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Rheology dependent on the distance to a propagating fault tip. the microstructural record of fault rocks at hypocentral depth
Rheology dependent on the distance to a propagating fault tip. the microstructural record of fault rocks at hypocentral depth
The microfabric of fault rocks from the base of the seismogenic zone, i.e., for the continental crust at depths of greenschist facies conditions, is crucial for the understanding of the seismic cycle. It provides information about the rheology during episodic deformation, controlling the strength of crustal rocks, and is thus relevant for large-scale geological processes in tectonic active regions. In this thesis, fault rocks of the Silvretta basal thrust in the central Alps and the Defereggen-Antholz-Vals (DAV) strike-slip shear zone in the Eastern Alps were analyzed to unravel the deformation, as well as stress and strain-rate histories within fundamentally different geological settings. Various fault rocks, containing pseudotachylytes and (ultra-)mylonites, were selected for the model case of the Silvretta basal thrust and compared to mylonitic pegmatites and cataclasites of a strike-slip tectonic setting (DAV), to evaluate the question of whether there is more than one transient high-stress deformation event recorded in one rock and the relation to the long-term shear zone activity. Fault rocks from both localities were deformed at greenschist facies conditions, i.e., they originate from the base of the seismogenic zone. The focus is on the microstructural record displaying a specific sequence of deformation mechanisms, to infer different stress and strain-rate conditions at hypocentral depth, not directly accessible for in situ measurements. The findings are compared to gneisses of the Vredefort impact structure in South Africa, which were shocked to relatively low shock conditions at depths comparable to those at the base of the seismogenic zone. These deformation conditions are compared and contrasted to those at hypocentral depth. The rock record of the different geological settings gives new insights into the deformation processes of rupturing events at hypocentral depth. The microstructures preserved within pseudotachylyte-related (fault) rocks indicate high stresses (>400 MPa), that prevail only transiently at the base of the seismogenic zone, and are diagnostic for coseismic deformation. These coseismically high stresses are associated with stress redistribution, controlled by the distance to the tip of the propagating fault instead of depending on local heterogeneities, as evident by systematic variations of deformation mechanisms at the same greenschist facies conditions. The specific stress and strain-rate conditions are decisive regarding the rheological rock behavior during seismic faulting, as opposed to a change in pressure and temperature. Deformation conditions during major earthquakes, revealed by these commonly modified microstructures, are comparable to those realized during impact cratering at relatively deep parts of the impact structures characterized by relatively low shock conditions and allow evaluation of the material behavior at non-steady state conditions. Pseudotachylytes, representing the last imprint after the passage of the rupture, are therefore not the only microstructural marker of coseismic deformation and ancient earthquakes. The findings from this thesis highlight the importance of the different strengths of crustal rocks and the time-dependent rheology at specific stress/strain-rate conditions during the seismic cycle at the base of the seismogenic zone.
Rheology, Pseudotachylyte, Fault rocks, Silvretta basal thrust
Brückner, Lisa Marie
2023
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Brückner, Lisa Marie (2023): Rheology dependent on the distance to a propagating fault tip: the microstructural record of fault rocks at hypocentral depth. Dissertation, LMU München: Faculty of Geosciences
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Abstract

The microfabric of fault rocks from the base of the seismogenic zone, i.e., for the continental crust at depths of greenschist facies conditions, is crucial for the understanding of the seismic cycle. It provides information about the rheology during episodic deformation, controlling the strength of crustal rocks, and is thus relevant for large-scale geological processes in tectonic active regions. In this thesis, fault rocks of the Silvretta basal thrust in the central Alps and the Defereggen-Antholz-Vals (DAV) strike-slip shear zone in the Eastern Alps were analyzed to unravel the deformation, as well as stress and strain-rate histories within fundamentally different geological settings. Various fault rocks, containing pseudotachylytes and (ultra-)mylonites, were selected for the model case of the Silvretta basal thrust and compared to mylonitic pegmatites and cataclasites of a strike-slip tectonic setting (DAV), to evaluate the question of whether there is more than one transient high-stress deformation event recorded in one rock and the relation to the long-term shear zone activity. Fault rocks from both localities were deformed at greenschist facies conditions, i.e., they originate from the base of the seismogenic zone. The focus is on the microstructural record displaying a specific sequence of deformation mechanisms, to infer different stress and strain-rate conditions at hypocentral depth, not directly accessible for in situ measurements. The findings are compared to gneisses of the Vredefort impact structure in South Africa, which were shocked to relatively low shock conditions at depths comparable to those at the base of the seismogenic zone. These deformation conditions are compared and contrasted to those at hypocentral depth. The rock record of the different geological settings gives new insights into the deformation processes of rupturing events at hypocentral depth. The microstructures preserved within pseudotachylyte-related (fault) rocks indicate high stresses (>400 MPa), that prevail only transiently at the base of the seismogenic zone, and are diagnostic for coseismic deformation. These coseismically high stresses are associated with stress redistribution, controlled by the distance to the tip of the propagating fault instead of depending on local heterogeneities, as evident by systematic variations of deformation mechanisms at the same greenschist facies conditions. The specific stress and strain-rate conditions are decisive regarding the rheological rock behavior during seismic faulting, as opposed to a change in pressure and temperature. Deformation conditions during major earthquakes, revealed by these commonly modified microstructures, are comparable to those realized during impact cratering at relatively deep parts of the impact structures characterized by relatively low shock conditions and allow evaluation of the material behavior at non-steady state conditions. Pseudotachylytes, representing the last imprint after the passage of the rupture, are therefore not the only microstructural marker of coseismic deformation and ancient earthquakes. The findings from this thesis highlight the importance of the different strengths of crustal rocks and the time-dependent rheology at specific stress/strain-rate conditions during the seismic cycle at the base of the seismogenic zone.