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Minds over Methods: Mineral reactions in the lab

CL image of a sheared quartz-muscovite gouge. Credit: André Niemeijer.

Mineral reactions in the lab

André Niemeijer, Assistant Professor, Department of Earth Sciences at Utrecht University, the Netherlands

In this blogpost we will go on a tour of the High Pressure and Temperature (HPT) Laboratory at Utrecht University and learn about some of the interesting science done there.

André Niemeijer next to a striated fault surface. Credit: André Niemeijer.

André’s main interest is fault friction and all the various processes that are involved in the seismic cycle. This includes the evolution of fault strength over long and short timescales, the evolution of fault permeability and the effects of fluids. His current research is aimed at understanding earthquake nucleation and propagation by obtaining a better understanding of the microphysical processes that control friction of fault rocks under in-situ conditions of pressure, temperature and fluid pressure.

Most of the deformation in the Earth’s brittle crust occurs on and along faults. Fault movement produces fine-grained wear material or gouge, which is very prone to fluid-rock interactions and mineral reactions (Wintsch, 1995). It has long been recognized that the presence of a fluid allows for deformation to occur at much lower differential stresses than without.

Pressure solution

One of the mechanisms by which this deformation occurs is pressure solution (alternatively termed “solution-transfer creep” or “dissolution-precipitation creep”). This mechanism operates through the dissolution of materials at sites of elevated stress, diffusion along grain boundaries and re-precipitation at low stress sites (e.g. pores). Pressure solution is an important diagenetic process in sandstones and carbonates as evidenced by the presence of stylolites in many carbonate rocks, which are often used as counter tops and floors (particularly in banks, I noticed). In addition, it has been suggested that pressure solution plays an important role in the accommodation of (slow) shear deformation of faults (Rutter & Mainprice, 1979) and possibly in controlling the recurrence interval of earthquakes (Angevine, 1982).

Fluid-rock interactions in the lab

Experimentally, it is challenging to activate pressure solution or mineral reactions in the laboratory, because they are typically slow processes. Moreover, it is difficult to find evidence of their operation. We have used a unique hydrothermal rotary shear apparatus, which is capable of temperatures up to 700 °C to activate pressure solution in fine-grained quartz gouges. We were able to prove that new material was precipitated by using a combination of state-of-art electron microscopy techniques that involve cathodoluminescence (CL).

The hydrothermal rotary shear apparatus at the HPT laboratory at Utrecht University, the Netherlands. Credit: André Niemeijer.

Signature of pressure solution

The CL signal of a mineral depends on the type and level of impurities and defects that are present. We used quartz derived from a single crystal which showed relatively uniform CL. Because our apparatus has various metal alloy parts, small amounts of aluminium are present in the fluid. Aluminium can be incorporated in newly precipitated quartz, which gives a different CL signal. This allows us to map the locations where quartz has newly formed and link this to the experimental data. Taken together, we can use these to derive and constrain microphysical models for fault slip that can be used to extrapolate to natural conditions (e.g. Chen & Spiers 2016, van den Ende et al., 2018).

RGB overlay of secondary electron and cathodoluminescence signals in a deformed quartz sample. Newly precipitated quartz shows up in a blue colour. Credit: Maartje Hamers.

Mineral reactions

Outcrops of natural faults often show evidence for enhanced mineral reactions with increasing shear strain. For instance, the Zuccale fault (Isle of Elba, Italy) has a high content of talc in the highest strained portion of the fault (Collettini & Holdsworth, 2004). Talc is a frictionally weak mineral and its presence in the Zuccale fault provides an explanation for the possibility of slip along this low-angle normal fault. We were able to produce talc experimentally from mixtures of dolomite and quartz in only 3-5 days of shearing at low velocity. This shearing was accompanied by major weakening, with friction dropping from 0.8 to as low as 0.3. The reaction to talc is sensitive to temperature and fluid composition. At slightly higher temperature, we produced diopside and forsterite which are frictionally unstable and generated audible laboratory earthquakes.

Identifying reaction products

We tried a whole range of different analytical techniques to identify the reaction products. Despite the obvious frictional weakening that we observed, talc was only observed in two samples with x-ray diffraction (XRD). Fourier-transform Infrared analysis, on the other hand, proved to be very sensitive to talc and has the big advantage that only a small amount of material is needed (~70 mg). Electron microscopy with EDS-analysis (Energy Dispersive X-ray Spectroscopy) proved helpful to some extent, because it shows the phase distribution. However, the small size of reaction products gives a mixed chemistry, which complicates the identification of reaction products. Finally, to positively identify the various phases in the different samples, we employed Raman mapping.

RGB overlays of EDS analyses of samples deformed at 300 °C (left) and 500 °C (right). Dolomite appears in yellow, quartz in blue, calcite in red, talc in cyan in the left image, while dolomite is orange, calcite is red, diopside is purple and forsterite is cyan in the right image. Credit: André Niemeijer.

Outlook

Our studies have shown that reactions can be quite rapid in fine-grained fault gouges. These reactions can have a profound effect on both fault strength and stability but are typically ignored in large-scale models of the seismic cycle. Incorporating reactions requires models that can account for the effect of stress and grain size reduction on the development of faults, which is not an easy task, but is a necessary ingredient to understand the long-term behavior of faults.

References

  • Angevine, C. L., Turcotte DL, Furnish MD. (1982) Pressure solution lithification as a mechanism for the stick-slip behavior of faults. Tectonics 1 (2), 151-160 doi:10.1029/TC001i002p00151.
  • Chen, J. and Spiers CJ. (2016) Rate and state frictional and healing behavior of carbonate fault gouge explained using microphysical model. Journal of Geophysical Research: Solid Earth 121 (12), 8642-8665 doi:10.1002/2016JB013470.
  • Collettini, C. and Holdsworth RE. (2004) Fault zone weakening and character of slip along low-angle normal faults: Insights from the Zuccale fault, Elba, Italy. Journal of the Geological Society 161 (6), 1039-1051 doi:10.1144/0016-764903-179.
  • E H Rutter, D H Mainprice (1979)On the possibility of slow fault slip controlled by a diffusive mass transfer process. Gerlands Beitr. Geophysik, Leipzig 88 (1979) 2, S. 154-162.
  • van den Ende, M. P. A., Chen J, Ampuero J., Niemeijer AR. (2018) A comparison between rate-and-state friction and microphysical models, based on numerical simulations of fault slip. Tectonophysics 733, 273-295 doi:10.1016/j.tecto.2017.11.040.
  • Wintsch, R. P., Christoffersen R, Kronenberg AK. (1995) Fluid-rock reaction weakening of fault zones. Journal of Geophysical Research: Solid Earth 100 (B7), 13021-13032 doi:10.1029/94JB02622.

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures

Minds over Methods: Massively dilatant faults in Iceland – from surface to subsurface structures
In this Minds over Methods we don’t have one, but two scientists talking about their research! Michael Kettermann and Christopher Weismüller, both from Aachen University, explain us about the multidisciplinary approach they use to understand more about massively dilatant faults. How do they form and what do they look like at depth?

Massively dilatant faults in Iceland – from surface to subsurface structures

Michael Kettermann & Christopher Weismüller, RWTH Aachen University

Michael (left) and Christopher (right) in the field. Credit: Michael Kettermann and Marianne Sophie Hollinetz.

Iceland is a volcanic island in a unique setting on the Mid-Atlantic Ridge, separating the Eurasian and North American plates. A deep mantle plume lies beneath Iceland, and the combination of rift and plume leads to very active basaltic volcanism. Ubiquitous features along the rift zone are normal faults, often exquisitely exposed at the surface. Normal faults in basalts are also common in many volcanic provinces, like Hawaii, the East African Rift, and along mid ocean ridges. These faults often form as massively dilatant faults (MDF), which show apertures up to tens of meters at the surface and supposedly have large volumes of open voids in the subsurface.

Figure 1. View along-strike a massively dilatant fault in layered basalt. The geometry of the vertical fault faces is prescribed by the cooling joints. Several basalt columns have been loosened and dropped into the fault, now being stuck in the fault (top) or filling the cavity (bottom). Opening width < 3 m. Credit: Michael Kettermann.

These openings form pathways for fluids like magma or hydrothermal waters and consequently are of importance for volcanic plumbing systems, mineralization and geothermal energy supply.

Iceland provides a perfect natural laboratory to study MDF. Due to its position on the Mid-Atlantic Ridge, Iceland is cut by extensional fault systems roughly from southwest to north. A wide range of oblique extensional to pure extensional faults can be observed mostly in flood basalts, but also in sub-glacially formed hyaloclastites (weaker volcanic sediments), pillow lavas and occasionally sediment layers formed in warmer times. Outcropping rocks in Iceland are younger than 20 Ma distal from the rift (eastern and western Iceland), while tectonic and corresponding volcanic activity at the ridge (central Iceland) constantly causes the formation of new rocks. The rough climate hinders soil formation and vegetation to overgrow faults, providing unique outcrop conditions.

While it is relatively easy to access and study the faults at surface level, investigations into the subsurface are much more challenging. Direct observations are only possible down to depths of some tens of meters by climbing into the fractures. Cavities are often filled with rubble, sediments, water or snow (Fig. 1). Steep, open fractures with meter-scale aperture are hard to detect with geophysical methods (seismic reflection/refraction, ground penetrating radar, electrical resistivity tomography) at depths greater than some meters.

We therefore started the massively dilatant fault project, a multidisciplinary, integrated project bringing together remote sensing, fieldwork, analogue modelling and numerical simulations. In essence, we utilize a modelling approach to recreate the structure and evolution of MDF at depth, using real 3D surface data as input and comparison data set.

 

Drone mapping and photogrammetry

In a first step, we capture and analyse the surface expressions of MDF at a number of representative fault areas in Iceland. To this end, we flew 27 drone surveys during our five weeks long field season in summer 2017, covering a total length of more than 42 km of faults. Luckily, in the Icelandic summer the days are very long, so the National Park Service allowed us to fly the drones early in the morning and late in the evening outside of tourist hours. For each area, we took several hundred to thousands of overlapping photographs (e.g. Fig. 2). We processed these sets with photogrammetry software, applying the Structure from Motion (SfM) technique. SfM is an increasingly popular, fast and cheap technique to reconstruct high-resolution 3D information from 2D images. This allows us to recreate digital elevation models and ortho-rectified photo-mosaics of the faults in resolutions better than 15 cm per pixel (Fig. 3). The largest area at the famous Thingvellir fissure swarm covers a length of almost 7 km with an average resolution of 11 cm per pixel.

We use these digital elevation models and ortho-photos to retrieve a wide range of structural data. Mapping the fault traces in a GIS software allows for the measurement of fault opening width, throw, orientation and length. From throw and aperture, we can then estimate the fault dip at depth. Digital elevation models further provide surface dip data that we then compare with observations from analogue models.

Figure 2. A drone photograph facing South of the Almannagjá fault in Thingvellir, where the Thingvallavegur road crosses the fault. The Almannagjá fault resembles the western shoulder of the Thingvellir graben system with locally > 50 m opening width and 40 m vertical offset. Credit: Christopher Weismüller. .

Figure 3. Digital elevation model created from drone photographs using photogrammetry software. It contains the faults at Sandvik on the Reykjanes Peninsula (SW Iceland). The detail panes (red square) show the DEM (right) at a higher zoom level and the corresponding ortho-rectified photograph (left). The bridge crossing the fault depicted in the detail panes is a famous touristic spot, known as „The bridge between the continents“, since the fault symbolically divides the North American and Eurasion plates. Credit: Michael Kettermann.

 

 

 

 

 

 

 

Figure 4. Sideview of an analogue model showing three timesteps of the development of a massivley dilatant fault and associated fractures in hemihydrate (Bücken, 2017). Note the tilted block developing at the surface of the model and the dilatant jogs and voids in the subsurface. The opening at the surface is not directly linked to the fault at depth, but caused by the rotation of the tilted block. Credit: Daniel Bücken..

Modelling approach

For the analogue modelling approach on the hundreds to thousand meter scale, we use cohesive powders as modelling material (Bücken, 2017). Especially hemihydrate powder has been proven suited to model dilatant fractures (Holland et al., 2006; Kettermann et al., 2016; van Gent et al., 2010) as it has a well characterized true cohesion and tensile strength. Faults in Iceland transform from opening mode fractures to shear mode faults at depth when overburden stress is high enough. As we are interested in the upper dilatant parts of the faults, i.e. above the shear mode faulting, we chose a basement-fault controlled approach, where a rigid basement represents the shear mode fault. It moves down-dip along a predefined surface, deforming the powder sieved on top. The basement fault dip follows the data we derived from the field and is set to 60° – 65°. The scale of the models calculates from strength and weight of the natural prototype and the modelling material. 1 cm of powder equals about 50 m of basalt.

 

Comparison of models and nature

Results show a close similarity between field and experiment at the surface structures. Open fractures form with large apertures at the surface and often we observe the formation of tilted blocks (Fig. 4). The existence and scaled dimensional similarity of fractures and tilted blocks in the field and using a scaled material suggest a validity of other observations in the models. Glass sidewalls in the analogue models provide the opportunity to examine how the faults evolve at depth. We observe that large caves form underneath these blocks and we predict that these must exist in the field as well, albeit potentially filled with rubble. Our models corroborate earlier predictions that extensional faults are open down to 800 – 1000 m (Gudmundsson and Bäckström, 1991). We also learned that below that, a hybrid failure zone exists where dilational jogs, open extensional fractures between shear mode faults, provide lateral pathways for magma or water, even at depths where the overburden stress prevents the formation of purely extensional faults.

 

Outlook

The previously shown experiments investigated MDF at a larger scale in purely dip-slip kinematics. However, faults at rifts often have strike-slip components, forming normal faults with oblique kinematics. In further experiments, we therefore explored the effect of varying basement fault obliquities, i.e. the range between dip-slip normal faults and strike-slip faults (Bitsch, 2017). As expected, early phases of faulting are dominated by Riedel shears. Surprisingly, the surface structure of mature faults, however, does not change distinctly up to obliquities of 60°, but the subsurface connectivity decreases with increasing obliquity.

Figure 5. Analogue model resembling successive layers of lava flows with cooling joints created by carefully stacking several layers of dried corn starch slurry (Winhausen, 2018). The dip of the basement fault is prescribed by the apparatus. The fault geometry generated in the model is very similar to the ones observed in Iceland. Large cavities develop and are partially refilled by loosened columns, as shown in figure 1. Tensile fractures develop on the surface of the footwall, similar to the hemihydrate model and the field. Credit: Lisa Winhausen.

Zooming in on the faults, an inherent mechanical anisotropy (orthotropy) of basalts gains more influence on the macroscale structure of faults. Due to the shrinking during cooling of flood basalts, polygonal to blocky columns form and present regular weak zones in the rockmass. Introducing mechanical anisotropy into a stronger modelling material (dried corn-starch slurry) beautifully illustrates how the small-scale structure of the faults is affected by the layering of flood basalts and cooling fractures therein (Fig. 5; Winhausen, 2018). Close to the surface the strong basalt does not fracture, but propagating faults rather localize at the pre-existing cooling joints. This causes a jagged structure of the fault, formation of caves, and eroded basalt columns filling the opening fractures.

We are currently working on implementing all these learning points into discrete element simulations, where we can adjust material properties in a way that allows for modelling deeper parts of the faults with better mechanical control.

 

 

References

Bitsch, N.D., 2017. Massively dilatant faults in oblique rift settings – an analogue modeling study (MSc Thesis). RWTH Aachen University, Germany, Aachen.

Bücken, D.H., 2017. Effect of mechanical stratigraphy on normal fault evolution – Insights from analogue models and natural examples in Iceland (MSc Thesis). RWTH Aachen University.

Gudmundsson, A., Bäckström, K., 1991. Structure and development of the Sveinagja graben, Northeast Iceland. Tectonophysics 200, 111–125. https://doi.org/10.1016/0040-1951(91)90009-H

Holland, M., Urai, J.L., Martel, S., 2006. The internal structure of fault zones in basaltic sequences. Earth Planet. Sci. Lett. 248, 301–315. https://doi.org/10.1016/j.epsl.2006.05.035

Kettermann, M., von Hagke, C., van Gent, H.W., Grützner, C., Urai, J.L., 2016. Dilatant normal faulting in jointed cohesive rocks: a physical model study. Solid Earth 7, 843–856. https://doi.org/10.5194/se-7-843-2016

van Gent, H.W., Holland, M., Urai, J.L., Loosveld, R., 2010. Evolution of fault zones in carbonates with mechanical stratigraphy – Insights from scale models using layered cohesive powder. J. Struct. Geol. 32, 1375–1391. https://doi.org/10.1016/j.jsg.2009.05.006

Winhausen, L., 2018. Influence of columnar joints on normal fault geometry and evolution An analog modeling study Master Thesis (MSc thesis). RWTH Aachen University.

Minds over Methods: What controls the shape of oceanic ridges?

Minds over Methods: What controls the shape of oceanic ridges?

In this edition of Minds over Methods, Aurore Sibrant, postdoc at Bretagne Occidentale University (France) explains how she studies the shape of oceanic ridges, and which parameters are thought to control this shape. By using laboratory experiments combined with observations from nature, she gives new insights into how spreading rates and lithosphere thickness influence the development of oceanic ridges. 

 

Credit: Aurore Sibrant

What controls the shape of oceanic ridges? Constraints from analogue experiments

Aurore Sibrant, Post-doctoral fellow at Laboratoire Géosciences Océans, Bretagne Occidentale University, France

Mid-oceanic ridges with a total length > 70 000 km, are the locus of the most active and voluminous magmatic activity on Earth. This magmatism directly results from the passive upwelling of the mantle and decompression melting as plates separate along the ridge axis. Plate separation is taken up primarily by magmatic accretion (formation of the oceanic crust), but also by tectonic extension of the lithosphere near the mid-ocean ridge, which modifies the structure of the crust and morphology of the seafloor (Buck et al., 2005). Therefore, the morphology of the ridge is not continuous but dissected by a series of large transform faults (> 100 km) as well as smaller transform faults, overlapping spreading centres and non-transform offsets (Fig. 1). Altogether, those discontinuities form the global shape of mid-ocean ridges. While we understand many of the basic principles that govern ridges, we still lack a general framework for the governing parameters that control segmentation across all spreading rates and induce the global shape of ridges.

Geophysical (Schouten et al., 1985; Phipps Morgan and Chen, 1993; Carbotte and Macdonald, 1994) and model observations (Oldenburg and Brune, 1975, Dauteuil et al., 2002, Püthe and Gerya, 2014) suggest that segmentation of oceanic ridges reflects the effect of spreading rate on the mechanical properties and thermal structure of the lithosphere and on the melt supply to the ridge axis. To understand the conditions that control the large-scale shape of mid-ocean ridges, we perform laboratory experiments. By applying analogue results to observations made on Earth, we obtain new insight into the role of spreading velocity and the mechanical structure of the lithosphere on the shape of oceanic ridges.

 

Laboratory experiments

The analogue experiment is a lab-scale, simplified reproduction of mid-oceanic ridges system. Our set-up yields a tank filled from bottom to top by a viscous fluid (analogous to the asthenosphere) overlain by the experimental “lithosphere” that can adopt various rheologies and a thin surface layer of salted water. This analogue lithosphere is obtained using a suspension of silica nanoparticles which in contact with the salted water emplaced on the surface of the fluid causes formation of a skin or “plate” that grows by diffusion. This process is analogous to the formation of the oceanic lithosphere by cooling (Turcotte and Schubert, 1982). With increasing salinity, the rheology of the skin evolves from viscous to elastic and brittle behaviour (Di Giuseppe et al., 2012; Sibrant and Pauchard, 2016).

The plate is attached to two Plexiglas plates moving perpendicularly apart at a constant velocity. The applied extension nucleates fractures, which rapidly propagate and form a spreading axis. Underlying, less dense, fresh fluid responds by rising along the spreading axis, forming a new skin when it comes into contact with the saline solution. By separately changing the surface water salinity and the velocity of the plate separation, we independently examine the role of spreading velocity and axial lithosphere thickness on the evolution of the experimental ridges.

 

Figure 2. Close up observations of analogue mid-oceanic ridges and schematic interpretation for different spreading velocity. The grey region is a laser profile projected on the surface of the lithosphere: the laser remains straight as long as the surface is flat. Here, the large deviation from the left to centre of the image reveals the valley morphology of the axis. Credit: Aurore Sibrant.

 

Analogue mid-oceanic ridges

Over a large range of spreading rates and salinities (Sibrant et al., 2018), the morphology of the axis is different in shape. The ridge begins with a straight axis (initial condition). Then during the experiment, mechanical instabilities such as non-transform offset, overlapping spreading centres and transform faults develop (Fig. 2) and cause the spreading axis to have a non-linear geometry (Fig. 3). A key observation is the variation of the shape of the analogue ridges with the spreading rate and salinities. For similar salinity and relative slow spreading rates, each segment is offset by transform faults shaping a large tortuous ridge (i.e. non-linear geometry). In contrast, at a faster spreading rate, the ridge axis is still offset by mechanical instabilities but remains approximately linear.

Figure 3. Ridge axis morphology observed in the experiments and schematic structural interpretations of the ridge axis, transform faults (orange ellipsoids) and non-transform faults (purple ellipsoids). Measurements of lateral deviation (LD) correspond to the length of the arrows. For comparison, white squares represent the size of closeup shows in Fig 2. Credit: Aurore Sibrant.

We can quantify the ridge shape by measuring the total lateral deviation, which is the total accumulated offset of the axis, when the tortuosity amplitude becomes stable. For cases with similar salinities, the results indicate two trends. First, the lateral deviation is high at slow spreading ridges and decreases within increasing spreading rate until reaching a minimum lateral deviation value for a given critical spreading rate (Fig 4A). Then the lateral deviation remains constant despite the increasing spreading rate. Experiments with different salinities also present a transition between tortuous and linear ridges. These two trends reflect how the lithosphere deforms and fails. In the first regime, the axial lithosphere is thick and is predominantly elastic-brittle. In such cases, the plate failures occur from the surface downwards through the development of faults: it is a fault-dominated regime. In contrast, for faster spreading rate or smaller salinities, the axial lithosphere is thin and is predominantly plastic. Laboratory inspection indicates that fractures in plastic material develop from the base of the lithosphere upwards: it is a fluid-intrusion dominated regime.

 

 

Comparison with natural mid-oceanic ridge

In order to have a complete understanding of the mid-oceanic ridge system, it is essential to compare the laboratory results with natural examples. Hence, we measure the lateral deviation of nature oceanic ridges along the Atlantic, Pacific and Indian ridges. The measurements reveal the same two regimes as found in laboratory data. The remaining step consists of finding the appropriate scaling laws to superpose the natural and experiment data. This exercise requires dynamics similarity between analogue model and real-world phenomena which is demonstrated using dimensionless numbers (Sibrant et al., 2018). Particularly, the “axial failure parameter – πF” describes the predominant mechanical behaviour of the lithosphere relative to its thickness. Low-πF accretion is dominated by fractures in a predominantly elastic-brittle lithosphere: the lateral deviation of the ridges is tortuous, while at higher pF, accretion is dominated by intrusion in a predominantly plastic lithosphere: the shape of the mid oceanic ridges is mostly linear (Fig 4B).

 

Figure 4. (A) Lateral deviation values measured in the experiments in function of the spreading rate velocities and salinities. (B) Evolution of the lateral deviation of the ridge axis, normalized by the critical axial thickness (Zc) relative to the axial failure parameter. Dark grey is the laboratory experiments and the colored circles are the Earth data. Adapted from Sibrant et al., 2018.

 

Our experiments give insight into the role of axial failure mode (fault-dominated or intrusion-dominated) on the shape of mid-oceanic ridges. In the future, we want to use this experimental approach to investigate the origin of mechanical instabilities, such as transform faults or overlapping spreading centres. This experimental development and results are a collaborative work between Laboratoire FAST at Université Paris-Saclay and Department of Geological Sciences at the University of Idaho and involves E. Mittelstaedt, A. Davaille, L. Pauchard, A. Aubertin, L. Auffray and R. Pidoux.

 

 

References
Buck, W.R., Lavier, L.L., Poliakov, A.N.B., 2005. Modes of faulting at mid-ocean ridges. Nature 434, 719-723.
Schouten, H., Klitgord, K.D., Whitehead, J.A., 1985. Segmentation of mid-ocean ridges. Nature 317, 225-229.
Carbotte, S.M., Macdonald, K. C., 1994. Comparison of seafloor tectonic fabric at intermediate, fast, and super fast spreading ridges: Influence of spreading rate, plate motions, and ridge segmentation on fault patterns. J. Geophys. Res. 99, 13609-13631.
Phipps Morgan, J., Chen, J., 1993. Dependence of ridge-axis morphology on magma supply and spreading rate. Nature 364, 706-708.
Oldenburg, D.W., Brune, J.N., 1975. An explanation for the orthogonality of ocean ridges and transform faults. J. Geophys. Res. 80, 2575-2585.
Dauteuil, O., Bourgeois, O., Mauduit, T., 2002. Lithosphere strength controls oceanic transform zone structure: insights from analogue models. Geophys. J. Int. 150, 706-714.
Püthe, C., Gerya, T., 2014. Dependence of mid-ocean ridge morphology on spreading rate in numerical 3-D models. Gondwana Res. 25, 270-283.
Turcotte, D., Schubert, G., Geodynamics (Cambridge Univ. Press, New York, 1982).
Di Giuseppe, E., Davaille, A., Mittelstaedt, E., Francois, M., 2012. Rheological and mechanical properties of silica colloids: from Newtonian liquid to brittle behavior. Rheologica Acta 51, 451-465.
Sibrant, A.L.R., Pauchard, L., 2016. Effect of the particle interactions on the structuration and mechanical strength of particulate materials. European Physics Lett., 116, 4, 10.1209/0295-5075/116/49002.
Sibrant, A.L.R., Mittelstaedt, E., Davaille, A., Pauchard, L., Aubertin, A., Auffray, L., Pidoux, R., 2018. Accretion mode of oceanic ridges governed by axial mechanical strength. Nature Geoscience 11, 274-279.

 

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.