DEFORMASI BATUAN

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 (1) Tegangan utama maksimum, (2) Tegangan utama menengah, dan (3). 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

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

Kekar gerus (Shear Joint)Kekar Tarik (Tension Joint)Kekar “release”(Release Joint)

Plumose Structures

Secondary StructuresShear 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 StructuresSlickensides, 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), adalah sudut yang dibentuk oleh bidang kekar/patahan/ apisan dengan bidang datar imajiner

c

Keterangan :ab - strike slipac - net slipad - dip slipae - vertical slip (throw)ed - harizontal slip (heave)

a

de

b

PATAHAN Patahan adalah pergeseran blok batuan mengikuti arah bidang kekar

1.PATAHAN TURUN adalah pergeseran bidang “dinding” (hanging wall) kearah turun

Distribusi Tegangan 3

PATAHAN 2. PATAHAN NAIK

1

Distribusi Tegangan 3

PATAHAN

3. PATAHAN GESER

Distribusi Tegangan 3

PATAHAN

4. PATAHAN DIAGONAL

3

Distribusi Tegangan 3

LIPATAN

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

higher temperatures; flow2.  Brittle deformation -- usually occurring shallower and with cooler

temperatures

Ductile deformation produces folds:1.     Anticline -- upwarping of rocks to produce an "A-like" structure2.    Syncline -- downwarping of rocks to produce "spoon-like" structure3.    Dome -- three-dimensional anticline resembling inverted cereal bowl4.    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 directionMonocline: a local steepening with homocline Structural terrace: local flattening of a uniform regional

dipCylindrical: 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 To distinguish between the different type

of folds Fig. 14.13 (after Fleuty 1964) is used.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 foldsFault-bend and fault-propagation folds: (: (Fig. 11.11) these type of folds associated with thrust fault