10:00 AM - 10:15 AM
[SCG46-05] Deformation of simulated quartz-feldspar faults at the brittle-ductile transition zone
Keywords:quartz, feldspar, gouge, Griggs shear experiment, microstructure
Continental earthquakes frequently nucleate in the brittle-ductile transition zone. Recently, experimental studies revealed that nano- or microscale deformation processes, such as amorphization (Peč et al., 2016) or intergranular dilatation (Verberne et al., 2017), may lead to nucleation of unstable slip hence runaway fault rupture. Since the continental upper crust is dominantly composed of granitoids, studying the rheological behaviors of fault rocks composed of quartz and feldspar is key for better understanding upper-crustal fault mechanics. Aiming to improve insight into the microphysical processes controlling fault slip in the continental crust, we conducted Griggs shear experiments on ~1 mm thick simulated faults composed of 50:50 wt% quartz-albite gouges. Temperatures and pressures in each experiment were varied to simulate conditions at 7, 8, 10, 13, and 18 km depths in the crust, assuming geothermal and lithostatic gradients of respectively 30℃/km and 2700 kg/m3. The bulk shear strain rate was sequentially stepped between ~10-3 and ~10-4 /s, achieving total bulk shear strains of up to 3.5 to 5.0. Recovered sheared samples were investigated using an optical and a scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) analyses in an SEM.
In general, when frictional regime dominates, shear resistance increases with increasing normal stress, whereas when flow regime dominates, shear resistance decreases with increasing temperature. Unlike this tendency, shear stresses obtained from the experiments showed positive dependences on both pressure and temperature. Since the shear resistances increased even with the increase in temperature, the mechanical data may suggest frictional regime was dominant in all experiments.
However, microstructural observations of recovered samples indicate a change in deformation style from brittle to partially plastic as the simulated depth condition increases. Specifically, backscattered electron micrographs showed that microcrack distribution changes as pressure and temperature increases. At the shallower conditions, large numbers of short cracks are observed in various directions, while at the deeper conditions, the cracks are less, and are seen to coalesce to constitute R1- and Y- shears. Using image analysis, we characterized the deformation properties of individual grains. At the shallower conditions, the long axes of K-feldspar marker grains, present in minor amounts, are distributed randomly, whereas at the deeper conditions these are aligned along the direction of maximum tensile strain. Our interpretation is that brittle crushing and rotation are dominant under shallow conditions, with a progressively more important role for plastic flow takes towards deeper conditions. This transition is consistent with a change in autocorrelation function (ACF) of K-feldspar grains with increasing temperature and pressure.
Our results indicate that even within the range of frictional behavior, plastic deformation gradually emerges on the scale of microstructure as increasing temperature and pressure.
References
- Peč, M., H. Stünitz, R. Heilbronner, and M. Drury (2016), Semi-brittle flow of granitoid fault rocks in experiments, J. Geophys. Res. Solid Earth, 121, 1677-1705, doi:10.1002/2015JB012513.
- Verberne, B. A., Chen, J., Niemeijer, A. R., de Bresser, J. H., Pennock, G. M., Drury, M. R., and Spiers, C. J. (2017), Microscale cavitation as a mechanism for nucleating earthquakes at the base of the seismogenic zone, Nat. Commun., 8, 1-8, doi: 10.1038/s41467-017-01843-3.
In general, when frictional regime dominates, shear resistance increases with increasing normal stress, whereas when flow regime dominates, shear resistance decreases with increasing temperature. Unlike this tendency, shear stresses obtained from the experiments showed positive dependences on both pressure and temperature. Since the shear resistances increased even with the increase in temperature, the mechanical data may suggest frictional regime was dominant in all experiments.
However, microstructural observations of recovered samples indicate a change in deformation style from brittle to partially plastic as the simulated depth condition increases. Specifically, backscattered electron micrographs showed that microcrack distribution changes as pressure and temperature increases. At the shallower conditions, large numbers of short cracks are observed in various directions, while at the deeper conditions, the cracks are less, and are seen to coalesce to constitute R1- and Y- shears. Using image analysis, we characterized the deformation properties of individual grains. At the shallower conditions, the long axes of K-feldspar marker grains, present in minor amounts, are distributed randomly, whereas at the deeper conditions these are aligned along the direction of maximum tensile strain. Our interpretation is that brittle crushing and rotation are dominant under shallow conditions, with a progressively more important role for plastic flow takes towards deeper conditions. This transition is consistent with a change in autocorrelation function (ACF) of K-feldspar grains with increasing temperature and pressure.
Our results indicate that even within the range of frictional behavior, plastic deformation gradually emerges on the scale of microstructure as increasing temperature and pressure.
References
- Peč, M., H. Stünitz, R. Heilbronner, and M. Drury (2016), Semi-brittle flow of granitoid fault rocks in experiments, J. Geophys. Res. Solid Earth, 121, 1677-1705, doi:10.1002/2015JB012513.
- Verberne, B. A., Chen, J., Niemeijer, A. R., de Bresser, J. H., Pennock, G. M., Drury, M. R., and Spiers, C. J. (2017), Microscale cavitation as a mechanism for nucleating earthquakes at the base of the seismogenic zone, Nat. Commun., 8, 1-8, doi: 10.1038/s41467-017-01843-3.