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
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.
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.
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.
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.
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.
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.