Axel Vollbrecht

Development of microcracks in granites during cooling and uplift: examples from the Variscan basement in NE Bavaria, Germany

Abstract Microcracks in two late-Variscan granites in NE Bavaria provide information about regional paleostress directions and P-T conditions during cooling and uplift. In both granites three prominent generations of healed microcracks (called CI, CII, CIII) constitute an orthorhombic fabric the orientation of which is closely related to the mesoscopic joint pattern. We present a model relating this orthorhombic crack fabric not only to external deviatoric stresses, but also to internal stresses on the grain scale resulting from the strong thermal contraction of quartz relative to the feldspar framework. At the deepest level (σ1 subvertical), steep CI cracks developed normal to σ3, and selectively in the quartz. Their formation caused a stress uncoupling of quartz aggregates from the external σ3 direction, so that further contraction of quartz produced steep CII cracks normal to σ2, forming bridges between CI cracks. At higher levels (σ2 subvertical), the same process reactivated the CI cracks and now formed horizontal crack bridges (CIII). P-T estimates based on fluid inclusion data and inferred geothermal gradients are 250–400°C at 1.5–3 kbar for CI–CII cracks and 150–200°C at 1–2 kbar for CIII cracks. The orientation of the younger open cracks which formed at higher crustal levels partly corresponds to the present regional stress field.

Structural model for the Saxothuringian-Moldanubian suture in the Variscan basement of the Oberpfalz (Northeastern Bavaria, F.R.G.) interpreted from geophysical data

Abstract The Moldanubian-Saxothuringian boundary ∗ marks a prominent subduction zone in the internal parts of the European Variscides. In the Oberpfalz area deep reflection seismic profiles show a high density of various reflectors mainly in the upper crust indicating a complex polyphase deformation during the collision process. Seismic structures and related data from surface geology suggest that the Moldanubian-Saxothuringian collision consisted of two main phases. The first phase is marked by extensive NW-directed overthrusting of Moldanubian onto Saxothuringian crust along SE-dipping master decollements, some of which can be traced down to the Moho. During an advanced stage progressive overthrusting led to the development of wedge structures associated with SE-directed backthrusting and backfolding in the upper structural levels. Subsequently some of these wedges and their related decollements were folded to form the main NE-striking anticlines. The first phase of crustal imbrication correlates with a regional low-pressure-high-temperature metamorphism (at about 320 Ma) decreasing in grade northwestward. The second phase is characterized by a reorientation of the main direction of crustal shortening from NW-SE to SW-NE which is interpreted as the result of the N-directed indentation of the Vindelician terrane into the Moldanubian. This event is recorded by conjugate systems of overthrusting and subsequent strike-slip faulting in the Oberpfalz and the Bavarian Forest and the Black Forest-Vosges area. In the area of the southern German block between these basement outcrops the indentation front is marked by the arc-shaped axes of gravimetric and magnetic anomalies and Permo-Carboniferous troughs.

The effect of oriented microcracks on seismic velocities in an ultramylonite

The influence of oriented microcracks on the directional dependence of P- and S-wave velocities has been investigated in an epidote-muscovite-ultramylonite. Optical and SEM studies revealed that the fabric of the mylonite is characterized by oriented intragranular and grain-boundary cracks. Comparison of orientation patterns of cracks with calculated elastic properties (Voigt average) of a model rock (isotropic matrix, ultramylonite crack geometry) and laboratory seismic data indicates that wave propagation is particularly affected by oriented grain-boundary cracks. Microcracks are most effective at low pressure and largely closed above about 200-300 MPa. The residual part of velocity anisotropy observed above this crack-closing pressure is mainly due to preferred mineral orientation, in particular of mica. Maximum concentration of poles to (001) mica planes corresponds to the maximum of poles to oriented grain-boundary cracks so that the effects of texture and oriented cracks on wave velocities line up, giving rise to extreme values of velocity anisotropy and shear-wave splitting at low-pressure conditions.

The Crustal Structure at the KTB Drill Site, Oberpfalz

The Variscan basement at the Continental Deep Drilling Site (KTB) Oberpfalz on the western margin of the Bohemian Massif is composed of three polyphase-deformed structural units: the Saxothuringian, the Moldanubian and MP to HP metamorphic nappe complexes of the Zone of Erbendorf-Vohenstraus (ZEV) and the Munchberg Massif (MM). The boundary between the Saxothuringian und Moldanubian, which is interpreted as a cryptic suture, is represented by the northwestern rim of the HT-mylonite belt of the Zone of Tirschreuth-Mahring (ZTM). This forms part of a formerly active continental margin, whereas the Saxothuringian terrane represents a formerly passive continental margin. Deformation of the Moldanubian active continental margin already began during the oceanic subduction stage and was therefore a longer-lasting, more penetrative deformational event than in the Saxothuringian, where the whole Variscan deformation is related only to the collisional stage.

Crystallographic orientation of microcracks in quartz and inferred deformation processes: a study on gneisses from the German Continental Deep Drilling Project (KTB)

Abstract This study was carried out on four gneiss samples from the German Continental Deep Drilling Project (KTB) taken at depths between 556 and 8633 m. The crystallographic orientation of microcracks in quartz was determined by a combination of the electron channelling pattern method (ECP) and U-stage microscopy. The distinct preferred crystallographic orientations of various crack generations point to different processes of crack initiation and propagation which partly seem to be depth-dependent. For older healed cracks, a crystalplastic initiation due to dislocation pile-up and related lattice distortion is indicated by cracking normal to prominent slip directions, which are 〈 a 〉 at lower or [ c ] at comparatively higher temperatures. In contrast, younger open cracks preferentially formed on crystallographic planes with low surface energy (rhombs and prisms), which is interpreted in terms of pure elastic crack mechanisms. Because of the thermoelastic anisotropy of quartz, internal stresses resulting from thermal contraction during cooling may be the main driving force for the initiation or further propagation of cracks parallel to the c -axis. Crack propagation preferentially affects grains with crystal lattices and operating crack mechanisms being in a direction appropriate to the applied external (tectonic) stresses. This selective cracking explains why cracks also show constant orientations with respect to geographic directions up to regional dimensions.

Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

Abstract The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust and was the subject of several review-type articles. The Ivrea–Verbano Zone was rotated into an upright position along the Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper continental crust crossing the Permian Cossato–Mergozzo–Brissagio Line (CMB Line). The CMB mylonites are locally overprinted by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are based on new 40Ar/39Ar hornblende ages, K–Ar biotite ages and zircon fission-track data along the NE–SW trending Valstrona section. A re-interpretation of existing U–Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are observed in proximity to the Pogallo Line (c. 292 Ma). Heat input by mafic intrusions was sufficient to reset the U–Pb monazite system, as is evidenced by the youngest ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are the result of complete resetting by a Permian thermal event. The 40Ar/39Ar hornblende ages and K–Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease from the Pogallo Line (hornblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (hornblende: 201 Ma vs. biotite: 156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting of the Ivrea–Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea–Verbano Zone during the Permian.

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.

Anisotropy of compressional wave velocities, complex electrical resistivity and magnetic susceptibility of mylonites from the deeper crust and their relation to the rock fabric

Abstract Laboratory measurements of compressional wave velocities ( V P ) complex electrical resistivity and magnetic susceptibility have been carried out on a mylonitic clinopyroxene amphibolite (MCA) and an epidote-muscovite-quartz ultramylonite (MQU) from the Ivrea Zone. Both mylonites exhibit significant anisotropies that are related to the rock fabric. At low pressure of about 50 MPa the anisotropy of V P for the MCA and MQU is 17 and 34%, respectively. It is due to oriented microcracks (intragranular, transgranular, grain boundary cracks and cleavage cracks), in addition to preferred orientation of anisotropic rock-forming minerals. At high pressure (600 MPa) where most of the cracks are closed a residual anisotropy of about 10 and 12%, respectively, is observed, that is mainly controlled by preferred orientations of mineral constituents. The anisotropies of the complex electrical resistivity measured with an electrolyte (tap water) at 0.1 MPa are 40 and 36% for the MCA and MQU, whereas at 80 MPa these values increase to 44% for both samples. In contrast, measured with an electrolyte of 0.1 M NaCl solution the resistivity is lower and gives anisotropies of 71% (MCA) and 63% (MQU) at 0.1 MPa. The anisotropies of the magnetic susceptibility (AMS) are about 27% (MCA) and 14% (MQU). For both mylonites the axes of the AMS ellipsoid coincide with the macroscopic fabric elements, where the main axis of magnetic susceptibility ( K max ) lies approximately parallel to the lineation. An intercorrelation of the different physical properties in terms of rock composition and anisotropic rock fabric is difficult. Nevertheless, the laboratory determination of anisotropic in-situ physical properties provide more objective data for the modelling of in-situ crustal conditions.

Complete seismic properties obtained from microcrack fabrics and textures in an amphibolite from the Ivrea zone, Western Alps, Italy

Abstract Laboratory seismic measurements of compressional wave velocities (Vp) and shear-wave splitting (ΔVs) have been carried out on a clinopyroxene amphibolite from the Ivrea Zone (Western Alps, Italy) at pressures up to 600 MPa. From the texture of the major minerals (hornblende, plagioclase and pyroxene) and the respective elastic constants, the elastic properties (Voigt average) and seismic anisotropies were calculated. At high confining pressure where the effect of microcracks is eliminated, Vp and shear-wave velocities (Vs, Vss) are in good agreement with the calculated velocities reflecting texture-induced anisotropies. Microcracks are most effective at low confining pressure, but are largely closed at about 200–300 MPa. Microscopic studies revealed that the crack fabric is characterized by oriented transgranular and cleavage cracks. The bulk anisotropy at low pressure conditions was computed by superposing the texture-induced anisotropy with the crack-induced anisotropy. These calculated values are consistent with the measured values, indicating that anisotropy of Vp and shear-wave splitting is affected by both texture and oriented microcracks. Based on detailed fabric analyses the presented method allows to describe the complete elastic and seismic properties of crystalline rocks, providing an ideal data basis for three-dimensional seismic modelling to investigate the influence of anisotropic media on the reflectivity.

Natural stone, weathering phenomena, conservation strategies and case studies: introduction

The weathering of historical buildings, as well as that of any monument or sculpture using natural stone (or man-made porous inorganic materials) is a problem identified since antiquity. Although much of the observed world-wide destruction of these monuments can be ascribed to war and vandalism, many other factors can contribute significantly to their deterioration. These threaten the preservation of the current inventory of historically, artistically or culturally valuable buildings and monuments. Furthermore, a drastic increase in deterioration has been observed on these structures during the past century. This prompted Winkler (1973) to make a pessimistic prediction, that at the end of the last millennium these structures would largely be destroyed because of predominantly anthropogenic environmental influences. Fortunately, this has proven not to be the case. There is a general belief that natural building stones are durable, and not for nothing does the Bible refer to the Rock of Ages. However, all rocks will weather and eventually turn to dust. If rocks are cut and used in buildings, the chance of deterioration increases because other factors come into play. To understand the complex interaction that the stone in a building suffers with its near environment, (i.e., the building, and the macro environment, the local climate and atmospheric conditions), requires an interdisciplinary approach with the work of geologists, mineralogists, material scientists, physicists, chemists, biologists, architects and art historians. Although most historical buildings use natural stone as the main construction material, other materials, such as mortars for masonry or rendering and ceramic roof tiles