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Dynamics of rock deformation at the brittle-plastic transition and the depth of earthquake faulting

Periodic Reporting for period 1 - RockDEaF (Dynamics of rock deformation at the brittle-plastic transition and the depth of earthquake faulting)

Reporting period: 2019-01-01 to 2020-06-30

The lithosphere is the thin outer shell of the Earth that supports the weight of mountains, plate tectonic forces, and stores the elastic energy that is released during earthquakes. The strength of the lithosphere directly controls the formation of tectonic plates and the generation and propagation of devastating earthquakes.
The strongest part of the lithosphere is where the deformation processes in rocks transition from brittle fracture to plastic flow. Earthquakes seem to stop below this transition, but we do not know exactly how deep they can propagate or nucleate. The transitional regime also marks the locus of the recently discovered low frequency earthquakes and tremors, which are currently not well explained.
Despite its fundamental importance, the transitional behaviour remains very poorly understood. In this regime, we still do not know how rock deformation processes and properties evolve with depth and, critically, time. We also do not know exactly where the transition occurs in nature, if and how it may move over time, and what are the prevailing conditions there.
The aim of this project is to provide unprecedented quantitative constrains on the key material properties and processes associated with deformation and fluid flow at the brittle-plastic transition, and arrive at a clear understanding of the prevailing conditions and the dynamics of fault slip at the depths where unconventional seismic motion (tremors, slow slip) is recorded in the lithosphere.
The project is interdisciplinary, at the interface between geology, materials science, and seismology.
I propose to conduct laboratory rock deformation experiments at the high pressure and temperature conditions relevant to the transitional (or semi-brittle) regime, by developing state-of-the-art high pressure instrumentation combined with novel microstructural observation techniques. I will determine the de- formation mechanisms, the evolution of rock physical properties, the effects of fluids, and the dynamics of strain localisation and semi-brittle fault slip. The laboratory data will be interpreted by developing micromechanical models that will allow me to extrapolate the observations to natural conditions, and determine the geophysical signature of the brittle-plastic transition and its relationship to earthquake dynamics.
"Since the start of the project in January 2019, extensive experimental work has been undertaken to upgrade an existing high pressure, high temperature rock deformation apparatus at UCL. Such work included setting up an ultrasonic monitoring system, the design and manufacture of new internal pistons, a new internal high temperature furnace, and a new control system. Final temperature calibrations and internal wiring are required to start the experimental campaign at the full machine capabilities, but promising data have been obtained in preliminary tests conducted at low temperature on Carrara Marble, a rock chosen for its representative properties of crustal materials in the semi-brittle regime. In parallel with this long-term laboratory developments which have yet to produce systematic data, a number of results have been obtained in other experimental setups.

A first series of laboratory tests were conducted to investigate in unprecedented level of detail the transition from localised (faulting) to distributed (ductile) deformation in crustal rock (using again marble as a representative rock type). We established a simple criterion that determines the partitioning of deformation between localised fault slip and off-fault, bulk ductile flow. The data highlight the key role played by strain hardening in the partitioning of deformation. This work was published in Geology in 2019.

We also teamed up with a group at the University of Oxford (Pr. Lars Hansen) and University of Utrecht (Dr. David Wallis) to investigate the role of intragranular cracking in the deformation of antigorite, a key mineral constituent of subduction zones, which typically deforms in a semi-brittle manner. The laboratory data and microstructural observations revealed that intragranular slip, most likely due to shear cracking, is the dominant mode of deformation in antigorite. This deformation mechanism is not widely known and might be more prevasive than previously thought in the high pressure, low temperature regime typical of the brittle-plastic transition zone. Our observations and data were published in the Phil. Trans. of the Royal Society.

In parallel, a significant effort has been devoted to developing new laboratory methods to measure accurately in situ fluid pressures during rock deformation. This new technology lead to the publication of a study demonstrating how dilatancy (the increase in crack volume occurring prior to and during faulting) can lead to dramatic fluid pressure drop during rock failure, providing the first direct experimental evidence of the so-called ""seismic sucction pump"" concept established in the 1980s.

The experimental work has been complemented systematically by theoretical investigations; current work includes the development of a new model to make quantitative predictions of rock rheology driven by intra- or inter-granular slip, inspired by our results on antigorite, by most likely generalisable to many rock types under high pressure and low temperature. These modelling efforts are currently pursued further to include more explicit coupling between shear cracking, tensional cracking and dislocation glide, with the aim of producing simple criteria for the partitioning of deformation between different deformation mechanisms."
It is expected that the new technological developments to (1) measure ultrasonic velocities at high pressure and temperature, and (2) measure fluid pressure in situ during rock deformation will lead to major breakthrough in our understanding of the semi-brittle flow of rocks. We are now in the position to test systematically the role of fluid pressure in the semi-brittle flow of rocks, which is a key problem at the transition between brittle and plastic behaviour. With the development of ultrasonic monitoring, we will be able to map systematically the regimes dominated by microcracking vs. intracrystalline plasticity, thus placing strong experimental constrains on deformation mechanism maps of crustal rocks.
Revised fault zone model including the role of plastic strain hardening in the brittle regime