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

shear zones

Shear zones involve volumes of rock deformed by shearing stress under brittle-ductile or ductile conditions, typically in subduction zones at depths down to 10-20 km. Shear zones often occur at the edges of tectonic blocks, forming discontinuities that mark distinct terranes. Shear zones can host ore bodies as a result of hydrothermal flow through orogenic belts, are commonly metasomatized, and often display some retrograde metamorphism from a peak metamorphic assemblage.

Close to the Earth's surface, cool rocks respond to tectonic stresses with fracture and faulting. At greater depths than ductile shear zones, migmatites result from high temperature/high pressure prograde Barrovian regional metamorphism, and at still higher temperatures, rocks melt to form magmas.

Transpression regimes, such as the Alpine Fault zone of New Zealand, form during oblique collision of tectonic plates and during non-orthogonal subduction. Transpression typically generates oblique-slip thrust faults, strike-slip faults, or transform faults. Microstructural evidence of transpressional regimes include rodding lineations, mylonites, augen-structured gneisses, and mica fish.

Transtension regimes are oblique tensional environments that result in oblique, normal geologic faults and detachment faults in rift zones. Microstructural evidence of transtension includes rodding or stretching lineations, stretched porphyroblasts, and mylonites.

Shear zones can extend from centimeters to several kilometres in width, and display deformation, folding, and foliations in dynamically altered rocks (breccias, cataclasites, mylonites, S-L-L-S breccia or cataclasite is formed, with the rock milled and broken into a mélange of random fragments.

Pseudotachylites form at depths from 5-10 km, where confining pressures are focused into discrete fault planes and are sufficient to prevent brecciation and milling. The frictional heating at these depths can melt the rock to form pseudotachylite glass or mylonite, and adjacent to these zones, can result in growth of new mineral assemblages.

At greater depths, angular breccias transform into ductile shear textures and mylonite zones, as ductile shear zones accommodate compressive stress through dislocation creep within minerals, fracturing of minerals and regrowth of sub-grain boundaries, or by lattice glide along preferred orientation foliation planes in phyllosilicates.

Within the depth range of 10-20km, ductile deformation conditions prevail and frictional heating is dispersed throughout shear zones, resulting in distributed deformation and a weaker thermal imprint. Here, deformation forms mylonites, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

◙ subduction zone magmas ◙

[links: images: animation: fabric in simple shear; shear zone experiment; formations: mylonitic migmatitic granite-gneiss in shear zone, Epupa Complex, S of Red Drum, NW Namibia; Golden Eagle Shear Zone, Yukon; melt enhanced shear zone, along the base of an intruding batholith; shear zone in the axial zone of the Pyrenees, Parc natural del Cap de Creus, Spain; dike cutting a shear zone, Snake Range, Nevada; sheath fold in boulder, Tarfala Valley, Sweden, and sheath folds, nSweden; fold in high strain zone, NZ; close-ups: ultramylonite core (~1 cm thick) from ductile shear zone of the Diana Syenite of the NW Adirondacks; shear zone related fold in the Kohistan Arc Complex, Northern Pakistan; rock texture in shear zone; rock in ductile shear zone; right-lateral, ductile shear zone; anorthosite in ductile shear zone, Adirondacks; close-up of dextral shear zone; leucosome cuts gneissic layering; 1.7 Ga foliated quartz monzonite of Boulder Creek batholith in Idaho Springs-Ralston shear zone with strong mylonitic (sheared) fabric that parallels the shear zone, 2, 3; 1.7 Ga metapelite (metamorphic marine claystone) that includes large porphyroblasts of pink quartz and andalusite (dull dark gray blocky crystals), and wavy alignment of porphyroblasts in this rock with a mylonitic fabric indicates a complex deformation history; with en echelon antithetic veins in dextral shear zone, Baraboo Quartzite; sigmoidal antithetic fractures in a dextral shear zone, Tiddiline Conglomerate, Bou Azzer inlier, Morocco; mylonitic marble in shear zone, Escambray Massif, Central Cuba; lower greenschist facies shear zone cutting basement schists, assymmetric clast of pegmatite, assymmetric pod of leucogranite in schist, ptygmatic folds of leucogranite in schist, assymmetric pod of schist, Cap De Creus, neSpain; thin-sections: thin section of Lower Ordovician Pinnak Sandstone showing multiple tectonic foliations, the most prominent of which is a crenulation cleavage that overprints an early fine foliation; euhedral staurolite (yellow pleochroic in PPL) overgrows shear zone between large light coloured plagioclase porphyroblasts (graphite inclusions outline shear zone, staurolite crystals postkinematic); garnet with spiral-shaped inclusion trails indicating synkinematic growth, and a dextral sense of shear; diagrams: cataclasite-mylonite in shear zone; block diagram - shear zone host for gold, geometric relationships between structural elements of zone and veins; region within macroscopic shear zone illustrating bimodal porosity distribution within shear zone; model of shear zone]