Long-term creep experiment on some rocks observed over three years

Since August 1974 the authors have been carrying out an experiment on creep by bending three small granite beams and three gabbro beams. While making measurements, an optical flat is set to produce interference fringes of Na-D light upon the upper polished surface of the beam, which was previously bent convex upward. By analyzing the fringes the profile of the surface is determined with in an accuracy of one-tenth of a wavelength. The routine determination of profiles gives a change in the amount of bending with time.A similar experiment on two large beams of granite was carried out for 21 years by Kumagai and Itô. It took 10 years in order to find out the secondary creep of granite. However, in this three-year experiment it has been found that the secondary creep of the granite specimens gives a viscosity of about 1 · 10²⁰ poise, which is comparable to that obtained over 20 years in the previous experiment (Kumagai and Itô). As for the gabbro specimens, the present authors cannot yet definitely recognize the secondary creep, because gabbro creeps much more slowly than granite.Both this and the previous experiments have revealed that the creep of rocks does not show a steady and monotonous progress, but a repeated “turn back” with various periods, some of which are longer than one year. In order to explain the creep accompanied by the “turn back” phenomenon, the authors venture to hypothesize that the elastic constants of a test-piece slightly vary with time during the advance of creep.     

Dynamic recrystallization of wet synthetic polycrystalline halite: dependence of grain size distribution on flow stress, temperature and strain

It is often observed that dynamic recrystallization results in a recrystallized grain size distribution with a mean grain size that is inversely related to the flow stress. However, it is still open to discussion if theoretical models that underpin recrystallized grain size–stress relations offer a satisfactorily microphysical basis. The temperature dependence of recrystallized grain size, predicted by most of these models, is rarely observed, possibly because it is usually not systematically investigated. In this study, samples of wet halite containing >10 ppm water (by weight) were deformed in axial compression at 50 MPa confining pressure. The evolution of the recrystallized grain size distribution with strain was investigated using experiments achieving natural strains of 0.07, 0.12 and 0.25 at a strain rate of 5×10⁻⁷ s⁻¹ and a temperature of 125 °C. The stress and temperature dependence of recrystallized grain size was systematically investigated using experiments achieving fixed strains of 0.29–0.46 (and one to a strain of 0.68) at constant strain rates of 5×10⁻⁷–1×10⁻⁴ s⁻¹ and temperatures of 75–240 °C, yielding stresses of 7–22 MPa. The microstructures and full grain size distributions of all samples were analyzed. The results showed that deformation occurred by a combination of dislocation creep and solution-precipitation creep. Dynamic recrystallization occurred in all samples and was dominated by fluid assisted grain boundary migration. During deformation, grain boundary migration results in a competition between grain growth due to the removal of grains with high internal strain energy and grain size reduction due to grain dissection (i.e. moving boundaries that crosscut or consume parts of neighbouring grains). At steady state, grain growth and grain size reduction processes balance, yielding constant flow stress and recrystallized grain size that is inversely related to stress and temperature. Evaluation of the recrystallized grain size data against the different models for the development of mean steady state recrystallized grain size revealed that the data are best described by a model based on the hypothesis that recrystallized grain size organizes itself in the boundary between the (grain size sensitive) solution-precipitation and (grain size insensitive) dislocation creep fields. Application of a piezometer, calibrated using the recrystallized grain size data, to natural halite rock revealed that paleostresses can vary significantly with temperature (up to a factor of 2.5 for T=50–200 °C) and that the existing temperature independent recrystallized grain size–stress piezometer may significantly underestimate flow stresses in natural halite rock.     

Strain measurements and tectonics of New Zealand

Measurements of shear strain from triangulation data have been made at 30 locations in New Zealand. The standard error of measurement in terms of strain rate is about ±1 · 10⁻⁷ y⁻¹ and values of up to 7 · 10⁻⁷ y⁻¹ are observed. Together with 22 fault-plane solutions for crustal earthquakes the measurements indicate broad-scale patterns of deformation. Between the Hikurangi and Flordland active margins is a 100-km-wide belt, the axial tectonic belt, with shear strain rate averaging 5 ± 1 · 10⁻⁷y⁻¹ and an azimuth of the principal axis of compression of 114 ± 8°. The rate of movement (45 mm y⁻¹) and direction (085°) between the Pacific and Indian plates from the Minster et al. pole can be accounted for by the measured strain in the axial tectonic belt through simple shear parallel to, and compression normal to, the belt. The similarity in the rates determined from triangulation data averaged over 20–100 years and from plate movement averaged over 5 m.y. indicates plate movement to be uniform in time. West of the axial tectonic belt in Nelson and Fiordland are two zones in which movement is highly oblique to plate movement, and can be explained by slip line deformation analogous to the deformation of Asia. The azimuth of the principal axis of compression in the Taupo rift and East Cape region is NE—SW, perpendicular to its direction in the axial tectonic belt, suggesting extension in the rift and East Cape region normal to the subduction zone.     

Deformation mechanism maps for feldspar rocks

Deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates. The maps display constant temperature contours in stress-grain size space for strain rates ranging from 10⁻¹⁶ to 10⁻¹² s⁻¹.Two fields of dominance of grain boundary diffusion-controlled creep and dislocation creep are separated by a strongly grain size-sensitive transition zone. For wet rocks, diffusion-controlled creep dominates below a grain size of about 0.1–1 mm, depending on temperature, stress, strain rate and feldspar composition. Plagioclase aggregates containing up to 0.3 wt.% water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks. The strength of water-bearing feldspar rocks is moderately dependent on composition and water fugacity.For a grain size range of about 10–50 μm commonly observed in natural ultramylonites, the deformation maps predict that diffusion-controlled creep is dominant at greenschist to granulite facies conditions. Low viscosity estimates of 10¹⁸–10¹⁹ Pa·s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures >900 °C or, at lower temperatures, diffusion creep of fine-grained rocks possibly localized in abundant high strain shear zones. For similar thermodynamic conditions and grain size, lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks.     

Reaction-induced weakening of plagioclase–olivine composites

The localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions. In order to study the effects of solid–solid mineral reactions on plastic deformation of rocks, we have investigated the shear deformation of plagioclase–olivine composites during the reaction plagioclase + olivine → orthopyroxene + clinopyroxene + spinel (± garnet). Microstructures of plagioclase–olivine composites were studied after shear deformation experiments in a Griggs apparatus. Experiments were performed on anorthite–forsterite (An–Fo) and labradorite–forsterite (Lab–Fo) composites at 900 °C, confining pressures between 1000–1600 MPa and with constant shear strain rates of 5 × 10⁻⁵ s⁻¹.In absence of reaction, Lab–Fo composites are stronger than pure olivine and labradorite end-members that deform with a high temperature plasticity mechanism. Lab–Fo composites strain–harden due to the inhibition of extensive recrystallisation by interphase boundaries.In An–Fo composites, the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine-grained (size < 0.5 μm) polyphase reaction products. Interconnecting layers of reaction products accommodate most of the applied strain by grain size sensitive creep. Recovery processes are pronounced during syndeformational reaction: original anorthite and olivine dynamically recrystallise by subgrain rotation and bulging recrystallisation. Presumably, the dynamic recrystallisation is caused by reduced stress conditions and partitioning of strain and strain rates between the new reaction products and the relict An–Fo grains. The results of our experiments are in good agreement with natural observations of shear localisation in the lower crust and upper mantle, and imply that anhydrous mineral reactions can be important causes for localisation of deformation.     

Vitrinite anisotropy under differential stress and high confining pressure and temperature: preliminary observations

The orientation of the optical indicating surface of vitrinite in reflected light has been determined following deformation at 350 and 500°C, confining pressures of 500 and 800 MPa and a strain rate of 10⁻⁵ s⁻¹. High temperature and large strain have facilitated reorientation of the indicating surface, increase in anisotropy (bireflectance) and an increase in maximum vitrinite reflectance. In a specimen deformed at 500°C and 23% axial strain the maximum vitrinite reflectance has been reoriented more than 70° from close to parallel to σ₁ in the undeformed state to perpendicular to σ₁ following deformation. Orientation of the optical indicating surface of some of the deformed specimens suggests the orientation of the maximum reflectance is a composite product of the original orientation of the indicating surface and an orientation produced during deformation.     

High-temperature flow of wet polycrystalline enstatite

Previous experiments by Raleigh et al. (1971) have shown that at strain rates of 10⁻².sec⁻¹ to 10⁻⁷.sec⁻¹ only slip occurs in dry enstatite at temperatures above 1300°C and 1000°C, respectively.The present experiments have been conducted on polycrystalline enstatite under wet conditions in this regime where enstatite only slips, polygonizes and recrystallizes. Slip occurs throughout the whole regime on the system (100)[001] and at strains greater than 40% the system (010)[001] is observed. Polygonization and intragranular recrystallization begin at about 1300°C and 10⁻⁴.sec⁻¹ and the orientation of these neoblasts is host-controlled. At lower strain rates intergranular neoblasts develop and their fabric is one of [100] maximum parallel with σ₁ and [010] and [001] girdles in the σ₂ = σ₃ plane, similar to those in natural enstatite tectonites.Dislocation substructures of experimentally deformed enstatite have been examined by transmission electron microscopy. The samples were deformed within the field in which slip polygonization and recrystallization are the dominant deformation mechanisms. Samples within this regime have microstructures that are characterized by stacking faults and partial dislocations. Under the conditions of steady-state flow in olivine, these microstructures inhibit the operation of recovery mechanisms in enstatite.Other samples deformed within the polygonization and recrystallization field have microstructures that confirm the optical observations of intragranular and intergranular growth of neoblasts. It is suggested that the former result from strain-induced tilt of subrains, whereas the latter may result from bulge nucleation into adjacent subgrains.Mechanical data from constant strain-rate experiments at steady state, stress relaxation and temperature-differential creep tests are best fit to a power-law creep equation with the stress exponent, n~3 and the apparent activation energy for creep, Q~65 kcal/mole. Extrapolation of this equation to a representative natural geologic strain rate of 10⁻⁴. sec⁻¹, over the temperature interval 1000–2000°C, gives an effective viscosity range of 10²⁰–10¹⁸ poise and stresses in the range of 7-0.1 bar, respectively. Comparison with corrected wet-olivine mechanical data (Carter, 1976) over the same environment indicates that olivine is consistently the weaker of the two minerals and will recrystallize whilst enstatite will only slip and kink, thus accounting for the different habits of olivine and enstatite in ultramafic tectonites.     

Microstructural development of fine-grained quartz aggregates by syntectonic recrystallization

Experimental study of syntectonic recrystallization of fine-grained quartz aggregates was carried out in order to simulate the development of some natural microstructures of quartz tectonites and to understand their formation condition. Agate was axially compressed with a constant-strain-rate apparatus. Experiments were conducted at 4 kbar solid confining pressure, 700–1000°C and 10⁻⁴-10⁻⁶ sec⁻¹ to 10%–45% strain. In all runs, deformation has proceeded under wet condition caused by dehydration of pyrophyllite used as pressure medium.Two different types of microstructure were distinguished in the deformed specimens. One is P-type which is characterized by equant, equidimensional, and polygonal grains. The other is S-type which is characterized by the highly oblate grains with the largest dimension perpendicular to the compression axis. The P-type microstructure is developed at higher temperatures and slower strain rates, while the S-type developed at lower temperatures and faster strain rates. The transition between the S- and P-types is found to be very sharp.     

Post-deformational annealing of calcite rocks

The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10^{− 4} s^{− 1} to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.     

Stress and viscosity in the asthenosphere

Stresses and effective viscosities in the asthenosphere to a depth of 400 km are calculated on the basis of Weertmans “temperature method” i.e., on relating viscosity to the ratio of the temperature to the melting point (=homologous temperature). Some oceanic and continental geotherms and two melting point—depth curves, the dry pyrolite solidus and the forsterite₉₀ melting curve are used for the conversion of the homologous temperature to the effective viscosity. Two creep laws are considered, the linear, grain-size-dependent Nabarro—Herring (NH) creep law, and a power creep law, in which the creep rate is proportional to the third power of the stress. A plate tectonic model yields creep rates of 2 · 10⁻¹⁴ s⁻¹ for the oceanic and 3 · 10⁻¹⁵ s⁻¹ for the continental asthenosphere. These values are held constant for the calculations and may be valid for regions inside plates.The dry pyrolite mantle model results in high homologous temperatures in the asthenosphere below oceans (0.9), very low stresses (a few bars and lower) and shows a low viscosity “layer” of about 200-km thickness. Below continental shields the homologous temperature has a maximum value of 0.73, stresses are around 5–20 bar and the low-viscosity region is thicker and less pronounced than in the oceanic case. The Fo₉₀ mantle model generally gives lower homologous temperatures (maximum value below oceans beside active ridges 0.75). The stresses in the asthenosphere beneath oceans vary from a few bars to about 50 bar and below continents to about 100 bar. The low-viscosity region seems to reach great depths without forming a “channel”. The Figs. 1 and 2 show the approximate viscosity—depth distribution for the two mantle models under study.Assuming a completely dry mantle and a mean grain size of 5 mm, power law creep will be the dominating creep process in the asthenosphere. However, grains may grow in a high-temperature—low-stress regime (i.e., below younger oceans), an effect which will further diminish the influence of NH creep. In the upper 100–150 km of the earth some fluid phases may affect considerably creep processes.     

Creep of olivine during hot-pressing

The densification curves for the hot-pressing of pure olivine powders were obtained as a function of grain size (5 μ–2000 μ), temperature (1000–1600°C), and compacting stress (166–298 bars). This range of variables was found to straddle two fields of hot-pressing behavior, one dominated by power-law creep, one by Coble creep. The time required to density a powder to 99% of the single crystal density could be represented by the shorter of the two times: t₁ = 2.2 · 10³σ^−3.4exp(85,000/RT)t₂ = 1.3 · 10⁴σ^−1.5(G)⁺³exp(85,000/RT) where the compacting stress or pressure, σ, is given in bars and the grain size, G, in centimeters. It was also possible to estimate the parameters appropriate to Coble creep in a solid polycrystalline aggregate from the hot-pressing data; and these were:

Full-size image (1K)

The strain rates computed from this formula are close to those predicted by Stocker and Ashby (1973) and those found by Twiss (1976).     

Single-layer folding in marble and limestone: An experimental study

Folding experiments have been carried out on single-layers of Carrara marble and Solnhofen limestone at a confining pressure of 275 bars, temperature of 400°C, and strain rates of 5.5×10⁻⁷ to 8.2×10⁻⁷. The marble and limestone layers were embedded in a rock-salt matrix and in a matrix of a mixture of 60% fine-grained halite and 40% fine-grained calcite, respectively, and deformed to different percentages of bulk shortening. Aspect ratios of the layers varied between 11.25 and 15. The stress-strain relationship reveals that strain increased with a very small increment in compressive stresses, once folding was initiated.With progressive deformation the bulk strain is compensated by folding along one fold hinge. The resulting folds are concentric and a combination of class 1a, 1b and 3 type. The changes in the arc length, layer thickness, limb dip and wavelength with progressive folding in marble layers, are discussed.The microstructure and texture of the folded marble and limestone layers have been investigated optically and by means of an X-ray texture goniometer. The inner fold arc exhibits a strong preferred orientation, whereas in the outer fold are the preferred orientation is poorly developed. Differences in the fabric in medium-grained marble and fine-grained limestone layers have been attributed to the difference in mechanism of deformation.     

Water assisted dynamic recrystallization and weakening in polycrystalline bischofite

Artificially prepared specimens of bischofite (MgCl₂-6H₂O) have been experimentally deformed at temperatures between 20 and 100°C, strain rates between 10⁻⁴ and 10⁻⁸⁸ s⁻¹, and confining pressures between 0.1 and 28 MPa. Development of microstructure with strain was studied by in-situ deformation experiments, and results of these were correlated with observations made on thin sections of deformed samples.In a first series of experiments the effect of grain size, impurity content and water content on the flow behaviour was investigated. Addition of about 0.1 wt.% water to dry samples was found to decrease the flow stress by a factor of 5. This effect was found to be associated with the formation of a thin fluid film on grain boundaries, strongly enhancing dynamic recrystallization due to the movement of high-angle grain boundaries, and possibly also to enhanced intracrystalline plasticity due to excess water present in the lattice. In a second series of experiments the strain-rate sensitivity of the flow stress of selected samples was investigated. Two regimes could be distinguished: one with a stress exponent n = 4.5 in the power law creep equation for values of the differential stress above 2.0 MPa, and one with n = 1.5 for stresses below this value.The main deformation mechanisms were intracrystalline slip, twinning, and grain-boundary sliding. Recrystallization occurred by subgrain rotation and high-angle grain-boundary migration. The rates of grain-boundary migration fell into two different regimes, one regime being distinguished by extremely fast migration rates. The applicability of the experimentally found flow law to the behaviour of bischofite rocks in nature is discussed.     

Quartz microstructures developed during non-steady state plastic flow at rapidly decaying stress and strain rate

Synseismic loading to very high stresses (>0.5 GPa) and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca. 300–350 °C, near the lower tip of an inferred once seismically active crustal scale fault, was proposed based on peculiar microstructures identified in rocks exposed over >100 km² in the Sesia Zone, European Western Alps. Here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small-scale shear zones. Sub-basal deformation lamellae and arrays of elongate subgrains on the TEM-scale indicate an early stage of glide-controlled deformation at high stresses. Distributed brittle failure is indicated by healed microcracks. Very fine-grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep. Locally, a fine-grained foam microstructure indicates a final stage of static grain growth at low differential stress. For the previously inferred peak stresses of about 0.5 GPa and given temperatures, initial strain rates on the order of 10⁻¹⁰ s⁻¹ are predicted by available flow laws for dislocation creep of quartz. We emphasise the importance of short-term non-steady state deformation in the uppermost plastosphere underlying seismically active upper crust. The related heterogeneous record of quartz is governed by the local stress history at constant temperature.     

Dynamic recrystallization near the brittle-plastic transition in naturally and experimentally deformed quartz aggregates

The electron backscattering diffraction technique (EBSD) was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates, both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains. For the Tonale mylonites we investigated, a temperature range of 300–380 °C, 0.25 GPa confining pressure, a flow stress range of ~ 0.1–0.2 GPa, and a strain rate of ~ 10^− 13 s^− 1 were estimated. Experimental samples of Black Hills quartzite were analyzed, which had been deformed in axial compression at 700 °C, 1.2–1.5 GPa confining pressure, a flow stress of ~ 0.3–0.4 GPa, a strain rate of ~ 10^− 6 s^− 1, and to 44% to 73% axial shortening. Using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development. Our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains. Grain boundary bulges are not only formed by local grain boundary migration, but they also display a lattice misorientation indicative of subgrain rotation. Dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation, i.e. with an orientation unsuitable for easy basal slip. In addition, Dauphiné twins within porphyroclasts are preferred sites for recrystallization. We interpret large misorientation angles in the experimental samples, which increase with increasing strain, as formed by the activity of fluid-assisted grain boundary sliding.     

Correlation between dissolved He concentration and Cl in groundwater at Äspö, Sweden

Tunnel excavation at Äspö Island, Sweden, has caused severe groundwater disturbance, gradually extending deeper into the tunnel as present-day Baltic seawater intrudes through fractures connecting to the surface. However, the paleo-hydrogeochemical conditions have remained in the deep highly saline waters that have avoided mixing. A correlation has been observed between dissolved ⁴He concentration and Cl⁻ ion concentration, measured every two years from 1995 to 2001 at Äspö. Groundwater mixing conditions can be examined by the correlations between 1/Cl, ³⁶Cl/Cl, and ³H concentrations. Subsurface production is responsible for the majority of the ³⁶Cl and excess dissolved ⁴He of interstitial groundwater in fractures. The secular equilibrium ratio of ³⁶Cl/Cl in rock was theoretically estimated to be (5.05 ± 0.82) × 10⁻¹⁴ based on the neutron flux intensity, a value comparable to the measured ³⁶Cl/Cl ratio in rock and groundwater. The degassing crustal ⁴He flux was estimated to be 2.9 × 10⁻⁸  1.3 × 10⁻⁶ (ccSTP/cm²a) using the HTO diffusion coefficient for the Äspö diorite. The ⁴He accumulation rate ranges from 6.8 × 10⁻¹⁰ (for the in situ accumulation rate) to 7.0 × 10⁻⁹ (ccSTP/(g_water · a) considering both ⁴He in situ production and the degassing flux, assuming ⁴He is accumulated constantly in groundwater. By comparing the subsurface ³⁶Cl increase with ⁴He concentrations in groundwater, the ⁴He accumulation rate was determined from data for groundwater arriving at the secular equilibrium of ³⁶Cl/Cl. The ⁴He accumulation rate was found to be (1.83 ± 0.72) × 10⁻⁸ ccSTP/(g_water · a) without determining the magnitude of degassing ⁴He flux.     

Large-strain deformation and strain partitioning in polyphase rocks: Dislocation creep of olivine–magnesiowüstite aggregates

Aggregates composed of olivine and magnesiowüstite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning, phase segregation and possible localization of deformation in a polyphase material. Samples with 20 vol.% of natural olivine and 80 vol.% of (Mg_0.7Fe_0.3)O were synthesized and deformed in a gas-medium torsion apparatus at temperatures of 1127 °C and 1250 °C, a confining pressure of 300 MPa and constant angular displacement rates equivalent to constant shear strain rates of 1–3.3 × 10^− 4 s^− 1. The samples deformed homogeneously to total shear strains of up to γ  15. During constant strain rate measurements the flow stress remained approximately stable at 1250 °C while it progressively decreased after the initial yield stress at the lower temperature. Mechanical data, microstructures and textures indicate that both phases were deforming in the dislocation creep regime. The weaker component, magnesiowüstite, controlled the rheological behavior of the bulk material and accommodated most of the strain. Deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowüstite and may have developed due to the presence of the second phase. At 1127 °C, olivine grains behaved as semi-rigid inclusions rotating in a viscous matrix. At 1250 °C, some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a-slip on (010) resulted in a weak foliation and a texture typical for pure dry olivine aggregates. Both a-slip and c-slip on (010) were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions.     

A comparison of intracrystalline deformation in naturally and experimentally deformed quartzites

A relatively undeformed quartzite sample from the Weverton formation was experimentally deformed in plane strain at a temperature of 700° C, a confining pressure of 15 kb and a constant strain rate of 10⁻⁶/sec, in a modified Griggs apparatus. A comparison of the known experimental strain for the sample with that measured from deformed rutile needles within the quartz grains shows fairly close agreement between the two values. This confirms the validity of using the needles as intracrystalline strain markers. A comparison has been made of the microstructures and preferred orientations in the experimentally deformed sample and a naturally deformed sample of the same quartzite which has undergone the same strain. The experimentally deformed sample exhibits more inhomogeneous intragranular deformation and a “double funnel” pattern of c axes, while the naturally deformed sample exhibits more homogeneous intragranular deformation and a broad great circle girdle of c axes normal to the foliation and lineation.     

On the anomalous land uplift and aseismic creep prior to the 1976 Tangshan earthquake

A spatio-temporal analysis based on the data of eleven repeated levellings around the Tangshan region prior to the 1976 earthquake indicates that an uplift lasting for 2 years, from 1968 through 1969. with a magnitude of 50 mm, occurred in the epicentral area.Aseismic creep superimposed on the accumulated strain has been found in the vicinity of Tangshan and Baodi along both the Tangshan and the Jiyunhe faults.Assuming uniform strain accumulation and elastic dislocation, theoretical values of displacement at the various dislocation sites have been calculated and, using the least squares method, the optimal values of strain accumulation and the parameters of the creep faults in different years have been determined.The creep fault under Tangshan, a right-lateral normal fault, strikes N47°E and dips S87°E. and is 8 km long and 6 km wide. The upper boundary of the fault lies 2 km deep. The strike-slip and dip-slip offsets are, respectively, 104 cm and 8cm. The average rate of strain accumulation amounts to 0.9 × 10⁻⁷/yr. Creep at the fault amounted to 18.6 cm/yr and 1.4 cm/yr, respectively, in the strike and dip directions over the period 1969–1975. The Jiyunhe fault, although of smaller dimensions, has experienced a greater rate of creep than the Tangshan fault.A correlation of the above-mentioned uplift and creep with that of the Tangshan earthquake suggests that the uplift might have been a manifestation of the early development of the earthquake and that aseismic creep may be one of the precursory phenomena of shallow earthquakes. The sequence of processes preceding the Tangshan earthquake may be described as: strain accumulation-land upliftaseismic creep-inverse land deformation (or decrease in creep rate)-earthquake.     

Finite-element folds of similar geometry

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.