Neville L. Carter

Flow properties of continental lithosphere

The occurrence of mechanically weak zones (σ1 − σ3 <10 MPa) at upper-, mid- and lower crustal depths, inferred from geological and geophysical observations and interpretations, is supported by empirically-determined steady-state flow properties of some common crystalline rocks. These zones are predicted to occur in the depth intervals 10–15 km, 20–28 km and 25–40 km, these intervals depending critically on rock type and tectonic province. In addition, the apparent widespread occurrence of ductile and semi-brittle fault zones suggests that weak zones may occur at virtually any depth below about 10 km. While data are not available for representative lower crustal materials, those likely to be closest in mechanical response support the common suggestion that the Moho is a mechanical discontinuity, with stronger peridotite below, but the magnitude of the discontinuity is shown to depend critically on temperature and strain rate. Mantle flow is currently best approximated by Chopra and Patersons (1981, 1984) wet Aheim dunite flow law. Combined with pyroxene thermobarometry and olivine paleopiezometry of mantle xenoliths, the wet flow law permits construction of viscosity-depth profiles that are in accord with other geophysical considerations. Yield envelopes commonly applied to geological and geophysical problems since the original one proposed for the lithosphere and upper asthenosphere by Goetze and Evans (1979) generally lead to serious overestimates of stress differences because of extrapolations both of Byerlees (1978) rock-friction relation and of steady-state flow laws to physical conditions beyond their range of validity. Modifications of the envelopes, will require careful additional experimental work on rock friction along with detailed delineations of boundaries between brittle, semi-brittle and ductile regimes, grainsize-sensitive domains, and their depth-dependencies. Once accomplished, realistic yield envelopes and refined deformation surfaces will lead to a much better understanding of the mechanical behavior and governing flow processes at depth in the continental lithosphere.

Stress in the lithosphere: Inferences from steady state flow of rocks

Mechanical data and flow processes from steady state deformation experiments may be used to infer the state of stress in the lithosphere and asthenosphere. Extrapolations of flow equations to a representative geologic strain rate of 10−14/sec. for halite, marble, quartzite, dolomite, dunite and enstatolite are now warranted because the steady state flow processes in the experiments are identical to those in rocks and because the geotherms are reasonably well established. More direct estimates are obtained from free dislocation densities, subgrain sizes and recrystallized grain sizes all of which are functions only of stress. Using the last of these techniques, we have estimated stress profiles as a function of depth from xenoliths in basalts and kimberlites, whose depths of equilibration were determined by pyroxene techniques, from four different areas of subcontinental and suboceanic upper mantle. The results are similar and indicate stress differences of about 200 to 300 bars at 40 to 50 km, decaying to a few tens of bars at depths betow 100 km. These stresses are reasonable and are in accord with extrapolations of the mechanical data provided that allowance is made for a general increase in strain rate and decrease in viscosity with depth.

Rheology of some continental lower crustal rocks

Abstract To gain insight into the rheology of selected feldspar-bearing rocks from the continental lower crust (CLC), three granulites from exposed lower crustal terrains and a microgabbro have been deformed to ca. ~10% strain at confining pressures of 0.8–1.0 GPa, temperatures of 600–900°C, and constant strain-rates of 10−4−10−7 s−1. The most extensive work was done on the mafic Pikwitonei and felsic Adirondack granulites. The Pikwitonei (Mg-hornblende-plag (An70−2 px) has only a limited T,ϵ range of apparent steady-state behavior, and strain-softens at T ⩾ 800°C at ϵ = 10−5−10−6s−1, apparently associated with developing shear zones. This strain-softening is pressure-insensitive from Pc = 0.8–1.0 GPa at T = 850°C, ϵ = 10−6s−1 even though the dominant strain accommodation mechanisms change from semibrittle/cataclastic to crystal plastic (including dynamic recrystallization) with increasing pressure. Plagioclase appears to bear more of the creep strain than amphibole at most experimental conditions except in those runs where both plagioclase and amphibole recrystallization occurs. Kapuskasing (plag-cpx-gt) granulite, deformed at 800°C and at 850°C, 10−6 s−1 has deformed in the steady-state at stress levels only slightly greater than the Pikwitonei. The Stillwater microgabbro and Adirondack granulite are appreciably weaker and the latter exhibits homogeneous semibrittle steady-state behavior over the entire range of conditions investigated. For the Adirondack (2 felds-2 px-gt-qtz) granulite, quartz (21%) appears to control the creep rate at these low strains, and steady-state mechanical data fit well a creep power law with A = 8 × 10−3 MPa−ns−1, Qc = 243 kJ/mol and n = 3.1. Extrapolation of this flow law and of preliminary power law creep relations for the Pikwitonei and microgabbro to a range of physical conditions assumed to encompass those at the continental Moho supports geological and geophysical inferences that a mechanical discontinuity occurs at this transition zone, with the lower crust generally being the weaker material. In addition, the stain-softening exhibited by the Pikwitonei may bear directly on the development of detachment zones at mid- to lower-crustal depths during prograde metamorphism.

Creep of rocksalt

Abstract A review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates. Included in the summary are discussions of: (a) microstructures induced during steady-state creep; (b) creep-rupture of rocksalt; (c) experiments associated with “Project Salt Vault” and more recent field studies; and (d) brine migration. A representative steady-state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics. While the mechanical behavior of rocksalt is probably better understood than for all other rock types, further investigations, especially on load path, stress history and creep-rupture are clearly mandated. Furthermore, additional investigations of brine migration and of bench and field-scale deformations are needed, the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock-mass response.

Upper limits of power law creep of rocks

At depths below a few kilometers, most rocks probably flow in the steady-state dominantly by diffusion-assisted dislocation creep mechanisms whereby strain rate is proportional to a low power of deviatoric stress. At high stress levels for metals, ceramics and rocks, the power law breaks down (PLB) to some other strain rate-stress functional relationship and this paper explores four methods by which the breakdown stress, σb, may be estimated. For metals, alloys and ceramics, σb ≅ 10−3μ, for non-silicate rocks, σb ≅ 5 × 10−3μ, and for silicates, σb ≅ 10−2μ. For silicates, the low stress exponent power law breaks down at high stresses to an exponential dependence of strain rate upon stress or to a Dorn law, the governing flow mechanisms generally being lattice-resistance controlled dislocation glide and mechanical twinning. PLB breakdown stresses for silicates are nearly strain-rate insensitive and extrapolated values of σb are so high as to suggest that low-temperature, high-stress plasticity may be suppressed entirely in geological deformations. PLB for olivine-rich rocks is discussed in some detail and new three-dimensional deformation mechanism surfaces for wet and dry olivine polycrystals are presented.

Rheology of rocksalt

We review progress in experimental determinations of transient and steady-state flow properties and processes of natural rocksalt aggregates, focusing primarily on results from Avery Island, Louisiana, domal salt. The steady-state flow field-established from constant stress and constant strain-rate tests at temperatures from 50 to 200°C, strain rates from 10−5 to 10−9 s−1 and differential stresses, σ, from 20.7 to 2.5 MPa-has been separated into two flow regimes, each fit by a power-law relation. At the higher stresses and strain rates this relation is e=1.6 ×10−4exp(−68.1RT.10−3)σ5.3 for which the pre-exponential constant is expressed in MPa−5.3 s−1 and the apparent activation energy is in J mol−1. Creep rates predicted by equation (A) do not differ appreciably from those predicted previously. The relatively low stress, low strain-rate data are very well fit by e=8.1 ×10−5exp(−51.6RT.10−3 σ2.4 which is comparable conditions, predicts creep rates higher and equivalent viscosities lower than does equation (A) by two orders of magnitude. The change in behavior from (A) to (B) is ascribed to a change in rate-limiting mechanism from cross-slip of screw dislocations to the climb of edge dislocations in this dry material. Subgrain formation dominates microstructural development during steady-state flow of salt and, from determinations of average subgrain diameters in crystals from 20 different rocksalt bodies, flow stress levels between 0.6 and 1.4 MPa have been estimated. These values do not bear directly on arguments concerning the nature of forces initiating salt pillow growth because, apparently, evidence relating to the early deformational history has been overprinted. At that stage, fluid-assisted grain boundary diffusional processes might dominate dislocation creep, leading to a linear stress-strain rate mechanical response. Considering buoyancy forces alone, behavior described by equation (B) rather than (A) reduces the relief necessary to initiate pillow growth but a 1000-fold amplification is required to produce stress differences near 1 MPa. That forces other than buoyancy are important is indicated by the occurrence of these same paleostress levels in bedded salts and in shallow offshore concordant intrusions. Differential loading generally provides the most plausible initial driving force for the growth of diapiric salt structures.

Transient creep and semibrittle behavior of crystalline rocks

We review transient creep and semibrittle behavior of crystalline solids. The results are expected to be pertinent to crystalline rocks undergoing deformation in the depth range 5 to 20 km, corresponding to depths of focus of many major earthquakes. Transient creep data for crystalline rocks at elevated temperatures are analyzed but are poorly understood because of lack of information on the deformation processes which, at low to moderate pressure, are likely to be semibrittle in nature. Activation energies for transient creep at high effective confining pressure are much higher than those found for atmospheric pressure tests in which thermally-activated microfracturing probably dominates the creep rate. Empirical transient creep equations are extrapolated at 200° to 600°C, stresses from 0.1 to 1.0 kbar, to times ranging from 3.17×102 to 3.17×108 years. At the higher temperatures, appreciable transient creep strains may take place but the physical significance of the results is in question because the flow mechanisms have not been determined. The purpose of this paper is to stimulate careful research on this important topic.

Stress dependence of recrystallized-grain and subgrain size in olivine

Abstract New experiments on Mt. Burnet dunite have been carried out to evaluate the effects of important physical parameters on recrystallized-grain size and subgrain size in olivine deforming under steady-state conditions. The experiments, done under both wet and dry conditions in a Griggs solid-pressure-medium apparatus, were conducted in constant strain rate, constant stress and stress relaxation modes at 10 kbar confining pressure, temperatures from 1000°C to 1300°C, strain rates from 10 −4 to 10 −8 /sec and stress differences of from 0.5 to 10 kbar. For dunite deformed under wet conditions, recrystallized-grain size is slightly temperature-dependent but under dry conditions it is only stress-dependent with D = 137 σ −1.27 for D in μm and σ in kbar. Subgrain sizes also depend only on stress; for the dry experiments d = 28 σ −0.62 and for the wet ones d = 15 σ −0.69 . Subgrain sizes decrease with increasing stress but do not increase with decreasing stress and hence record only maximum stress levels. Recrystallized-grain sizes adjust to both increasing and decreasing stress levels, at minimal strains and times, and thus record the stress history. Because of this and of the inherent stability of recrystallized grains, this technique is regarded as more reliable than the subgrain size and free dislocation density and curvature methods for estimating stress magnitudes in tectonites having deformed in the steadystate.

Rheology of the upper mantle: Inferences from peridotite xenoliths

Abstract Stress estimates as a function of depth are obtained for peridotite xenoliths from the upper mantle of three types of tectonic environments by applying revised recrystallizedgrain-size paleopiezometry and pyroxene thermobarometry. The general increase in grain size with depth and hence decrease in deviatoric stress, observed previously, is confirmed but reversals in these trends are now established and remain enigmatic. Stresses and temperatures obtained are combined with a representative creep-flow law to calculate strainrate and viscosity profiles that appear to be physically reasonable. Profiles for the highthermal-gradient rift/ridge environments show a complexity that is interpreted as.a rheological discontinuity resulting from the emplacement of asthenospheric diapirs during late stages of continental rifting. Profiles for broad continental extension zones (C.E.Z.), believed to be most representative of oceanic upper mantle, fluctuate between 50 and 80 km, with a general small increase in strain rate and decrease in viscosity with depth; deepest samples apparently come from the base of the lithosphere. Profiles for the infracratonic mantle of southern Africa show nearly a uniform increase in strain rate to values greater than 10 −14 /sec, and a decrease in viscosity to lower than 10 21 poise, at a depth of 230 km. These profiles may transect the mechanically defined lithosphere—asthenosphere transition at about 200 km and, if so, there is no evidence for a mechanical discontinuity at the boundary. This observation, coupled with evidence that the sense of shear is homogeneous for all mantle profiles constructed, clearly favors a model whereby lithospheric plates are dragged by thermal convection of the asthenosphere below. Sea-floor spreading rates and relative plate-velocity estimates are consistent with this interpretation but do not independently permit a definitive choice between the two favored models advanced to explain the driving force for plate motions.

Finite-element folds of similar geometry

Abstract Model folds of similar geometry have been produced by using the finite-element method and the constitutive relations of a layer of wet quartzite embedded in a marble matrix with an initially sinusoidal configuration and a 10° limb dip. The power law for steady-state flow of Yule Marble (Heard and Raleigh, 1972) is used for the matrix and our new law for Canyon Creek quartzite deformed in the presence of water is used for the layer. The equiv- alent viscosity of the wet quartzite is highly temperature-sensitive, giving rise to a strong temperature dependence of the quartzite: marble viscosity ratio which, at a strain rate of 10 −14 /sec, drops from 543 at 200° to 0.13 at 800°C. At 375°C ( η q / η m = 10), concentric folds develop at all strains to 80% natural shortening and stress, finite strain and viscosity distributions are somewhat similar to those found previously. Raising the temperature to 550° C ( η q / η m = 1), at any stage of prior amplification, causes the folds to flatten with increasing strain, accompanied by attenuation of limbs and thickening of hinges, leading to folds with similar geometries and isoclinal folds at extreme strains. The effects are more pronounced at higher temperatures and at 700° C ( η q / η m = 0.3) limb attenuation is so severe as to give rise to unrealistic geometries. At temperatures below about 600° C ( η q / η m = 2), similar folds do not form. It thus appears as if a viscosity contrast near unity is required to produce similar folds in rocks, under the conditions simulated and different temperature dependencies of viscosities of materials in layered sequences is one important means of reducing viscosity contrasts.