Microstructure, micro-inclusions, and mineralogy along the EGRIP ice core – Part 1: Localisation of inclusions and deformation patterns

Impurities deposited in polar ice enable the reconstruction of the atmospheric aerosol concentration of the past. At the same time they impact the physical properties of the ice itself such as its deformation behaviour. Impurities are thought to enhance ice deformation, but observations are ambiguous due to a shortage of comprehensive microstructural analyses. For the first time, we systematically analyse micro-inclusions in polar fast flowing ice, i.e. from the East Greenland Ice Core Project ice core drilled through the Northeast Greenland Ice Stream. In direct relation to the inclusions we derive the crystal preferred orientation, fabric, grain size, and microstructural features at 10 depths, covering the Holocene and Late Glacial. We use optical microscopy to create microstructure maps to analyse the in situ locations of inclusions in the polycrystalline, solid ice samples. Micro-inclusions are more variable in spatial distribution than previously observed and show various distributional patterns ranging from centimetre-thick layers to clusters and solitary particles, independent of depth. In half of all samples, micro-inclusions are more often located at or close to the grain boundaries by a slight margin (in the areas occupied by grain boundaries). Throughout all samples we find strong indications of dynamic recrystallisation, such as grain islands, bulging grains, and different types of sub-grain boundaries. We discuss the spatial variability in micro-inclusions, the link between spatial variability and mineralogy, and possible effects on the microstructure and deformation behaviour of the ice. Our results emphasise the need for holistic approaches in future studies, combining microstructure and impurity analysis.

Microstructure, Micro-inclusions and Mineralogy along the EGRIP ice core – Part 1: Localisation of inclusions and deformation patterns

Impurities deposited in polar ice allow the reconstruction of the atmospheric aerosol concentration of the past. At the same time they impact the physical properties of the ice itself such as its deformation behaviour. Impurities are thought to enhance ice deformation, but observations are ambiguous due to a shortage of comprehensive microstructural analyses. For the first time, we systematically analyse micro-inclusions in polar fast flowing ice, i.e. from the East Greenland Ice Core Project ice core drilled trough the Northeast Greenland Ice Stream. In direct relation to the inclusions we derive the crystal-preferred orientation, fabric, grain size, and microstructural features at ten depths, covering the Holocene and Late Glacial. We use optical microscopy to create microstructure maps to analyse the in situ locations of inclusions in the polycrystalline, solid ice samples. Micro-inclusions are more variable in spatial distribution than previously observed, and show various distributional patterns ranging from centimetre-thick layers to clusters and solitary particles, independent of depth. Analysing the area occupied by grain boundaries in the respective samples shows that micro-inclusions are slightly more often located at or close to grain boundaries in half of all samples. Throughout all samples we find strong indications of dynamic recrystallisation, such as grain islands, bulging grains and different types of subgrain boundaries. We discuss the spatial variability of micro-inclusions, the link between spatial variability and mineralogy, and possible effects on the microstructure and deformation behaviour of the ice. Our results emphasise the need for holistic approaches in future studies, combining microstructure and impurity analysis.

Twin Induced Reduction of Seismic Anisotropy in Lawsonite Blueschist

Lawsonite is an important mineral for understanding seismic anisotropy in subducting oceanic crust due to its large elastic anisotropy and prevalence in cold subduction zones. However, there is insufficient knowledge of how lawsonite twinning affects seismic anisotropy, despite previous studies demonstrating the presence of twins in lawsonite. This study investigated the effect of lawsonite twinning on the crystal preferred orientation (CPO), CPO strength, and seismic anisotropy using lawsonite blueschists from Alpine Corsica (France) and the Sivrihisar Massif (Turkey). The CPOs of the minerals are measured with an electron backscatter diffraction instrument attached to a scanning electron microscope. The electron backscatter diffraction analyses of lawsonite reveal that the {110} twin in lawsonite is developed, the [001] axes are strongly aligned subnormal to the foliation, and both the [100] and [010] axes are aligned subparallel to the foliation. It is concluded that the existence of twins in lawsonite could induce substantial seismic anisotropy reduction, particularly for the maximum S-wave anisotropy in lawsonite and whole rocks by up to 3.67% and 1.46%, respectively. Lawsonite twinning needs to be considered when determining seismic anisotropy in the subducting oceanic crust in cold subduction zones.

Role of the Adria plate structural heterogeneities on the dynamics of the Central-Western Mediterranean region

<p>In the geodynamic context of the slow Africa-Europe plates convergence, the Central-Western Mediterranean region has been involved in a complex subduction process, which in the last 30 Myr was characterized by the rapid retreat of the Ionian slab, the opening of back-arc extensional basins (i.e., Liguro-Provençal, Algerian, Alboran, and Tyrrhenian basins) and episodes of slab lateral tearing, segmentation and break-off.  A proper modelling of 3-D mantle flow evolution beneath the Mediterranean could provide important clarifications about the complex mantle dynamics of this region and help us understanding the interaction between surface tectono-magmatic processes and mantle convection patterns. </p><p>The mantle flow and its relations with plate horizontal and vertical motions can be determined by measuring seismic anisotropy generated by strain-induced lattice/crystal preferred orientation (LPO/CPO) of intrinsically anisotropic minerals. Seismic anisotropy is widespread in the Mediterranean and it shows an intricate pattern that likely has some relations with the recent (20-30 Myr) behavior of subducting slabs. The extrapolation of the mantle flow from seismic anisotropy is neither simple nor always warranted, especially at subduction zones where complex and non-steady-state 3D flow patterns may establish.  A promising approach, which helps reducing the number of plausible models that can explain a given anisotropy dataset, is to compare seismic measurements with predictions of numerical and experimental flow models (Long et al.,2007). Recently, Faccenda and Capitanio (2013) and Faccenda (2014) have extended this methodology to account for the non-steady state evolution typical of many subduction zones, yielding mantle fabrics that are physically consistent with the deformation history.</p><p>In this study, we apply a similar modelling approach to the complex Central-Western Mediterranean convergent margin. We use the wealth of observations from the Mediterranean region available in the literature to design and calibrate 3D thermo-mechanical subduction modelling. We test different initial configurations defined at 30 Ma according to the paleogeographic and tectonic reconstructions derived from (Lucente and Speranza, 2001; Carminati et al., 2012; van Hinsbergen et al., 2014) in order to optimize the fit between predicted and observed slabs position and obtain a final model configuration resembling the present-day surface and deeper structures.</p><p>In particular, here we want to evaluate the influence on rollback rates, trench shape and the occurrence and timing of slab tears (Mason et al., 2010) of structural heterogeneities within the Adria plate as proposed by (Lucente and Speranza, 2001). In all models, subduction migrates south-eastward driven by the subducting oceanic lithosphere, and slab lateral tearing or break-off occurs when a continental margin enters the trench. More importantly, we show that the presence of a stiffer continental promontory in central Adria together with a thinned continental margin in the Umbria-Marche region plays a fundamental role on (i) the development of a slab window below the Central Apennines, (ii) the retreat of the Northern Apenninic trench till the Adriatic Sea, and (iii) the retreat of the Ionian slab till the present-day position.</p>

Twin induced attenuation of seismic anisotropy in lawsonite blueschist

<p>Lawsonite is an important mineral to understand seismic anisotropy in subducting oceanic crust because of its large elastic anisotropy and prevalence in cold subduction zones. However, there is a lack of knowledge on how lawsonite twinning affects seismic anisotropy despite previous reports showing the existence of twins in lawsonite. We thus investigated the effect of twins in lawsonite on crystal preferred orientation (CPO), fabric strength, and seismic anisotropy of lawsonite using the lawsonite blueschists from Alpine Corsica (France) and Sivrihisar Massif (Turkey). CPOs of minerals were measured by using the electron backscattered diffraction (EBSD) facility attached to scanning electron microscope. The EBSD analyses of lawsonite revealed that {110} twin in lawsonite is developed and [001] axes are strongly aligned subnormal to the foliation and both [100] and [010] axes are aligned subparallel to the foliation. It is found that the existence of twins in lawsonite could induce a large attenuation of seismic anisotropy, especially for the maximum S-wave anisotropy up to 18.4 % in lawsonite and 24.3 % in the whole rocks. Therefore, lawsonite twinning needs to be considered in the interpretation of seismic anisotropy in the subducting oceanic crust in cold subduction zones.</p>

Seismic Properties of a Unique Olivine-Rich Eclogite in the Western Gneiss Region, Norway

Investigating the seismic properties of natural eclogite is crucial for identifying the composition, density, and mechanical structure of the Earth’s deep crust and mantle. For this purpose, numerous studies have addressed the seismic properties of various types of eclogite, except for a rare eclogite type that contains abundant olivine and orthopyroxene. In this contribution, we calculated the ambient-condition seismic velocities and seismic anisotropies of this eclogite type using an olivine-rich eclogite from northwestern Flemsøya in the Nordøyane ultrahigh-pressure (UHP) domain of the Western Gneiss Region in Norway. Detailed analyses of the seismic properties data suggest that patterns of seismic anisotropy of the Flem eclogite were largely controlled by the strength of the crystal-preferred orientation (CPO) and characterized by significant destructive effects of the CPO interactions, which together, resulted in very weak bulk rock seismic anisotropies (AVp = 1.0–2.5%, max. AVs = 0.6–2.0%). The magnitudes of the seismic anisotropies of the Flem eclogite were similar to those of dry eclogite but much lower than those of gabbro, peridotite, hydrous-phase-bearing eclogite, and blueschist. Furthermore, we found that amphibole CPOs were the main contributors to the higher seismic anisotropies in some amphibole-rich samples. The average seismic velocities of Flem eclogite were greatly affected by the relative volume proportions of omphacite and amphibole. The Vp (8.00–8.33 km/s) and Vs (4.55–4.72 km/s) were remarkably larger than the hydrous-phase-bearing eclogite, blueschist, and gabbro, but lower than dry eclogite and peridotite. The Vp/Vs ratio was almost constant (avg. ≈ 1.765) among Flem eclogite, slightly larger than olivine-free dry eclogite, but similar to peridotite, indicating that an abundance of olivine is the source of their high Vp/Vs ratios. The Vp/Vs ratios of Flem eclogite were also higher than other (non-)retrograded eclogite and significantly lower than those of gabbro. The seismic features derived from the Flem eclogite can thus be used to distinguish olivine-rich eclogite from other common rock types (especially gabbro) in the deep continental crust or subduction channel when high-resolution seismic wave data are available.

Quantitative texture analysis by neutron diffraction in deformed crystalline aggregates: Contín Shear Zone, Morais Complex, N Portugal.

<p>Two samples of mylonitic-ultramylonitic ortogneisses collected along the Contín shear zone were investigated for crystal preferred orientation and seismic anisotropy. Neutron diffraction data obtained at the D1B beamline at ILL (Institute Laue-Langevin, Grenoble) were analyzed with the Rietveld method as implemented in the code MAUD, to obtain the orientation distribution functions (ODF) of the principal phases (quartz, K-feldspar, plagioclase, phlogopite, muscovite and riebeckite). Texture and microstructure are compatible with the plastic deformation of the aggregates under medium to low-temperature conditions. Kinematic analysis supports a top-to-the SE sense of shear, suggesting a thrust character. Using preferred orientation data and single crystal elastic tensors, P and S-waves velocities and elastic anisotropy have been calculated. We have explored the role of several factors controlling the elastic properties of rocks, particularly the role of strain state and mineral changes in a shear zone. Those factors have a direct impact on the medium impedance and consequently on the interphase reflectivity. P-wave velocities, S-wave splitting and anisotropy increase with muscovite content. Seismic anisotropy is linked with the texture symmetry, which can result in large deviations between actual anisotropy and that measured along Cartesian XYZ sample directions (lineation/foliation reference frame). This is significant for the prediction and interpretation of seismic data. (Research support CGL2016-78560-P)</p>