Paleomagnetic results from Permian rocks of the northern Saharan craton and motions of the Moroccan Meseta and Pangea

The characteristic magnetization of redbed samples from the upper part of the Série d'Abadla (probably Early Permian 31°N, 2.7°W) has a mean direction derived from 13 sites of D=129°, I=11°, k=59, α₉₅=6° and a corresponding south paleopole at 29°S, 60°E, A₉₅=5°. All directions have reversed polarity. The paleolatitude of the northern fringe of the Saharan craton was 6°±3°S, which is in excellent agreement with that for the Moroccan Meseta. Therefore, in all probability, there has been no paleolatitudinal displacement greater than about 500 km of the Moroccan Meseta relative to Africa since Permian time. Comparison of results from sedimentary rocks shows no evidence for relative rotation of the Moroccan Meseta since Permian time. Small apparent rotations are indicated by evidence from massive trachyandesite lavas from Morocco, but we argue that these could have arisen from the incomplete averaging of secular variation and uncertainties in estimates of paleohorizontal, rather than from true tectonic rotations. The combined latest Carboniferous/Early Permian paleopole for the Saharan craton and the Meseta differs form the path of apparent polar wandering for North America when the continents are assembled in Wegener's Pangea (Pangea A, in which northwest Africa is opposite North America). It is in reasonable agreement when the continents are assembled in the Pangea B configuration (northwest Africa opposite Europe).     

Variations in Ion-Component Pressure during Dipolarization in the Near-Earth Magnetotail Plasma Sheet

Geomagnetism and Aeronomy - The dynamics of fluxes of thermal and suprathermal H+ and O+ ions and the pressure variations of these components during 11 events of magnetic field dipolarization in...     

Constraining the links between the Himalayan belt and the Central Myanmar Basins during the Cenozoic:An integrated multi-proxy detrital geochronology and trace-element geochemistry study

Geological and tectonic analysis of the Eastern Himalayan basins has given rise to a decade-long debate on the geodynamic evolution of the Burmese terrane and on the extent of reorganization of the main SE Asia drainage systems.However,the influence of the Himalayan belt on the Central Myanmar Basin(CMB)system remains poorly documented,although it is key to providing more accurate models for the evolution of the Himalayan-Burmese orogen.In this contribution,we present geochronological,isotopic and geochemical analysis from 2500 zircon,1700 titanite,700 rutile and 850 apatite detrital grains from fifteen Cenozoic siliciclastic samples and one Cretaceous igneous rock.The samples were collected within the fore-and back-arc basins of the Central Myanmar Basin domain(CMB)to constrain the provenance,maximum depositional ages,and depositional environments of the west Burma terrane.Nine key lithological units,the Sadwingyi,Ketpanda,Wabo Chaung,Gwegon,Minwun,Padaung,Okmintaung and Irrawaddy formations have detrital age spectra spanning from the Miocene to Paleoarchean.The entire data set has common age peaks at ca.20,40,60,90,100 Ma,with about 80%of the U–Pb ages younger than ca.140 Ma and only ca.1%of the grains predating ca.3.0 Ga.Our results shed light on the current ambiguities on the transport pathways of Himalayan detritus in the CMB.They show that the fore-arc basin was open to the trench and fed by the unroofing of both the Wuntho Popa volcanic arc to the east and possibly from the Burmese basement and/or from Himalayan-derived Bengal Fan detritus to the west during the Eocene,from at least ca.44 Ma to before ca.39 Ma.We show that the west Burma Terrane was partitioned into pull-apart basins such as the Minwun Basin,which during the Oligocene recorded the first evidence of a new source contribution into the CMB at ca.27 Ma.This new source is characterized by detritus highly compatible with the SE Asia basement rocks,which we suggest corresponds to the initiation of the palaeo–Irrawaddy River.This geodynamic evolution does not require any Yarlung Tsangpo-Irrawaddy-Brahmaputra paleodrainage reorganization,since from the Oligocene to the Early Miocene,the Irrawaddy River fed an internally drained basin,and from the Late Miocene onwards,the Yarlung drained into the Brahmaputra in the Bengal Basin.     

等离子体片中能量离子的空间分布——Cluster/RAPID观测数据分析

本文根据Cluster卫星上的粒子成像质谱仪（RAPID）探测器在穿越地球等离子体片过程中的观测数据,统计研究了等离子体片中能量离子能量密度的空间分布（氢离子能量范围从40keV到1500keV,氦离子和氧离子从10keV到1500keV）,并且给出了离子能量密度在不同地磁活动时期随GSE-Z向分布的剖面.研究表明能量离子的能量密度以及能量密度的梯度与地磁活动指数Kp之间存在近似线性的关系.观测结果表明形成这种分布变化的主要原因是在地磁活动期间在电流片附近离子能量密度的增加,特别是其中的重离子成分增加更为显著.本文通过一个简化的电流片模型的数值计算,定性地研究了形成能量离子空间分布的机理.计算表明重离子在电流片中可以获得更多的能量,电流片加速可能是形成能量密度分布变化的一种可能的机制.     

Magnetic storm acceleration of radiation belt electrons observed by the Scintillating Fibre Detector (SFD) onboard EQUATOR-S

On the basis of the currents induced by electron fluxes in the Scintillating Fibre Detector (SFD) onboard the EQUATOR-S satellite launched on 2 December 1997, an in-situ acceleration of radiation belt electrons is found to possibly contribute to the increase of the flux of electrons with energies greater than 400 keV. The data acquired between 16 December 1997 and 30 April 1998 on the 500–67300 km, 4° inclination EQUATOR-S orbit show that the increase of the energetic electron flux corresponds to the enhanced geomagnetic activity measured through the Dst index.     

Recovery and curation of the Winchcombe (CM2) meteorite

The Winchcombe meteorite fell on February 28, 2021 and was the first recovered meteorite fall in the UK for 30 years, and the first UK carbonaceous chondrite. The meteorite was widely observed by meteor camera networks, doorbell cameras, and eyewitnesses, and 213.5 g (around 35% of the final recovered mass) was collected quickly—within 12 h—of its fall. It, therefore, represents an opportunity to study very pristine extra-terrestrial material and requires appropriate careful curation. The meteorite fell in a narrow (600 m across) strewn field ~8.5 km long and oriented approximately east–west, with the largest single fragment at the farthest (east) end in the town of Winchcombe, Gloucestershire. Of the total known mass of 602 g, around 525 g is curated at the Natural History Museum, London. A sample analysis plan was devised within a month of the fall to enable scientists in the UK and beyond to quickly access and analyze fresh material. The sample is stored long term in a nitrogen atmosphere glove box. Preliminary macroscopic and electron microscopic examinations show it to be a CM2 chondrite, and despite an early search, no fragile minerals, such as halite, sulfur, etc., were observed.     

Crustal tomographic imaging of a transitional continental rift: the Ethiopian rift

In this study we image crustal structure beneath a magmatic continental rift to understand the interplay between crustal stretching and magmatism during the late stages of continental rifting: the Main Ethiopian Rift (MER). The northern sector of this region marks the transition from continental rifting in the East African Rift to incipient seafloor spreading in the southern Red Sea and western Gulf of Aden. Our local tomographic inversion exploits 172 broad-band instruments covering an area of 250 × 350 km of the rift and adjacent plateaux. The instruments recorded a total of 2139 local earthquakes over a 16-month period. Several synthetic tests show that resolution is good between 12 and 25 km depth (below sea level), but some horizontal velocity smearing is evident along the axis of the Main Ethiopian Rift below 16 km. We present a 3-D P -wave velocity model of the mid-crust and present the first 3-D Vp / Vs model of the region. Our models show high P -wave velocities (6.5 km s⁻¹) beneath the axis of the rift at a depth of 12–25 km. The presence of high Vp / Vs ratios (1.81–1.84) at the same depth range suggest that they are cooled mafic intrusions. The high Vp / Vs values, along with other geophysical evidence, suggest that dyking is pervasive beneath the axis of the rift from the mid-crustal depths to the surface and that some portion of partial melt may exist at lower crustal depths. Although the crustal stretching factor across the Main Ethiopian Rift is ∼1.7, our results indicate that magma intrusion in narrow zones accommodates a large proportion of extensional strain, with similarities to slow-spreading mid-ocean ridge processes.     

Shock metamorphism in plagioclase and selective amorphization

Plagioclase feldspar is one of the most common rock‐forming minerals on the surfaces of the Earth and other terrestrial planetary bodies, where it has been exposed to the ubiquitous process of hypervelocity impact. However, the response of plagioclase to shock metamorphism remains poorly understood. In particular, constraining the initiation and progression of shock‐induced amorphization in plagioclase (i.e., conversion to diaplectic glass) would improve our knowledge of how shock progressively deforms plagioclase. In turn, this information would enable plagioclase to be used to evaluate the shock stage of meteorites and terrestrial impactites, whenever they lack traditionally used shock indicator minerals, such as olivine and quartz. Here, we report on an electron backscatter diffraction (EBSD) study of shocked plagioclase grains in a metagranite shatter cone from the central uplift of the Manicouagan impact structure, Canada. Our study suggests that, in plagioclase, shock amorphization is initially localized either within pre‐existing twins or along lamellae, with similar characteristics to planar deformation features (PDFs) but that resemble twins in their periodicity. These lamellae likely represent specific crystallographic planes that undergo preferential structural failure under shock conditions. The orientation of preexisting twin sets that are preferentially amorphized and that of amorphous lamellae is likely favorable with respect to scattering of the local shock wave and corresponds to the “weakest” orientation for a specific shock pressure value. This observation supports a universal formation mechanism for PDFs in silicate minerals.     

Introduction to the special issue of Meteoritics & Planetary Science on the Winchcombe meteorite

This issue of Meteoritics & Planetary Science celebrates the science of the Winchcombe meteorite, which fell to Earth on the 28th February 2021 close to the town of Winchcombe, Gloucestershire in the UK.     

On the penetration of a hot diapir through a strongly temperature-dependent viscosity medium

Summary. The ascent of a hot spherical body through a fluid with a strongly temperature-dependent viscosity has been studied using an axisymmetric finite element method. Numerical solutions range over Peclet numbers of 10⁻¹– 10³ from constant viscosity up to viscosity variations of 10⁵. Both rigid and stress-free boundary conditions were applied at the surface of the sphere. The dependence of drag on viscosity variation was shown to have no dependence on the stress boundary condition except for a Stokes flow scaling factor. A Nusselt number parameterization based on the stress-free constant viscosity functional dependence on the Peclet number scaled by a parameter depending on the viscosity structure fits both stress-free and rigid boundary condition data above viscosity variations of 100. The temperature scale height was determined as a function of sphere radius. For the simple physical model studied in this paper pre-heating is required to reduce the ambient viscosity of the country rock to less than 10²² cm² s⁻¹ in order for a 10 km diapir to penetrate a distance of several radii.