Timing of deformation phases within the South Finland shear zone, SW Finland

The Palaeoproterozoic Svecofennian crust in southern and central Fennoscandia was established about 1.8 Ga ago after a prolonged history of accretion and intrusion. During late stages of the Svecofennian orogeny, deformation was partitioned into several crustal-scale shear zones in present-day Finland, Sweden and Estonia. One such major ductile deformation zone, ‘the South Finland shear zone’ (SFSZ) extends for almost 200 km through the Åland archipelago in southwestern Finland, and further along the southern and southwestern coast of Finland. This more than a kilometer wide transpressional zone appears to have been repeatedly reactivated. The deformation started with a period of regional, ductile dextral shearing of igneous rocks, producing striped granodioritic and tonalitic gneisses. The ductile phases are locally overprinted and followed by ductile to semi-ductile deformation evidenced by mylonite zones of variable width. The last stage of tectonic activity along the shear zone is recorded by pseudotachylites. Within this study, we dated zircons (SIMS U–Pb) and titanites (ID-TIMS U–Pb) from eight rock samples, and two pseudotachylite whole-rock samples (⁴⁰Ar/³⁹Ar) in order to reconstruct the deformation and (re)activation history of the shear zone.The results suggest that the medium-grained gneisses underwent three distinct deformation phases separated by time intervals without regional deformation. The ductile deformation within the study area initiated at 1.85 Ga. A second, more intensive deformation phase existed around 1.83 Ga, by which the shear zone was already well developed. Finally, the last ductile event is recorded by 1.79 Ga metamorphic titanites in relatively granoblastic granitoid gneisses that nevertheless already display protomylonitic textures, suggesting the initiation of large-scale mylonitisation around or soon after this time. The age of a pseudotachylite sample and, hence, the brittle deformation is bracketed between 1.78 and 1.58 Ga based on the age of pegmatites cut by pseudotachylites as well as ⁴⁰Ar/³⁹Ar minimum ages for the pseudotachylite, respectively. The data imply that the rocks within the study area entered the ductile–brittle transition zone due to rapid cooling and exhumation of the crust after 1.79 Ga.     

The generation of ductile and brittle shear bands in a low-angle mylonite zone

In the Saint-Barthélemy Massif, French Pyrenees, a ductile thrust zone developed in gneisses during retrogression from lower amphibolite facies conditions to the upper greenschist facies. The last major structures formed in the zone are isolated shear bands, divided into three types.Anastomosing, inhomogenous ultramylonitic shear bands (Type I) are subparallel to the mylonitic foliation in the gneiss (S_g). Most of these bands developed by ductile deformation processes only.Planar, homogeneous ultramylonite bands (Type II) are usually oblique to S_g. They generated as pseudotachylyte bands by brittle fracturing and underwent strong subsequent ductile deformation.Type III shear bands are planar and oblique to S_g. They consist of pseudotachylyte, weakly affected by ductile deformation.Type I, II and III bands seem to represent progressively younger structures on a local scale, linked to falling P-T conditions. The systematic variation in orientation of the different shear bands with respect to S_g is interpreted as being due to a different response of brittle and ductile structures to the orientation of the kinematic frame and the rock anisotropy.     

Progressive development of structures in a ductile shear zone

In the Singhbhum Shear Zone of eastern India successive generations of folds grew in response to a progressive ductile shearing. During this deformation a mylonitic foliation was initiated and was repeatedly transposed. The majority of fold hinges were formed in an arcuate manner at low angles to the Y-axis in an E-W trending subhorizontal position and major segments of the fold hinges were then rotated towards the down-dip northerly plunging X-axis. The striping and intersection lineations were rotated in the same manner. The down-dip mylonitic lineation is a composite structure represented by rotated early lineations and newly superimposed stretching lineations. The consistent asymmetry of the folds, the angular relations between C and S surfaces and the evidence of two-dimensional boudinage indicate that the deformation was non-coaxial, but with a flattening type of strain with λ₁ ⪢ λ₂. The degree of non-coaxiality varied both in space and time. From the progressive development of mesoscopic structures it is concluded that the 2–3 km wide belt of ductile shear gave rise to successive anastomosing shear zones of mesoscopic scale. When a new set of shear lenses was superimposed on already sheared rocks, the preexisting foliation generally lay at a low angle to the lenses. No new folds developed where the acute angle was sympathetic to the sense of shear displacements. Where the acute angle was counter to the sense of shear, the pre-existing foliation, lying in the instantaneous shortening field, was deformed into a set of asymmetric folds.     

Heterogeneous constrictional deformation in a ductile shear zone resulting from the transposition of a lineation-parallel fold

We use new (micro-)structural, petrofabric, strain and vorticity data to analyze the deformation path in a mesoscopic quartz mylonite zone. The mylonite zone resulted from the complete transposition of a stretching lineation-parallel isoclinal fold. Symmetric cleft-girdle quartz c-axis fabrics were recorded in the middle domain, which occupies the inner limbs of the precursor isoclinal fold, while asymmetric cleft- and crossed-girdle fabrics were observed in the upper and lower domains that represent the outer limbs. Constrictional strain, with increasing k values towards the middle domain, is inferred from petrofabric and 3D strain data. Oblique grain shape fabrics yield vorticity estimates of 0.72–0.90 in the zone. However, in the middle domain, pure shear dominated deformation is suggested by orthorhombic crystallographic fabrics. Strain rate is constant throughout the zone; a strain decrease towards the zone center implies that deformation ceased earlier in the middle domain. The data indicates that fold transposition and subsequent mylonitization started as pure-shear-dominated constrictional deformation and progressively changed to simple-shear-dominated, plane strain. During this flow path the asymmetric quartz c-axis fabrics likely developed by depopulation of cleft-girdle maxima rather than from the synthetic rotation of fabric maxima itself.     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带,至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关,应变能常沿断裂集中,形成方解石脉;然后,经由北往南的逆冲推覆,形成(叠瓦状)逆冲推覆构造;最后,由于山体重力势能的作用,伸展跨塌。该剪切带成为藏南拆离系的组成部分。     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带,至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关,应变能常沿断裂集中,形成方解石脉;然后,经由北往南的逆冲推覆,形成(叠瓦状)逆冲推覆构造;最后,由于山体重力势能的作用,伸展跨塌。该剪切带成为藏南拆离系的组成部分。     

西藏拉轨岗日构造带孔木韧性剪切带特征

在拉轨岗日构造带佩枯错新发现了一条韧性剪切带，至少存在3期变形。韧性剪切作用可能与应变能转化的热能有关，应变能常沿断裂集中，形成方解石脉；然后，经由北往南的逆冲推覆，形成(叠瓦状)逆冲推覆构造；最后，由于山体重力势能的作用，伸展跨塌。该剪切带成为藏南拆离系的组成部分。     

韧性剪切变形对岩石地球化学行为的制约——以北阿尔金巴什考供韧性剪切带为例

韧性剪切变形对岩石地球化学行为的制约一直是地质学家们探讨的课题。本文以构成北阿尔金红柳沟——拉配泉俯冲碰撞杂岩带与北阿尔金地块边界的巴什考供斜向逆冲型韧性剪切带为例,通过对韧性剪切带内花岗岩变形前后不同变形强度构造岩的地球化学组成进行对比,确定等比线斜率,探讨韧性变形对岩石体积和成分变异的影响。计算结果表明,在糜棱岩化过程中,糜棱岩化花岗岩体积亏损21％,花岗质糜棱岩体积亏损31％。质量平衡计算结果和等比线图表明,韧;陛剪切作用导致SiO2,流失量最大,A12O3、K2O及Ba、Rb、Sr等都有不同程度的丢失,显示出较强的活动性,MnO、P2O5、Sc位于等比线上或附近,表现出相对的稳定性。岩石中活动组分的变异是流体渗滤作用引起的,不活动组分的变异是体．积亏损造成的。     

Structures and fabrics in a crustal-scale shear zone,Betic Cordillera,SE Spain

A broad zone of dominantly ductile high-strain deformation lies beneath the Aguilón nappe in the Sierra Alhamilla, southern Spain. It forms part of a crustal-scale movement zone, traceable through much of the Betic Cordillera, which separates the Higher Betic Nappes from the underlying Nevado-Filabride Complex. The zone is characterized in outcrop by a distinctive platy foliation and a strong NNE-trending stretching lineation. Microstructural characteristics include quartz ribbons, mica fish, augen of feldspar and other minerals in a matrix of dynamically recrystallized quartz, and extensional crenulation cleavages. Narrow bands of ultramylonite and cataclasite occur within and on the margins of the movement zone. Deformation occurred under lower greenschist-facies conditions and was accompanied by retrogression of earlier higher-grade mineral assemblages.Structures in the movement zone developed in a temporal sequence, beginning with isoclinal folding and transposition of older foliations. This was followed by the formation of extensional crenulation cleavages, and the progressive localization of strain into the ultramylonite bands. Mylonitic foliation in these bands is deformed by syn-mylonite folds restricted to the bands. All these structures were then deformed by S- to SE-vergent small-scale folds restricted to the movement zone as a whole. Cataclasis, associated with alteration, is localized along the ultramylonite bands and indicates a transition to late-stage brittle deformation. The lower boundary of the movement zone is gradational: strain decreases, recrystallized grainsize and the degree of recrystallization of quartz increases, and pressure solution becomes the dominant deformation mechanism in mica-schist.Asymmetric quartz fabrics in the movement zone indicate a NNE sense of shear; but variations in the degree of asymmetry suggest that flow was partitioned, with the ultramylonite bands taking up much of the shear strain, and the intervening rocks deforming more slowly and with a lower degree of non-coaxiality. Diffuse fabrics in the fine-grained ultramylonite bands may indicate a switch to a grainsize-sensitive deformation mechanism, and an overall downward increase in the opening angle of crossed-girdle fabrics may reflect increased water activity at depth.     

Contrasted metamorphic and structural evolutions across a major ductile/brittle displacement zone in NW Connemara,western Ireland

NW Connemara is transected by a steeply dipping, E-W to ESE-WNW trending structure, the Renvyle-Bofin Slide (RBS), which shows evidence of both ductile and brittle displacement. Structural observations indicate a polyphase D1-D4 deformation history in the adjacent rock units, dominated by the development during D2 of a regional L-S fabric comprising an E-W to ESE-WNW striking foliation and a strike-parallel stretching lineation. On the north side of the RBS the stretching lineation is locally oriented N-S and associated with a system of linked ductile duplex structures in the contact aureole of the Dawros-Currywongaun-Doughruagh Complex (DCDC). Folding of the N-S stretching lineation by F3 folds suggests it too formed during D2 synchronous with the more strike-parallel stretching lineation. »Peak« metamorphic conditions and qualitative PTt paths derived from GIBBS method thermodynamic modelling of garnet zoning indicate both sides of the RBS underwent burial during D2, but that the south side of the RBS was buried more (c. 1.5 kbar, 5 km). It is suggested that D2 involved predominantly sinistral displacement on the RBS, but east of Tully Mountain around the DCDC there was substantial dipslip displacement resulting from transpression where the slide markedly bends around Tully Mountain. Modelling of metamorphic mineral assemblages developed across the RBS and analysis of published mineral ages provide evidence that the RBS moved repeatedly during and after regional metamorphism, that the D2 dip-slip component around the DCDC may have been substantial during garnet growth assisting burial of rocks on the southern side of the RBS, and that dip-slip movements were probably dominant and variable during exhumation.

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Zusammenfassung NW Connemara ist durch eine steil einfallende E-W bis ESE-WNW streichende Struktur zerteilt, die Renvyle-Bofin Slide (RBS), welche sowohl Hinweise für eine duktile als auch für spröde Verwerfung zeigt. Tektonische Beobachtungen weisen auf eine mehrphasige D1-D4 Deformationsgeschichte in den umliegenden Gesteinsserien hin, welche von der Entwicklung während D2 des regionalen L-S Gefüges dominiert wird. Diese enthält eine E-W bis ESE-WNW streichende Bänderung, sowie eine parallel zum Streichen verlaufende Streckungslineation. Auf der nördlichen Seite des RBS ist die Streckungslineation lokal N-S orientiert, und wird in Zusammenhang mit einem System von verbundenen duktilen Duplex-Strukturen in der Kontaktaureole des Dawros-Currywongaun-Doughruagh Complexes (DCDC) gebracht. Faltung der N-S gerichteten Streckungslineation durch F3-Falten, legt die Vermutung nahe, daß auch sie von der D2 zeitgleich mit der eher parallel zum Streichen verlaufenden Streckungslineation gebildet wurden. Metamorphe Spitzenbedingungen und qualitative PTt Pfade die von GIBBS Methode abgeleitet wurden, zeigen an Hand von thermodynamischen Mustern, daß beide Seiten des RBS einer Versenkung während D2 unterzogen wurden; die Südseite wurde dabei aber tiefer versenkt (ca. 1,5 kb; 5 km). Es ist naheliegend, daß D2 überwiegend sinistrale Verwerfung des RBS mit sich brachte. Aber östlich von Tully Mountain, in der Nähe der DCDC finden sich erhebliche Aufschiebungen die von der Druckbeanspruchung herrührt, bei der die Überschiebung um Tully Mountain herumbiegt. Modellieren der metamorphen Mineralvergesellschaftungen welche quer über die RBS entwickelt sind, sowie Analysen veröffentlichter Mineralalter liefern einen Hinweis darüber, daß sich die RBS wiederholt während und nach der Regionalmetamorphose bewegt hat. Weiterhin zeigt sich, daß die D2-Sprunghöhe um den DCDC relativ groß während des Granatwachstums war, und unterstützend auf die Versenkung der Gesteine der Südseite des RBS wirkte; sowie weiterhin, daß die Verwerfungsbewegungen während der Freilegung wahrscheinlich dominant und variabel waren.

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Résumé La partie nord-ouest du Connemara est traversée par un accident d'allure redressée, orienté E-W à ESE-WNW: le Renvyle-Bofin Slide (RBS) qui montre à la fois des indices d'un déplacement ductile et cassant. Les observations structurales révèlent, dans les roches avoisinantes, une histoire déformative polyphasée (D1 à D4); celle-ci est dominée par le développement, au cours de la phase D2, d'une fabrique L-S régionale, comportant une foliation E-W à ESE-WNW et une linéation d'étirement parallèle à cette direction. Du côté nord de la RBS, la linéation d'étirement est localement orientée N-S et associée à un système de structures duplex ductiles conjuguées dans l'auréole de contact du complexe de Dawros-Currywongaun-Doughruagh (DCDC). Le plissement de cette linéation N-S par les plis F3 suggère qu'elle aussi a été engendrée par la phase D2 en même temps que la linéation parallèle à la direction. Les conditions du pic du métamorphisme et les trajets (PTt) qualitatifs déduits de la modélisation, par la méthode thermodynamique de Gibbs, du zonage des grenats indiquent que les régions situées de part et d'autre du RBS ont été le siège d'un enfouissement au cours de D2, mais que cet enfouissement était plus important pour la partie sud (environ 1,5 Kbar soit 5 Km). Il semble que la phase D2 a engendré essentiellement un déplacement horizontal sénestre le long du RBS, mais à l'est du Mont Tully, autour du DCDC, il y eut une composante verticale marquée en raison d'une transgression à l'endroit où l'accident contourne le Mont Tully. La modélisation des associations minérales métamorphiques développées en travers du RBS, ainsi que l'étude des datations publiées montrent que le RBS a joué, à diverses reprises pendant et après le métamorphisme régional; elles montrent aussi que la composante verticale de D2 autour du DCDC doit avoir été importante pendant la croissance des grenats qui a marqué l'enfouissement des roches au sud de l'accident et que les mouvements verticaux étaient probablement prédominants et variables au cours de l'exhumation.

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- , E-W ESE-WNW , , , . (D1-D4) , L-S D-2. -W ESE-WNW , . N-S Dawros-Currywongaun-Doughruagh = DCDC. N-S F-3 , D 2. PTt, Gibb's, , , , D 2 , ( 1,5 ; 5 ). , D 2 . Tully Mountain, DCDC , , Tully Mountain. , , . , DCDC D 2 , , , .

     

宁陕断裂带韧性剪切变形终点——来自龙脖子剪切带活动时代的限定

南秦岭宁陕断裂镇安段北缘的龙脖子剪切带记录了宁陕断裂带左行走滑韧性剪切变形过程。带内3类石英脉体和方解石脉体的ESR年龄分别为125.6～88.7Ma、56.7～32.9Ma和19.8～14.6Ma。其中第一类产出于构造片理和A型褶皱核部的石英脉体,代表左行走滑韧性剪切变形结束、脆性构造活跃的时代。研究表明,宁陕左行走滑剪切带在晚三叠世早期开始活动,且可能持续到早—中侏罗世。第一类脉体年龄的确定表明,宁陕断裂带左行走滑韧性剪切变形最晚可持续到早白垩世;晚白垩世—始新世,宁陕断裂带以伸展-走滑脆性或韧-脆性剪切变形为主。因此,早白垩世是宁陕断裂带韧性剪切变形向脆性剪切变形转换的关键时期。宁陕断裂带经历了晚三叠世—中侏罗世晚期快速冷却阶段、晚侏罗世—白垩纪缓慢冷却阶段和古近纪以来快速冷却阶段。宁陕断裂带在缓慢冷却晚期(早白垩世)实现韧性剪切变形向脆性剪切变形转换说明,早白垩世也是秦岭造山带陆内变形机制转变的关键时期。     

Progressive deformation across a ductile shear zone: an example from the Singhbhum Shear Zone,eastern India

Structural investigations in the Precambrian Singhbhum Shear Zone of eastern India document an intimate relationship between micro- to meso-scale structures and the deformation history. Shear zone rocks are characterized by composite foliation, a well-developed stretching lineation, folds, shear planes, and quartz veins. These structures reflect thrusting of the Proterozoic north Singhbhum hanging wall block over the Archaean south Singhbhum footwall block. Microstructural analysis of multiple foliation and mylonitic rocks within the shear zone helps to define its progressive evolution. During progressive deformation, overprinting of microstructures resulted in incomplete transposition or complete erasing of previously formed structures and mineral assemblages, allowing room for new dynamic equilibrium structures to form. The dominant deformation mechanism was dissolution–recrystallization, with locally important fluid circulation responsible for transformation of the quartzo-feldspathic mass into phyllonite, and quartzites and schists into mylonite. Textural features suggest that the bulk deformation was non-coaxial, evolving from dominant pure shear in the early stage followed by simple shear in a single progressive strain history of the Singhbhum Shear Zone.     

龙门山韧性剪切带主要矿物结构水含量与变形的关系

矿物中的各种水对变形有显著影响。本文研究了龙门山中央断裂带映秀-北川断裂南段韧性剪切带花岗质岩中石英和长石的变形和水含量的关系。通过显微镜下统计石英、长石的粒度和轴比,得出剪切带中花岗质岩石的变形程度分为:弱变形带和过渡带的花岗片麻岩以及强变形带的初糜棱岩。其中弱变形和过渡变形样品中有细粒化强变形条带。根据石英动态重结晶粒度与流动应力关系,计算了剪切带的流动应力约15～200MPa。利用稳态流变方程,估算出韧性剪切带的变形温度范围400～550℃。花岗质岩石和细粒化剪切带的全岩化学成分分析显示,强变形导致SiO2、K2O减小,Fe2O3、CaO、MgO、LOI增大。Fe、Mg含量增大,K含量降低。显然说明长石含量降低,铁镁质矿物含量增多,初步认为是长石经水解反应发生云母化导致的。利用傅里叶变换红外吸收光谱仪(FTIR)对剪切带花岗质岩石中的主要矿物石英和长石进行了结构水含量的分析,结果表明长石的水含量高于石英的水含量,弱变形的粗粒长石和石英的水含量低于强烈变形的细粒长石和石英的含水量,即随着变形程度的增强,矿物中的含水量呈增加趋势。因此,在剪切带中,强烈剪切变形导致长石和石英晶体位错密度变大,形成点缺陷和缺陷,这些缺陷中被OH充填,形成结构水。这种结构水促进了剪切带中岩石的变形。     

京北云蒙山地区四合堂韧性逆冲型剪切带的变形特征及早期变形时限约束

云蒙山杂岩是华北克拉通内记载了晚中生代构造演化的重要构造单元之一。云蒙山地区区域构造格架主要由四合堂背斜推覆体、四合堂逆冲型韧性剪切带、云蒙山背形、河防口正断层及水峪伸展型韧性剪切带等组成。对于四合堂逆冲型剪切带的活动时限及其与水峪剪切带之间的关系,是长期争论的课题之一。在四合堂韧性剪切带中广泛发育有剪切演化各个阶段就位的花岗质岩脉。本文研究云蒙山四合堂地区剪切变形特征及广泛发育的岩脉与构造变形之间关系,将岩脉厘定为构造期前(剪切前)岩脉、同构造期(同剪切)岩脉和构造期后(剪切后)岩脉等6期。不同类型的同构造岩脉锆石U-Pb定年获得了岩脉结晶年龄为170~150Ma。从构造-岩浆关系分析角度考虑,本文认为四合堂剪切带韧性逆冲作用早期的活动始于约170Ma,并持续到约150Ma。     

Dating metamorphic reactions and fluid flow: application to exhumation of high-P granulites in a crustal-scale shear zone, western Canadian Shield

The Legs Lake shear zone is a crustal‐scale thrust fault system in the western Canadian Shield that juxtaposes high‐pressure (1.0+ GPa) granulite facies rocks against shallow crustal (< 0.5 GPa) amphibolite facies rocks. Hangingwall decompression is characterized by breakdown of the peak assemblage Grt + Sil + Kfs + Pl + Qtz into the assemblage Grt + Crd + Bt ± Sil + Pl + Qtz. Similar felsic granulite occurs throughout the region, but retrograde cordierite is restricted to the immediate hangingwall of the shear zone. Textural observations, petrological analysis using P–T/P–M_H2O phase diagram sections, and in situ electron microprobe monazite geochronology suggest that decompression from peak conditions of 1.1 GPa, c. 800 °C involved several distinct stages under first dry and then hydrated conditions. Retrograde re‐equilibration occurred at 0.5–0.4 GPa, 550–650 °C. Morphology, X‐ray maps, and microprobe dates indicate several distinct monazite generations. Populations 1 and 2 are relatively high yttrium (Y) monazite that grew at 2.55–2.50 Ga and correspond to an early granulite facies event. Population 3 represents episodic growth of low Y monazite between 2.50 and 2.15 Ga whose general significance is still unclear. Population 4 reflects low Y monazite growth at 1.9 Ga, which corresponds to the youngest period of high‐pressure metamorphism. Finally, population 5 is restricted to the hydrous retrograded granulite and represents high Y monazite growth at 1.85 Ga that is linked directly to the synkinematic garnet‐consuming hydration reaction (KFMASH): Grt + Kfs + H₂O = Bt + Sil + Qtz. Two samples yield weighted mean microprobe dates for this population of 1853 ± 15 and 1851 ± 9 Ma, respectively. Subsequent xenotime growth correlates with the reaction: Grt + Sil + Qtz + H₂O = Crd. We suggest that the shear zone acted as a channel for fluid produced by dehydration of metasediments in the underthrust domain.     

The transition from brittle to ductile deformation in the Sambagawa metamorphic belt, Japan

The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism.     

Theoretical and natural strain patterns in ductile simple shear zones

A simple empirical model representing the variation of shear strain throughout a simple shear zone allows us to determine the evolution of finite strain as well as the progressive shape changes of passive markers. Theoretical strain patterns (intensity and orientation of finite strain trajectories, deformed shapes of initially planar, equidimensional and non-equidimensional passive markers) compare remarkably well with patterns observed in natural and experimental zones of ductile simple shear (intensity and orientation of schistosity, shape changes of markers, foliation developed by deformation of markers).The deformed shapes of initially equidimensional and non-equidimensional passive markers is controlled by a coefficient P, the product of

1. (1) the ratio between marker size and shear zone thickness

2. (2) the shear gradient across the zone.

For small values of P (approximately P < 2), the original markers change nearly into ellipses, while large values of P lead to “ retort” shaped markers.This theoretical study also allows us to predict, throughout a simple shear zone, various relationships between the principal finite strain trajectory, planar passive markers and foliations developed by deformation of initially equidimensional passive markers.     

Late Palaeozoic deformation of post-Variscan lower crust: shear zone widening due to strain localization during retrograde shearing

The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite-facies Permian shear zone that developed after the emplacement of a deep-seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature range from ~800 to ~400°C. During the earlier high-grade deformation stage, which was accompanied by decompression from ~7 to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ≥1 km wide. Strain became increasingly localized as temperature decreased, but rather than reactivating pre-existing shear zones as commonly expected, younger mylonites expanded into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the SLSZ suggests that pre-existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones.     

Evolution of a crustal-scale transpressive shear zone in the Albany–Fraser Orogen, SW Australia: 1. P–T conditions of Mesoproterozoic metamorphism in the Coramup Gneiss

The Albany–Fraser Orogen in southwestern Australia preserves an important thermo‐tectonic record of Australo‐Antarctic cratonic assembly during the Mesoproterozoic. New petrologic and thermobarometric data from the Coramup Gneiss (a 10 km wide zone of high strain rocks within the NE‐trending eastern Albany–Fraser Orogen) indicate at least two high‐grade metamorphic events during 1345–1140 Ma convergence and amalgamation of the West Australian and Mawson cratons. The first event (M1) involved c. 1300 Ma granulite facies metamorphism of the Coramup Gneiss (M1a: 800–850 °C, 5–7 kbar), followed by burial and recrystallization under high‐P conditions (M1b: 800–850 °C, c. 10 kbar) prior to high‐T decompression (M1c: 700–800 °C, 7–8 kbar) and the 1290–1280 Ma emplacement of Recherche Granite sills. The second event (M2) entailed high‐T, low‐P metamorphism within dextral D2 shear zones (M2a: 750–800 °C, 5–6 kbar), followed by fluid‐present amphibolite facies M2b retrogression. Subsequent sinistral D3 mylonites and pseudotachylites are considered contemporaneous with similar structures in the adjacent Nornalup Complex that postdate the c. 1140 Ma Esperance Granite. Our petrological and thermobarometric data permit two end‐member P–T‐time relationships between M1 and M2: (1) a single post‐M1b event involving continuous M1b–M1c–M2a–M2b cooling and decompression, and (2) a two‐stage post‐M1b evolution involving M1c metamorphism during the waning stages of an event unrelated causally or temporally to subsequent M2a metamorphism and D2 deformation. In a companion paper, new structural and U–Pb SHRIMP zircon data are presented to support a two‐stage P–T evolution for the Coramup Gneiss, with M1 and M2, respectively, reflecting thermo‐tectonic activity during Stage I (1345–1260 Ma) and Stage II (1215–1140 Ma) of the Albany–Fraser Orogeny.     

Dissolution and precipitation processes in deformed amphibolites: an example from the ductile shear zone of the Ryoke metamorphic belt, SW Japan

The granitic mylonite zone in the Cretaceous Ryoke metamorphic belt contains deformed amphibolites as thin layers. The amphibolite layers do not exhibit pinch‐and‐swell or boudinage structures, even when contained in a high‐strain granitic mylonite. This mode of occurrence suggests that they were deformed as much as the surrounding granite mylonite. In the highly deformed zone, strongly foliated amphibolites contain Ti‐rich brown amphibole porphyroclasts rimmed by Ti‐poor green amphibole, titanite and chlorite. These porphyroclasts are elongated, forming shear surfaces defined by preferential distribution of the chlorite and titanite. Porphyroclastic plagioclase in the strongly foliated amphibolites consists of two components: an anorthite‐rich core and an anorthite‐poor rim. Based on these observations, the mass‐balanced reaction occurring during deformation is defined as As the reaction products form a weak interconnected matrix, the strain rate of the amphibolites may be controlled by the rate of dissolution–precipitation through fluids. Weakly foliated amphibolites in the low‐strain zone exhibit cataclastic microstructures, whereas the strongly foliated amphibolites do not exhibit such features. These microstructural and chemical changes suggest that high‐strain amphibolites were initially deformed by cataclasis, followed by deformation through metamorphic reactions. During the metamorphism/deformation, old plagioclase grains with high X_an were not stable and dissolved, and new plagioclase grains with low X_an crystallized at the old plagioclase rim. Dissolution of old plagioclase and precipitation of new plagioclase occurred normal to and parallel to the foliation, respectively, reflecting incongruent pressure solution due to differential stress and changes in P–T–H₂O conditions. The development of incongruent pressure solution is attributed to increased fluid flux in the strongly foliated amphibolites, as evidenced by the greater abundance of hydration‐reaction products in the strongly foliated amphibolites than in the weakly foliated ones.