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Tectonics and Structural Geology

Experiments

Minds over Methods: Experimental seismotectonics

Minds over Methods: Experimental seismotectonics

For our next Minds over Methods, we go back into the laboratory, this time for modelling seismotectonics! Michael Rudolf, PhD student at GFZ in Potsdam (Germany), tells us about the different types of analogue models they perform, and how these models contribute to a better understanding of earthquakes along plate boundaries.

 

Credit: Michael Rudolf

Experimental seismotectonics – Seismic cycles and tectonic evolution of plate boundary faults

Michael Rudolf, PhD student at Helmholtz Centre Potsdam – German Research Centre for Geosciences GFZ

The recurrence time of large earthquakes that happen along lithospheric-scale fault zones such as the San Andreas Fault or Chile subduction megathrusts, is very long (≫100 yrs.) compared to human timescales. The scarcity of such events over the instrumental record of around 60 years is unfortunate for a statistically sound analysis of the earthquake time series.

So far, only few megathrust events have been monitored in detail with near-field seismic and geodetic networks. To circumvent this lack of observational data, we at Helmholtz Tectonic Laboratory use analogue modelling to understand plate boundary faulting on multiple time-scales and the implications for seismic hazard. We use models of strike-slip zones and subduction zones, to investigate several aspects of the seismic cycle. Additionally, numerical simulations accompany and complement each experimental setup using experimental parameters.

 

Seismotectonic scale models
In my project, we develop experiments that can model multiple seismic cycles in strike-slip conditions. Our study employs two types of experimental setups both are using the same materials. The first is simpler (ring shear setup) and is able to show the on-fault rupture propagation. The second is geometrically more similar to the natural system, but only the surface deformation is observable.

To model rupture propagation, we introduce deformable sliders in a ring shear apparatus. Two cylindrical shells of ballistic gelatine (Ø20 cm), representing the side walls, rotate against each other, with a thin layer (5 mm) of glass beads (Ø355-400µm) in between representing an annular fault zone. A see-through lid connected to force sensors holds the upper shell in place, whereas the machine rotates the lower shell. Through the transparent lid and upper shell, we directly observe the fault slip. We can vary the normal stress on the fault (<20 kPa) and the loading velocity (0.0005 – 0.5 mm/s).

The next stage of analogue model, features depth-dependent normal stress and a rheological layering mimicking the strike-slip setting in the uppermost 25-30 km of the lithosphere (see also Mehmet Köküm’s blog post). A gelatine block (30x30cm) compressed in uniaxial setting represents the elastic upper crust under far-field forcing. Embedded in the block is a thin fault filled with quartz glass beads. The ductile lower crust is modelled using viscoelastic silicone oil. The model floats in a tank of dense sugar solution, to guarantee free-slip, stress-free boundaries.

 

Figure 2 – Setup and monitoring technique during an experiment. Several cameras record the displacement field and the ring shear tester records the experimental results. Credit: Michael Rudolf

 

Analogue earthquakes
Both setups generate regular stick-slip cycles including minor creep. Long phases of quiescence, where no slip or very slow creep occurs, alternate with fast slip events sometimes preceded by slow slip events. The moment magnitude of analogue earthquake events is Mw -7 to -5. The cyclic recurrence of slip events is an analogue for the natural seismic cycle of a single-fault system.

 

Figure 3 – Detailed setup and results from the ring shear tester experiments. The upper right image shows a snapshot of an analogue earthquake rupture along the fault zone. The plot shows the recorded shear forces and slip velocities over one hour of experiment. Credit: Michael Rudolf

 

Optical cameras record the slip on the fault and the deformation of the sidewalls. Using digital image correlation techniques, we are able to visualize accurately deformations on the micrometre scale at high spatial and temporal resolution. Accordingly, we can verify that analogue earthquakes behave kinematically very similar to natural earthquakes. They generally nucleate where shear stress is highest, and then propagate radially until the seismogenic width is saturated. In the end, the rupture continues laterally along the fault strike. Our experiments give insight into the role of viscoelastic relaxation, interseismic creep, and slip transients on the recurrence of earthquakes, as well as the control of loading conditions on seismic coupling and rupture dynamics.

 

Figure 4 – Setup and Results for the strike-slip geometry. The surface displacement field is very similar to natural earthquakes. The plot shows that due to technical limitations of this setup, fewer events are recorded but the slip velocities are higher. Credit: Michael Rudolf

 

Future developments
Together with our partners in the Collaborative Research Centre (CRC1114 – Scaling Cascades in Complex Systems) we employ a new mathematical and numerical description of the fault system, to simulate our experiments and get a physical understanding of the empirical friction laws. In the future, we want to use this multiscale spatial and temporal approach to model complex fault networks over many seismic cycles. The experiments serve as benchmarks and cross-validation for the numerical code, which in the future will be using natural parameters to get a better geological and mathematical understanding of earthquake slip phenomena and occurrence patterns in multiscale fault networks.

Minds over Methods: Making ultramylonites

Minds over Methods: Making ultramylonites

“Summer break is over, which means we will continue with our Minds over Methods blogs! For this edition we invited Andrew Cross to write about his experiments with a new rock deformation device – the Large Volume Torsion (LVT) apparatus. Andrew is currently working as a Postdoctoral Research Associate in the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA. He did his PhD at the University of Otago, New Zealand, although he is originally from the UK. His main research interest lies in understanding how micro-scale deformation processes influence the evolution of Earth’s lithosphere and tectonic plate boundaries. Hopefully we will be seeing more of him in the very near future” – Subhajit Ghosh.

Credit: Andrew Cross

Investigating strain-localisation processes in high-strain laboratory deformation experiments

Andrew Cross, Postdoctoral Research Associate at the Department of Earth and Planetary Sciences, Washington University in St. Louis, USA.

Below the upper few kilometres of the Earth’s surface – where rocks break and fracture under stress – elevated temperatures and pressures enable solid rocks to flow and bend, like a chocolate bar left outside on a warm day. This ductile flow of rocks and minerals plays a crucial role in many large-scale geodynamic processes, including mantle convection, the motion of tectonic plates, the flow of glaciers and ice sheets, and post-seismic and post-glacial rebound.

Fig. 1: Creep deformation occurs over very long timescales in the Earth. To replicate these processes on observable timescales, we must increase the rate of deformation in the laboratory. Credit: Andrew Cross

Unlike seismogenic slip that periodically accommodates large displacements over very short timescales, ductile flow occurs continuously, and at an almost imperceptibly slow rate: for example, rocks in the Earth’s interior creep at a rate roughly 10 billion times slower than that of the long-running pitch drop experiment1. Since few researchers are willing to wait millions of years to observe creep deformation in nature, we need ways of replicating these processes on much shorter timescales. Fortunately, by increasing temperature and the rate of deformation in the laboratory, we can generate creep behaviour in small samples of rock over timescales of a few hours, days, or weeks (Fig. 1).

In the Experimental Studies of Planetary Materials (ESPM) group at Washington University in St. Louis, we have spent the last couple of years developing a new rock deformation device – the Large Volume Torsion (LVT) apparatus (Fig. 2) – for performing torsion (twisting) experiments on geologic materials. By twisting small, disk-shaped rock samples, we are able to apply much more deformation (“strain”) than by squashing cylindrical samples end-on: this enables us to replicate deformation processes that operate in high-strain regions of the Earth (along the boundaries between tectonic plates, for instance).

Fig. 2: The Large Volume Torsion (LVT) apparatus. A 100-ton hydraulic ram applies a confining pressure, while electrical current passes through a graphite tube around the sample, generating heat through its electrical resistance. A screw actuator (typically used to raise and lower drawbridges) is used to rotate the lower platen and twist the sample, held between two tungsten-carbide anvils. Credit: Andrew Cross

Using the LVT apparatus, we are starting to investigate the microstructural and mechanical processes that lead to the formation of mylonites and ultramylonites: intensely deformed rocks that comprise the high-strain interiors of ductile shear zones and tectonic plate boundaries. It is widely thought that dramatic grain size reduction during (ultra)mylonite formation causes strain localisation, since strain-weakening deformation mechanisms (i.e., diffusion creep and grain boundary sliding) dominate at small grain sizes. However, grain size reduction (and therefore strain-weakening) is counteracted by the tendency of grains to grow over time, in the same way that bubbles in soapy water merge and grow over time.

An effective way of limiting grain growth is through “Zener pinning”, whereby the intermixing of grains of different mineral phases prevents grain boundary migration (and therefore growth). However, despite its suspected importance for ultramylonite formation and the occurrence of localised deformation on Earth (and possibly other planetary bodies), the processes leading to interphase mixing remain somewhat poorly understood and quantified.

Fig. 3: A comparison between our experimentally deformed calcite-anhydrite samples2 (backscattered electron (BSE) images), and natural metagranodiorite mylonites from Gran Paradiso, Western Alps3 (quartz grains, in black, mapped using electron backscatter diffraction (EBSD). Credit: Andrew Cross and Kilian et al., 2011.

To investigate phase mixing processes, we recently performed torsion experiments on mixtures of calcite and anhydrite. By deforming these mixtures to different amounts of strain, and then analysing the deformed samples in a scanning electron microscope, we were able to observe and quantify the evolution of deformation microstructures and mechanisms leading to ultramylonite formation. Backscattered electron (BSE) images show that clusters of the different minerals stretch out to form very thin, fine-grained layers, similar to foliation in natural shear zones (Fig. 3). At relatively large shear strains (17 < γ < 57) those layers disaggregated to form a fine-grained and homogeneously mixed aggregate. Electron backscatter diffraction (EBSD) analysis showed that calcite crystals became progressively more randomly oriented during phase mixing, indicative of a transition to the strain-weakening diffusion creep and grain boundary sliding regime.

The fact that a large amount of strain is required for phase mixing – and therefore strain-weakening – suggests that 1) only mature (highly-strained) shear zones are likely to maintain their weakness over long periods of geologic time, and 2) these features are therefore more likely to be reactivated after periods of quiescence. Inherited, long-lived mechanical weakness may well explain why tectonic plate boundaries are often reactivated over multiple cycles of continent accretion and rifting.

 

http://smp.uq.edu.au/content/pitch-drop-experiment

 Cross, A. J., & Skemer, P. (2017). Ultramylonite generation via phase mixing in high‐strain experimentsJournal of Geophysical Research: Solid Earth122(3), 1744-1759.

 3 Kilian, R., Heilbronner, R., & Stünitz, H. (2011). Quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creepJournal of Structural Geology33(8), 1265-1284.