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1
Hanmer, Simon, Randy Parrish, Michael Williams, and Chris Kopf. "Striding-Athabasca mylonite zone: Complex Archean deep-crustal deformation in the East Athabasca mylonite triangle, northern Saskatchewan." Canadian Journal of Earth Sciences 31, no. 8 (August 1, 1994): 1287–300. http://dx.doi.org/10.1139/e94-111.
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The geophysically defined Snowbird tectonic zone is manifested in northernmost Saskatchewan as a deep-crustal, multistage mylonitic structure, the East Athabasca mylonite triangle. The triangle, located at the northeastern apex of a stiff, crustal-scale "lozenge," is composed of mid-Archean annealed mylonites and late Archean ribbon mylonites, formed during two granulite facies events (850–1000 °C, 1.0 GPa). The flow pattern in the mylonites is geometrically and kinematically complex, and corresponds to that expected adjacent to the apex of a stiff elliptical volume subjected to subhorizontal regional extension parallel to its principal axis. The late Archean mylonites are divided into an upper structural deck, entirely occupied by a dip-slip shear zone, and an underlying lower deck. The latter is divided into two upright conjugate strike-slip shear zones, separated by a low-strain septum, which deformed by progressive coaxial flow. The flow pattern in the mid-Archean mylonites is compatible with that of the late Archean mylonites, and suggests that the crustal-scale lozenge influenced deformation since the mid-Archean. In the interval ca. 2.62–2.60 Ga, deformation in the upper and lower decks evolved from a granulite facies pervasive regime to a more localized amphibolite facies regime. With further cooling, deformation was localized within very narrow greenschist mylonitic faults at the lateral limits of the lower deck. By the late Archean, the East Athabasca mylonite triangle was part of a deep-crustal, intracontinental shear zone. This segment of the Snowbird tectonic zone was not the site of an Early Proterozoic suture or orogen.
2
Pearson, A., F. Heidelbach, and H. R. Wenk. "Texture Analysis of Quartz in a Granite Mylonite by EBSP-Orientation Imaging Microscopy." Textures and Microstructures 29, no. 3-4 (January 1, 1997): 185–99. http://dx.doi.org/10.1155/tsm.29.185.
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The crystallographic preferred orientation (texture) of the quartz phase in a mylonitic leucogranite from the Santa Rosa mylonite zone was investigated using automated analysis of electron backscattering patterns (EBSP) in the scanning electron microscope (SEM). The separation of the quartz diffraction patterns from patterns of other constituents (feldspar, mica, etc.) in this polymineralic rock was achieved using an image quality parameter. The quartz phase displays a texture typical for high temperature mylonites (c-axis maximum in the intermediate strain direction). The misorientation distribution between next neighbors is dominated by Dauphiné twins.
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V, Thirukumaran, Biswal T.K, Sundaralingam K, Sowmya V, Boopathi S, and Mythili R. "Strain Pattern Analysis of Mylonites From Sitampundi-Kanjamalai Shear Zone, Thiruchengode, South India." International Journal of Civil, Environmental and Agricultural Engineering 1, no. 1 (May 30, 2019): 25–34. http://dx.doi.org/10.34256/ijceae1914.
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This study aims to investigate the petrography and strain pattern of mylonites from parts of N-S trending Sitampundi-Kanjamalai Shear Zone (SKSZ) around Thiruchengode. The petrographic study indicates the presence of recrystallized quartz, K-feldspar, plagioclase, biotite and some hornblende. The kinematic analysis of Mylonites was 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 as a product of partitioning of both strain and vorticity between domains. These all indicates the simple shear extension along E-W direction and the mylonitic foliation shows the pure shear compression along N-S direction. Further the study of bulk strain analysis by Flinn plot method using L and T section of mylonite shows k<1 which lies in the field of flattening zone of finite strain. The kinematic vorticity number is calculated by Rxz/β method which gives the value of 0.36 indicating the general shear. The rigid grain graph shows that the pure shear component is more dominant than the simple shear component. The analysis leads to the conclusion that the mylonite has experienced a high temperature shearing of above 700°cat deep crustal level.
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Owen, J. V. "Geochemical changes accompanying the mylonitization of diverse rock types from the Grenville Front zone, eastern Labrador." Canadian Journal of Earth Sciences 25, no. 9 (September 1, 1988): 1472–84. http://dx.doi.org/10.1139/e88-140.
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The relationship between mineralogy and bulk composition was examined in mylonites developed in a variety of rock types from the Grenville Front zone of eastern Labrador. Mylonites developed from olivine-, pyroxene-, and (or) amphibole-bearing protoliths are the most altered, typically being enriched in K2O and volatiles and depleted in CaO compared with the protolith. Bulk-compositional modification was accompanied by the formation of almandine-rich garnet and (or) relatively ferruginous hornblende in medium-grade (epidote–amphibolite to lower amphibolite facies) mylonites derived from both mafic and quartzo-feldspathic rocks. Protoliths containing biotite as the principal or sole Fe–Mg phase were the least altered during deformation. Apart from variations in Ti content, biotite in mylonitic assemblages derived from these rocks is compositionally similar to that in the low-strain precursor.Bulk-compositional changes recognized in narrow mylonite zones appear to have occurred in Grenvillian tectonites developed on a regional scale.
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MALATESTA, C., L. FEDERICO, L. CRISPINI, and G. CAPPONI. "Fluid-controlled deformation in blueschist-facies conditions: plastic vs brittle behaviour in a brecciated mylonite (Voltri Massif, Western Alps, Italy)." Geological Magazine 155, no. 2 (January 25, 2017): 335–55. http://dx.doi.org/10.1017/s0016756816001163.
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AbstractA blueschist-facies mylonite crops out between two high-pressure tectono-metamorphic oceanic units of the Ligurian Western Alps (NW Italy). This mylonitic metabasite is made up of alternating layers with different grain size and proportions of blueschist-facies minerals.The mylonitic foliation formed at metamorphic conditions of T = 220–310 °C and P = 6.5–10 kbar. The mylonite shows various superposed structures: (i) intrafoliar and similar folds; (ii) chocolate-tablet foliation boudinage; (iii) veins; (iv) breccia.The occurrence of comparable mineral assemblages along the foliation, in boudin necks, in veins and in breccia cement suggests that the transition from ductile deformation (folds) to brittle deformation (veining and breccia), passing through a brittle–ductile regime (foliation boudinage), occurred gradually, without a substantial change in mineral assemblage and therefore in the overall P–T metamorphic conditions (blueschist-facies).A strong fluid–rock interaction was associated with all the deformative events affecting the rock: the mylonite shows an enrichment in incompatible elements (i.e. As and Sb), suggesting an input of fluids, released by adjacent high-pressure metasedimentary rocks, during ductile deformation. The following fracturing was probably enhanced by brittle instabilities arising from strain and pore-fluid pressure partitioning between adjacent domains, without further external fluid input.Fluids were therefore fixed inside the rock during mylonitization and later released into a dense fracture mesh that allowed them to migrate through the mylonitic horizon close to the plate interface.We finally propose that the fracture mesh might represent the field evidence of past episodic tremors or ‘slow earthquakes’ triggered by high pore-fluid pressure.
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SARKARINEJAD, KHALIL, and SOMAYE DERIKVAND. "Structural and kinematic analyses of the basement window within the hinterland fold-and-thrust belt of the Zagros orogen, Iran." Geological Magazine 154, no. 5 (November 4, 2016): 983–1000. http://dx.doi.org/10.1017/s0016756816000558.
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AbstractThe Zagros hinterland fold-and-thrust belt is located in the central portion of the Zagros Thrust System and consists of the exhumed basement windows associated with NW-striking and NE-dipping flexural duplex structures that contain in-sequence thrusting and related folds. Mylonitic nappes of the basement were exhumed along deep-seated sole thrusts of the Zagros Thrust System. Lattice preferred orientation (LPO) c-axes of quartz show asymmetric type-1 crossed girdles that demonstrate a non-coaxial deformation under plane strain conditions. Based on the opening angles of quartz c-axis fabric skeletons, deformation temperatures vary from 425±50°C to 540±50°C, indicating amphibolite facies conditions. The estimated mean kinematic vorticity evaluated from quartz c-axis of the quartzo-feldspathic mylonites (Wm = 0.55±0.06) indicates the degree of non-coaxiality during mylonite exhumation. The estimated angle θ between the maximum instantaneous strain axis (ISA1) and the transpressional zone boundary is 17°, and the angle of oblique convergence is 57° in the M2 nappe of the basement involved. This indicates that the mylonitic nappe was formed by a combination of 62% pure shear and 38% simple shear during oblique convergence.
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Hanmer, Simon. "Textural map units in quartzo-feldspathic mylonitic rocks." Canadian Journal of Earth Sciences 24, no. 10 (October 1, 1987): 2065–73. http://dx.doi.org/10.1139/e87-195.
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The classical macrotextural subdivision of quartzo-feldspathic mylonitic rocks yields only three rock types: protomylonite, mylonite, and ultramylonite. This restriction impedes detailed mapping of the internal textural transitions common in wide, deep-seated, crustal-scale shear zones, where such transitions may occur over kilometres and involve several clearly mappable textural types. The introduction of two objectively defined field mapping terms, "homoclastic" and "heteroclastic," describing the macroscopic grain-size distribution within the porphyroclast population provides descriptive flexibility without changing the matrix–porphyroclast basis of the established classification. This allows the description of textural paths other than protomylonite → mylonite → ultramylonite and facilitates the consideration of textural paths in terms of strain partitioning between the constituent grains of the deforming aggregate, rather than as a simple function of finite strain.
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Owen, J. Victor. "Determination of the finite-strain ellipsoid from deformed porphyroblastic mineral aggregates and preferentially oriented feldspars in a mylonitized metamafic dyke." Canadian Journal of Earth Sciences 26, no. 11 (November 1, 1989): 2333–40. http://dx.doi.org/10.1139/e89-199.
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Strain in a narrow mylonite zone has been estimated from deformed garnetiferous porphyroblastic aggregates and from preferentially oriented plagioclase porphyroclasts with high aspect ratios. In the undeformed metamafic dyke hosting the mylonite, the mineral aggregates have spheroidal to slightly oblate shapes, and plagioclase is nearly randomly oriented. In the mylonite, the mineral aggregates are prolate ellipsoids, and plagioclase in the aggregates and matrix is symmetrically oriented about the mylonitic planar fabric. Comparison with the radii of spheres of equal volume shows that the ellipsoidal mineral aggregates underwent triaxial strain, with maximum extension of 50–140% parallel to X and with shortening of up to −30 and −45% parallel to Y and Z, respectively. The maximum strain ratio varies between 1.9 and 4.2 (mean of 10 measurements = 3.1). The orientation and aspect ratios of elongate plagioclase grains measured in the X–Z plane indicate an intermediate value (2.7) for the strain ratio. Plagioclase deformation was apparently accommodated by dislocation glide on (010), recovery processes (subgrain rotation), and microcracking. The effects of mechanical anisotropy in plagioclase, however, were subordinate to the strain regime, strain ratio, and initial aspect ratio of grains in determining the final aspect ratio and rest position of these porphyroclasts.Both the deformed garnetiferous aggregates and the plagioclase porphyroclasts record state of strain in the mylonite. This suggests that the preferred orientation of densely packed feldspars of high aspect ratio potentially may be used to estimate strain in tectonites.
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Festa, Vincenzo, Annamaria Fornelli, Francesca Micheletti, Richard Spiess, and Fabrizio Tursi. "Ductile Shearing and Focussed Rejuvenation: Records of High-P (eo-)Alpine Metamorphism in the Variscan Lower Crust (Serre Massif, Calabria—Southern Italy)." Geosciences 12, no. 5 (May 17, 2022): 212. http://dx.doi.org/10.3390/geosciences12050212.
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In the present study, we unveil the real significance of mylonitic reworking of the polymetamorphic crystalline basement in the Serre Massif of Calabria (Southern Italy). We use a multidisciplinary approach to comprehend the structural, microstructural and petrologic changes that occurred along a, so far, not much considered shear zone affecting the Variscan lower crustal rocks. It was never before studied in detail, although some late Cretaceous ages were reported for these mylonites, suggesting that this shear zone is of prime importance. Our observations reveal now that the formation of the new structural fabric within the shear zone was accompanied by changes in mineral assemblages, in a dominant compressive tectonic regime. During this tectono-metamorphic event, high-P mylonitic mineral assemblages were stabilized, consisting of chloritoid, kyanite, staurolite, garnet and paragonite, whereas plagioclase became unstable. Average peak P–T conditions of 1.26–1.1 GPa and 572–626 °C were obtained using THERMOCALC software. These data question (i) that the Serre Massif represents an undisturbed continuous section of the Variscan crust, as generally suggested in the literature, and (ii) highlight the role of (eo-)Alpine high-P tectonics in the Serre Massif, recorded within mylonite zones, where the Variscan basement was completely rejuvenated.
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Farkašovský, Roman, Katarína Bónová, and Marián Košuth. "Microstructural, modal and geochemical changes as a result of granodiorite mylonitisation – a case study from the Rolovská shear zone (Čierna hora Mts, Western Carpathians, Slovakia)." Geologos 22, no. 3 (September 1, 2016): 171–90. http://dx.doi.org/10.1515/logos-2016-0019.
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Abstract Strong tectonic remobilisation and shear zone development are typical features of the easternmost part of the Veporicum tectonic unit in the Western Carpathians. The granodiorite mylonites in the area of the Rolovská shear zone (Čierna hora Mts) underwent a complex polystage evolution during the Hercynian and Alpine orogenies. Deformation during the latter reached greenschist facies under metamorphic conditions. Mylonites are macroscopically foliated rocks with a stretching lineation and shear bands. Structurally different mylonite types, ranging from protomylonites to ulramylonites with typical grainsize reduction from the margins towards the shear zone centre, have been assessed. The modal mineralogy of the different mylonite types changes considerably. Typical is a progressive decrease in feldspar content and simultaneously the quartz and white mica content increases from protomylonites towards the most strongly deformed ultramylonites. The deformation had a brittle character in less deformed rocks and a ductile one in more deformed tectonites. Obvious chemical changes occur in mesomylonites and ultramylonites. During mylonitisation, the original biotite granodiorite was depleted of Mg, Fe, Na, Ca and Ba, while K, Rb and mainly Si increased considerably. Other (major and trace) elements reflect erratic behaviour due to lateral mobility. Chemical changes indicate the breakdown and subsequent recrystallisation of biotite and feldspars and, in turn, the crystallisation of albite and sericite. REE decrease in ultramylonites due to the breakup of accessory minerals during deformation and alteration.
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Soda, Yusuke, Taku Matsuda, Sachio Kobayashi, Motoo Ito, Yumiko Harigane, and Takamoto Okudaira. "Reversely zoned plagioclase in lower crustal meta-anorthosites: An indicator of multistage fracturing and metamorphism in the lower crust." American Mineralogist 105, no. 7 (July 1, 2020): 1002–13. http://dx.doi.org/10.2138/am-2020-7284.
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Abstract This paper describes the formation mechanism of reversely zoned plagioclase, which has been observed frequently in lower crustal shear zones and is indicative of multistage fracturing and meta-morphism in the lower crust, by studying the microstructural and chemical characteristics of plagioclase in sparsely fractured anorthosites and anorthositic mylonites from the Eidsfjord shear zone, Langøya, northern Norway. Based on the field relationship between sparsely fractured anorthosite and anorthositic mylonite, the fracturing of anorthosite occurred before the formation of mylonite. In sparsely fractured anorthosites, transgranular fractures are observed; hydration-reaction products, including Na-rich plagioclase, occur within cracks and fractures, suggesting that hydration reactions occurred during or after fracturing. The hydration reactions in sparsely fractured anorthosites are estimated to have occurred at higher-pressure (P) amphibolite-facies conditions (~0.9–1.0 GPa and ~550–700 °C). In anorthositic mylonites, which are considered to have initiated by fracturing and subsequent hydration metamorphism at lower-P amphibolite-facies conditions (~0.7 GPa and ~600 °C), recrystallized plagioclase grains often show compositional zoning with an Na-rich core and a Ca-rich rim. Because the compositions of metamorphic plagioclase grains in the sparsely fractured anorthosites and those of the Na-rich cores of the reversely zoned plagioclase in anorthositic mylonites are similar to each other, the Na-rich cores of the matrix plagioclase in the anorthositic mylonites have recrystallized under higher-P amphibolite-facies conditions and then been overgrown or replaced by the Ca-rich rims under lower-P conditions. Consequently, the reversely zoned plagioclase observed frequently in lower crustal shear zones is an indicator of multistage brittle fracturing and subsequent hydration metamorphism during exhumation, providing information relevant to understanding the deep rupture process caused by repeated seismicity alternating with aseismic creep below the seismogenic zone.
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SARTINI-RIDEOUT, C., J. A. GILOTTI, and W. C. McCLELLAND. "Geology and timing of dextral strike-slip shear zones in Danmarkshavn, North-East Greenland Caledonides." Geological Magazine 143, no. 4 (May 17, 2006): 431–46. http://dx.doi.org/10.1017/s0016756806001968.
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The North-East Greenland eclogite province is divided into a western, central and eastern block by the sinistral Storstrømmen shear zone in the west and the dextral Germania Land deformation zone in the east. A family of steep, NNW-striking dextral mylonite zones in the Danmarkshavn area are geometrically and kinematically similar to the ductile Germania Land deformation zone, located 25 km to the east. Amphibolite facies deformation at Danmarkshavn is characterized by boudinage of eclogite bodies within quartzofeldspathic host gneisses, pegmatite emplacement into the boudin necks and subsequent deformation of pegmatites parallel to gneissosity, a widespread component of dextral shear within the gneisses, and localization of strain into 10–50 m thick dextral mylonite zones. The gneisses and concordant mylonite zones are cut by a swarm of weakly to undeformed, steeply dipping, E–W-striking pegmatitic dykes. Oscillatory-zoned zircon cores from two boudin neck pegmatites give weighted mean 206Pb/238U sensitive, high mass resolution ion microprobe (SHRIMP) ages of 376 ± 5 Ma and 343 ± 7 Ma. Cathodoluminescence images of these zircons reveal complex additional rims, with ages from ranging from c. 360 to 320 Ma. Oscillatory-zoned, prismatic zircons from two late, cross-cutting pegmatites yield weighted mean 206Pb/238U SHRIMP ages of 343 ± 5 Ma and 332 ± 3 Ma. Zircons from the boudin neck pegmatites record a prolonged growth history, marked by fluid influx, during amphibolite facies metamorphism beginning at c. 375 Ma. The cross-cutting pegmatites show that dextral deformation in the gneisses and ductile mylonite zones had stopped by c. 340 Ma. Ultrahigh-pressure metamorphism in the eastern block at 360 Ma requires that the Greenland Caledonides were in an overall contractional plate tectonic regime. This, combined with 20% steep amphibolite facies lineations in the eclogites, gneisses and mylonites suggests that dextral transpression may have been responsible for a first stage of eclogite exhumation between 370 and 340 Ma.
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O'Neill, J. Michael, Jeff D. Lonn, David R. Lageson, and Michael J. Kunk. "Early Tertiary Anaconda Metamorphic Core Complex, southwestern Montana." Canadian Journal of Earth Sciences 41, no. 1 (January 1, 2004): 63–72. http://dx.doi.org/10.1139/e03-086.
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A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting.
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Bianco, Caterina. "The Capo Castello Shear Zone (Eastern Elba Island): Deformation at the Contact between Oceanic and Continent Tectonic Units." Geosciences 10, no. 9 (September 10, 2020): 361. http://dx.doi.org/10.3390/geosciences10090361.
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Low-grade mylonitic shear zones are commonly characterized by strain partitioning, with alternating low strain protomylonite and high strain mylonite and ultramylonite, where the shearing is most significant. In this paper the capo Castello shear zone is analyzed. It has developed along the contact between continental quartzo-feldspathic, in the footwall, and oceanic ophiolitic units, in the hangingwall. The shear zone shows, mostly within the serpentinites, a heterogeneous strain localization, characterized by an alternation of mylonites and ultramylonites, without a continuous strain gradient moving from the protolith (i.e., the undeformed host rock) to the main tectonic contact between the two units. The significance of this mylonitic shear zone is examined in terms of the dominant deformation mechanisms, and its regional tectonic frame. The combination of the ultramafic protolith metamorphic processes and infiltration of derived fluids caused strain softening by syntectonic metamorphic reactions and dissolution–precipitation processes, leading to the final formation of low strength mineral phases. It is concluded that the strain localization, is mainly controlled by the rock-fluid interactions within the ophiolitic level of the Capo Castello shear zone. Regarding the regional setting, this shear zone can be considered as an analogue of the initial stage of the post-collisional extensional fault, of which mature stage is visible along the Zuccale fault zone, a regional structure affecting eastern Elba Island.
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Young, Erik M., Christie D. Rowe, and James D. Kirkpatrick. "Shear zone evolution and the path of earthquake rupture." Solid Earth 13, no. 10 (October 26, 2022): 1607–29. http://dx.doi.org/10.5194/se-13-1607-2022.
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Abstract. Crustal shear zones generate earthquakes, which are at present unpredictable, but advances in mechanistic understanding of the earthquake cycle offer hope for future advances in earthquake forecasting. Studies of fault zone architecture have the potential to reveal the controls on fault rupture, locking, and reloading that control the temporal and spatial patterns of earthquakes. The Pofadder Shear Zone exposed in the Orange River in South Africa is an ancient, exhumed, paleoseismogenic continental transform which preserves the architecture of the earthquake source near the base of the seismogenic zone. To investigate the controls on earthquake rupture geometries in the seismogenic crust, we produced a high-resolution geologic map of the shear zone core mylonite zone. The core consists of ∼ 1–200 cm, pinch-and-swell layers of mylonites of variable mineralogic composition, reflecting the diversity of regional rock types which were dragged into the shear zone. Our map displays centimetric layers of a unique black ultramylonite along some mylonite interfaces, locally adding to thick composite layers suggesting reactivation or bifurcation. We present a set of criteria for identifying recrystallised pseudotachylytes (preserved earthquake frictional melts) and show that the black ultramylonite is a recrystallised pseudotachylyte, with its distribution representing a map of ancient earthquake rupture surfaces. Pseudotachylytes are most abundant on interfaces between the strongest wall rocks. We find that the geometry of lithologic interfaces which hosted earthquakes differs from interfaces lacking pseudotachylyte at wavelengths of ≳ 10 m. We argue that the pinch-and-swell structure of the mylonitic layering, enhanced by viscosity contrasts between layers of different mineralogy, is expected to generate spatially heterogeneous stress during viscous creep in the shear zone, which dictated the path of earthquake ruptures. The condition of rheologically layered materials causing heterogeneous stresses should be reasonably expected in any major shear zone, is enhanced by creep, and represents the pre-seismic background conditions through which earthquakes nucleate and propagate. This has implications for patterns of earthquake recurrence and explains why some potential geologic surfaces are favored for earthquake rupture over others.
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Næraa, Tomas, Jens Konnerup-Madsen, Bjørn Hageskov, and Lalu Prasad Paudel. "Structure and petrology of the Dadeldhura Group, far western Nepal, Himalaya." Journal of Nepal Geological Society 35 (December 31, 2007): 21–28. http://dx.doi.org/10.3126/jngs.v35i0.23631.
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The granites, phyllites, schists, and gneisses of the Dadeldhura Group exhibit a significant Himalayan metamorphic imprint. The rocks of the group constitute a synform and the group is delimited by thrusts. The North Dadeldhura Thrust (NDT) zone is dominated by granitic mylonite with subordinate quartzite, quartz-chlorite schist, and amphibolite. In the quartz-chlorite schist from the NDT zone, relict kyanite is observed, which together with recrystallisation textures in the granitic mylonite indicate that low-temperature syn-tectonic retrogression has affected the thrust zone. Prograde garnets with spiral structures from a zone 2–4 km structurally above the base of the NDT are associated with mylonite-like rocks, and indicate distinct prograde shear zones in this area. This suggested that prograde thrust stacking has affected about 4 km wide north belt of the Dadeldhura Group. Rim thermobarometry from the garnet holding rocks shows that the minerals were re-equilibrated at 440–550 °C and 6.5–9.5 kbar. In the southern part of the Dadeldhura Group, textures in quartz and feldspar from the Saukhark Granite-Gneiss indicate that temperatures during recrystallisation were around 450–550 °C. These P–T estimates suggest that most of the now exposed rocks in the Dadeldhura Group experienced prograde epidoteamphibolite facies metamorphism during early orogenic build up and thrusting of the Dadeldhura Nappe. A subsequent syntectonic retrograde phase is observed in the NDT zone as low temperature recrystallisation of quartz and feldspar in mylonitic rocks, and chlorite and quartz in chlorite-grade rocks holding relict kyanite. Retrograde trusting was also focused along a distinct “back thrusting” zone within the northern part of the group.
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Aksoyoglu, S. "Cesium sorption on mylonite." Journal of Radioanalytical and Nuclear Chemistry Articles 140, no. 2 (June 1990): 301–13. http://dx.doi.org/10.1007/bf02039502.
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Leiss, B., S. Siegesmund, and K. Weber. "Texture Asymmetries as Shear Sense Indicators in Naturally Deformed Mono- and Polyphase Carbonate Rocks." Textures and Microstructures 33, no. 1-4 (January 1, 1999): 61–74. http://dx.doi.org/10.1155/tsm.33.61.
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The microstructural and quantitative texture analyses of a naturally deformed calcite mylonite, a dolomite mylonite and a dolomitic calcite mylonite reveal different texture asymmetries for comparable deformation conditions. Calcite shows a c-axis maximum rotated against the shear sense with regard to the main shear plane. In contrast, the dolomite shows a c-axis maximum rotated with the shear sense. In accordance with the experimental and simulated textures from the literature, this difference proves e-twinning and r-slip for calcite and f-twinning and c-slip for dolomite as the main deformation mechanisms. The dolomitic calcite mylonite shows for both the calcite and the dolomite a c-axis maximum rotated against the shear sense. On account of the microstructure of this sample, the dolomite texture has been passively overtaken from the deformation texture of calcite during a late-deformative dolomitization. The results significantly contribute to the interpretation that the sampled shear zone is a transpressive strike–slip fault.
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Cooper, Frances J., John P. Platt, and Whitney M. Behr. "Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes." Solid Earth 8, no. 1 (February 21, 2017): 199–215. http://dx.doi.org/10.5194/se-8-199-2017.
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Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.
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Li, Kunmeng, Yuanhui Li, Shuai Xu, Zhipeng Xiong, and Long An. "Research on the Failure and Fracture Evolution of Mylonite with a Prefabricated Circular Opening by Discrete Element Method." Advances in Civil Engineering 2021 (May 11, 2021): 1–10. http://dx.doi.org/10.1155/2021/5510150.
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In this paper, the failure and fracture process of mylonite with a prefabricated circular opening under biaxial loading is studied by PFC2D code. Firstly, the hoop stress change law of opening wall in the process of loading is theoretically analyzed and three fracture patterns of rocks are proposed. Secondly, the biaxial loading tests of mylonite for numerical simulation are carried out, and the failure and fracture are analyzed from three aspects of space-time evolution of microcracks, energy conversion process, and final damage patterns. As the load progresses, the microcracks start to initiate at the side wall of the opening and the growth velocity of microcracks develops from the slow to fast and then slows down again. The final damage pattern of mylonite with a prefabricated circular opening belongs to shear fracture. The fracture zones start with the side wall spalling and then gradually extend to the border of the rock, which widen from the opening boundary to the border of the rock and slightly twist in the middle. The final fault zone width is about 6 times wider than the average size of simulation particles. Finally, based on the uniaxial compressive strength of mylonite in the laboratory, it is inferred that the fracture pattern of mylonite with a prefabricated circular opening by theoretical analysis is indeed shear failure, which is consistent with the result of numerical simulation.
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VAUGHAN, ALAN P. M., SIMON P. KELLEY, and BRYAN C. STOREY. "Mid-Cretaceous ductile deformation on the Eastern Palmer Land Shear Zone, Antarctica, and implications for timing of Mesozoic terrane collision." Geological Magazine 139, no. 4 (July 2002): 465–71. http://dx.doi.org/10.1017/s0016756802006672.
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Ar–Ar dating of high-strain ductile mylonites of the Eastern Palmer Land Shear Zone in the southern Antarctic Peninsula indicates that reverse movement on the shear zone occurred in late Early Cretaceous times (Albian), and not latest Jurassic times as previously supposed. The Eastern Palmer Land Shear Zone forms a major tectonic boundary, separating suspect arc terranes from rocks of Gondwana continental affinity. The dated mylonites are developed in Lower Jurassic plutonic rocks at Mount Sullivan, eastern Palmer Land, and form part of a zone of ductile reverse deformation up to 25 km wide. Biotite from a fine-grained mafic mylonite yields an Ar–Ar cooling age of 102.8±3.3 Ma. Movement of this age on the Eastern Palmer Land Shear Zone is coeval with circum-Pacific deformation, possibly related to a mantle superplume event, and provides support for allochthonous-terrane models for the Antarctic Peninsula with accretion in post-Early Cretaceous times.
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Culshaw, Nicholas, Christopher Gerbi, and Laura Ratcliffe. "Macro- and microstructural analysis of the North Tea Lake Mylonite Zone: an extensional shear zone in the Central Gneiss Belt, Grenville Province, Ontario." Canadian Journal of Earth Sciences 52, no. 11 (November 2015): 1027–44. http://dx.doi.org/10.1139/cjes-2015-0009.
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The North Tea Lake Mylonite Zone is a late extensional ductile fault that is concordant with and has reworked fabrics of the North Tea Lake Shear Zone, the frontal thrust shear zone of the upper amphibolite–granulite facies Kiosk domain within the interior of the Central Gneiss Belt. North Tea Lake Mylonite Zone fabric is an anomalously fine-grained mylonite compared to Central Gneiss Belt gneisses, and consists of three microstructural domains that display progressive recrystallization and grain size refinement of the protolith granitoid. On the basis of petrography and electron backscatter diffraction, these microdomains are inferred to represent a transition from dominantly dislocation creep to diffusion creep and diffusion-accommodated grain boundary sliding at elevated stress (>100 MPa), low fluid activity, and temperatures ∼500 °C. The North Tea Lake Mylonite Zone is interpreted to mark a step in the progressive transition in deformation mode during late- to post-Ottawan extension and cooling of the Grenville orogen from weak, wide, wet, and warm shear zones to Rigolet-phase cooler, narrow, ultrafine, high-stress shear zones reworking dry protoliths.
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Chadwick, B., and C. R. L. Friend. "The high-grade gneisses in the south-west of Dove Bugt: an old gneiss complex in a deep part of the Caledonides of North-East Greenland." Rapport Grønlands Geologiske Undersøgelse 152 (January 1, 1991): 103–11. http://dx.doi.org/10.34194/rapggu.v152.8163.
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The complex range of orthogneisses has been subdivided on the basis of field characteristics into an old polyphase group, grey phlebitic gneisses, younger varieties of pink granitic gneisses that occur principally as extensive sheets, mixed orthogneisses with schlieric facies and undifferentiated gneisses with dioritic facies. Mafic sheets, now amphibolites, were emplaced at various stages in the evolution of the gneisses. Enclaves and sheets of supracrustal rocks include paragneisses, graphitic schists, marble, amphibolite and stratiform gabbroic complexes with anorthosite. Nappes with curvilinear hinge lines and belts of mylonite developed in high amphibolite conditions after emplacement of the sheets of pink granitic rocks (now variably deformed gneisses) into the old orthogneisses and supracrustal rocks. Principal displacements in the mylonites were shallow and N-directed. The nappes and mylonites were deformed by upright-inclined folds with north-westerly vergence. The nappes, mylonites and north-westerly verging folds are presumed to be Caledonian. N- and S-directed extensions indicated by small-scale ductile structures younger than the regional folding suggest that longitudinal collapse predominated in this part of the Caledonian belt in the south-west of Dove Bugt.
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RÉGNIER, JEAN-LUC, JOCHEN E. MEZGER, and CEES W. PASSCHIER. "Metamorphism of Precambrian–Palaeozoic schists of the Menderes core series and contact relationships with Proterozoic orthogneisses of the western Çine Massif, Anatolide belt, western Turkey." Geological Magazine 144, no. 1 (October 19, 2006): 67–104. http://dx.doi.org/10.1017/s0016756806002640.
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The tectonic setting of the southern Menderes Massif, part of the western Anatolide belt in western Turkey, is characterized by the exhumation of deeper crustal levels onto the upper crust during the Eocene. The lowermost tectonic units of the Menderes Massif are exposed in the Çine Massif, where Proterozoic basement orthogneisses of the Çine nappe are in tectonic contact with Palaeozoic metasedimentary rocks of the Selimiye nappe. In the southern Çine Massif, orthogneiss and metasedimentary rocks are separated by the southerly dipping Selimiye shear zone, preserving top-to-the-S shearing under greenschist facies conditions. In contrast, in the western Çine Massif, the orthogneiss is deformed and mylonitic near the contact with the metasedimentary rocks. The geometry of the mylonite zone and the observed shear directions change from north to southwest. In the north, the mylonite zone dips shallowly to the north, with top-to-the-N shear sense indicators showing northward thrusting of the orthogneiss over the metasedimentary rocks. In the southwest, the mylonite zone resembles a steep N–S striking strike-slip shear zone associated with top-to-the-SSW sense of shear. Overall, the geometry of the mylonite shear zone is consistent with northward movement of the orthogneiss relative to the metasedimentary rocks. Different shear senses are attributed to strain partitioning.AFM diagrams and P–T pseudosections with mineral parageneses of metasedimentary rocks of the Selimiye nappe and metasedimentary enclaves within the orthogneiss of the Çine nappe indicate a single Barrovian-type metamorphism. An earlier higher pressure phase is evident from staurolite–chloritoid inclusions in garnets of the Çine nappe, suggesting a clockwise P–T path. A similar path is inferred for the Selimiye nappe. Index minerals and the sequence of mineral parageneses point to a single amphibolite facies metamorphic event affecting metasedimentary rocks of both nappes, which predates Eocene emplacement of the high pressure–low temperature Lycian and Cycladic blueschist nappes. Northward thrusting of the orthogneiss onto the metasedimentary rocks of the Selimiye nappe is coeval with amphibolite facies metamorphism. Recently postulated polymetamorphism cannot be supported by this study. Petrological data provide no evidence for burial of the lower units of the Menderes Massif to depth greater than 30 km during closure of the Neo-Tethys. A major pre-Eocene tectonic event associated with top-to-the-N thrusting and Barrovian-type metamorphism could lend support to the idea of a Neo-Tethys (sensu stricto) suture south of the Menderes Massif and below the Lycian nappes.
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CHATTOPADHYAY, A., L. KHASDEO, R. E. HOLDSWORTH, and S. A. F. SMITH. "Fault reactivation and pseudotachylite generation in the semi-brittle and brittle regimes: examples from the Gavilgarh–Tan Shear Zone, central India." Geological Magazine 145, no. 6 (August 20, 2008): 766–77. http://dx.doi.org/10.1017/s0016756808005074.
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AbstractIn the sheared and foliated granitoids of the Proterozoic Gavilgarh–Tan Shear Zone (GTSZ) in central India, two types of pseudotachylite (Pt-M and Pt-C) are recognized. Pt-M layers are interbanded with mylonite and ultramylonite, show strong internal plastic deformation and buckle folding concurrent with the host rocks, and appear to have formed within the greenschist facies (300–400 °C) in the brittle–plastic transitional (semi-brittle) regime. Pt-C layers show sharp contacts with the host rock, exhibit abundant coeval cataclasis, preserve no evidence of subsequent plastic deformation, and formed at shallower depths, at temperature < 300 °C. Sulphide droplets and embayment of quartz grain margins in the pseudotachylite (Pt-C) matrix indicates a melt origin. Ductile shear sense criteria in the host mylonites are consistently sinistral, while those associated with the deformed pseudotachylite (Pt-M) layers are dextral. It appears therefore that the host mylonite/ultramylonite foliation experienced reactivated slip movement in the ‘semi-brittle’ zone when pseudotachylite was generated and subsequently ductilely deformed. The brittle pseudotachylite (Pt-C) layers were generated later at a shallower level, and at a lower temperature. They are spatially associated with a set of foliation-parallel brittle shears with sinistral-sense displacements. The multiple episodes of frictional melt generation within the Gavilgarh–Tan Shear Zone illustrate that it has a complex history of multiple reactivations. It therefore represents an important new area for the study of seismic behaviour of the upper crust along pre-existing structures and may facilitate a better geological understanding of the present seismic activity in the central Indian Shield.
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Schwerdtner, W. M. "Interplay between folding and ductile shearing in the Proterozoic crust of the Muskoka – Parry Sound region, central Ontario." Canadian Journal of Earth Sciences 24, no. 8 (August 1, 1987): 1507–25. http://dx.doi.org/10.1139/e87-148.
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Grenville gneiss of the central Georgian Bay region was subjected to ductile deformation that produced narrow mylonite zones as well as three sets of superimposed folds differing greatly in structural signature, size, and orientation. Some mylonite zones are concordant to gneissosity and are repeatedly folded, others cut gneissosity and postdate the folding. Gneissosity was generated as a regionally subhorizontal feature, either by crustal thinning or, like the early mylonite zones, by low-angle reverse shearing. An attempt is made to account for the initially subhorizontal gneissosity, the mylonite zones, and the folds in a regime of large-scale reverse shearing that strikes parallel to the Grenville Front.Upright northwest–southwest to north–south buckle folds dominate the map pattern and are subperpendicular to the reverse Grenville Front boundary fault. These set-2 folds cannot be attributed to reverse simple shearing but require a large component of east–west compression. Such stress could have been generated in a northwest–southeast zone of sinistral ductile shear caused by temporary locking of the southern segment of the Grenville Front boundary fault (now under Lake Huron).All structural facts can be explained without large differential translations of crustal slices. For example, most discordances in the regional gneissosity pattern could have been created by décollement and repeated buckling. Detailed geobarometry and petrologic studies may be required to settle the question of large-scale thrusting within the Grenville gneiss terrane.
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Ralser, S., B. E. Hobbs, and A. Ord. "Experimental deformation of a quartz mylonite." Journal of Structural Geology 13, no. 7 (January 1991): 837–50. http://dx.doi.org/10.1016/0191-8141(91)90008-7.
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Buick, I. S. "Mylonite fabric development on Naxos, Greece." Journal of Structural Geology 13, no. 6 (January 1991): 643–55. http://dx.doi.org/10.1016/0191-8141(91)90027-g.
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Baird, Graham B. "Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone." Geosphere 16, no. 3 (March 19, 2020): 844–74. http://dx.doi.org/10.1130/ges02155.1.
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Abstract Crustal-scale shear zones can be highly important but complicated orogenic structures, therefore they must be studied in detail along their entire length. The Carthage-Colton mylonite zone (CCMZ) is one such shear zone in the northwestern Adirondacks of northern New York State (USA), part of the Mesoproterozoic Grenville province. The southern CCMZ is contained within the Diana Complex, and geochemistry and U-Pb zircon geochronology demonstrate that the Diana Complex is expansive and collectively crystallized at 1164.3 ± 6.2 Ma. Major ductile structures within the CCMZ and Diana Complex include a northwest-dipping penetrative regional mylonitic foliation with north-trending lineation that bisects a conjugate set of mesoscale ductile shear zones. These ductile structures formed from the same 1060–1050 Ma pure shear transitioning to a top-to-the-SSE shearing event at ∼700 °C. Other important structures include a ductile fault and breccia zones. The ductile fault formed immediately following the major ductile structures, while the breccia zones may have formed at ca. 945 Ma in greenschist facies conditions. Two models can explain the studied structures and other regional observations. Model 1 postulates that the CCMZ is an Ottawan orogeny (1090–1035 Ma) thrust, which was later reactivated locally as a tectonic collapse structure. Model 2, the preferred model, postulates that the CCMZ initially formed as a subhorizontal mid-crustal mylonite zone during collapse of the Ottawan orogen. With continued collapse, a metamorphic core complex formed and the CCMZ was rotated into is current orientation and overprinted with other structures.
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Kimpe, Christian De, Chang Wang, Marc Laverdière, and Pierre LaSalle. "Variability within a saprolite deposit near Quebec City, Canada." Canadian Journal of Earth Sciences 22, no. 5 (May 1, 1985): 781–85. http://dx.doi.org/10.1139/e85-083.
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Samples were collected in two trenches opened in a saprolite developed in a mylonite derived from biotite–garnet gneiss near Quebec City in order to investigate the spatial variability from the points of view of morphological, physical, mineralogical, and chemical properties. Phyllosilicate assemblages were: (a) kaolinite, (b) mica, (c) kaolinite + mica, and (d) mica + interstratified minerals (+ smectite). Grouping of the samples according to prominent colour in the matrix showed a relation between chroma and kaolinite content. Total chemical analyses were in good agreement with mineral assemblages. The spatial distribution of these assemblages corresponded to the degree of transformation of the gneiss into mylonite.
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Zhang, Xiaoli, Jinxian He, Zeqiang Ren, Taotao Zhou, Wenjie Cao, and Ben Xu. "Analysis of the Submicrostructural Deformation of Amphibole in a Ductile Shear Zone Based on the TEM Technique." Journal of Nanoscience and Nanotechnology 21, no. 1 (January 1, 2021): 765–71. http://dx.doi.org/10.1166/jnn.2021.18466.
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Deformed amphibole in the plagioclase amphibolite mylonite of the Guandi Complex, Xishan, Beijing, is the research object in this study. The amphibole nanodeformation under the middle crust was analyzed using microstructural analysis and high-resolution transmission electron microscopy (TEM). Microscope observations show that the amphibolite deformations in the plagioclase amphibolite mylonite are δ and σ type porphyroclasts, and the porphyroclast tail is composed of new long-columnar crystals. Using transmission electron microscopy (TEM, and this acronyms would be defined only once), the authors observed the nanodeformation characteristics of the amphibole porphyroclast core and mantle. Dislocation tangles are dominant in the porphyroclast core, and inside the new crystal, there is little or no dislocation. Swelled new crystals surrounded by dislocation can be observed in the transition zone between the porphyroclasts and new crystals. The deformed amphibole microstructure and submicrostructure represent typical brittle–ductile transitional deformation. The deformation process can be divided into two stages: the disordered dislocation increment stage and the dislocation reduction and ordering stage. Crystalline plastic deformation occurs in the amphibole in the plagioclase amphibolite mylonite of the Xishan area, Beijing. The crystalline plastic deformation temperature in amphiboles is higher than that in plagioclase.
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Ashton, K. E., J. F. Lewry, L. M. Heaman, R. P. Hartlaub, M. R. Stauffer, and H. T. Tran. "The Pelican Thrust Zone: basal detachment between the Archean Sask Craton and Paleoproterozoic Flin Flon Glennie Complex, western Trans-Hudson Orogen." Canadian Journal of Earth Sciences 42, no. 4 (April 1, 2005): 685–706. http://dx.doi.org/10.1139/e04-035.
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The Pelican Thrust Zone is a 37 km-wide recrystallized mylonite zone, along which Paleoproterozoic arc volcano-plutonic and derived sedimentary rocks of the Flin Flon Glennie Complex were thrust over an Archean package (Jan Lake Complex) consisting of ca. 3.1 Ga calc-alkaline orthogneisses, pelitic migmatites, and a ca. 2.45 Ga tholeiitic charnockitenorite intrusive suite. A regional northeast-plunging stretching lineation and a variety of kinematic indicators imply southwesterly transport, matching that of other coeval shallower crustal-level structures observed throughout the northern Flin Flon Domain and southern flank of the Kisseynew Domain (e.g., SturgeonWeir and Annabel Lake shear zones). Subsequent east-side-up displacement on the Tabbernor Fault, together with domal fold interference, has exposed the Pelican Window and mantling mylonite zone, where it has been seismically profiled along Lithoprobe transects. Tectonic windows also expose Archean rocks in the Glennie Domain to the west, where similar southwest-verging, recrystallized mylonite zones have been documented. Together, these zones record collision and underplating of the Flin Flon Glennie Complex by the Archean Sask Craton during prolonged protocontinentcontinent collision over a minimum 18261805 Ma interval.
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Gibbons, Wes. "Basement–cover relationships around Aberdaron, Wales, U.K.: the fault-reactivated northwestern margin of the Welsh Basin." Geological Magazine 126, no. 4 (July 1989): 363–72. http://dx.doi.org/10.1017/s0016756800006567.
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AbstractOn mainland North Wales basement rocks emerge from beneath a Lower Ordovician cover along the west side of the Llyn peninsula. The basement contains steeply dipping mylonites (Llŷn shear zone) that separate plutonic and gneissic rocks (Sarn Complex) from a melange (Gwna mélange). The western edge of the Ordovician outcrop follows the basement shear zone, and new trenching data confirm that only a faulted relationship exists between cover and basement along this northwestern extremity of the Welsh Basin. Deformation along this margin has propagated into the Arenig cover to produce southeasterly verging thrusts, asymmetric folds and northwesterly dipping cleavage. A prominent steep fault (Daron Fault) cutting the Ordovician succession follows an eastern splay of the Llŷn shear zone and again therefore records brittle reactivation of an underlying mylonite belt. The likelihood of syn-Arenig fault movements is provided by the presence of a prominent late Arenig coarse clastic unit, containing boulders of the basement mylonites, that is found only to the west of the Daron Fault. Steep basement structures such as the Llŷn shear zone, initially generated as major transcurrent faults, are interpreted as having exerted a strong control over the deposition and subsequent deformation of the Ordovician cover sequence.
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Fawzy, Mona M., Mohamed S. Kamar, and Gehad M. Saleh. "Physical processing for polymetallic mineralization of Abu Rusheid mylonitic rocks, South Eastern Desert of Egypt." International Review of Applied Sciences and Engineering 12, no. 2 (May 29, 2021): 134–46. http://dx.doi.org/10.1556/1848.2021.00200.
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AbstractIn this study, the mineralogical content of Abu Rusheid mylonite sample was investigated and revealed that the sample is essentially composed of quartz and feldspar (72.14% mass), muscovite (16.6% mass), and contains heavy economic polymetallic minerals of about 2.65% by mass. By studying the differences in the physical properties of this mineral content, a proposed flow sheet was set up to explain the successive physical upgrading steps for concentrating and separating the valuable minerals content and getting rid of the associated gangue minerals. Industrial, economic and strategic polymetallic minerals were identified at Abu Rusheid mylonite sample, including cassiterite, titanite, brass, kasolite, monazite, and uranothorite. A group of sulfide minerals also existed as pyrite, arsenopyrite, galena, and molybdenite in addition to the presence of fluorite and iron oxides bearing rare earth elements (REEs) and base metals. Using dry high intensity magnetic separation followed by wet gravity separation and flotation, three concentrates were obtained; heavy paramagnetic concentrate (monazite, columbite, brass, and jarosite), heavy diamagnetic concentrate (zircon, kasolite, uranothorite, cassiterite, and sulphide minerals) and muscovite concentrate for industrial uses. Physical processing of Abu Rusheid mylonite sample was carried out to produce high grade mineral concentrate used as a raw material for chemical treatment to extract economic elements that necessary for several industries.
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Kidder, Steven, David J. Prior, James M. Scott, Hamid Soleymani, and Yilun Shao. "Highly localized upper mantle deformation during plate boundary initiation near the Alpine fault, New Zealand." Geology 49, no. 9 (June 3, 2021): 1102–6. http://dx.doi.org/10.1130/g48532.1.
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Abstract Peridotite xenoliths entrained in magmas near the Alpine fault (New Zealand) provide the first direct evidence of deformation associated with the propagation of the Australian-Pacific plate boundary through the region at ca. 25–20 Ma. Two of 11 sampled xenolith localities contain fine-grained (40–150 μm) rocks, indicating that deformation in the upper mantle was focused in highly sheared zones. To constrain the nature and conditions of deformation, we combine a flow law with a model linking recrystallized fraction to strain. Temperatures calculated from this new approach (625–970 °C) indicate that the observed deformation occurred at depths of 25–50 km. Calculated shear strains were between 1 and 100, which, given known plate offset rates (10–20 mm/yr) and an estimated interval during which deformation likely occurred (<1.8 m.y.), translate to a total shear zone width in the range 0.2–32 km. This narrow width and the position of mylonite-bearing localities amid mylonite-free sites suggest that early plate boundary deformation was distributed across at least ∼60 km but localized in multiple fault strands. Such upper mantle deformation is best described by relatively rigid, plate-like domains separated by rapidly formed, narrow mylonite zones.
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Hippertt, J. F., and F. D. Hongn. "Deformation mechanisms in the mylonite/ultramylonite transition." Journal of Structural Geology 20, no. 11 (November 1998): 1435–48. http://dx.doi.org/10.1016/s0191-8141(98)00047-9.
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Wex, Sebastian, Neil S. Mancktelow, Friedrich Hawemann, Alfredo Camacho, and Giorgio Pennacchioni. "Inverted distribution of ductile deformation in the relatively “dry” middle crust across the Woodroffe Thrust, central Australia." Solid Earth 9, no. 4 (July 11, 2018): 859–78. http://dx.doi.org/10.5194/se-9-859-2018.
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Abstract. Thrust fault systems typically distribute shear strain preferentially into the hanging wall rather than the footwall. The Woodroffe Thrust in the Musgrave Block of central Australia is a regional-scale example that does not fit this model. It developed due to intracontinental shortening during the Petermann Orogeny (ca. 560–520 Ma) and is interpreted to be at least 600 km long in its E–W strike direction, with an approximate top-to-north minimum displacement of 60–100 km. The associated mylonite zone is most broadly developed in the footwall. The immediate hanging wall was only marginally involved in the mylonitization process, as can be demonstrated from the contrasting thorium signatures of mylonites derived from the upper amphibolite facies footwall and the granulite facies hanging wall protoliths. Thermal weakening cannot account for such an inverse deformation gradient, as syn-deformational P–T estimates for the Petermann Orogeny in the hanging wall and footwall from the same locality are very similar. The distribution of pseudotachylytes, which acted as preferred nucleation sites for shear deformation, also cannot provide an explanation, since these fault rocks are especially prevalent in the immediate hanging wall. The most likely reason for the inverted deformation gradient across the Woodroffe Thrust is water-assisted weakening due to the increased, but still limited, presence of aqueous fluids in the footwall. We also establish a qualitative increase in the abundance of fluids in the footwall along an approx. 60 km long section in the direction of thrusting, together with a slight decrease in the temperature of mylonitization (ca. 100 °C). These changes in ambient conditions are accompanied by a 6-fold decrease in thickness (from ca. 600 to 100 m) of the Woodroffe Thrust mylonitic zone.
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Culshaw, N., T. Brown, P. H. Reynolds, and J. WF Ketchum. "Kanairiktok shear zone: the boundary between the Paleoproterozoic Makkovik Province and the Archean Nain Province, Labrador, Canada." Canadian Journal of Earth Sciences 37, no. 9 (September 1, 2000): 1245–57. http://dx.doi.org/10.1139/e00-035.
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The polyphase Kanairiktok shear zone (KNSZ) separates gneissic rocks of the Archean Nain craton from their reworked equivalents in the Paleoproterozoic Kaipokok domain of the Makkovik Province. In its early stages, the KNSZ bounded the Kaipokok domain as it was thermally softened by 1895-1870 Ma Andean-type magmatism, accompanied by dextral oblique convergence and resultant penetrative deformation. The amphibolite-facies tectonite that developed in this stage was widely overprinted by greenschist-facies mylonite. Laserprobe and spectral 40Ar/39Ar ages of recrystallized and porphyroclastic muscovite, from the greenschist-facies mylonite and from muscovite in a syntectonic quartz vein, bracket the age of deformation between 1740 and 1710 Ma with the best estimate at 1715 Ma. These ages are similar to those of A-type granites within the Makkovik Province and amphibole cooling ages from the province interior. Together with the petrological similarity of the greenschist-facies mylonite to localized low-grade shear zones elsewhere in the Makkovik Province, they are suggestive of a widespread, lithosphere-scale event. The 40Ar/39Ar data do not provide good constraints on the early activity of the KNSZ. However, preservation of relationships between granitoid sheets correlated with the 1895-1870 Ma Island Harbour Bay plutonic suite and early fabrics imply that the granites were emplaced syntectonically in the KNSZ. Thus, the KNSZ was a major, long-lived structure in the Makkovik Province that decoupled events in the reactivated Nain craton from an inert cratonic region.
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Lynch, Gregory, and Peter S. Giles. "The Ainslie Detachment: a regional flat-lying extensional fault in the Carboniferous evaporitic Maritimes Basin of Nova Scotia, Canada." Canadian Journal of Earth Sciences 33, no. 2 (February 1, 1996): 169–81. http://dx.doi.org/10.1139/e96-016.
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The Ainslie Detachment occurs near the base of the Carboniferous Windsor Group, forming a regional flat-lying extensional fault distributed across 10 000 km2. New mapping has delineated the structure through southwestern Cape Breton Island and into central Nova Scotia. Shearing is concentrated at the top of the basal Macumber limestone along its contact with overlying evaporites and younger allochthonous units. The highly contrasting rheologies of the formations created an anisotropic zone of weakness which acted as an upper crustal stress guide, stratigraphically controlling the trajectory of the detachment through the basin. The detachment is characterized by an approximately 3–10 m thick calc-mylonite zone, with an intense planar fabric featuring alternating very fine grained shear planes and coarser annealed layers. Coarser layers are boudinaged into pinch and swell structures, locally producing segmented augen. Highly strained intraclasts, ooids, and peloids, recrystallized carbonate boudins, and carbonate vein segments are included in the calc-mylonite as semirigid inclusions and rotated porphyroclasts. Thick zones of fault breccia straddle portions of the detachment and overprint the mylonite, demonstrating an evolution to brittle conditions during progressive shear. Listric faults in the hanging wall of the detachment feature a ramp and flat geometry, with an upper detachment occurring along the upper contact of the Windsor Group with the overlying Namurian Mabou Group. Locally up to 2 km of the stratigraphic succession has been removed, with faults cutting downsection in a westerly direction producing rollover in the hanging wall.
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Passchier, C. W., R. F. Bekendam, J. D. Hoek, P. G. H. M. Dirks, and H. de Boorder. "Proterozoic geological evolution of the northern Vestfold Hills, Antarctica." Geological Magazine 128, no. 4 (July 1991): 307–18. http://dx.doi.org/10.1017/s0016756800017581.
Full textAbstract:
AbstractThe presence of polyphase shear zones transected by several suites of dolerite dykes in Archaean basement of the Vestfold Hills, East Antarctica, allows a detailed reconstruction of the local structural evolution. Archaean and early Proterozoic deformation at granulite facies conditions was followed by two phases of dolerite intrusion and mylonite generation in strike-slip zones at amphibolite facies conditions. A subsequent middle Proterozoic phase of brittle normal faulting led to the development of pseudotachylite, predating intrusion of the major swarm of dolerite dykes around 1250 Ma. During the later stages and following this event, pseudotachylite veins were reactivated as ductile, mylonitic thrusts under prograde conditions, culminating in amphibolite facies metamorphism around 1000–1100 Ma. This is possibly part of a large-scale tectonic event during which the Vestfold block was overthrust from the south. In a final phase of strike-slip deformation, several pulses of pseudotachylite-generating brittle faulting alternated with ductile reactivation of pseudotachylite.
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Davis, George H., Anne F. Gardulski, and Gordon S. Lister. "Shear zone origin of quartzite mylonite and mylonitic pegmatite in the Coyote Mountains metamorphic core complex, Arizona." Journal of Structural Geology 9, no. 3 (January 1987): 289–97. http://dx.doi.org/10.1016/0191-8141(87)90053-8.
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Kim, Sung-Wook, Eun-Kyeong Choi, Tae-Sun Yang, and Kyu-Hwan Lee. "Engineering Properties of Mylonite in the Youngju Area." Journal of the Korean Geotechnical Society 27, no. 10 (October 31, 2011): 35–43. http://dx.doi.org/10.7843/kgs.2011.27.10.035.
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Casey, Martin, and Danielle Williams. "Micromechanical control of rheological anisotropy in quartz mylonite." Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy 25, no. 2 (January 2000): 127–32. http://dx.doi.org/10.1016/s1464-1895(00)00021-1.
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Burlini, L., and K. Kunze. "Fabric and seismic properties of carrara marble mylonite." Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy 25, no. 2 (January 2000): 133–39. http://dx.doi.org/10.1016/s1464-1895(00)00022-3.
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Trimby, Patrick W., David J. Prior, and John Wheeler. "Grain boundary hierarchy development in a quartz mylonite." Journal of Structural Geology 20, no. 7 (July 1998): 917–35. http://dx.doi.org/10.1016/s0191-8141(98)00026-1.
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Hanmer, Simon. "Initiation of cataclastic flow in a mylonite zone." Journal of Structural Geology 11, no. 6 (January 1989): 751–62. http://dx.doi.org/10.1016/0191-8141(89)90009-6.
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Healy, David. "Dehydration-induced brittle vein in a serpentinite mylonite." Journal of Structural Geology 33, no. 5 (May 2011): 757. http://dx.doi.org/10.1016/j.jsg.2009.05.004.
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OTOH, SHIGERU. "Calc-mylonite in the Ashidachi area, Okayama Prefecture." Journal of the Geological Society of Japan 92, no. 9 (1986): 691–94. http://dx.doi.org/10.5575/geosoc.92.691.
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Munro, M. "Mylonite zones in the Insch ‘Younger Basic’ Mass." Scottish Journal of Geology 22, no. 1 (May 1986): 132–36. http://dx.doi.org/10.1144/sjg22010132.
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Zagorevski, A., and V. McNicoll. "Evidence for seamount accretion to a peri-Laurentian arc during closure of Iapetus 1This article is one of a series of papers published in CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.2 Geological Survey of Canada Contribution 20100465." Canadian Journal of Earth Sciences 49, no. 1 (January 2012): 147–65. http://dx.doi.org/10.1139/e11-016.
Full textAbstract:
The Red Indian Line is the fundamental Iapetus suture zone in the Newfoundland Appalchians along which the main tract of the Iapetus Ocean was consumed. Despite being the site of the closure of a wide ocean, few vestiges of the Iapetus plate have been accreted along Red Indian Line. Ordovician rocks in the Notre Dame Bay area preserve the only evidence for accretion of a seamount in Newfoundland. The seamount is characterized by alkali basalt and hypabyssal rocks that are juxtaposed with Darriwilian peri-Laurentian volcanic arc rocks (466 ± 4 and 467 ± 4 Ma) along a major mylonite zone. The mylonite zone lacks sedimentary rocks suggesting that the seamount was accreted to the arc along a sediment-starved interface and that significant subduction erosion took place along the Laurentian margin. Identification of subduction erosion indicates that an accretionary prism did not exist outboard of Laurentia in Newfoundland, in contrast to the well developed accretionary prisms of the Caledonides.