Combining synchrotron and acoustic emission techniques to reveal the secrets of high PT faulting

Numerous studies have illustrated that mineral transformations have the capability to induce faulting at elevated pressure and temperature (PT), circumstances in which ductile flow would typically dominate. This mechanism, commonly known as transformational faulting, emerges as a plausible explanation for the puzzling phenomenon of deep-focus earthquakes occurring at depths up to 700 km. Currently, the debate partly revolves around determining why certain phase transformations lead to faulting while others do not. To better understand this phenomenon, we can compare different transformations taking place in similar experimental conditions and see how they do or do not cause strain localization and faulting. In this regard, we conducted a series of five deformation experiments in the large volume press at the PB61 beamline at DESY synchrotron. Two of these experiments involved deforming germanium-olivine samples as they transformed into ringwoodite (the high-pressure phase). The other three experiments were carried out on quartzite (novaculite) samples while they were transforming to coesite. Throughout the experiments, we collected X-ray diffraction patterns and images concurrently with the collection of Acoustic Emissions (AEs).

The results indicate, in both quartz and olivine experiments, the growth of the high-pressure phase at various rates depending on PT conditions and equilibrium overstep. Specifically, we observed rapid olivine-ringwoodite kinetics at elevated PT, far from equilibrium, while slower kinetics were noted for the quartz-coesite transformation. Thousands of AEs were collected in each experiment, and their locations reconstructed using arrival times on the six transducers used. Interestingly, the spatial distribution of these AEs revealed that for some quartz-coesite experiments, AEs originated from fault planes that formed within the initially intact rock cores. Furthermore, an analysis of the AE catalogues, focusing on the magnitude-frequency distribution, revealed a wide range of b-values influenced by varying PT conditions and transformation kinetics. This observation underscores the different underlying mechanisms since the obtained b-values are high when transformation and strain are distributed and lower when strain is localized (i.e., when a fault plane develops).

Our study supports the major role of mineral transformations in inducing faulting under high PT. These findings will help better quantify the intricate relationships between mineral transformations and faulting and in turn contribute to a better understanding of the fundamental geological processes behind deep and intermediate earthquakes.