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  • 1
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    Twinned calcite as an indicator of high differential stresses and low shock pressure conditions during impact cratering (2023)
    Details
    Publication Date: 2023-11-02
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Shock‐related calcite twins are characterized in calcite‐bearing metagranite cataclasites within crystalline megablocks of the Ries impact structure, Germany, as well as in cores from the FBN1973 research drilling. The calcite likely originates from pre‐impact veins within the Variscan metagranites and gneisses, while the cataclasis is due to the Miocene impact. Quartz in the metagranite components does not contain planar deformation features, indicating low shock pressures (〈7 GPa). Calcite, however, shows a high density (>1/μm) of twins with widths 〈100 nm. Different types of twins (〈italic toggle="no"〉e〈/italic〉‐, 〈italic toggle="no"〉f‐〈/italic〉, and 〈italic toggle="no"〉r〈/italic〉‐twins) crosscutting each other can occur in one grain. Interaction of 〈italic toggle="no"〉r〈/italic〉‐ and 〈italic toggle="no"〉f〈/italic〉‐twins results in 〈italic toggle="no"〉a〈/italic〉‐type domains characterized by a misorientation relative to the host with a misorientation angle of 35°–40° and a misorientation axis parallel to an 〈italic toggle="no"〉a〈/italic〉‐axis. Such 〈italic toggle="no"〉a〈/italic〉‐type domains have not been recorded from deformed rocks in nature before. The high twin density and activation of different twin systems in one grain require high differential stresses (on the order of 1 GPa). Twinning of calcite at high differential stresses is consistent with deformation during impact cratering at relatively low shock pressure conditions. The twinned calcite microstructure can serve as a valuable low shock barometer.〈/p〉
    Description: Bavarian Natural History Collections
    Keywords: ddc:549 ; Ries impact structure ; twinned calcites ; cataclasis
    Language: English
    Type: doc-type:article
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  • 2
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    Rheology Dependent on the Distance to the Propagating Thrust Tip—(Ultra‐)Mylonites and Pseudotachylytes of the Silvretta Basal Thrust (2023)
    Details
    Publication Date: 2024-04-25
    Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉To evaluate how the presence of pseudotachylytes affects the strength of crustal rocks, deformed pseudotachylytes and their relationship with pristine pseudotachylytes at the base of the Silvretta nappe are analyzed. Pseudotachylytes formed associated with high‐stress crystal plasticity (〈italic〉σ〈/italic〉〈sub〉〈italic〉d〈/italic〉〈/sub〉 > 400 MPa), as indicated by twinned amphiboles in gneisses. Mylonitic quartz clasts enclosed within deformed pseudotachylytes and mylonitic vein‐quartz, hosting folded pseudotachylyte injection veins, reflect creep at lower stresses (ca. 100 MPa) after seismic rupturing. Deformed pseudotachylytes can be crosscut by pristine pseudotachylytes, indicating a second, independent stage of coseismic rupturing after creep. The evidence of dynamic dislocation creep of quartz and the presence of stilpnomelane and epidote associated with all fault rocks indicate similar ambient greenschist facies conditions during all deformation stages. Whereas the intermediate stage of creep is interpreted to represent deformation at large distance to the propagating thrust tip, the pristine pseudotachylytes represent the last stage of rupturing eventually leading to nappe decoupling from its basement. Gneiss clasts in an ultramylonitic matrix (i.e., deformed pseudotachylyte) reveal that pseudotachylytes have a lower strength during creep in relation to the hosting gneisses. In contrast, during coseismic high‐stress crystal plasticity, the coarse gneisses accumulate a higher amount of strain. This strength‐relationship explains that only those rocks rupture, which have not been previously deformed before. The study demonstrates the importance of different strengths of crustal rocks at specific stress‐ and strain‐rate conditions in dependence on the distance to the propagating fault tip.〈/p〉
    Description: Key Points: 〈list list-type="bullet"〉 〈list-item〉 〈p xml:lang="en"〉Ultramylonites (deformed pseudotachylytes) and mylonites represent creep at large distance to the propagating thrust tip〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Pristine pseudotachylytes represent final deformation at the tip of the propagating thrust fault associated with nappe decoupling〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Pseudotachylytes are weak during aseismic creep and strong during coseismic high‐stress plasticity〈/p〉〈/list-item〉 〈/list〉 〈/p〉
    Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Description: https://doi.org/10.17632/xhh2ktks9g.4
    Description: https://nano.oxinst.com/products/aztec/
    Description: https://www.horiba.com/aut/scientific/products/detail/action/show/Product/labspec-6-spectroscopy-suite-software-1843/
    Keywords: ddc:551.8 ; (deformed) pseudotachylytes ; (ultra‐)mylonites ; creep ; multiple high‐stress events ; seismic cycle ; nappe decoupling
    Language: English
    Type: doc-type:article
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  • 3
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    Dissolution precipitation creep versus crystalline plasticity in high-pressure metamorphic serpentinites (2011)
    In:  Geological Society Special Publication 360: 129-149.
    Details
    Publication Date: 2011-11-17
    Description: Serpentinite is widely assumed to constitute weak material in subduction zones and to play an essential role for the development of a subduction channel. Information on deformation mechanisms and appropriate rheological models to describe these large-scale flow processes can only be obtained from natural serpentinites exhumed from ancient subduction zones. We examine the microstructural record of HP-metamorphic (P c. 2±0.5 GPa, T c. 550±50 °C) serpentinites exposed in the Zermatt–Saas zone, Western Alps, using optical and scanning electron microscopy with electron backscatter diffraction (EBSD). The schistose and compositionally layered rocks show pervasive small-scale folding. There is no evidence for any significant deformation by dislocation creep. Instead, the microfabrics including strain shadows and crenulation cleavage indicate that high strain is accumulated by dissolution precipitation creep. In terms of rheology, this suggests Newtonian behaviour and a low viscosity for the long-term flow of serpentinites in deeper levels of subduction zones. This does not preclude dislocation creep and a power law rheology at higher stress levels, as realized at local sites of stress concentration and transient episodes of post-seismic creep.
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  • 4
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    Episodic deformation and reactions in mylonitic high-grade metamorphic granulites from Dronning Maud Land, Antarctica (2020)
    Elsevier
    Details
    Publication Date: 2020-12-01
    Print ISSN: 0191-8141
    Electronic ISSN: 1873-1201
    Topics: Geosciences
    Published by Elsevier
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  • 5
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    Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps (2019)
    Copernicus
    Details
    Publication Date: 2019-01-17
    Description: Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (
    Print ISSN: 1869-9510
    Electronic ISSN: 1869-9529
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 6
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    Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps (2018)
    Copernicus
    Details
    Publication Date: 2018-08-21
    Description: Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate the rheologically dominant feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures parallel to the main shortening direction indicated by the foliation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. Polyphase aggregates at sites of dilation indicate dissolution-precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a sawtooth-shaped interface. This replacement is interpreted to be by interface-coupled dissolution-precipitation driven by a solubility difference between K-feldspar and albite and is not controlled by strain. In contrast, albite porphyroclasts are replaced at sites of shortening by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent, kinked and twinned albite. No indication of effective dislocation climb with dynamic recovery, for example by the presence of subgrains, a crystallographic preferred orientation or sutured grain boundaries was observed. We interpret the grain size reduction of albite at sites of shortening to be the result of coupled fracturing, dislocation glide and strain-induced grain boundary migration. This strain-induced replacement by nucleation and growth leads, together with granular flow, to the monophase albite layers. The associated quartz layers in contrast, show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures that correlate with strain. Samples with lower strain are characterized by layers of a few hundreds of µm width, with coarse-grained quartz and layers with isometric, fine-grained feldspar. Higher strained samples are characterized by narrow alternating layers of some tens of µm width composed of fine-grained quartz and coarse albite grains elongated parallel to the stretching lineation, respectively. These observations indicate that grain size reduction by strain-induced replacement of albite, granular flow assisted by fracturing and dissolution-precipitation together with dislocation creep of quartz are rheologically dominant.
    Electronic ISSN: 1869-9537
    Topics: Geosciences
    Published by Copernicus on behalf of European Geosciences Union.
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  • 7
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    Low-temperature plasticity of olivine during high stress deformation of peridotite at lithospheric conditions — An experimental study (2011)
    Elsevier
    Details
    Publication Date: 2011-11-01
    Print ISSN: 0012-821X
    Electronic ISSN: 1385-013X
    Topics: Geosciences , Physics
    Published by Elsevier
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  • 8
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    Kick-and-cook experiments on peridotite: simulating coseismic deformation and post-seismic creep (2011)
    Wiley
    Details
    Publication Date: 2011-12-01
    Print ISSN: 0954-4879
    Electronic ISSN: 1365-3121
    Topics: Geosciences
    Published by Wiley
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  • 9
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    The deformation record of olivine in mylonitic peridotites from the Finero Complex, Ivrea Zone: Separate deformation cycles during exhumation (2015)
    American Geophysical Union
    Details
    Publication Date: 2015-12-01
    Print ISSN: 0278-7407
    Electronic ISSN: 1944-9194
    Topics: Geosciences
    Published by American Geophysical Union
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  • 10
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    Crystal–plastic deformation and recrystallization of peridotite controlled by the seismic cycle (2012)
    Elsevier
    Details
    Publication Date: 2012-03-01
    Print ISSN: 0040-1951
    Electronic ISSN: 1879-3266
    Topics: Geosciences , Physics
    Published by Elsevier
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