# FAULTS AND FAULTING Dr. Masdouq Al-Taj

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FAULTS AND FAULTING Dr. Masdouq Al-Taj
CHAPTER 8 FAULTS AND FAULTING Dr. Masdouq Al-Taj

FAULTS A fault is any surface or zone in the Earth across which measurable slip (shear displacement) develops. Faults are fractures on which slip develops primarily by brittle deformation processes. Fault zone is a brittle structure in which loss of cohesion and slip occurs on several faults within a band of definable width. Shear zone: occurs at depth without definable displacement on the surface

We used four relative scales of observations
- Micro: optical scale (microscope or even electron microscope). - Meso: single outcrop (personal scale). - Macro: regional scale (mountain range). - Mega: continental scale (plate dimensions).

Fractured feldspar grain in photomicrograph
Fault trace in aerial photo Mesoscopic faults in outcrop

Fault components Rocks adjacent to the fault surface is the wall of the fault, and the body of rocks that moved as consequence of slip on the fault is a fault block. If the fault is not vertical, we can distinguish between the hanging-wall block, which is the rock body above the fault plane, and the footwall block, which is the rock body below the fault plane.

Foot wall and Hanging Wall

HOW TO DESCRIBE THE ATTITUDE OF FAULT
We need to measure: Strike Dip angle, dip direction and hade Net slip vector (rake) Strike-slip component Dip-slip component (Heave and throw)

Foot wall and Hanging Wall

Note that the rake angle is measured from the horizontal to the direction of net-slip on the fault plane

Fault types 1. Dip-slip faults Normal (Listric)
The most common types of faults are: 1. Dip-slip faults Normal (Listric) Reverse or Thrust (if dip angle <45º) 2. Strike-slip faults 3. Oblique-slip faults Other faults: Scissors (Rotational).

listric fault

If the fault moves rock of much different strength together, differential erosion may create a fault scarp. Such scarps may have dips opposite to that of the underlying fault.

Concepts of extensional and contractional faults

Reverse fault

Thrust Sheet Diagram Window (fenster) shows of the autochthon through the eroded allochthon Klippe is a piece of allochthon surrounded by autochthon

Window (Fenster) Thrust faults are often thin sheets, and erosion may open holes in them A hole through a thrust sheet is called a fenster, or window Fenster: An eroded area of a thrust sheet that displays the rocks beneath the thrust sheet Triangular teeth point outward fenster are used on a map

Klippe If erosion leaves an isolated remnant of thrust sheet, surrounded by exposed footwall, the remnant is called a klippe (German for cliff) Klippe are indicated on a map by inward pointing teeth

Definitions Autochthon: A body of rocks that remains at its site of origin, where it is rooted to its basement. Although not moved from their original site. Allochthon: A mass of rock that has been moved from its place of origin by tectonic processes, as in a thrust sheet Many allochthonous rocks have been moved so far from their original sites that they differ greatly in facies and structure from those on which they now lie

DESCRIPTION OF FAULT DIP
0° →horizontal fault 0 -10 →sub horizontal fault=Detachment:A regional, low-angle, listric normal fault formed during crustal extension 10-30→shallowly dipping faults 30-60→modertly dipping fault 60-80→steeply dipping faults 80-90→sub vertical fault →vertical fault

SEPARATION The distance between the separated parts of the marker horizon is the separation, which is not the same as the net slip unless the line along which separation is measured happens to parallel the net-slip vector.

Components of Separation
Separation can be divided into seven components: Stratigraphic separation Heave Throw Strike separation Vertical Separation Horizontal separation Dip separation

Stratigraphic Separation
Offset measured perpendicular to bedding

Horizontal Separation
Horizontal separation (H) - Offset measured in a horizontal direction along a line perpendicular to the offset surface.

Vertical Separation Vertical separation (V) - Distance between two points on the offset bed as measured in a vertical direction If borehole data is used, it is vertical separation that is measured between two parts of an offset marker horizon.

Dip Separation Dip separation (D): The distance between the offset horizons measured in the dip direction Strike separation (S): Distance between the offset horizons measured along the strike direction.

Components of Separation
Dip separation has two components; 1. Heave: Horizontal component of the dip separation 2. Throw: Vertical component of the dip separation Strike separation: Distance between the offset horizons measured along the strike direction.

Change in Fault attitude (Fault bends)
Fault bends or steps along strike-slip faults cause abrupt changes in the strike of the fault and in the associated structural features. Where movement across a segment of a strike-slip fault results in some compression, we say that transpression (restraining bends) is occurring across the fault (form Pressure ridges); But where movement results in some extension, we say that transtension (releasing bends) is occurring across the fault (form sag pond (local area) or pull-apart basin (regional scale, example is the Dead Sea).

rele

San Andreas Fault Ridge
Ridge created by transpression along the fault Striped white and gray rocks are basement rocks pushed up relative to dark sedimentary cover.

Sag Pond Prominent scarps with sag ponds are found along the Denali fault trace. The ground is weakened on the fault trace and has the tendancy to sag and erode more easily than surrounding land. Image:

Change in attitude vertically
Fault segments may parallel bedding in either the footwall or hanging wall, but cut across bedding in the opposite block.

Bedding and Fault Plane Orientation
Fault segments may parallel bedding in either the footwall of the hanging wall, but cut across bedding in the opposite block.

Pressure and Temperature Influence Faulting
Changes due to burial depth Shallow faults, < 5 km Intermediate, between 3-5 km and km Brittle faults end at 15 km depth

Age relationship between different faults and their termination relation
Like joints, fault must terminate, and can do so in several different ways The Principle of Cross-Cutting Relationships can be used to determine the relative ages. A fault may terminate where it has been cut by a younger structure, such as another fault (C & D), an unconformity (E), or an intrusion (B), or at the ground surface (A)

Representation of Faults on map and cross section.

Cutoffs Faults which cross geologic contacts will displace the contact, unless the net-slip vector is exactly parallel to the fault-contact intersection The point of intersection on either a map or cross-section is called a cutoff.

Death of a Fault Faults can also split, to form an anastamosing array, which may merge and diverge several times along its length A fault splay may develop, with the fault splitting and dying out – these are called horsetails (B) A fault dies when its displacement becomes less and less, finally reaching zero near the tip, in a zone of plastic deformation (C).

Emergent Fault Faults can also terminate at the ground surface, or appear to The San Andreas fault does terminate at the ground surface, and is called an emergent fault. Image: San Andreas Carrizo dis01bigb.jpg

Exhumed Faults Other faults, blind when they formed, may be exposed by erosion to become exhumed faults. It may ACTIVE or INACTIVE.

Blind Faults Blind faults are faults that terminated before reaching the surface.

Blind Fault Effects Blind faults, by definition, do not directly affect the surface Nevertheless, surface elevations can be changes, as monoclinal folds

Fault Length and Displacement
This is a general relationship, supported by research within the last two decades. The longer the fault, the greater the displacement The best fit to the data is D = C • Ln, with C =0.03, and n = 1.06 Where D=displacement, L= fault length, C is a constant, and n is called the fractal dimension.

Prediction of Fault Length or Displacement
In (a) the offset of XX’ is small. As the fault grows with time (from t1 to t2), the offset of XX’ increases.

Slickensides and slip lineation
Slickensides are the fault surfaces features that have been polished and scratch (groove lineation, striations) by the process of frictional sliding.

Fault Breccia

Indurated Breccia Photomicrograph
Photomicrograph of fault breccia in the Antietam Formation, Blue Ridge province Breccias form when rocks are extensively fractured in fault zones and are cemented together when minerals precipitate in the cracks and fractures Image: Note the angular fragments (fr) of quartz sandstone in a matrix of fine-grained iron oxide cement (ic) Field of View 4 x 2.7 mm, Cross Polarized Light

Fault Gouge Photo Continued movement along the fault may form gouge.
Image:

Pseudotachylyte Photo
Silicate rocks are excellent insulators, and heat generated by friction does not escape Temperatures in excess of 1000ºC are possible Tachylyte is a type of volcanic glass, and the prefix pseudo means false, so the name literally means false volcanic glass Image: Newer pseudotachylyte injection vein cuts the older one.

Argille Scagliose Photos
Argille scagliose melange associated with obducted ophiolite. Image top: Image bottom:

Cataclasite photo Image: Foliated cataclasite in the core of the San Gabriel fault, San Andreas System, California.

Slip Fibers Slip fibers on fault surface
Note Brunton compass for scale Steps indicate sense of shear.

Quartz Fibers Quartz fibers in ductile shear zone.
Image: Quartz fibers in ductile shear zone.

Formation of Pits Any step on the fault surface subjected to pressure solution experiences more pressure than the areas around them.

Slickolites Restraining steps become pitted by pressure solution, orming styolites. Releasing steps become the locus of grain growth.

San Andreas and Subsidiary Faults
San Andreas Fault to left; Hayward Fault to right of SF Bay

Clay Experiment We can model the situation by placing a clay layer over two wooden blocks, and then moving one block opposite the other, as shown in the figure The clay will accommodate some of the strain, but will then rupture.

Formation of Reidel Shears
The first fractures are short, shear fractures inclined to the trace of the through-going fault They are called Reidel shears, and generally occur as a conjugate pair The acute bisectrix of the Reidel shears gives the local orientation of σ1.

Fault-Related Folding
We have several types of fault-related folding: Fault-propagation folds. Fault-bend folds Folding accompanies faulting (in fault zone) Detachment folds Drag fold, or drape folds or forced folds

Fault-propagation folds. Development of Folds
1. Stages in development of folds, leading to a fault. 2. This might reflect simply an increase in the regional strain rate, or it might reelect a “lock-up” 3. Lock-up means that the folds reach a point where continued folding is very difficult

Fault-Propagation Fold Photo
Fault propagation fold in Mesozoic sedimentary rocks in the Salt Range, northern Pakistan. Image:

Fault-Bend Folds A bend in the fault surface may cause folding of strata that move past the bend The moving layers must accommodate the bend, without gaps or overlaps Folds that form in this manner are called fault-bend folds They develop in association with all kinds of faults, but have been most studied in dip-slip faults.

Diagram of Fault-Bend Fold Development
Image: Steps in the formation of a fault-bend fold

Fault-Bend Folds Folds in a shear zone Detachment folds Drag fold, or drape folds

3. Strike-slip Fault systems
1. Normal Fault systems 2. Reverse Fault systems 3. Strike-slip Fault systems

Types of fault Arrays a. Parallel array b. Anastamosing array
c. en echelon array d. Relay array e. Conjugate array f. Random array

1.Normal Fault systems a. Half-Graben Blocks
Rotation of the hanging-wall block tilts the surface of the hanging-wall toward the fault, which creates a half-graben Half-graben blocks are bounded by a fault on one side only.

Basin and Range Province
In the Basin and Range Province, most of the blocks are half-grabens The ranges are the tilted tips of the fault blocks Image:

Bear Island, Norway Half graben tilting: Beds dipping about 30º to the west. Cross section indicates the dip is most likely due to half-graben development

b. Horst and Graben Normal Fault systems
When two adjacent normal faults dip toward each other, the central block slides down to form a graben The remaining high ground in between is called a horst This type of faulting is common in rift systems.

2. Reverse Fault System a. Imbricate Fan
Thrust fault arrays are usually either parallel or relay arrays If there is no upper confining layer, an imbricate fan forms These faults die out up dip.

2. Reverse Fault System b. Duplex Structures
If an upper and lower confining layers are part of the thrust, the intermediate lays form duplex structures, where the thrust spans the gap between the lower and upper thrusts sheets The lower confining layer is the floor thrust, and the upper confining layer is the roof thrust.

3. Strike-slip system Flower Structures
Their cross-sectional view looks like the head of a flower, so they are called flower-structures. They are of two types positive flower structure and negative flower structure.

Relation of Faulting to Stress
If faults initiate as Coulomb shear fractures, they will form at about 30° to the σ1 direction and continued at the σ2 direction. The ratio of shear stress to normal stress on planes orientated at about 30 ° to σ1 is at a maximum.

FAULTS AND SOCIETY 1. Earthquakes

2. Mining

3. Oil Reservoirs

4. Groundwater