4. DEFORMASI BATUAN

Tegangan yang terjadi akibat gerakan tektonik tersebut

mempunyai arah sejajar permukaan bumi (mendatar) dari segala arah

dan tegangan yang lain berasal dari dalam bumi ke arah permukaan

bumi (vertikal). Kekuatan tegangan-tegangan tersebut berbeda satu

sama lain baik arah mendatar maupun yang berarah vertikal, oleh

karenanya tegangan ini sering dikenal dengan tegangan utama

(principal stress). Berdasarkan perbedaan kekuatan tegangan tersebut

dibedakan menjadi 3 bagian yaitu (s1) Tegangan utama maksimum,

(s2) Tegangan utama menengah, dan (s3). Faktor lain yang

berpengaruh adalah sifat fisik dan mekanik batuan, seperti misalnya

bila batuan bersifat plastis maka batuan akan mengalami pelipatan

teta[I bila btuan bersifat tegar (rigid) maka batuan akan retak/pecah.

4. DEFORMASI BATUAN

Gambar 1 : Sistem tegangan utama yang terjadi di litosfera

s2` s2` s3`

s1`

s1` s3`

s2`

s1`

slate sandstone limestone

STRUKTUR GEOLOGI

Deformasi batuan adalah perubahan lapisan kerak bumi yang

diakibatkan adanya tegangan yang bersasal dari gerakan

tektonik

Hasil dari proses deformasi batuan dikenal dengan Struktur geologi.

Struktur geologi adalah bentuk arsitektural lapisan batuan yang muncul

dipermukaan bumi.

Struktur geologi dapat dibedakan menjadi 4 macam yaitu :

1. Struktur kekar (joint)

2. Struktur patahan (fault)

3. Struktur lipatan (fold)

4. Struktur ketidakselarasan (unconformity)

Fundamental

Structures

• Contacts: are the

most basic structures,

they separate one rock

unit from another -

depositional,

unconformities,

faults, intrusive, shear

zones.

Fundamental Structures

• Primary Structures: These are sedimentary

structures that may be in strata prior to

deformation. They may be quite useful as strain

markers (giving us an initial state) and as way-

up indicators, etc.

• They must not be mistaken for secondary

structures, which are the result of deformation.

Bedding Laminations

Graded Bedding

up

Primary Structures

Cross-Beds (asymmetric)

Oscillation Ripples (symmetric)

up

up

up

up

Mud Cracks

Rain Drops / Footprints

Load Casts

Tool Marks

up

up

Root Casts / Worm Burrows

Stromatolites

up

up

KEKAR (JOINT) KEKAR (JOINT) adalah retakan yang mempunyai pola dan arah

tertentu sesuai dengan tegangan penyebabnya.

Gambar 2 : Kekar yang terjadi akibat tegangan dari bawah

s1`

Kekar gerus (Shear Joint) Kekar Tarik (Tension Joint)

Kekar “release”(Release Joint)

Plumose Structures

Secondary Structures

Shear fractures:

• Form in response to a very slight shearing

movement parallel to the plane of the fracture.

• Commonly found in conjugate sets, in rocks

that have been folded or faulted.

Secondary Structures

Slickensides, slickenlines:

• Effectively, they are small scratches that form

in response to motion on a fault.

• May be the result of very large or very small

displacements.

• Lines indicate direction of motion. Steps in

rock or mineral coatings may indicate sense of

slip.

KEKAR NON TEKTONIK

1. Kekar Tiang (columnar joint)

2. Kekar lembar (sheeting joint)

PATAHAN Kekar yang terbentuk akan membentuk suatu bidang dengan arah dan

kemiringan tertentu. Oleh karena tegangan yang ditimbulkan oleh

gerakan lempeng tektonik bekerja terus maka akan terjadi pergeseran

mengikuti arah bidang kekar. Kekar yang mengalami pergeseran

disebut dengan patahan atau sesar (fault).

Arah dan kemiringan bidang kekar/patahan/lapisan mempunyai

orientasi terhadap arah azimut bumi. Arah dan kemiringan dapat

digambarkan seperti dibawah ini :

PATAHAN Strike dan dip

Strike (jurus), s adalah garis potong bidang kekar/patahan/ lapisan dengan bidang datar imajiner Dip (kemiringan), a adalah sudut yang dibentuk oleh bidang kekar/patahan/ apisan dengan bidang datar imajiner

a

c

Keterangan : ab - strike slip ac - net slip ad - dip slip ae - vertical slip (throw) ed - harizontal slip (heave)

a

d e

b

PATAHAN Patahan adalah pergeseran blok batuan mengikuti arah bidang kekar

1.PATAHAN TURUN adalah pergeseran bidang “dinding” (hanging

wall) kearah turun

s1`

Distribusi Tegangan s1`>> s2`= s3

PATAHAN

2. PATAHAN NAIK

s1

Distribusi Tegangan s1`>> s2`= s3

PATAHAN

3. PATAHAN GESER

Distribusi Tegangan s1`>> s2`>> s3

s1`

PATAHAN

4. PATAHAN DIAGONAL

s1`

s2`

s3

Distribusi Tegangan s1`>> s2`>> s3

LIPATAN

There can be two (2) resulting responses to stress: 1. Ductile deformation -- usually occurring deeper and with higher

temperatures; flow 2. Brittle deformation -- usually occurring shallower and with cooler

temperatures Ductile deformation produces folds: 1. Anticline -- upwarping of rocks to produce an "A-like" structure 2. Syncline -- downwarping of rocks to produce "spoon-like" structure 3. Dome -- three-dimensional anticline resembling inverted cereal bowl 4. Basin -- three-dimensional syncline resembling upright cereal bowl * When brittle deformation occurs and rocks fracture, they can simply

crack producing a fracture with no offset, called a joint (= kekar). When brittle deformation occurs and rocks fracture, they can also crack producing a fracture with offset, called a fault (= patahan = sesar).

Homocline: rocks that dip uniformly in one direction

Monocline: a local steepening with homocline

Structural terrace: local flattening of a uniform regional dip

Cylindrical: The hinges are parallel every where and the fold can be generated by moving the fold axis parallel to itself (Fig. 14.9)

Non-cylindrical: The hinges are not parallel and can converge in one point (Fig.14.9)

Sheath folds: are non-cylindrical and closed at one end the fold hinges curve within axial surface (Fig. 14.10)

Upright folds: have vertical axial surface (Fig. 14.11)

Overturned folds: have one inverted limb (Fig. 14.11)

Reclined folds: axes plunge at nearly same angle as the dip of the axial surface, plunge of the axis normal or at high angle to the strike of the axial plane (Fig. 14.11)

Recumbent folds: Have horizontal axes and axial surfaces.

Isoclinal folds: are tight folds wherein axial surfaces and limbs are parallel

To distinguish between the different type of folds Fig. 14.13 (after Fleuty 1964) is used.

(Fig. 14.14)

Parallel folds: folds maintain constant thickness (Fig. 14.14)

Concentric folds: parallel folds in which folded surfaces define circular arcs and maintain the same center of curvature.

Ptygmatic folds: nearly concentric shape, attenuated limbs and intestinal appearance.

Similar folds: maintain the same shape throughout a section but not necessarily with the same thickness.

Chevron and kink folds: have sharp angular hinges and straight limbs.

Disharmonic: shape or wavelength changes from one layer to another.

Supratenuous folds: synclines are thickened and anticlines are thinned. These folds are usually non-tectonic form in unconsolidated sediments and when uplift is taking place.

Fault-bend and fault-propagation folds: (Fig. 11.11) these type of folds associated with thrust fault