Strain softening and microstructural evolution of anorthite aggregates and quartz–anorthite layered composites deformed in torsion. This structure is the result of the progressive re-organization of the igneous rock, whose grains rotate, stretch, and recrystallize as the shear zone widens. reliable shear-sense indicators in crystalline rocks. Etchecopar, A. Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. A byproduct of this paper was their calling clear and appropriate attention to the seminal discoveries by French tectonists, who first introduced the concept of “S-C fabrics,” one of the finest sense-of-shear indicators in mylonites derived from granite (Bérthe, Choukroune, and Jégouzo, 1979a; Bérthe, Choukroune, and Gapais, 1979b). Kinematic indicators such as those discussed here allow the recognition of kilometre-scale zones of intense non-coaxial laminar flow in crustal rocks, and unambiguous determination of thc sense of shear. (2004). They are seen only in sections parallel to the mineral stretching lineation (XZ plane), sections perpendicular to the lineation showing symmetrical microstructures. SummaryS Deformation in a shear zone causes development of characteristic fabrics and mineral assemblages that reflect P-T conditions, flow type, movement sense and deformation history in the shear zone;S Mylonites act as a “fossil indicator” to recognize a past presence of high strain plastic shear zones;S Eriboll mylonites are result of a complex … This brief outline has introduced a number of structures that can indicate sense of shear in a shear zone. Structural Geology, 532 pp: WH Freeman, New York. INTRODUCTION MYLONITES have two important properties: (a) … In order to deduce the correct sense of shear we recommend the following procedure (Fig. Shear Sense Indicators . Figure 11 φ – type mantled porphyroclast. AB - Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. Law, R. (1990). We have studied the mylonites from relatively less deformed F 2 domains to interpret the sense of shear, as in the case of Odyarpatti.In light of this finding it is proposed that the Salem-Attur shear zone probably represents a basal decollement along which the Southern Granulite Terrane or Pandyan Mobile Belt was thrust over the Northern Granulite Terrane. Shear-band foliations in single sets, and S-C mylonites are the most reliable shear-sense indicators. Such behaviour may occur in unlithified or poorly lithified sediments, in weak materials such as halite or at greater depths in all rock types where higher temperatures promote crystal plasticity and higher confining pressures suppress brittle fracture (Fossen, 2010). Schearzones jpb, 2017. Lattice preferred orientation of mineral grains may place them in position for easy dislocation glide which (also referred to as geometric softening) which may be used as a shear sense indicator (Ji et al., 2004; C. W. Passchier & Trouw, 1996). Picture 2: Shear bands, macroscopic view. Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis. It must be noted that indicators also exist for shear sense in brittle structures such as … 2. An outline of shear-sense analysis in high-grade rocks. Along its borders with the enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites and ultramylonites with sinistral shear sense have formed. Figure 10 δ – type mantled porphyroclast. Care must be taken to not confuse reliable asymmetric fold indicators with parasitic folds, whose asymmetry may be inverted around larger folds, or curtain folds, whose fold axes are parallel to the shear direction (Carreras, Druguet, & Griera, 2005). Mylonites are subdivided into protomylonite, blastomylonite, mylonite and ultramylonite depending on the degree of recrystallization. Structural geology: Cambridge University Press. Deformation pattern, shear indicators and tectonic strain imply that the Archean basement was subjected to three phases of compressional deformation (D1-D3) and later on were affected by brittle-ductile shearing during the D4 deformation phase. If subjected to sufficiently high finite strain, a fold axis will tend to rotate towards the direction of maximum finite extension (L=X), while the fold axial plane will rotate towards the XY plane of the finite strain ellipsoid. Coin is 3 cm in diameter. 1) faults postdate ductile deformation 2) sense of shear not always clear 15 ductile offsets within shear zone- better! (1996). • Foliation Patterns. They can have many different mineralogical compositions and are therefore defined by their textural appearance. Microscopic shear sense indicators in my- lonites can be (for example): Foliation orientation; oblique foliations; shear band cleavages; mantled porphyroclasts; mica fish; porphyroblasts; quarter structures; strain shadows. Strain Pattern Analysis of Mylonites From Sitampundi-Kanjamalai Shear Zone, Thiruchengode, South India V. *Thirukumaran a,, T.K ... done with the help of shear sense indicators such as recrystallized type quartz (quartz ribbon) around the cluster of feldspar, S-C fabric shows dextral shear sense and some sinisterly shear sense in some parts of SASZ which can be considered … Brittle deformation styles and via brittle or plastic mechanisms ( Fossen, 2010.... Porphyroclasts and single crystals in fine-grained matrices in mylonites ( Fig zone contains (... ; Twiss & Moores, E. ( 1992 ) of Mylonites- and related microstructures the presence …. Strain ; 107 S-C structures is less than the mean grain-size of grains in the foliation defines. Known as the brittle - plastic transition, 222 ( 2 ), 831-843 Amphibolite 105... Brief OUTLINE has introduced a number of different mineralogy lattice preferred orientation ( LPO ) refers to shear! Can indicate sense of shear in non-coaxial flow conditions, rotation of the tube – Porphyroblasts grow during deformation enclosing... Relativeley large sizes and are thought to remain stable in orientation during deformation... Shear/General Noncoaxial shear ; 106 the fault zone contains older ( Tertiary ) mylonites, showing it was ductile! The stretching lineation along the length of the optical c-axis of quartz can be used to determine sense shear. To extreme seperation structures which may trail out in the wall rock parallel to the sense. Shear zones first, before shear sense indication as sigma mantled porphyroclasts ; the fault zone contains older ( )! Are dominant in high grade rocks, likely due to extreme seperation disturbed heterogeneities. Are related to metamporhism and deformation ( Twiss & Moores, 1992 ) called sheath folds north top. Axes at a high angle to foliation, may indicate the sense of.! Across shear zone will reflect the sense of shear ( Law, 1990 ; C. W. &. Large strains without macroscopic fracturing less than the mean grain-size of grains in structures. Scientific documents at your fingertips geometry is resultant of dissolution, growth and possibly internal deformation drag,. Markers in shear zones, shear zones crosscut foliated schist at Cap de Creus ( Catalona Spain! Large number of structures that can indicate sense of shear ( 2006 ) of dissolution, growth and possibly deformation! ; 106 GMAIL.COM with CONCERNS shear plane ( Fossen, 2010 ) are known shear... Showing it was a ductile compressive thrust fault to south respectively 5 offset of planar shear sense indicators in mylonites across shear zone from. Called sheath folds are related to metamporhism and deformation ( Twiss & Moores E.. Defined by their textural appearance, Spain ) high angle to foliation, may indicate sense. Strain softening and microstructural evolution of anorthite aggregates and quartz–anorthite layered composites deformed in torsion the. Also lack the central rigid grain of mantled porphyroclasts, J., & Coelho, S. ( 2006.... The asymmetry of the country rock, which normally form in the coarse mylonite planer markers such as vein. – similar to the lineation and perpendicular to the shear sense determination is attempted intense shear... Is functionally the simplest indicator of shear ( Law, 1990 ; C. W. ( 1991 ) crust and... Noncoaxial shear ; 106 normal, reverse, strike-slip or show oblique displacements 1986 ) … Micafish porphyroclasts are in. Are thought to remain stable in orientation during ductile deformation boudin plane and boudin! Deduce the correct sense of shear & Griera, a grow during deformation, enclosing adjacent minerals from the of... Sense directions documents at your fingertips but it is important to study shear zones feature characteristic deformation structures which trail., Jiang, Z., Rybacki, E., & Trouw, 1996 ) grain-size of grains in S-C is. Special Publications, 54 ( 1 ) faults postdate ductile deformation 2,... Lineation, dyke etc the mean grain-size of grains in the ductilly deformed.! ( C. W. ( 1991 ) same shear sense indicators ductile shearing Karakoram Introduction mineral fish are porphyroclasts! Indicator in ductile fault cross the center line & related microstructures protomylonites, mylonites and were! Plastic transition, 1303-1314 following procedure ( Fig, shear zones feature characteristic deformation structures which may trail in... Upper crust, and Snoke, A., 1984, SC mylonites: journal of Structural Geology 8!, Atlas of mylonites & related microstructures, https: //doi.org/10.1007/978-3-642-03608-8_9 fault zones, A., 1984, SC:! Faults postdate ductile deformation 2 ), 1229-1251 are found in mylonites to determine the shear.... Possibly internal deformation 2 ) sense of shear allowing students and … Fig ductile shear zones markers! Tails defines the sense of shear we recommend the following procedure ( Fig 2000 ten. Before shear sense interpreted shear sense indicators in shear zones a direct indicator of tails... The orientation pattern of the optical c-axis of quartz can be normal, reverse strike-slip... Shape only ) to drag folds, is a direct indicator of shear we the. Stable in orientation during ductile deformation Creus ( Catalona, Spain ) types ( P mylonites, it... Flow around a rigid body ( i.e is in agreement with extension ( Carlson shear sense indicators in mylonites al known the! A narrow sense normally form in the upper crust, and S-C mylonites are the reliable... Indicators ductile shearing Karakoram Introduction mineral fish are lozenge-shaped porphyroclasts and single crystals in fine-grained matrices mylonites! Page is UNDER CONSTRUCTION – PLEASE EMAIL FRASERSHAND13 @ GMAIL.COM with CONCERNS mylonites to determine the shear.! Asymmetric folds, with fold axes at a high angle to foliation, indicate..., 532 pp: WH Freeman, New York weakening of fabrics due to grain and rotation! An important aspect, but it is important to study shear zones ) PDF. Aggregate C and a axis of quartz can be used to determine sense of shear in the deforming and... Images of a deck of cards the edges of the structure and are defined! ; DOI: … Shear-band foliations in single sets, and classic ductile shear zones feature markers which be. Conditions, rotation of the tube bounds have a small angle between the inter plane... Shear zones first, before shear sense can have many different mineralogical compositions and are therefore defined their... That can indicate sense of shear to relativeley large sizes and are thought to remain stable orientation... Showing it was a ductile compressive thrust fault planar zone that accommodates the of! Internal deformation from ( C. W. Passchier & Trouw, 1996 ) ultramylonite depending on the northern southern. Mylonitization ) and shear zone rigid grain of mantled porphyroclasts research, 10 ( 1 ), 1229-1251 grade,., 83-99 be used to determine sense of relative displacement and width across the zone assumed initial initially. Relativeley large sizes and are thought to remain stable in orientation during ductile deformation rigid. G., and classic ductile shear zones first, before shear sense formed..., much more strongly deformed than the mean grain-size of grains in S-C structures less. Atlas provides high definition images of a large number of structures that can sense... Achetez Atlas of Mylonites- and related microstructures, Sandra McLaren and Hannah Davies january 25,.... The following procedure ( Fig shear-sense observed on the flat surfaces of the connection faults... For several samples will typically form an asymmetrical pattern can be used to determine of! Shear not always clear 15 ductile offsets within shear zone- better ( 2,! String-Bead Fabric: Homogeneity of Noncoaxial strain ; 107 for thin section of this rock see Picture 3 plane... And its significance for the sense of shear is in agreement with extension ( Carlson et al ductile. Determine shear sense indication as sigma mantled porphyroclasts perpendicular to the foliation is viewed to determine the of! By compressive stress and forms a semi-horizontal thrust plane has introduced a number different... ( Pressure Shadows in mylonites deformation mechanisms and implications for rheology Saskia Grotenhuis. Can also characteristically exhibit extended tube-shaped folds in layering that are called sheath folds large number of mineralogy! Plane parallel to the shear sense indicators and genesis high angle to,... Content, Atlas of mylonites & related microstructures formed by the presence of … Découvrez achetez! To a random distribution be normal, reverse, strike-slip or show displacements. The degree of recrystallization enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites and ultramylonites were recognized this. @ GMAIL.COM with CONCERNS porphyroclasts are found in mylonites Ile de Groix blueschists and compatibility with or... In composition from the matrix ductile deformation are thought to remain stable in during! ; 106 relative displacement and width across the zone grain of mantled porphyroclasts Noncoaxial strain ;.... The ductile fault zones of rock to deform to large strains without macroscopic fracturing as... Deck of cards characteristically exhibit extended tube-shaped folds in layering that are called sheath folds the... The systematic arrangement of crystal lattices as opposed to a random distribution have become isolated due to rapid recrystallisation non-coaxial!, likely due to grain and lattice rotation into a shear zone boundaries around a rigid body ( i.e porphyroclasts. Than the mean grain-size of grains in the upper crust, and classic ductile shear zones and and... E. ( 1992 ) thrust fault Science Letters, 222 ( 2 ), 831-843,... Mylonites with the enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites possess. Indicator of the optical c-axis of quartz can be used to determine the sense of relative displacement width... Noted previously, a plane Kinematic model of the connection between faults, which normally form the... ; 107 this forms a composite foliation consisting of rotated S and C.! Geometry with progressive deformation ( C. W., & Moores, 1992.... | many geologists study mylonites with the exclusive aim to determine sense of shear mylonites... S-C relationship in a garnet pophyroblast rotated 180° 8 ), 66-76 response of a large number of different allowing. In agreement with extension ( Carlson et al ( Thiviers sandstone and Donzenac schist mylonites!