Correspondence: Reply to ‘Numerical modelling of the PERM anomaly and the Emeishan large igneous province’

Tectonic plates and plate boundaries migrate substantially through time and mantle plumes are generally accepted to be mobile within the convecting mantle, but it has been proposed that large low shear velocity provinces (LLSVPs) could have been fixed and rigid for as much as 540 million years (Myr). The hypotheses of fixed and rigid LLSVPs cannot be easily tested in the absence of constraints on the past location of lowermost mantle structures. We evaluated the hypothesis9of lower mantle thermochemical structure fixity with numerical experiments. As in earlier studies, we argue that the location of lower mantle thermochemical structures has changed through time

Propagation of social representations

Based on a minimal formalism of social representations as a set of associated cognems, a simple model of propagation of representations is presented. Assuming that subjects share the constitutive cognems, the model proposes that mere focused attention on the set of cognems in the field of common conscience may replicate the pattern of representation from context into subjects, or, from subject to subject, through actualization by language, where cognems are represented by verbal signs. Limits of the model are discussed, and evolutionist perspectives are presented with the support of field data

Origin and evolution of the deep thermochemical structure beneath Eurasia

A unique structure in the Earth’s lowermost mantle, the Perm Anomaly, was recently identified beneath Eurasia. It seismologically resembles the large low-shear velocity provinces (LLSVPs) under Africa and the Pacific, but is much smaller. This challenges the current understanding of the evolution of the plate–mantle system in which plumes rise from the edges of the two LLSVPs, spatially fixed in time. New models of mantle flow over the last 230 million years reproduce the present-day structure of the lower mantle, and show a Perm-like anomaly. The anomaly formed in isolation within a closed subduction network ∼22,000 km in circumference prior to 150 million years ago before migrating ∼1,500 km westward at an average rate of 1 cm year^(−1), indicating a greater mobility of deep mantle structures than previously recognized. We hypothesize that the mobile Perm Anomaly could be linked to the Emeishan volcanics, in contrast to the previously proposed Siberian Traps

Parallel flow in Hele-Shaw cells with ferrofluids

Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow with relative velocity parallel to the interface between them. The interface is unstable due to a Kelvin-Helmholtz type of instability in which fluid flow couples with inertial effects to cause an initial small perturbation to grow. Large amplitude disturbances form stable solitons. We consider the effects of applied magnetic fields when one of the two fluids is a ferrofluid. The dispersion relation governing mode growth is modified so that the magnetic field can destabilize the interface even in the absence of inertial effects. However, the magnetic field does not affect the speed of wave propagation for a given wavenumber. We note that the magnetic field creates an effective interaction between the solitons.Comment: 12 pages, Revtex, 2 figures, revised version (minor changes

Circum-Arctic mantle structure and long-wavelength topography since the Jurassic

The circum-Arctic is one of the most tectonically complex regions of the world, shaped by a history of ocean basin opening and closure since the Early Jurassic. The region is characterized by contemporaneous large-scale Cenozoic exhumation extending from Alaska to the Atlantic, but its driving force is unknown. We show that the mantle flow associated with subducted slabs of the South Anuyi, Mongol-Okhotsk, and Panthalassa oceans have imparted long-wavelength deflection on overriding plates. We identify the Jurassic-Cretaceous South Anuyi slab under present-day Greenland in seismic tomography and numerical mantle flow models. Under North America, we propose the “Farallon” slab results from Andean-style ocean-continent convergence around ~30°N and from a combination of ocean-continent and intraoceanic subduction north of 50°N. We compute circum-Arctic dynamic topography through time from subduction-driven convection models and find that slabs have imparted on average <1–16 m/Myr of dynamic subsidence across the region from at least 170 Ma to ~50 Ma. With the exception of Siberia, the main phase of circum-Arctic dynamic subsidence has been followed either by slowed subsidence or by uplift of <1–6 m/Myr on average to present day. Comparing these results to geological inferences suggest that subduction-driven dynamic topography can account for rapid Middle to Late Jurassic subsidence in the Slave Craton and North Slope (respectively, <15 and 21 m/Myr, between 170 and 130 Ma) and for dynamic subsidence (<7 m/Myr, ~170–50 Ma) followed by dynamic uplift (<6 m/Myr since 50 Ma) of the Barents Sea region. Combining detailed kinematic reconstructions with geodynamic modeling and key geological observations constitutes a powerful tool to investigate the origin of vertical motion in remote regions

Rotating Hele-Shaw cells with ferrofluids

We investigate the flow of two immiscible, viscous fluids in a rotating Hele-Shaw cell, when one of the fluids is a ferrofluid and an external magnetic field is applied. The interplay between centrifugal and magnetic forces in determining the instability of the fluid-fluid interface is analyzed. The linear stability analysis of the problem shows that a non-uniform, azimuthal magnetic field, applied tangential to the cell, tends to stabilize the interface. We verify that maximum growth rate selection of initial patterns is influenced by the applied field, which tends to decrease the number of interface ripples. We contrast these results with the situation in which a uniform magnetic field is applied normally to the plane defined by the rotating Hele-Shaw cell.Comment: 12 pages, 3 ps figures, RevTe

Phase transitions in a ferrofluid at magnetic field induced microphase separation

In the presence of a magnetic field applied perpendicular to a thin sample layer, a suspension of magnetic colloidal particles (ferrofluid) can form spatially modulated phases with a characteristic length determined by the competition between dipolar forces and short-range forces opposing density variations. We introduce models for thin-film ferrofluids in which magnetization and particle density are viewed as independent variables and in which the non-magnetic properties of the colloidal particles are described either by a lattice-gas entropy or by the Carnahan-Starling free energy. Our description is particularly well suited to the low-particle density regions studied in many experiments. Within mean-field theory, we find isotropic, hexagonal and stripe phases, separated in general by first-order phase boundaries.Comment: 12 pages, RevTex, to appear in PR

An Extended Network of Genomic Maintenance in the Archaeon Pyrococcus abyssi Highlights Unexpected Associations between Eucaryotic Homologs.

In Archaea, the proteins involved in the genetic information processing pathways, including DNA replication, transcription, and translation, share strong similarities with those of eukaryotes. Characterizations of components of the eukaryotic-type replication machinery complex provided many interesting insights into DNA replication in both domains. In contrast, DNA repair processes of hyperthermophilic archaea are less well understood and very little is known about the intertwining between DNA synthesis, repair and recombination pathways. The development of genetic system in hyperthermophilic archaea is still at a modest stage hampering the use of complementary approaches of reverse genetics and biochemistry to elucidate the function of new candidate DNA repair gene. To gain insights into genomic maintenance processes in hyperthermophilic archaea, a protein-interaction network centred on informational processes of Pyrococcus abyssi was generated by affinity purification coupled with mass spectrometry. The network consists of 132 interactions linking 87 proteins. These interactions give insights into the connections of DNA replication with recombination and repair, leading to the discovery of new archaeal components and of associations between eucaryotic homologs. Although this approach did not allow us to clearly delineate new DNA pathways, it provided numerous clues towards the function of new molecular complexes with the potential to better understand genomic maintenance processes in hyperthermophilic archaea. Among others, we found new potential partners of the replication clamp and demonstrated that the single strand DNA binding protein, Replication Protein A, enhances the transcription rate, in vitro, of RNA polymerase. This interaction map provides a valuable tool to explore new aspects of genome integrity in Archaea and also potentially in Eucaryotes

The International Workshop on Osteoarthritis Imaging Knee MRI Segmentation Challenge: A Multi-Institute Evaluation and Analysis Framework on a Standardized Dataset

Purpose: To organize a knee MRI segmentation challenge for characterizing the semantic and clinical efficacy of automatic segmentation methods relevant for monitoring osteoarthritis progression. Methods: A dataset partition consisting of 3D knee MRI from 88 subjects at two timepoints with ground-truth articular (femoral, tibial, patellar) cartilage and meniscus segmentations was standardized. Challenge submissions and a majority-vote ensemble were evaluated using Dice score, average symmetric surface distance, volumetric overlap error, and coefficient of variation on a hold-out test set. Similarities in network segmentations were evaluated using pairwise Dice correlations. Articular cartilage thickness was computed per-scan and longitudinally. Correlation between thickness error and segmentation metrics was measured using Pearson's coefficient. Two empirical upper bounds for ensemble performance were computed using combinations of model outputs that consolidated true positives and true negatives. Results: Six teams (T1-T6) submitted entries for the challenge. No significant differences were observed across all segmentation metrics for all tissues (p=1.0) among the four top-performing networks (T2, T3, T4, T6). Dice correlations between network pairs were high (>0.85). Per-scan thickness errors were negligible among T1-T4 (p=0.99) and longitudinal changes showed minimal bias (<0.03mm). Low correlations (<0.41) were observed between segmentation metrics and thickness error. The majority-vote ensemble was comparable to top performing networks (p=1.0). Empirical upper bound performances were similar for both combinations (p=1.0). Conclusion: Diverse networks learned to segment the knee similarly where high segmentation accuracy did not correlate to cartilage thickness accuracy. Voting ensembles did not outperform individual networks but may help regularize individual models.Comment: Submitted to Radiology: Artificial Intelligence; Fixed typo

Seismological structure of the 1.8 Ga Trans-Hudson Orogen of North America

Precambrian tectonic processes are debated: what was the nature and scale of orogenic events on the younger, hotter, and more ductile Earth? Northern Hudson Bay records the Paleoproterozoic collision between the Western Churchill and Superior plates—the ∼1.8 Ga Trans-Hudson Orogeny (THO)—and is an ideal locality to study Precambrian tectonic structure. Integrated field, geochronological, and thermobarometric studies suggest that the THO was comparable to the present-day Himalayan-Karakoram-Tibet Orogen (HKTO). However, detailed understanding of the deep crustal architecture of the THO, and how it compares to that of the evolving HKTO, is lacking. The joint inversion of receiver functions and surface wave data provides new Moho depth estimates and shear velocity models for the crust and uppermost mantle of the THO. Most of the Archean crust is relatively thin (∼39 km) and structurally simple, with a sharp Moho; upper-crustal wave speed variations are attributed to postformation events. However, the Quebec-Baffin segment of the THO has a deeper Moho (∼45 km) and a more complex crustal structure. Observations show some similarity to recent models, computed using the same methods, of the HKTO crust. Based on Moho character, present-day crustal thickness, and metamorphic grade, we support the view that southern Baffin Island experienced thickening during the THO of a similar magnitude and width to present-day Tibet. Fast seismic velocities at >10 km below southern Baffin Island may be the result of partial eclogitization of the lower crust during the THO, as is currently thought to be happening in Tibet