Evidence for Small-Scale Mantle Convection in the Upper Mantle beneath the Baikal Rift Zone

Inversion of teleseismic P wave travel time residuals collected along a 1280-km-long profile traversing the Baikal rift zone (BRZ) reveals the existence of an upwarped lithosphere/asthenosphere interface, which causes a travel time delay of about 1 s at the rift axis ( central high ). An area with early arrivals relative to the stable Siberian platform of up to 0.5 s is observed on each side of the rift, about 200 km from the rift axis ( flank lows ). While the location of the central high is approximately fixed in the vicinity of the rift axis, those of the flank lows vary as much as 200 km with the azimuth of the arriving rays. We use three techniques to invert the travel time residuals for velocity anomalies beneath the profile. Two of the techniques assume an isotropic velocity structure, and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry. We use independent geophysical observations such as gravity, active source seismic exploration, and crustal thickness measurements to compare the applicability of the models. Other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one, which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2.5% lateral velocity reduction, and a velocity increase due to transverse isotropy with a vertical axis of symmetry. We consider the anisotropy to be the result of the vertical component of a lithosphere/asthenosphere small-scale mantle convection system that is associated with the rifting

Asymmetric Upwarp of the Asthenosphere beneath the Baikal Rift Zone, Siberia

In the summer of 1991 we installed 27 seismic stations about lake Baikal, Siberia, aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the Baikal rift zone and adjacent regions. Sixty-six teleseismic events with high signal-to-noise ratio were recorded. Travel time and Q analysis of teleseisms characterize an upwarp of the lithosphere-asthenosphere boundary under Baikal. Theoretical arrival times were calculated by using the International Association of Seismology and Physics of the Earth\u27s interior 1991 Earth model, and travel time residuals were found by subtracting computed arrival times from observed ones. A three-dimensional downward projection inversion method is used to invert the P wave velocity structure with constraints from deep seismic sounding data. Our results suggest that (1) the lithosphere-asthenosphere transition upwarps beneath the rift zone, (2) the upwarp has an asymmetric shape, (3) the velocity contrast is -4.9% in the asthenosphere, (4) the density contrast is -0.6%, and (5) the P wave attenuation contrast t* is 0.1 s

S K S Splitting beneath Continental Rift Zones

We present measurements of S K S splitting at 28 digital seismic stations and 35 analog stations in the Baikal rift zone, Siberia, and adjacent areas, and at 17 stations in the East African Rift in Kenya and compare them with previous measurements from the Rio Grande Rift of North America. Fast directions in the inner region of the Baikal rift zone are distributed in two orthogonal directions, NE and NW, approximately parallel and perpendicular to the NE strike of the rift. In the adjacent Siberian platform and northern Mongolian fold belt, only the rift-orthogonal fast direction is observed. In southcentral Mongolia, the dominant fast direction changes to rift-parallel again, although a small number of measurements are still rift-orthogonal. For the axial zones of the East African and Rio Grande Rifts, fast directions are oriented on average NNE, that is, rotated clockwise from the N-S trending rift. All three rifts are underlain by low-velocity upper mantle as determined from teleseismic tomography. Rift-related mantle flow provides a plausible interpretation for the rift-orthogonal fast directions. The rift-parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small-scale mantle convection with rift-parallel flow. The agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift-parallel fast directions are caused by oriented magmatic cracks

Reply [to “Comment on “SKS Splitting beneath Continental Rifts Zones” by Gao et al.”]

Vauchez et al. [this issue] (hereinafter refered to as VBN) interpret the petrologic, tomographic, and anisotropy data from continental rifts to support a model of continental rifting [Nicolas, 1993; Nicolas et al., 1994] in which the lithosphere splits along the rift axis and asthenosphere flows in from the sides to fill the resulting gap. We suggest here that the data can also be described by a model in which the lower lithosphere is modified or eroded by active mantle upwelling over a region of significantly greater dimensions than the rift graben and that partial melt developing in the upwelling region can account for the widespread volcanism, as well as the seismic properties. Nicolas [1993] argued that rift-aligned anisotropy could be explained by rift-parallel mantle flow. We thank VBN for bringing this relevant paper to our attention. Volcanism about the East African Rift and the Rio Grande is not confined to the rifts but extends hundreds of kilometers from the rift axes (Mount Kilimanjaro, Mount Elgon, Mount Kenya in East Africa, The Jemez Lineament on the Rio Grande) in regions uplifted relative to their surroundings. The low-velocity tomographic anomalies also extend beneath the uplifted regions and are thought to be related to the uplift possibly supporting it by thermostatic buoyancy. The size of the P and S velocity contrasts and attenuation of high frequencies have led to the suggestion that large regions of the anomalous bodies have temperatures at or above the solidus [Achauer et al, 1994; Slack et al., 1994, 1996]. The wide extent of the anomalous regions is not explicable as resulting from an abyssal lithospheric dike beneath the rift intruded by asthenosphere. The extension of the East African, Baikal, and Rio Grande rift grabens has been estimated to be about 10 km [Baker et al., 1972; Baldridge et al., 1984; Morgan and Golombek, 1984; Logatchev and Florensov, 1978]. Passive influx of asthenosphere into a 10 km lithospheric dike is insufficient to explain the tomographic anomalies [Davis, 1991]. In addition, the amount of finite strain from lithospheric diking is insufficient to explain the anisotropy anomalies. Active replacement or modification of lower lithosphere either prior to, or contemporaneous with, rifting could generate tomographic anomalies of this magnitude

Dectin-2 is a Syk-coupled pattern recognition receptor crucial for Th17 responses to fungal infection

Innate immune cells detect pathogens via pattern recognition receptors (PRRs), which signal for initiation of immune responses to infection. Studies with Dectin-1, a PRR for fungi, have defined a novel innate signaling pathway involving Syk kinase and the adaptor CARD9, which is critical for inducing Th17 responses to fungal infection. We show that another C-type lectin, Dectin-2, also signals via Syk and CARD9, and contributes to dendritic cell (DC) activation by fungal particles. Unlike Dectin-1, Dectin-2 couples to Syk indirectly, through association with the FcRγ chain. In a model of Candida albicans infection, blockade of Dectin-2 did not affect innate immune resistance but abrogated Candida-specific T cell production of IL-17 and, in combination with the absence of Dectin-1, decreased Th1 responses to the organism. Thus, Dectin-2 constitutes a major fungal PRR that can couple to the Syk–CARD9 innate signaling pathway to activate DCs and regulate adaptive immune responses to fungal infection

Multiorgan MRI findings after hospitalisation with COVID-19 in the UK (C-MORE): a prospective, multicentre, observational cohort study

Introduction: The multiorgan impact of moderate to severe coronavirus infections in the post-acute phase is still poorly understood. We aimed to evaluate the excess burden of multiorgan abnormalities after hospitalisation with COVID-19, evaluate their determinants, and explore associations with patient-related outcome measures. Methods: In a prospective, UK-wide, multicentre MRI follow-up study (C-MORE), adults (aged ≥18 years) discharged from hospital following COVID-19 who were included in Tier 2 of the Post-hospitalisation COVID-19 study (PHOSP-COVID) and contemporary controls with no evidence of previous COVID-19 (SARS-CoV-2 nucleocapsid antibody negative) underwent multiorgan MRI (lungs, heart, brain, liver, and kidneys) with quantitative and qualitative assessment of images and clinical adjudication when relevant. Individuals with end-stage renal failure or contraindications to MRI were excluded. Participants also underwent detailed recording of symptoms, and physiological and biochemical tests. The primary outcome was the excess burden of multiorgan abnormalities (two or more organs) relative to controls, with further adjustments for potential confounders. The C-MORE study is ongoing and is registered with ClinicalTrials.gov, NCT04510025. Findings: Of 2710 participants in Tier 2 of PHOSP-COVID, 531 were recruited across 13 UK-wide C-MORE sites. After exclusions, 259 C-MORE patients (mean age 57 years [SD 12]; 158 [61%] male and 101 [39%] female) who were discharged from hospital with PCR-confirmed or clinically diagnosed COVID-19 between March 1, 2020, and Nov 1, 2021, and 52 non-COVID-19 controls from the community (mean age 49 years [SD 14]; 30 [58%] male and 22 [42%] female) were included in the analysis. Patients were assessed at a median of 5·0 months (IQR 4·2–6·3) after hospital discharge. Compared with non-COVID-19 controls, patients were older, living with more obesity, and had more comorbidities. Multiorgan abnormalities on MRI were more frequent in patients than in controls (157 [61%] of 259 vs 14 [27%] of 52; p<0·0001) and independently associated with COVID-19 status (odds ratio [OR] 2·9 [95% CI 1·5–5·8]; padjusted=0·0023) after adjusting for relevant confounders. Compared with controls, patients were more likely to have MRI evidence of lung abnormalities (p=0·0001; parenchymal abnormalities), brain abnormalities (p<0·0001; more white matter hyperintensities and regional brain volume reduction), and kidney abnormalities (p=0·014; lower medullary T1 and loss of corticomedullary differentiation), whereas cardiac and liver MRI abnormalities were similar between patients and controls. Patients with multiorgan abnormalities were older (difference in mean age 7 years [95% CI 4–10]; mean age of 59·8 years [SD 11·7] with multiorgan abnormalities vs mean age of 52·8 years [11·9] without multiorgan abnormalities; p<0·0001), more likely to have three or more comorbidities (OR 2·47 [1·32–4·82]; padjusted=0·0059), and more likely to have a more severe acute infection (acute CRP >5mg/L, OR 3·55 [1·23–11·88]; padjusted=0·025) than those without multiorgan abnormalities. Presence of lung MRI abnormalities was associated with a two-fold higher risk of chest tightness, and multiorgan MRI abnormalities were associated with severe and very severe persistent physical and mental health impairment (PHOSP-COVID symptom clusters) after hospitalisation. Interpretation: After hospitalisation for COVID-19, people are at risk of multiorgan abnormalities in the medium term. Our findings emphasise the need for proactive multidisciplinary care pathways, with the potential for imaging to guide surveillance frequency and therapeutic stratification