This lecture includes the brief description of types of fractures especially shear. contractional and tension fractures. Classification of faults

1. Amit K. mishra
Assistant Professor
Department of Geology
Banasthali Univrsity

2. • Strictly speaking, a fracture is any planar or
subplanar discontinuity that is very narrow in
one dimension compared to the other two and
forms as a result of external (e.g. tectonic) or
internal (thermal or residual) stress.
• Fractures are discontinuities in displacement and
mechanical properties where rocks or minerals
are broken, and reduction or loss of cohesion
characterizes most fractures.
• They are often described as surfaces, but at
some scale there is always a thickness involved.
• Fractures can be separated into shear fractures (slip surfaces) and opening or
extension fractures (joints, fissures and veins).
• In addition, closing or contraction fractures can be defined.

3. • A shear fracture or slip surface is a fracture along which the
relative movement is parallel to the fracture.
• The term shear fracture is used for fractures with small (mm- to
dm-scale) displacements, while the term fault is more
commonly restricted to discontinuities with larger offset.
• The term slip surface is used for fractures with fracture-parallel
movements regardless of the amount of displacement and is
consistent with the traditional use of the term fault.
• Fractures are commonly referred to as cracks in material
science and rock mechanics oriented literature.
Fractures are very narrow zones, often thought of as surfaces, associated with
discontinuities in displacement and mechanical properties (strength or stiffness)

4. • Extension fractures are fractures that show extension
perpendicular to the walls.
• Joints have little or no macroscopically detectable
displacement, but close examination reveals that most joints
have a minute extensional displacement across the joint
surfaces, and therefore they are classified as true extension
fractures.
• Extension fractures are filled with gas, fluids, magma or
minerals. When filled with air or fluid we use the term fissure.
• Mineral-filled extension fractures are called veins, while
magma-filled fractures are classified as dikes. Joints, veins
and fissures are all referred to as extension fractures.
Extension fractures

5. Contractional planar features
• Contractional planar features (anticracks) have
contractional displacements across them and are filled
with immobile residue from the host rock.
• Stylolites are compactional structures characterized by
very irregular, rather than planar, surfaces.
• Some geologists now regard stylolites as contraction
fractures or closing fractures, as they nicely define one of
three end-members in a complete kinematic fracture
framework together with shear and extension fractures.
• Such structures are known as anticracks in the
engineering-oriented literature.

7. • Faults are rupture along which the opposite walls
have moved past each other.
• The essential feature is differential movement
parallel to the surface of fracture.
• Some faults are only few inches long, and total
displacement is measured in fractions of an inch.
At the other extreme, there are faults that are
hundreds of miles long with a displacement
measured in miles and even tens of mile.

8. 1. Fault plane- The surface or plane along which two blocks
have moved relative to each other.
2. Fault zone- Some times fracture distributes over a wide zone,
known as fault zone (may range from centimeters to few
meters).
3. Hanging wall (block) and foot wall (block) - In case of
inclined fault plane the block lying above the fault plane is
hanging wall and the one below is foot wall. In normal fault
hanging wall down up and foot wall goes up. In reverse fault
the hanging wall goes up and foot wall goes down.
4. Up-throw and down-throw (block)- This term refers to the
relative movements between the blocks. The block which
seems to have gone up is the up-throw side and one which
appears to gone down is the down thrown block.

11. 5. Strike of fault plane- It is the line formed by the
intersection of the fault plane and imaginary
horizontal plane.
6. Dip of the fault plane- It is the angle between
the fault plane and imaginary horizontal plane.
7. Hade of fault – angle made between the fault
plane and vertical plane (obsolete term)
8. Net slip- It is the total displacement along a fault
and measured on the fault plane between the two
formerly attached points, now situated on
opposite blocks.

12. 9. Strike slip- Displacement parallel to the
strike of the fault plane.
10. Dip slip- Displacement parallel to the dip of
the fault.
11. Throw- Vertical component of displacement
of two formerly attached points, separated by
faulting.
12.Heave- Horizontal component of
displacement of two formerly attached points,
separated by faulting.

13. Classification of fault
• The faults are classified on the basis of their
geometric relationship with the fault plane and
their mode of origin.
1. Geometric classification
2. Genetic classification

14. Geometric classification of fault
Bases of classification
• Faults like joints are classified on the basis of their
geometry and their genesis.
• The bases of five different geometric classification
are-
I. The rake of the net slip
II. The altitude of fault relative to altitude of net slip
III. Pattern of fault
IV. Angle at which fault dips and
V. Apparent movement on the fault

15. 1. Strike slip fault: A strike slip fault is one in which
net slip is parallel to the strike of the fault; i.e. the
strike equals to the net slip and there is no dip slip
component. The rake of net slip is therefore zero.
They usually have very steep vertical dip.
• Sinistral (left lateral): If
opposite blocks have moved to
the left.
• Dextral (right lateral): If
opposite blocks have moved to
the right.

16. 2. Dip slip fault: A dip slip fault is the one in which the net slip is up
or down the dip of fault; i.e. the net slip is equal to the net slip and
there is no strike-slip component. Many types of dip slip fault
exists.
• Normal slip fault : If hanging wall goes down with respect to foot
wall.
• Reverse slip faults: In case of this type of fault the hanging wall
goes up with respect to the foot wall.
• Low angle normal slip fault: Many normal slip dip at moderate to
steep angle 60°-80°. Normal slip faults if dip around 30° or less
and hanging wall goes down, are known as low angle normal slip
faults or ‘Lags’.
• Low angle reverse slip faults: In case of there faults the fault plane
dips 30° or less and hanging wall goes up. These are also called
‘THRUSTS’.

19. 3. Diagonal slip fault/ oblique slip
faults: A diagonal slip fault is the
one in which blocks have move
diagonally and we found
components of the dip slip and
strike slip.
4. Rotational fault (Scissor like) :
These are scissor like faults which
seems to have rotated along some
axis. The magnitude of slip and
rotation changes all along the fault.
It may be normal slip for a part of
length and reverse slip for other
part of length.

21. Classification on the relationship with the
attitude of the bed
• Strike fault:Is the fault
which runs parallel to the
strike of the bed.
• Dip fault: Is the fault that
runs parallel to the dip of
the bed, cut across the
strike of the bed.
• Bedding fault : Is the
fault which is parallel to
the bedding of the rock i.e.
Displacement along or
parallel to a bedding plane.
NOTE : Dip slip and strike slip fault is the
movement along the dip or strike of the fault
plane. While in case of dip and strike fault is
the movement along the dip and strike of the
bed.

22. a. Parallel fault
b. Step fault
c. Horst
d. Graben
e. Radial fault
f. Peripheral fault
g. Enechelon fault

24. • Parallel Fault : A series of faults running more or
less parallel to one another and all handing in the
same direction, are called “parallel faults”
• Step Fault : It is consists of those parallel faults
where down throw of all are in the same direction
and it gives a step like arrangement.
• Graben or rift fault : When two normal faults
fade towards each other and the beds between
them are thrown down in the from of a wedge, the
structure is called graben or rift fault
• Horst : A horst consists of a central block on the
both sides of which adjacent beds appear to have
been faulted down

25. • Radial Fault : A number of faults exhibiting a
radial pattern are descried as radial faults.
• Peripheral fault : Curved faults of more or
less circular, or are like outcrops on level
surface are called peripheral faults.
• Enechelon fault are comparatively short faults
which overlap each other.

26. Classification based on attitude of
the fault relative to the attitude of
adjacent beds
• Longitudinal Fault : A longitudinal fault strikes parallel to
the strike of the reagional structure, abcd of fig in the next
slide .
• Along most of its course it is a strike, but locally the
adjacent rocks may strike at high angle to the fault.
• A Transverse fault strikes perpendicularly or diagonally to
the strike of a regional structure.
• Ef and gh in next slide.
• Along most of its course a transverse fault is dip or diagonal
fault but locally the adjacent rocks may strike parallel to the
fault.

28. Genetic Classification
• Most satisfactory genetic classification at
present is based on the nature of relative
movement along the fault. viz.
• Thrust fault
• Normal fault/ Gravity fault
• Strike- slip fault

29. Thrust Geometry
• Where thrusts affect a set of bedded rocks that are near-
horizontal in attitude, the thrusts generally follow a
staircase path made up of alternating ramps and flats
(Figure).
• The flats are where the thrust sheet slides along a
relatively weak bedding plane (often called a detachment
or decollement plane) and the ramps are sections where
the thrust cuts upwards through the stratigraphic sequence
at an angle of typically around 30° to the horizontal.
• In uninverted strata, thrust displacements of this type
always place older strata upon younger strata .

30. • Structures and terminology of
thrust zones. A. Shape of
thrust surface: ramps and
flats. B. Hangingwall
geometry: a fold in the
hangingwall must result from
a ramp. C. Piggyback thrust
sequence (new thrusts
develop in the footwall). D.
Overstep thrust sequence
(new thrusts develop in the
hangingwall). E. Structure of
a duplex: imbricate thrust
slices are contained between
a floor thrust and a roof
thrust. F. Pop-up structure
formed by backthrusting (see
text). G. Triangle zone
formed by backthrusting (see
text). (After Butler, RWH
(1983) Journal of Structural
Geology, 4, 239-45.)