Bernd Leiss

Mechanisms of dynamic recrystallization in naturally deformed dolomite inferred from EBSP analyses

Abstract In order to investigate the mechanisms of dynamic recrystallization in dolomite, single grain orientation measurements on a mantled porphyroclast have been carried out by means of electron back-scattered patterns (EBSPs). The local orientations in the grain fabric were analysed to establish meaningful orientation families. In addition to a twinned grain, the porphyroclast consists of two domains, each with a specific orientation pattern. Both domains are polygonized by subgrain rotation, which gives great- and small-circle dispersion patterns of the crystal orientation data in the pole figures. However, even the simple and well-defined dispersion patterns for which one can assume a single slip system are not consistent with any of the theoretical dispersion patterns of the known slip systems. Additional mechanisms like rigid body rotation, nucleation and/or combinations of different slip systems are the likely explanation. No definitive orientation relationship has been found between the porphyroclast and the mantle. The lack of such a relationship, together with the shape of the grain boundary, point to grain boundary migration. The texture of the mantle agrees with the global texture of the sample, indicating that mechanisms of crystal plasticity were active during flow around the clast. The results demonstrate the potential of EBSP data for the deduction of active mechanisms, but a proper interpretation requires a better theoretical foundation. This could be achieved by modelling dispersion patterns for different mechanisms and different combinations, and by including orientation correlations.

Fabric anisotropy and its influence on physical weathering of different types of Carrara marbles

Abstract To evaluate the texture-controlled part on the thermally induced degradation of marbles, the anisotropic thermal dilatation was calculated from texture analyses of four exemplary samples from the Carrara area in Italy and compared to experimentally measured dilatation coefficients. The thermal dilatation as determined in the experiment is controlled by an intrinsic part (anisotropic single crystal properties and texture) and an extrinsic part (e.g. thermally induced microcracks). As expected from theoretical calculations, there is a correlation between the strongest dilatation and the c -axis maxima and the weakest dilatation and the a- axis maxima according to the single crystal data of calcite. However, a quantitative correlation could not be established. Obviously, other fabric parameters like the grain size, grain shape anisotropies, grain boundary geometries and microcrack formation during heating modify the texture-controlled part significantly. After thermal treatment up to 130°C, all samples show a residual strain. However, the magnitude and directional dependence is remarkably different and is unequivocally correlated to both the microstructure and the texture. Since the number of parameters controlling the physical weathering is very large, a comprehensive quantification of fabrics is indispensible for the understanding of thermally controlled degradation processes of physical weathering in marbles.

Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile

Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S1. As the F1 folds became slightly overturned to the west, S1 was folded about west-vergent open F2 folds and an S2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by Rf/ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-to-the-east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c. 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c. 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.

‘High-temperature’ texture in naturally deformed Carrara marble from the Alpi Apuane, Italy

Abstract From deformation experiments and numerical modelling, a large type variety of crystallographic preferred orientations (textures) are known for calcite. In contrast, naturally deformed samples usually show the ‘low-temperature (LT)’-texture type with only minor texture variations. The ‘high-temperature (HT)’-texture type is rarely described and mostly not very well defined. Based on neutron diffraction measurements and a quantitative texture analysis by means of the iterative series expansion method and the texture component model, this study gives evidence for the HT-texture type in a deformed marble from the Alpi Apuane in Italy. The microstructure of the sample shows elongated grains with long/short axis ratios of up to 10:1. The long axes of the grains are oriented parallel to the general direction of transport indicating prolate strain, but no shear sense. From the texture, a shear sense can be deduced that cannot be fully brought in line with the regional deformation and thermal history. The results indicate a larger texture variety of naturally deformed calcite rocks than generally assumed. This should stimulate further systematic texture studies for a better understanding of the texture forming mechanisms and the closely connected understanding of the kinematic significance of textures for the analysis of regional deformation histories.

Initiation of left-lateral deformation along the Ailao Shan–Red River shear zone: new microstructural, textural, and geochronological constraints from the Diancang Shan metamorphic massif, SW Yunnan, China

The Diancang Shan metamorphic massif, the northwestern extension of the Ailao Shan Massif, is a typical metamorphic complex situated along the NW–SE-trending Ailao Shan–Red River shear zone. Diancang Shan granitic and amphibolitic mylonites collected from sheared high-grade metamorphic rocks were studied using petrographic and electron-backscatter diffraction techniques. Sensitive high-resolution ion microprobe U–Pb dating of zircon grains from the granitic mylonites constrains the timing of shearing. Macro- and microstructural and textural analysis reveals intense plastic deformation of feldspar, quartz, and amphibole under amphibolite-facies conditions, all consistently document left-lateral shearing. Porphyritic monzogranitic mylonite within the shear zone possesses evidence supporting a sequential, progressive process from crystallization during magma emplacement, through submagmatic flow to solid-state plastic deformation. We suggest that the early-kinematic pluton subsequently underwent strong left-lateral strike–slip shearing. The development of complex textures of quartz, feldspar, and amphibole from the granitic and amphibolitic mylonites apparently records successive variation of conditions attending coherent, solid-state high-temperature ductile deformation during regional left-lateral shearing. All magmatic zircons from the mylonitized porphyritic monzogranite give U–Pb ages of 30.95 ± 0.61 million years for the crystallization of the granite. This age provides the timing of onset of left-lateral shearing along the Ailao Shan–Red River shear zone in the Diancang Shan high-grade metamorphic massif.

Textures and microstructures of naturally deformed amphibolites from the northern Cascades, NW USA: methodology and regional aspects

Abstract Neutron texture analyses of quartz-bearing and quartz-free amphibolite mylonites from the Windy Pass thrust, Cascades Crystalline Core (Washington/USA) reveal pronounced textures of plagioclase and clino-amphiboles (hornblende, cummingtonite) but no preferred orientation of quartz. A reliable strategy for amphibolite fabric analysis is presented by a systematic analytical approach to the experimental diffraction data processing. Clino-amphiboles show transitional textures between ideal single crystal orientations and axial symmetric great circle distributions. Plagioclase reveals a-axes distributions scattering along a great circle approximating the foliation plane as well as a-axes maxima close to the macroscopic lineation. Correlation of the textures with grain shape anisotropies of horn-blende and plagioclase and comparison with data from the literature suggest that the texture variations are due to different strain regimes rather than due to different crystallographic reorientation mechanisms. The kinematic directions deduced from the microfabric correlate well with the regional tectonic interpretations. In contrast, individual deformation paths are not yet established for the different tectonic units, as the significance of the separating Windy Pass thrust requires further structural analysis and fabric studies.

Potential thermal expansion of calcitic and dolomitic marbles from Andalusia (Spain)

Marble has historically been used as an ornamental stone because of its aesthetic appeal, ease of polishing and ex;cellent physical properties. One of the main factors affecting the durability of marbles is their thermal behaviour. Although marble is used extensively in Spain as a building and decorative material, little research has been done into its thermal behaviour. In this work, the textural and microstructural properties of seven calcitic and dolomitic marbles from Andalusia (southern Spain) were characterized to assess how these properties affect their thermal response. Rock fabric properties (grain morphology, boundaries and micro-crack populations) were studied by polarized microscopy and lattice preferred orientation using X-ray texture goniometry. Elastic properties were measured by ultrasound, thermal properties were determined by a six-rod dilatometer (thermal expansion) and the opening of micro-cracks was observed using environmental scanning electron microscopy. For each marble, thermal coefficients of calcite and dolomite crystals were calculated using thermo-X-ray diffraction, a novel application to characterize historic marbles. The results show that marble thermal expansion coefficients are related to preferred crystallographic orientation, which can help to identify the directions along which decay occurs. The results also show for the first time that the thermal expansion coefficient of the main components of marble, i.e. calcite and dolomite, is specific to each marble, and plays a key role in their different thermal behaviours. Thermal properties also depend on mineral composition, the existence of micro-cracks and hydric properties.

Structure and kinematics of an outcrop-scale fold-cored triangle zone

Triangle zones are widespread structural elements that link fold and thrust belts with their foreland basins. We present a structural analysis of an outcrop-scale (~6 m [20 ft] wide) triangle zone that is exposed in the siliciclastic Carboniferous strata of the Harz Mountains in northern Germany. The geometry of the triangle zone is critically compared with larger outcrop and seismic-scale structures. The external form of the triangle zone is the same as that proposed for larger examples, with two bounding detachments that dip in opposing directions. However, the interior of the triangle zone is characterized by tight to isoclinal folds. We demonstrate how the triangle zone probably evolved from a fault-bend fold, which accreted further folds behind it. This is an alternative fold-based interpretation for a structure that is commonly modeled as a duplex stack. We present the resulting consequences for seismic interpretation and hydrocarbon reservoir evaluation.

Textures and Microstructures in Peridotites from the Finero Complex (Ivrea Zone, Alps) and their Influence on the Elastic Rock Properties

In order to quantify differences in the elastic rock properties as a result of fabric differences, peridotite samples from the Finero complex were investigated with respect to their mineral textures and elastic properties. Our data indicate only weak to intermediate texture strengths and weak elastic anisotropy, which is too small to produce a significant acoustic contrast in a seismic experiment. Consequently, internal structures from peridotite bodies in the Earths crust with such fabric characteristics cannot be resolved reliably.

Complex fabric development in Paleoproterozoic meta-quartzites of the Västervik Basin, SE Sweden

Abstract The metamorphic siliciclastics of the Palaeoproterozoic Västervik Basin locally display a complex fabric development caused by Si-metasomatism and ductile deformation. This is in contrast to large areas where primary sedimentary fabrics are remarkably well preserved. Fabric and texture analyses indicate that significant volumes of almost pure quartzites originated from arkosic sandstones due to metasomatic replacement of K-feldspar and plagioclase by quartz. These rheologically “soft” quartzites were preferentially affected by subsequent ductile deformation indicated by dynamic recrystallization fabrics and strongly developed quartz textures. It is suggested that both Si-metasomatism and ductile deformation are related to structural aureoles around minor granitic intrusions.