Faults PDF

Secondary Structures Compiled By: Prof. Fathy Hassan  Secondary Structures - Deformation Structures ▪ Secondary structures - deformation structures produced by tectonic forces and other stresses in crust. ▪ Principle types: ▪ Fractures/joints ▪ Faults ▪ folds ▪ cleavage/foliati...

Secondary Structures Compiled By: Prof. Fathy Hassan  Secondary Structures - Deformation Structures ▪ Secondary structures - deformation structures produced by tectonic forces and other stresses in crust. ▪ Principle types: ▪ Fractures/joints ▪ Faults ▪ folds ▪ cleavage/foliation/lineation ▪ shear zones ▪ Secondary structures are of primary interest in structural geology Fractures and Joints ▪ Fractures – surfaces along which rocks have broken and lost cohesion ▪ Joints - fractures with little or no displacement parallel to failure surface ▪ indicate brittle deformation of rock Joints in younger Granites, Sinai, Egypt Joints Smooth, planar cracks that cause loss of cohesion of the rock, and upon which there has been almost imperceptible movement. They typically occur in families and swarms. Basic joints are tensional fractures. Their surfaces often have plumose structures that can indicate direction of crack propagation. Shear fractures form in response to a very slight shearing movement parallel to the plane of the fracture. Commonly found in conjugate sets, in rocks that have been folded or faulted. Fractures Joint Sets Multiple Joints in Sudr Chalk- Southern Sinai  Joint Sets Joint sets in younger Granite, Wadi Hawashia, Eastern Desert, Egypt Joint Sets Basic Dykes Cross-cutting Younger Granite of Wadi Hawashia, Eastern Desert  Columnar Joints Chevron Fold  Plumose Fracture Plumose structure or hackle plume , named for its resemblance to the shape of feather. Its is more clearly displayed in rocks of uniform fine-grained texture ex. Mudstone and chalk. The main joint face displays the hackle plume (subtle ridges and grooves) with hackle lines that diverge from an axis. Fringe faces are restricted to the edge of a joint surface. In some cases, curvilinear features called rib marks and ripple marks cross the lines of hackle on the fracture surface. The direction of divergence of the hackle lines is the direction in which the fracture propagated. The hackle is usually found to radiate from a single point. Rib marks are interpreted to be arrest lines where fracture propagation halted temporarily. Ripple marks are interpreted to form during very rapid fracture propagation. Plumose Fracture  Fracture Surface in Sudr Chalk, Sinai. Chevron Fold.. Plumose Fracture Faults Compiled By: Prof. Fathy Hassan  Faults Faults are fractures along which there has been visible offset by shear displacement, parallel to the fault surface. Fault zones are a system of closely spaced anastomosing fault surfaces. Faults are fundamentally brittle structures, they are discontinuites in rock, and the primary mechanism for achieving shear displacement at shallow levels. At deep levels (high P&T) faults grade into shear zones, which are ductile plastic equivalents of faults.  Faults Faults are discrete fractures or discontinuities along which some amount of offset occurred, in the plane parallel to the discontinuity. ▪ single fault plane ▪ fault zone - set of associated shear fractures ▪ shear zone - zone of ductile shearing Physical Character of Faults: Scarps Fault motion produces offset in both natural and man-made objects. The surface expression of a fault is a fault scarp. Scarps often do not mark the exact location of a fault, but rather may be modified by erosion, and therefore mark the approximate location of the fault. Even when the fault does not make it to the surface there might be a surface expression of the fault - perhaps a fold scarp. With the presence of a fault that juxtaposes 2 different lithologies there can be the appearance of significant vertical offset, when in fact it is the result of differential erosion. The original fault scarp is gradually replaced by a fault-line scarp. When planar fault scarps are dissected by erosion they sometimes yield triangular facets, which are the remnants of the fault plane - quite common on normal faults. Fault Scarp Triangular Facets – Wadi Matallah , Sinai - Egypt  Slickensides and Slickenlines Faults often have polished surfaces called slickensides. The ability of a surface to “take a polish” is probably a function of lithology. Some slickensided surfaces are shiny because of thin neomineral coatings (like chrome plating). Often slickenside surfaces will have variably developed striations or grooves upon them that are called slickenlines. Slickenlines are generally straight, fine-scale striations that record the direction of fault motion. Many slickenlines are striations caused by frictional abrasion during slip. Scratches and grooves are caused by motion past asperities, which are irregular bumps or protrusions on the fault surface. An asperity may break off and then ‘float’ along the fault, further scratching the fault surface. We often find streaks of fine-grained material which pile up in front and behind asperities.  Asperities and fault surface scratching Fault Terminology Slickensides ; Slickenlines & Fault Steps Naming of Faults: Slip & Separation ▪ Slip is the actual relative displacement on a fault. ▪ Separation is the apparent relative displacement on a fault. ▪ If we can establish the slip then we can name the fault. Strike-slip, dip-slip and oblique-slip faults - definitions ▪ Separation refers to the apparent sense and magnitude of relative displacement on the fault, regardless of the actual relative displacement. ▪ Its what we see without having to really think about what really happened (It is descriptive not interpretive). Fault Types Dip-slip Hanging Wall Strike-slip Hanging Wall Displacement, Slip and Separation The vector connecting two points that were connected prior to faulting indicates the local displacement vector or net slip direction (Figure 8.6). Ideally, a strike-slip fault has a horizontal slip direction, while normal and reverse faults have displacement vectors in the dip direction. A series of displacement vectors over the slip surface gives us the displacement field or slip field on the surface. 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 (Figure 8.7). 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). Displacement, Slip and 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. 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.  c a d b 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 ▪ A fault that affects a layered sequence will, in three dimensions, separate each surface (stratigraphic interface) so that two fault cutoff 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 cutoff 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  Dip slip Fault with oblique component Types of Faults ▪ There are different types of fault. These can be classified on the basis of the direction of displacement relative to the orientation of the fault plane at the time of displacement. ▪ Classified by relative motion 1. Normal Fault 1. Dip - Slip 2. Reverse Fault 2. Strike - Slip high angle low angle – (Thrust 3. Oblique - Slip Fault) Orientation of the main stress axes Fault types classified on the basis of the direction of displacement relative to the orientation of the fault plane. 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. Based on Angelier (1994).  Normal Faults Normal faults generally occur in places where the lithosphere is being stretched. Consequently, they are the chief structural components of many sedimentary rift basins (e.g. the North Sea) where they have major significance for hydrocarbon exploration. Most active normal faults can be shown to dip at angles steeper than 50 degrees, there are examples of very low- angle normal faults. These are often termed "detachments" - although this is a pretty vague term! Detachments show gentle dips and often expose high grade metamorphic rocks in their footwalls. These footwalls can be termed metamorphic core complexes. Normal faulting is now thought to be an important way in which metamorphic rocks come to be at the earth's surface today. Normal Fault Normal Fault Plunging Anticline Fossil Evidences  (a) Kink band ,where the bisecting surface, Normal Fault in Paleozoic i.e. the surface dividing the interlimb angle in two, is different from the axial surface. (b) Rocks, Southern Sinai, Egypt Chevron folds (harmonic). More fracturing along Normal Fault zone facilitates upward fluid migration and further communicatio n (fracture intensity)  Fracture intensity, consequently porosity and permeability increase towards fault zone  Chevron Deformation in the footwall of the Markha Fold transfer fault, Southern Sinai, Egypt. Interpretation sketch after McClay , 2008. 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 horst (a), symmetric graben (b) and asymmetric graben (c), also known as a half-graben. Antithetic and synthetic faults are shown. Physical model of horst and graben structures Horst & Graben Three stages in rift development. a Plunging vs. (a) Early extension creating or rejuvenating deep-going fractures. Strain is low at this stage, and non- plunging magma locally fills deep fractures as dikes. (b) The stretching phase, during which major fault complexes and arrays form. Synrift sediments not shown. (c) Postrift subsidence and sedimentation. Compactional b faults in postrift sequence due to differential compaction. Three stages in rift development. (a) Early extension creating or rejuvenating deep-going fractures. Strain is low at this stage, and magma locally fills deep fractures as dikes. (b) The stretching phase, during which major fault c complexes and arrays form. Synrift sediments not shown. (c) Postrift subsidence and sedimentation. Compactional faults in postrift sequence due to differential compaction. Red Sea Rift Valley Reverse -Thrust Faults Thrusts are reverse faults and commonly dominate the structure of collision mountain belts. Some thrusts have moved a long way - many mountain belts have thrusts that have moved many tens of kilometers. The regional rock packages that make up thrust belts are called thrust sheets. They are allochthonous - meaning that they are not in their original position and underlain by faults. The rocks that the thrust sheets move upon are autochthonous - in place. Regional thrusts separate allochthons from autochthons.  Regional thrust autochthons Reverse-Thrust Faults Reverse - Thrust Faults  Reverse Fault Drag folds on a subvertical reverse High- angle Reverse fault in Entrada Fm., Grand Staircase- fault reverse fault Escalante National Monument, Utah ▪ The main, regional zone that separates allochthonous from autochthonous a rocks is called the basal detachment, or decollement (although technically decollement is a process). ▪ A basal detachment is a discrete fault, and a decollement is a zone of shearing on some sort b of weak layer (i.e., salt, shale, etc.). ▪ Ramps are places where the fault cuts up-section (generally at an angle around 30°). ▪ Flats are where the hanging wall moves horizontally, generally along a bedding plane. ▪ The consequences of moving up a c staircase of ramps and flats. ▪ Folds that form above the thrust ramps are called ramp anticlines or fault-bend folds.  Klippe ▪ An isolated thrust block is called Klippe ▪ The term nappe refers to allocthonous thrust sheets (thrust nappe), often recumbent to Proterozoic overturned folds (fold nappe). Nappe is a term mostly used in Cretaceous Europe, born from Nappe Alpine geologists... Fault-bend fold ▪ Classical produce fault- bend fold very simple structures where the bed dips simply reflect the angle of the ramps through the stratigraphy. ▪ If beds were horizontal before thrusting then ramp angles are commonly less than 30 degrees to bedding. ▪ Consequently fault bend folds should be open antiforms. ▪ Furthermore, the footwall should remain undeformed.  Simple Fault-bend Fold Model Ramp-flat geometry allows us to define a few different fault-contact types Fault-bend fold & Fault- Propagation Fold Hope's Nose tip- line fold , Devon. The bedding in the hanging wall defines a tight fold with over-steepened bedding cut-offs. Special Folds ▪ Fault Related Folds ▪ Fault Bend – generated by fault curvature ▪ Fault Propagation – generated by fault terminations on a ramp Fault Bend ▪ Detachment Folds – Generated by fault terminations on a flat. Fault Propagation Thrust Faults: Definitions Imbricate fans ▪ Horses: fault-bounded slices of rock, carried along and within the fault zone. ▪ There are 2 fundamental arrangements of thrusts: Duplex ▪ 1) Imbricate fans: marked by a sole (basal) thrust that has several branching splays off of it - the ‘fan’ opens upward, and; ▪ 2) Duplexes: like an imbricate fan, but instead of the faults splaying upward, they are cut by another fault - the roof thrust. In this case, the basal thrust is called the floor thrust.  Normal duplexes Antiformal duplexes Forwarded-dipping duplexes Duplex Types Duplex Types ▪ As with imbricate fans, you can have different types of duplexes, depending on when the new horses are formed relative to the displacement of the older horses. ▪ a) If new horses are formed at the front (in the slip direction), the older horses are tilted to the back and you get a hinterland-dipping duplex. This is the most common of the two types. ▪ b) However, if the new horse only forms after the older horses have moved over it, one gets a foreland-dipping duplex. Thrust belts ▪ Thrust belts commonly mark the outer edge of collision mountain ranges such as the Alps and Himalayas. In these situations the thrusts don't just appear on their own but in herds! The thrusts can interact to make wonderfully complicated cross-sections. ▪ In general thrusts are directed outward, away from mountain ranges. But sometimes thrusts can be found, directed back towards the mountain Back thrusts generated above a ramp in the ranges - these are termed "back- Caledonian sole thrust. The main thrusting direction is toward the right. North of Oslo, thrusts". Norway. Based on Morley (1986). Inversion tectonics The reuse of faults during contraction is often best expressed when normal faults are reversed. This phenomenon, and the opposite (reverse faults being reactivated as normal faults) is commonly referred to as inversion. Normal faults tend to be steeper than their contractional counterparts, particularly in their upper part. This makes them less favourable for reactivation when σ1 is switched to horizontal. Contraction therefore commonly results in more gently dipping shortcut faults, particularly in the Example of inversion model, upper and steepest part of normal faults. Contractional faults shown in red. Based on structures mapped in northern Chile by Amilibia et al. (2008). Null Point Inversion tectonics Inverted Basin East Razzak Area, Western Desert, Egypt  The main tectonic trends and structural elements of Egypt (modified and simplified after Youssef 1968) o 1- NW (Red Sea or Suez trend or Erytherean trend) o 2- NNE (Aqaba trend) o 3- E – W (Tethyan trend) o 4- N-S (Nubian or East African trend) o 5- W - NW (Darag trend) o 6- E– NE (Syrian Arc trend) o 7- NE (Aualitic or Tibesti trend)  Strike-slip Faults Strike-slip faults include some of the world's most famous - or infamous structures, including the San Andreas Fault system and the North Anatolian Fault system. Both of these are renowned for devastating earthquakes. the Strike-slip faults are those where relative displacement is parallel to the strike of the fault. Strike-slip fault zones are commonly, but by no means exclusively, steep and can be rather difficult to recognize on cross-sections. commonly However, active strike-slip faults are associated with spectacular tectonic landforms, such as the narrow basin and abrupt range-edge seen in the photograph. Strike-slip Faults Strike-slip faults that connect half-grabens of Opposite polarity are a kind of transfer fault. Such transfer Faults are common in rifts.  Fault bends and Stepovers When individual fault segments overlap and link in map view, a fault stepover or fault bend forms. Contractional or extensional structures form in such bends, depending on the sense of slip on the fault relative to the sense of stepping. Contractional structures include stylolites, cleavages, folds and reverse faults, and form in restraining bends. The restraining bend in Figure 18.13 is located where a sinistral fault steps to the right. Subparallel reverse or oblique-slip contractional faults bounded by the two strike-slip segments can form and are called contractional strike-slip duplexes.  Extensional duplex (transtension) and contractional duplex (transpression) developed at bends or stepovers along a strike-slip fault system. Large-scale examples may lead to basin formation and local orogeny. Fault Bends Fault bends and Stepovers ▪ Bends along strike-slip faults are exciting structures that contain extensional or contractional structures depending on the sense of slip and stepping (right or left). Releasing bends form where a sinistral strike-slip fault steps to the left, or a dextral fault steps to the right. Such bends produce extensional structures such as normal faults and extension fractures. Series of parallel extensional faults bounded on both sides by strike-slip faults, are called extensional strike-slip duplexes. The Dead Sea is a famous example of such a basin, created in an overlap zone between two strike-slip transforms. Releasing- bend basins along strike-slip faults are called pull-apart basins.  Structures in Bends Fault Pull-apart Bends basin Pop-up structure Flower Structures  Flower Structures A characteristic feature of bends along strike slip faults is their tendency to split and widen upward. These structures are called flower structures. Flower structures that are associated with restraining bends are called positive, and those associated with releasing bends are called negative flower structures. Strike-slip faults thus bifurcate and widen toward the surface, particularly in restraining and releasing bends.  Transpression and Transtension We have seen that bends in strike-slip faults can produce local components of contraction or extension. The type of deformation occurring in such bends is referred to as transpression and transtension. In general, transpression is the spectrum of combinations of strike-slip and coaxial strain involving shortening perpendicular to the zone, and transtension encompasses the combinations of strike-slip and perpendicular extension.  Transpression and transtension connect contraction, strike-slip and extension. Oblique – Slip Fault Dip - Slip and Strike - Slip movement  k an Th