Shaocheng Ji

Ductility of garnet as an indicator of extremely high temperature deformation

Discoidal shaped garnets (Alm63 Prp33 Grs3 Sps1) occur in the foliation plane of lineated quartzofeldspathic mylonites of the Morin shear zone located along the eastern boundary of the Morin anorthosite complex (Grenville Province, Quebec). The flattened shape of garnets records a component of coaxial strain (X = Y ⪢ Z), different from the dominant rotational shear strain (X >Y >Z) in this mylonite. TEM observations indicate dislocation slip and recovery to be the main mechanism for the deformation of garnet. Extrapolation of available experimentally-determined flow laws to natural strain-rates shows an inversion of flow strength between quartz ± feldspar and garnet: garnet, much stronger than quartz and feldspar at temperatures below 700°C, becomes weaker than quartz and feldspar above 900°C. The coaxial ductile strain recorded by the garnets could be produced by a local, uncharacterized deformation, under extremely high temperature conditions at the roof of the rising anorthositic diapir (∼1155 Ma). Subsequent, lower temperature rotational deformation is related to strike-slip movement along the Morin shear zone (∼1020 Ma).

Microstructures, petrofabrics and seismic properties of ultra high-pressure eclogites from Sulu region, China: implications for rheology of subducted continental crust and origin of mantle reflections

Ultra high-pressure (UHP) eclogites from Sulu region (China) represent mafic components of the continental crust, which were first subducted to mantle depths greater than 100 km and then exhumed to the earth’s surface. Detailed investigation of microstructures, chemical compositions, petrofabrics and seismic properties of the UHP eclogites can provide important information on the operating deformation mechanisms and rheology of subducted continental crust and on the origin of seismic reflections within the upper mantle. We present here results from field, optical and TEM observations, electron back-scattered diffraction (EBSD) measurements and numerical computations of the seismic properties of UHP eclogites collected from fresh surface outcrops at the drill site (Maobei, Donghai County, Jiangsu Province) of the Chinese Continental Scientific Drilling Program (CCSD). Two types of eclogites have been distinguished: Type-1 (coarse-grained) eclogites deformed by recovery-accommodated dislocation creep at the peak metamorphic conditions, and Type-2 (fine-grained) eclogites which are composed of reworked Type-1 materials during recrystallizationaccommodated dislocation creep in shear zones which were active during the exhumation of the UHP metamorphic rocks. Both garnet and omphacite in these eclogites deformed plastically and the flow strength contrast between these two constituent minerals is apparently much less than an order of magnitude under the UHP metamorphic conditions. Plasticity of eclogites under UHP conditions can effectively facilitate channeled flow along the interplate shear zone. The

Recrystallization and Fabric Development in Plagioclase

The microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite-granulitic gneisses from the lower crust have been studied. Special interest in the effects of dynamic recrystallization and deformation fabrics has been taken. The plagioclase feldspars (An40-50) have undergone extensive ductile deformation by intracrystalline slip and twinning. They show strong crystallographic preferred orientation which was produced by dominant (010) [001] slip. Dynamic recrystallization is indicated by fine-grained new grains (neoblasts) interspersed with deformed relicts (porphyroclasts). The neoblasts have formed preferentially along grain boundaries, kink band boundaries (KBBs), intracrystalline shear zones, and around solid inclusions, such as garnet and pyroxene. Optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low strain. The orientation of neoblasts is closely related to the kinematic framework of flow (X-Y-Z), not to the original orientation of their host grains. The stable preferred orientation developed by dynamic recrystallization favours the easy glide of dislocations on the (010) [001] system. We suggest that recrystallization-accommodated dislocation creep is an important process in the development of preferred orientations in plagioclase feldspars, which will result in geometric softening.

Shear-wave velocities, anisotropy and splitting, in high-grade mylonites

Shear-wave velocities have been measured for 10 high-grade, granulite- and upper amphibolite-facies mylonites from the Snowbird tectonic zone (Canada) at various pressures up to 600 MPa in order to explore the kinematic significance of shear-wave splitting in the lower crust. At 600 MPa, Vs anisotropy ranges from 11% in amphibolitic mylonites to 2% in granulite-facies pyroxene-bearing and tonalitic mylonites. Hornblende-bearing granitic mylonites display intermediate Vs anisotropy (6–7%). Vs anisotropy is lower than Vp anisotropy in hornblende-rich mylonites, whereas Vs anisotropy is higher than Vp anisotropy in feldspar-rich mafic, tonalitic, granitic and diatexitic mylonites. Shear-wave splitting is pronounced in rocks displaying strong S-wave anisotropy. In most of the high-grade mylonites studied, the polarization direction of the first arrival, Vs1, corresponds to the extension lineation (X) when propagation is parallel to Y (perpendicular to lineation and parallel to foliation), or to the Y-direction when propagation is parallel to X. For all other propagation-directions, the polarization direction of Vs1 is oblique to the X-, Y- and Z-directions. Therefore, it may be difficult to infer the structural implications of shear-wave splitting in lower continental crustal shear zones if one does not know (1) the orientation of the propagation and fast vibration-directions with respect to in-situ lineation and foliation, and (2) the lattice preferred orientation of in-situ rock-forming minerals.

Bulk flow strength of forsterite–enstatite composites as a function of forsterite content

Creep experiments have been conducted to investigate the effect of varying forsterite content (VFo) on the bulk flow strength of dry forsterite–enstatite (Fo–En) aggregates in order to evaluate the applicability of existing theoretical models to two-phase rocks, as well as to understand the rheology of polyphase systems in general. The experiments were performed at temperatures of 1423–1593 K, stresses of 18–100 MPa, oxygen fugacities of 10 � 14 –10 � 2.5 MPa and 0.1 MPa total pressure. The fine-grained (Fo: 10–17 mm; En: 14–31 mm) composites of various Fo volume fractions (VFo=0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) were synthesized by isostatically hot-pressing in a gas-medium apparatus at 1523 and 350 MPa. Our experiments show that flow strength contrasts between Fo and En are in the range of 3–8 at the given experimental conditions, with Fo as the stronger phase. The measured stress exponent (n) and activation energy (Q) values of the Fo–En composites fall between those of the endmembers. The n values show a nearly linear increase from 1.3 to 2.0, while the Q values display a non-linear increase from 472 to 584 kJ/mol with En volume fraction from 0 to 1.0. There is no clear dependence of creep rates on oxygen fugacity for the Fo–En composites. The mechanical data and TEM microstructural observations suggest no change in deformation mechanism of each phase when in the composites, compared to when in a single-phase aggregate, the En deformed mainly by dislocation creep while the Fo deformed by dislocation-accommodated diffusion creep for our grain sizes and experimental conditions. Comparisons between the measured composite strengths and various theoretical models indicate that none of the existing theoretical models can give a precise predication over the entire VFo range from 0 to 1. However, the theoretical models based on weak-phase supported structures (WPS) yield a good prediction for the flow strengths of the composites with VFo 0.6. No model gives a good prediction for the bulk strength of two-phase composites in the transitional regime (VFo=0.4–0.6). Applications of the WPS- and SPS-based models in the transitional regime result in under- and over-estimations for the composite flow strength, respectively. Thus, the effect of rock microstructure should be taken into consideration in modeling the bulk flow strengths of the crust and upper mantle using laboratory-determined flow laws of single-phase aggregates. D 2001 Elsevier Science B.V. All rights reserved.

Sense of shear in high-temperature movement zones from the fabric asymmetry of plagioclase feldspars

Abstract A new method for determining the sense of shear in plagioclase-bearing tectonites from the (010) orientation of plagioclase feldspar is presented. The method is based on the asymmetry of the (010) plane with respect to the structural frame (foliation and lineation) and the dominant activity of the (010) slip plane in the high-temperature plasticity of plagioclase feldspar. Using examples from the Zabargad gneisses (Red Sea) the method is applied to plagioclases of An25–An45 and compared with other methods of shear-sense determination (quartz c -axis fabrics and microstructural criteria).

Elasticity of six polycrystalline silicate garnets at pressure up to 3.0 GPa

Abstract The elasticities of six polycrystalline silicate garnets (almandine, grossular, pyrope, uvarovite, andradite, and Prp25Alm56Spe19) have been experimentally studied at pressures up to 3.0 GPa using a phase comparison method with an ultrasonic interferometer in a liquid cell piston-cylinder apparatus. Complete elasticity data sets (P- and S-wave velocities, bulk moduli Ks, shear moduli G, and their first pressure derivatives KS′ and G′) have been obtained for all six garnets, and are used together with an up-to-date compilation of garnet elasticity data to examine composition-elasticity systematics of garnets. Our results suggest that pyralspite and ugrandite have different relationships between bulk sound velocity (Vφ) and mean atomic weight (M̅0), between Poisson’s ratio (σ) and density (ρ), and between G/Ks and KS/ρ ratios. A large error may occur when the systematics are applied across different garnet groups.

Obliquity between seismic and electrical anisotropies as a potential indicator of movement sense for ductile shear zones in the upper mantle

Teleseismic shear-wave splitting and magnetotelluric experiments across the Grenville front, between the Archean craton and the Proterozoic Grenville province in the regions of the Pontiac subprovince and northwestern Grenville province (Canada), show a consistent obliquity between the polarization direction of the fast split shear wave Φ and the most electrically conductive direction (ΦMT) in the upper mantle transcurrent shear zones. At all well-recorded stations, Φ is nearly N103°E, and ΦMT is approximately N80°E. The obliquity may be considered a potential kinematic indicator, because the seismic and electrical anisotropies are thought to be controlled by lattice-preferred and shape-preferred orientations of mantle minerals (mainly olivine), respectively. The dextral movement sense of the transcurrent shear zones in the mantle, inferred from the observed obliquity, is consistent with that inferred from surface geology of the crustal shear zones. This consistency implies that deformation of the crust and the subcrustal upper mantle in the lithosphere was largely coherent in the study region.

Refinements of shear-lag model and its applications

Abstract Since it was developed by Cox [Cox, H.L., 1952. The elasticity and strength of paper and other fibrous materials. Br. J. Appl. Phys. 3, 72–79.], the shear-lag model (SLM) or fiber-loading theory has been widely used by material scientists and structural geologists as a powerful analytical method for analyzing mechanical interactions between hard and soft phases (layers) in composites and two-phase rocks. We found that several approximations in previous mechanical derivations of the model can be improved in terms of mechanical equilibrium and boundary conditions. Our refinements make the model more rigorous and elaborate. Applications of the improved model include: (1) prediction of the axial tensile stress in strong inclusions embedded in a continuous weak matrix, and consequently interpretation of extension fracture boundinage; (2) modeling of the distribution of shear stresses in the weak matrix surrounding the strong inclusions and, accordingly, interpretation of the variations of dislocation density and of recrystallized grain size in composites and polyphase rocks; and (3) prediction of mechanical properties of industrial composites and two-phase rocks. The analytical results of the improved model agree approximately with those obtained from both experimental investigations and numerical calculations.

Seismic reflectivity of a finely layered, granulite-facies ductile shear zone in the southern Grenville Province (Quebec)

Abstract Lithoprobe reflection profile 54 across the granulite-facies Morin shear zone (MSZ) in the Grenville Tectonic Province (Quebec, Canada) reveals a west-dipping zone of strong reflections. The origin of these reflections has been investigated by detailed field mapping, laboratory measurements of P-wave velocities and densities of representative rock samples from the main lithological units, and forward synthetic modeling. Petrofabric and microstructural analyses were performed in order to interpret the measured seismic properties. Synthetic seismograms generated from the velocity and density data demonstrate that reflections originate from lithologic thin layers and thin-layer clusters, likely resulted from intensive tectonic transposition and metamorphic differentiation. Lattice-preferred-orientation (LPO)-induced anisotropy of granulite-facies mylonites (except for those containing abundant sillimanite, biotite or amphibole) is very low, and hence unlikely to be the primary cause for reflections. Confining pressure does not significantly affect the reflectivity of the shear zone because the reflection coefficients are not sensitive to pressure up to at least 600 MPa. Since composite reflections of thin-layer clusters can cause high reflectivity, the strong reflectivity from some deep crustal sections with low refraction velocities may be interpreted to be due to the presence of thin-layer, mafic clusters within dominantly felsic rocks.