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Ductile Deformation (ductile + deformation)
Selected AbstractsOn strike-slip faulting in layered mediaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002Maurizio Bonafede Summary We study the effects of structural inhomogeneities on the stress and displacement fields induced by strike-slip faults in layered media. An elastic medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space characterized by different elastic parameters. Shear cracks with assigned stress drop are employed as mathematical models of strike-slip faults, which are assumed to be vertical and planar. If the crack is entirely embedded within the lower medium (case A), a Cauchy-kernel integral equation is obtained, which is solved by employing an expansion of the dislocation density in Chebyshev polynomials. If the crack is within the lower medium but it terminates at the interface (case B), a generalized Cauchy singularity appears in the integral kernel. This singularity affects the singular behaviour of the dislocation density at the crack tip touching the interface. Finally, the case of a crack crossing the interface is considered (case C). The crack is split into two interacting sections, each placed in a homogeneous medium and both open at the interface. Two coupled generalized Cauchy equations are obtained and solved for the dislocation density distribution of each crack section. An asymptotic study near the intersection between the crack and the interface shows that the dislocation densities for each crack section are bounded at the interface, where a jump discontinuity is present. As a corollary, the stress drop must be discontinuous at the interface, with a jump proportional to the rigidity contrast between the adjoining media. This finding is shown to have important implications for the development of geometrical complexities within transform fault zones: planar strike-slip faults cutting across layer discontinuities with arbitrary stress drop values are shown to be admissible only if the interface between different layers becomes unwelded during the earthquake at the crack/interface junction. Planar strike-slip faulting may take place only in mature transform zones, where a repetitive earthquake cycle has already developed, if the rheology is perfectly elastic. Otherwise, the fault cannot be planar: we infer that strike-slip faulting at depth is plausibly accompanied by en-echelon surface breaks in a shallow sedimentary layer (where the stress drop is lower than prescribed by the discontinuity condition), while ductile deformation (or steady sliding) at depth may be accommodated by multiple fault branching or by antithetic faulting in the upper brittle layer (endowed with lower rigidity but higher stress). [source] Three-dimensional finite strain analysis in the high-grade part of the Sanbagawa Belt using deformed meta-conglomerateISLAND ARC, Issue 2 2002Yoshinori Moriyama Abstract Regional ductile deformation of the Sanbagawa belt is generally thought to be characterized by constrictional strain, based on strain analysis using deformed radiolarians in the low-grade regions. Similar strain analysis could not be carried out in the medium- to high-grade zones, because it is very difficult to identify individual radiolarians after strong recrystallization. However, discovery of the first known meta-conglomerate in the high-grade region of the Sanbagawa Belt allows quantitative 3-D strain to be estimated in this region. Using a development of the Rf-, method, an evaluation of appropriate errors for this estimate can be determined. The principal strain ratios and estimated errors are X/Y = 5.4,6.6 and Y/Z = 3.8,3.9 implying deformation in the flattening field and refuting the idea of uniform constrictional strain. Semi-quantitative markers of the shape of the strain ellipse throughout the high-grade regions suggest that the deformation of the Sanbagawa Belt is dominantly in the flattening field. The difference with the earlier results may be due to late-stage overprinting by upright folding of the main ductile fabric in the low-grade region of western Shikoku. [source] Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb,Sr and fission-track ages of gneissose granitoid and numerical modelingISLAND ARC, Issue 2 2001Takamoto Okudaira Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb,Sr whole-rock,mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb,Sr ages fall in a range of ca 89,87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1, error), respectively. Combining the newly obtained ages with previously reported (Th,)U,Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is , 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1,0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis. [source] Petrology of corundum-spinel-sapphirine-anorthite rocks (sakenites) from the type locality in southern MadagascarJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2008M. M. RAITH Abstract ,Sakenites' constitute a unique association of corundum-, spinel- and sapphirine-bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum-, spinel- and sapphirine-bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite-sillimanite-cordierite gneisses of the Ihosy granulite unit in the NW of the Pan-African Bongolava-Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite-rich sediments or calcareous bauxites that were altered by Ca or Si infiltration-metasomatism prior to or coeval with the development of the anorthite-corundum assemblage. P,T pseudosection modelling of metapelitic gneisses suggests peak-conditions around 800 °C and 6,7 kbar for the regional high-grade metamorphism and deformation in the NW part of the Bongolava-Ranotsara shear zone. The well-annealed granoblastic-polygonal textures indicate complete chemical and textural re-equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P,T conditions (750,700 °C, 6 kbar), the influx of Mg-, Si- and K-bearing fluids into the anorthite-corundum rocks caused significant metasomatic changes. In zones infiltrated by ,primary' potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg-Al minerals, thereby producing corundum-, spinel- and sapphirine-bearing phlogopitites. Further advance of the resulting ,residual' Mg- and Si-bearing fluids into anorthite-corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava-Ranotsara ductile shear zone c. 530,500 Ma ago. [source] A comparative U,Th,Pb (zircon,monazite) and 40Ar,39Ar (muscovite,biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross OrogenJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2007G. DI VINCENZO Abstract Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U,Th,Pb system in zircon and monazite and of the 40Ar,39Ar system in micas during ductile deformation. Meso- and micro-structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top-to-the-SW and -NE, respectively) and developed under upper greenschist facies conditions. In situ U,Pb dating by laser-ablation inductively coupled plasma-mass spectrometry of zircon areas with well-preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross,Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U,Th,Pb ages of c. 440 Ma, in agreement with in-situ Ar laserprobe ages of syn-shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal-chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U,Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ,ghost' structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U,Pb ages of c. 450 and 410 Ma. The c. 450-Ma date matches within errors most monazite U,Th,Pb ages and in-situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician,Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear-zone system formed in response to ,NE,SW-directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician,Late Devonian Lachlan Fold Belt. [source] Reaction localization and softening of texturally hardened mylonites in a reactivated fault zone, central ArgentinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2005S. J. WHITMEYER Abstract The Tres Arboles ductile fault zone in the Eastern Sierras Pampeanas, central Argentina, experienced multiple ductile deformation and faulting events that involved a variety of textural and reaction hardening and softening processes. Much of the fault zone is characterized by a (D2) ultramylonite, composed of fine-grained biotite + plagioclase, that lacks a well-defined preferred orientation. The D2 fabric consists of a strong network of intergrown and interlocking grains that show little textural evidence for dislocation or dissolution creep. These ultramylonites contain gneissic rock fragments and porphyroclasts of plagioclase, sillimanite and garnet inherited from the gneissic and migmatitic protolith (D1) of the hangingwall. The assemblage of garnet + sillimanite + biotite suggests that D1-related fabrics developed under upper amphibolite facies conditions, and the persistence of biotite + garnet + sillimanite + plagioclase suggests that the ultramylonite of D2 developed under middle amphibolite facies conditions. Greenschist facies, mylonitic shear bands (D3) locally overprint D2 ultramylonites. Fine-grained folia of muscovite + chlorite ± biotite truncate earlier biotite + plagioclase textures, and coarser-grained muscovite partially replaces relic sillimanite grains. Anorthite content of shear band (D3) plagioclase is c. An30, distinct from D1 and D2 plagioclase (c. An35). The anorthite content of D3 plagioclase is consistent with a pervasive grain boundary fluid that facilitated partial replacement of plagioclase by muscovite. Biotite is partially replaced by muscovite and/or chlorite, particularly in areas of inferred high strain. Quartz precipitated in porphyroclast pressure shadows and ribbons that help define the mylonitic fabric. All D3 reactions require the introduction of H+ and/or H2O, indicating an open system, and typically result in a volume decrease. Syntectonic D3 muscovite + quartz + chlorite preferentially grew in an orientation favourable for strain localization, which produced a strong textural softening. Strain localization occurred only where reactions progressed with the infiltration of aqueous fluids, on a scale of hundreds of micrometre. Local fracturing and microseismicity may have induced reactivation of the fault zone and the initial introduction of fluids. However, the predominant greenschist facies deformation (D3) along discrete shear bands was primarily a consequence of the localization of replacement reactions in a partially open system. [source] P,T conditions of decompression of the Limpopo high-grade terrane: record from shear zonesJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2001C. A. Smit Abstract The Southern Marginal Zone of the late Archean Limpopo Belt of southern Africa is an example of a high-grade gneiss terrane in which both upper and lower crustal deformational processes can be studied. This marginal zone consists of large thrust sheets of complexly folded low-strain gneisses, bound by an imbricate system of kilometre-wide deep crustal shear zones characterized by the presence of high-strain gneisses (,primary straight gneisses'). These shear zones developed during the decompression stage of this high-grade terrane. Low- and high-strain gneisses both contain similar reaction textures that formed under different kinematic conditions during decompression. Evidence for the early M1/D1 metamorphic phase (> 2690 Ma) is rarely preserved in low-strain gneisses as a uniform orientation of relict Al-rich orthopyroxene in the matrix and quartz and plagioclase inclusions in the cores of early (M1) Mg-rich garnet porphyroblasts. This rare fabric formed at >,820 °C and >,7.5 kbar. The retrograde M2/D2 metamorphic fabric (2630,2670 Ma) is well developed in high-strain gneisses from deep crustal shear zones and is microscopically recognized by the presence of reaction textures that formed synkinematically during shear deformation: M2 sigmoid-shaped reaction textures with oriented cordierite,orthopyroxene symplectites formed after the early M1 Mg-rich garnet porphyroblasts, and syn-decompression M2 pencil-shaped garnet with oriented inclusions of sillimanite and quartz formed after cordierite under conditions of near-isobaric cooling at 750,630 °C and 6,5 kbar. The symplectites and pencil-shaped garnet are oriented parallel to the shear fabric and in the stretching direction. Low-strain gneisses from thrust sheets show similar M2 decompression cooling and near-isobaric cooling reaction textures that formed within the same P,T range, but under low-strain conditions, as shown by their pseudo-idioblastic shapes that reflect the contours of completely replaced M1 garnet and randomly oriented cordierite,orthopyroxene symplectites. The presence of similar reaction textures reflecting low-strain conditions in gneisses from thrust sheets and high-strain conditions in primary straight gneisses suggests that most of the strain during decompression was partitioned into the bounding shear zones. A younger M3/D3 mylonitic fabric (< 2637 Ma) in unhydrated mylonites is characterized by brittle deformation of garnet porphyroclasts and ductile deformation of the quartz,plagioclase,biotite matrix developed at <,600 °C, as the result of post-decompression shearing under epidote,amphibolite facies conditions. [source] Super-silicic garnet microstructures from an orogenic garnet peridotite, evidence for an ultra-deep (>6 GPa) originJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2000Van Roermund We report the field, petrographic and mineral chemical characteristics of relict super-silicic (=majoritic) garnet microstructures from the Otrøy peridotites in the Western Gneiss Region, Norway. The evidence for the former existence of super-silicic garnet consists of two-pyroxene exsolution microstructures from garnet. Estimates of the initial composition of the super-silicic garnet imply pressures of 6,6.5 GPa, indicating that the Otrøy garnet peridotites were derived from depths >185 km. The garnet peridotites consist of inter-banded variable compositions with c. 50% garnet peridotite and 50% garnet-free peridotite. Two distinct garnet types were identified: (a) normal matrix garnet, grain-size ,4 mm, and (b) large isolated single garnet crystals and/or (polycrystalline) garnet nodules up to 10 cm in size. Large garnet nodules occur only within limited bands within the garnet peridotites. The relicts of super-silicic garnet were exclusively found in some (not all) of the larger garnet nodules. Petrographic observations revealed that the microstructure of nodular garnet consists of the following four characteristic elements. (1) Individual garnet nodules are polycrystalline, with grain sizes of 2,8 mm. Garnet grain boundaries are straight with well-defined triple junctions. (2) Some garnet triple junctions and garnet grain boundaries are decorated by interstitial orthopyroxene. (3) Cores of larger polycrystalline garnet contain two-pyroxene exsolution microstructures. (4) Precipitation-free rims (2 mm thick) surround garnet cores containing the exsolved pyroxene microstructure. Pyroxene exsolution from super-silicic garnet was subsequently followed by brittle,ductile deformation of garnet. Both exsolved pyroxene needles and laths become undulous or truncated by fractures. Simultaneous garnet plasticity is indicated by the occurrence of high densities of naturally decorated dislocations. Transmission electron microscopy observations indicate that decoration is due to Ti-oxide precipitation. Estimates of the P,T conditions for mineral chemical equilibration were obtained from geothermobarometry. The mineral compositions equilibrated at mantle conditions around 805±40 °C and 3.2±0.2 GPa. These P,T estimates correspond to cold continental lithosphere conditions at depths of around 105 km. From a combination of both depth estimates it can be concluded that the microstructural memory of the rock extends backwards to twice as great a depth range as obtained by thermobarometric methods. Available geochronological and geochemical data of Norwegian garnet peridotites suggest a multi-stage, multi-orogenic exhumation history. [source] Orogenic Gold Mineralization in the Qolqoleh Deposit, Northwestern IranRESOURCE GEOLOGY, Issue 3 2007Farhang Aliyari Abstract The Qolqoleh gold deposit is located in the northwestern part of the Sanandai-Sirjan Zone, northwest of Iran. Gold mineralization in the Qolqoleh deposit is almost entirely confined to a series of steeply dipping ductile,brittle shear zones generated during Late Cretaceous,Tertiary continental collision between the Afro-Arabian and the Iranian microcontinent. The host rocks are Mesozoic volcano-sedimentary sequences consisting of felsic to mafic metavolcanics, which are metamorphosed to greenschist facies, sericite and chlorite schists. The gold orebodies were found within strong ductile deformation to late brittle deformation. Ore-controlling structure is NE,SW-trending oblique thrust with vergence toward south ductile,brittle shear zone. The highly strained host rocks show a combination of mylonitic and cataclastic microstructures, including crystal,plastic deformation and grain size reduction by recrystalization of quartz and mica. The gold orebodies are composed of Au-bearing highly deformed and altered mylonitic host rocks and cross-cutting Au- and sulfide-bearing quartz veins. Approximately half of the mineralization is in the form of dissemination in the mylonite and the remainder was clearly emplaced as a result of brittle deformation in quartz,sulfide microfractures, microveins and veins. Only low volumes of gold concentration was introduced during ductile deformation, whereas, during the evident brittle deformation phase, competence contrasts allowed fracturing to focus on the quartz,sericite domain boundaries of the mylonitic foliation, thus permitting the introduction of auriferous fluid to create disseminated and cross-cutting Au-quartz veins. According to mineral assemblages and alteration intensity, hydrothermal alteration could be divided into three zones: silicification and sulfidation zone (major ore body); sericite and carbonate alteration zone; and sericite,chlorite alteration zone that may be taken to imply wall-rock interaction with near neutral fluids (pH 5,6). Silicified and sulfide alteration zone is observed in the inner parts of alteration zones. High gold grades belong to silicified highly deformed mylonitic and ultramylonitic domains and silicified sulfide-bearing microveins. Based on paragenetic relationships, three main stages of mineralization are recognized in the Qolqoleh gold deposit. Stage I encompasses deposition of large volumes of milky quartz and pyrite. Stage II includes gray and buck quartz, pyrite and minor calcite, sphalerite, subordinate chalcopyrite and gold ores. Stage III consists of comb quartz and calcite, magnetite, sphalerite, chalcopyrite, arsenopyrite, pyrrhotite and gold ores. Studies on regional geology, ore geology and ore-forming stages have proved that the Qolqoleh deposit was formed in the compression,extension stage during the Late Cretaceous,Tertiary continental collision in a ductile,brittle shear zone, and is characterized by orogenic gold deposits. [source] 40Ar/39Ar Dating of Deformation Events and Reconstruction of Exhumation of Ultrahigh-Pressure Metamorphic Rocks in Donghai, East ChinaACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2003LI Jinyi Abstract Recent investigations reveal that the ultrahigh-pressure metamorphic (UHPM) rocks in the Donghai region of East China underwent ductile and transitional ductile-brittle structural events during their exhumation. The earlier ductile deformation took place under the condition of amphibolite facies and the later transitional ductile-brittle deformation under the condition of greenschist facies. The hanging walls moved southeastward during both of these two events. The 40Ar/39Ar dating of muscovites from muscovite-plagioclase schists in the Haizhou phosphorous mine, which are structurally overlain by UHPM rocks, yields a plateau age of 218.0±2.9 Ma and isochron age of 219.8Ma, indicating that the earlier event of the ampibolite-facies deformation probably took place about 220 Ma ago. The 40Ar/39Ar dating of oriented amphiboles parallel to the movement direction of the hanging wall on a decollement plane yields a plateau age of 213.1 ± 0.3 Ma and isochron age of 213.4±4.1 Ma, probably representing the age of the later event. The dating of pegmatitic biotites and K-feldspars near the decollement plane from the eastern Fangshan area yield plateau ages of 203.4±0.3 Ma, 203.6±0.4 Ma and 204.8±2.2 Ma, and isochron ages of 204.0±2.0 Ma, 200.6±3.1 Ma and 204.0±5.0 Ma, respectively, implying that the rocks in the studied area had not been cooled down to closing temperature of the dated biotites and K-feldspars until the beginning of the Jurassic (about 204 Ma). The integration of these data with previous chronological ages on the ultrahigh-pressure metamorphism lead to a new inference on the exhumation of the UHPM rocks. The UHPM rocks in the area were exhumed at the rate of 3,4 km/Ma from the mantle (about 80,100 km below the earth's surface at about 240 Ma) to the lower crust (at the depth of about 20-30km at 220 Ma), and at the rate of 1,2 km/Ma to the middle crust (at the depth of about 15 km at 213 Ma), and then at the rate of less than 1 km/Ma to the upper crust about 10 km deep at about 204 Ma. 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