Fault anatomy

Fault anatomy

kacimo samy   5:30 ص

Fault anatomy

Faults drawn on seismic or geologic sections are usually portrayed as single lines of even thickness. In detail, however, faults are rarely simple surfaces or zones of constant thickness. In fact, most faults are complex structures consisting of a number of structural elements that may be hard to predict. Because of the variations in expression along, as well as between, faults, it is not easy to come up with a simple and general description of a fault. In most cases it makes sense to distinguish between the central fault core or slip surface and the surrounding volume of brittlely deformed wallrock known as the fault damage zone, as illustrated in Figure 8.10.

Simplified anatomy of fault.

The fault core can vary from a simple slip surface with a less than millimeter-thick cataclastic zone through a zone of several slip surfaces to an intensely sheared zone up to several meters wide where only remnants of the primary rock structures are preserved. In crystalline rocks, the fault core can consist of practically non-cohesive fault gouge, where clay minerals have formed at the expense of feldspar and other primary minerals. In other cases, hard and flinty cataclasites constitute the fault core, particularly for faults formed in the lower part of the brittle upper crust. Various types of breccias, cohesive or non-cohesive, are also found in fault cores. In extreme cases, friction causes crystalline rocks to melt locally and temporarily, creating a glassy fault rock known as pseudotachylyte. The classification of fault rocks is shown in heading below.
In soft, sedimentary rocks, fault cores typically consist of non-cohesive smeared-out layers. In some cases, soft layers such as clay and silt may be smeared out to a continuous membrane which, if continuous in three dimensions, may greatly reduce the ability of fluids to cross the fault. In general, the thickness of the fault core shows a positive increase with fault throw, although variations are great even along a single fault within the same lithology. 

The damage zone is characterized by a density of brittle deformation structures that is higher than the background level. It envelops the fault core, which means that it is found in the tip zone as well as on each side of the core. Structures that are found in the damage zone include deformation bands, shear fractures, tensile fractures and stylolites, and Figure below shows an example of how such small-scale structures (deformation bands) only occur close to the fault core, in this case defining a footwall damage zone width of around 15 meters.

Damage zone in the footwall to a normal fault with 150–200 m throw. The footwall damage zone is characterized by a frequency diagram with data collected along the profile line. A fault lens is seen in the upper part of the fault. Entrada Sandstone near Moab, Utah.

The width of the damage zone can vary from layer to layer, but, as with the fault core, there is a positive correlation between fault displacement and damage zone thickness (Figure below a). Logarithmic diagrams such as shown in Figure below are widely used in fault analysis, and straight lines in such diagrams indicate a constant relation between the two plotted parameters. In particular, for data that plot along one of the straight lines in this figure, the ratio between fault displacement D and damage zone thickness DT is the same for any fault size, and the distance between adjacent lines in this figure represents one order of magnitude. Much of the data in Figure below a plot around or above the line D=DT, meaning that the fault displacement is close to or somewhat larger than the damage zone thickness, at least for faults with displacements up to 100 meters. We could use this diagram to estimate throw from damage zone width or vice versa, but the large spread of data (over two orders of magnitude) gives a highly significant uncertainty. 

A similar relationship exists between fault core thickness (CT) and fault displacement (Figure below b). This relationship is constrained by the straight lines D=1000CT and D-10CT, meaning that the fault core is statistically around 1/100 of the fault displacement for faults with displacements up to 100 meters.

(a) Damage zone thickness (DT) (one side of the fault) plotted against displacement (D) for faults in siliciclastic sedimentary rocks. (b) Similar plot for fault core thickness (CT). Note logarithmic axes. Data from several sources.

Layers are commonly deflected (folded) around faults, particularly in faulted sedimentary rocks. The classic term for this behavior is drag, which should be used as a purely descriptive or geometric term. The drag zone can be wider or narrower than the damage zone, and can be completely absent. The distinction between the damage zone and the drag zone is that drag is an expression of ductile fault-related strain, while the damage zone is by definition restricted to brittle deformation. They are both part of the total strain zone associated with faults. In general, soft rocks develop more drag than stiff rocks.

Fault rocks

When fault movements alter the original rock sufficiently it is turned into a brittle fault rock. There are several types of fault rocks, depending on lithology, confining pressure (depth), temperature, fluid pressure, kinematics etc. at the time of faulting. It is useful to distinguish between different types of fault rocks, and to separate them from mylonitic rocks formed in the plastic regime. Sibson (1977) suggested a classification based on his observation that brittle fault rocks are generally non-foliated, while mylonites are well foliated. He further made a distinction between cohesive and non-cohesive fault rocks. Further subclassification was done based on the relative amounts of large clasts and fine-grained matrix. Sibson’s classification is descriptive and works well if we also add that cataclastic fault rocks may show a foliation in some cases. Its relationship to microscopic deformation mechanism is also clear, since mylonites, which result from plastic deformation mechanisms, are clearly separated from cataclastic rocks in the lower part of the diagram. 

Fault breccia is an unconsolidated fault rock consisting of less than 30% matrix. If the matrix fragment ratio is higher, the rock is called a fault gouge. A fault gouge is thus a strongly ground down version of the original rock, but the term is sometimes also used for strongly reworked clay or shale in the core of faults in sedimentary sequences. These unconsolidated fault rocks form in the upper part of the brittle crust. They are conduits of fluid flow in non-porous rocks, but contribute to fault sealing in faulted porous rocks.

Pseudotachylyte consists of dark glass or microcrystalline, dense material. It forms by localized melting of the wall rock during frictional sliding. Pseudotachylyte can show injection veins into the sidewall, chilled margins, inclusions of the host rock and glass structures. It typically occurs as mm- to cm-wide zones that make sharp boundaries with the host rock. Pseudotachylytes form in the upper part of the crust, but can form at large crustal depths in dry parts of the lower crust. 

Crush breccias are characterised by their large fragments. They all have less than 10% matrix and are cohesive and hard rocks. The fragments are glued together by cement (typically quartz or calcite) and/or by microfragments of mineral that have been crushed during faulting.

Cataclasites are distinguished from crush breccias by their lower fragment–matrix ratio. The matrix consists of crushed and ground-down microfragments that form a cohesive and often flinty rock. It takes a certain temperature for the matrix to end up flinty, and most cataclasites are thought to form at 5km depth or more. 

Mylonites, which are not really fault rocks although loosely referred to as such by Sibson, are subdivided based on the amount of large, original grains and recrystallised matrix. Mylonites are well foliated and commonly also lineated and show abundant evidence of plastic deformation mechanisms rather than frictional sliding and grain crushing. They form at greater depths and temperatures than cataclasites and other fault rocks; above 300 C for quartz-rich rocks. The end-member of the mylonite series, blastomylonite, is a mylonite that has recrystallized after the deformation has ceased (postkinematic recrystallization). It therefore shows equant and strain-free grains of approximately equal size under the microscope, with the mylonitic foliation still preserved in hand samples.

Credits: Haakon Fossen (Structural Geology)

Folds and Foliations

Folds and Foliations

kacimo samy   8:24 ص

Folds and Foliations

Geometry of Folds Imagine a carpet lying flat on the floor. Push on one end of the carpet, and it will wrinkle or contort into a series of wavelike curves. Stresses developed during mountain building can similarly warp or bend bedding and foliation (or other planar features) in rock. The result a curve in the shape of a rock layer is called a fold.

Geometric characteristics of folds.

Not all folds look the same some look like arches, some look like troughs, and some have other shapes. To describe these shapes, we must first label the parts of a fold (figure above a). The hinge refers to a line along which the curvature is greatest, and the limbs are the sides of the fold that display less curvature. The axial surface is an imaginary plane that contains the hinges of successive layers and effectively divides the fold into two halves. With these terms in hand, we distinguish among the following: 

• Anticlines, synclines, and monoclines: Folds that have an arch-like shape in which the limbs dip away from the hinge are called anticlines (figure above a), whereas folds with a trough-like shape in which the limbs dip toward the hinge are called synclines (figure above b). A monocline has the shape of a carpet draped over a stair step (figure above c). 
• Non-plunging and plunging folds: If the hinge is horizontal, the fold is called a non-plunging fold, but if the hinge is tilted, the fold is called a plunging fold (figure above d). 
• Domes and basins: A fold with the shape of an overturned bowl is called a dome, whereas a fold shaped like an upright bowl is called a basin (figure above e, f). Domes and basins both display circular outcrop patterns that look like bull’s-eyes the oldest layer occurs in the centre of a dome, whereas the youngest layer is located in the centre of a basin. 

Characteristics of folds on outcrops and in the landscape.

Using these terms, now see if you can identify the various folds shown in figure above a–e.

Formation of Folds 

Fold development in flexural-slip and passive flow-folding.

Folds develop in two principal ways (figure above a, b). During formation of flexural-slip folds, a stack of layers bends, and slip occurs between the layers. The same phenomenon happens when you bend a deck of cards to accommodate the change in shape, the cards slide with respect to each other. Passive-flow folds form when the rock, overall, is so soft that it behaves like weak plastic and slowly flows; these folds develop simply because different parts of the rock body flow at different rates. 

Folding is caused by several different processes, as illustrated by the following cross sections.

Why do folds form? Some layers wrinkle up, or buckle, in response to end-on compression (figure above a–d). Others form where shear stress gradually shifts one part of a layer up and over another part. Still others develop where rock layers move up and over step-like bends in a fault and must curve to conform with the fault’s shape. Finally, some folds form when new slip on a fault causes a block of basement to move up so that the overlying sedimentary layers must warp.

Tectonic Foliation in Rocks 

In an undeformed sandstone, the grains of quartz are roughly spherical, and in an undeformed shale, clay flakes press  together into the plane of bedding so that shales tend to split parallel to the bedding. During ductile deformation, however, internal changes take place in a rock that gradually modify the original shape and arrangement of grains. For example, quartz grains may transform into cigar shapes, elongate ribbons, or tiny pancakes, and clay flakes may recrystallize or reorient so that they lie at an angle to the bedding. Overall, deformation can produce inequant grains and can cause them to align parallel to each other. We refer to layering developed by the alignment of grains in response to deformation as tectonic foliation. 

The development of tectonic foliation in rock.

We introduced foliation, such as slaty cleavage, schistosity, and gneissic layering, while discussing the effects of metamorphism. Here we add to the story by noting that such foliation forms in response to flattening and shearing in ductilely deforming rocks in other words, foliation indicates that the rock has developed a strain under metamorphic conditions (figure above a, b).
Credits: Stephen Marshak (Essentials of Geology)

Brittle Structures

Brittle Structures

kacimo samy   7:58 ص

Brittle Structures 

Joints and Veins 

Examples of joints and veins.

If you look at the photographs of rock outcrops, you’ll notice thin dark lines that cross the rock faces. These lines represent traces of natural cracks along which the rock broke and separated into two pieces during brittle deformation.  Geologists refer to such natural cracks as joints (figure above a, b). Rock bodies do not slide past each other on joints. Since joints are roughly planar structures, we define their orientation by their strike and dip, as described in (Describing the Orientation of Geologic Structures).

Joints develop in response to tensile stress in brittle rock: a rock splits open because it has been pulled slightly apart. Joints may form for a variety of geologic reasons. For example, some joints form when a rock cools and shrinks because the process makes one part of a rock pull away from the adjacent part. Others develop when rock formerly at depth undergoes a decrease in pressure as overlying rock erodes away, and thus changes shape slightly. Still others form when rock layers bend.

Geotechnical engineers, people who study the geologic setting of construction sites, pay close attention to jointing when recommending where to put roads, dams, and buildings. Water flows much more easily through joints than it does through solid rock, so it would be a bad investment to situate a water reservoir over rock containing lots of joints the water would leak down into the joints. Also, building a road on a steep cliff composed of jointed rock could be risky, for joint-bounded blocks separate easily from bedrock, and the cliff might collapse. 

If groundwater or hydrothermal fluids seep through cracks in rocks, minerals such as quartz or calcite may precipitate out of the groundwater and fill the crack. A mineral-filled crack is called a vein. Veins may look like white stripes cutting across a body of rock (figure above c). Some veins contain small quantities of valuable metals, such as gold.

Faults: Surfaces of Slip 

After the San Francisco earthquake of 1906, geologists found a rupture that had torn through the land surface near the city. Where this rupture crossed orchards, it offset rows of trees, and where it crossed a fence, it broke the fence in two; the western side of the fence moved northward by about 2 m. The rupture represents the trace of the San Andreas Fault. As we have seen, a fault is a fracture on which sliding occurs, and slip events, or faulting, can generate earthquakes. Faults, like joints, are planar structures, so we can represent their orientation by strike and dip.

Faults have formed throughout Earth history. Some are currently active in that sliding has been occurring on them in recent geologic time, but most are inactive, meaning that sliding on them ceased long ago. Some faults, such as the San Andreas, intersect the ground surface and thus displace the ground when they move. Others accommodate the sliding of rocks in the crust at depth and remain invisible at the surface unless they are later exposed by erosion.

Fault Classification 

Not all faults result in the same kind of crustal deformation some accommodate horizontal shear, some accommodate shortening, and some accommodate stretching. It’s important for geologists to distinguish among different kinds of faults in order to interpret their tectonic significance. Fault classification focuses on two characteristics of faults: (1) the dip of the fault surface (see Describing the Orientation of Geologic Structures) the dip can be vertical, horizontal, or any angle in between; and (2) the shear sense across the fault by shear sense, we mean the direction that material on one side of the fault moved relative to the material on the other side. With this concept in mind, let’s consider the principal kinds of faults. 

The different categories of faults.

• Strike-Slip Faults: A strike-slip fault is a fault on which the slip direction is parallel to a strike line, a horizontal line on the fault surface (see Describing the Orientation of Geologic Structures). This means that the block on one side of the fault slips sideways, relative to the block on the other side, and there is no up-or-down motion. Most strike-slip faults have a steep to vertical dip. Geologists distinguish between two types of strike-slip faults based on the shear sense as viewed when you are facing the fault and looking across it. If the block on the far side slipped to your left, the fault is a left-lateral strike-slip fault, and if the block slipped to the right, the fault is a right-lateral strike-slip fault (figure above a). The faults that occur at transform plate boundaries are strike-slip faults. 
• Dip-Slip Faults: On a dip-slip fault, movement is parallel to the dip line, a line parallel to the slope of the fault surface  (see Describing the Orientation of Geologic Structures), so the hanging-wall block, meaning the material above the fault surface, slides up or down relative to the footwall block, the material below the fault surface (figure above b). If the hanging-wall block slides up, the fault is a reverse fault; a reverse fault with a gentle dip is also known as thrust fault. Reverse faults accommodate shortening of the crust, as happens during continental collision. If the hanging-wall block slides down, the fault is a normal fault. Normal faults accommodate stretching of the Earth’s crust, as happens during rifting. 
• Oblique-Slip Faults: On an oblique-slip fault, sliding occurs diagonally on the fault plane. In effect, an oblique-slip fault is a combination of a strike-slip and a dip-slip fault (figure above c).

Recognizing Faults 

Recognizing fault displacement in the field.

How do you recognize a fault when you see one? The most obvious criterion is the occurrence of displacement, meaning the movement across a fault plane. Displacement offsets the layers in rocks, so that layers on one side of a fault are not continuous with layers on the other side (figure above a, b).

Faults may also leave their mark on the landscape. Those that intersect the ground surface while they are active can displace natural landscape features such as stream valleys or glacial moraines (figure above c), and human-made features such as highways, fences, or rows of trees in orchards. Displacement on a dip-slip or oblique-slip fault will make a step on the ground surface; this step is called a fault scarp (figure above a). And because faults tend to break up and weaken rock, the “fault trace” (the line of intersection between the fault and the ground surface) preferentially erodes to become a linear valley.

Faulting under brittle conditions may crush or break rock. If this shattered rock consists of visible angular fragments, then it is called fault breccia (figure above b), but if it consists of a fine powder, then it is called fault gouge. Some fault surfaces are polished and grooved by the movement on the fault. Polished fault surfaces are called slickensides, and linear grooves on fault surfaces are slip lineations or fault striations (figure above c). We specify the orientation of a slip lineation by giving its plunge and bearing (see Describing the Orientation of Geologic Structures).

Features of exposed fault surfaces.

Describing the Orientation of Geologic Structures

Specifying the orientation of planar and linear structures.

When discussing geologic structures, it’s important to be able to communicate information about their orientation. For example, does a fault exposed in an outcrop at the edge of town continue beneath the nuclear power plant 3 km to the north, or does it go beneath the hospital 2 km to the east? If we knew the fault’s orientation, we might be able to answer this question. To describe the orientation of a geologic structure, geologists picture the structure as a simple geometric shape, then specify the angles that the shape makes with respect to a horizontal plane (a flat surface parallel to sea level), a vertical plane (a flat surface perpendicular to sea level), and the north direction (a line of longitude). 

Let’s start by considering planar structures such as faults, beds, and joints. We call these structures planar because they resemble a geometric plane. A planar structure’s orientation can be specified by its strike and dip. The strike is the angle between an imaginary horizontal line  (the strike line) on the plane and the direction to true north (figure above a, b). We measure the strike with a special type of compass (figure above c). The dip is the angle of the plane's slope more precisely, it is the angle between a horizontal plane 

and the dip line (an imaginary line parallel to the steepest slope on the structure), as measured in a vertical plane perpendicular to the strike. We measure the dip angle with a clinometer, a type of protractor. A horizontal plane has a dip of 0°, and a vertical plane has a dip of 90°. We represent strike and dip on a geologic map using the symbol shown in figure above b. 

A linear structure resembles a geometric line rather than a plane; examples of linear structures include scratches or grooves on a rock surface. Geologists specify the orientation of a line by giving its plunge and bearing (figure above d). The plunge is the angle between a line and horizontal in the vertical plane that contains the line. A horizontal line has a plunge of 0°, and a vertical line has a plunge of 90°. The bearing is the compass heading of the line, meaning the angle between the projection of the line on the horizontal plane and the direction to true north.

Credits: Stephen Marshak (Essentials of Geology)

Fault Terminology

Fault Terminology

kacimo samy   6:10 ص

Faults are much more complex and compound features that can accommodate large amounts of strain in the upper crust. The term fault is used in different ways, depending on geologist and context. A simple and traditional definition states:

A fault is any surface or narrow zone with visible shear displacement along the zone.

This definition is almost identical to that of a shear fracture, and some geologist use the two terms synonymously. Sometimes geologists even refer to shear fractures with millimeter- to centimeter-scale offsets as microfaults. However, most geologists would restrict the term shear fracture to small-scale structures and reserve the term fault for more composite structures with offsets in the order of a meter or more. 

The thickness of a fault is another issue. Faults are often expressed as planes and surfaces in both oral and written communication and sketches, but close examination of faults reveals that they consist of fault rock material and subsidiary brittle structures and therefore have a definable thickness. However, the thickness is usually much smaller than the offset and several orders of magnitude less than the fault length. Whether a fault should be considered as a surface or a zone largely depends on the scale of observation, objectives and need for precision.

Faults tend to be complex zones of deformation, consisting of multiple slip surfaces, subsidiary fractures and perhaps also deformation bands. This is particularly apparent when considering large faults with kilometerscale offsets. Such faults can be considered as single faults on a map or a seismic line, but can be seen to consist of several small faults when examined in the field. In other words, the scale dependency, which haunts the descriptive structural geologist, is important. This has led most geologists to consider a fault as a volume of brittlely deformed rock that is relatively thin in one dimension:

A fault is a tabular volume of rock consisting of a central slip surface or core, formed by intense shearing, and a surrounding volume of rock that has been affected by more gentle brittle deformation spatially and genetically related to the fault.

The term fault may also be connected to deformation mechanisms (brittle or plastic). In a very informal sense, the term fault covers both brittle discontinuities and ductile shear zones dominated by plastic deformation. This is sometimes implied when discussing large faults on seismic or geologic sections that penetrate much or all of the crust. The term brittle fault (as opposed to ductile shear zone) can be used if it is important to be specific with regard to deformation mechanism. In most cases geologists implicitly restrict the term fault to slip or shear discontinuities dominated by brittle deformation mechanisms, rendering the term brittle fault redundant:

A fault is a discontinuity with wall-parallel displacement dominated by brittle deformation mechanisms.

By discontinuity we are here primarily referring to layers, i.e. faults cut off rock layers and make them discontinuous. However, faults also represent mechanical and displacement discontinuities.  A kinematic definition, particularly useful for experimental work and GPS-monitoring of active faults can therefore be added:

 Faults appear as discontinuities on velocity or displacement field maps and profiles. The left blocks in the undeformed map  a) and profile (b) are fixed during the deformation. The result is abrupt changes in the displacement field (arrows) across faults.

A fault is a discontinuity in the velocity or displacement field associated with deformation.

Faults differ from shear fractures because a simple shear fracture cannot expand in its own plane into a larger structure. In contrast, faults can grow by the creation of a complex process zone with numerous small fractures, some of which link to form the fault slip surface while the rest are abandoned.

Geometry of faults

Normal (a), strike-slip (sinistral) (b) and reverse (c) faults. These are end-members of a continuous spectrum of oblique faults. The stereonets show the fault plane (great circle) and the displacement vector (red point).

Non-vertical faults separate the hanging wall from the underlying footwall. Where the hanging wall is lowered or down thrown relative to the footwall, the fault is a normal fault. The opposite case, where the hanging wall is up thrown relative to the footwall, is a reverse fault. If the movement is lateral, i.e. in the horizontal plane, then the fault is a strike-slip fault. Strike-slip faults can be sinistral (left-lateral) or dextral (right-lateral) (from the Latin words sinister and dexter, meaning left and right, respectively).

Although some fault dip ranges are more common than others, with strike-slip faults typically occurring as steep faults and reverse faults commonly having lower dips than normal faults, the full range from vertical to horizontal faults is found in naturally deformed rocks. If the dip angle is less than 30 the fault is called a low-angle fault, while steep faults dip steeper than 60 . Low-angle reverse faults are called thrust faults, particularly if the movement on the fault is tens or hundreds of kilometres. 

Listric normal fault showing very irregular curvature in the sections perpendicular to the slip direction. These irregularities can be thought of as large grooves or corrugations along which the hanging wall can slide.

A fault that flattens downward is called a listric fault, while downward-steepening faults are sometimes called antilistric. The terms ramps and flats, originally from thrust fault terminology, are used for alternating steep and sub-horizontal portions of any fault surface. For example, a fault that varies from steep to flat and back to steep again has a ramp-flat-ramp geometry. 

Irregularities are particularly common in the section perpendicular to the fault slip direction. For normal and reverse faults this means curved fault traces in map view, as can be seen from the faults of the extensional oil field. Irregularities in this section cause no conflict during fault slippage as long as the axes of the irregularities coincide with the slip vector. Where irregularities also occur in the slip direction, the hanging wall and/or footwall must deform. For example, a listric normal fault typically creates a hanging-wall rollover.

The main faults in the North Sea Gullfaks oil field show high degree of curvature in map view and straight traces in the vertical sections (main slip direction). Red lines represent some of the well paths in this field. 

A fault can have any shape perpendicular to the slip direction, but non-linearity in the slip direction generates space problems leading to hanging or footwall strain.

The term fault zone traditionally means a series of sub-parallel faults or slip surfaces close enough to each other to define a zone. The width of the zone depends on the scale of observation – it ranges from centimetres or meters in the field to the order of a kilometre or more when studying large-scale faults such as the San Andreas Fault. The term fault zone is now also used inconsistently about the central part of the fault where most or all of the original structures of the rock are obliterated, or about the core and the surrounding deformation zone associated with the fault. This somewhat confusing use is widespread in the current petroleum related literature, so any use of the term fault zone requires clarification.

A horst (a), symmetric graben (b) and asymmetric graben (c), also known as a half-graben. Antithetic and synthetic faults are shown.

Two separate normal faults dipping toward each other create a down thrown block known as a graben. Normal faults dipping away from each other create an up thrown block called a horst. The largest faults in a faulted area, called master faults, are associated with minor faults that may be antithetic or synthetic. An antithetic fault dips toward the master fault, while a synthetic fault dips in the same direction as the master fault. These expressions are relative and only make sense when minor faults are related to specific larger-scale faults.

Displacement, slip and separation 

 Illustration of a normal fault affecting a tilted layer. The fault is a normal fault with a dextral strike-slip component (a), but appears as a sinistral fault in map view (b, which is the horizontal section at level A). (c) and (d) show profiles perpendicular to fault strike (c) and in the (true) displacement direction (d).

Displacement, slip and separation

The vector connecting two points that were connected prior to faulting indicates the local displacement vector or net slip direction. Ideally, a strike-slip fault has a horizontal slip direction while normal and reverse faults have displacement vectors in the dip direction. In general, the total slip that we observe on most faults is the sum of several increments (earthquakes), each with its own individual displacement or slip vector. The individual slip events may have had different slip directions. We are now back to the difference between deformation sensu stricto, which only relates the undeformed and deformed states, and deformation history. In the field we could look for traces of the slip history by searching for such things as multiple striations.

Classification of faults based on the dip of the fault plane and the pitch, which is the angle between the slip direction (displacement vector) and the strike.

A series of displacement vectors over the slip surface gives us the displacement field or slip field on the surface. Striations, kinematic indicators and offset of layers provide the field geologist with information about direction, sense and amount of slip. Many faults show some deviation from true dip-slip and strike-slip displacement in the sense that the net slip vector is oblique. Such faults are called oblique-slip faults. The degree of obliquity is given by the pitch (also called rake), which is the angle between the strike of the slip surface and the slip vector (striation).

Unless we know the true displacement vector we may be fooled by the offset portrayed on an arbitrary section through the faulted volume, be it a seismic section or an outcrop. The apparent displacement that is observed on a section or plane is called the (apparent) separation. Horizontal separation is the separation of layers observed on a horizontal exposure or map, while the dip separation is that observed in a vertical section. In a vertical section the dip separation can be decomposed into the horizontal and vertical separation. Note that this horizontal separation is different from. These two separations recorded in a vertical section are more commonly referred to as heave (horizontal component) and throw (vertical component). Only a section that contains the true displacement vector shows the true displacement or total slip on the fault.

The relationship between a single fault, a mapped surface and its two fault cutoff lines. Such structure contour maps are used extensively in the oil industry where they are mainly based on seismic reflection data.

A fault that affects a layered sequence will, in three dimensions, separate each surface (stratigraphic interface) so that two fault cut off lines appear. If the fault is non-vertical and the displacement vector is not parallel to the layering, then a map of the faulted surface will show an open space between the two cut-off lines. The width of the open space, which will not have any contours, is related to both the fault dip and the dip separation on the fault. Further, the opening reflects the heave (horizontal separation) seen on vertical sections across the fault.

Stratigraphic separation

 (a) Missing section in vertical wells (well C) always indicates normal faults (assuming constant stratigraphy). (b) Repeated section (normally associated with reverse faults) occurs where the normal fault is steeper than the intersecting well bore (well G).

Drilling through a fault results in either a repeated section or a missing section at the fault cut (the point where the wellbore intersects the fault). For vertical wells it is simple: normal faults omit stratigraphy (figure a), while reverse faults cause repeated stratigraphy in the well. For deviated wells where the plunge of the well bore is less than the dip of the fault, such as the well G (figure b), stratigraphic repetition is seen across normal faults. The general term for the stratigraphic section missing or repeated in wells drilled through a fault is stratigraphic separation. Stratigraphic separation, which is a measure of fault displacement obtainable from wells in subsurface oil fields, is equal to the fault throw if the strata are horizontal. Most faulted strata are not horizontal, and the throw must be calculated or constructed.

Credits: Haakon Fossen (Structural Geology)

Types of fractures

Types of fractures

kacimo samy   1:27 م

What is a fracture? 

Strictly speaking, a fracture is any planar or sub-planar discontinuity that is very narrow in one dimension compared to the other two and forms as a result of external (e.g. tectonic) or internal (thermal or residual) stress. Fractures are discontinuities in displacement and mechanical properties where rocks or minerals are broken, and reduction or loss of cohesion characterizes most fractures. They are often described as surfaces, but at some scale there is always a thickness involved. Fractures can be separated into shear fractures (slip surfaces) and opening or extension fractures (joints, fissures and veins). In addition, closing or contraction fractures can be defined.

Three types of fracture.

The orientation of various fracture types with respect to the principal stresses.

Fractures are very narrow zones, often thought of as surfaces, associated with discontinuities in displacement and mechanical properties (strength or stiffness).

A shear fracture or slip surface is a fracture along which the relative movement is parallel to the fracture. The term shear fracture is used for fractures with small (mm- to dm-scale) displacements, while the term fault is more commonly restricted to discontinuities with larger offset. The term slip surface is used for fractures with fracture-parallel movements regardless of the amount of displacement and is consistent with the traditional use of the term fault. Fractures are commonly referred to as cracks in material science and rock mechanics oriented literature.

Extension fractures are fractures that show extension perpendicular to the walls. Joints have little or no macroscopically detectable displacement, but close examination reveals that most joints have a minute extensional displacement across the joint surfaces, and therefore they are classified as true extension fractures. Extension fractures are filled with gas, fluids, magma or minerals. When filled with air or fluid we use the term fissure. Mineral-filled extension fractures are called veins, while magma-filled fractures are classified as dikes. Joints, veins and fissures are all referred to as extension fractures. 

Contractional planar features (anticracks) have contractional displacements across them and are filled with immobile residue from the host rock. Stylolites are compactional structures characterized by very irregular, rather than planar, surfaces. Some geologists now regard stylolites as contraction fractures or closing fractures, as they nicely define one of three end-members in a complete kinematic fracture framework together with shear and extension fractures. Such structures are known as anticracks in the engineering-oriented literature. Rock mechanics experiments carried out at various differential stresses and confining pressures set a convenient stage for studying aspects of fracture formation. 

Experimental deformation structures that develop under extension and contraction. Initial elastic deformation is seen for all cases, while ductility increases with temperature (T) and confining pressure (Pc). YP, yield point.

Similarly, numerical modeling has added greatly to our understanding of fracture growth, particularly the field called linear elastic fracture mechanics. In the field of fracture mechanics it is common to classify the displacement field of fractures or cracks into three different modes. Mode I is the opening (extension) mode where displacement is perpendicular to the walls of the crack. Mode II (sliding mode) represents slip (shear) perpendicular to the edge and Mode III (tearing mode) involves slip parallel to the edge of the crack. Modes II and III occur along different parts of the same shear fracture and it may therefore be confusing to talk about Mode II and Mode III cracks as individual fractures. Combinations of shear (Mode II or III) fractures and tension (Mode I) fractures are called hybrid cracks or fractures. Furthermore, the term Mode IV (closing mode) is sometimes used for contractional features such as stylolites. The mode of displacement on fractures is an important parameter, for instance when fluid flow through rocks is an issue.

Mode I, II, III and IV fractures.

Extension fractures and tensile fractures 

Extension fractures ideally develop perpendicular to s3 and thus contain the intermediate and maximum principal stresses (2y ¼ 0 ). In terms of strain, they develop perpendicular to the stretching direction under tensile conditions, and parallel to the compression axis during compression tests. Because of the small strains associated with most extension fractures, stress and strain axes more or less coincide.

Joints are the most common type of extension fracture at or near the surface of the Earth and involve very small strains. Fissures are extension fractures that are more open than joints, and are characteristic of the uppermost few hundred meters of the solid crust, where they may be up to several kilometers long. 

Fissures formed in Thingvellir, Iceland, along the rift axis between the Eurasian and Laurentian plates. The fissures are open extension fractures in basalt, but the vertical displacement (right-hand side down) indicates a connection with underlying faults.

Extension fractures are typical for deformation under low or no confining pressure, and form at low differential stress. If extension fractures form under conditions where at least one of the stress axes is tensile, then such fractures are true tensile fractures. Such conditions are generally found near the surface where negative values of s3 are more likely. They can also occur deeper in the lithosphere, where high fluid pressure reduces the effective stress. Many other joints are probably related to unloading and cooling of rocks.

Shear fractures 

Shear fractures show fracture-parallel slip and typically develop at 20–30 to s1, as seen from numerous experiments under confined compression. Such experiments also show that they commonly develop in conjugate pairs, bisected by s1. Shear fractures develop under temperatures and confining pressures corresponding to the upper part of the crust. They can also form near the brittle–plastic transition, where they tend to grow into wider bands or zones of cataclastic flow. Such shear factures result in strain patterns otherwise typical for plastic deformation.

While extension fractures open perpendicular to s3, shear fractures are oblique to s3 by an angle that depends mostly on rock properties and state of stress.

Brittle and plastic deformation show different stress-strain curves (blue versus red curves): the more ductile the deformation, the greater the amount of plastic deformation prior to fracturing. It is also interesting to note the relationship between confining pressure (depth) and strain regime (contractional or extensional). The experimental data indicate that the brittle-plastic transition occurs at higher confining pressure under extension than under contraction. 

Lineations in the brittle regime

Lineations in the brittle regime

kacimo samy   5:57 ص

Lineations in the Brittle Regime

Some lineations occur only on fracture surfaces. They are not fabric-forming elements and are more characteristic of the brittle regime in the upper crust. These lineations form by mineral growth in extension fractures, as striations carved on the walls of shear fractures and faults, by intersections between fractures and by fracture curvatures that form early during fracturing. 

Mineral lineations in the brittle regime tend to be restricted to fiber lineations, where minerals have grown in a preferred direction on fractures. The growth of minerals on fractures usually requires that the fractures open to some extent, either as true extension fractures or as shear fractures with a component of extension. Furthermore, the minerals must grow in a preferred direction for a lineation to be defined. Minerals such as quartz, antigorite, actinolite, gypsum and anhydrite may appear fibrous on fractures. 

Two perpendicular sets of mineral lineations on a fault surface in serpentinite. The two directions indicate movements under two different stress fields. Leka Ophiolite, Scandinavian Caledonides.

Mineral fibers are found in many extensional or Mode I fractures. The orientation of the fibers is commonly taken to represent the extension direction. Curved fibers are sometimes seen, implying that the extension direction has changed during the course of deformation, or that shear has occurred during or after the formation of the fibers. 

Fiber lineation (talk) in extension fracture where the fibers have grown perpendicular to the fracture walls as the fracture opened.

Even though extension is involved in the formation of fibrous mineral lineations, this does not imply that such lineations are restricted to extension fractures. Because of the irregular shapes of most shear fractures, a component of extension may occur in extensional stepovers, and minerals may grow as walls separate. The mineral fibers then grow on the lee side of steps or other irregularities, precipitated from fluids circulating on the fracture network. Thus,the senseof slipis detectablefrom the relation between fiber growth and fracture geometry.

Formation of fiber lineation in irregular shear fractures. (a) Early stage. (b) Final stage. In (c) the upper wall is removed for inspection. Groove lineations (striations) are found on surfaces that have not opened during faulting.

Non-planar shear fractures can contain extensional (pull-apart) segments where fibers can grow and form a local lineation.

Striations or slickenlines are lineations found on shear fractures and form by physical abrasion of hanging-wall objects into the footwall or vice versa. The smooth and striated slip surface itself is called a slickenside. 

Slickensides with slickenlines, formed by cataclastic shearing of epidote on a post-Caledonian normal fault in the Precambrian of West Norway.

Slickensides tend to be shiny, polished surfaces coated by a 1 mm thick layer of crushed, cohesive fault rock. Hard objects or asperities can carve out linear tracks or grooves known as fault grooves. The term groove lineation can be used for this type of slickenlines. 

Fault grooves on striated fault surface separating limestone from sandstone. Moab Fault, Utah.

Such mechanically formed slickensides may show similarities with glacially striated surfaces. Close examination of many slickensided slip surfaces reveals that they are formed on mineral fill or that they actually are fiber lineations.

There are two principal types of slickenlines: those that form by mechanical abrasion (striations) and those formed by fibrous growth (slip fiber lineations).

Minerals may grow during the movement history of a fault, and it is common to find a combination of fiber lineations and striations. Such lineations may be of the same or different ages, and sometimes two or more different sets of lineations of different minerals exist that may or may not be striated due to mechanical abrasion.

Geometric striae relate to the irregular or corrugated shape of a slip surface. Such irregularities may have a preferred orientation or axis in the slip direction and appear as lineations on an exposed wall. A special type of geometric striae is the cigar-shape seen on the walls of deformation band cluster zones. Geometric striae and physical striae or slickenlines commonly coexist. 

Lineations in a zone of deformation bands. The lineation points down dip and is an expression of the cigarshaped geometry of undeformed
volumes of rock between the deformation bands. Compass for scale. Entrada Sandstone, San Rafael Desert, Utah.

Intersection lineations are found on fractures where the main slip plane is intersected by secondary fractures such as Riedel fractures or tensile fractures. The lines of intersection typically (but not necessarily) form a high angle to the slip direction, in marked contrast to striae and mineral lineations that tend to parallel the slip direction. Finally, we mention a type of lineation that is typical for some deformed limestones. It forms perpendicular to well-developed pressure solution seams where shortening occurs across the fracture surface and is composed of tubular structures known as slickolites. These structures tend to point in the direction of contraction and slickolites are thus a kind of lineation that is kinematically different from the other lineations.
Credits: Haakon Fossen (Structural Geology)

Lineations related to plastic deformation

Lineations related to plastic deformation

kacimo samy   3:38 ص

Plastic deformation lineations

Strain data can be represented in (a) the Flinn diagram (linear or logarithmic axes) or (b) the Hsu ¨ diagram. The same data are plotted in the two diagrams for comparison. 

Penetrative lineations are found almost exclusively in rocks deformed in the plastic regime. Where the lineation forms the dominating fabric element so that the S-fabric is weak or absent, the rock is classified as an L-tectonite. It can be seen from rocks with strain markers that most L-tectonites plot in the constrictional field of the Flinn diagram, i.e. X >> Y Z (Figure above). A balanced combination of a foliation (S-fabric) and a penetrative lineation (L-fabric) is more common, and such a rock is referred to as an LS-tectonite. LS-tectonites tend to plot close to the diagonal in the Flinn diagram. S-tectonites, which have no or just a hint of linear fabric, typically plot in the flattening field of the Flinn diagram.

Mineral lineations 

A penetrative linear fabric is typically made up of aligned prismatic minerals such as amphibole needles in an amphibolite, or elongated minerals and mineral aggregates such as quartz–feldspar aggregates in gneiss. Mineral lineations can form by several processes:

Minerals and mineral aggregates can form a linear fabric by means of recrystallization, dissolution/ precipitation or rigid rotation.

Physical rotation of rigid prismatic minerals in a soft matrix can in some cases occur during deformation. An example is amphibole or epidote crystals in micaschist, where statically grown amphiboles become aligned in zones of localized deformation. In most cases the competence contrast between elongate-shaped minerals and their matrix is not high enough for rotation to be important. Instead, synkinematic recrystallization by means of plastic deformation mechanisms or a dissolution/precipitation process reshapes minerals and mineral aggregates. In addition, precipitation of quartz in pressure shadows or strain shadows is a common way to facilitate growth of minerals or mineral aggregates in a preferred direction. Even crushing or cataclasis of brittle minerals and mineral aggregates enclosed in a ductile matrix can reshape mineral aggregates to linear fabric elements.

Cataclasis, pressure solution and recrystallization all contribute to change the shape of minerals and mineral aggregates during deformation.

In a homogeneously strained rock, if a mineral aggregate had a spherical shape at the onset of deformation, its shape after deformation would represent the strain ellipsoid. In most cases the original shape is unknown so that the final shape only gives us a qualitative impression of the shape of the strain ellipsoid. Nevertheless, deformed mineral aggregates in gneisses have been used for strain analysis, although the difference in viscosity between the aggregates and their surroundings may add uncertainty to the results. In cases where the initial shape is known and the competence contrast is small, such analyses are particularly useful. Deformed conglomerates or oolites are examples of rocks where linear shapes can quantitatively be related to strain. These and other lineations defined by the shape of deformed objects are named stretching lineations and the related fabric is called a shape fabric. Stretching lineations and shape fabrics are extremely common in plastically deformed rocks such as gneisses.

Stretching lineation in quartzite conglomerate. The long axes of the pebbles are plunging to the right. The Bergsdalen Nappes, West Norway Caledonides.

Stretching of minerals and mineral aggregates into a penetrative stretching lineation forms the most common type of lineation in deformed metamorphic rocks.

Quartz, calcite and some other common minerals can grow well-aligned fibrous crystals that define linear elements. Such mineral lineations are referred to as mineral fiber lineations. Fibers may grow in the instantaneous stretching direction (ISA1), but may also grow perpendicular to the face of the opening walls. Besides, once formed they may rotate away from this direction as a result of progressive deformation. In addition to their occurrence in veins in retrograde metamorphic rocks and un-metamorphosed sedimentary rocks such as over-pressured mudstones, they are commonly found in strain shadows of porphyroclasts in metamorphic rocks during low-grade metamorphism. Fibers do not form if pressure and temperature get too high, and seldom above middle greenschist-facies conditions. This rather restricted occurrence makes fiber lineations less common and also less penetrative than many other lineation types. 

Rodding describes elongated mineral aggregates that are easily distinguished from the rest of the rock. Quartz rods are common in micaschists and gneisses where striped quartz objects occur as rods or cigars in the host rock. Rods are often considered as stretching lineations, but are commonly influenced by other structure-forming processes. They may represent isolated fold hinges, or be related to boudinage or mullion structures, or to deformed veins with an originally elongated geometry.

Intersection lineations 

Intersection lineations appearing on bedding or foliation surfaces that are intersected by a later foliation.

Many deformed rocks host more than one set of planar structures. A combination of bedding and cleavage is a common example. In most cases such planar structures intersect, and the line of intersection is regarded as an intersection lineation. Where the first tectonic cleavage (S1) cuts the primary layering or bedding (S0), the resulting intersection lineation (L1) appears on the bedding planes. Intersection lineations formed by the intersection of two tectonic foliations are also common. In most cases intersection lineations are related to folding, with the lineation running parallel to the axial trace and the hinge line. Note that for transected folds  there will be an angle between the intersection lineation and the axial trace. 

In some deformed rocks an intersection lineation appears only locally. In most cases, however, their frequency and distribution are large enough that the lineation can be considered penetrative. Like other lineations, intersection lineations may be folded about later folds, showing their use in tracing the deformation history of a rock.

Fold axes and crenulation lineations 

Fold axes are generally regarded as linear structures, despite being theoretical lines related to the geometrical shape of the folded surface. Some rocks have a high enough density of parallel fold axes that they constitute a fabric. This is often the case with phyllosilicate-rich metamorphic rocks, where small-scale folds or crenulations constitute a crenulation lineation. Crenulation lineations are thus composed of numerous millimetre to centimetre-scale fold hinges of low-amplitude folds. They are commonly seen in multiply deformed phyllites, schists and in micaceous layers in quartz-schists, mylonites and gneisses. Crenulation lineations are closely associated with intersection lineations but are different in that they are comprised of fold hinges identifiable to the naked eye. During folding of layered rocks crenulation cleavages and crenulation lineations form at an early stage, while larger folds form later on during the same process. It is therefore of interest to compare the orientation of early crenulation lineations with related but slightly younger fold axes. If there is a difference in orientation this could be related to how layers rotated during deformation. We are already familiar with the concept of transected folds, where the lineation makes an angle to the hinge line.

Cleavage transecting the axial surface of a transected fold.

Boudinage 

Cylindrical pinch-and-swell structures (above) and boudins (below) represent linear elements in many deformed rocks.

Boudins are competent rock layers that have been stretched into segments. Individual boudins are commonly much longer in one dimension than the other two and thus define a lineation. Such linear boudins form where the X-axis of the strain ellipsoid is significantly larger than Y. Chocolate-tablet boudins can form when X Y. When occurring in folded layers, boudins typically appear on the limbs of the fold with their long axes oriented in the direction of the fold axis. 

Common connection between folding and boudinage. The fold hinges are thickened while the limbs are extended and boudinaged. The strain ellipse is indicated.

In general, boudinage structures are most easily recognized in sections perpendicular to the long axes of the boudins. Because of this fact they may be difficult to recognize as linear features in deformed rocks. It is also true that boudins are restricted to competent layers and therefore more restricted in occurrence than most other lineations.

Mullions 

Mullion is the name that structural geologists use for linear deformation structures that are restricted to the interface between a competent and an incompetent rock. The term mullion has been used in several different ways in the literature, ranging from striations on fault surfaces (fault mullions) to layer-interface structures formed during layer-parallel extension as well as contraction. We will relate the term to layer-interface structures where the viscosity contrast is significant. In such cases the cusp shapes of mullions always point into the more competent rock, i.e. the one with the higher viscosity at the time of deformation. 

Mullion structures form lineations at the interface between rocks of significantly different competence (viscosity).

Such mullions are closely related to buckle folds in the sense that their formation is predicted by a contrast in viscosity, they form by layer parallel shortening, and their characteristic wavelength is related to the viscosity contrast. But they differ from buckle folds in having shorter wavelengths and they are restricted to a single layer interface. A common place to find mullion structures in metamorphic rocks is at the boundary between quartzite and phyllite or micaschist. Mullions also occur on the surface of quartz pods in micaschists.

Pencil structures 

The formation of pencil structures occurs as a result of discrete interference between compaction cleavage and a subsequent tectonic cleavage, or between two equally developed tectonic cleavages. Pencil structures have a preferred orientation and form a lineation in un-metamorphosed and very low-grade metamorphic rocks.

Credits: Haakon Fossen (Structural Geology)