Pressure Solution Creep Research Articles

Kinetics of pressure solution creep in quartz: theoretical considerations

The kinetics of pressure solution creep are formulated using chemical potentials generalized to nonhydrostatic states. Solving a coupling equation of diffusion and reaction on a spherical quartz grain with diameter d and grain boundary width w, the flow law of pressure solution creep is derived. As extreme cases, the flow law becomes: ϵ ̇ = (αν 2 SiO 2 KDw)(ν H 2O RTd 3) −1σ for the diffusion-controlled case and becomes: ϵ ̇ = (βν 2 SiO 2 κ +)(ν H 2O RTd) −1σ for the reaction-controlled case, where ϵ is strain rate, σ is deviatoric stress, ν is the molar volume, D is the diffusion coefficient through a wet grain boundary, K is the equilibrium constant, κ + is the rate constant of dissolution, R is the gas constant, T is temperature, and α and β are shape factors. Using the reaction constants determined by Rimstidt and Barnes (1980) and the grain boundary diffusion coefficients estimated by Nakashima (1995), the strain rate of pressure solution creep in metamorphic conditions for quartzose rocks is estimated as 10 −9∼13, 10 −8∼11, and 10 −7∼11 s −1 at 150, 250, and 350°C, respectively. These values, compared with the duration of regional metamorphism, suggest rapid pressure solution and dewatering in subduction zones followed by fluid-absent metamorphism.

Viscous-plastic finite-element models of fault-bend folds

In finite-element models of fault-bend folds, viscous and plastic material properties are used to simulate pressure solution creep and cataclasis, respectively. Plastic deformation occurs primarily above ramp hinges. However, a band of high plastic strain is created in the lower hanging wall as material moves over the ramp hinges. Viscous deformation dominates above the ramp and flats, where plastic deformation is negligible. Hanging-wall material undergoes layer-parallel shortening on the lower flat and lower part of the ramp, layerparallel extension on the upper part of the ramp and upper flat, and layer-parallel shortening farther along the upper flat. The stress invariant √ J′ 2 is highest above the ramp hinges and below the ramp, whereas stress invariant J 1 is highest above and below the ramp. The high J 1 below the ramp suppresses plastic deformation, so that footwall deformation is predominantly viscous. In multilayer models, strain is concentrated in the weak layers, especially on the limbs of the hanging-wall anticline. In models with bedding-parallel slip surfaces in the hanging wall, the strain on fold limbs is accommodated by interlayer slip, and layers between slip surfaces develop individual neutral surfaces. The models indicate that the relative and absolute amounts of deformation in fault-bend folds resulting from pressure solution creep and cataclasis vary as a function of structural position and history.

Experimental pressure solution of Halite by an indenter technique

An indenter technique is used to study pressure solution of halite immersed in brine. Two types of dissolution features are observed involving dissolution around or under the indenter: (1) free‐face pressure solution driven by elastic and plastic strain energy; and (2) water‐film diffusion controlled by normal stress. These different types of dissolution features are related to changes in the initial saturation of the brine. Free‐face pressure solution is only observed at slightly undersaturated initial conditions. Under fully saturated conditions, a linear relation of displacement rate versus stress is obtained confirming that the major driving force is the normal stress. However, both types of pressure solution mechanisms (and the associated pressure solution creep laws) can be expected to occur in natural deformation as various conditions of temperature, pressure and fluid composition change during progressive deformation.

Fluid overpressures and pressure solution in the crust

Abstract In this paper, we examine the coupled problem of fluid overpressure build-up, permeability and local mass transfer in the crust. Fluid flow in the crust is described by means of an advection-diffusion equation derived from the poro-elastic theory. A source term is added to the equation which could result from different processes: i.e., porosity reduction or fluid injection. The capability of the crust to develop and maintain fluid overpressures depends mainly on the magnitude of the source term and on the permeability. We start from a permeability model based on the statistical description of a population of cracks and we assume that permeability variation with depth is controlled both by crack closure and fracture density depth variation. Then, we focus our attention on the effect of porosity reduction. Pressure solution creep occurs on asperities on crack surfaces and this leads to a porosity reduction. The kinetic of porosity reduction is derived from a microstructural description of asperities. We have performed 1-D simulations in order to discuss the conditions under which fluid overpressures are expected. We review the mechanical properties of the upper crust and infer time constant for fracturation within the crust. Consequences for electrical and acoustic properties are also briefly discussed.

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 * = Z 0 exp (−Δ H/RT ), a thermally activated term representing effective diffusivity along grain boundaries. With the aim of determining Z 0 , Δ H 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 Z 0 = (2.79 ± 1.40) × 10 −15 m 3 s −1 , Δ H = 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.

Mobilization of cryogenic ice in outer Solar System satellites

Voyager images of the uranian satellites show a diversity of geological features, including clear evidence for the ‘softening’ and mobilization of ice on Miranda and Ariel. Some of these features are similar to those seen on jovian and saturnian satellites, where the mobile material is believed to be water or a water–ammonia mixture. However, the extremely low temperatures and probable unavailability of large energy sources within the uranian satellites lead us to consider flow mechanisms that operate at very low temperature (T ≤ 100 K). We propose here a form of pressure-solution creep, in which very fine-grained water ice or clathrate hydrate is mobilized by a small amount of intergranular cryogenic fluid (CH_4, CO or N_2). Viscosities as low as 10^(12) P are possible for a limited time, sufficient to allow flooding of rift valleys and perhaps even substantial lateral flows (glaciers).

Basement diapirism associated with the emplacement of major ophiolite nappes: Some constraints

The association of basement uplifts with major ophiolite nappes in some Phanerozoic orogenic belts suggests that gravitational instability results in the local diapiric uplift of the basement following ophiolite emplacement. In previous analyses of diapirism in crustal silicate rocks, viscous behaviour of rocks has been assumed. It is argued that this assumption is not valid. An alternative analysis is offered to determine whether or not the stress would be sufficient for diapirism to occur. The negative buoyancy stress resulting from the emplacement of an ophiolite nappe 5–15 km thick onto continental basement may be in the range 10–50 MPa. If the horizontal deviatoric stress is zero, this will be the maximum principal compressive stress. After ophiolite emplacement the thermal profile through the ophiolite and the basement will relax from a saw-tooth form to an equilibrium profile. If the ophiolite is young and thick there will be a zone of ductile strain in the lower part of the ophiolite and in the upper part of the continental basement. Results from steady-state creep experiments suggest that temperatures in this zone may be high enough for a short time after ophiolite emplacement (3 Ma or more) for the rocks in this zone to deform at geologically significant strain rates (10 −14 or greater) in response to the negative buoyancy stress. A thin ophiolite or rapid erosion will result in this ductile zone being absent or too short-lived for significant strain. Aquaeous fluids may reduce the strength of brittle rocks by decreasing the effective normal stress or by encouraging pressure solution creep. Evidence suggests that the deviatoric stress across presently active faults may be as low at 10 MPa. Thus diapirism in response to ophiolite emplacement may occur through brittle strain. Gravity spreading within the ophiolite is an alternative mechanism for accommodating the gravitational instability. The critical evidence lies in the field.

The role of an intracrustal asthenosphere on the behavior of major strike‐slip faults

Measurements of strain accumulation adjacent to the San Andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault. Such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake. The restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike‐slip faults has been attributed to the damping effect of a viscoelastic asthenosphere. However, to explain the narrowness of the zone on the San Andreas fault, it is necessary to conclude that the thickness of the lithosphere is 10–20 km. This does not appear to be consistent with the relatively low surface heat flow measurements. In this paper an alternative four layer model is proposed to explain the behavior of the San Andreas fault. An upper elastic plate extends to a depth of about 15 km, the depth of the deepest seismicity on and adjacent to the fault. Beneath this upper elastic plate is a soft, intracrustal asthenosphere that exhibits a viscoelastic behavior. The viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz, which is known to have a very soft rheology, or the presence of water that can lead to pressure solution creep or to both effects. Beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks. And beneath this second elastic layer is the asthenosphere. There is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in California and elsewhere. We show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the San Andreas fault.

“Pressure solution” creep: Some causes and mechanisms

Despite the widespread evidence of stress‐controlled dissolution and precipitation in diagenetically altered and low‐grade metamorphic rocks, a great deal of controversy remains concerning the driving forces and transport mechanisms involved. To clarify the various driving forces, the free enthalpy equation is expanded here to allow identification of different terms contributing to the overall phenomenon. It is argued that under diagenetic conditions, stress concentrations at grain‐to‐grain contacts will be the largest source of chemical potential gradients and that upon burial and cementation these inhomogeneities decline and the orientation dependence of normal stress in a quasi‐homogeneous stress field becomes important as well. These mechanisms operate efficiently enough under these relatively cold, H2O‐rich conditions that stresses can remain below the threshold for crystal plastic deformation. Water on grain boundaries provides at the very least a high diffusivity path and in cases of large volume losses must also contribute directly through fluid flow. Most experimental work on this phenomenon has not distinguished carefully between stress‐enhanced solubility and solubility enhancement due to plastic deformation or microcracking. A new thermodynamic analysis of the results of some experiments by Sprunt and Nur suggests that in at least some of their experiments, true pressure solution creep has been activated. A related phenomenon, volume transfer creep during phase transformations which involve significant volume change, displays many of the characteristics of pressure solution.

Compositional Layering in Alpine Peridotites: Evidence for Pressure Solution Creep in the Mantle

Field evidence from the Josephine (southwestern Oregon) and Red Mountain (New Zealand) peridotites indicates that compositional layering in alpine-type peridotites pre-dates emplacement into the crust, and is dissimilar in origin to layering in stratiform intrusions. Field, geochemical, and textural evidence all suggest that the layering formed during anatexis and upward flow of the peridotite in the mantle. A cumulus origin for the layering is rejected as there is strong geochemical evidence to suggest that alpine-type peridotites are the residues of partial fusion in the mantle. Although the layering superficially resembles layering in stratiform intrusions, there are no other features present which suggest a magmatic origin. The orientation of the layering, however, closely resembles that of strain-slip (or crenulation) cleavage in the axial zones of old mountain belts suggesting a deformation related origin. It also appears unlikely that the layering is solely the product of a mechanical segregation accompanying deformation ("flow layering") inasmuch as evidence for prolonged high-temperature creep of the peridotites is found throughout the entire peridotite whereas the layering is not. The most likely mechanism for the origin of the layering appears to be metamorphic differentiation accompanying deformation, pressure solution creep, and anatexis of the peridotite. This suggests that pressure-solution creep may be a principal creep mechanism in areas of ascending athenosphere such as below mid-ocean ridges.