Christopher J. Spiers

Weakening of rock salt by water during long-term creep

The rheological properties of rock salt are of fundamental importance in predicting the long-term evolution of salt-based radioactive waste repositories and strategic storage caverns, and in modelling the formation of salt diapirs and associated oil traps1,2. The short-term, high-stress rheology of rock salt is well known from laboratory experiments; however, extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures. Furthermore, experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt. Here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine. Trace brine is always present in natural salt but sometimes escapes during experiments. Our tests on dry dilated salt show more or less conventional dislocation creep behaviour, but brine-bearing samples show marked weakening at low strain rates. This is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep. Because natural rock salt always contains some brine, these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long-term rheological behaviour of salt in nature.

Frictional‐viscous flow of phyllosilicate‐bearing fault rock: Microphysical model and implications for crustal strength profiles

It is widely believed that around the brittle-ductile transition, crustal faults can be significantly weaker than predicted by conventional two-mechanism brittle-ductile strength envelopes. Factors contributing to this weakness include the polyphase nature of natural rocks, foliation development, and the action of fluid-assisted processes such as pressure solution. Recently, ring shear experiments using halite/kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional-viscous behavior (i.e., both normal stress and strain rate sensitive behavior) involving the combined effects of pressure solution and phyllosilicates. This behavior was accompanied by the development of a mylonitic microstructure. A quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model. In this paper, a microphysical model for shear deformation of foliated, phyllosilicate-bearing fault rock by pressure solution-accommodated sliding along phyllosilicate foliae is developed. The model predicts purely frictional behavior at low and high shear strain rates and frictional-viscous behavior at intermediate shear strain rates. The mechanical data on wet halite + kaolinite gouge compare favorably with the model. When applied to crustal materials, the model predicts major weakening with respect to conventional brittle-ductile strength envelopes, in particular, around the brittle-ductile transition. The predicted strength profiles suggest that in numerical models of crustal deformation the strength of high-strain regions could be approximated by an apparent friction coefficient of 0.25-0.35 down to depths of 15-20 km.

The effective viscosity of rocksalt: implementation of steady-state creep laws in numerical models of salt diapirism

A steady-state creep law for rocksalt, describing the two parallel mechanisms of dislocation creep and fluid-enhanced grain-boundary diffusion creep, has been used in numerical models of salt diapirism, to study the effective viscosity of rocksalt. Typical models included a 3-km-thick sedimentary layer on top of 1 km of rocksalt. The grain size of the salt has been varied between 0.5 and 3 cm and the geothermal gradient between 25 and 35 K/km. For strain rates of 10−12−10−15 s−1, typical of salt diapirism driven by buoyancy alone, the diffusion creep mechanism dominates at the fine grain sizes, with dislocation creep becoming important in coarsely grained salt. The effective viscosity ranges from 1017 Pa s for small grain size and high-temperature salt to 1020 Pa s for large grain size and low-temperature salt. The viscosity is strongly dependent on grain size and moderately dependent on temperature. For the larger grain sizes, the dislocation creep mechanism is most effective during the diapiric stage, but the non-Newtonian effects in the salt are not important in determining the growth rate and geometry of the diapirs. The estimates for the Newtonian viscosity of salt that have traditionally been used in modelling of salt dynamics are at the lower end of the range that we find from these numerical experiments.

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

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.

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

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.

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

While the fluid transport properties of rocks are well understood under hydrostatic conditions, little is known regarding these properties in rocks undergoing plastic deformation. In this study the influence of macroscopic plastic deformation on permeability has been investigated experimentally using synthetic salt rock. Dilatometric triaxial deformation experiments performed on this material, at room temperature, confining pressures (Pc) in the range 5–20 MPa and strain rates of ∼ 4 × 10−5 s−1 to total strains of ∼ 15%, exhibited work hardening behaviour with minor amounts of dilatancy at Pc < 18 MPa. Microstructural observations confirmed that deformation occurred by dislocation glide, with grain boundary microcracking in the dilatant field. At the lowest pressures, deformation-induced dilatancy of only 0.1–0.2 vol.% produced extremely rapid initial increases in permeability (from ≤ 10−21 m2 to ∼ 2 × 10−16 m2), suggesting critical behaviour of the type described by percolation theory. This rapid permeability development with dilatancy is well described by crack linkage models based on percolation theory, provided a broad range of fluid conductance is incorporated. The study has shown that minor dilatancy (< 0.2 vol.%) during plastic deformation of salt rock can lead to very large increases in permeability. This is of direct interest with regard to the behaviour of salt rock in waste disposal and storage systems, and may have important implications for the transport of fluids through, and the interaction of fluids with, crystalline rocks undergoing crystal plastic deformation in nature.

Experimental investigation into the microstructural and mechanical evolution of phyllosilicate-bearing fault rock under conditions favouring pressure solution

Abstract Mature crustal fault zones are known to be zones of persistent weakness. This weakness is believed to result from microstructural modifications during deformation, such as grain-size reduction and foliation development. Around the brittle–ductile transition, phyllosilicates are expected to have a significant effect on fault strength, in particular under conditions favouring pressure solution. To study such effects, we performed rotary shear experiments on brine-saturated halite/kaolinite mixtures, aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate. The results show significant strain weakening, and a transition with progressive strain towards more rate-sensitive and less normal stress-sensitive behaviour. This was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate, asymmetric clasts in a fine-grained, foliated matrix. The results demonstrate that strain weakening and the development of a typical ‘mylonitic’ microstructure can occur as a consequence of grain-size reduction by cataclasis, and a transition to pressure solution accommodated deformation, even in the absence of dislocation creep. The data raise questions regarding the reliability of microstructures as rheology indicators, as well as on the use of low strain, monomineralic flow laws for modelling crustal dynamics.

Compaction creep of quartz sand at 400-600°C: Experimental evidence for dissolution-controlled pressure solution

Intergranular pressure solution (IPS) is an important compaction and deformation mechanism in quartzose rocks, but the kinetics and rate-controlling process remain unclear. The aim of the present study is to test microphysical models for compaction creep by IPS against isostatic hot pressing experiments performed on quartz sand under conditions expected to favor pressure solution (confining pressure 300 MPa, pore water pressure 150–250 MPa, temperature 400–600°C). Microstructural observations revealed widespread intergranular indentation features and confirmed that intergranular pressure solution was indeed the dominant deformation mechanism under the chosen conditions. For porosities down to 15%, the mechanical data agree satisfactorily with a microphysical model incorporating a previously determined kinetic law for dissolution of loose granular quartz, suggesting that the rate-limiting mechanism of IPS was dissolution. The model also predicts IPS rates within one order of magnitude of those measured in previous experiments at 150–350°C, and thus seem robust enough to model sandstone compaction in nature. Such applications may not be straightforward, however, as the present evidence for dissolution control implies that the compositional variability of natural pore fluids may strongly influence IPS rates in sandstones.

Mantle shear zones and their effect on lithosphere strength during continental breakup

Abstract The Erro-Tobbio lherzolite in the ophiolitic Voltri Massif of northwestern Italy includes several thrusted fragments of lithospheric mantle which were first exhumed to the ocean floor during Jurassic rifting and breakup, and at a later stage became emplaced in the Alpine collisional stack during Tertiary convergence between Africa and Europe. Coherent slices of these mantle rocks contain several sets of major shear zones generated during the Jurassic rift evolution. One such shear zone, several kilometres wide and formed at temperatures between 920 and 1040°C, is transected by up to 200 m wide, ultra-fine-grained hydrated mylonite zones formed at temperatures in the range 990-550°C. All these structures are cut by MORB-type gabbroic and basaltic dykes. The microstructures of the mylonite zones are interpreted to reflect progressive, reaction-related grainsize reduction plus localization of the deformation during the early stages of continental breakup. In view of experimental evidence that wet olivine rocks weaken considerably with decreasing grainsize, in response to a change from grainsize-insensitive dislocation creep to grainsize-sensitive creep mechanisms, it is proposed that shear localization and allied grainsize reduction may have resulted in a drastic decrease in strength of the upper mantle during rifting. In order to obtain an order of magnitude estimate of this rheological effect, we present a layered rheological model of the Piemonte-Ligurian lithosphere, based on the observed microstructures (i.e., grainsizes) and pressure-temperature data, and including appropriate rheological laws for grainsize-sensitive and -insensitive creep in wet olivine. The model calculations suggest strength values for the uppermost mantle up to four orders of magnitude lower than those expected for homogeneous deformation exclusively controlled by dislocation creep of dry olivine.

Influence of phyllosilicates on fault strength in the brittle-ductile transition: insights from rock analogue experiments

Abstract Despite the fact that phyllosilicates are ubiquitous in mature fault and shear zones, little is known about the strength of phyllosilicate-bearing fault rocks under brittle-ductile transitional conditions where cataclasis and solution-transfer processes are active. In this study we explored steady-state strength behaviour of a simulated fault rock, consisting of muscovite and halite, using brine as pore fluid. Samples were deformed in a rotary shear apparatus under conditions where cataclasis and solution transfer are known to dominate the deformation behaviour of the halite. It was found that the steady-state strength of these mixtures is dependent on normal stress and sliding velocity. At low velocities (<0.5 µm s−1) the strength increases with velocity and normal stress, and a strong foliation develops. Comparison with previous microphysical models shows that this is a result of the serial operation of pressure solution in the halite grains accommodating frictional sliding over the phyllosilicate foliation. At high velocities (>1 µm s−1), velocity-weakening frictional behaviour occurs along with the development of a structureless cataclastic microstructure. Revision of previous models for the low-velocity behaviour results in a physically realistic description that fits our data well. This is extended to include the possibility of plastic flow in the phyllosilicates and applied to predict steady-state strength profiles for continental fault zones containing foliated quartz-mica fault rocks. The results predict a significant reduction of strength at mid-crustal depths and may have important implications for crustal dynamics and seismogenesis.