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

Mesoscopic faults in outcrop Fault trace in aerial photo

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

The most common types of faults are:

1. Dip-slip faults

• Normal (Listric)

• Reverse or Thrust (if dip angle <30º)

2. Strike-slip faults

3. Oblique-slip faults

4. Other faults: Scissors (Rotational).

Listric fault

Fault Scarp

Misleading Scarps(Fault-line scarp)

• 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

Thrust Sheet Diagram

• Window (fenster) shows of the autochthonautochthon through the eroded allochthonallochthon

• Klippe is a piece of allochthon surrounded by autochthon

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

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

DESCRIPTION OF FAULT DIP 0° →horizontal fault 0 -10 →sub horizontal fault=Detachment: a regional, low-angle, listric at depth and overthrust faults .10-30→shallowly dipping faults 30-60→modertly dipping fault 60-80→steeply dipping faults 80-90→sub vertical fault 90 →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 several components: Stratigraphic separation Horizontal separation ( from offset in map view) Dip separation (Heave and Throw) Strike separation Vertical Separation (from cross section)

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.

Dip Separation

Dip separation (D): The distance between the offset horizons measured in the dip direction.

Dip separation has two components;

1. Heave: Horizontal component of the dip separation

2. Throw: Vertical component of the dip separation

Strike separation (S): Distance between the offset horizons measured along the strike direction.

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.

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.

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

Flat and Ramp

Pressure and Temperature

Influence Faulting

• Changes due to burial depth• Breccia or gouge< 5 km• thin between 3-5 km and 10-15 km• Brittle faults end at 15 km depth• more wide at depth

• 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)

Age relationship between different faults and their

termination relation

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).

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.

• Cutoff: The point of intersection on either a map or cross-section.

Emergent Fault• Faults can also

terminate at the ground surface.

• The San Andreas fault does terminate at the ground surface, and is called an emergent fault.

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

Exhumed Faults

• Other faults, blind when they formed, may be exposed by erosion to become exhumed faults.

• Faults may ACTIVE or INACTIVE.

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 isD = C • Ln, with C =0.03, and n = 1.06Where 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

• 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.

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

• Newer pseudotachylyte injection vein cuts the older one.

Argille Scagliose

Photos

• Argille scagliose melange associated with obducted ophiolite.

Cataclasite photo

• 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.

Formation of Pits• Any step on the fault surface

subjected to pressure solution experiences more pressure than the areas around them.

Stylolites

• Restraining steps become pitted by pressure solution, forming stylolites.

• Releasing steps become the locus of grain growth.

San Andreas transform fault 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.

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

• Steps in the formation of a fault-bend fold

Detachment folds Drag fold, or drape folds

Folds in a shear zoneFault-Bend Folds

Fault systemsFault 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 systemsa. 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

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

• 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.

Normal Fault systems

2. Reverse Fault Systema. 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 systemFlower 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.

Negative flower structure

positive 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 SOCIETY1. Earthquakes

2. Mining

3. Oil Reservoirs

4. Groundwater