Fractures

Fractures

FRACTURE: is defined by Twiss and Moores (1992) as “..surfaces along which rocks or minerals have broken; they are therefore surfaces across which the material has lost cohesion”

Characteristics of fractures according to Pollard and Aydin (1988)

Ø fractures have two parallel surfaces that meet at the fracture front

Ø these surfaces are approximately planar

the relative displacement of originally adjacent points across the fractures is small compared to the fracture length.

Types of Fractures

Extensional Fracture

In extensional fractures the Fracture plane is oriented parallel to σ1 and σ 2 and perpendicular to σ 3.

Three types of fractures have been identified:

n Mode I fractures (joints) it is the extensional fractures and formed by opening with no displacement parallel to the fracture surface.

n Mode II and Mode III are shear fractures. These are faults like fractures one of them is strike -slip and the other is dip-slip

Same fracture can exhibit both mode II and mode III in different parts of the region.

* Conjugate  fractures : paired fracture systems, formed in the same time, and produced by tension or shear. Many of them intersect at an acute angle which will be bisected by the  

* Curved   fractures : occur frequently and may be caused by the textural and compositional differences within a thick bed or large rock mass or they may a result of changes in stress direction or analysis.

Characteristics of Fractures

n Plumose structure: is the structures formed on the joint surface during its propagation and provides information about the joint propagation direction.

Hackle marks: indicate zones where the joint  propagate rapidly.

Arrest line: forms perpendicular to the direction of propagation and is parallel to the advancing edge of fractures.

n 

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Faults

Faults

Faults defined as :  ruptures along which the opposite walls have moved past each other , the essential feature is differential movement parallel to the surface of the fracture.

Another definition :  it is a secondary planer or curve planar structure element ,formed due to tectonic processes  , it is a fracture which its displacement can be detected "measurable".

# Understanding faults is useful in design for long-term stability of dams, bridges, buildings and power plants. The study of fault helps understand mountain building.

 Faults may be hundred of meters or a few centimeters in length.# 

                                                              

Parts of the Fault                                                

1) Horizontal and vertical displacement.

2)  Fault plane: it's the plane where the opposite walls move about it.

3) Strike: it's the trend of horizontal line on the fault plane.

4)  Dip: it's the angle between a horizontal surface and the fault plane .

5)  Hade : it's the complement angle of the dip .

6)  Hanging wall : it's the block above the plane.

7)  Foot wall : it's the block blow the fault plane .

8)  Up thrown – down thrown side. 

     

 10) slip : Describes the movement parallel to the fault plane.

11) Dip slip: Describes the up and down movement parallel to the dip direction of the fault.

12) Strike slip: Applies where movement is parallel to strike of the fault plane

Oblique slip: Is a combination of strike slip and dip slip . 13)

14) Net slip (true displacement): Is the total amount of motion measured parallel to 

the direction of motion .

 

Natural of movement along the fault

1)  Translation and rotational movements:-

_ In the translational movement , no rotation of the blocks relative to each other , all straight lines on the opposite sides of the fault which were parallel before the displacements, are parallel afterwards.

_ in the rotational movements,  are those in which some straight lines on the opposite sides of the fault which were parallel before displacement are not parallel 

afterwards

Geometric classification :-

According to rake of net slip :   A_

1) strike slip fault : net slip =strike slip and dip slip equal to zero.

2) dip slip fault: net slip = dip slip and strike slip equal to zero.

3) Oblique slip fault: net slip =component of strike and dip slip.

B_ According to attitude of fault relative to adjacent beds:

1) Strike fault: which its strikes are essentially parallel to the strikes of the adjacent rocks.

2)bedding fault: which its strikes are parallel to the bedding planes.

3) Dip fault: which its strikes are essentially parallel to the directios of dip the adjacent rocks.

4) Oblique fault: is that one where its strikes obliquely to the strike of the adjacent 

rocks.

C_ According to fault pattern :

Such classification is based on the pattern on a map or cross section.

1)  Parallel faults: the have essentially the same dip and strike

2) en-echelon faults : are relatively short fault that overlap each other.

3) Peripheral faults : are circular faults that bound a circular area or part of a circular area.

4) radial faults : belong to a system of faults that radiate out from a point .

D_classification based on value of dip fault :

1) Low angle faults : dip less than45 o

2) high angle faults : dip is greater than 45 o and less than 90 o

3) Vertical faults: dip is equal to 90 o

E_ classification according to the apparent movement:

1) Normal fault : when the hanging wall has apparently gone down relative to the foot wall .

2) Reverse fault : is one in which the hanging wall has apparently gone up relative to 

the foot wall .

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Folds

folds
What is fold?

- Undulations or waves in rocks formed by stratified formation such as sedimentary and volcanic rocks or either metamorphosed equivalents.

1- Axial plane: the plane that divides the fold symmetrically as possible and includes hinge line

2- ¨Hinge line: line formed by the intersection of axial plane and curved part of fold

May be classified according to:

Shape (straight , curved , irregular)

Position (verticle , inclined , horizontal)

3- Limbs: the two sides of a fold that are divided by axial plane

4- Hinge point: the point of maximum curvature of a fold

5- Fold axis: the geometric lines on both sides of fold that are parallel to hinge line

6- Axial trace : the line produced by intersection of axial plane and any other plane

Based on shape: 

¨Anticline fold: convexing upward and has older rocks in the core

¨Syncline fold: concaving upward and has younger rocks in the core 

¨Synform  anticlinars e: it appesyncline but it has older rocks in core like anticline fold

•      Dome form: layers dip in all directions  away from the central point

•      Basin form: layers dip inward toward central point

•      Monocline: local steepening with homocline

•      chevorn

Based on tightness (Fleuty classification):

•      Gentle fold : 170 : 180

•      Open  fold   : 90 : 170

•      Tight fold     : 10 : 90

•      Isoclined fold : 0 : 10

Based on scale:

  folds can present in all scales:-

   1-Microscopic (require magnification)

   2-Mesoscopic  (specimen and outcrop size)

   3-Macroscopic(larger scale)

•    Fold orders:

First order : largest fold in given area

Second order : smaller folds found on the limbs of first order fold

Mechanism of fold

•      Folds : are the signatures of deformation forces that result from plate tectonic. The term fold implies that an originally planer structure, such as sedimentary bed, has been bent

•      In plate tectonic(convergence) continental with continental will not occur subduction but occur colloid to each other forming mountain range of folded rocks.                                        

•      Another example of convergence of plat

•      tectonic: oceanic with oceanic , in it will occur subduction among the two layers.

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Sedimentary Structures

Sedimentary Structures

Definition of primary sedimentary structures

:- it is that structure which formed at the phase of deposition and before solidification of rocks.

*Secondary structures:

1-It is that forms undergo deformation after solidification of rocks.

2- Rocks architectures that produced as a result of deformation of primary sedimentary structures/ distortion under effect of external processes.

Penecontemporaneous; means after deposition and before solidification of rock.

Internal sedimentary structure: it is that structure appears in completely horizontal planes called as bedding.

Definition of bedding: it is the basic structural unit in sedimentary succession, has an upper and lower surfaces, lateral extension. Each unit characterized by its own unique chemical and lithologic composition

Classification of bedding:

Bedding plane may be;

-Sharp/planar

-Irregular

-Wavey

Cross bedding classified according to the geometry of foreset as follows

1-Tabular/planar; foreset form acute angel with the upper and lower sets

2- Tangential; tangent to the top and bottom sets.

3- trough cross bedding; approximately parallel  to the sole of beding

Cross bedding classified according to the geometry of foreset as follows

1-Tabular/planar; foreset form acute angel with the upper and lower sets

2- Tangential; tangent to the top and bottom sets.

3- trough cross bedding; approximately parallel  to the sole of beding

Determination of depositional currents (direction)

 1- Wedge shape cross bedding

Wannwing current

2- Herring bone

Cross beds form as sand dunes or ripples migrate due to wind or water currents. Herringbone cross bedding forms when current periodically flows in opposing directions such as in a tidal flat or tidal  channel. The result is alternating layers of cross beds dipping in opposite directions. The pattern created resembles the backbone and ribs of a herring or fish.

3- Gradded bedding ;- progressive change in clast grain size from base to top

 - Normal grading:  fining from base to top (most common)

2- Reverse, or inverse, grading: coarsening from base to top

generally results from deposition from turbidity flows

5- Fissility

Fissility refers to the property of rocks to split along planes of weakness into thin sheets.^[1] This is commonly observed in shales, which are sedimentary rocks, and in slates and phyllites, which are foliated metamorphic rocks. The fissility in these rocks is caused by the preferred alignment of platy phyllosilicate grains due to compaction, deformation or new mineral growth

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