Scaling laws for stagnant-lid convection with a buoyant crust

SUMMARY Stagnant-lid convection, where subduction and surface plate motion is absent, is common among the rocky planets and moons in our solar system, and likely among rocky exoplanets as well. How stagnant-lid planets thermally evolve is an important issue, dictating not just their interior evolution but also the evolution of their atmospheres via volcanic degassing. On stagnant-lid planets, the crust is not recycled by subduction and can potentially grow thick enough to significantly impact convection beneath the stagnant lid. We perform numerical models of stagnant-lid convection to determine new scaling laws for convective heat flux that specifically account for the presence of a buoyant crustal layer. We systematically vary the crustal layer thickness, crustal layer density, Rayleigh number and Frank–Kamenetskii parameter for viscosity to map out system behaviour and determine the new scaling laws. We find two end-member regimes of behaviour: a ‘thin crust limit’, where convection is largely unaffected by the presence of the crust, and the thickness of the lithosphere is approximately the same as it would be if the crust were absent; and a ‘thick crust limit’, where the crustal thickness itself determines the lithospheric thickness and heat flux. Scaling laws for both limits are developed and fit the numerical model results well. Applying these scaling laws to rocky stagnant-lid planets, we find that the crustal thickness needed for convection to enter the thick crust limit decreases with increasing mantle temperature and decreasing mantle reference viscosity. Moreover, if crustal thickness is limited by the formation of dense eclogite, and foundering of this dense lower crust, then smaller planets are more likely to enter the thick crust limit because their crusts can grow thicker before reaching the pressure where eclogite forms. When convection is in the thick crust limit, mantle heat flux is suppressed. As a result, mantle temperatures can be elevated by 100 s of degrees K for up to a few Gyr in comparison to a planet with a thin crust. Whether convection enters the thick crust limit during a planet’s thermal evolution also depends on the initial mantle temperature, so a thick, buoyant crust additionally acts to preserve the influence of initial conditions on stagnant-lid planets for far longer than previous thermal evolution models, which ignore the effects of a thick crust, have found.

East Asian orogenic collapse caused by oblique subduction and reduced boundary force

<p>East Asia experienced compressional deformation in the early Mesozoic, across the South China Block, North China Craton (NCC) and the part of the Central Asian Orogenic Belt to the north of the NCC. Deformation and magmatism resulted from Triassic collisions that accreted the continental blocks, and also Izanagi (Paleo-Pacific) Plate subduction from the east. We suggest that there was a single East Asian orogenic plateau by the Middle Jurassic, from NE Russia to SW China, with a length of ~4000 km. The causes and timings of the destruction of this plateau are unclear, especially loss of the lower lithosphere of the NCC. Here, we synthesize evidence for late Mesozoic and early Cenozoic crustal thinning via extension and denudation, to quantify the previous crustal thickness. We find that there was a ~50 km thick crust by the Middle Jurassic across much of the area between NE Asia and SW China, which has since undergone ~30% thinning. A force balance indicates that the buoyancy force produced by the gravitational potential energy of this thick crust drove extension from the latest Jurassic - Early Cretaceous (~145 Ma), when a rapid switch from orthogonal to oblique subduction at the Asia-Izanagi plate margin reduced the compressive boundary force by ~30%. Mantle lithosphere thinning of the NCC exceeds crustal thinning by a factor of ~2; extensional collapse cannot be the only cause of cratonic destruction, but played a major role, and potentially triggered mantle instability. Early Cretaceous extension was accompanied by a flare-up in volcanism along East Asia, which we speculate contributed to the Cretaceous hothouse climate.</p>

No mafic layer in 80 km thick Tibetan crust

All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp < 6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.

Crusted scabies in patients with long-term use of oral corticosteroid with different underlying diseases – case series

People with a weakened immune system may develop the severe form of scabies called crusted scabies. We have two cases of female patients with history of using long-term oral corticosteroid and then developing scabies infection. The first patient had other comorbidities which caused her become more susceptible while the second patient had been diagnosed with bullous pemphigoid. Both patients complained thick crust on some parts of their bodies, accompanied with itchy papules. Based on microscopic examination from lesion scraping, we found the adult form of sarcoptes scabiei and crusted scabies diagnosis was established. The lesions were improving when they got combination of topical sulfur precipitate, salicylic acid, and permethrine 5%. Crusted scabies is characterized by hyperkeratosis and crusting of the skin due to the profuse proliferation of mites resulting from an altered host response. Combination of topical antiscabies and keratolytic can make the crusts thinner and improve topical absorption.

Penetrometer tests on 109 pit latrines in Kibera, Nairobi, Kenya

Pit latrine contents can have varying consistencies that make the pits challenging to empty. It has been assumed that solids sink to the bottom of the pit with a more liquid layer above this and a scum on the top. This implies that it would be possible to remove the uppermost layers with a pump. However, recent studies have found alternative sludge profiles, including a thick crust on the top. This paper presents penetrometer data from a large number (109) of pit latrines in Kibera, Nairobi, Kenya. The penetrometer consists of a weight, dropped onto an anvil that drives a cone into the pit. The depth of penetration following each impact was measured. This was converted into plots of depth in the pits against penetration. Each pit was analysed using Spearman’s rank correlation coefficient, and categorised into positive or negative correlation, significant or non-significant. Significant, negative correlation was assumed to correspond to pits where the solids had sunk to the bottom. 38% of pits were classified in this way, which suggested that while it is common that the number of impacts required to penetrate the sludge increases with depth in the pit, this is not always the case. Pit emptiers should be prepared to deal with pits with a variety of sludge profiles, including those with a solids layer at the top.

NORWEGIAN SCABIES IN AIDS PATIENT: A CASE REPORT

Scabies is a skin infection caused by Sarcoptes scabiei var. hominis. This disease may present severe clinical manifestations in immune-compromised patient, well-known as Norwegian scabies or crusted scabies.A 36-year old man with AIDS had chief complaint thick crust almost all over his body in this case. History of household member infected by scabies before was present. Clinical findings show hyperpigmented macules unsharply marginated, covered with thick scales and accompanied by papules, fissures, and erotion. T cell CD4 level was 12 cell/ µL. Scraping examination showed scabies infection and so did the histopathology examination. This patient was treated by topical Permethrin 5% combined with 2-4 ointment application in between permethrin usage. Before topical scabicide was given, thick crust was previously treated by topical urea 10% and wet dressing by normal saline. On day 14 after the patient first came there was lesion improvement.