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Tectonics and Structural Geology

Features from the field

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.

Features from the field: Folding

Features from the field: Folding

Folding is one of the most common geologic phenomena in the world. I should start with defining the term ‘deformation’ in order to understand the folding process better.

In geology, deformation is an alteration of the size or shape of rocks. Deformation is caused by stress, the scientific term for force applied to a certain area. Stresses on rocks can stem from various sources, such as changes in temperature or moisture, shifts in the Earth’s plates, sediment buildup or even gravity.

Z folds in the Alba Syncline. Did they really make it? They are geologist so they can 🙂 Photo credit: by Erin Kennedy distrubted via  geology.blogs.brynmawr.

There are three types of rock deformation. Elastic deformation is temporary and is reversed when the source of stress is removed. Ductile deformation is irreversible, resulting in a permanent change to the shape or size of the rock that persists even when the stress stops. A fracture is considered as brittle deformation, whereas folding is considered as ductile deformation. The third one type is viscous deformation is the behavior of the fluids such as magma.

Certain factors determine which type of deformation rocks will exhibit when exposed to stress. These factors are rock type, strain rate, pressure and temperature. For instance, higher temperatures and pressures encourage ductile deformation. This is common deep within the Earth, where, due to higher temperatures and pressure than nearer the surface, rocks tend to be more ductile.

But, nowadays we find rocks from deep regions exposed at the surface. How? The answer is ‘uplift’, the balance between the rate of magma intrusion into the crust, erosion, and the relative densities of the continental crust and the mantle.

Anticline Trap. Anticline is a structural trap for petroleum.  Image reproduced from original source.

Folding is a manner for sedimentary and metamorphic rocks. Different layers in those rocks help geologist to understand structures.

Last but not least, Anticlines (type of folding) are important types of “structural traps” in petroleum geology.

To sum it up, Folds are significant structures for either in structural or economic geology. They are, moreover, remarkable phenomenon for people due to their great looking like many other geologic structure.

Strike Slip Faults Classification

Strike Slip Faults Classification

A strike slip faults is a fault on which most of the movement is parallel to the fault strike (Bates and Jackson, 1987). The term ‘wrench fault’ is also popularized in some researchers. Sylvester (1988) suggest not using wrench fault term for defining strike slip fault as general term because wrench fault was defined by Anderson (1905) as deep seated, regional and vertical faults. Many major strike slip faults; however, are not vertical and do not cut the lithosphere on the continental crust.

Strike slip faults are clas sify by two major groups by Sylvester (1988) with regard to where they occur: Transform faults are general term that cut the whole lithosphere and Transcurrent faults are general term do not cut the lithosphere.

Sylvester (1988) classification of the strike slip faults is the most used and convenient way to determine the type of the strike slip faults.

Table 1. Classification Strike Slip Faults by Sylvester (1988).

Figure 1. Plate tectonic setting of major classes of strike slip faults by Sylvester (1988).

Figure 2. Plate tectonic setting of major classes of strike slip faults by Sylvester (1988).

Features from the Field: Growth Faults

Features from the Field: Growth Faults

Growth faults are syndepositional or syn-sedimentary extensional faults. Growth faults develop when sediments are being deposited, are key elements in understanding deformation processes. Indeed, successively deposited sedimentary layers are involved in the different stages of the growth of the structure and produce a record of the deformation history. Their fault plane dips mostly toward the basin and has long-term continuous displacement.

As the fault grows upward, it cuts through the newly formed sedimentary layers at the top. Therefore, the overall displacement along the fault plane is not the same. Further, the lowermost layer has higher displacement than the uppermost layer while the intermediate layer displacement lies in between (See Figure). Because the fault plane flattens into décollement, the downthrown block moves basinward and the displaced sedimentary layer of the downthrown block bends close to the fault plane forming rollover anticline, synthetic and antithetic faults.

Soft Sediment Structures: Slumps and Flames

Soft Sediment Structures: Slumps and Flames

Today’s topic in Features of the Field is the well-known soft-sediment deformation; one of the most common phenomena which develop during, or shortly after deposition. The sediments; for this reason, need to be “liquid-like” or unsolidified for the deformation to occur. The most common places for soft-sediment deformations to form are deep water basins with turbidity currents, rivers, deltas, and shallow-marine areas with storm impacted conditions. Because these environments have high deposition rates, the sediments are allowed to be packed loosely.

Types of soft-sediment deformation structures;

Slumps; they generally occur in sandy shales and mudstones, but may also be present in limestones, sandstones, and evaporates. Thickness of slumps varies between 90 cm and 130 cm; their shapes can clearly be seen to be folds (Figure 1). Axes of these folds are horizontal or nearly horizontal (recumbent). They are a result of the displacement and movement of unconsolidated sediments in areas with steep slopes and fast sedimentation rates. Slump structures are related to tectonic activity.

Flame structures; they are mainly formed in sands, muds, and marls. The structures range from 5 to 30 cm in size (Figure 2) and are developed by mudstones which are injected into overlying sandstones. This injection is the result of large differences in dynamic viscosity between sediment layers. This makes fine-grained sediments behave as diapiric intrusions.

Soft-sediment deformation structures related to seismically induced liquefaction or fluidization are named as Seismites. Some researchers have been working on Seismites to reveal seismic history of an area.

In the field, some may define soft sediment structures as folds or something else by mistake. We should pay attention to the layers above and below these structures in order to avoid this mistake. This is because soft-sediment deformation structures are confined by non-deformed layers of the same formation.

Have fun..!

 

Figure 2. Developing of Soft Sediment Structures

Figure 2. Developing of Soft Sediment Structures

Features from the field: Slickenside Lineations

Features from the field: Slickenside Lineations

In this Tectonics and Structural Geology blog we will use different categories for our blog-posts. The first category we present to you is all about field geology: “Features from the field”. One of our bloggers, Mehmet Köküm, spends a lot of time in the field for his PhD and will share some of the features used in structural geology with us. This edition of ‘Features of the Field’ will be all about Slickenside lineations!

Paleostress Studies Reveals Deformation Mechanism 

It is assumed that faults are formed as pure strike slip or dip-slip faults. However, we widely come across oblique faults. If they are formed as pure strike-slip or dip-slip faults, then something should have affected its behavior. This can be done by many things, such as a change in tectonic regime or a block rotation. Many areas in the world have experienced several different tectonic regimes in the past. Faults should have been affected by these tectonic regime changes. A normal fault could have worked as a reverse fault in the past or vice versa. In other words, if we may figure out a faults’ past behavior, we could figure out the evolution of tectonic regimes in the related area.

Within this blog I will explain how structural geologists determine the behavior of a fault in the past and present. The principle purpose of my PhD project is to determine the deformation mechanism and the relation between past and present behavior of the East Anatolian Fault (EAF) by using paleostress analysis. The EAFZ is one of the most active intracontinental transform faults in Turkey.

During a field trip as part of my PhD project, one of the goals was to find slickenside lineation on a slip surface along the East Anatolian Fault in Turkey. Slicken-lines are series of parallel lines on a fault plane and represent the direction of relative displacement between the two blocks separated by the fault. Hence, direction and sense of slip can be obtained from slickenside lineation on a fault plane. Knowing this for numerous faults helps us to understand previous and present behavior of faults.

The aim of using slickenside lineation is to calculate the paleostress tensor. Paleostress tensors provide a dynamic interpretation (in terms of stress orientation) to the kinematic (movement) analysis of brittle features. Paleostress tensor analysis enables identification of the stress history of a studied area.

There are two principal types of slicken-lines: those that form by mechanical abrasion (striations) and those formed by mineral fibrous growth (mineral fiber lineations). The former can occur either in relief or groove on a fault surface. It can be a small quartz grain or larger grain causing striations on a fault surface. The latter developed due to crystal growth fibres or other grains being crystallized during fault slip. Most are made of calcite, quartz, gypsum etc. These two types of lineations are reliable criteria for calculating the paleostress tensor and common in low-grade metamorphic rocks and sedimentary rocks.

In this work, the key issue is to find and collect as much fault slip data sets as possible. In that sense, it is important to know what kind of rocks may include slicken-lines. Striations or slicken-lines are particularly found on limestone, sandstone and claystone. Moreover, mineral fiber lineations are seen most in limestone. Therefore, limestone should be investigated in more detail to collect fault slip data.

Paleostress studies require great care, effort, and attention in the field, but its outcomes for the behavior of the faults are important, since they reveal the tectonic evolution of the area. For this reason, many structural geologist touch on palestress studies in their work in order to relate observed structures to the causative tectonic forces.