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Deformation of Fine-Grained Granitoid Fault Rocks Microstructures, Deformation Processes & Rheology

English title Deformation of Fine-Grained Granitoid Fault Rocks Microstructures, Deformation Processes & Rheology
Applicant Herwegh-Züger Marco
Number 192124
Funding scheme Project funding (Div. I-III)
Research institution Institut für Geologie Universität Bern
Institution of higher education University of Berne - BE
Main discipline Geology
Start/End 01.10.2020 - 30.09.2024
Approved amount 546'929.00
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All Disciplines (3)

Discipline
Geology
Geophysics
Geochemistry

Keywords (8)

rheology; fluid; frictional-viscous-transition; deformation-mechanisms; seismicity; ultramylonites; polymineralic; mineral reactions

Lay Summary (German)

Lead
Infolge Plattentektonik bauen sich entlang Plattenrändern, aber auch im Innern der Platten, Gesteinsspannungen auf, welche zu lokalisierter Deformation in Störungszonen führen. Die Art der Deformation variiert je nach Tiefe (Temperatur, Druck) aber auch dem Vorhandensein von "Wasser und anderen Fluide". Wenn einmal ausgebildet, werden solche Störungszonen immer wieder aktiviert. Die Verformug führt im allgemeinen zu einer Korngrössenreduktion in den Gesteinen. Unsere Hypothese ist, dass die Festigkeit dieser ultrafeinkörnigen Gesteine sehr klein sein muss. Wie klein, ist im Moment unbekannt und soll im Rahmen dieses Projektes untersucht werden.
Lay summary
Inhalt und Ziel

Unser Ziel ist es, mittels Experimenten Kennwerte über die Festigkeiten und das Deformationsverhalten der polymineralischen, ultrafeinkörnigen Gesteinen zu erhalten. Im Rahmen eines internationalen Forschungskonsortiums (Universitäten von Bern, Orleans und Utrecht, ETHZ) wird die Problematik einerseits durch Tieftemperatur/Niedrigdruck andererseits durch Hochtemperatur/Hochdruck Deformationsexperimenten angegangen. Mit diesem Ansatz wird das Deformationsverhalten dieser Gesteine in der seichten Kruste (spröde) und der mittleren Kruste (fliessfähig) simuliert, als auch der Übergangsbereich dazwischen. Dies ist insofern von grosser Wichtigkeit, als genau in diesem Übergangsbereich das Ablösen von schnellen seismischen zu langsamen aseismischen Prozessen stattfindet. Die erzielten experimentellen Mikrostrukturen sollen mit solchen aus der Natur verglichen und auf Ihre Anwendbarkeit hin überprüft werden. Die neuen Erkenntnisse werden zum Projektende in numerische Modellierungen einfliessen, um die oben gestellte Hypothese in geodynamische Modelle integrieren zu können.



Wissenschaftlicher Kontext

Unsere Erkenntnis wird wichtige neue Einblicke in das Deformationsverhalten der Krustengesteine und der Verformungslokalisierung generieren. Dies ist wichtig, für ein besseres Verständnis der Erdbebentätigkeit aber auch der Fliesswege von Wasser in die Tiefe und wieder zurück an die Oberfläche. Letzteres ist vor allem in Hinblick auf die Nutzung von Geothermie von grossem gesellschaftlichem Interesse.

Direct link to Lay Summary Last update: 26.06.2020

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169055 Structure and evolution of an antiformal nappe stack (Aar massif, Central Alps): Formation of mechanical anisotropies and their bearing on natural risks 01.10.2016 Project funding (Div. I-III)

Abstract

In the Earth’s middle to upper crust (consisting often of granitoid rocks), deformation strongly localizes in fault/shear zones, in which the deformation processes and structures strongly depend on the physico-chemical properties of the crust forming rocks (e.g. rheology, permeability, reactivity). Since their properties change with composition, pressure, temperature, stress, strain rate and fluid presence/absence; deformation mechanisms and microstructures change with depth resulting in frictional and viscous creep processes, for the upper and middle crust, respectively, separated by the frictional viscous transition. Rheological strength profiles have been constructed for the major granitoid-crust-forming minerals, namely quartz and feldspar. Instead of localizing deformation in layers of these two rock forming minerals, however, recent studies reveal the formation of ultrafine-grained polymineralic mixtures of quartz, feldspar and phyllosilicates over a depth range of >20 kilometres. Field evidence suggests much lower rock strengths of these fault rocks compared to that of the aforementioned end member minerals. Moreover, their deformation is highly fluid-sensitive (pore fluid pressures, dissolution-precipitation and mass transfer processes) resulting in switches between fast seismic and slow aseismic deformation. We therefore hypothesize that fault zones in the granitoid continental crust are mechanically much weaker than anticipated; yet, the associated rheology of these so important fault rocks is unknown.The goal of the suggested research project is to close the existing knowledge gap by performing a combination of studies on experimentally and naturally deformed polymineralic, ultrafine-grained fault rocks. This includes the determination of deformation mechanisms by high-resolution quantitative microstructural analyses as well as the generation of new constitutive equations for the deformation under (i) dominantly viscous, (ii) dominantly frictional and (iii) combined mode granular flow processes. Special attention will be paid to the effect of fluids in terms of mass transfer processes but also on mechanics. Four work packages (WP) are defined, which will be covered by two PhD projects: WP-A (PhD-1) and WP-B (PHD-2), respectively, dealing with experimental rock deformation in the viscous (Griggs rig deformation apparatus, with Prof. H. Stunitz, University of Tromsø) and frictional (hydrothermal ring shear apparatus using non-cohesive starting materials, with Profs. A. Niemeijer and C. Spiers, Utrecht University) deformation regimes. Both studies meet at the frictional-viscous transition, which will be investigated in a collaborative manner. In WP-C naturally deformed ultramylonites (PhD-1) and gouges/cataclasites with granitoid composition will be analysed in terms of microstructures but also fault geometries. WP-D applies the findings of WP-A-C into a numerical modelling environment. WP-C/D allow the validation of experimental results in light of applicability to nature as well as to investigate up-scaling relations for predicting rheological changes in crustal-scale fault strength profiles of these fault rocks. Both experimental and natural samples will undergo rigorous quantitative microstructural analyses applying conventional as well as cutting edge high-resolution techniques. The suggested research approach generates new quantitative information on: (1) the rheology of polyphase, ultrafine-grained granitoid fault rocks; (2) the evolution of dynamic pore structures and the effect of fluids (pore pressure, mass transfer/healing processes) as well as (3) continuous versus discontinuous deformation and related cyclical changes (interseismic-seismic). All these are controlling parameters for understanding seismicity, fluid flow and related mineral reactivity in granitoid fault/shear zones. Fundamental knowledge on these parameters is mandatory to improve input parameters required for geodynamic numerical modelling, reactive transport modelling but also for understanding shear zone hosted ore deposits. New insights at the low temperature end of the study will also have high relevance for an improved understanding on fault-hosted hydrothermal systems, which are currently targeted for the sustainable use of geothermal energy in basement units.
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