boudinage

Boudinage refers to structures deformed by extension in ductile shear zones. Boudinage structures contain a rigid tabular body that has been stretched and deformed where embedded within more deformable (less competent) rocks.

Competent tabular bodies that are susceptible to boudinage include veins and strata such as sandstones. Where conditions favor brittle fracture rather than ductile deformation, imbricate (overlapping) fracturing occurs.


In boudinage, the competent bed break ups into sausage-shaped boudins – forming structures such as ribbon-like boudins or chocolate-tablet boudins (depending upon the axis and isotropy of extension).


[links: images: Amphibolite boudins in gneisses; formations: Zoroaster Veining, boudins composed of quartz and plagioclase, boudin of metagabbro (HP mafic granulite) in tonalitic gneiss]

| 0 Guide-Glossary

deformation

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

enclaves

Magmatic enclaves are volumes of rock surrounded by emplaced host rock of related but distinct composition and of separated genesis (incomplete magmatic mixing).

Enclaves are distinguished from xenoliths, which are fragments of metamorphically altered older country rock that fell into magma or lava and became enveloped within igneous rock. Enclaves are also distinguished from schlieren, which are concentrations of mafic material that have crystallized out of a single magma.

Microgranitic (mafic) enclaves are common components of plutonic bodies and may represent the result of mingling of mafic and felsic magmas. These enclaves are often ellipsoidal or elongate in shape.

Studies on mafic enclave-host granite pairs indicate that enclaves and their host granites share compositional characteristics indicating their close relationship, but that the two groups of rocks are not cogenetic. The enclave-host relationship was probably acquired through pervasive mechanical and chemical interaction (especially differential interdiffusion) between two originally independent magmas. Microgranitoid enclaves typically show microstructural
evidence that suggests that prior to incorporation of parent magma globules into the host magma (during magma mingling), the enclaves underwent magma mixing, in a setting where the mafic magma was more abundant than the felsic magma.

The most mafic enclaves are generally the most stable enclaves with respect to disruption by entraining flow, and proto-enclaves with silica contents close to those of the host granite are highly unstable. Enclaves are often interpreted as strain markers. However, most deformation of microgranitic enclaves probably occurs at relatively high temperatures (950 -1050 °C), so the enclaves record magmatic strain of the host over only a limited temperature-time range in the host’s cooling history. Observation of apparent deformation of enclaves in a liquid regime implies that magmatic flow velocities are likely to be below 10 m/yr in enclave-bearing plutonic systems. [r, 2, 3]

[links: images: Mafic microgranular enclave (MME) dense zone in the upper part ofthe Kurobegawa Granitic Pluton, Japan, and mafic microgranular enclaves (MME ) mingled with filsic crystal mush mainly consisting of porphyritic plagioclase and quartz; Rock dragon enclave; magmatic enclave rich in biotite and relatively fine-grained; magmatic enclave at Elephant Rocks; mafic enclave dike; mafic magmatic enclave (Nikia lavas) with quench rims; mafic enclaves in tonalite with leucocratic halos around the mafic enclave, and magmatic fabric in tonalite indicated by prefered orientation of long axis of mafic enclave; diorite enclaves in Ross of Mull Granite show rounded and lobate forms and more angular forms. This varitety suggests that the diorite and granite magmas coexisted as liquids, although it is possible that the granite was partially crystallized when the diorite magma was intruded. Some show complex veining structures, and many have conspicuous K-feldspar megacrysts not present in the main diorite. These textures show that the two magmas have interacted extensively in some cases.; Silvermines Granite at Tiemann Shut-Ins with a variety of enclaves shapes, from rounded / diffuse to sharp / angular; melanocratic enclave in a leucocratic granite; enclave from ERSNA and the ERB, feldspars within this enclave show both rapakivi (left) and anti-rapakivi (right); rock, 2; mafic enclave, Brazil; mafic enclaves in tonalite of Lake Mary, mafic dike in tonalite of Lake Mary containing mafic enclaves similar in composition of that of the mafic dike, 2, mafic dike with dark, chilled margin in tonalite of Lake Mary, concentration of mafic enclaves in tonalite of Lake Mary, mafic enclaves with plagioclase xenocrysts inherited from the tonalite of Lake Mary host, differential weathering of mafic enclaves in tonalite of Lake Mary, aplite dike cutting cluster of rounded mafic enclaves; close-up: spheroidal mafic enclave in granodiorite of Summit Lake; Desolation Valley granodiorite with mafic enclave, surrounding granodiorite has anhedral interstitial K-feldspar and rim myrmekite, but the mafic enclave is not penetrated and replaced by K-feldspar; change in colour related to minute changes in mineralogical composition that may indicate the presence of a magmatic banding; thin-sections: portion of mafic enclave from Desolation Valley granodiorite which contains hornblende (tan), biotite (reddish), albite-twinned and zoned plagioclase (black and white), and quartz (cream); webpages: Igneous Processes in the Moruya Batholith, Igneous and Metamorphic Geology Field Trip.]

fault attributes

Faults 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.



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.


Crustal 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).

faulting

Complex 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.

Shear 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]

| 0 Guide-Glossary

folding

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