Definitions and images to illustrate geological terms, links to images and website articles


fault attributesfaultingfault (shear) zonesfold anatomyfolding (attitude of fold structures, anticline, monoclines, overturned and recumbent folds, ptygmatic folds in migmatites, slumps, syncline) ▪ foliated textureforearc basinfractional crystallizationfragmental texturefrost quakefusibles and refractories

fault attributes

diagram of fault attributesFaults can move horizontally to the left or right and/or vertically, and the rake of a fault results from a combination of vertical (dip) and horizontal vectors. Rake = angular slip direction of hanging wall block measured in the fault plane and from the dip vector.

Net-slip comprises dip vector component and horizontal vector components.
a. net-slip = total slip of fault.
b. dip-slip = dip-parallel slip component.
c. strike-slip = strike-parallel slip component.
d. vertical-throw = vertical component of net-slip.
e. horizontal-throw = horizontal component of net-slip.
f. heave = stratigraphic heave = apparent horizontal component of the net-slip.

diagram comparing vertical vs stratigraphic throw

Vertical throw, which is the vertical component of net-slip, is different for dipping strata than stratigraphic throw, which is the vertical offset of faulted strata.

If the hanging wall, which lies above the fault, moves downward relative to the footwall, then the fault (as in diagram) is a normal fault. A detachment fault is a regional, low-angle, listric normal fault formed during crustal extension. Listric faults are curved normal faults in which the fault surface in concave upwards because the main detachment fracture following a curved path rather than a planar path. Slumps are listric faults.

diagram of paired horst and graben faultsCrustal extension stresses create sunken graben blocks bounded by parallel normal faults and lying between horst blocks that lie higher than the sunken graben blocks. Compressional forces during orogenies can elevate horst blocks.

If the hanging wall is thrust upward above the footwall, then the fault is a reverse fault. A thrust fault is the special case of low dip-angle faults that formed during regional compressional deformation.

If the predominant movement is in the horizontal direction, then the fault is a strike-slip fault (transform fault or wrench fault). The term tear fault can be used to indicate a steeply-dipping wrench fault that bounds or cuts the hanging wall of a thrust or normal fault (also used for mode III faults).

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diagram of main fault typesComplex deformation with failure of strata and accompanying movement of one rock body relative to another creates geologic faults, fault lines, or simply faults. The fault zone is that area of complex deformation that is associated with the fault plane.

Whether in a normal or a reverse fault, the hanging wall is defined as the rock body above the line of the fault, while the footwall lies below the fault.

fault in shalesShear stresses that cause faulting in shear zones may be associated with tectonic compression at plate boundaries, with tectonic extension, with impact compression, or with compression by overburden.

Stresses build in rocks where friction prevents simple slippage. Stresses initially cause deformation of rock structures, and only when accumulated potential energies exceed the strain threshold will rock bodies fail and relative motion occur across the fault.

Initially, rock failure may occur on a small scale (microfracturing, microseismicity). However, continued compression can cause the large-scale slippage associated with seismic events (tremors and earthquakes).

Relative movement (slip) determines the type of fault that occurs:
▪ strike-slip or transform faults, such as the San Andreas Fault
▪ normal
▪ reverse (thrust) faults such as the hugh Lewis Thrust

[link: images: animation: model of earthquake centered on Hayward Fault; small-scale faulting: small fault in Delmar formation; small fault in Mosaic Canyon, Death Valley, and recumbent folding; fault; low-angle thrust fault in moraine; moderate-scale faulting: normal fault; fault; growth fault with well developed clay smear exposed in cliff face near Albuquerque New Mexico; fault in rocks (left slid down relative to right), Mosaic Canyon, Death Valley, and limestone/marble fault breccia; Early Carboniferous quartz-cemented, mineralised fault breccia in a fault strand of the Billefjorden Fault Zone, Gråkammen, Austfjorden, Spitsbergen; breccia; fault breccia, Haughton impact structure; normal fault, 2, fault, 2, Birmingham Shale, Bakerstown Station, PA; Blue Anchor, Somerset, 2; labelled fault; Surpise Cliff Fault in Ricardo Group sediments, close-up, and aerial view of fault; fault, Black Hills; Internal Oriskany Hanging Wall Deformation; Devil's Elbow fault separates red Tertiary megabreccia (hanging wall) from strongly deformed Proterozoic gneiss in (footwall), another footwall block in right foreground, and close-up; San Gabriel Fault (where the light rock meets the dark rock); fault in complex zone of oblique thrusting on the Alpine Fault (mylonite thrust up to 2 km westwards over Quaternary gravels, NZ); fault; detachment fault; thrust fault, Melabakkar; Early Tertiary folded and thrust Permian and Carboniferous strata with Cretaceous dolerite sills; view subparallel with thrust movement, and isoclinally folded dolerite sill between two thrust faults, Midterhuken, Bellsund, Spitsbergen; early Tertiary thrust ramp (left) and box folds (right) in Triassic strata, typical foreland structures of marginal fold-thrust belts, Engadinerberget, Wedel Jarlsberg Land, Spitsbergen; synsedimentary faulting: extensional duplex or synsedimentary growth faults in the Triassic Botneheia Formation, Eistradalen, Agardhdalen, Eastern Spitsbergen; collapse of a sandstone plateau or delta front. Festnigen sandstone, base of Helvetiafjellet Formation (Barrême), and (closer) Festnigen sandstone, base of Helvetiafjellet Formation (Barrême), KvalhovdenArea, Kvalvågen, Eastern Spitsbergen; large-scale faulting: San Andreas Fault, 2, from Shuttle, topographic map, diagram, folding; Keraf Suture in Sudan Collision Zone; Lone Pine, CA; Fault line in Southern California; Alaska's Denali Fault, map, 2, Denali Fault Earthquake Fact Sheet; Fault Scarp, Rock Creek, and closer view, 2; Furnace Creek Fault; Transform Strike-Slip Faults across Atlantic Mid-Ocean Ridge; Faults and Fault Zones in Ross Sea; webpages: picture gallery of Svalbard, Jan Mayen, and Dronning Maud Land, Antarctica]

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fold anatomy

diagram of fold componentsFolding buckles strata away from the linear, planar, and horizontal.

Folds may be symmetrical or asymmetrical. In a symmetrical fold (image at left), the axial plane is vertical and the limbs (sides) dip symmetrically from the axis. In an asymmetrical fold, the axial plane is tilted from the vertical with one limb dipping more steeply than the other.

  • ▪ the axial plane of a fold is an imaginary plane surface that divides a fold as symmetrically as possible.
  • ▪ the fold axis is a line drawn along the points of maximum curvature of a layer of a fold (parallel to the hinge in anticlines and synclines).
  • ▪ the plunge of a fold is the angle between the horizontal plane and the fold axis (when this is not not horizontal).
  • ▪ the plunge direction is the geographic quadrant towards which the plunge is directed

  • By convention, plunge direction is expressed as a three-digit number, and the plunge by a two-digit number. Thus, a line plunging 35º toward the azimuth 35º will be noted: 45º->035

    Monoclinal folding drapes strata as though over a ledge, whereas hinge folding wraps the limbs of a fold as though hinged around the fold axis.

    diagram of adjacent anticlinal and synclinal foldsBecause the eroded surface of a plain could fail to indicate the sub-surface folding relationships of ridges of resistant strata, folding is most easily elucidated by examining structures eroded through folds (side view of diagram at left).

    diagram of orthogonal versus oblique cuts through a fold

    However, the apparent shape of a fold may be distorted by the angle of erosional exposure relative to the fold axis (image at left).

    (Try this with a stick of celery.)

    .. On geological maps, the stike and dip of bedding, rather than axes and folds, are indicated.

    faultingfault attributes – net-slip, dip-slip, dip vector, strike-slip, vertical-throw, horizontal-throw, horizontal vector, heave, rake

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    Geological folding involves the plastic deformation (bending, buckling) of a single or multiple (stack) strata, such as sediments and rocks, which were originally planar horizontal surfaces. Although even brittle rocks may undergo plastic deformation when stresses are applied over considerable periods of time (low strain rate). Beyond plastic deformation, rocks fail structurally and faulting occurs.

    Folds may be isolated or may occur in extensive fold trains, and folding may range from the microscopic scale to mountain-sized folds in orogenic belts. Folds are classified according to size, fold shape, tightness, and dip of the axial plane.

    Folding can occur by flexural slip, buckling, or mass displacement under under varying conditions of stress, hydrostatic pressure, pore pressure, and temperature.

    Slumping of material before deformation causes synsedimentary folds. Folding orientation can produce:

  • anticlines

  • synclines

  • ▪ domes

  • ▪ basins
  • décollement folds

  • monoclines

  • overturned and recumbent folds

  • slumps

  • ptygmatic folds

  • links: images: plunging anticlines and synclines north of Moab, UT; Folding Satellite Images gallery; Capitol Reef, UT, satellite image and false color image of a monocline and syncline crossed by transverse stream; Capitol Reef Fold, Capitol Reef from ground, 3D anaglyphic images (needs 3D glasses), wp; anticline, 2; folds, 2; fold axes; fold and unconformity; kink fold; kink fold in mountains; refolded; overturned fold; synclinal; close folding and lineations, White Mountains, NH; complex folds in shale and sandstone, Cumana, Venezuela; ductile thrust faulting and large-scale fold nappes, 2, and complex fold interference patterns produced during formation of the fold nappes; Lower Palaeozoic Gondwanan sediments (ca. 500 My) metamorphosed and folded during Variscan Orogenesis (ca 350 My), Aiguilles Rouges, Lac d'Emosson; folding in schist; chevron fold, road cut Kingston-Rhinecliff Bridge, NY; fold, 2, road cut, Catskill, NY; California fold; sheath fold with strongly curved hinge line; glaciotectonic fold, Melabakkar, and close up, 2, 3; ptygmatic folding in quartz vein in Archean metasediments; folded Cambrian limestones and shales, Bay of Islands, Newfoundland ; large folds (Osceola) indicate laterally directed strain away from 4 km distant transient crater (Weaubeau structure of southwestern Missouri); folds in the Barranquin Formation, northeastern Venezuela; webpages: Geology Gallery : Folds : Geological Structures : Secondary Rock Structures : Mouser

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    fractional crystallization

    Fractional crystallization (fractionation) is that process of magmatic differentiation that accompanies the failure of early-forming crystals to react to the melt that remains. The process of fractional crystallization is responsible for the bulk of differentiation that is occurs in igneous rocks.

    As ascending melts cool and react with country rock, those minerals in the melt that have the highest melting points or the lowest solubilities (quick-freezing refractories, like olivine and pyroxene) crystallize out first, leaving minerals with the lowest melting points or solubilities (quick-melting fusibles, like silica) behind in the melt to freeze out last.

    Latent heat associated with phase change is released by the crystallization of refractories, replacing heat lost by conduction to the surrounding country rocks, lost to melting of country rock, and lost to the assimilation of fusibles in the country rock. Fusibles enter and refractories leave the melt at characteristic temperatures and pressures, and these exchanges tend to occur at specific depths along the ascent. The remaining melt loses volume as it rises, rendering its fusibles increasingly concentrated. Thus, exchanges within ascending magma leave behind a trail of solid refractories and country rock alterations.

    Gravitative differentiation is the commonest form of fractionation, and results from the phenomenon that most solid minerals are denser than their parent melts. As denser crystals settle to the bottom of the magma body, they become segregated from the residual melt. Rocks that are formed by settling crystals are termed cumulates, and the rocks are often zoned, with the densest, first-formed crystals accumulated at the base of the magma chamber. Cumulates formed by the lighter crystals occasionally float to the top, with the lightest at the very top. This process produces layering in igneous rocks. The crystals of cumulate rocks are typically cemented by residual magmatic fluids.

    Bowen's Reaction Series

    [links: animations: fractional crystallization/magmatic settling; webpages: Kurt Hollocher's webpage on Greenland's Eocene Skaergaard Intrusion has a gallery of excellent photographs of microrhythmic layering (and other interesting phenomena); Mining: Rock Formation]

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    fusibles and refractories

    Fusibles are rocks or minerals that melt easily, and are the opposite of melting-resistant refractories.

    Sedimentary rocks, which are stable at the Earth's cool surface, tend to be fusible because they consist mostly of stable minerals that have resisted weathering. Crystalline rocks tend to be refractory and to resist melting because they consist mostly of minerals that crystallized out of melts. The most refractory rocks, such as gabbro and peridotite, are stable in the lower crust and the mantle.

    Bowen's Reaction Series

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