University of Grenoble

About: University of Grenoble is a education organization based out in Saint-Martin-d'Hères, France. It is known for research contribution in the topics: Population & Context (language use). The organization has 25658 authors who have published 45143 publications receiving 909760 citations.

Roughness of fault surfaces over nine decades of length scales

Abstract: [1] We report on the topographic roughness measurements of five exhumed faults and thirteen surface earthquake ruptures over a large range of scales: from 50 μm to 50 km. We used three scanner devices (LiDAR, laser profilometer, white light interferometer), spanning complementary scale ranges from 50 μm to 10 m, to measure the 3-D topography of the same objects, i.e., five exhumed slip surfaces (Vuache-Sillingy, Bolu, Corona Heights, Dixie Valley, Magnola). A consistent geometrical property, i.e., self-affinity, emerges as the morphology of the slip surfaces shows at first order, a linear behavior on a log-log plot where axes are fault roughness and spatial length scale, covering five decades of length-scales. The observed fault roughness is scale dependent, with an anisotropic self-affine behavior described by four parameters: two power law exponents H, constant among all the faults studied but slightly anisotropic (H∥ = 0.58 ± 0.07 in the slip direction and H⊥ = 0.81 ± 0.04 perpendicular to it), and two pre-factors showing variability over the faults studied. For larger scales between 200 m and 50 km, we have analyzed the 2-D roughness of the surface rupture of thirteen major continental earthquakes. These ruptures show geometrical properties consistent with the slip-perpendicular behavior of the smaller-scale measurements. Our analysis suggests that the inherent non-alignment between the exposed traces and the along or normal slip direction results in sampling the slip-perpendicular geometry. Although a data gap exists between the scanned fault scarps and rupture traces, the measurements are consistent within the error bars with a single geometrical description, i.e., consistent dimensionality, over nine decades of length scales.

Ultrastructural aspects of the acetylation of cellulose

Abstract: An ultrastructural study of the acetylation of cellulose was achieved by subjecting well characterized cellulose samples fromValonia cell wall and tunicin tests to homogeneous and heterogeneous acetylation. The study involved transmission electron microscopy observations on negatively stained microcrystals as well as diffraction contrast images of the cross sections of wall fragments at various stages of the reaction. These observations showed that the acetylation of crystalline cellulose proceeds by a reduction of the diameters of the crystals while their lengths are reduced to a lower extent. These results were corroborated by electron and X-ray diffraction experiments that showed that during the reaction there was a rapid decrease in the intensities of the equatorial diffraction spots of cellulose, whereas those located on the meridian or close to the meridian stayed constant. A model of acetylation of the cellulose crystal is presented. It is based on a non swelling reaction mechanism that affects only the cellulose chains located at the crystal surface. In the case of homogeneous acetylation, the partially acetylated molecules are sucked into the acetylating medium as soon as they are sufficiently soluble. In heterogeneous conditions the cellulose acetate remains insoluble and surrounds the crystalline core of unreacted cellulose.

What Makes for Good Views for Contrastive Learning

Abstract: Contrastive learning between multiple views of the data has recently achieved state of the art performance in the field of self-supervised representation learning. Despite its success, the influence of different view choices has been less studied. In this paper, we use theoretical and empirical analysis to better understand the importance of view selection, and argue that we should reduce the mutual information (MI) between views while keeping task-relevant information intact. To verify this hypothesis, we devise unsupervised and semi-supervised frameworks that learn effective views by aiming to reduce their MI. We also consider data augmentation as a way to reduce MI, and show that increasing data augmentation indeed leads to decreasing MI and improves downstream classification accuracy. As a by-product, we achieve a new state-of-the-art accuracy on unsupervised pre-training for ImageNet classification ($73\%$ top-1 linear readout with a ResNet-50). In addition, transferring our models to PASCAL VOC object detection and COCO instance segmentation consistently outperforms supervised pre-training. Code:http://github.com/HobbitLong/PyContrast

Lithospheric, Cratonic, and Geodynamic Setting of Ni-Cu-PGE Sulfide Deposits

Abstract: The location of magmatic Ni-Cu-PGE sulfide deposits is related to lithospheric architecture, particularly that of the subcontinental lithospheric mantle (SCLM). At crustal levels, this relationship is manifest by a close proximity to craton and paleocraton margins. Deposits are associated with mafic-ultramafic rocks and many show a close spatial relationship with a coeval large igneous province (LIP). Metal quantities and tenors observed in deposits require segregation of a magmatic sulfide melt from a large volume of parental ultramafic melt. Generation of these parental melts requires melting of upwelling mantle rising to depths of 100 km or less. The timing and tectonic setting of deposits indicates that this most likely occurs when mantle plumes impact on the base of the SCLM and are channeled laterally to areas of thinnest SCLM, where they undergo decompression melting. Alternatively, the setting of some smaller deposits suggests that upwelling may be induced by syn-to post-collisional lithospheric delamination. Craton margins are generally zones of relatively thin lithosphere and are the focus of strain during regional tectonism, providing points of dilation along active translithospheric faults. These faults facilitate melt introduction into the crust. The craton margin zone of thin lithosphere and active faulting may be adjacent to a neighboring block of continental lithosphere (paleocraton margin) or adjacent to a flanking narrow marginal basin (underlain by asthenosphere). All deposits form during periods of active regional tectonism, most commonly under mildly compressional to transpressional conditions. These different settings and conditions may result in differing depths of melting and differing depths and degrees of crustal interaction. The latter is believed to be a key factor for the development of a metal-rich sulfide melt, which is ultimately emplaced in the deposit environment. These variations can account for the observed range of ore and host rock types. Most large deposits are associated with intracontinental settings or with (former) passive margins at the edge of small marginal basins. No significant deposits are associated with the margins of large oceans or with supra-subduction zone environments, possibly reflecting poor preservational potential or (with the latter) limitations to plume interaction with continental lithosphere. A craton-margin model for the genesis of magmatic Ni-Cu-PGE deposits is proposed. This model provides a framework for further examination of deposit-forming processes and also provides a coherent predictive framework for mineral exploration.