complex in Central Alps (Malenco region) comprising slivers deformed at ca. 30 km depth, in the frame of the Alpine subduction interface. This area is potentially the missing element linking deformation patterns in depths of the Arosa-Engadine units (10-20 km) in Central Alps and Dent Blanche-Sesia massifs (40-60 km) in Western Alps. This study aims to answer to the following questions:
• What is the characteristic deformation along the subduction interface during subduction of continental slivers and what is its timing? Can deformation fabrics and their timing elucidate active deformation conditions of the transition zone, at the downdip end of the seismogenic zone?
The final major set of questions that this thesis tackles is related to the deformation of the interface, in the case of a subduction mélange. In particular:
• What is the effective rheology of a subduction interface, in the special case that it is not a homogeneous medium, but rather characterized by a block-in-matrix geometry? How does the concentration of blocks affect the bulk deformation of the mélange? Can the effective rheology of a mixed material reflect small-scale complexities, rendering it thus useful for implementation in large-scale geodynamic models?
All the above questions are tackled individually in the main chapters of the thesis (Chapters 3, 4 and 5, respectively).
1.7 Scope of the thesis
The aim of the thesis is to review and investigate how deformation is taken up by the rock record along a subduction zone interface and what its effective rheology is. Two case studies are considered, the first concerning deformation of exhumed continental slivers, the second a generic mélange unit; these questions are tackled with two different approaches. The first in-cludes field work, as well as petrological observations and geochronological analyses of exhumed continental slivers in the Central Alps. The results of this study are presented in Chapter 4:
"Deformation along the roof of a fossil subduction interface in the transition zone
below seismogenic coupling: The Austroalpine case and new insights from the Malenco Massif (Central Alps)". It is published in the Journal Geosphere as:
Ioannidi, P.I., Angiboust, S., Oncken, O., Agard, P., Glodny, J., and Sudo, M., 2020, Defor-mation along the roof of a fossil subduction interface in the transition zone below seismogenic coupling: The Austroalpine case and new insights from the Malenco Massif (Central Alps):
Geosphere, v. 16, no. X, p. 1-23, https://doi.org/10.1130/GES02149.1.
Samuel Angiboust and Onno Oncken designed the project. Paraskevi Io Ioannidi made sub-stantial contribution to the field work, data analysis,40Ar/39Ar dating and their interpretation and prepared the draft of the manuscript. Samuel Angiboust performed a major part of the thermodynamic modelling and contributed to the draft and the field work. Onno Oncken and Philippe Agard took part in the field work, discussion and interpretation of the data. Johannes Glodny and Masafumi Sudo performed geochronology and contributed to the discussion and interpretation of age data.
The second approach includes the numerical study of a subduction mélange, where strong basaltic blocks are embedded within a quartzitic matrix. The study focuses mainly on the effect of block concentrations on the effective rheology of the mélange, assuming the rheol-ogy of each material is characterized by brittle failure and dislocation creep. From literature review as well as the aforementioned in the Central Alps, field observations point to dissolution-precipitation creep as the dominant deformation mechanism for a quartz- and phyllosilicate-rich matrix (Bachmann et al., 2009b; Fagereng and Cooper, 2010; Fagereng, 2011; Grigull et al., 2012; Wassmann and Stöckhert, 2013b); however, the detailed processes of this deformation mechanism are not well constrained yet (see also Wallace et al., 2012). Moreover, most numer-ical simulations use dislocation creep to model large scale subduction zone processes. Using dislocation creep in our models makes our models comparable to large scale modelling studies.
Moreover, dislocation creep is usually active at higher stresses; therefore, by using this mecha-nism instead of a dissolution-precipitation or diffusion creep law, and our results can be seen as placing an upper limit for the stresses (Stöckhert, 2002; Wallace et al., 2012), or a lower limit for the strain rates. This work is presented in Chapter 5: "Effective rheology of a two-phase subduction shear zone: insights from numerical simple shear experiments and implications for subduction zone interfaces" and is currently submitted at EPSL
1.7. Scope of the thesis 25
as:
Ioannidi, P.I., Le Pourhiet, L., Agard, P., Angiboust, A., Oncken, O., Effective rheology of a two-phase subduction shear zone: insights from numerical simple shear experiments and implications for subduction zone interfaces.
Laetitia Le Pourhiet and Paraskevi Io Ioannidi designed the project. Paraskevi Io Ioannidi designed and processed the models, calculated the effective rheology and wrote the draft of the manuscript. Laetitia Le Pourhiet supervised the numerics of the study. Philippe Agard, Onno Oncken and Samuel Angiboust provided the field examples modelled and insight into deforma-tion processes of natural rocks. All co-authors contributed to the discussion and interpretadeforma-tion of the results.
Finally, I summarize the most commonly observed deformation mechanisms along exhumed sub-duction interfaces and their interpretation in the frame of actively recorded deformation, such as episodic tremor and slip (ETS) and slow slip events (SSEs). I additionally discuss the most common numerical implementations for modelling the rheology of a subducting slab and their limitations. The results are presented in Chapter 3: "Viscous deformation mechanisms along the subduction interface".
Chapter 2
Methods
Different methods were used in this study and are shortly presented here. Estimation of P-T conditions, thermodynamic modelling, as well as isotopic analyses of Rb/Sr and Ar/Ar comprise the methods employed for the field data. Additionally, the Finite Element Method (FEM) was applied in order to investigate the rheological properties of a subduction mélange. Some of the presently mentioned methods are also presented in Chapters 4 and 5, since these form parts of one accepted and one submitted publication.
2.1 Field work
Three field campaigns were undertaken in the area of Val Malenco, North Italy, during the summer months of 2015 and 2016. The aim of these campaigns was to identify deformation patterns at various scales and collect samples. These samples, obtained exclusively from the continental slivers and sediments described in the area, were used to perform petrological, geochemical and geochronological analyses. In total, more than twenty samples were collected and twelve were analyzed, in order to characterize the textures and the type of micro-scale deformation and also to constrain the P-T-t evolution of the study area.
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