A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones

Triaxial compressive creep experiments have been conducted over a range of hydrous conditions to investigate the effect of water fugacity on the creep behavior of olivine aggregates in the dislocation creep regime. Samples synthesized from powders of San Carlos olivine were deformed at confining pressures of 100 to 450 MPa and temperatures between 1473 and 1573 K. Water was supplied by the dehydration of talc. Water fugacities of ∼80 to ∼520 MPa were obtained by varying the confining pressure under water-saturated conditions with the oxygen fugacity buffered at Ni/NiO. Sancles were deformed at differential stresses of ∼20 to 230 MPa. The transition from diffusion creep to dislocation creep occurs near 100 MPa for both the hydrous case and the anhydrous case. Under hydrous conditions creep experiments yield a stress exponent of n ≈ 3 and an activation energy of Q ≈ 470 kJ/mol. The creep rate of olivine is enhanced significantly with the presence of water. At a water fugacity of ∼300 MPa, samples crept ∼5–6 times faster than those deformed under anhydrous conditions at similar differential stresses and temperatures. Within the range of water fugacity investigated, the strain rate is proportional to water fugacity to the 0.69 to 1.25 power, assuming values for the activation volume of 0 to 38×10−6 m3/mol, respectively. We argue that water influences creep rate primarily through its effect on the concentrations of intrinsic point defects and hence on ionic diffusion and dislocation climb. With increasing water fugacity the charge neutrality condition changes from [FeMe•] = 2[VMe″] to [FeMe•] = [HMe′]. For the latter charge neutrality condition the concentration of silicon interstitials is proportional to fH2O1, suggesting that under hydrous conditions dislocation climb is rate limited by diffusion of Si occurring by an interstitial mechanism. Our experimentally determined constitutive equation permits extrapolation from laboratory to mantle conditions in order to assess the rheological behavior of regions of the upper mantle with different water contents, such as beneath a mid-ocean ridge and in the mantle wedge above a subducting slab.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000JB900179

Influence of water on plastic deformation of olivine aggregates 2. Dislocation creep regime

CBM and CO2-ECBM related sorption processes in coal: A review

The ongoing collision of the Indian and Asian continents has created the Himalaya and Tibetan plateau through a range of deformation processes. These include crustal thickening, detachment of the lower lithosphere from the plate (delamination) and flow in a weakened lower crust 1‐6 . Debate continues as to which of these processes are most significant 7 . In eastern Tibet, large-scale motion of the surface occurs, but the nature of deformation at depth remains unresolved. A large-scale crustal flow channel has been proposed as an explanation for regional uplift in eastern Tibet 6,8,9 , but existing geophysical data 10,11 do not constrain the pattern of flow. Magnetotellurics uses naturally occurring electromagnetic waves to image the Earth’s subsurface. Here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20-40 km. The channels extend horizontally more than 800 km from the Tibetan plateau into southwest China. The electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12,13 that permits flow on a geological timescale. These findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux. Our results reveal the previously unknown complexities of these patterns of crustal flow. Many previous studies of the IndiaAsia

/pdf/crustal-deformation-of-the-eastern-tibetan-plateau-revealed-2a0ri1x1my.pdf

Crustal deformation of the eastern Tibetan plateau revealed by magnetotelluric imaging

Adsorption of methane and carbon dioxide on gas shale and pure mineral samples

Abstract Theoretical models for compaction creep of porous aggregates, and for conventional creep of dense aggregates, by grain boundary diffusion controlled pressure solution are examined. In both models, the absolute rate of creep is determined by the phenomenological coefficient Z* = Z0exp (−ΔH/RT), a thermally activated term representing effective diffusivity along grain boundaries. With the aim of determining Z0, ΔH and hence Z* for pressure solution creep in rocksalt, compaction creep experiments have been performed on wet granular salt. Compaction experiments were chosen since theory indicates that pressure solution creep is accelerated in this mode. The tests were performed on brine-saturated NaCl powder (grainsize 100–275 μm) at temperatures of 20–90°C and applied stresses of 0.5–2.2 MPa. The mechanical data obtained show excellent agreement with the theoretical equation for compaction creep. In addition, all samples exhibited well-developed indentation, truncation and overgrowth microstructures. We infer that compaction did indeed occur by diffusion controlled pressure solution, and best fitting of our data to the theoretical equation yields Z0 = (2.79 ± 1.40) × 10−15 m3s−1, ΔH = 24.53 kJ mol−1. Insertion of these values into the theoretical model for conventional creep by pressure solution leads to a preliminary constitutive law for pressure solution in dense salt. Incorporation of this creep law into a deformation map suggests that flow of rocksalt in nature will tend to occur in the transition between the dislocation-dominated and pressure solution fields.

Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution

A hypothesis is advanced that dynamic recrystallization of Earth materials undergoing solid state flow may represent a balance between grain size reduction and grain growth processes occurring directly in the boundary between the dislocation and diffusion creep fields. Accordingly, the recrystallized grain size (D) and flow stress (σ) at steady state will be related by the equation delineating the field boundary, which in general is temperature dependent. Creep experiments on a metallic rock analogue, Magnox, yielded D=10 1.12 exp[29.3/RT]σ 1.2:3 and demonstrated that D (μm) decreases with increasing σ (MPa) and increasing temperature (T) in a manner which is in agreement with the field boundary hypothesis. If the model applies to rocks, the widely accepted idea that dynamic recrystallization can lead to major rheological weakening in the Earth may not hold. Moreover, empirical D-σ relations, used in paleo-piezometry, will need to be modified to account for temperature effects.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GL02690

On dynamic recrystallization during solid state flow: Effects of stress and temperature

Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock

[1] Properties of partially molten rocks depend strongly on the grain-scale melt distribution. Experimental samples show a variety of microstructures, such as melt lenses, layers, and multigrain melt pools, which are not readily explained using the theory for melt distribution based on isotropic interface energies. These microstructures affect the melt distribution and the porosity-permeability relation. It is still unclear how the melt distribution changes with increasing melt fraction. In this study, electrical conductivity measurements and microstructural investigation with scanning electron microscopy and electron backscatter diffraction are combined to analyze the melt distribution in synthetic, partially molten, iron-free olivine rocks with 0.01–0.1 melt fraction. The electrical conductivity data are compared with the predictions of geometric models for melt distribution. Both the conductivity data and the microstructural data indicate that there is a gradual change in the melt distribution with melt fraction (Xm) between 0.01 and 0.1. At a melt fraction of 0.01, the melt is situated in a network of triple junction tubes, and almost all grain boundaries are free from melt layers. At 0.1, the melt is situated in a network of grain boundary melt layers, as well as occupying the triple junctions. Between melt fractions 0.01 and 0.1, the number of grain boundary melt layers increases gradually. The electrical conductivity of the partially molten samples is best described by Archie's law (σsample/σmelt = CXmn) with parameters C = 1.47 and n = 1.30.

/pdf/melt-distribution-in-olivine-rocks-based-on-electrical-462h1j4g48.pdf

Colin J. Peach

Papers

Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution

On dynamic recrystallization during solid state flow: Effects of stress and temperature

Influence of crystal plastic deformation on dilatancy and permeability development in synthetic salt rock

Melt distribution in olivine rocks based on electrical conductivity measurements

Frictional-viscous flow of simulated fault gouge caused by the combined effects of phyllosilicates and pressure solution