dykes in Vestfjella and the Ferrar Province, suggest that Antarctica had already drifted/rifted significantly away from southern Africa from ca. 207 Ma to ca. 180 Ma, earlier than previously thought. The data, if correct, require a re-evaluation of the ages ascribed to ocean-floor anomalies used to constrain reconstructions of Gondwana and may provide insight into the history of microcontinental blocks including the Falkland Islands, Haag nunataks, Ellsworth-Whitmore block and Maurice Ewing Bank.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5612838

Discovery of Deccan Inclination Anomaly and its possible geodynamic implications over the Indian Plate

The rapid northward drift of the Indian plate during Deccan volcanism assumes a gradual shallowing of paleomagnetic inclinations in subsequent lava flow formations. A comparison of palaeomagnetic data produced during the last six decades reveals an inclination anomaly during Chron C29r (66.398 - 65.688 Ma) along with brief clockwise-counter-clockwise rotations during and after the main phase Deccan eruption. This interval temporally coincides with i) an accelerated Indian ocean spreading rates, ii) brief incursion of an inland ‘seaway’ and iii) a major drop in the sea level at the southern tip of the Indian Peninsula. Furthermore, the restoration of tilt later during C29n agrees with the withdrawal of the inland seaway and the development of a regional southward dip of the Deccan lava flow formations. Here, we produce an evolutionary model to postulate the interaction of the Réunion plume with the Indian lithospheric plate with coincident geological evidence demanding further exploration.

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 Role of Astronomy and Feng Shui in the Planning of Ming Beijing

Present day Beijing developed on the urban layout of the Ming capital, founded in 1420 over the former city of Dadu, the Yuan dynasty capital. The planning of Ming Beijing aimed at conveying a key political message, namely that the ruling dynasty was in charge of the Mandate of Heaven, so that Beijing was the true cosmic centre of the world. We explore here, using satellite imagery and palaeomagnetic data analysys, symbolic aspects of the planning of the city related to astronomical alignments and to the feng shui doctrine, both in its “form” and “compass” schools. In particular, we show that orientations of the axes of the “cosmic” temples and of the Forbidden City were most likely magnetic, while astronomy was used in topographical connections between the temples and in the plan of the Forbidden City in itself.

Northward motion of the Burma Terrane alongside India during the Cenozoic.

<p>Recent paleomagnetic data from early Late Cretaceous and late Eocene rocks from Myanmar (1,2) demonstrate that the Burma Terrane (BT) underwent an important northward translation alongside India in the Cenozoic. We present new paleomagnetic results from Paleocene to Eocene sediments that confirm the slightly southern to equatorial paleolatitudes during the Paleocene to mid Eocene. However, these paleomagnetic results imply a new paleogeography not compatible with the typical view of the geology of Myanmar as an andean-type margin above an active subduction of the Tethys/India oceanic crust below Sundaland.  Most previous models proposed an active subduction below Myanmar during the Paleogene but a slab anchored in the mantle would impede the large northward motion of the BT implied by our paleomagnetic data. We thus review the geology of the BT in light of the new latitudinal constraints provided by the paleomagnetic data. The BT contains >10km thick Cenozoic basins (Central Myanmar Basins (CMBs)) recording the Cenozoic geological evolution of the BT. The CMBs were previously interpreted with sediment sources located within the Myanmar magmatic arc and to the east in Sibumasu. The numerous studies on detrital zircons from the Late Cretaceous - Paleogene sediments  of  the CMBs highlight a clear correlation in the distribution of the ages of the pre-Cretaceous zircons (~40% of the zircons in the sediments) with the one from the Triassic turbidites (Pane Chaung Formation) of the Indo-Burman Ranges and the Triassic sediments from the Tethyan Himalaya (Langjiexue Fm.). Thus, the source of sediments is unlikely to be in Sibumasu but proposed to be in an actively eroding north-western extension of the Indo-Burman ranges (Greater Burma block, (2)) possibly linked to the Tethyan Himalaya and consistent with a BT position within the India plate during the Cenozoic. In any case, we find little evidence for a nearby active magmatic arc in the detrital zircon record supporting the hypothesis of an active subduction below the BT. Thus this review of the geology of the BT supports a rapid northward moving BT alongside India during the Cenozoic. We will discuss the implication of this new paleogeography on the India-Asia collision models.</p><p>(1) Westerweel et al. « Burma Terrane Part of the Trans-Tethyan Arc during Collision with India According to Palaeomagnetic Data ». Nature Geoscience 12, no 10 (octobre 2019): 863‑68. https://doi.org/10.1038/s41561-019-0443-2.</p><p>(2) Westerweel et al. « Burma Terrane Collision and Northward Indentation in the Eastern Himalayas Recorded in the Eocene‐Miocene Chindwin Basin (Myanmar) ». Tectonics 39, no 10 (octobre 2020). https://doi.org/10.1029/2020TC006413.</p>

Records of the Laschamps geomagnetic polarity excursion from Black Sea sediments: magnetite versus greigite, discrete sample versus U-channel data

SUMMARY Magnetostratigraphic investigation of sediment cores from two different water depths in the SE Black Sea based on discrete samples, and parallel U-channels in one of the cores, yielded high-resolution records of geomagnetic field variations from the past about 68 ka. Age constrains are provided by three tephra layers of known age, accelerator mass spectrometry 14C dating, and by tuning element ratios obtained from X-ray fluorescence scanning to the oxygen isotope record from Greenland ice cores. Sedimentation rates vary from a minimum of ∼5 cm ka−1 in the Holocene to a maximum of ∼50 cm ka−1 in glacial marine isotope stage 4. Completely reversed inclinations and declinations as well as pronounced lows in relative palaeointensity around 41 ka provide evidence for the Laschamps geomagnetic polarity excursion. In one of the investigated cores also a fragmentary record of the Mono Lake excursion at 34.5 ka could be revealed. However, the palaeomagnetic records are more or less affected by greigite, a diagenetically formed magnetic iron sulphide. By definition of an exclusion criterion based on the ratio of saturation magnetization over volume susceptibility, greigite-bearing samples were removed from the palaeomagnetic data. Thus, only 25–55 per cent of the samples were left in the palaeomagnetic records obtained from sediments from the shallower coring site. The palaeomagnetic record from the deeper site, based on both discrete samples and U-channels, is much less affected by greigite. The comparison of palaeomagnetic data shows that the major features of the Laschamps polarity excursion were similarly recovered by both sampling techniques. However, several intervals had to be removed from the U-channel record due to the presence of greigite, carrying anomalous directions. By comparison to discrete sample data, also some directional artefacts in the U-channel record, caused by low-pass filtering of the broad magnetometer response functions, averaging across fast directional and large amplitude changes, can be observed. Therefore, high-resolution sampling with discrete samples should be the preferred technique when fast geomagnetic field variations, such as reversals and excursions, shall be studied from sedimentary records in the very detail.

Constraining the Eruption History of the Rangitoto Volcano using Palaeomagnetic Data

<p>Rangitoto is an island volcano situated outside the city of Auckland, New Zealand. The volcano is the youngest and largest volcano in the monogenetic Auckland Volcanic Field (AVF), with the last eruption occurring about 550-500 calibrated years BP, a date determined from studying historical records. The eruption history of Rangitoto is unknown, however all other volcanoes in the AVF have a brief eruption history. In February 2014 a core spanning 127 metres in length was recovered, consisting of 53 lava flows varying in thickness from 1 to 15 metres. Radiocarbon dates taken from marine sediments found at the bottom of the core, underneath the Rangitoto’s lava flows, suggest that there was early activity as far back as 6000BP, after which Rangitoto may have been dormant until the main shield building phase at around 600BP. Magnetic mineralogy analysis has also shown that much of the core is a reliable recorder of the past geomagnetic field. 156 samples have been analysed for palaeodirectional data and 21 acceptable palaeointensity estimates have been accrued from a range of depths throughout the core length. The collected palaeomagnetic data are used to reconstruct variations in the geomagnetic field, which in turn are used to constrain the eruption rate. Preliminary results suggest that the palaeomagnetic data are incompatible with a short eruption duration of the shield building phase implied by the radiocarbon data (under 100 years) and more compatible with a longer duration of shield building for Rangitoto Island. We discuss alternative explanations for this discrepancy and potential implications of our results in regard to improving hazard planning in Auckland.</p>