mylonites

Mylonites are fine-grained, compact, laminated, metamorphic rocks with textures greatly modified by predominantly ductile, plastic flow due to dynamic recrystallization in fault zones. Mylonites develop predominantly in ductile shear zones where high rates of strain are focused into thin layers. Thus, mylonites are are the deep counterparts to the breccias and pseudotachylites generated by shallower, cataclastic, brittle faults. In ductile deformation, rock deeper than 10-20 km is subjected to enormous lithostatic stress, and the high temperatures of burial render the hot rock softer and more malleable. At these depths, in the lower continental crust and mantle, rock undergoes plastic deformation and flows in response to application of a differential stress that is stronger than its yield strength. Rock undergoes ductile deformation by gradual creep along crystal grain boundaries and planes within crystal lattices. Mylonites form at depth in ductile shear zones at temperatures above 300ºC in continental and oceanic crust. Depending upon the protolith, mylonites can have many different mineralogical compositions, ranging from quartzo-feldspathic to olivine-pyroxenite composition. Classification as a mylonite depends upon a flow texture resulting from dynamic recrystallization in response to shear stress, stress and deformation. As mylonites are compacted, minerals recrystallize, becoming smaller in grain size and reducing the free energy state of grain boundaries. Mylonites may also exhibit some component of milling of fragments. The microstructures that develop during mylonitization depend upon applied stresses, strain, strain rate, confining pressure, temperature, original mineralogy and modal compositions, and presence or absence of fluids. Mylonites can be L-tectonites, S-tectonites, or L-S tectonites. ◙ L- tectonites are highly deformed rocks with a very strong linear fabric. ◙ S- tectonites are highly deformed rock with a very strong planar (schistose) fabric. Blastomylonites are coarse grained, often sugary in appearance, often without a strong fabric evident, and lack distinct tectonic banding. Thermal recovery, static recrystallization produced a late stage or subsequent increase in grain size. Ultramylonites have undergone extreme grainsize reduction, so they are hard, dark, cherty to flinty in appearance and may be confused with volcanic rocks, pseudotachylite and obsidian. Ultramylonites commonly have porphyroclastic inclusions. Phyllonites are phyllosilicate rich mylonites comprising predominantly chlorite or mica. Phyllonites may have highly strained inclusions, and typically have a simple shear fabric or well-developed secondary shear (C') fabric. Phyllonites often have distinctive composite fabrics that indicate the simple shear component. Because micas can grow and recrystallize relatively easily, phyllonites often lack notable grain size reduction. Interpretation of the shear sense of stresses that generated mylonites relies upon identifying the S-plane, C-plane and the C-S intersection angle. Rotated porphyroblasts, and rotated clasts and their mantles may provide indicators of shear sense. Dikes and veins, crenulation cleavage, asymmetric folding, fragmented porphyroclasts, mica fish, and mantled porphyroclasts can provide offset markers. Previous orientation may be recorded by poikiloblasts and other relict structures, but their deformation obscures details of the origin of the protolith.

links: images: formations: Cap de Creus; mylonitic foliations; mylonites derived from metasediments, Knockan, Scotland, and Moine mylonite, Moine Thrust Zone, Knockan Crag; mylonites; mylonite; Brevard Zone Mylonite; thrust-nappes in tonalite mylonite in the western footwall to the Isua Greenstone Belt; Sigmoidal Antithetic Fractures in a Dextral Shear Zone, Tiddiline Conglomerate, Bou Azzer inlier, Morocco; En Echelon Antithetic Veins in a Sinistral Shear Zone in block that moved down and to the left, Baraboo Quartzite; hand-specimens: mylonite; strongly deformed leucogranite lenses in mylonitic metasedimentary rocks (lower Indian cover sequence) between Garol and Gashit, near the western margin of the Jalhari Shear Zone; close-ups: mylonite exhibiting recrystallization of feldspar (lightest crystals) and ribbons of quartz (grey ) in a dark, biotite-rich matrix; Mylonites of the Main Mantle Thrust, eastern margin of the massif on the Indus Gorge, near village of Silbu, and granitoid protolith S/C mylonite from Chhichi Nullah, and S/C mylonite of Rupal-Chhichi Shear Zone, showing oblique west side up (thrust) and dextral motion, Rupal-Chhichi Shear Zone, in Chhichi Nullah, ~5km-wide shear zone forms the southeastern margin of the central Nanga Parbat massif (eastern fault); mylonite with typical S/C structure, without labels, texture S/C with combination of two types of deformation - "microfailles" of Riedel twisted into sigmoid “crochons” (diagram), 2, 3; sigmoidal lenses of quartz define shear bands mylonitic and cataclastic rock and more mylonitic and cataclistic rock in the southeastern San Bernardino Mountains, southern California – these rocks occur in the lower plate of the eastern Peninsular Ranges Shear Zone, a zone of crustal movement and dislocation that occurs along the east margin of the San Jacinto and Santa Rosa Mountains and in various locations within and east of the Borrego Desert. The original rock type was plutonic granodiorite and tonalite that has been subjected to high-strain squeezing, stretching, and grinding of the mineral crystals to yield a rock whose original grain-to-grain relationships have been completely modified; such a high-strain rock is known as cataclasite or mylonite, depending on the brittle or ductile nature of the deformation style; micaceous minerals that crystallize during early faulting have been folded and broken during later fault movement in the Piedmont; mylonite, Bonavista Bay, Canada; Mary Opx-bearing granite mylonite, East Athabasca mylonite triangle, NW Saskatchewan, Canada; mylonite, 2, 3, 4; tailed clast in talc mylonite, Urals; mylonite; granitic proto-mylonite; Hornsilver Ridge mylonite; marble mylonite; fine-textured ultramylonite derived from coarse-grained migmatitic gneiss, dextral Goat Rock Fault zone in the Appalachian hinterland, Georgia; thin-sections: mylonite, Sigma GrainTail Complex in mylonite - dextral shear, 2; Ensay Shear Zone mylonite, t-s of mylonite derived from granite, showing dextral shear, Livingstone Creek; western Piedmont phyllite contains isolated fold noses (outlined by light colored quartz-rich zones) that imply that the rock has been intensely sheared; mylonite, Ragged Ridge, NC; webpages: Foliation and Lineation; Structure Photo Gallery; Greenland Research Snapshots; Brittle Structures; Deformation; Ductile Structures; Fabrics: Folications and Lineations; Faults - normal, reverse, strike-slip; Folds; Joints; Mohr's Circle for Stress; Strain Rate and Rheology; Shear Zones; Stress; Veins;