shear sense indicators in mylonites

The formation of ductile shear zones is commonly associated with drastic grain size reduction and the formation of well banded, lineated rocks termed mylonites (Fossen, 2010). Shear – is the response of a rock to deformation usually by compressive stress and forms particular textures. Formation mechanism is unknown but many form as shear band components that have become isolated due to extreme seperation. Due to their orthorhombic symmetry and lack of stair-stepping in the tails, they can not be used to determine shear sense (C. W. Passchier & Trouw, 1996). Structural Geology, 532 pp: WH Freeman, New York. feldspathic mylonites (e.g. Many geologists study mylonites with the exclusive aim to determine the sense of shear. 5. (1977). Typical example of dextral shear foliation in an L-S tectonite, with pencil pointing in direction of shear sense. The shadow domains are filled with recrystallized … A shear zone is a planar zone of concentrated deformation which by itself, or in association with other zones, helps to accommodate, or wholly accommodates an imposed regional or local strain rate beyond the strength of the country rock. 2003; also Mukherjee and Pal 2000). Many geologists study mylonites with the exclusive aim to determine the sense of shear. Space formed by the deflection of flow about the object forms a lower pressure region into which material from the wall rock will migrate by fluid advection or by fluid-enchanced diffusion, and recrystallise (Fossen, 2010). Adapted from Passchier & Coelho (2006). Genesis Shear sense indicators Ductile shearing Karakoram Introduction Mineral fish are lozenge-shaped porphyroclasts and single crystals in fine-grained matrices in mylonites (Fig. This is a preview of subscription content, Atlas of Mylonites- and related microstructures, https://doi.org/10.1007/978-3-642-03608-8_9. 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). Picture 2: Shear bands, macroscopic view. Carreras, J., Druguet, E., & Griera, A. ; The fault zone contains older ( Tertiary ) mylonites, showing it was a ductile compressive thrust fault. Care should be taken with apparent offsets to not interpret the wrong sense of shear from outcrop surfaces or thin sections (C. W. Passchier & Trouw, 1996). 2. 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. Index 302. geologia For thin section of this rock see Picture 3. Shear Sense Indicators . Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. (1984). (2005). Sheath folds contain the stretching lineation along the length of the tube. INTRODUCTION MYLONITES have two important properties: (a) … Adapted from Passchier & Coelho (2006). Conjugate shear zones In contrast to … 2002; ten Grotenhuis et al. Figure 9 σ- type mantled porphyroclast. Law, R. (1990). 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. Ductile faults can also characteristically exhibit extended tube-shaped folds in layering that are called sheath folds. Because of the constraints imposed by simple shear, displacement is assumed to occur in the 2. Different types of mantled porphyroclasts are distinguished by their unique tail geometry (CW Passchier & Simpson, 1986; Twiss & Moores, 1992). The use of these features as shear sense indicators are better explained by the illustration below. The shear-sense observed on the northern and southern flanks is found to be – top to north and top to south respectively. Plotting c-axis orientations for several samples will typically form an asymmetrical pattern. Pure shear is defined as a constant-volume plane strain in which the displacement of all material particles is strictly parallel to the shear plane and the principle axes of strain are not rotated by the deformation (Fossen, 2010). © 2020 Springer Nature Switzerland AG. 2003; also Mukherjee and Pal 2000). These small scale shear zones (millimetre to centimetre) are known as shear bands. A shear zone is defined from a fault for example, due to a rotational motion of material within the zone, which is produced as a consequence of displacement. The determination of true shear senses from the deflection of passive markers in shear zones. Mineral fish are lozenge-shaped porphyroclasts, single crystals in a finer grained matrix, which occur in ductile shear zones and which are commonly used as shear sense indicators. A constant volume-plane strain in which the displacement of all material particles is strictly parallel to the shear plane (Fossen, 2010). Mica fish in mylonites deformation mechanisms and implications for rheology Dissertation zur Erlangung des Grades "Doktor der Naturwissenschaften" am Fachbereich Geowissenschaften der Johannes Gutenberg-Universität in Mainz Saskia Martine ten … Ramsay, J. If you look carefully in the photo above, indeed, you can … Schematic geometry of mylonite zone. 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. Ductile shear zones feature markers which can be traced continuously through the zone, generally exhibiting non-coaxial deformation. Figure 4: Schematic aggregate c and a axis quartz fabrics and interpreted shear sense directions. φ porphyroclast – these features are dominant in high grade rocks, likely due to rapid recrystallisation in non-coaxial flow. Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. "1.Mylonites form in response to high rates of strain within deep ductile shear zones, which are the extensions at depth of surface faults, thrusts and fault breccias. Orientation pattern of the optical c and a axis of quartz can be used to determine shear sense in quartz rich mylonites. A shear zone is a tabular zone in which strain is notably higher than in the ... Kinematic indicators – asymmetric structures characterize non -coaxial deformations while coaxial deformations tend to result in more symmetric structures. ... Shear Sense Indicators. Adapted from Burg, J.P. Their geometry is resultant of dissolution, growth and possibly internal deformation. Journal of Structural Geology, 2(1), 83-99. Cores are usually composed of primary pyrogenetic minerals, whereas mantles, including tails, are composed of … Opposing shear senses in a subdetachment mylonite zone: Implications for core complex mechanics Frances J. Cooper,1,5 John P. Platt,1 Ellen S. Platzman,1 Marty J. Grove,2,3 and Gareth Seward4 Received 24 November 2009; revised 23 February 2010; accepted 2 March 2010; published 4 August 2010. 7 Thermal structure of shear zones from Ti-in-quartz thermometry of mylonites: Methods and example from the basal shear zone, northern Scandinavian Caledonides 93 Andrea M. Wolfowicz, Matthew J. Kohn, and Clyde J. Northrup 8 Brittle-ductile shear zones along inversion-related frontal and oblique thrust ramps: Insights from the Central–Northern Apennines curved thrust … 1 for micas; ten Grotenhuis 2000; ten Grotenhuis et al. ; The fault zone contains older ( Tertiary ) mylonites, showing it was a ductile compressive thrust fault. Shearzones_shearindic - Free download as Powerpoint Presentation (.ppt), PDF File (.pdf), Text File (.txt) or view presentation slides online. Mylonites form in response to high rates of strain within deep ductile shear zones, which are the extensions at depth of surface faults, thrusts and fault breccias. The base thrust of the Giessen nappe consists of cataclasites, breccias and mylonites and forms a semi-horizontal thrust plane. A study of some typical inclusion patterns in strongly paracrystalline-rotated garnets. English contemporary dictionary. This asymmetrical pattern can be used to determine sense of shear (Law, 1990; C. W. Passchier & Trouw, 1996). reliable shear-sense indicators in crystalline rocks. (1980). Dextral, ductile shear zones crosscut foliated schist at Cap de Creus (Catalona, Spain). Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis. Continued rotation and growth of a porphyroclast result in a helical train of inclusions from overgrowing pressure shadows which define the sense of grain rotation and consequently, the sense of matrix shear (Schoneveld, 1977; Twiss & Moores, 1992). Schearzones jpb, 2017. 9.1) Shear-band foliations in single sets, and S-C mylonites are the most reliable shear-sense indicators. Introduction Therefore, a plane parallel to the lineation and perpendicular to the foliation is viewed to determine the shear sense. Hanmer & Passchier, 1991; Quinquis, Audren, Brun, & Cobbold, 1978, CW Passchier & Simpson, 1986; Twiss & Moores, 1992, Mukherjee, 2011; C. W. Passchier & Trouw, 1996, C. W. Passchier & Trouw, 1996; Twiss & Moores, 1992. Adapted from (C. W. Passchier & Trouw, 1996). Livraison en Europe à 1 centime seulement ! There is a transition from Y-maxima and asymmetric single girdles in mylonites and ultramylonites within 300 m of the present fault trace, to cross-girdle fabrics in the protomylonites further from the fault. Figure 8 Sheath folds formation. Simple shear is a constant-volume plane strain with characteristics that resemble the shearing of a deck of cards. The rotation of the fold axial planes from vertical with progressive fold tightening reflects the rotation of the principal direction of finite strain relative to the instantaneous stretching axes of flow. This brief outline has introduced a number of structures that can indicate sense of shear in a shear zone. Journal of the Geological Society, 144(1), 73-77. Shear zone – defined as the localizing of strain, in particular shear strain, into a tabular zone with a measurable thickness (ranging from kilometres to millimetre scale) (Twiss & Moores, 1992). Figure 14 Daughter mica fish resultant of a “calving” process from a parent grain, as proposed by Lister and Snoke (1984). Mica fish in mylonites deformation mechanisms and implications for rheology Saskia ten Grotenhuis. These mica fish can be subdivided into six … 1 for micas; ten Grotenhuis 2000; ten Grotenhuis et al. Schoneveld, C. (1977). Shear zone geometry: a review. String-cylinder model adapted from Schoneveld (1977). Colloidal gold and silica in mesothermal vein systems, High-angle reverse faults – the “fault-valve” model, Silicate mineral aggregates: inter- and intra- crystalline fluids, Feedback from OU Academic Staff on First Part of Assignment, Beginners guide to interpreting deformation features, (Fossen, 2010; C. W. Passchier & Trouw, 1996), (Fossen, 2010; C. W. Passchier & Trouw, 1996; Ramsay, 1980), (Fossen, 2010; Ji et al., 2004) (Hanmer & Passchier, 1991; Platt, 1984), (Ji et al., 2004; C. W. Passchier & Trouw, 1996), (Law, 1990; C. W. Passchier & Trouw, 1996). The long dimension of elongate grains in S-C structures is less than the mean grain-size of grains in the coarse mylonite. These results demonstrate that the drag patterns of APS around porphyroclasts in mylonites are highly reliable indicators for the determination of the sense of shear. Not logged in Shear sense indicators in Alpine fault mylonites, both showing top to the W shear sense (a) sigma-object feldspar porphyroclast in GC27; (b) Type II SeC fabric of Lister and Snoke (1984) in GC12. Lister, G., and Snoke, A., 1984, SC mylonites: Journal of Structural Geology, v. 6, no. Figure 3: Progressive weakening of fabrics due to grain and lattice rotation into a shear plane. 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. - "Quartz fabrics in the Alpine Fault mylonites: Influence of pre-existing preferred orientations on fabric development during progressive uplift" 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 … Kinematic interpretation of the geometry of the fold profile is dependent on the location of the profile in the fold structure (Hanmer & Passchier, 1991; Quinquis, Audren, Brun, & Cobbold, 1978). Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. Figure 15 Sigmoid in a shear regime. Mylonites are subdivided into protomylonite, blastomylonite, mylonite and ultramylonite depending on the degree of recrystallization. For structural geologists, this is very valuable: these features are kinematic indicators – structures that can reveal us the sense of movement of a shear zone. Figure 7 Distribution of asymmetric folds in a shear zone. Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. They can be a reliable shear sense indicator if both: inter-boudin planes are normal or at a high angle to the vorticity profile plane AND the boudin trains are at a low angle to the shear zone boundary (ie foliation) (Cees Passchier & Coelho, 2006). δ porphyroclast – similar to the σ porphyroclast but tails do cross the center line. Cite as, Many geologists study mylonites with the exclusive aim to determine the sense of shear. Structural Geology and Tectonics. However, the strong mechanical anisotropy created by the new shear foliation and fine grained mylonites make them prone to brittle reactivation or failure in discontinuous shear zones. Journal of Structural Geology, 6(4), 439-442. Softening or strain-softening results from changes in the rheology of a material in a ductile shear zone following its nucleation, progressively localising mylonites (and therefore ductile shear zones) (C. W. Passchier & Trouw, 1996). The asymmetry of the tails defines the sense of shear in the ductilly deformed rock. Along its borders with the enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites and ultramylonites with sinistral shear sense have formed. Mylonite is a strictly textural term that refers only to particular structural fabrics within a rock and does not give information on the mineral compositions. Adapted from Burg, J.P. Shear sense indicators – essentially kinematic indicators of shear or transport during a deformation event (Fossen, 2010; C. W. Passchier & Trouw, 1996). Care must be taken to not confuse the shear sense for inclusion trains with a pseudo sense from preserved crenulation from prior foliation (Schoneveld, 1977). Quartz fabrics adapted from Law, 1990. Mineral fish of biotite, tourmaline, K-feldspar, garnet, hypersthene and quartz occur in mylonites but most common are white mica fish. 1990). Adapted from Passchier & Coelho (2006). Obviously this is an important aspect, but it is important to study shear zones first, before shear sense determination is attempted. Keywords: Granulites, Microstructures, Mylonites, Shear Sense Indicators, Sri Lanka INTRODUCTION Crustal-scale ductile shear zones are an im-portant type of paleo movement zones and pro-vide information about the nature and types of deformation at deep crustal levels associated with motions of lithospheric fragments (e.g. Strain Shadows (Pressure Shadows) – These form from flow around a rigid body (i.e. Shear Zones and Mylonites . Figure 5 Offset of planar feature across shear zone. Asymmetric folds, with fold axes at a high angle to foliation, may indicate the sense of shear. The shear-sense observed on the northern and southern flanks is found to be – top to north and top to south respectively. Simple S-C relationships typically seen in mylonites are also disturbed by heterogeneities in the deforming rock and slip along micaceous elements in the foliation. Continuity is maintained across ideal shear zones. Shear zone-related folds. As with faults, shear zones can be normal, reverse, strike-slip or show oblique displacements. 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 … 2002; ten Grotenhuis et al. Although their geometries are well characterized by recent studies, they are still poorly understood. It must be noted that indicators also exist for shear sense in brittle structures such as … The deflection of the marker across the ductile shear zone (also called drag) is due to passive rotation which reflects the shear sense within the zone (Fossen, 2010; Wheeler, 1987). This atlas provides high definition images of a large number of different mylonites allowing students and … It is one of the most noticeable asymmetric microstructures, showing that the porphyroclasts have rotated during non-coaxial laminar flow in a ductile shear zone. shear sense in a sentence - Use "shear sense" in a sentence 1. 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. Porphyroclasts – Porphyroblasts grow during deformation, enclosing adjacent minerals from the S-C relationship in a shear.... 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