At the start of the project, 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.
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. Since early 2021 the experimental apparatus is fully functional. In 2022 we completed a suite (around 100 tests) of high pressure, high temperature experiments aimed at analysing the brittle-plastic transition in calcite marble, notably looking into the role of grain size, microcracks, twinning and dislocations in the hardening behaviour of the material. Our dataset combines ultrasonic monitoring under lower-crustal deformation conditions, and showed the key role of grain size in the strength of rocks in the semi-brittle regime. We followed this study by a systematic work quantifying the evolution of key microstructural parameters during semi-brittle flow, which allowed us to determine the key state variables controlling semi-brittle strength.
In parallel with the experimental programme, we conducted theoretical work which lead to (1) a complete analysis of the role of microscale friction in the formation of microcracks in the semi-brittle regime, and (2) a first micromechanical model including feedbacks between brittle and plastic mechanisms, capable of reproducing key features of the stress-strain behaviour of rocks in the semi-brittle regime.
We also devoted a significant effort to developing new laboratory methods to measure in situ fluid pressures during rock deformation. This new technology allowed us to conduct work 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. In follow up work we demonstrated experimentally that (1) dilatancy has the potential to stabilise shear rupture and fault slip in rocks, which had been suspected in theory but never observed so far; (2) post-rupture fluid pressure changes lead to substantial fault slip, which had never been reported before.
Overall, the work from the project lead to 17 publications in internatial peer-reviewed journals, with 9 more manuscripts submitted or in advanced stages of preparation. We communicated results in major international conferences. Our work has lead to major technical progress in the field of experimental geophysics, with the development of ultrasonic monitoring techniques at elevated pressure and temperature, and in situ fluid pressure monitoring sensors for use in high pressure rock deformation apparatus. In the course of the project, we generated unique, systematic datasets documenting the behaviour of rocks across the brittle-plastic transition and the role of fluids on the dynamics of faulting, and developed new physical models to quantify and upscale our results.
