Presentation on theme: "FAULTS AND FAULTING Dr. Masdouq Al-Taj"— Presentation transcript:
1FAULTS AND FAULTING Dr. Masdouq Al-Taj CHAPTER 8FAULTS AND FAULTINGDr. Masdouq Al-Taj
2FAULTSA 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
4We 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).
5Fractured feldspar grain in photomicrograph Fault trace in aerial photoMesoscopic faults in outcrop
6Fault componentsRocks 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 distinguishbetween the hanging-wall block, which is the rockbody above the fault plane, and the footwall block,which is the rock body below the fault plane.
11Note that the rake angle is measured from the horizontal to the direction of net-slip on the fault plane
12Fault types 1. Dip-slip faults Normal (Listric) The most common types of faults are:1. Dip-slip faultsNormal (Listric)Reverse or Thrust (if dip angle <45º)2. Strike-slip faults3. Oblique-slip faultsOther faults: Scissors (Rotational).
19Misleading 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.
28Thrust Sheet DiagramWindow (fenster) shows of the autochthon through the eroded allochthonKlippe is a piece of allochthon surrounded by autochthon
29Window (Fenster)Thrust faults are often thin sheets, and erosion may open holes in themA hole through a thrust sheet is called a fenster, or windowFenster: An eroded area of a thrust sheet that displays the rocks beneath the thrust sheetTriangular teeth point outward fenster are used on a map
30KlippeIf 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
31DefinitionsAutochthon: 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 sheetMany 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
34DESCRIPTION OF FAULT DIP 0° →horizontal fault0 -10 →sub horizontal fault=Detachment:A regional, low-angle, listric normal fault formed during crustal extension10-30→shallowly dipping faults30-60→modertly dipping fault60-80→steeply dipping faults80-90→sub vertical fault→vertical fault
35SEPARATIONThe 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.
36Components of Separation Separation can be divided into seven components:Stratigraphic separationHeaveThrowStrike separationVertical SeparationHorizontal separationDip separation
37Stratigraphic Separation Offset measured perpendicular to bedding
38Horizontal Separation Horizontal separation (H) - Offset measured in a horizontal direction along a line perpendicular to the offset surface.
39Vertical SeparationVertical separation (V) - Distance between two points on the offset bed as measured in a vertical directionIf borehole data is used, it is vertical separation that is measured between two parts of an offset marker horizon.
40Dip SeparationDip separation (D): The distance between the offset horizons measured in the dip directionStrike separation (S): Distance between the offset horizons measured along the strike direction.
41Components of Separation Dip separation has two components;1. Heave: Horizontal component of the dip separation2. Throw: Vertical component of the dip separationStrike separation: Distance between the offset horizons measured along the strike direction.
42Change 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).
44San Andreas Fault Ridge Ridge created by transpression along the faultStriped white and gray rocks are basement rocks pushed up relative to dark sedimentary cover.
45Sag PondProminent 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:
46Change in attitude vertically Fault segments may parallel bedding in either the footwall or hanging wall, but cut across bedding in the opposite block.
47Bedding 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.
49Pressure and Temperature Influence Faulting Changes due to burial depthShallow faults, < 5 kmIntermediate, between 3-5 km and kmBrittle faults end at 15 km depth
50Age relationship between different faults and their termination relation Like joints, fault must terminate, and can do so in several different waysThe 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)
51Representation of Faults on map and cross section.
52CutoffsFaults which cross geologic contacts will displace the contact, unless the net-slip vector is exactly parallel to the fault-contact intersectionThe point of intersection on either a map or cross-section is called a cutoff.
53Death of a FaultFaults can also split, to form an anastamosing array, which may merge and diverge several times along its lengthA 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).
55Emergent FaultFaults can also terminate at the ground surface, or appear toThe San Andreas fault does terminate at the ground surface, and is called an emergent fault.Image: San Andreas Carrizo dis01bigb.jpg
56Exhumed FaultsOther faults, blind when they formed, may be exposed by erosion to become exhumed faults.It may ACTIVE or INACTIVE.
57Blind FaultsBlind faults are faults that terminated before reaching the surface.
58Blind Fault EffectsBlind faults, by definition, do not directly affect the surfaceNevertheless, surface elevations can be changes, as monoclinal folds
59Fault Length and Displacement This is a general relationship, supported by research within the last two decades.The longer the fault, the greater the displacementThe 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.
60Prediction 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.
61Slickensides and slip lineation Slickensides are the fault surfaces features that have been polished and scratch (groove lineation, striations) by the process of frictional sliding.
66Indurated Breccia Photomicrograph Photomicrograph of fault breccia in the Antietam Formation, Blue Ridge provinceBreccias form when rocks are extensively fractured in fault zones and are cemented together when minerals precipitate in the cracks and fracturesImage: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
67Fault Gouge Photo Continued movement along the fault may form gouge. Image:
68Pseudotachylyte Photo Silicate rocks are excellent insulators, and heat generated by friction does not escapeTemperatures in excess of 1000ºC are possibleTachylyte is a type of volcanic glass, and the prefix pseudo means false, so the name literally means false volcanic glassImage:Newer pseudotachylyte injection vein cuts the older one.
70Cataclasite photoImage:Foliated cataclasite in the core of the San Gabriel fault, San Andreas System, California.
71Slip Fibers Slip fibers on fault surface Note Brunton compass for scaleSteps indicate sense of shear.
72Quartz Fibers Quartz fibers in ductile shear zone. Image:Quartz fibers in ductile shear zone.
73Formation of PitsAny step on the fault surface subjected to pressure solution experiences more pressure than the areas around them.
74SlickolitesRestraining steps become pitted by pressure solution, orming styolites.Releasing steps become the locus of grain growth.
75San Andreas and Subsidiary Faults San Andreas Fault to left; Hayward Fault to right of SF Bay
76Clay ExperimentWe 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 figureThe clay will accommodate some of the strain, but will then rupture.
77Formation of Reidel Shears The first fractures are short, shear fractures inclined to the trace of the through-going faultThey are called Reidel shears, and generally occur as a conjugate pairThe acute bisectrix of the Reidel shears gives the local orientation of σ1.
79Fault-Related Folding We have several types of fault-related folding:Fault-propagation folds.Fault-bend foldsFolding accompanies faulting (in fault zone)Detachment foldsDrag fold, or drape folds or forced folds
80Fault-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
81Fault-Propagation Fold Photo Fault propagation fold in Mesozoic sedimentary rocks in the Salt Range, northern Pakistan.Image:
82Fault-Bend FoldsA bend in the fault surface may cause folding of strata that move past the bendThe moving layers must accommodate the bend, without gaps or overlapsFolds that form in this manner are called fault-bend foldsThey develop in association with all kinds of faults, but have been most studied in dip-slip faults.
83Diagram of Fault-Bend Fold Development Image:Steps in the formation of a fault-bend fold
89Fault-Bend FoldsFolds in a shear zoneDetachment foldsDrag fold, or drape folds
903. Strike-slip Fault systems 1. Normal Fault systems2. Reverse Fault systems3. Strike-slip Fault systems
91Types of fault Arrays a. Parallel array b. Anastamosing array c. en echelon arrayd. Relay arraye. Conjugate arrayf. Random array
921.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-grabenHalf-graben blocks are bounded by a fault on one side only.
93Basin and Range Province In the Basin and Range Province, most of the blocks are half-grabensThe ranges are the tilted tips of the fault blocksImage:
94Bear Island, NorwayHalf graben tilting: Beds dipping about 30º to the west.Cross section indicates the dip is most likely due to half-graben development
95b. Horst and Graben Normal Fault systems When two adjacent normal faults dip toward each other, the central block slides down to form a grabenThe remaining high ground in between is called a horstThis type of faulting is common in rift systems.
962. Reverse Fault System a. Imbricate Fan Thrust fault arrays are usually either parallel or relay arraysIf there is no upper confining layer, an imbricate fan formsThese faults die out up dip.
972. 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 sheetsThe lower confining layer is the floor thrust, and the upper confining layer is the roof thrust.
1003. 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.
102Relation 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.