Conference Agenda

Deformation localisation can lead to a variety of structures, such as shear zones and bands that range from grain to crustal scale, from discrete zones to anastomosing networks, and shear zone related folds.

We present numerical simulations of the deformation of an intrinsically anisotropic material with a single maximum crystal preferred orientation (CPO) in simple shear. We use the Viscoplastic Full-Field Transform (VPFFT) crystal plasticity code coupled with the modelling platform ELLE to achieve very high strains. The VPFFT-approach simulates deformation by dislocation glide, taking into account the different available slip systems and their critical resolved shear stresses. We vary the anisotropy of the material from isotropic to highly anisotropic, as well as the orientation of the initial CPO. To visualize deformation structures, we use passive markers, for which we also systematically vary the initial orientation.

At low strains the amount of strain rate localisation and resulting deformation structures depend on the initial CPO in all anisotropic models. Three regimes can be recognised: distributed shear localisation, synthetic shear bands and antithetic shear bands. However, at very high strains localisation behaviour always tends to converge to a similar state, independent of the initial CPO.

Shear localisation is often detected by folded layers, which may be parallel to the anisotropy (e.g. cleavage formed by aligned mica), or the deformation of passive layering, such as original sedimentary layers. The resulting fold patterns vary strongly, depending on the original layer orientation. This can result in misleading structures that seem to indicate the opposite sense of shear.

Quartz crystallographic preferred orientations (CPOs) in natural mylonites can vary to such an extent that they apparently give opposite senses of shear within a single thin section. Many qualitative explanations have been proposed. Here, we take a multiscale numerical approach to investigate the variation of quartz CPOs resulting from flow partitioning in a ductile shear zone environment. We couple our self-consistent Eshelby formalism for power-law viscous composite materials with the visco-plastic self-consistent (VPSC) model for simulating CPOs in crystalline aggregates. In a quartz-bearing polyphase mylonite, we regard quartz aggregates in the rock as microscale Eshelby inhomogeneities embedded in a macroscale medium (the polyphase continuum). The effective rheology of the continuum is represented by a hypothetical homogeneous equivalent medium and is obtained self-consistently from the constituent phases (e.g., quartz, feldspar and mica grains). We obtain the partitioned flow fields in each quartz aggregate first, using our own Eshelby formalism, and then use the partitioned fields to simulate quartz CPOs, using the VPSC model. We reproduced the observed quartz CPOs and found out that the CPO variation actually reflects a macroscale finite strain gradient rather than vorticity-sense reversal as previously thought. We demonstrated that, despite the microscale flow fields varying from one quartz aggregate to another and from the macroscale flow, the sense of vorticity in all quartz RDEs remains the same as the macroscale vorticity. Our work suggests that quartz c-axis fabrics cannot be effectively used to estimate the macroscale flow vorticity.

An overview of the deformation and recrystallization mechanisms that are active in the North Greenland Eemian Ice Drilling (NEEM) ice core is given, based on microscale models, light microscopy and cryogenic electron backscatter diffraction (cryo-EBSD).

The Holocene ice (0-1419 m depth) deforms by dislocation creep with basal slip accommodated by non-basal slip. The amount of non-basal slip is controlled by the extent of strain induced boundary migration (SIBM). The most important recrystallization mechanisms and processes in the Holocene ice are grain dissection, strain induced boundary migration (SIBM), and bulging nucleation.

In the glacial ice (1419-2207 m of depth) basal slip is accommodated by both non-basal slip and grain boundary sliding (GBS). Rotation recrystallization is more important, while SIBM is less important in the glacial ice compared to the Holocene ice.

In the Eemian ice (2207-2540 m depth), which is at high temperature, different microstructures occur depending on the impurity content of the ice. The difference in microstructure and deformation mechanisms, between interglacial and glacial ice can have important consequences for ice rheology and ice sheet dynamics.

An accurate flow law for dislocation creep of quartzite is critical for the understanding of continental rheology and geodynamic models. Despite many years of effort, existing creep experiments have yielded very different quartz flow law parameters. We demonstrate that the difference can be explained by considering the pressure effect on the activation enthalpy and the slip system dependence of the stress exponent. We carefully examine high-quality experimental data of wet quartzite corresponding to steady-state regimes 2, and 3 dislocation creep together with related quartz c-axis fabrics and identify two end-member quartz flow laws corresponding to dominant prism <a> slip and dominant basal <a> slip respectively. The parameters of two flow laws are determined from experiments through a self-consistent iterative approach. The flow law for dislocation creep by dominant prism <a> slip is ε=2.5×10^-14·f^2.6·exp(-(132000+35.8P)/RT)·σ^4 and that by dominant basal <a> slip is ε=6.3×10^-12·f^1.7·exp(-(126000+23.1P)/RT)·σ^2.5. Quartz c-axis from both experiments and nature, suggest that there is a continuous spectrum from dominant prism <a> slip to a mixture of prism <a> and basal <a> slip to dominant basal <a> slip. In such a case, quartzite is a polycrystal aggregate in which the slip system varies among grains. On a suitable representative volume element, the overall rheology of a quartzite must be obtained through a self-consistent micromechanics approach. The results are shown as a plot of overall strength versus temperature and contributions of dominant slip systems in a 3D profile. We also discussed the significance and implications of multiple flow behaviors for continental strength.

How fine-grained, well-mixed (ultra)mylonitic layers in the upper mantle form is still unclear. Here, microfabrics displaying the transition from porphyroclasts to mixed assemblages were analyzed regarding their phase assemblage, grain size and shape, mixing patterns and crystallographic orientations for insights into the interplay of phase mixing and recrystallization.

Olivine recrystallization starts in porphyroclastic textures and continues until ultramylonites. Ol microfabrics are divided in neoblasts in multiphase assemblages, recrystallized porphyroclasts and neoblast layers. Both latter microfabrics are ±monomineralic (ol~90 area%) with low mixing intensities (>90% ol-ol boundaries). However, olivine is the dominant mixing phase during opx/cpx porphyroclast recrystallization.

Up to mylonitic textures orthopyroxenes occur as porphyroclasts and bands. In mylonites both recrystallize, forming an instant fine-grained mixture of ol (~56 area%, ~13µm) and opx (~37 area%, ~14µm). The phase mixing intensity is high (63% phase boundaries).

Clinopyroxene porphyroclasts recrystallize from protomylonites onwards forming an instant mixture of cpx (~61 area%, ~21µm) and ol (~31 area%, ~18µm) in mylonites. Cpx generally displays bigger grain sizes and higher abundances than opx in ol+opx assemblages. Mixing intensities are high (66% phase boundaries).

Beside few thoroughly mixed areas, cpx/opx+ol bands are still distinguishable in (ultra)mylonitic layers. They can largely be traced back to recrystallized opx/cpx porphyroclasts. At their boundaries, high abundances of ol neoblasts are present, mixing with host porphyroclast neoblasts. Opx and cpx porphyroclasts that recrystallized isolated from ol-bearing matrix have significantly lower mixing intensities. Phase mixing is therefore assumed to occur simultaneous during porphyroclast recrystallization, and to be strongly dependent on ol nucleation.

“Ballen structures” of quartz and cristobalite aggregates have been observed in impactites from a number of terrestrial impact structures, predominantly from impact melt rocks, suevites, and target rock clasts affected by high post-shock temperatures (e.g., CARSTENS, 1975; FERRIÈRE et al., 2010). The aggregates range from few hundreds of microns to 5mm in size and have a peculiar fracture pattern. For their formation, phase transformations from cristobalite to quartz after shock have commonly been proposed (e.g., CARSTENS, 1975; FERRIÈRE et al., 2010). Raman-spectroscopy, light and electron microscopy, as well as electron back scattered diffraction (EBSD) analysis of quartz and cristobalite ballen aggregates in impact breccias from the Ries crater, however, suggest that they form by dehydration of diaplectic silica glass and a “diaplectic silica melt”, respectively. The polycrystalline quartz ballen aggregates partly preserve the crystallographic orientation and shape of the original quartz grain from the target granitic gneisses. Cristobalite with ballen structure is comprised by radiating polycrystalline aggregates. We suggest that the original quartz rich in fluid inclusions transformed into diaplectic glass or highly viscous supercooled “diaplectic melt” as a result of shock loading, where the volatiles from the inclusions dissolved into the glass or melt, and the glass retained remnants of the quartz structure guiding the subsequent topotactic crystallization.

During decompression and cooling, dehydration causes strain concentration followed by fracturing into small globular ballen. The fluid is expelled along the fractures, comparable to perlitic structures in silica rich volcanic rocks, which can be described as circular crack networks (e.g., MARAKUSHEV et al., 1987). Whereas quartz with ballen structure crystallized from the dehydrated diaplectic glass, cristobalite with ballen structure is suggested to have crystallized rather from the “diaplectic melt” with no crystallographic or shape memory, probably generated due to a locally higher fluid availability. Dendritic and radiating cristobalite at the rim of quartz ballen aggregates in contact to vesicles is suggested to have crystallized from a melt enriched in the expelled fluids from the diaplectic silica glass/melt. Prismatic cristobalite in contact to feldspar in the matrix surrounding the ballen aggregates is interpreted to have crystallized from a silica-rich melt.

We derived the relationship between AMS and strain in a marble shear zone by combining rock magnetic studies, detailed microstructural analysis and CPO-based numerical modelling of the AMS. AMS ellipsoids are characterised by k1 orientation as being at a high angle to the primary fabric and experience gradual rotation towards the SZ plane with increasing strain. The k1 orientation is consistent with the calcite c-axes preferred orientation, which is considered to represent an "inverse" AMS fabric, because the "normal" AMS fabric should show k3 parallel to c-axes. Moreover, the AMS shows an angular deviation from the local macroscopic fabric observed in the shear zone. The microstructural evolution related to the shear zone development is characterised by dynamic recrystallization of a primary coarse-grained calcite microstructure. The increasing strain is accommodated by increasing the amount of recrystallized matrix at the porphyroclasts expense. To interpret inverse magnetic fabric and observed strain-AMS relationship we have implemented numerical modelling. Models are constructed based on microstructure, CPO, modal and chemical composition of constituting minerals.

The localization of deformation at P-T conditions of dislocation creep leads to the contemporaneous evolution of two microstructural subfabrics in the marble. The combination of their respective magnetic signals results in distinct orientation of total AMS and local macroscopic fabric. This means that neither strain magnitude nor its orientation derived from macroscopic fabric orientation would correspond to estimates deduced from AMS. The combination of magnetic signal of distinct subfabrics also influences the shape and strength of magnetic anisotropy.

We show that due to localization of deformation, AMS is indirectly dependent on the magnitude and character of deformation. In order to decipher the AMS-strain relationship, AMS studies should be accompanied by microstructural analyses combined with numerical modelling of magnetic fabric.

The description of drilling probes due to their porosity is an important research subject in terms of searching for nuclear waste repositories. Th current techniques use raster electron microscope pictures together with various edge detection and growing seed algorithms coming from the field of image processing. Those algorithms suffer from certain shortcomings. The BGR has started the project ITERATOR to overcome those shortcomings with the usage of artificial intelligence, mainly with deep neural networks. The first step is to produce enough REM images and use them within the second step to train a neural network. The first results of the used neural network are promising. Within the speech the starting material and the used network will be described. A closer look inside the network will be provided to better demonstrate the function of the neural network. To complete the presentation of the project ITERATOR the next steps within the project (3D-Models and detection of rock grains).

The nature of the shallow-crustal active faults has been widely studied to elucidate their stress accommodation mechanisms and response to the fault movement in terms of stress localizations and ultimately stress release by rupture or creep mechanisms. The Main Frontal Thrust (MFT) and its hinterland area are at present the most tectonically active zones within the Himalayan orogen accommodating northward convergence of India against the Eurasian plate with periodic release of accumulating strain energy through moderate–large earthquakes. This work is a first report on detailed microstructural and mineralogical studies from fault gouges of the Nahan Thrust (NT) occurring in the vicinity of MFT. The NT exhibits a ~150 m wide fault core-damage zone in the studied area, where the fault core consists of grey-green, red and black gouge layers. The intact rocks comprise alternations of arenitic and argillaceous sandstone, while the damage zone consists of crackle to mosaic and foliated breccia. The modal mineral analysis of the fault core rocks and the intact rocks determined in conjunction with SEM-BSE, EPMA, and image processing by ImageJ software (Schneider et al., 2012), suggests that the original rock of the grey-green and black gouge layers is the arenitic sandstone, while that of the red gouge layers is the argillaceous sandstone. Microstructures indicate dominance of stress localization and cataclasis in the grey-green gouge, while pressure solution creep in the red gouge. Additionally, the calcite vein fragments present in the red gouge show development of Type-I deformation twins indicating the overall temperature of deformation ~170℃ (Rowe and Rutter, 1990; Burkhard, 1993; Ferrill et al., 2004). However, the black gouge layer shows development of ultracataclasite-rich principal slip zones with degassing bubbles and clay clast aggregates (CCA), indicating localized frictional heating and possible seismic faulting (Boutareaud et al., 2008; Tesei et al., 2014). Hence, this study proposes that the arenitic sandstone suffered seismic rupture, while the argillaceous sandstone deformed aseismically, implying a strong lithological dependence on the seismic cycle within the Nahan thrust.