Chapter 6 - Geological Structures
-
Upload
zulaikha-kamal -
Category
Documents
-
view
227 -
download
1
Transcript of Chapter 6 - Geological Structures
-
8/13/2019 Chapter 6 - Geological Structures
1/40
Chapter 6: Geological Structures
-
8/13/2019 Chapter 6 - Geological Structures
2/40
INTRODUCTION
Over the past thousand million years of Earth history the crust of the Earth has beenmobile.
As a consequence many of the rocks that we see nownear of at the surface, nomatter what their origin have been squashed, stretched orfractured; they have beendeformed.
Deformation arisesbecause large parts of the Earth(lithospheric plate) have beenmoving relative to each other throughout geological time.
The movementof these platesgenerate stressesthat lead to both compression(collide) and tension(break apart or stretched).
The rocks comprising the crustrespond to such stressesby undergoing changes of
shape (strain), therefore various geological structuresare developedwhich provide arecord of type of deformation.
Compressional, tensional and shearing forcesacting on rocks may cause them toform:
(a) Fold
(b) Fractures(c) Joints
-
8/13/2019 Chapter 6 - Geological Structures
3/40
Deformation of rocks
There are threedifferent kind of stress (Table 1):
(1) Compression
(2) Tension
(3) Shear
Every rock has a limitbeyond which it cannot continue to respond tostress by bendingand will therefore fractureas (Table 2):
(1) Brittle
(2) Ductile
The factorsthat govern the way a rock deformare as follows (Table 3):
(1) Confining pressure and temperature
(2) Time over which the stress is applied
-
8/13/2019 Chapter 6 - Geological Structures
4/40
Table 1
Stresses Description
Compression compressor squeezethe rock bodysuch as
in theconvergent tectonic plate.
Tension forces pullingthe rock apartsuch as in the
divergent of the continent.
Shear results from forces acting parallelbut in
opposite directionssuch as in folds.
-
8/13/2019 Chapter 6 - Geological Structures
5/40
Table 2
Fracture Description
Brittle rocks which simply breaksrather thandeform
plasticallyby application of stress.
Ductile undergo considerable smooth deformationbeforethey rupture.
-
8/13/2019 Chapter 6 - Geological Structures
6/40
Table 3
Factors Description
Confining
pressure and
temperature
Rock may behave in a brittlemanner when
near the surface of Earthwhere the
confining pressure and temperatureare
relatively low.
Time over
which the
stress is
applied
A rock may not respond plasticallyif the
stress applied is rapid, but may undergo
extensive plastic deformationif the stress
applied is lowbut long sustained.
-
8/13/2019 Chapter 6 - Geological Structures
7/40
Types of deformation
6.2.1 Fractures
6.2.2 Strike and dip 6.2.3 Joints
-
8/13/2019 Chapter 6 - Geological Structures
8/40
Fractures
Faultsare fractureswhichhavehad displacement of therocks along them.
The adjacent rock massesslipped past one another in
response totension, compression or shearing stress.
Fault planeis the plane of dislocation along whichmovements occurduring faulting.
Fault commonlycreate zones of broken ground - weakerand less stable than the adjacent rock.
Sudden movements along faultsmay causeearthquakes.
-
8/13/2019 Chapter 6 - Geological Structures
9/40
Fault
-
8/13/2019 Chapter 6 - Geological Structures
10/40
Fault
-
8/13/2019 Chapter 6 - Geological Structures
11/40
Fault categories
Categories of faults:
(a) Normal fault
(b) Reverse fault
(c) Lateral fault
(d) Oblique slip fault
Note: (a)and (b)are also known as dip - slip faultsand(c)are known as strike slip - fault.
-
8/13/2019 Chapter 6 - Geological Structures
12/40
Normal fault
Occurs most frequently in rocksthat have been subjectedto
horizontal tensional force.
One side of the layer move downwardsrelative to the other.
-
8/13/2019 Chapter 6 - Geological Structures
13/40
Reverse fault
Occurs when the crustsare compressed and one side of the layer
moved upwardsrelative to the other.
-
8/13/2019 Chapter 6 - Geological Structures
14/40
Lateral fault
Involves the horizontal movementalong the strike of the fault plane.
-
8/13/2019 Chapter 6 - Geological Structures
15/40
-
8/13/2019 Chapter 6 - Geological Structures
16/40
Strike and dip
Strikeand dipis to describe the compass
directionand the degree of inclinationof a
rock mass.
Outcropis an exposure of rock at the
surface(or the area of a rock lying directly
beneath a soil cover).
-
8/13/2019 Chapter 6 - Geological Structures
17/40
Contd
Definition(refer to Figure 6.7)
(1)Strike:-
The line formedby the intersection of horizontal plane(the water surface) and an inclined plane(the surface ofthe rock layer).
(2) Dipordip angle:-
The maximum angular deviation of the inclined layerfrom horizontal.
In other words, the maximal angle of slope of a tiltedstratummeasured directly downward from the horizontalplane.
The directionof dipis perpendicularto the strike.
-
8/13/2019 Chapter 6 - Geological Structures
18/40
Figure 6.7: Dip and strike
-
8/13/2019 Chapter 6 - Geological Structures
19/40
Natural example of strikeand dip. The strikeof the dipping rock surface
is marked by its intersection with the water surface
-
8/13/2019 Chapter 6 - Geological Structures
20/40
Joints
These are rock fractureswith no movementalong themand tend to break a rock massinto a network of blocks.
They are formed by tectonic stressingand are developed
in nearly all rocks.
Dominant fractureswithin sedimentary rocksare usuallythe bedding planes.
Many bedding planesare very thin bands orpartings ofshale or claybetween units of stronger rocks.
Massive rocks have less fractures, joints or structuralweaknesses.
-
8/13/2019 Chapter 6 - Geological Structures
21/40
Jointing in a folded stratum
-
8/13/2019 Chapter 6 - Geological Structures
22/40
Joints in granite slope
-
8/13/2019 Chapter 6 - Geological Structures
23/40
Erosion along paralleljointin
Arches National Park, Utah
-
8/13/2019 Chapter 6 - Geological Structures
24/40
Folds
Foldsis a bendor flexurein layered rocks.
It is the most common kind of deformation in layered rocksusually well collision of developed in great mountain systemsdueto the collisions of tectonic plates.
Upward foldsare anticlinesor downwardsynclines.
An anticlineis an up - archedor convex upward foldwith theoldest rock layers in its core.
A synclineis a down - archedor concave upward foldin whichthe youngest rock layers are in its core.
They may be gentle, moderate or strong.
Foldsmay be roundedor angular.
-
8/13/2019 Chapter 6 - Geological Structures
25/40
Folded rocksin the Calico of southern California. Three foldsare visible from left to right: a syncline, an
anticlineand another syncline. We can infer compressionwas responsible for these folds.
S li d ti li h i th
-
8/13/2019 Chapter 6 - Geological Structures
26/40
Syncline and anticline showing the
axial plane, axis and fold limbs
-
8/13/2019 Chapter 6 - Geological Structures
27/40
Foldsand their relationshipto topography. Cross section illustrating that anticlinesdo not necessarily
correspond to high and low areas of the surface. Notice that the foldseven underlie the rather flat area.
-
8/13/2019 Chapter 6 - Geological Structures
28/40
A synclineis the peak of this mountainin Kootenay National Park,
British Columbia, Canada. Lower on the left flank of the mountain, an
synclineand another anticlineare also visible.
-
8/13/2019 Chapter 6 - Geological Structures
29/40
Kink band fold
-
8/13/2019 Chapter 6 - Geological Structures
30/40
Recumbent fold
-
8/13/2019 Chapter 6 - Geological Structures
31/40
Folds categories
Categoriesof foldare:
(1) Monocline
(2)Anticline
(3) Syncline
(4) Overturned anticline and syncline
-
8/13/2019 Chapter 6 - Geological Structures
32/40
Monocline
Are foldsin which horizontal orgently dipping bedare modified by
simple steplike bends.
-
8/13/2019 Chapter 6 - Geological Structures
33/40
Anticline
Up-arched rocksin which the older rocks are in the centerand the
younger rocks are on the flanks.
-
8/13/2019 Chapter 6 - Geological Structures
34/40
Syncline
Folded downwardsin which the younger beds in the centerand the
older rocks on the flanks diagram of folds.
-
8/13/2019 Chapter 6 - Geological Structures
35/40
Overturned anticline and syncline
Major fold types and elements of fold
-
8/13/2019 Chapter 6 - Geological Structures
36/40
Major fold types and elements of fold
-
8/13/2019 Chapter 6 - Geological Structures
37/40
Competent and Incompetent Strata
Foldinginvolves brittleand ductile deformation.
Competent roc ksare folded rock stratawhich behaveas brittle material, competent bedsare folded by
retaining their original thickness.
Incom petent rocksare folded rock stratawhich flow asductile materialand usually composed of soft rocks or
thinly bedded shales or thin beds of sandstone. They areusually strongly distortedand show rapid changes initialthicknessupon folding.
-
8/13/2019 Chapter 6 - Geological Structures
38/40
Unconformity
This is theplaneor break betweentwosequencesor rocks with different dips.
It indicates a period of earth movementsandtectonic deformationbetween times of sedimentdeposition.
It formsa major structural break- the older rocksmust be more lithifiedand perhaps moremetamorphosed, than the younger rocks aboveunconformity.
-
8/13/2019 Chapter 6 - Geological Structures
39/40
Unconformity boundary shows by
the different rock structures
-
8/13/2019 Chapter 6 - Geological Structures
40/40
Q & A
End of the Chapter 6