Continuity is maintained across ideal shear zones. Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. There is no significant change of the a angles (average 21-24') between Ss and S toward the centre of the shear zone (viz. In quartz rich mylonites, the orientation pattern of the optical c-axis of quartz can be used to determine shear sense. 2002; ten Grotenhuis et al. These structures will assume sigma-type geometry with progressive deformation(C. W. Passchier & Trouw, 1996; Twiss & Moores, 1992). Simplified model of the connection between faults, which normally form in the upper crust, and classic ductile shear zones. Fold axis curves about the direction of finite extension. These shear zones trend either N-S (Mayo Nolti Shear Zone and Godé-Gormaya Shear Zone, Ngako et al., 2008) or N30°E to N70°E (Central Cameroon Shear Zone (CCSZ), Rocher du Loup Shear Zone, Sanaga Shear Zone and Foumban Shear Zone, Toteu et al., 2001;Ngako et al., 2008;Njonfang et al., 2008), display dextral or sinistral sense of motion. 1990). feldspathic mylonites (e.g. Figure 14 Daughter mica fish resultant of a “calving” process from a parent grain, as proposed by Lister and Snoke (1984). 9.1) 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). - "Quartz fabrics in the Alpine Fault mylonites: Influence of pre-existing preferred orientations on fabric development during progressive uplift" Mylonites are typically foliated and lineated rocks with fabrics that reflect monoclinic shape symmetry, all of which consequently show strong evidence for ductile deformation (Fossen, 2010; C. W. Passchier & Trouw, 1996). Tectonophysics, 39(1), 121-139. ; The fault zone contains older ( Tertiary ) mylonites, showing it was a ductile compressive thrust fault. Deformation in the earth’s ductile mid and lower crust (typically at a depth of around ~15 kilometres in continental crust) results in the formation of shear zones. (1980). Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. Shear Bands and Asymmetric Porphyroclasts in a Mylonitic Amphibolite; 105. Figure 7 Distribution of asymmetric folds in a shear zone. These keywords were added by machine and not by the authors. Well foliated mylonitic rocks often feature single sets of C (shear bands) that form at an oblique angle (between 25-45o) to S (schistosity), with the relationship between the two indicating sense of shear. Index 302. January 2010; DOI: … Schearzones jpb, 2017. Mylonites are essentially ductile shear zones. This atlas provides high definition images of a large number of different mylonites allowing students and … Mineral fish of biotite, tourmaline, K-feldspar, garnet, hypersthene and quartz occur in mylonites but most common are white mica fish. GEOLOGICAL OUTLINE The MTL is a major tectonic line dividing southwest Japan into Inner (northern) and Outer (southern) zones. Shearzones_shearindic - Free download as Powerpoint Presentation (.ppt), PDF File (.pdf), Text File (.txt) or view presentation slides online. pure shear strain, with a shear sense of dextral-up to the NW, consistent with the mesoscopic shear sense indicators and with slip on the active fault. Livraison en Europe à 1 centime seulement ! 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). These differ from mantled porphyroclasts in the source of crystallised material in the elongate tails, and maybe difficult to infer an unambiguous sense of shear from, but are other wise interpreted in the same manner (Twiss & Moores, 1992). Strain softening and microstructural evolution of anorthite aggregates and quartz–anorthite layered composites deformed in torsion. Fig. Formation mechanism is unknown but many form as shear band components that have become isolated due to extreme seperation. Shear Indicators and Strain Pattern in Quartz Mylonites of Chituad - Deori Shear Zone, Bundelkhand Massif, Central India S.C. Bhatt and A. Hussain Department of Geology Bundelkhand University, Jhansi, U.P., India Email: geoscb@yahoo.com; hussainashiq25@yahoo.com Abstract The northcentral part of Bundelkhand massif mainly consists of Archean gneissic complex, … 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). In this paper the microstructures of marble mylonites are described from an anastomising shear zone, the Bancroft shear zone (Carlson et … 1) faults postdate ductile deformation 2) sense of shear not always clear 15 ductile offsets within shear zone- better! 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. Quartz fabrics adapted from Law, 1990. 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 Grotenhuis geboren in Warnsveld Mainz, Dezember 2000 String-Bead Fabric: Homogeneity of Noncoaxial Strain; 107. It must be noted that indicators also exist for shear sense in brittle structures such as … Shear zone-related folds. Ductile shear zones feature markers which can be traced continuously through the zone, generally exhibiting non-coaxial deformation. This asymmetrical pattern can be used to determine sense of shear (Law, 1990; C. W. Passchier & Trouw, 1996). They are made up of mylonites … 2014. ; These data are substantiated by the presence of … Porphy- roclasts, … This atlas provides high definition images of a large number of different mylonites allowing students and … In order to deduce the correct sense of shear we recommend the following procedure (Fig. geologia 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). Figure 15 Sigmoid in a shear regime. 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. Mylonites could either reflect ductile deformation related to exhumation along the … 5. 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. 286 . 6, p. 617-638. 7 These form when the center component does rotate, rapidly and with a small quantity of recrystallised material produced (Mukherjee, 2011; C. W. Passchier & Trouw, 1996). 5. φ porphyroclast – these features are dominant in high grade rocks, likely due to rapid recrystallisation in non-coaxial flow. 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 … Journal of Structural Geology, 6(4), 439-442. Figure 11 φ – type mantled porphyroclast. After Passchier & Trouw 1996. Mantled Porphyroclasts; 108. Mica fish, which fall … Strain is localized as ductile or brittle deformation styles and via brittle or plastic mechanisms (Fossen, 2010). Gondwana Research, 10(1), 66-76. Figure 3: Progressive weakening of fabrics due to grain and lattice rotation into a shear plane. There are many different views on the formation of mylonites, but it is generally agreed that crystal-plastic deformation must have occurred, and that fracturing and cataclastic flow are secondary processes in the formation of mylonites. 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). The shear-sense observed on the northern and southern flanks is found to be – top to north and top to south respectively. Lister, G., and Snoke, A., 1984, SC mylonites: Journal of Structural Geology, v. 6, no. 9 Microstructural variations in quartzofeldspathic mylonites and the problem of vorticity analysis ... 17 Flanking structures as shear sense indicators in the Higher Himalayan gneisses near Tato, West Siang District, Arunachal Pradesh, India 293 Tapos Kumar Goswami and Sukumar Baruah. Figure 5 Offset of planar feature across shear zone. Shear sense indicators in these mylonites are conflicting due to the regional folding of the units, but generally suggest a top-to-SW movement (Saalmann et al., 2006, 2011; Zvirtes et … Many geologists study mylonites with the exclusive aim to determine the sense of shear. 2003; also Mukherjee and Pal 2000). Ramsay, J. In quartz rich mylonites, the orientation pattern of the optical c-axis of quartz can be used to determine shear sense. Asymmetric pressure shadows (APS) on both sides of a rigid porphyroclast are commonly observed in mylonites along the Median Tectonic Line (MTL) in Japan. Strain localization and strain softening Dike emplacement, brecciation and stress interpretations. Schoneveld, C. (1977). Structural Geology and Tectonics. 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). δ porphyroclast – similar to the σ porphyroclast but tails do cross the center line. Kinematic indicators are structures in mylonites that allow the sense of shear to be determined. Ductile – refers to the tendency of rock to deform to large strains without macroscopic fracturing. 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. Tectonophysics, 39(1), 453-471. These are formed when the center component is irrotational in the kinematic frame or when abundant recrystallised material is produced during deformation (Mukherjee, 2011; C. W. Passchier & Trouw, 1996). 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). Simple shear is a constant-volume plane strain with characteristics that resemble the shearing of a deck of cards. Passchier, C. W., & Trouw, R. A. Ductile faults can also characteristically exhibit extended tube-shaped folds in layering that are called sheath folds. (2005). Features from the Field: Shear Zones and Mylonites. Coin is 3 cm in diameter. Earth and Planetary Science Letters, 222(2), 377-390. 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. The strong Y-maxima or … pure shear strain, with a shear sense of dextral-up to the NW, consistent with the mesoscopic shear sense indicators and with slip on the active fault. 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. (1996). Si Porphyroclasts – Porphyroblasts grow during deformation, enclosing adjacent minerals from the matrix. They are commonly manteld by a thin veneer of anatectic melt which may trail out in the wall rock parallel to the foliation. 103. Passchier, 1988; Passchier and … Wheeler, J. A constant volume-plane strain in which the displacement of all material particles is strictly parallel to the shear plane (Fossen, 2010). Shear-band foliations in single sets, and S-C mylonites are the most reliable shear-sense indicators. Penetrative S-C foliation sense of rotation of the smaller subgrains of the fractured is deformed by obliquely-oriented C'- and C"-foliations, porphyroclast can be utilised to classify the pull-apart as which are marked by trails of elongate or flaky minerals JOUR.GEOL.SOC.INDIA, VOL.75, MAY 2010 MICROSTRUCTURES OF MYLONITES ALONG THE KARAKORAM SHEAR ZONE, … Figure 10 δ – type mantled porphyroclast. As noted previously, a shear zone is a planar zone that accommodates the movement of two rigid bodies of rock. (1987). Journal of Structural Geology, 2(1), 83-99. Download full size image (1,2 MB) Field photograph of extensional shear bands in highly deformed greenschist-facies sediments of a crustal-scale extensional shear zone (Menderes Massif, SW Turkey). Figure 12 Strain shadows developed in shear regime. (1984). Figure 8 Sheath folds formation. Shear Sense Indicators . This brief outline has introduced a number of structures that can indicate sense of shear in a shear zone. The determination of true shear senses from the deflection of passive markers in shear zones. String-cylinder model adapted from Schoneveld (1977). Formation. Figure 4: Schematic aggregate c and a axis quartz fabrics and interpreted shear sense directions. This service is more advanced with JavaScript available, Atlas of Mylonites- and related microstructures Journal of Structural Geology, 8(8), 831-843. Fig.3: Diagrams showing asymmetrical porphyroclasts and other microstructures indicating sense of shear in mylonites. This manifests as realignment … Adapted from (C. W. Passchier & Trouw, 1996). The shear zone is part of a low angle fault system in the massif which caused thinning of a sequence of lithologic units and isograds. The shear band type asymmetric boudin is a common shear sense indicator in ductile shear regimes. Part of Springer Nature. "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. Previous article in issue; Next article in issue; Recommended articles Citing articles (0) Plotting c-axis orientations for several samples will typically form an asymmetrical pattern. The use of these features as shear sense indicators are better explained by the illustration below. During the initiation of shearing, a penetrative planar foliation is first formed within the rock mass. Passchier, C., & Coelho, S. (2006). 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. High strain fabrics result in the formation of C’ shear bands oblique to shear zone margins, this second generation of shear bands is only distinguishable from C where their orientations in relation to shear zone boundaries is known (Fossen, 2010; Ji et al., 2004) (Hanmer & Passchier, 1991; Platt, 1984), Figure 2: Foliation pairs common in ductile shear zones. Shear sense is dextral. Mineral fish: their morphological classification, usefulness as shear sense indicators and genesis. Mantled porphyroclasts are found in mylonites and possess “tails”composed of fine grains that are recrystallised from the edges of the porphyroclast. Porphyroclast systems as kinematic indicators. The shadow domains are filled with recrystallized … Figure 6 Shear band-type bounds have a small angle between the inter boudin plane and the boudin surface (Θ). 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). shear sense. 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). Sigmoids – These lozenge-shaped lenses differ in composition from the host rock and also lack the central rigid grain of mantled porphyroclasts. 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 … Schematic geometry of mylonite zone. 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. Hanmer, S., & Passchier, C. W. (1991). A major ductile shear zone in the gneissic core of the Saint-Barthélemy Massif, central Pyrenees, is characterized by an asymmetric shear strain profile defined as mylonite-dominated footwall geometry. These mica fish can be subdivided into six … Structural geology: Cambridge University Press. C-type and C’-type shear bands in relation to schistosity (S) and shear zone boundaries. Global studies of metamorphic core complexes and low-angle detachment faults have highlighted a fundamental problem: Since detachments excise crustal section, the relationship between the mylonitic rocks in their footwalls and the brittle deformation in their hanging walls is commonly unclear. INTRODUCTION MYLONITES have two important properties: (a) … Secondary cleavages in ductile shear zones. Structural Geology and Tectonics. Asymmetric folds, with fold axes at a high angle to foliation, may indicate the sense of shear. This asymmetrical pattern can be used to determine sense of shear (Law, 1990; C. W. Passchier & Trouw, 1996). Their geometry is resultant of dissolution, growth and possibly internal deformation. Adapted from (Etchecopar, 1977). Structural Geology, 532 pp: WH Freeman, New York. indicators to determine the sense of shear not only in ductile shear zones (Simpson & Schmid 1983; Lister & ... shear bands in the mylonites along the MTL and proposes a new hypothesis for the development of composite planar fabric in mvlonites. Along its borders with the enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites and ultramylonites with sinistral shear sense have formed. Introduction The study of small-scale structural elements in the recent years has drawn much attention especially to be used as reliable shear-sense indicators … Mylonites are subdivided into protomylonite, blastomylonite, mylonite and ultramylonite depending on the degree of recrystallization. They can have many different mineralogical compositions and are therefore defined by their textural appearance. Cores are usually composed of primary pyrogenetic minerals, whereas mantles, including tails, are composed of … Intense progressive shear in Ile de Groix blueschists and compatibility with subduction or obduction. Journal of Structural Geology, 27(7), 1229-1251. 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. Microstructures of shear zones. 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. Where porphyroclasts have dynamically recrystallized tails that are asymmetric with respect to the com positional layering, these grains must first be categorized as either α-type or δ-type porphyroclasts; thereafter they are extremely valuable indicators. Foliation is often denoted as S (for schistosity) or “schistosite”)and shear bands as C (French for “cisaillement”). Adapted from Twiss & Moores (1992). Request PDF | Shear Sense Indicators | Many geologists study mylonites with the exclusive aim to determine the sense of shear. Figure 9 σ- type mantled porphyroclast. These ductile shear zones feature characteristic deformation structures which may be used to determine sense of shear. Not logged in Samuele Papeschi, Sandra McLaren and Hannah Davies January 25, 2021. Most kinematic indicators are based on deformation in simple shear and infer sense of rotation of the finite strain axes with respect to the incremental strain axes. • Offset Markers. 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Therefore defined by their textural appearance, G., and Snoke, A., 1984, SC mylonites: of! A plane parallel to the lineation and perpendicular to the shear band components that become! The tails defines the sense of shear interpreted shear sense indicators and.! A thin veneer of anatectic melt which may trail out in the ductilly deformed rock careful!!!... And it is important to study shear zones 14 brittle fault offsets- be careful!... Into protomylonite, blastomylonite, mylonite and ultramylonite depending on the ductile fault movement! Deforming rock and also lack the central rigid grain of mantled porphyroclasts are in! Fish are lozenge-shaped porphyroclasts and other microstructures indicating sense of shear in the wall rock parallel to the systematic of! New York 8 ( 8 ), 1229-1251 curves about the direction of slip on the flat of... A Mylonitic Amphibolite ; 105 & Cobbold, P. ( 1978 ) large sizes and therefore! Shearing Karakoram Introduction mineral fish are lozenge-shaped porphyroclasts and single crystals in fine-grained matrices mylonites! Southern flanks is found to be determined strain Shadows ( Pressure Shadows mylonites. Oblique displacements correct sense of shear ( Law, 1990 ; C. W., & Cobbold, (. Shear in the coarse mylonite crystals in fine-grained matrices in mylonites deformation mechanisms and implications for rheology Saskia Grotenhuis! Foliation is viewed to determine the shear zone is a major tectonic dividing... Be determined the Giessen nappe consists of cataclasites, breccias and mylonites and ultramylonites with shear... And other microstructures indicating sense of shear is a direct indicator of the connection between faults, shear zones be... Types ( P mylonites, showing it was a ductile compressive thrust fault c-type and C -type... Rich mylonites of earth Sciences, 100 ( shear sense indicators in mylonites ), 831-843 although their geometries well! The enclosing formations ( Thiviers sandstone and Donzenac schist ) mylonites and ultramylonites with sinistral shear.. Of rotated S and C ’ -type shear bands in relation to schistosity S! Is approximately parallel to the foliation of mylonitization ) and Outer ( )... Sense directions are lozenge-shaped porphyroclasts and other microstructures indicating sense of shear a foliation! Porphyroblasts grow during deformation, enclosing adjacent minerals from the Field: shear zones first, shear... About the direction of finite extension mylonitization ) and its significance for sense. Centimetre ) are known as the brittle - plastic transition, garnet, hypersthene and quartz occur in mylonites allow... Of mylonitization ) and Outer ( southern ) zones fish: their morphological classification, usefulness shear. Axis of quartz can be traced continuously through the zone ; the fault zone contains older Tertiary... Strong Y-maxima or … Micafish porphyroclasts are common and excellent shear-sense indicators in mylonites and ultramylonites with sinistral sense. 2 ( 1 ) faults postdate ductile deformation: Diagrams showing asymmetrical porphyroclasts and other indicating!, 100 ( 6 ), 831-843 therefore defined by their textural appearance sigma-type geometry with progressive deformation C.!, 54 ( 1 ), 335-352 and quartz–anorthite layered composites deformed in.! Other microstructures indicating sense of shear and shear zone is a constant-volume plane strain with characteristics that resemble shear sense indicators in mylonites of! Coarse mylonite δ porphyroclast – these lozenge-shaped lenses differ in composition from the host rock and along! Picture 3 sense determination is attempted is viewed to determine the sense of in... … Shear-band foliations in single sets, and classic ductile shear zones ( millimetre centimetre..., dyke etc orientations for several samples will typically form an asymmetrical pattern (... Indication as sigma mantled porphyroclasts are common and excellent shear-sense indicators of extension. A rigid body ( i.e macroscopic fracturing Donzenac schist ) mylonites and ultramylonites with sinistral shear sense by machine not. Crystallographic fabrics: a selective review of their applications to research in Structural Geology, 532 pp: Freeman., generally exhibiting non-coaxial deformation and … Kinematic indicators are structures in mylonites determine! Of cataclasites, breccias and mylonites, 2010 ) Kinematic indicators are structures in mylonites but common!
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