Heterogeneous subgreenschist deformation in an exhumed sediment‐poor mélange

Many described subduction complexes (or mélanges) exhumed from seismogenic depths comprise thick, turbidite‐dominated sequences with deformed zones containing clasts or boudins of more competent sandstone and/or basalt. In contrast, many active subduction zones have a relatively small thickness of sedimentary inputs (<2 km), turbidite sequences are commonly accreted rather than subducted, and the role of pelagic sediments and basalt (lavas and hyaloclastites) in the deforming zone near the plate interface at <20 km depth is poorly understood. Field investigation of Neoproterozoic oceanic sequences accreted in the Gwna Complex, Anglesey, UK, reveals repeated lenticular slices of variably sampled ocean plate stratigraphy (OPS) bounded by thin mélange‐bearing shear zones. Mélange matrix material is derived from adjacent OPS lithologies and is either dominantly illitic, likely derived from altered siliciclastic sediment, or chloritic, likely derived from altered volcanics. In the illitic mélange, mutually cross‐cutting phyllosilicate foliation and variably deformed chlorite‐quartz‐calcite veins suggest ductile creep was cyclically punctuated by transient, localized fluid pulses. Chlorite thermometry indicates the veins formed at 260 ± 10°C. In the chloritic mélange, recrystallized through‐going calcite veins are deformed to shear strains of 4–5 within a foliated chlorite matrix, suggesting calcite veins in subducting volcanics may localize deformation in the seismogenic zone. Shear stress‐strain rate curves constructed using existing empirical relationships in a simplified shear zone geometry predict that slip velocities varied depending on pore fluid pressure; models predict slow slip velocities preferentially by frictional sliding in chlorite, at pore fluid pressures greater than hydrostatic but less than lithostatic

Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks

Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.Funding was provided by NERC to the CRIUS consortium (NE/F004699/1), Shell Global Solutions, for GR as part of the Center for Nanoscale Controls on Geologic CO₂ (NCGC), an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under Award # DE-AC02-05CH11231, and DECC, which provided a CCS Innovation grant for completion of this work

Tensor Operators for Uh(sl(2))

Tensor operators for the Jordanian quantum algebra Uh(sl(2)) are considered. Some explicit examples of them, which are obtained in the boson or fermion realization, are given and their properties are studied. It is also shown that the Wigner-Eckart's theorem can be extended to Uh(sl(2)).Comment: 11pages, LaTeX, to be published in J. Phys.

Enabling large-scale hydrogen storage in porous media – the scientific challenges

Expectations for energy storage are high but large-scale underground hydrogen storage in porous media (UHSP) remains largely untested. This article identifies and discusses the scientific challenges of hydrogen storage in porous media for safe and efficient large-scale energy storage to enable a global hydrogen economy. To facilitate hydrogen supply on the scales required for a zero-carbon future, it must be stored in porous geological formations, such as saline aquifers and depleted hydrocarbon reservoirs. Large-scale UHSP offers the much-needed capacity to balance inter-seasonal discrepancies between demand and supply, decouple energy generation from demand and decarbonise heating and transport, supporting decarbonisation of the entire energy system. Despite the vast opportunity provided by UHSP, the maturity is considered low and as such UHSP is associated with several uncertainties and challenges. Here, the safety and economic impacts triggered by poorly understood key processes are identified, such as the formation of corrosive hydrogen sulfide gas, hydrogen loss due to the activity of microbes or permeability changes due to geochemical interactions impacting on the predictability of hydrogen flow through porous media. The wide range of scientific challenges facing UHSP are outlined to improve procedures and workflows for the hydrogen storage cycle, from site selection to storage site operation. Multidisciplinary research, including reservoir engineering, chemistry, geology and microbiology, more complex than required for CH4 or CO2 storage is required in order to implement the safe, efficient and much needed large-scale commercial deployment of UHSP.This work was stimulated by the GEO*8 Workshop on “Hydrogen Storage in Porous Media”, November 2019 at the GFZ in Potsdam (Germany). NH, AH, ET, KE, MW and SH are funded by the Engineering and Physical Sciences Research Council (EPSRC) funded research project “HyStorPor” (grant number EP/S027815/1). JA is funded by the Spanish MICINN (Juan de la Cierva fellowship-IJC2018-036074-I). JM is co-funded by EU INTERREG V project RES-TMO (Ref: 4726 / 6.3). COH acknowledges funding by the Federal Ministry of Education and Research (BMBF, Germany) in the context of project H2_ReacT (03G0870C).Peer reviewe

Rotary shear experiments on glass bead aggregates: Stick-slip statistics and parallels with natural seismicity

The goal of predicting earthquakes remains elusive despite decades of instrumental observations and research, and a much longer historical record. Even the practice of seismic hazard analysis is a topic of heated debate, in part due to our inability to accurately determine the rate and size distribution of earthquakes that a fault can produce. The main reason for these deficiencies is the lack of a validated, physics-based theory of earthquakes. Heuristic attempts to discover patterns in seismicity based on its phenomenology have produced ambiguous and sometimes contradicting results, due to the relatively short instrumental record of big earthquakes compared to their rate of occurrence. From a geodynamics perspective, earthquakes are bursts of energy release as the lithosphere is loaded due to the motion of tectonic plates at rates of a few centimeters per year. Similar behavior, known as crackling, is observed when shearing granular aggregates. Loosely packed particles behave collectively as a fluid, giving rise to small instabilities only. At a critical packing fraction, the size distribution of the instabilities approaches power law scaling. This suggests that the aggregate is at a phase transition and that long-range correlations are a key characteristic of its macroscopic behavior. Above the critical packing fraction, the collective behavior of the particles is similar to that of a solid. In that solid-like regime, the aggregates alternate between power law distributed event sizes and quasi-periodic stick-slip. A significant number of laboratory studies have employed granular media to explore the dynamics of critical systems in the context of seismicity and fault gouge rheology. These studies have been performed either at low normal stress (< 1 MPa) or to limited shear displacements (< 50 mm), and often under dry conditions. It is not known whether the macroscopic behavior of granular aggregates remains the same under higher normal stress and larger displacements, or in the presence of pressurized water. If not, is it possible to determine what mechanisms are responsible for the change? The rotary shear experiments presented in this thesis expand the envelope of the experimentally tested conditions up to 8 MPa normal stress and 165 mm of shear displacement, \textit{simultaneously}. This enabled us to infer the emergence of correlations under these conditions, through changes in the statistics of granular avalanches. Because the elevated stress conditions do not allow direct visual observation of the glass bead samples, a specially developed AE monitoring system was used to detect and locate the source of crackling. The findings of this thesis highlight the importance of emergent, long range correlations in sheared granular media, as a function of experimental conditions. We infer that the key parameter that determines the scaling of avalanche statistics is the packing fraction, which in turn depends on normal stress, wear rate, and particle size distribution

Embedded Multigrid Approach For Real-Time

Abstract. Finding efficient and physically based methods to interactively simulate deformable objects is a challenging issue. The most promising methods addressing this issue are based on finite elements and multigrid solvers. However, these multigrid methods still suffer, when used to simulate large deformations, from two pitfalls, depending on the kind of grids hierachy used. If embedded grids are used, approximating complex geometries becomes difficult, whereas when unstructured grids hierarchy is used, solving speed-up is reduced by the necessity to update coarser levels stiffness matrices. We propose a framework that combines embedded grids solving with fast remeshing. We introduce a new hierarchical mesh generator which can build a hierarchy of topologically embedded grids approximating a complex geometry. We also show how to take advantage of the knowledge of the stiffness matrix sparsity pattern to efficiently update coarse matrices. These methods are tested on interactive simulation of deformable solids undergoing large deformations.

Surface microstructures developed on polished quartz crystals embedded in wet quartz sand compacted under hydrothermal conditions

Intergranular pressure solution plays a key role as a deformation mechanism during diagenesis and in fault sealing and healing. Here, we present microstructural observations following experiments conducted on quartz aggregates under conditions known to favor pressure solution. We conducted two long term experiments in which a quartz crystal with polished faces of known crystallographic orientation was embedded in a matrix of randomly oriented quartz sand grains. For about two months an effective axial stress of 15 MPa was applied in one experiment, and an effective confining pressure of 28 MPa in the second. Loading occurred at 350 °C in the presence of a silica-saturated aqueous solution. In the first experiment, quartz sand grains in contact with polished quartz prism (1¯¯¯010) faces became ubiquitously truncated against these faces, without indenting or pitting them. By contrast, numerous sand-grain-shaped pits formed in polished pyramidal (176¯¯¯3) and (4¯¯¯134) crystal faces in the second experiment. In addition, four-leaved and (in some cases) three-leafed clover-shaped zones of precipitation formed on these prism faces, in a consistent orientation and pattern around individual pits. The microstructures observed in both experiments were interpreted as evidence for the operation of intergranular pressure solution. The dependence of the observed indentation/truncation microstructures on crystal face orientation can be explained by crystallographic control of stress-induced quartz dissolution kinetics, in line with previously published experimental and petrographic data, or possibly by an effect of contact orientation on the stress-induced driving force for pressure solution. This should be investigated in future experiments, providing data and microstructures which enable further mechanism-based analysis of deformation by pressure solution and the effect of crystallographic control on its kinetics in quartz-rich sands and sandstones