The Marwar Supergroup (NW Peninsular India) is thought to be of Ediacaran-Cambrian age, based on previous paleontological and geochronological studies. However, direct constraints on the onset of sedimentation within the Marwar basin are still scarce. In this study, we report U–Pb zircon, LA-ICP-MS, and SIMS ages from the Chhoti Khatu felsic volcanic rocks, interlayered with the Jodhpur Group sandstones (Lower Marwar Supergroup). The cathodoluminescence images of the zircons indicate complex morphologies, and core-rim textures coupled with the wide range of ages indicate that they are likely inherited or in the case of thin poorly indurated ash-beds, detrital in origin. The age spectra of 68 zircon analyses from our sampling display a dominant 800–900 Ma age peak corresponding to the age of basement "Erinpura granite" rocks in the region. The youngest inherited zircon from a felsic ash layer yielded a U–Pb age of 651 Ma ± 18 Ma that, together with previous studies and paleontological evidence, indicates a post-Cryogenian age for the initiation of Marwar sedimentation following a ~125 Ma hiatus between the end of Malani magmatism and Marwar deposition.
Abstract The sudden eruption of the Tonga underwater volcano (20.53°S, 175.38°W) on 15 January 2022 generated explosions that triggered blast waves traveling away from the eruption. In this study, the analysis of the geomagnetic field observations on the ground shows that the eruption perturbed the E ‐region current density by 22–55 mA/m within a radius of 8,000 km away from the eruption. The perturbation evolved into large scales of ∼5 hr and thousands of kilometers as it traveled away. The traveling speed of the leading front is ∼740 m/s that is near acoustic in the ionosphere. The magnetic fields and total electron content observations suggest that the dynamics changes further induced significant ionospheric disturbances that lasted ∼10 hr after the eruption. The examination of the Tonga volcanic eruption inspires us that a near‐surface perturbation can change the dynamics of the upper atmosphere.
We report a summary of geochronological data derived from detrital zircons in the Marwar Supergroup, Rajasthan, India. Although the age of deposition for the Marwar sequence was inferred to be post-Cryogenian, there is a lack of any direct age data. Two recent papers reached different conclusions based on geochronological data. A Rb-Sr age on a felsic flow within the Sonia sandstone yielded a whole-rock isochron age of 700 Ma. That age seemed consistent with geochronological data from the underlying Malani rhyolites and hinted that initiation of Marwar basin subsidence occurred concurrently with the waning phases of Malani volcanism. If correct, then the Marwar Supergroup sedimentation last from ~700-Cambrian. This conclusion was challenged by the discovery of detrital zircons within the Sonia sandstone with ages ~622 Ma which is consistent with earlier conjectures of a post-Cryogenian age. In this contribution, we detail our attempts to date the felsic volcanics at Chhoti Khatu. Although we did not obtain a crystallization age, the youngest zircon retrieved is 651 Ma. A total of 68 zircon analyses yielded a similar spectra compared to previous results and we conclude that the Marwar basin initiation was post-Cryogenian and the hiatus between Malani volcanism and sedimentation is on the order of 125 Ma.
Abstract Solar eclipse is a daytime phenomenon that significantly disturbs the ionosphere, but whether the eclipse induces ionospheric irregularities in the nighttime remains unknown. In this study, we analyzed the dense total electron content (TEC) observations from the ground‐based Global Navigation Satellite System receivers over East and South Asia to examine the development of the irregularities in the nighttime on the day of the 21 June 2020 annular solar eclipse. The rate of TEC index (period <5 min) indicates the occurrence of the irregularities that evolve from the large or coarse structures with a period ranging from hours to dozens of minutes in the nighttime due to the eclipse. We take advantage of the data‐adaptive analysis method, Hilbert‐Huang transform, to derive the instantaneous amplitude and frequency of the TEC time series, which exposes the temporal and spatial evolutions of the irregularities from larger structures continuously.
The Qinling Orogenic Belt (QOB) is one of the most important orogens in Eastern Asia formed by the collision between the North China Block (NCB) and the South China Block (SCB). The evolution history of the QOB is essential to the assembly processes of the major blocks in China and the evolution history of the Proto-Tethys Ocean (Shangdan Ocean). Paleomagnetism can quantitatively restore the paleo-position of blocks, which is key to studying the related tectonic evolution. Hindered by the complex tectonic process, few paleomagnetic results have been reported from the QOB. Here we reported a primary paleomagnetic study from the northern QOB by conducting both rock magnetic and paleomagnetic experiments on the early Devonian Lajimiao pluton (~413Ma) in the North Qinling belt (NQB), to constrain its paleo-position and the evolution of the QOB during the early Paleozoic period. 253 cores from 28 sites were drilled by portable gasoline drills, and oriented by a magnetic compass and also a sun compass if possible. Rock magnetic experiments indicate that the main magnetic mineral in most of the samples is mainly magnetite in a pseudo-single domain or multi-domain state. Both thermal demagnetization and alternating-field demagnetization were applied to obtain the characteristic remanent magnetization. The Fisher-mean direction of the low-temperature/coercivity component is roughly consistent with the present geomagnetic field (PGF), suggesting that it is probably a viscous remanent magnetization caused by the PGF. The high-temperature/coercivity component yielded a Fisher-mean direction Ds/ Is = 355.8°/19.1° in stratigraphic coordinates, corresponding to a paleomagnetic pole of 65.8°N/299.9°E (A95=2.4°). It is the first Devonian paleomagnetic pole among the scarce paleomagnetic results from the QOB. This pole indicates that the NQB may have been located at a low latitude at the early Devonian, probably in proximity to both the North China and South China blocks. However, the difference between the coeval paleomagnetic poles from the three blocks (NQB, NCB, SCB) may hint the assembly process of the several major blocks is not simple and direct. Anyway, the newly obtained paleomagnetic pole from the NQB would be able to refine our understanding of the tectonic evolution of the QOB and the Proto-Tethys Ocean.
Abstract The North China craton is encircled by four successive triple-conjugated rifts, which are respectively the centres of large igneous provinces (LIPs) of bimodal compositions, i.e. Xiong'er rift (south, c. 1.78 Ga Taihang LIP), Yanliao rift (north, c. 1.32 Ga Yanliao LIP), Xuhuai rift (east, c. 1.23 Ga Licheng and c. 0.92 Ga Dashigou LIPs) and Langshan rift (west, c. 0.82 Ga Qianlishan LIP). These rifts are genetically related to their contemporaneous LIPs based on their consistent geometry. Spatial migration of these rifts and LIPs indicates their propagation from along one marginal side to the opposite side of the craton, which may have resulted in the sequential breakup of the proto-North China craton from one side to the other during 1.8–0.8 Ga. However, the observation that the lithosphere under the LIP-associated rift regions is less destroyed (decratonized) in the Mesozoic indicates a possible role of LIPs in strengthening intracratonic steady state. This study shows that LIPs may change craton stability in either direction.
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