Tectonics and Structural Geology

Features from the field

Features from the field: Folding


The folds of the rocks. The folds of the rocks at position Apoplystra, between the village of St. Paul and the sandhills (Greece) are remarkably colorful.Photo credit: © by G. Shuttleworth via geologyin.


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.


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