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Jumat, 31 Juli 2009

Struktur Geologi : Lipatan

Lipatan merupakan Struktur berbentuk menyerupai gelombang yang umumnya berkembang pada batuan berlapis yang mengalami gaya kompresi horisontal atau gaya Vertikal.

Bagian Lipatan:

Sayap Lipatan (Limbs) : Bidang miring yang membangun struktur antiklin atau sinklin

Inflection Point-

limb point where sense of curvature changes

Axial Surface (Axial Plane)-

connects series of hinge lines within a fold

Fold Axis-

Imaginary straight line that when moved parallel to itself reproduces the form of a fold

Occurs only with Cylindrical folds

Hinge Line-

connects points of maximum fold curvature

Cylindrical vs. Non-Cylindrical Folds

Cylindrical Folds-

Straight hinge lines

Contains fold axes

Non-Cylindrical Folds-

Curved hinge lines

Does not contain fold axes

Cylindrical Non-Cylindrical

Non-Plunging Folds

Plunging Folds

Landsat TM image of nortwest trending Mariscal Mountain Anticline, Big Bend National Park

Limestone beds of Torcer Formation in syncline of Malone Mountains, south of Torcer station. Hudspeth County, Texas. Syncline plunging away from observer. USGS photo.

Fold Symmetry

Lipatan Simetri - Bidang Sumbu Tegak

Non-Simetri - Bidang Sumbu Miring

Lipatan Simetrik Lipatan Asimetrik

Form vs Lithologic age relations:

Antiform- Melipat Cembung keatas

Synform- Melipat Cekung kebawah

These terms used when stratigraphic age relations are unclear between folded layers.

Anticline-convex upward fold wherein the oldest stratigraphic units occupy core of fold

Syncline- concave upward fold wherein the youngest stratigraphic units occupy fold core

Sideling Hill Syncline (photo by N. Heywood)

Green Pond Inlier folds, Newfoundland, NJ

Fold Classification:

Dip of Axial Surface

Upright: 700-900

Inclined: 100-700.

Recumbent 00-100.

Interlimb Angle

A. Gentle: 1200-1800

B. Open: 600-1200

C. Tight: 100-600

D. Isoclinal: 0-100.

Dip Isogon Analysis-

Dip isogons connect points of equal dip on the upper and lower boundary of a folded layer

Measure bed thickness change throughout fold

Class 1- convergent dip isogons

Class 1A- limb thickening

Class 1B (Parallel)-

equal bed thickness throughout the fold

Class 1C- slight hinge thickening

Class 2 (Similar)-

parallel dip isogon patterns-

limb thinning; hinge thickening

Class 3:

divergent dip isogon pattern-

limb thinning, hinge thickening

Distinctive Fold types:




Chevron Folds

Mechanics of Folding:

Passive Folding (Shear Folding)

slip on surfaces not parallel to rock layering

rock layering (black layer) does not influence fold development

black bedding layer merely acts as a “passive” recorder of strain

card deck analogy in which slip occurs on individual cards;

the surface of individual card records no strain

produces similar (Class 2) folds

Passive folding in layered leucogabbro, Siroua, Morocco

Flexural Slip Folding

Folding is controlled by rock layering

Slip (bending) occurs between rock layers

No strain on the bedding plan surface

Circles become ellipses in the profile plane

Amount of slip increases from the hinge line to the inflection point

produces parallel (Class 1B) folds

Flexural Slip Folding

Neutral Surface Folding – produces concentric folds

X-axis is normal to hinge line along top of folded layer

Neutral surface (no strain) occurs in middle of folded layer

X-axis is parallel to hinge line along bottom of folded layer

produces parallel (Class 1B) folds

Superposed Folds- refolded folds

Specimen of refolded isoclinal folds in schist. Riverside Mountains, Riverside County, California. January 1967. Photo by Warren B. Hamilton, USGS.

Transposed Folds – hinges distended from limbs

Extensional Folds: folds may also be generated by tension

Reverse Drag

Soft Sediment Deformation- Lastly, folds may occur due to soft sediment deformation, commonly in turbidity current environments

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