tectonites

Tectonites are rocks in which microstructure (texture) has been distorted by tectonic shearing forces in areas of dynamic metamorphism. Stress is defined in terms of a force and an area across which it acts to strain a rock mass – compression and extension are simple stresses. Shear is a stress that results from the opposition of forces that are not aligned. Strain is a measure of deformation and is the deflection divided by the original dimension. The modulus of elasticity is stress divided by strain. Tectonites form by constrictional deformation (strain ellipsoids). When mineral grains are platy (oblate) the fabric belongs to an S-tectonite. The planar fabric is typically defined by platy minerals such as micas or distorted grains such as quartz, so flattened as to develop schistosity. Schistosity results from a combination of platy minerals and more equant phases such as feldspar and quartz. The term is most commonly applied to medium-grade rocks such as schists of nearly any bulk composition. Gneissic foliation results from equant minerals such as feldspars and quartz. When the minerals are aligned and cigar shaped (prolate), the fabric belongs to an L-tectonite that has deformed under a prolate strain ellipsoid. Linearity is typically defined by either linear minerals, such as hornblende, or by stretched grains, such as quartz. Fabrics in which minerals are distorted into both linear alignment and planar foliation are found in L-S tectonites. Most metamorphic rocks are L-S tectonites that form from a more general strain history in which stresses summate from several directions (shown as an oblique in image above). A porphyroclast is a relict grain larger than the other grains surviving in a deformed rock. Whereas porphyroblasts are larger than surrounding grains because they grew within the rock, porphyroclasts are larger because they have survived the size reduction suffered by surrounding grains. Deformed porphyroclasts act as shear sense indicators – grain tail complexes align with shear, sigma clasts have wedge shaped tails that do not cross the reference plane of shear, and delta clasts have curved tails that cross the reference plane of shear. Mica fish are large isolated mica crystals within a fine-grained recrystallized mylonitic matrix; they are named for their appearance of a lozenge shape and monoclinic shape symmetry with one curved and one planar side, and they can also show stair-stepping. Mica fish lie with their long axis in the extensional quadrant of the deformation, showing steeper inclination to the fabric attractor than mylonitic foliation. This, together with their asymmetry and stair-stepping of trails can be used as a shear sense indicator. The evolution of mica fish probably result from a combination of slip on the basal plane, rigid body rotation, boudinage, and recrystallization at the edges. Slickensides are polished, parallel striations on rock surfaces that were produced by relative motion across opposite sides of fault planes. Although slickensides may appear similar to glacial striations, they pass into the body of the rock. Slickensides can be very helpful in determining the direction of movement.
S-tectonites	S-tectonites are rocks in which all the grains are pancake shaped (oblate). The "S" indicates schistosity, or planar rock fabric. A penetrative planar foliation develops throughout the rock mass at the initiation of shearing. Incipient shear foliation typically orients normal (perpendicular) to the direction of principal shortening as minerals flatten, and the shear foliation is diagnostic of the direction of shortening.
L-tectonites	L-tectonites show lineations due to grains that are aligned and cigar shaped (prolate). In rock masses that undergo large degrees of lateral movement the strain ellipse will lengthen into a cigar shaped volume, and shear foliations distort into a rodding lineation or a stretch lineation.
L-S tectonites	L-S-tectonites have both a linear alignment and a foliation. Within assymmetric shear zones, minerals are flattened and skewed. Pronounced displacements within shear zones may cause shear foliation at a shallow angle to the gross plane of the shear zone. Such foliation at a shallow angle to the main shear foliation can manifest as a sinusoidal set of foliations that curve into the main shear foliation. S/C fabrics result from deformations sufficiently strong for S-surfaces to rotate into parallelism with C-surfaces (discrete planes of concentrated simple shear).
links: images: animation: formation of a delta clast; formations: linear fabric in mylonitic granite from Norway, with clearly stretched K-feldspar grains in a fine-grained matrix of quartz, plagioclase, and biotite, and close-up; linear fabric in mylonitic gneiss; strongly lineated massive gneiss; shear zone; distorted xenoliths in shear zone; slickensides, 2, 3; slickenlines; slickenside lineations on serpentinized ultramafic rocks, Pembine-Wausau volcanic terrane, Wisconsin; contact between massive gneiss (bottom) and megacrystic gniess (top) with parallel lineation, in which the megacrystic gneiss cuts the lineation developed in the massive gneiss; foliation with planar cleavage in slates; slate outcrop; pencil slate; schist; wavy cleavage; C and S foliation; S/C fabric; Rapikivi granite metamorphosing into gneiss (Norway); gneissic banding, Siroua, Morocco; crenulation cleavage in phyllite, Baraboo, Wisconsin; crenulated metamorphic fabric, 2; contact between Tertiary fanglomerates and retrograde mylonitized tectonites, Clara Peak; cleavage refraction between phyllite (left) and quartzite (right), Van Hise Rock, Baraboo, Wisconsin; metaquartzite outcrop; stretched conglomerate; metaconglomerate outcrop; mylonite; pseudotachylite (item); serpentinite; large boudin; highly sheared Carboniferous gypsum layer (even at relatively shallow 'brittle' depths, gypsum undergoes ductile deformation); shear, 2; phyllonite with well developed composite surfaces (yellow dots, NSC; green dots DSF), Late Alleghanian dextral shear zones, Piedmont region; tiny feldspar porphyroclasts in ultramylonite; garnet schist with quartz porphyroblast, Wisconsin; distorted porphyroclasts: assymetric K-spar porphyroclast, 2; sigma clast showing sinistral (left-lateral) shear; sigma clast; pyrite sigma clast; delta clast; contact mylonite-ultramylonite in quartzofeldspathic rock, the ultramylonite matrix is very fine grained and composed of mixed quartz, mica, and K-feldspar with a large albitic content; porphyroclasts in garnet gneiss; thin-sections: micas that form foliations and cleavages in foliated schist; crenulation cleavage; crenulation cleavage that has formed as a result of shortening at a low angle to a pre-existing cleavage; dextral shear in mylonitic quartzofeldspathic rock, blue line indicates discrete C-zone and the green line shows orientation of S-surface in recystallized quartz; distorted porphyroclast; peridotite ultramylonite with extreme grain size reduction associated with localization into 5 mm to 30 m wide shear zones after relatively large and uniform strain, North Pyrenees; mica fish, 2; asymmetric muscovite porphyroclast (mica fish), dextral shear, Goat Rock Fault Zone mylonite in the Georgia Piedmont; sheared amphibolite with assymetric feldspar porphyroclast (blue) and an adjacent NSC surface that has possibly separated the two feldspar grains - brown flaky material is biotite, and green material is hornblende.