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The origin of pseudotachylytes in seismogenic fault zones and their role during slow interseismic creep


Project Description

The aim of this project is to gain insight into the processes that control the seismic cycle by investigating the relationship between pseudotachylytes and associated mylonites in present-day as well as ancient seismogenic fault zones.

Earthquakes are natural events that occur on a daily basis in the Earth’s crust. Large earthquakes represent a serious societal hazard because they can damage buildings, and result in landslides and tsunami. The challenge of constraining their occurrence and magnitude can be tackled by studying the mechanical, physical and chemical properties of rocks that form along seismogenic fault zones. Sibson (1975) was the first to provide convincing field evidence that melting by frictional heating can actually occur in nature during seismic slip and form pseudotachylytes. These rocks are regarded as the sole testimony of ancient earthquakes, and may provide key information on earthquake mechanics and the seismic cycle (e.g. Giulio Di Toro et al. 2005).

Despite their importance, the origin of pseudotachylytes has been the focus of a long-lasting debate (H. W. Green II et al. 2015) and their relationship with mylonitic rocks pseudotachylytes are often found associated with, is to date not well understood. It has been suggested that the fine grain sizes that often characterize pseudotachylytes, due to devetrification of unstable glass, could favour deformation by grain boundary sliding and therefore lead to significant weakening (Drury et al., 2011), and potentially to cyclic transitions from frictional sliding to plastic flow (Menegon et al., 2013). However, the processes that might facilitate such transitions remain unclear.
This research project will improve our understanding of the links between rupture in the seismogenic crust and slow slip in the deeper crust.

Specific objectives are:
1. To characterize geometry, microstructure and chemistry of pseudotachylytes in two major fault zones in the Ivrea-Verbano Zone, NW Italy and in the Alpine Fault Zone of New Zealand;
2. To study the relationships between pseudotachylytes and mylonites, cataclasites and gouges and investigate how a switch from brittle to creep deformation may occur;
3. To test experimentally the reactivation of deformation in pseudotachylyte-rich natural samples.
The goals of this project will be achieved through a combination of fieldwork, analytical techniques and laboratory experiments.
Detailed geological mapping in the Ivrea-Verbano Zone will sample pseudotachylytes associated with the mylonites of two major faults. The microstructure and chemistry of all samples will be analysed using electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) in the scanning electron microscope (SEM) as well as X-Ray diffraction (XRD). Experiments will be carried out using a Griggs apparatus, under mid to lower crustal P-T conditions. Samples will be prepared by coring pseudotachylyte and mylonite bearing rocks in order to test their mechanical behaviour upon reactivation of the deformation. All experimental samples will be analysed using EBSD, EDS and XRD.
Year 1: Field work in NW Italy: geological mapping and sampling; analysis of pseudotachylytes in Alpine Fault Zone core samples; conference attendance; Thesis plan;
Year 2: EBSD, EDS and XRD analyses of all samples; sample preparation for Griggs rig experiments; manuscript write-up; conference attendance; Thesis plan and write-up;
Year 3: Griggs rig experiments; conference attendance; thesis write-up; manuscript write-up.

To apply for this opportunity please visit: https://www.liverpool.ac.uk/study/postgraduate-research/how-to-apply/ and click the ‘Apply online’ button.

Funding Notes

Full funding (fees, stipend, research support budget) is provided by the University of Liverpool for 3.5 years for UK or EU citizens. Formal training is offered through partnership between the Universities of Liverpool and Manchester. Our training programme will provide all PhD students with an opportunity to collaborate with an academic or non-academic partner and participate in placements.

References

Drury, M.R., Avé Lallemant, H.G., Pennock, G.M. & Palasse, L.N. 2011. Crystal preferred orientation in peridotite ultramylonites deformed by grain size sensitive creep, étang de lers, pyrenees, france. Journal of Structural Geology, 33 (1-2), 1776-1789.
Giulio Di Toro, Stefan Nielsen & Pennacchioni., G. 2005. Earthquake rupture dynamics frozen in exhumed ancient faults. Nature, 436, 1009-1012, doi: 10.1038/nature03910.
H. W. Green II, F. Shi, K. Bozhilov, G. Xia & Reches., Z. 2015. Phase transformation and nanometric flow cause extreme weakening during fault slip. Nature Geoscience, 8, 484-490, doi: 10.1038/NGEO2436.
Kovarik, L., Stevens, A., Liyu, A., Browning, N.D. 2016. Implementing an accurate and rapid sparse sampling approach for low-dose atomic resolution STEM imaging. Applied Physics Letters, 109(16),164102.
Menegon, L., Stünitz, H., Nasipuri, P., Heilbronner, R. & Svahnberg, H. 2013. Transition from fracturing to viscous flow in granulite-facies perthitic feldspar (lofoten, norway). Journal of Structural Geology, 48, 95-112.
Sibson, R.H. 1975. Generation of pseudotachylyte by ancient seismic faulting. Geophysical Journal International, 43, 775–794.

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