Brian Evans

Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere

We deformed cores of peridotite with ∼10%–15% lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation. The strength, the pressure dependence of strength, and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites. These results indicate that deformation is accommodated primarily by serpentine, while olivine, despite being the more abundant component, remains nominally undeformed. On the basis of these data and previous work, we determine that the transition from a “strong,” dilatant dunite rheology to a “weak,” nondilatant serpentinite rheology is not a linear function of the degree of serpentinization. Instead, an abrupt transition in strength is observed at low degrees of serpentinization. The pressure of the transition from localized to distributed deformation also decreases abruptly, from >1000 MPa to 150–350 MPa. The change in rheological behavior occurs at a serpentine content of 10%–15% or less, which corresponds to published compressional seismic velocity of >7.8–7.5 km/s at a pressure of 200 MPa. The seismic velocity of the oceanic lithosphere, particularly of that formed at slow spreading ridges, can thus provide constraints on its mechanical properties at depth. Because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite, significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis.

Effects of serpentinization on the lithospheric strength and the style of normal faulting at slow-spreading ridges

Abstract Rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere. Strength versus depth profiles, calculated for conditions appropriate for slow-spreading ridges, indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30%. Results from flexural fault models indicate that if serpentinization is isolated to fault zones, strain localization should be enhanced, providing an explanation for the variations in the style of normal faulting along slow-spreading ridge segments. At segment centers, where serpentinites are scarce, deformation is accommodated on closely spaced faults with small throws. At the segment ends, where serpentinites are most abundant, faults are widely spaced and have large throws.

Nondilatant brittle deformation of serpentinites: Implications for Mohr-Coulomb theory and the strength of faults

We conducted deformation experiments to investigate the strength, deformation processes, and nature of the brittle-ductile transition of lizardite and antigorite serpentinites. A transition from localized to distributed deformation occurs as confining pressure increases from ∼200 to ∼400 MPa at room temperature. Deformation in both brittle (localized) and ductile (distributed) regimes is accommodated by shear microcracks, which form preferentially parallel to the (001) cleavage. Axial microcracks (mode I) are infrequently observed. Volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant, consistent with the shear-dominated microcracking. Three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow: (1) a lack of evidence for crystal plastic deformation, (2) a positive pressure dependence of the maximum differential stress, and (3) abundant evidence for brittle microcracking. The weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the (001) cleavage, combined with the low pressure dependence of strength. The transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite, rather than the more general Byerlees law. If brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity, serpentinites define an end-member style of nondilatant brittle deformation. This deformation style may result in extremely weak faults in nature, and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present.

Healing of microcracks in quartz: Implications for fluid flow

Microcracks in quartz {approximately} 100{mu}m in length and < {approximately}10 {mu}m in width heal in 4 h at 600 C and water pressure of 200 MPa (fluid pressure (P{sub f}) = confining pressure (P{sub c})). Healing is thermally activated; the activation energy is estimated to be between 80 and 35 kJ/mol, depending on the model assumed. Rates also show dependence on fluid pressure, chemistry, and crack dimensions. Faster healing rates are observed in smaller cracks. Thus, when new cracks are not being produced in rocks at elevated temperatures and pressures, fractures will have a vast range of lifetimes: macrofractures transport most of the fluid volume and seal relatively slowly, whereas microcracks allow pervasive penetration of fluid into the rock mass but heal quickly.

Permeability-porosity Relationships in Rocks Subjected to Various Evolution Processes

It is well known that there is no “universal” permeability-porosity relationship valid in all porous media. However, the evolution of permeability and porosity in rocks can be constrained provided that the processes changing the pore space are known. In this paper, we review observations of the relationship between permeability and porosity during rock evolution and interpret them in terms of creation/destruction of effectively and non-effectively conducting pore space. We focus on laboratory processes, namely, plastic compaction of aggregates, elastic-brittle deformation of granular rocks, dilatant and thermal microcracking of dense rocks, chemically driven processes, as a way to approach naturally occurring geological processes. In particular, the chemically driven processes and their corresponding evolution permeability-porosity relationships are discussed in relation to sedimentary rocks diagenesis

On the rheologically critical melt fraction

Abstract With increasing melt fraction (φ), the strength of partially molten granite decreases from a value characteristic of solid, competent rock to a value nearly equal to that of the melt. Previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes. Thus, the pressure in the melt is expected to be important in determining strength. When the volume changes during deformation, strength and fluid flow will be coupled by such parameters as permeability (k), storage capacity per unit volume or storativity (βs), melt compressibility (βf), grain size (d), fluid viscosity (η), and strain rate ( ϵ ). Experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k/(ηβs). An evaluation of published experimental data suggests that this relation also holds for partially molten granites, indicating that the strength of these rocks depends on their transport properties. Since the permeability is related to φ, below a critical melt fraction (φrcmf), the melt pressure will change if deformation is not iso-volumetric. By assuming a power-law relationship between k and φ, we estimate that φ rcmf ∝( ϵ ηβ f /d m ) 1/(n−1) where m and n relate permeability to grain size and φ, respectively.

LABORATORY STUDY OF FAULT HEALING AND LITHIFICATION IN SIMULATED FAULT GOUGE UNDER HYDROTHERMAL CONDITIONS

Abstract Seismic data, geologic observations and laboratory friction studies suggest that faults lithify and strengthen (heal) during the interseismic period of the earthquake cycle. We report on experiments to investigate the influence of healing duration and temperature on the strength and healing rate of simulated faults. Layers of μm-sized quartz powder were used to simulate granular fault gouge. Gouge layers were sheared within Sioux quartzite in a triaxial pressure vessel at elevated pressures ( P c = 250 MPa), elevated temperatures (230–636°C), and in the presence of water ( P H 2 O = 75 MPa). We performed ‘hold-slide’ experiments in which samples were subjected to a period of healing under hydrostatic load, followed by shear deformation. Healing times ranged from a few hundred s to 10 5 s. To isolate the effects of lithification and temperature on friction, we ran two types of hold-slide experiments: (1) samples were subjected to healing and shear deformation at elevated temperature; and (2) following healing at elevated temperature, temperature was reduced prior to shear. We also ran slide-hold-slide experiments in which samples were healed under shear load at elevated temperature (636°C). Experiments in which deformation was carried out at a lower temperature than healing show that both the static and sliding coefficient of friction increase with heal time. Samples healed and deformed at higher temperature showed lower peak strengths, and within the scatter of our data, did not exhibit time-dependent strengthening. These data are interpreted to result from enhanced lithification rates at elevated temperature. Samples healed under shear load exhibited a log-linear decrease in friction with hold time. Modeling using rate- and state-dependent friction laws indicates velocity-strengthening behavior, with friction a - b values of 0.05 to 0.08. Our data indicate that at high temperature, the effective state-evolution term b is negative. Moreover, the data indicate the rate of frictional strengthening varies significantly for hydrostatic and non-hydrostatic load conditions. Our measurements of healing are consistent with seismic estimates of fault-healing rates, if stress drop is a fraction of the total shear stress and effective fault normal stress is 60 to 100 MPa.

Experimental pressure solution in halite: the effect of grain/interphase boundary structure

To investigate the mechanisms and kinetics of pressure solution, we measured deformation at contacts between polished convex lenses of halite and polished flat lenses of either halite or fused silica in saturated brine at 50.2 ± 0.2°C and fluid pressures of 0.1 MPa. Loads, applied using small springs, ranged from 0.1–4.2 N; mean effective normal stresses within the contact zone ranged from 1–14 MPa. The geometry and diametric growth rate of the contact spot (neck growth) and the rate at which the lenses approached one another (convergence) were monitored during deformation using transmitted and reflected light photomicrography. Time-dependent convergence did not occur at measurable rates when two halite lenses were pressed together in saturated brine but did occur when halite and fused silica lenses were pressed together in brine. Convergence rates in the halite/silica experiments were 0.01–0.05 μm/day. Although the initial deformation during loading involved elastic and plastic processes, control experiments without brine showed no time-dependent convergence, indicating that dislocation creep did not contribute to the observed rates. No undercutting or cataclasis was observed in any experiments. Residual fluid inclusions were formed along grain boundaries between two halite lenses loaded in brine, indicating non-zero wetting angles.

Grain growth in synthetic marbles with added mica and water

Evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder, through primary recrystallization and normal grain growth, to a size stabilized by second phases. To form the marbles, reagent grade CaCO3 was mixed with 0, 1 and 5 volume% mica and heat-treated under pressure with added water. Densification with negligible recrystallization occurred within one hour at 500° C and 500 MPa confining pressure. Primary recrystallization occurred at 500–550° C, causing increases of grain size of factors of 2–5. Resulting samples had uniform grain size, gently curved grain boundaries, and near-equilibrium triple junctions; they were used subsequently for normal grain growth studies. Normal grain growth occurred above 550° C; at 800° C, grain size (D) increased from 7 μm (D0) to 65 μm in 24 hours. Growth rates fit the equation, Dn-D0n=Kt, where K is a constant and n≃2.6. Minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size, Dmax, which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0.3 and 1. Once Dmax is reached, normal growth continues only if second phases are mobile or coarsen, or if new driving forces are introduced that cause unpinning of boundaries. Normal grain growth in Solnhofen limestone was significantly slower than in pure synthetic marble, suggesting that migration is also inhibited by second phases in the limestone.

Chapter 10 Growth of Grain Contacts in Halite by Solution-transfer: Implications for Diagenesis, Lithification, and Strength Recovery

Lithification of a sediment to form a rock may involve cementation, diagenetic reactions, or compaction under load. In these experiments, convex halite lenses were pressed against fiat halite plates at 50° C in a specially designed microscope stage. A saturated brine surrounded the samples, which were observed during the experiment in transmitted and reflected light. No time-dependent convergence was observed between the two crystals, even at mean normal stresses of up to 14 M Pa at the contact. In all experiments, however, the contact (or neck) between the two crystals grew with time as material dissolved from the surrounding lens surfaces, diffused through the pore fluid, and precipitated at the neck. Neck growth rates did not appear to correlate with the applied load, but did systematically increase as the misorientation between the two crystals decreased. Our analysis of the shapes of fluid inclusions formed along the grain boundary within the neck suggests that the grain boundary energy is about 1.8 times greater than the fluid-solid interfacial energy. Neck growth appears to be driven by the reduction of interfacial energy rather than by mechanical loads. Assuming that the interfacial energy is isotropic, and incorporating some geometric simplifications, two models of neck growth were formulated. The rate-controlling steps in the models were either precipitation or diffusion in the pore fluid. The data fit either model equally well. Both models predict that neck growth rate will be rapid at first but will decrease with time, as was observed. Neck growth will lead to an increase in real area of contact between grains in a granular aggregate even without the introduction of supersaturated solutions and may be important in the induration of sediments and the strengthening of fault gouge.