Projects

An extended DEM approach with multi-scale aggregates and healing algorithms will be used to study structures on the grain and single-band scale, whereas the reservoir scale flow properties will be determined with continuum models. Codes will be developed and tested simultaneously and natural examples from our rock collection and field examples can be used in the other projects. We will then develop an algorithm for the healing of fractured grains and will finally approach the large scale and look at the influence of deformation bands on the permeability of aquifers. Here we will vary mechanical content in bands, deformation conditions from shear to compaction, compactional and extensional shear, and finite strain.

Understanding the interplay between brittle and ductile deformation mechanisms at the grain scale in mylonites is essential for understanding shear localization at depth in the continental crust. This interplay also has a strong influence on the length-scale and velocity of fluid transfer below the base of the seismogenic crust, and on the seismic cycle itself. The main goal of the project is to understand the origin of discrete zones of recrystallization (DZR) in quartz as potential indicators of microfracturing during the incipient stages of mylonitization. Such structures are developed in quartz veins from the Schober Group (Hohe Tauern mountains in the Central Eastern Alps), which were deformed at c. 450-500°C, and will be used as a key study. More generally, the project aims to improve the understanding on interaction of different deformation mechanisms (micro-fracturing, subgrain rotation and grain boundary migration, mechanical Dauphiné twinning, dissolution-precipitation, grain boundary sliding) during the initial formation of DZR structures and, how their significance changes with progressive development of the mylonitic and ultramylonitic microstructures. Without an integrated approach, using different up-to-date techniques of high-resolution microstructural and microchemical (trace element) analysis, interpretations of the quartz deformation microstructures detailed above are destined to remain speculative. The project includes integrated micro- and nano-analyses on fine-grained microstructures by means of: electron backscatter diffraction (EBSD), SEM orientation contrast imaging (channeling contrast), SEM cathodoluminescence (CL), transmission electron microscopy (TEM) and secondary ion mass spectrometry (SIMS and NanoSIMS) for Ti-in-Quartz analysis. Additional, new developments in high resolution analysis (down to atomic scale) by atom probe will be applied to obtain information about (sub)grain-scale diffusion processes (especially of Ti) during localized rock deformation. Furthermore this project will test the applicability of a newly developed Near Field Microscope with NanoFTIR capability to detect intragranular water in quartz at nano- to micrometer scales. This test will be accompanied by (OH- molecular ions) analysis in quartz using NanoSIMS technique. If these two independent methods prove successful, it will open up a new era of measuring water in fine-grained minerals (not only quartz) and, further, could specifically address the measurement of water along grain boundaries, subgrain boundaries and even dislocations structures. Combined with the Ti distribution analysed by the atom probe, this would help in recognizing processes such as dislocation pipe diffusion or diffusion along subgrain boundaries and their effects on the resetting of the Ti-in-Quartz system.

Im Rahmen von Untersuchungen zur Struktur des Untergrundes im Zielgebiet des GAB-Forschungsprojektes „Petrotherm“ wurden neben den schon bekannten NW-SE verlaufenden Störungen im Fränkischen Becken auch N-S verlaufende Störungen als bedeutende Lineamente beschrieben. Die Kinematik dieser Strukturen im Paläo- und im in situ Spannungsfeld und damit ihr Reaktivierungspotential sind bislang nicht ausreichend untersucht. In diesem Kurzprojekt (12 Monate) soll eine detaillierte Datenerhebung zum Störungs- und Kluftsystem in einem kleinräumigen Gebiet um die Bohrung Obernsees durchgeführt werden, um das strukturelle Inventar zu erfassen. Mittels der RACOS^®-Methode soll an einem Bohrkern aus der Bohrung Obernsees eine Bestimmung der herrschenden in situ-Spannungen durchgeführt werden.

Diese Methodenkombination wurde im Projekt „Petrotherm“ bislang nicht angewendet, wären aber für eine Bewertung des Reaktivierungspotentials von Störungsflächen im Fränkischen Becken und seines Untergrundes von Bedeutung. Mit diesem Kurzprojekt soll daher die Machbarkeit und Nützlichkeit für die Thematik „Ermittlung von Spannungszuständen im Zielgebiet und das Bruchverhaltens ausgewählter Gesteinstypen“ des Gesamtprojektes getestet und die Datenbasis zu dieser Thematik verdichtet werden.

Despite a wealth of data about seismic fault zones there is an ongoing discussion about the possibility of frictional melting of quartzitic rocks. In the present study we analysed fault vein bearing fault zones within quartzitic rocks within the Schneeberg Normal Fault Zone (SNFZ), Southern Tirol, Italy. Electron microscopy (scanning electron microscopy, SEM, including electron back scatter diffraction, EBSD, and cathodoluminescence, CL, analysis  in combination with transmission electron microscopy, TEM) analyses revealed that the fault veins (0.5-2 mm thick) are not ultracataclastic zones as presumed initially (see original title of the project WA 1010/11-1). Instead an extensive melting and subsequent quenching of quartz is evident. These quenched friction-induced melts along a fault during seismic slips are so-called tectonic pseudotachylytes and record paleo-earthquakes.  Pseudotachylytes are typically considered to be representative for the brittle upper crust and in association with cataclasites. However the Schneeberg NFZ quartzites show clear evidence of crystal plasticity and dynamic recrystallization resulting in ultrafine-grained (1-2 µm) aggregates along microshear zones (50-150 µm thick) in the host rock adjacent to pseudotachylyte veins. Ar-Ar dating of the Schneeberg NFZ pseudotachylyte reveal an age of 60-66 Ma and indicates that the coseismic event is younger than the greenschist facies metamorphism of the Schneeberg NFZ (76 Ma, exiting data from the literature). Thus pseudotachylyte formation should has occurred after exhumation of the Schneeberg NFZ into the brittle crust under far field ambient temperatures conditions <250-300 °C. The occurrence of such fine recrystallized quartz was also reported in other pseudotachylytes-bearing faults, but these microstructures have been overlooked in most works on pseudotachylytes (also considering that they are hardly visible with standard optical methods) and a detailed electron microscopy study including crystallographic preferred orientation analysis of the microstructure was missing. In this project we carried out a direct comparison between the deformation microfabrics of quartz in two different pseudotachylyte-bearing faults both showing the development of ultrafine-grained recrystallization aggregates: the Schneeberg NFZ quartzite and the Adamello Gole Larghe Fault Zone(GLFZ) tonalite (Southern Alps). The observations of this study suggest that the association of ultrafine recrystallization and frictional melting is a systematic feature of most pseudotachylyte-bearing faults and could yield a more complete information on the mechanics of coseismic slip. Based on thermal models we suggest that crystal plastic deformation of quartz accompanied by dramatic grain size refinement by dynamic recrystallization occurs during seismic faulting at the base of the brittle crust as a result of the high temperature transients (> 800°C) related to frictional heating in the host rock selvages of the slip surface. These localised high deformation temperatures made possible that the process of dynamic recrystallization, including recovery processes, could occur in a time lapse of a few tens of seconds.

In order to verify these modeled quartz deformation temperatures we applied the Ti-in-quartz geothermometer by measuring the Ti content in quartz by nanoSIMS. The geochemical analysis for both pseudotachylyte-bearing samples (Schneeberg NFZ and Adamello GFZL) showed that during the seismic-related development of ultrafine-grained dynamic recrystallized quartz aggregates the pre-seismic host Ti signal is inherited. Therefore no temperature related resetting of the Ti content occurs during seismically-induced quartz recrystallization. However the steep increase of Ti in quartz in the direct vicinity (1-2 µm) of melt-related submicron-sized Ti-bearing particles gives evidence of Ti diffusion and points to short-timed high temperature transient, which is consistent with the thermal modelling of pseudotachylyte vein and its host rock margin.