Siegfried Siegesmund

Anisotropy of VP and VS in an amphibolite of the deeper crust and its relationship to the mineralogical, microstructural and textural characteristics of the rock

Abstract Laboratory seismic measurements of compressional ( V p )- and shear-wave ( V s ) velocities have been carried out on a deformed amphibolite from the Ivrea Zone at pressures and temperatures up to 600 MPa and 600°C, respectively. Amphibolite is considered to be an important rock of the deeper crust. From crystallographic orientations of the component minerals (hornblende, plagioclase and diopside) and the respective elastic constants, we calculated the average elastic constants (Voigt average) for the rock specimen. Compressional- and shear-wave velocities in three orthogonal directions (normal and parallel to foliation) measured at high confining pressure (600 MPa) are in good agreement with the calculated velocity surface. This suggests that seismic-wave velocity anisotropy is controlled by preferred orientations of minerals in rocks (texture) at high confining pressure, where the effect of microcracks on seismic wave velocities is eliminated. In the amphibolite sample, the velocity anisotropy is dominated by the texture of hornblende because of the strong preferred orientation and large volume fraction of this mineral species. A large S-wave splitting is documented in the directions parallel to the foliation (particularly in the lineation) which strongly argues against the microcrack origin of S-wave splitting, at least at conditions of high effective pressure.

Geochronological constraints on the evolution of the southern Dom Feliciano Belt (Uruguay)

Abstract: New U–Pb sensitive high-resolution ion microprobe, K–Ar and Ar–Ar data from the southernmost Dom Feliciano Belt allow the identification of four major events. Orthogneisses from the Punta del Este Terrane indicate a magmatic episode at c. 770 Ma and high-grade metamorphism at 641 ± 17 Ma. Granitoid emplacement at 627 ± 23 Ma was roughly coeval with peak metamorphism. Volcaniclastic rocks of the Las Ventanas Formation dated at 573 ± 11 Ma can be correlated with the peripheral foreland basin (571 ± 8 Ma). Transpression and coeval high-K calc-alkaline magmatism is recorded in the Maldonado granite dated at 564 ± 7 Ma. The following events are postulated: (1) magmatism at 850–750 Ma related to rifting; (2) metamorphism and granite emplacement at 650–600 Ma; (3) molasse sequences and foreland basins at c. 573 Ma; (4) late magmatism at 580–560 Ma associated with transpression. The data indicate that (1) the Punta del Este Terrane could be a portion of the Coastal Terrane of the Kaoko Belt, (2) granitoid emplacement at 650–600 Ma in the Punta del Este and Nico Pérez terranes favours westward subduction, and (3) widespread post-collisional synkinematic magmatism occurred in the Dom Feliciano and Kaoko belts between 580 and 550 Ma. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18369.

N2-BET specific surface area of bentonites

The specific surface areas (SSA(N2BET)) of 36 different bentonites had larger values for Ca(2+)/Mg(2+) bentonites than for Na(+) bentonites. This trend could not be explained by the different d(001) values nor by the different microstructures. The investigation of Cu-triene-exchanged smectites, which on drying at 105 degrees C still had a d(001) value accounting for approximately 13A, proved that the SSA(N2BET) of low-charged smectites increased more than that of high-charged smectites. This could be explained by: (i) more space between the permanent charge sites in the case of low-charged smectites and (ii) the fact that the layers of Cu-triene smectites do not collapse at 105 degrees C. In contrast the SSA(N2BET) of Ca(2+)-exchanged bentonites could not be related to the layer charge density (LCD) as in the case of the Cu-triene-exchanged bentonites which is probably due to the varying number of collapsed layers. In conclusion, the SSA(N2BET) of bentonites which is known to be largely variable is probably determined by microporosity resulting from the quasi-crystalline overlap region and accessible areas of the interlayer. The number of layers per stack and the microstructure are supposed to play a subordinate role. The larger SSA(N2BET) of Ca/Mg bentonites compared to Na bentonites probably can be explained by the larger space between the charges in the case of the presence of divalent cations.

Neoproterozoic to Early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina)

The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c. 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c. 496 Ma El Peñón pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208Pb/232Th monazite age of 478 Ma for a migmatite from the Nogolí Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c. 500 Ma and c. 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

Crustal Provenance and Cooling of the Basement Complexes of the Sierra de San Luis: An Insight Into the Tectonic History of the Pro to-Andean Margin of Gondwana

Abstract The Sierra de San Luis constitutes the southernmost tip of the Eastern Sierras Pampeanas. Its Palaeozoic metamorphic basement units define a key location for the understanding of the accretional history along the proto-Andean margin of Gondwana. Although, it is largely accepted that the polyphase accretional history of the Sierras Pampeanas is preluded by the docking of the Pampean Terrane followed by the Famatinian Orogenic Cycle that involves subduction along the margin of Gondwana and the accretion of the Precordillera (Cuyania) Terrane and finally ceased with the collision of the Chilenia terrane, a vast amount of controversial information concerning the timing and mode of collisions as well as the origin of the different involved crustal fragments within the Eastern Sierras Pampeanas is published. In this paper, those different hypothesis are presented and evaluated under the light of new isotopic data of the Sierra de San Luis. Nd-systematics of the metasedimentary sequences of the Sierra de San Luis indicate that the studied sequences were developed on the Pampean Terrane. An Amazonian origin of the Pampean Terrane that was probably detached from the Arequipa Antofalla Craton is proposed. Furthermore, the correlation of two low-grade phyllitic belts (San Luis Formation) with the widespread Puncoviscana Formation is not supported by Sm-Nd data. It is suggested that the sedimentary precursors of the Pringles Metamorphic Complex and the topping phyllites were sourced on the Pampean Orogen and accommodated in a newly formed back arc basin during the early Famatinian. The cooling history of the basement complex is recorded by an extensive amount of K-Ar muscovite and biotite ages. A high variability in muscovite ages is only partly related to different intrusion times of two pegmatoid generations. Post Famatinian to Achalian crustal scale mylonite formation (-359 Ma) and a rotational exhumation of the central basement unit are causal for the observed K-Ar muscovite age pattern in the range from 395 Ma to 447 Ma. Therefore, the decrease in metamorphic degree from west to east is the result of the erosion level of a crustal profile from the mid lower crust to the upper crust. An even higher variability in K-Ar biotite cooling ages covering the range from 315 Ma to 418 Ma is related to the slow cooling after the Famatinian Orogenic Cycle or reheating during the Achalian Orogenic Cycle and consequent variable reset of the isotopic system. However, ages recorded by biotite booklets substantiate the hypothesis of a differential exhumation of the basement of the Sierra de San Luis.

Tectonic aspects of the Alpine-Dinaride-Carpathian system

The Alps, Carpathians and Dinarides form a complex, highly curved and strongly coupled orogenic system. Motions of the European and Adriatic plates gave birth to a number of ‘oceans’ and microplates that led to several distinct stages of collision. Although the Alps serve as a classical example of collisional orogens, it becomes clearer that substantial questions on their evolution can only be answered in the Carpathians and Dinarides. Our understanding of the geodynamic evolution of the Alpine-Dinaride-Carpathian System has substantially improved and will continue to develop; this is thanks to collaboration between eastern and western Europe, but also due to the application of new methods and the launch of research initiatives. The largely field-based contributions investigate the following subjects: pre-Alpine heritage and Alpine reactivation; Mesozoic palaeogeography and Alpine subduction and collision processes; extrusion tectonics from the Eastern Alps to the Carpathians and the Pannonian Basin; orogen-parallel and orogen-perpendicular extension; record of orogeny in foreland basins; tectonometamorphic evolution; and relations between the Alps, Apennines and Corsica.

Physical and Mechanical Properties of Rocks

Since early antiquity dimension stones have been used as building materials due to their natural beauty and availability, and the diversity of their applications has increased ever since. As any other building material, dimension stones today have to fulfill the physical and technical requirements demanded by architects. This chapter focuses on the physical and mechanical properties of dimension stones, while emphasizing that stones are an old, but still modern building material. Among the parameters discussed here are water absorption, thermal conductivity and expansion, hygric and hydric properties, strength, abrasion, the more modern aspect of breaking load at the dowel hole, and ultrasonic wave velocities. Extensive data sets and a variety of case studies reveal relationships between the physical properties and the internal fabric elements of the dimension stones, such as sedimentary layering, metamorphic foliation, pores, and microcracks. In addition, these fabric elements are often responsible for the weathering behavior of the dimension stones, which not only affects the heritage but also the safety of modern buildings. This is illustrated through laboratory experiments and case studies.

The emplacement of the Rieserferner Pluton (Eastern Alps, Tyrol): constraints from field observations, magnetic fabrics and microstructures

Abstract Within the alpine chain several Tertiary intrusions (e.g. Bergell, Adamello) intruded along the Periadriatic Lineament. This study attempts to show how the Rieserferner Pluton (Vedrette di Ries) is related to the Defereggen-Antholz-Vals (DAV)-Line. For a survey of the fabric pattern over the pluton the anisotropy of the magnetic susceptibility (AMS) was determined on nearly 170 samples. Low bulk susceptibilities of K vol –6 SI and a total degree of anisotropy P′

Complete texture analysis of a deformed amphibolite: comparison between neutron diffraction and U-stage data

Abstract This paper describes a new technique of analysing complex neutron diffraction patterns, allowing the quantitative texture analysis of each rock-forming mineral up to trielinic lattice symmetry in polymineralic rocks. The method is demonstrated using hornblende and plagioclase fabrics in Alzenau amphibolite from the Spessart Mountains (Mid-German Crystalline Rise). Hornblende and plagioclase exhibit a pronounced shape preferred orientation with respect to the macroscopic fabric elements. For control purposes, crystallographic preferred orientations were also determined independently by individual grain measurements using a U-stage. From the neutron texture analyses we obtain only superposed pole figures (hornblende and hornblende, hornblende and plagioclase, and plagioclase with plagioclase) of low-indexed lattice planes. Therefore, the critical crystallographic directions were recalculated by the orientation distribution function (ODF) which is given by texture components. The optical and neutron-derived pole figures are in a good agreement. All pole figures display strong preferred orientation, although the plagioclase texture is less pronounced. The hornblende texture consists of a [001] maximum parallel to the stretching lineation, with [010] showing a tendency to develop a girdle around the lineation. [100]-axes are distributed with a clearly split maximum representing the monoclinic angle between [100] and [001]. The plagioclase texture is a rare one with (001) subparallel within the foliation and [100] oriented subparallel to the lineation. [010] of plagioclase is concentrated normal to lineation within the foliation. The technique outlined here is potentially the most useful one for texture analysis of polyphase rocks, independent of crystal symmetry.

Thermal degradation of marble: indications from finite-element modelling

Microstructure-based finite-element simulations were used to study the thermo-mechanical behaviour of calcite marbles. Given the same microstructure, a strongly textured marble had smaller thermal stresses, smaller elastic strain energy and a distinct directional dependence of thermal degradation compared to a weakly textured marble. This result of modelling coincides with observations from thermal expansion experiments. Different boundary conditions assumed in the model calculations have an important influence on the magnitude and pattern of thermally induced microcracks. Microstructure-based finite element simulations are considered an excellent tool for elucidating influences of the rocks fabric on the thermal degradation of marbles.