Chapter 6 - Geological Structures

8/13/2019 Chapter 6 - Geological Structures

1/40

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


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


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.


5/40

Table 2

Fracture Description

Brittle rocks which simply breaksrather thandeform

plasticallyby application of stress.

Ductile undergo considerable smooth deformationbeforethey rupture.


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.


7/40

Types of deformation

6.2.1 Fractures

6.2.2 Strike and dip 6.2.3 Joints


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.


9/40

Fault


10/40

Fault


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.


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.


13/40

Reverse fault

Occurs when the crustsare compressed and one side of the layer

moved upwardsrelative to the other.


14/40

Lateral fault

Involves the horizontal movementalong the strike of the fault plane.


15/40


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).


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.


18/40

Figure 6.7: Dip and strike


19/40

Natural example of strikeand dip. The strikeof the dipping rock surface

is marked by its intersection with the water surface


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.


21/40

Jointing in a folded stratum


22/40

Joints in granite slope


23/40

Erosion along paralleljointin

Arches National Park, Utah


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.


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


26/40

Syncline and anticline showing the

axial plane, axis and fold limbs


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.


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.


29/40

Kink band fold


30/40

Recumbent fold


31/40

Folds categories

Categoriesof foldare:

(1) Monocline

(2)Anticline

(3) Syncline

(4) Overturned anticline and syncline


32/40

Monocline

Are foldsin which horizontal orgently dipping bedare modified by

simple steplike bends.


33/40

Anticline

Up-arched rocksin which the older rocks are in the centerand the

younger rocks are on the flanks.


34/40

Syncline

Folded downwardsin which the younger beds in the centerand the

older rocks on the flanks diagram of folds.


35/40

Overturned anticline and syncline

Major fold types and elements of fold


36/40

Major fold types and elements of fold


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.


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.


39/40

Unconformity boundary shows by

the different rock structures


40/40

Q & A

End of the Chapter 6

Chapter 6 - Geological Structures

Documents

Transcript of Chapter 6 - Geological Structures