Differences in grain growth of calcite: a field-based modeling approach

Normal grain growth of calcite was investigated by combining grain size analysis of calcite across the contact aureole of the Adamello pluton, and grain growth modeling based on a thermal model of the surroundings of the pluton. In an unbiased model system, i.e., location dependent variations in temperature-time path, 2/3 and 1/3 of grain growth occurs during pro- and retrograde metamorphism at all locations, respectively. In contrast to this idealized situation, in the field example three groups can be distinguished, which are characterized by variations in their grain size versus temperature relationships: Group I occurs at low temperatures and the grain size remains constant because nano-scale second phase particles of organic origin inhibit grain growth in the calcite aggregates under these conditions. In the presence of an aqueous fluid, these second phases decay at a temperature of about 350°C enabling the onset of grain growth in calcite. In the following growth period, fluid-enhanced group II and slower group III growth occurs. For group II a continuous and intense grain size increase with T is typical while the grain growth decreases with T for group III. None of the observed trends correlate with experimentally based grain growth kinetics, probably due to differences between nature and experiment which have not yet been investigated (e.g., porosity, second phases). Therefore, grain growth modeling was used to iteratively improve the correlation between measured and modeled grain sizes by optimizing activation energy (Q), pre-exponential factor (k0) and grain size exponent (n). For n=2, Q of 350kJ/mol, k0 of 1.7×1021μmns−1 and Q of 35kJ/mol, k0 of 2.5×10-5μmns−1 were obtained for group II and III, respectively. With respect to future work, field-data based grain growth modeling might be a promising tool for investigating the influences of secondary effects like porosity and second phases on grain growth in nature, and to unravel differences between nature and experimen

The effect of different second-phase particle regimes on grain growth in two-phase aggregates: insights from in situ rock analogue experiments

The aim of the study was to investigate the effect of rigid second phases on grain growth of a matrix phase. For this purpose, variable mixtures of norcamphor as the matrix phase, with glass beads (0.08-0.51 volume fraction) as second phase, were used to perform see-through rock-analogue experiments under static conditions at constant temperatures (50°C). Irrespective of the second-phase content, grain-size evolution of all mixtures can be subdivided into a stage of continuous grain growth, a transient stage and a stage of a finally stabilized grain size. On the grain-scale, the second phases affect the migrating grain boundaries either by pinning by single particles, by multiple particles or even by particle clusters. Summed up over the entire aggregate, these pinning regimes affect the average bulk grain size of the matrix grains, such that the changes in matrix grain size directly correlate with the amount of second phases, their dispersion and their degree of clustering. In this way, the matrix grain size decreases with increasing second-phase content, which can be expressed as a Zener relationship. Originating from the modification of an ordinary grain growth law, a new mathematical expression is defined, which allows the calculation of changes in the matrix grain size as a function of different second-phase volume fractions and particle sizes. Such models will be helpful in the future to predict microstructural changes in polymineralic rocks at dept

Neotectonics in the Swiss Alps—A late Alpine to postglacially active fault at the Gemmi Pass

The area of the central and western Swiss Alps reveals not only the highest uplift rates of Switzerland (1.5mma−1 near Brig, Schlatter & Marti 2002), but also shows a strong concentration of earthquakes (e.g. Deichmann et al. 2004). This raised the question, whether the region hosts any linear topographic expressions that can be attributed to motion along potentially seismogenic faults. The area was therefore chosen for the investigation of postglacially active lineaments. Firstly, aerial photographs from the entire area were searched for linear features, which could be of gravitational or tectonic origin. Subsequently, selected lineaments were visited in the field to study their origin. We found scarce but positive evidence for neotectonic fault movements. One particular lineament that exhibited the most promising exposures was investigated in greater detail. This lineament is a prominent NW–SE striking fault located at the Gemmi Pass, runs perpendicularly to the regional fold axes and cuts through the Helvetic nappe stack. The position and orientation of the fault discounts gravitational reactivation. A close examination of the fault rocks reveals a long term evolution of this fault starting already at a late stage of Alpine nappe emplacement and related deformation.conferenc

225 Jahre Naturforschende Gesellschaft in Bern

Ein Überdauern von 225 Jahren ist eine lange Zeit, in der viel Wasser die Aare hinuntergeflossen ist – «Wasser bewegt», so das Motto unseres Jubiläumsjahres. In dieser Zeit war und ist die NGB einem kontinuierlichen Wandel der wissenschaftlichen Tätigkeiten, aber auch des gesellschaftlichen Umfelds unterworfen. Der folgende Artikel soll einen kurzen Überblick über die Entstehungsgeschichte und die Entwicklung der NGB in diesen 225 Jahren liefern. Als wichtigste Grundlagen dienten beim Recherchieren die bereits bestehenden Zusammenfassungen zum 100-, 150- und 200-jährigen Bestehen der NGB (siehe Referenzliste). Auch die in den «Mitteilungen» publizierten Jahresberichte der Präsidenten stellten wichtige Zeitzeugnisse dar, welche exemplarisch konsultiert und da und dort mit eingearbeitet wurden. Daneben finden sich in unserem Archiv Hunderte von Sitzungsprotokollen, welche ein Durchforsten und historisches Aufbereiten verdient hätten, was aber aus Zeitgründen vom Autor nicht zu bewerkstelligen war. Es liegt also ein spannendes historisches Forschungsgebiet brach, welches einer zukünftigen Bearbeitung harrt. Nebst dem Abhandeln des geschichtlichen Ablaufes war der Autor auch bestrebt, den historisch bedingten Ursprung verschiedenster Aktivitäten der NGB zu beleuchten und den Aktualitätsbezug herzustellen

Influence of rheologically weak layers on fault architecture: insights from analogue models in the context of the Northern Alpine Foreland Basin

We present a series of analogue models inspired by the geology of the Zürcher Weinland region in the NorthernvAlpine Foreland Basin of Switzerland to explore the influence of rheological weak, i.e. (partially) ductile layers on the 3D evolution of tectonic deformation. Our model series test the impact of varying weak layer thickness and rheology, as well as different kinematics of an underlying “basal fault”. Model analysis focuses on deformation in the weak layer overburden and, uniquely, within the weak layer itself. We find that for low to moderate basal fault displacements, the above-mentioned parameters strongly influence the degree of coupling between the basal fault and the weak layer overburden. Coupling between the basal fault and overburden decreases by reducing the strength of the weak layer, or by increasing the weak layer’s thickness. As a result, basal fault displacement is less readily transferred through the weak layer, leading to a different structural style in the overburden. By contrast, increasing the amount, or rate, of basal fault slip enhances coupling and leads to a more similar structural style between basal fault and overburden. Moreover, dip-slip displacement on the basal fault is more readily transferred to the overburden than strike-slip displacement of the same magnitude. Our model results compare fairly well to natural examples in the Northern Alpine Foreland Basin, explaining various structural features. These comparisons suggest that rheological weak layers such as the Jurassic Opalinus Clay have exerted a stronger control on fault zone architecture than is commonly inferred, potentially resulting in vertical fault segmentation and variations in structural style. Furthermore, the novel addition of internal marker intervals to the weak layer in our models reveals how complex viscous flow within these layers can accommodate basal fault slip. Our model results demonstrate the complex links between fault kinematics, mechanics and 3D geometries, and can be used for interpreting structures in the Alpine Foreland, as well as in other settings with similar weak layers and basal faults driving deformation in the system

Tracing wedge-internal deformation by means of strontium isotope systematics of vein carbonates

Radiogenic strontium isotopes (87Sr/86Sr) of vein carbonates play a central role in the tectonometamorphic study of fold-and-thrust belts and accretionary wedges and have been used to document fluid sources and fluxes, for example, along major fault zones. In addition, the 87Sr/86Sr ratios of vein carbonates can trace the diagenetic to metamorphic evolution of pore fluids in accreted sediments. Here we present 87Sr/86Sr ratios of vein carbonates from the Infrahelvetic flysch units of the central European Alps (Glarus Alps, Switzerland), which were accreted to the North Alpine fold-and-thrust belt during the early stages of continental collision. We show that the vein carbonates trace the Sr isotopic evolution of pore fluids from an initial seawater-like signature towards the Sr isotopic composition of the host rock with increasing metamorphic grade. This relationship reflects the progressive equilibration of the pore fluid with the host rock and allows us to constrain the diagenetic to low-grade metamorphic conditions of deformation events, including bedding-parallel shearing, imbricate thrusting, folding, cleavage development, tectonic mélange formation and extension. The strontium isotope systematics of vein carbonates provides new insights into the prograde to early retrograde tectonic evolution of the Alpine fold-and-thrust belt and helps to understand the relative timing of deformation events. © The Author(s), 2022. Published by Cambridge University Press