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dwzwy: Geology : 200607

STAG1032: PHYSICAL GEOLOGY (Geologi Fizik)

DYNAMIC EARTH (2 hours)

•Force, Stress and Strain. •Formation of folds, faults & Joints

Dr. Wan Zuhairi Wan YaacobProgram Geologi, UKM

Principal of original horizontality?

Horizontal layers of sediments accumulation

Rock Deformation

• Deformation (de = out ; forma = form)– Changes in shape / size /volume of rock

• Stress (tegasan)– the result of force applied to a given area of rock

• Strain (terikan)– deformation caused by stress (i.e. intensity of stress

> rock’s internal strength)• Deformation ~ strain• 3 types of stress:

– (1) Compression (mampatan)– (2) Tension (regangan)– (3) Shear (ricihan)

StressF

• Stresses (tegasan); P = F/A– Depending on the direction of

forces– (1) Compression

• Forces directed toward one another

• Acts to compress objects– (2) Tensional

• Forces acting in opposite direction

• Acts to stretch objects– (3) Shear

• Forces act parallel but in opposite direction

Higher stressLow stress

The same force, applied to a rock over a broad surface area results in a low applied stress, whereas the same force, applied over a smaller area produces a much higher stress.

Types of Deformation (Strain)

• Three types of strain depending on the response of rocks to stress:-

• (1) Elastic deformation– Recoverable deformation– Directly proportional to applied load– When elastic limit is exceeded, permanent deformation

• (2) Plastic deformation– Non recoverable deformation– Ductile (folding & flowing)

• Rupture after plastic deformation

• (3) Brittle Deformation– Rupture before plastic deformation– Faults– Brittle failure of rock solids

Elastic-plastic deformation(Ductile rock)

Plastic fracture

Material A

Types of stress application and their resultant strain.

STRAINSTRESS

Shorteningcompression

lengtheningtensional

rotating

StrainType of strain depends on:--• (1) Kind of stress applied• (2) The amount of temperature & pressure

– Rocks near surface; low T & P ; brittle solid– Brittle (bryttian = to shatter)– At depth, high T & P ; rocks exhibit ductile behavior

• (3) The rock type– Rocks are considered either ductile / brittle depending on the amount of

plastic deformation– Influence by mineral composition and texture– Crystalline rocks (granit, basalt, quartzite)

• strong internal bonds• Fail by brittle fracture

– Sedimentary rocks (weakly cemented); metamorphic rocks (foliation)• Fail by ductile deformation / flow

• (4) Length of time the rock is subjected to stress– Small stress; longer time – plastic deformation– Large stress applied rapidly – fracture

An example of plastic deformation due to small stressbut was applied in a longer time span

DEFORMATION AND GEOLOGIC FRACTURES

• Folds (lipatan)

• Faults (sesar)

• Joints (kekar)

Type of rock?Type of stress ?

Ductile rocksCompression

Brittle rocksCompression/Tensional/shear

Brittle rocksCompression/Tensional

Primary vs. Secondary structures ??

(a) lamination

(d) Folds

faults

FAULTS the terminology

• Fault– Fracture along which blocks on opposite

sides of the fracture move parallel with the fracture surface (fault plane)

• Fault scarp– A cliff formed as a result of vertical

movement (quickly modified by erosion)

• Fault breccia– Crushed and shattered rock (angular

blocks)

• Slickenside – Scratch or polish surface of a fault plane

• Hanging wall block– block of rock overlying the fault

• Foot wall block– Lies beneath the fault plane

footwall

Fault drag

Some manifestations of faults

(PEPARIT)

(GOREKAN)

(muka upam)

slickensides on the fault plane discovered at Jeram Besu, Benta Pahang

dwzwy: Geology : 200607 (KERUKAN)(BREKSIA)

A fault zone/plane with crushed and shattered angular blocks

Clay-like material that is produced by the movement of a fault.

Dip and strike?

• Fault planes are characterized by dip and strike similar to rock layers

• Strike:– Direction of a line formed

by intersection of a horizontal plane with an inclined plane

• Dip:– A measure of an inclined

plane’s inclination from horizontal

Strike and dip(jurus dan kemiringan)

Types of faults

• Movement parallel to the dip of the fault plane – Dip Slip Faults

• Movement in the direction of the fault plane’s strike – Strike Slip faults

• Combination of dip-slip and strike-slip movements is called oblique-slip fault.

FAULTS (SESAR)

• DIP SLIP FAULTS– Normal faults– Reverse faults

• Dip>45 ; Reverse faults• Dip<45 ; Thrust faults

• STRIKE SLIP FAULTS– Right lateral strike slip fault– Left lateral strike slip fault– Transform fault; eg: San Andreas Fault

Normal Fault (Sesar normal)

Reverse Fault (Sesar songsang)

Thrust fault (Sesar sungkup)

Lateral fault (Sesar mendatar)

Some examples of faults

(1) DIP SLIP FAULTSNormal faultsReverse faults

Dip>45 ; Reverse faultsDip<45 ; Thrust faults

(2) STRIKE SLIP FAULTSRight lateral strike slip faultLeft lateral strike slip fault

dwzwy: Geology : 200607Bedding A

Bedding A

Normal fault

Reverse fault

dwzwy: Geology : 200607REVERSE fault

dwzwy: Geology : 200607Penjom Gold Mine, K.Lipis Pahang

Reverse fault

San Andreas Fault

• “California has its fault”• Transform fault ; i.e. motion between two

plates• Total length = 1300 kms• Types : Right lateral strike slip fault• Total displacement = 560 kms !!• 1906 earthquake (5m displacement)

Conservative: San Andreas Fault

Right lateral offset of a gully by the San Andreas fault in southern California. The gully is offset about 21m

offset

FOLDS(Lipatan)

• Monocline• Anticline• Syncline• Overturned• Isocline• Recumbent

Assignment !!!: Due next week; Discuss Monocline and Isocline

Simple folds (upright/inclined)

Upright --- inclined --- overturned – isocline --- recumbent

Folds (Lipatan)

• Compression• Variable in size:--

– Minor – a few mm’ s– Major – up to 1-2km

• Syncline– arch downwards (palung)– Oldest rock layers in its core

• Anticline– arch upwards– Youngest rock layers in its core

Size (Scale)

MINOR MAJOR

>20m ?

Micro structure / folds

Fold (Terminology)

(the axis of the fold penetratesinto the ground)

Anatomy of folds

Fold (Terminology)

(the greatest curvature exist)

Fold (Terminology)

Overturned folds (Terbalik)- axial planes inclined. – limbs on either side of the axial plane dip in the same direction

Recumbent folds (Rebah)- axial planes and limbs on either side of the plane are sub-horizontal

Types of folds

The tightness of folds:Gentle Fold (B > 120)open (limbs dip gently; B = 120 to 70o)close fold (B = 70-30o)tight (limbs dip steeply; B = <30o)isoclinal (limbs are parallel; B = 0o).

Satah paksi (axial plane)

Garis paksi (axial line)

B = angle between two limbs orinterlimb angle

Dip of axial plane:-0-12 ---- Recumbent fold (rebah)13-89 --- Inclined/overturned fold (terbalik)90 --- upright fold (tegak)

Limb - sayap

Upright (axial plane vertical, limbs symmetric), Overturned (axial plane moderately inclined, one limb overturned-terbalik), Recumbent (axial plane near horizontal, one limb inverted - songsang).

Types of folds

(axis is vertical)

(axis is inclined)

Types of folds

Some examples of folds

dwzwy: Geology : 200607Upright anticline and syncline

Anticline (Asymmetrical fold / inclined fold)

dwzwy: Geology : 200607Syncline

OVERTURNED fold

Overturned FOLD

dwzwy: Geology : 200607OVERTURNED fold

Recumbent fold

dwzwy: Geology : 200607Recumbent fold

RECUMBENT fold (lipatan REBAH)

dwzwy: Geology : 200607 Recumbent fold

Isoclinal fold

The limbs and axial plane are parallel and interlimb angle is zero

JOINTS (Kekar)

• Fractures along which no movement has occurred• Random or parallel groups (i.e. orientation)• 3 types of joints:-

– 1. Columnar Jointing – cooling of igneous rocks and create shrinkage fractures

– 2. Sheeting• Expansion of rock as erosion removes the

overlying material (i.e. pressure release)• Exfoliation

– 3. Tensional joints• Joints associated with folds

2 sets of JOINTS

Polygonal jointing in basaltic rock

Columnar Joints – cooling of igneous rocks and create shrinkage fractures

Basalt from Kuantan, PahangTensional stress due to the contractions of the magma

Sheeting (or exfoliation) @ pengelupasan kulit bawangExpansion of rock as erosion removes the overlying material

Discontinuities / Ketakselanjaran

• planes of weaknesses within the rock mass

• Bedding / Perlapisan

• Faults / Sesar

• Folds / Perlipatan

• Joints / kekar

The END

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