Toru Takeshita

Pure shear and simple shear calcite textures. Comparison of experimental, theoretical and natural data

Abstract The pattern of lattice preferred orientation (texture) in deformed rocks is an expression of the strain path and the acting deformation mechanisms. A first indication about the strain path is given by the symmetry of pole figures: coaxial deformation produces orthorhombic pole figures, while non-coaxial deformation yields monoclinic or triclinic pole figures. More quantitative information about the strain history can be obtained by comparing natural textures with experimental ones and with theoretical models. For this comparison, a representation in the sensitive three-dimensional orientation distribution space is extremely important and efforts are made to explain this concept. We have been investigating differences between pure shear and simple shear deformation incarbonate rocks and have found considerable agreement between textures produced in plane strain experiments and predictions based on the Taylor model. We were able to simulate the observed changes with strain history (coaxial vs non-coaxial) and the profound texture transition which occurs with increasing temperature. Two natural calcite textures were then selected which we interpreted by comparing them with the experimental and theoretical results. A marble from the Santa Rosa mylonite zone in southern California displays orthorhombic pole figures with patterns consistent with low temperature deformation in pure shear. A limestone from the Tanque Verde detachment fault in Arizona has a monoclinic fabric from which we can interpret that 60% of the deformation occurred by simple shear.

Anisotropy of VP and VS in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural and textural characteristics of the rock

Abstract Laboratory seismic measurements of compressional ( V p )- and shear-wave ( V s ) velocities have been carried out on a deformed amphibolite from the Ivrea Zone at pressures and temperatures up to 600 MPa and 600°C, respectively. Amphibolite is considered to be an important rock of the deeper crust. From crystallographic orientations of the component minerals (hornblende, plagioclase and diopside) and the respective elastic constants, we calculated the average elastic constants (Voigt average) for the rock specimen. Compressional- and shear-wave velocities in three orthogonal directions (normal and parallel to foliation) measured at high confining pressure (600 MPa) are in good agreement with the calculated velocity surface. This suggests that seismic-wave velocity anisotropy is controlled by preferred orientations of minerals in rocks (texture) at high confining pressure, where the effect of microcracks on seismic wave velocities is eliminated. In the amphibolite sample, the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species. A large S-wave splitting is documented in the directions parallel to the foliation (particularly in the lineation) which strongly argues against the microcrack origin of S-wave splitting, at least at conditions of high effective pressure.

A new estimate of the conditions for transition from basal 〈a〉 to prism [c] slip in naturally deformed quartz

Abstract Quartz c -axis fabrics of naturally deformed quartz in metacherts from the Ryoke metamorphic belt in the Yanai district, southwestern Japan, suggest that the prism [ c ], prism 〈 a 〉 and basal 〈 a 〉 slip systems were activated during the plastic deformation under high-temperature metamorphic conditions. With increasing temperature, quartz c -axis fabric patterns change from girdle type, through X -maximum with a faint crossed girdle, to X -maximum. The dominant orientation of subgrain boundaries in the quartz also changes from prism to basal with increasing temperature, closely correlated with the change in quartz c -axis fabric. The fabric transition is also accompanied by an abrupt increase of recrystallized quartz grain size. Moreover, observations with transmission electron microscopy (TEM) clearly identify the dominant activation of [ c ] slip through the presence of basal subgrain boundaries and free dislocations with the [0001] Burgers vector in the quartz from the metachert samples exhibiting X -maximum c -axis fabric. Based on these observations it is concluded that the mechanism switch from basal 〈 a 〉 to prism [ c ] slip systems occurred with increasing temperature in the metacherts. The transition temperature is petrologically estimated to be ca. 550–600°C at the natural strain rate.

Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U–Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts

Abstract In this paper we present new U–Pb zircon ages determined with Sensitive High-Resolution Ion MicroProbe (SHRIMP) for nine plutonic rocks or orthogneisses and one paragneiss from the Higo and Maana belts (here referred to as the paleo-Ryoke belt) in southwest Japan, and the South Kitakami belt in northeast Japan. Both belts are Paleozoic–Mesozoic continental terranes discontinuously distributed along the Median Tectonic Line (MTL), and structurally lying on the Sambagawa belt in the Japanese Islands. Three groups of U–Pb zircon ages showing the timing of different magmatic events were determined for the plutonic rock or orthogneiss samples: ca. 500 Ma (Early Ordovician, two samples), a mean age of 292.0±12.4 Ma (Late Carboniferous–Early Permian, one sample) and ca. 110 Ma (Early Cretaceous, six samples). Furthermore, zircons with core–rim microstructures from a garnet–biotite–cordierite paragneiss in the Higo belt were also dated with SHRIMP. The cores yielded 2155–184 Ma inherited U–Pb ages (mostly varying between 330 and 184 Ma), while the rims yielded a mean age of 116.5±18.7 Ma (Early Cretaceous) U–Pb recrystallization ages comparable with the igneous ages, limiting the sedimentary age of the protolith to between Early–Middle Jurassic (ca. 180 Ma) and Early Cretaceous time. These SHRIMP U–Pb ages of plutonism, metamorphism and sedimentation, together with previously reported radiometric ages, have revealed a strong similarity in the tectonic histories of the paleo-Ryoke and South Kitakami and Abukuma belts, suggesting that these belts can be correlated. At present the paleo-Ryoke belt and the South Kitakami and Abukuma belts are separated by more than 1000 km. However, these belts may have originally comprised a continuous continental terrane that was later displaced and juxtaposed, together with the underlying Sambagawa belt, against the Ryoke belt by large-scale sinistral faulting along the MTL and associated major strike–slip faults during the Latest Cretaceous.

Quartz plastic segregation and ribbon development in high-grade striped gneisses

Quartz microstructures and c-axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high-grade Alem Paraiba shear zone, in southeastern Brazil, are documented. Cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process, where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons. These ribbons are separated by feldspar-rich domains devoid of quartz. The stage at which individual, stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode, which is reflected by an abrupt increase in the smoothness of the ribbon boundaries. This change is interpreted to represent a strain-softening kink in the stress-strain-time path. Progressive ribboning is accompanied by strengthening of the c-axis fabric Z-maximum, indicative of continued plastic flow by basal glide. Operation of basal glide at these high-temperature conditions (680–700°C) is interpreted to be a consequence of relatively dry deformation conditions. A model is then proposed for development of straight quartz ribbons in high-grade striped gneisses, where scattered quartz grains are continuously stretched and segregated by crystal–plastic processes. The small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement. Pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons.

Dislocation microstructures in naturally deformed silicate garnets

Abstract Dislocation microstructures of naturally deformed silicate garnets and olivines in garnet-peridotites and silicate garnets in eclogites from four localities have been observed with a transmission electron microscope (TEM) to clarify the dislocation characteristics of silicate garnets. We have obtained the following results: (1) dislocation densities of garnets in all the garnet-peridotites (ρ = 105−107 cm−2) are always nearly an order of magnitude lower than those of co-existing olivines; (2) dislocation densities of garnets in eclogites (ρ = 105−108 cm−2) which are embedded in garnet-peridotites are almost an order of magnitude higher than those of garnets in the surrounding garnet-peridotites; (3) the dominant Burgers vector, b, of mainly edge dislocations in garnet is 〈100〉 for specimens with dislocation density ρ = 105−106 cm−2, while b = 1 2 〈111〉 for specimens with ρ = 107−108 cm−2. Result (1) indicates that the observed dislocations in garnets were formed by plastic deformation under the same stresses as for co-existing olivines, and that there is a similar relationship between applied stress and dislocation density for garnets as for olivines. Result (2) suggests that the stress concentration occurred around eclogites embedded in garnet-peridotites, and the resulting differential stress in garnets in eclogites was further elevated by the interlocking of neighboring hard garnet grains. Finally, result (3) indicates that the dominant Burgers vector of mainly edge dislocations in garnet changes from 〈100〉 to 1 2 〈111〉 with increasing applied differential stress.

Plastic anisotropy and geometrical hardening in quartzites

Abstract Lattice rotation in crystals which creates a characteristic preferred orientation in deformed tectonites takes place whenever slip (dislocation glide) is involved in the deformation process. The crystal lattice either rotates to an orientation favorable for further slip or to one that is unfavorable, resulting in an overall hardening or softening of the polycrystal during deformation. This effect of plastic anisotropy which arises due to changes in the crystal orientation distribution is analyzed quantitatively for quartz polycrystals using the Taylor model. The results indicate that in easy {basal}〈a〉 slip region (low temperature), quartzites become softer in flattening with increasing strain, whereas in easy {prism} 〈a〉 slip region (high temperature), quartzites become softer in elongation with increasing strain. Accordingly, deformation is facilitated by flattening at low temperature and by elongation at high temperature if it can be heterogeneous to a certain degree. This is borne out by geological observations, which indicate that at low metamorphic grade flattening to plane strain is the most frequently observed strain mode (e.g., Moine thrust), whereas at high metamorphic grade constriction is favored (e.g., Sambagawa belt). The strain distribution in a large geological body may depend on internal anisotropic plastic properties such as preferred slip systems as well as imposed external boundary conditions. Heterogeneities are also observed on the hand specimen scale. Grains in orientations where easier slip systems are difficult to activate remain relatively undeformed in a ductile matrix consisting of grains in which strain is accommodated mostly by easier slip systems. Some aspects of the Taylor model for creep rate sensitivity are discussed.

Development of preferred orientation and microstructure in sheared quartzite: comparison of natural data and simulated results

Abstract c-axis fabric and microstructures in a quartzite sample, sheared and extensively recrystallized under greenschist facies conditions, have been analyzed and compared with theoretical predictions using a viscoplastic self-consistent model modified to incorporate the effects of dynamic recrystallization. An asymmetric small-circle c-axis fabric about the finite shortening z-axis with a small half opening angle (35°) is present in the sample; it consists of four orientation components which are represented by host grain c-axis orientations (referred to as A, B, C and D): A and B are at high angles to the foliation plane, displaced against and with the sense of shear, respectively; C is in an intermediate direction between the Y- and Z-axis of finite strain, and D forms a subsidiary concentration around the intermediate strain (Y-) axis. B- and C-grains are favorably oriented for basal (0001) and pyramid {10 1 1}〈a〉 slip, respectively, and strongly deformed, while A- and D-grains are unfavorably oriented for the slip systems and little or moderately deformed. Some of A-grains are even fractured. The degree of dynamic recrystallization increases with increasing strain undergone by differently oriented grains (in the sequence of A-, D-, C- and B-grains). Microstructural evidence and theoretical predictions indicate that harder A-, C- and D-grains were significantly consumed by the grain boundary migration of the softer recrystallized B-component (although the consumption of A-grains was not really documented in the quartzite sample). The conclusion is supported by the fact that the B-component is much more dominant in the recrystallized than in the host c-axis fabric. Hence, the c-axis maximum nearly perpendicular to the shear plane and apparently displaced with the sense of shear commonly found in naturally sheared quartzites (correlated with the B-component) is presumably developed by the growth of soft orientations for basal (0001) slip by grain boundary migration at large strains.

c-Axis fabrics and microstructures in a recrystallized quartz vein deformed under fluid-rich greenschist conditions

The quartz v-axis orientations and microstructures have been analyzed in an originally single-crystal buckled quartz vein of greenschist metamorphic grade from the Sambagawa metamorphic belt, southwest Japan. The basal (0001) plane was inclined by ca 10–20 to the quartz vein surface before deformation, and the flexure (essentially kink) which produced the fold was accomplished by basal (0001) slip alone in the hinges (i.e. flexural-slip folding). In addition to crystal plasticity, dissolution microstructures are ubiquitous in the quartz vein. Dynamic recrystallization resulted from both subgrain rotation and grain-boundary migration. Rotation recrystallization associated with basal (0001) slip resulted in host-controlled v-axis orientation distribution along a great circle, whereas migration recrystallization, perhaps assisted by intergranular fluid, resulted in a penetrative development of a weak X (elongation)-maximum v-axis fabric nowhere rotated by the folding. The migration recrystallization is evidenced by large v-axis misorientations across the grain boundaries, as well as irregular grain shape. Based on the weak v-axis fabric development and lack of grain shape fabric, solution-precipitation creep may have dominated in the fluid-assisted grain-boundary migration regime, and hence it is inferred that the transition from rotation to migration recrystallization was caused by a local decrease of differential stress in the buckled quartz vein.

Simulation of dislocation-assisted plastic deformation in olivine polycrystals

Studies on mantle convection provide insight into the geodynamics of the Earth. Convection models have relied on flow laws for olivine (e.g. Hager 1984, Christensen 1987), which is believed to be the major constituent of the upper mantle. The mantle is assumed to deform in a regime of steady-state creep in which diffusion and dislocation glide occur. When dislocation glide is present, polycrystalline materials develop anisotropy due to rotations of crystals during straining, which produce a preferred orientation distribution. Strong seismic anisotropy has been observed in the upper mantle underneath oceanic crust (e.g. Hess 1964) and continental crust (e.g. Fuchs 1983), and it is generally accepted that at high pressure where microfractures are closed, seismic anisotropy can be due to crystallographic preferred orientation or texture (e.g. Christensen 1984). Whereas Arrhenius-type flow laws are adequate to describe the diffusion-controlled plastic deformation (e.g. Karato et al. 1986), they do not take account of plastic anisotropy due to slip. We attempt to introduce a generalized description of anisotropic plastic flow of olivine based on polycrystal plasticity theory which takes into account the deformation history. It should be emphasized that all arguments brought forward relate to processes in which dislocation movements are involved and do not apply to recrystallization, which is also important in deformation of olivine (e.g. Ave Lallemant & Carter 1970). Deformation by grain-boundary sliding, such as during superplastic flow, does not produce texture.