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


décollement foldsdeformationdiapirdiatremedikes, dikelets (aplite dikes, pegmatite dikes) ▪ dipdip directiondirection (dip, plunge, strike) ▪ disconformitydiscordantduplexdykes

décollement folds

Décollement (detachment) folds develop during folding, secondary to separation of a (more competent) layer from an underlying (less competent) layer as deformation proceeds.

The most spectacular décollement folds develop when a homogeneous layer of uniform thickness overlies a less viscous layer. The incompetent layer can comprise evaporates, shales, or heated lower crust (viscosity inversion with depth).

Décollement folds involve a mismatch between horizontal dimensions of layer above versus below the detachment zone, with the upper layer larger in at least one dimension. Such a dimensional mismatch can result when:
▪ the upper layer is stretched as in diapiric folds,
▪ the upper layer has broken away and slid off an uplift, or
▪ the lower layer has been shortened by thrusting or subduction (thrusting at the margins - Sichuan basin; thin-skinned décollement driven forward along a thrust plane in low-viscosity salt and gypsum deposits - Jura Mtns; subduction - Appalachians, Anti Atlas; continental telescoping and thickening shortened the lower layer in the Western Overthrust Belt of the central Rocky Mountains.)

The Sichuan basin of central China includes some of the best samples of décollement deformation.

subduction zone magmas

[links: images: formations: view perpendicular to thrust movement along Early Tertiary décollement folds in Tertiary strata, décollement at base of black shales (Bravaisberget Formation, Midterhukfjellet, Bellsund, Spitsbergen; décollement fold, Reed Wash area, western San Rafael Swell; folds created by "thin-skinned" thrusting over a décollement in underlying rocks, Mt. Kidd, Kananaskis Mountains, Alberta, Canada; extensive outcrops of Upper Jurassic gypsum-anhydrite décollement layer and siliciclastic strata, Sierra Madre Oriental (SMO) salient, near Galeana; décollement, Montagne Noir; close-up: décollement folds in Khosh Yeilagh fm. (N Iran); webpages: gallery of geological phenomena]

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Sediments and rock structures are subject to deformation under the influence of imposed stresses.

Stress is defined as a force applied over an area, F/A.
Stress may be uniform and equal from all directions:
  • confining pressure of overburden
  • ▪ release from confining pressure due to exposure by erosion (diagenesis and retrograde metamorphism).

    Alternatively, stress may be unequal from different directions (differential):
  • ▪ compression
  • ▪ extension (tensional)
  • shear stress (applied obliquely)

    Causes of stress include
  • ▪ uniform confining, lithostatic stress due to overburden (burial)
  • tectonic stress
  • ▪ expansion of water that has frozen in rock cracks or soils (cryoseism)

    When rocks deform in response to imposed stress they exhibit strain, which is the differential change in size, shape, or volume of a material. Materials differ in their responses to stress, depending upon composition, conditions of temperature and confining pressure, and strain rate. However, regardless of intrinsic degrees of brittle or ductile qualities, all strained materials pass through 3 successive stages of deformation: elastic, ductile, and fracture (failure, or brittle deformation). Provided that the strain rate is sufficiently slow to allow minerals to accommodate structurally, minerals can adjust to applied stresses by a variety of mechanisms.

    Forms of deformation include:
    unconsolidated sediments
  • slumping
  • folding (fold anatomy)
  • ▪ mass wasting and landslides
  • faulting (fault attributes)

  • consolidated rock

  • brittle, ductile, or elastic deformation due to lithostatic or tectonic stresses
  • --earthquakes
  • --faulting
  • --folding
  • --cataclism, milling, and brecciation
  • --orogenesis

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    diapir bulging upward through overlying strataA diapir or piecement structure results from the upward intrusion of a more buoyant material into/through overlying strata. Diapirs are most commonly composed of evaporitic salt deposits (salt domes) or gas charged muds, but may be igneous.

    As ancient seas evaporated they left salt deposits that were buried by sediment. Because the salt deposits were less dense than overlying rock the buoyant mass of salt ballooned upward, intruding into the overlying rocks through weak spots. The intruding “salt bubble” is called a salt diaper, and in most environments, salt diapirs erode rapidly on reaching the surface, leaving craters such as the ones shown below left (Salt Dome & Craters on Melville Island). In arid regions, salt domes may persist (below right - click to enlarge - Salt Dome in the Zagros Mountains, Iran ).

    Salt Dome & Craters on Melville IslandSalt Dome in the Zagros Mountains, IranIf the rising plug of salt (called a salt diapir) breaches the surface, it can become a flowing salt glacier (bottom right - click to enlarge image - Iran's salt glaciers).

    Iran's salt glaciers

    images Earth Observatory : Subscribe to the Earth Observatory :

    [links: images: formations: gabbroic anorthosite diapir, west side of Wagers Peak in LZb (postulated mechanisms of formation are 1. that this is a diapir of liquid or mush that pushed aside and broke the overlying layers and then caused turbulence in the magma that caused deposition of the disturbed bed, or 2. that this body is actually a surface deposit of relatively hydrous, plagioclase-rich residual liquid that percolated upward through the crystal mush floor cumulates.)

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    Diatremes are breccia-filled volcanic pipes formed by gaseous explosions.

    Diatremes may
    ● breach the surface, producing a tuff cone of consolidated volcanic ash
    ● form filled relatively shallow craters known as a maars
    ● form other volcanic pipes

    The term diatreme sometimes refers to any concave body of broken rock or tuff-breccia, generally formed by explosive or hydrostatic forces, whether or not the structure is related to volcanism. Some diatreme, phreatic explosions result from the interaction of hot magma with relatively shallow groundwater.

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    Dikes or dykes are discordant tabular or sheet-like bodies of magma that cut vertically or almost vertically through and across strata, though some dikes are steeply inclined.

    Hundreds of dikes can invade the cone and inner core of a volcano. Dikes may occur in swarms of parallel dikes, particularly where there has been crustal extension. In regions of crustal extension, fracturing may open the route for filling by magma from a deep source, or intrusive magma may promote the fracturing and extension of the crust. Outcrops of dikes can range from a few metres to many kilometres in length, and can spread lateral distances from a few centimetres wide to over 100 m. Very thin dikes or dikelets are sometimes called veins. The Great Dyke of Zimbabwe is a gabbroic mass nearly 500 km long and about 8 km wide [sat. image, 2, 3].

    Because dikes intrude relatively cool country rocks, they frequently display a chilled margin, with grain size becoming coarser towards the centre where the rate of cooling has been slower. If the dike cooled very slowly at great depth, the large crystals of pegmatite dikes have had time to form.

    Pegmatite dikes represent crystallization from a residual melt fraction, but pegmatites are formed from a water-rich fluid, so are very coarse grained. Most pegmatites contain quartz, alkali feldspar, micas, and tourmaline. However, some pegmatites contain minerals such as tourmaline, garnets, apatite, beryl, topaz, spodumene, magnetite, sphene (titanite), and zircon, and various other rare minerals. This occurrence of rare minerals results from progressive concentration of trace elements into the last fraction of melt because these elements have not been removed by earlier crystallization during the solidification of the bulk of the magma. [image Pegmatite vein in granite boulder, Glenwood Canyon; Tanco is perhaps the most fractionated igneous body on earth, and a super giant among chemically complex pegmatites]

    Aplite dykes are commonly found in granitic bodies. Aplites are light coloured, fine to medium grained, and equigranular. Aplites formed from the ultimate residual melt after most of the crystallization of the granitoid was completed, so aplites are rich in quartz and alkali feldspar and sometimes muscovite. [image links of aplite rock and formations]

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    X-section illustrating imbricate thrusting in Himalayas (courtesy of USGS) In structural geology, a duplex is a system of imbricate (overlapping) thrusts that branch off from a single fault below and merge with a thrust fault above. Duplexes form stacks of thrust-bounded rock bodies, which are bounded by roof and floor thrusts. The rock body that is bounded by faults above and below is called a horse.

    Duplexes are formed through continued thrusting along a floor thrust with successive collapse of thrust ramps. Antiformal stacks are defined as systems of totally overlapping thrust horses that are characterized by a coincident trailing branch line. Antiformal stacks result when the forward motion of a forward-breaking thrust sequence is interrupted or completely blocked from regular forward development of a foreland-vergent duplex system. Antiformal stacks commonly occur in the cores of mountain chains, mainly in continent-continent or arc-continent collision zones, where the subducted plate acts as an obstacle, forcing faulting upward.

    The Lewis thrust forms a 450 km long fault with thrusting of Precambrian limestone over the top of Cretaceous shale (taking place 160-145 Ma). This Alberta-Montana thrust sheet has a duplex structure that exhibits the geometries of both a hinterland-dipping duplex and an antiformal stack, and that contains inclined and stacked thrust horses that are bounded by the main fault traces.

    The Lewis thrust surface is a low-angle thrust fault with ramp-flat geometry, indicating that the thrust moved horizontally, stepping upwards through stratigraphic layers. The higher thrust faults in the Lewis duplex are folded over lower faults ramps and their associated horses, indicating that slip on the higher thrusts occurred first, as the formation of thrusts progressed downward and toward the foreland.

    subduction zone magmas

    [link: images: animations: animation of duplex formation; panoramas: Tarndale, New Zealand, Wairau Quicktime panorama (along the fault zone); Death Valley Quicktime Panorama; Interactive 360 degree view of the San Andreas Fault at Wallace Creek; Earth Revealed, courtesy of Anneberg Media, requires Windows media Player; horse: horse wedged between two faults; formations: remarkable internal imbrication or "duplexing" of a single layer (turbidite sequence somewhere in Middle East, from AAPG Bulletin); extensional duplex; duplex in late Paleozoic limestone, Crows Nest Pass, southern Canadian Cordillera; viewing a duplex reverse fault structure, Redwall Fault in Sinclair Canyon, Kootenays, Rockies; beautifully developed duplex structure in glacial lakebeds at Skardu, Karakoram-Himalaya; outcrop scale extensional duplex, Triassic rocks, coast of Chile; thrust fault duplex, near Albany, NY, within the Hudson Valley fold and thrust belt; isoclinal upright anticline (lower part of the Lønstrup Klint Formation in the frontal part of the Stortorn Section) with right limb that constitutes an imbricate duplex formed by connecting thrust-fault splays (white dot-and-dash lines). The fold is interpreted as a hanging-wall anticline developed during fault propagation and successive imbricate stacking; complex duplexing at least three imbricates have been sheared off from and shoved under the continuation of the layer above, in sandstone layer in Tertiary turbidites of Olympic accretionary prism, Washington; imbricated slump structures, imbricated slump structures in sandstone beds and closer view;duplex in sedimentary sequence deformed under partially consolidated conditions, Pliocene tuffaceous mudstones and sandstones, south of Tokyo Japan; duplex in Albany limestone, VT; geomorphic disruption in response to the Hope Fault oblique strike-slip duplex in the Lottery-Mt Lyford area, North Canterbury, NZ; Valley and Ridge, duplex, Pellissippi Parkway; diagrams: fault propagation fold duplex; footwall imbrication; duplex-unit model for fault-bend folding of duplex segments, wp; duplex thrust interpretation of the effect of the downdip step in the plate interface, Hikurangi Subduction Interface, NZ; Gunnison thrust, duplex structure of fault-bend-fold, central Utah; webpages: Thrust Faults; DeGray Spillway, I, II, III]

    image of imbricate thrusting in the Himalayas courtesy of USGS

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