Using earthquake-like laboratory experiments to reveal ancient seismicity in the rock record.

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Griffiths, James Rhys (James)

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Currently, pseudotachylytes are the only unequivocal indicator of seismic slip in the rock record, but pseudotachylytes do not form in certain litholo- gies (e.g. carbonates) and are mainly restricted to depths of >5 km. Seis- mically active carbonate-bearing areas such as central Italy, Greece and the Himalayan belt would benefit from a better understanding of how ancient seismicity is preserved in the rock record, to better evaluate the future haz- ard. Using a unique dataset of synthetic fault rocks produced in earthquake-like laboratory experiments, this project aimed to quantify the microstructures produced in mixed calcite-dolomite gouges. Specifically, the project focuses on the microstructural evolution of samples deformed under identical seis- mic conditions (Max. slip vel. = 1 msô€€€1, n = 18 MPa) but taken to increasing displacements of 0.03 to 0.4 m (representative of average coseis- mic slip during approx. Mw 4 - 6 earthquakes). Quantitative analysis of SEM images, combined with chemical and opti- cal analysis, shows that grain size and gouge fabric evolve systematically with increasing displacement. The bulk of the microstructural changes oc- cur within the first c. 0.1 m of slip, during a transient strengthening and dynamic weakening phase during which grain comminution occurs by cat- aclasis. The weaker calcite phase wraps around relatively rigid dolomite clasts to define coarse foliations formed entirely by brittle processes. After c. 0.1 m of slip, strain has localised within the gouge layers and tempera- ture sensitive processes (e.g. thermal decomposition, grain-scale plasticity iii and recrystallization) become increasingly important near the localized slip surface. During this phase, cataclastic processes in the bulk gouge layer slow down or cease but the coarse foliations continue to rotate slowly to- wards parallelism with the localized slip surface. The results indicate that relatively small-displacement, seismic slip events in upper-crustal fault rocks could generate foliated fault rocks paired with a sharp principal slip zone on which carbonate thermal decomposition prod- ucts may be present. This may leave a lasting microstructural signature of seismic slip in the rock record.

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xiv, 149 pages A4

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2014Griffiths

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http://download.otagogeology.org.nz/temp/Abstracts/2014Griffiths.pdf

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Griffiths, James Rhys (James), “Using earthquake-like laboratory experiments to reveal ancient seismicity in the rock record.,” Otago Geology Theses, accessed December 4, 2021, http://theses.otagogeology.org.nz/items/show/578.

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