No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Shear-zone systems and melts: Feedback relations and self-organization in orogenic belts
Abstract
In orogenic belts, the common spatial and temporal association of granites with crustal-scale shear-zone systems suggests melt transfer from source to upper crust was the result of a feedback relation. In this relation, the presence of melt in the crust profoundly affects the rheology, and induces localization of strain within shear-zone systems. Consequently, melt is moved out of the source preferentially along high-strain zones, which helps the system to accommodate strain. Because actively deforming orogenic belts are non-equilibrium systems, they may generate dissipative structure by self-organization; we interpret crustal-scale shear-zone systems and their associated granites as the manifestation of this self-organization. The architecture and permeability structure are controlled by the type of shear-zone system (transcurrent, normal, reverse or oblique); this is the primary control on melt transfer in orogenic belts. During active deformation, movement of melt is by percolative flow and melt essentially is pumped through the system parallel to the maximum principal finite elongation direction. If a build-up of melt pressure occurs, melt-enhanced embrittlement enables tensile and dilatant shear fracturing, and transfer of melt is by channelized flow.
... Relações de causa e efeito são argumentadas de ambos os lados, como a focalização da migração de magmas ao longo de zonas de cisalhamento (Brown, 1994) vs. a concentração de deformação sobre corpos magmáticos parcialmente cristalizados (Tommasi et al., 1994, Neves et al., 1996. Brown & Solar (1998) propõem de forma conciliadora que estes sejam pontos de vistas distintos, mas não excludentes, de um processo de retroalimentação. Destaca-se a importância de rochas magmáticas sintectônicas como bons marcadores temporais da progressão da deformação transpressiva (e.g. ...
... As encaixantes apresentam geometria sinclinal em ambas as margens do plúton, relacionada a um detatchment superior e a um empurrão inferior, o que permitiu a ascensão do plúton. Brown & Solar (1998 Brown & Solar (1998) propõem uma distinção no padrão estrutural desenvolvido em granitos sintectônicos em zonas de cisalhamento transcorrentes e de empurrão. Onde o eixo finito de elongação máxima é (sub-) horizontal, o desenvolvimento de estruturas do tipo S-C é comum, atestando a ascensão concomitante a deformação. ...
... As encaixantes apresentam geometria sinclinal em ambas as margens do plúton, relacionada a um detatchment superior e a um empurrão inferior, o que permitiu a ascensão do plúton. Brown & Solar (1998 Brown & Solar (1998) propõem uma distinção no padrão estrutural desenvolvido em granitos sintectônicos em zonas de cisalhamento transcorrentes e de empurrão. Onde o eixo finito de elongação máxima é (sub-) horizontal, o desenvolvimento de estruturas do tipo S-C é comum, atestando a ascensão concomitante a deformação. ...
Transpressão representa a deformação por uma combinação entre contração e cisalhamento simples direcional e ao longo do mergulho (empurrão) de zonas de cisalhamento, em diferentes proporções. A deformação transpressiva é uma característica comum em diferentes orógenos, ao longo de margens convergentes de placas, e também em outros ambientes tectônicos. São revisados conceitos fundamentais para a compreensão da deformação transpressiva, em uma perspectiva histórica da evolução do conhecimento, a partir do acúmulo de observações de exemplos naturais e sofisticação dos modelos teórico-matemáticos. Além disso, a relação entre transpressão com alguns aspectos orogênicos importantes, como o magmatismo sintectônico, a partição da deformação e a exumação, também são abordados. Um estudo de caso, no limite entre ante-país e além-país no setor norte do Cinturão Dom Feliciano (Santa Catarina, Brasil), onde a deformação transpressiva regional culminou na partição da deformação em domínios estruturais, é apresentado como forma de ilustrar estes conceitos e relações.
... Migmatites are heterogeneous rocks which form in areas of medium-to high-grade metamorphism (typically T>700 O C) by processes associated with partial melting, melt transport, and melt accumulation (Brown, 2013). Widespread migmatite terrains are very common in the exhumed midto lower-crustal portions of both ancient and modern orogenic belts (e.g., Brown, 1994;Brown and Solar, 1998b;Slagstad et al., 2005;Searle et al., 2009;Weinberg et al., 2013;Cavalcante et al., 2021;Wang et al., 2021). The availability of H2O plays a fundamental role in lowering the melting temperature of rocks; therefore, if an aqueous fluid is present, then water-fluxed melting can occur at relatively low temperatures with the involvement of quartz and plagioclase plus K-feldspar or biotite (Cruden and Weinberg, 2018). ...
... A further distinction can therefore be made between in-situ migmatites, i.e., those that have formed by partial melting in their present position, and injection migmatites, i.e., those where the granitic leucosome has been extracted from deeper source rocks and migrated upwards through the crust to be injected into the overlying country rocks. Regions of orogenic belts that were exposed to high-temperature metamorphism (T>700 O C), and which have Age of the Abbabis Complex, Damara Orogen | Jones et al., 8th August 2023 -PhD research 8 typically therefore been exhumed from mid-to lower-crustal depths, are often dominated by widespread migmatite terrains (e.g., Brown, 1994;Brown and Solar, 1998b;Slagstad et al., 2005;Searle et al., 2009;Weinberg et al., 2013;Cavalcante et al., 2021;Wang et al., 2021). ...
... The leucosomes in these migmatites show evidence for "petrographic continuity" with larger, often discordant, intrusive granite dykes and plutons. The concept of petrographic continuity is routinely interpreted as robust evidence that two or more melt-bearing structures once formed a continuous melt-bearing network in the crust (e.g., Brown, 2007;Brown, 2013), and detailed observations of these features in the field has allowed the pathways of magma migration through the crust to be mapped out in detail (e.g., Brown, 1994;Collins and Sawyer, 1996;Brown and Solar 1998a;Brown and Solar, 1998b;Searle, 1999;Searle et al., 2009;Reichardt and Weinberg, 2012;Martini et al., 2019). The observation of petrographic continuity in this case strongly implies that the metatexite and diatexite migmatites of the Abbabis Complex observed along the lower Swakop River are approximately the same age as the larger leucogranite dykes and sills, and that these features all represent part of a magma migration network which accommodated the ascent of granitic melt through the crust. ...
The Abbabis Complex outcrops at the deepest exposed level of the southern Central Zone of the Damara Orogen and consists primarily of granitic gneisses, including augen gneisses, along with extensive granitic intrusions and some subordinate metasedimentary and metavolcanic horizons. Most previous studies have interpreted the Abbabis Complex as a c. 1 or 2 Ga granitic basement complex onto which the overlying Damara metasedimentary Supergroup was deposited unconformably at c. 870-590 Ma. However, in this study, we present extensive new field evidence from the lower Swakop River, which shows clearly and consistently that granitic rocks of the underlying Abbabis Complex are universally intrusive into the overlying Damara Supergroup at all locations visited. These findings do not support the interpretation that the Damara metasedimentary Supergroup was deposited unconformably onto the Abbabis Complex. We propose that the Abbabis Complex instead represents a syn-orogenic granite-migmatite complex which formed exclusively from granite intrusion and accumulation at c. 550-500 Ma in the mid-crust of the Damara Orogen. This was facilitated by high-temperature granulite-facies regional metamorphic conditions which peaked at c. 520 Ma in the southern Central Zone. The timing of this high-temperature metamorphism and coeval granite intrusion, lagging the onset of collision in the Central Zone of the Damara Orogen by several tens of millions of years, is consistent with numerical simulations of heat produced from long-lived radioactive decay in thickened orogenic crust. The interpretation of the Abbabis Complex as a syn-orogenic granite-migmatite terrain in the mid-crust of the Damara Orogen, rather than as an older basement complex, is therefore consistent with 1) the detailed description of field relationships reported in this study, 2) the spatial and temporal evolution of metamorphism and magmatism within the orogen, and 3) increasingly sophisticated tectonic and geodynamic models presented in the wider scientific literature to explain the evolution of orogenic belts.
... Les conditions expérimentales pourraient donc surestimer l'impact du liquide sur la perte de viscosité. Cependant, de nombreuses études documentent l'initiation et la propagation de fractures dans des migmatites en raison du changement de volume positif qui accompagne réactions de fusion (vapeur absente) (Bons et al., 2004(Bons et al., , 2008Brown, 2004Brown, , 2013Brown & Solar, 1998Clemens & Mawer, 1992 ;Clemens et al., 1997 ;Petford et al., 2000 ;Yakymchuk et al., 2013). Lorsque la quantité de liquide dans la roche atteint 20 à 40% (RCMP, « Rheological Critical Melt Percentage ») une autre chute brutale de près de dix ordre de grandeur est observée qui correspond à la perte du réseau solide, on passe alors des métatexite au diatexites (Sawyer et al., 2011) lesquelles ont un comportement rhéologique proche de celui Chapitre 1. Problématique scientifique : les relations entre fusion partielle, déformation et transferts de matière au cours de l'orogénèse. ...
... En raison des forces gravitaires et tectoniques, le drainage du liquide silicaté via des processus de compaction ou de cisaillement est inévitable et peut s'opérer à des échelles de temps très courtes ( Fig. 3 ; Jackson et al., 2003 ;Scaillet & Searle, 2006). La fusion partielle et la déformation sont des mécanismes intimement liés (Hollister 1993 ;Brown & Solar, 1998) L'effet du paramètre temps sur la production et la ségrégation de liquide peut aussi dépendre de la tectonique à grande échelle. Plus la roche va stagner longtemps aux conditions P-T favorables, plus la quantité de liquide produit sera grande. ...
... Continent-scale strike-slip shear zones are very efficient conduit along which ductile and brittle deformation drives the segregation and transport of melt from deep to shallow crustal levels where syn-kinematic plutons emplaced (e.g. D' Lemos et al., 1992;Tikoff and Teyssier, 1994;Tommasi et al., 1994;Brown and Solar, 1998;Brown, 2013). Large(continental)-scale shearing may last over tens of m.y. ...
A l’échelle des orogénèses, la déformation et la rhéologie de la lithosphère continentale sont fortement contrôlées par les processus de migmatisation et les phénomènes magmatiques au cours de l’évolution des grandes chaines continentales. La déformation de la croûte moyenne assistée par les mécanismes de fusion partielle et la présence de liquides silicatés joue un rôle majeur dans l’accommodation de grands déplacements horizontaux et verticaux au sein de l’orogenèse en construction. Ces fluages de la croûte partiellement fondue sont décrits dans les grands systèmes orogéniques actuels (Himalaya et Andes) et anciens (Chaîne Varisque). Dans le détail, la déformation syn-orogénique, les processus de ségrégation des liquides silicatés et leur migration dans la croûte orogénique sont des phénomènes inter-opérant de manière complexe, à plusieurs échelles de temps et d’espace.Par l’étude structurale, pétrologique et géochronologique des massifs Varisques de l’Agly (Pyrénées) et des Aiguilles-Rouges (Alpes), nous abordons cette problématique en ciblant les interactions entre partitionnement de la déformation et fusion partielle dans une croûte orogénique à haute-température. En parallèle, nous proposons de repositionner ces massifs dans leur cadre tectonique Varisque afin d’améliorer la compréhension des processus de fluage à l’origine de leur construction et leur structuration.L’analyse structurale du massif de l’Agly et la datation des déformations indiquent que la fusion partielle commence dès 325 Ma et se termine aux alentours de 300 Ma. Cette migmatisation est contemporaine du développement d’une fabrique planaire subhorizontale interprétée comme le fluage horizontal gravitaire de la croûte partiellement fondue. Ce fluage est localement perturbé par une zone de cisaillement dextre kilométrique drainant les liquides magmatiques et permettant le transfert et l’emplacement de plutons dans la croûte supérieure. Cette déformation dextre est interprétée comme une des branches nord d’une zone de cisaillement d’échelle crustale représentée par la zone axiale des Pyrénées.Dans les Aiguilles-Rouges, la reconstitution du trajet P-T-t-D des éclogites et de métapélites de moyen grade indiquent un enfouissement en conditions de MP/MT jusqu’à des conditions de haut grade atteignant des conditions de pression-température maximales de l’ordre de 1.75 GPa et 710 °C à 340-330 Ma. L’existence d’une ancienne zone de subduction océanique dans les Massifs Cristallins Externes (ECMs) potentiellement associée à ces conditions de HP-HT est questionnée et discutée. Nos données indiquent qu’entre 340 et 330 Ma, la croûte inférieure est exhumée à la faveur d’un fluage horizontal de la croûte partiellement fondue pendant que la croûte moyenne/supérieure subit un enfouissement, dans un régime global en transpression dextre. Ces deux unités sont ensuite juxtaposées et exhumées par les mouvements transcurrents dextres le long de l’Eastern Variscan Shear Zone (EVSZ). Nos données suggèrent que la nucléation des zones de cisaillement verticales composant l’EVSZ s’opère dès les premiers stades de migmatisation et préférentiellement dans les orthogneiss subissant une fusion hydratée. La EVSZ évolue ensuite sous la forme d’un large réseau de zones de cisaillement anastomosées favorisant le drainage des fluides, augmentant la production de liquides à l’origine de la formation de plutons syn-cinématiques, mis en place dans les zones de dilatance d’échelle crustale.La synthèse de nos résultats nous permet de proposer une évolution géodynamique des ECMs s’intégrant dans l’évolution de la chaîne Varisque Européenne et nous conduisent à présenter une nouvelle reconstitution paléogéographique de la branche Sud-Est Varisque à la fin du Carbonifère. Cette nouvelle vision du positionnement des ECMs, et la comparaison avec le massif de l’Agly nous permettent de discuter de l’évolution spatiale et temporelle du fluage de la croûte fondue au cours du Carbonifère.
... High-strain zones are an example of such sites of focussed melt flow. It has been shown that highstrain zones may act as important melt migration pathwaysthey are recognised in the field as high-strain zones containing high proportions (>10%) of macroscopic, internally undeformed, felsic or leucocratic material that is inferred to, at least in part, represent the crystallisation of former melt (Brown and Solar, 1998a;Carvalho et al., 2016Carvalho et al., , 2017Hasalová et al., 2011;Marchildon and Brown, 2003;Piazolo et al., 2020;Schulmann et al., 2008;Tommasi et al., 1994;Weinberg and Mark, 2008). However, if melt-bearing high-strain zones cut sub-solidus rocks but lack noticeable felsic or leucocratic components at the outcrop scale (e.g., dykes and lenses), they are challenging to recognise as meltmigration pathways (e.g., Gardner et al., 2020;Stuart et al., 2018aStuart et al., , 2018b. ...
... The strongly layered morphology is attributed to transposition of leucosome during melt present high-strain deformation (Park, 1983), although stromatic migmatites have been shown to form in low-strain settings (Johannes and Gupta, 1982). Likewise, some kilometre-scale regions of stromatic migmatite hosted in diatexite (a migmatite with high melt fraction; Brown, 1973) have been interpreted as crustal-scale high-strain magma transfer zones involving migration and/or draining of melt (e.g., Scott and Stevenson, 1986;Sleep, 1974) from adjacent less deformed migmatite, i.e., supra-solidus wall rocks (Brown and Solar, 1998a;Hasalová et al., 2011;Marchildon and Brown, 2003;Schulmann et al., 2008;Weinberg and Mark, 2008). Field studies link melt ascent and eventual emplacement of plutons based on the close association between regional deformation, migmatisation, dyking, and zones of strain localisation (e. g., Brown, 2013;Brown and Solar, 1998b;Castro, 1986;de Saint Blanquat et al., 1998;Hutton, 1988;Pitcher, 1979;Rosenberg, 2004;Vernon et al., 2012;Vigneresse, 1995;Zibra et al., 2014). ...
... High-strain zones that display high proportions (>10%) of felsic or leucocratic material in outcrop, where the leucocratic material lacks internal sub-solidus deformation microstructures, are distinguished from migmatite subsequently deformed under sub-solidus conditions by the field geologist and therefore recognised as having experienced meltpresent deformation. Such zones are reported from areas of regional supra-solidus migmatite domains containing overall high leucosome content (e.g., Brown and Solar, 1998a). However, few such high-strain zones are reported to occur in sub-solidus host rocks where the highstrain zone contains low proportions of felsic or leucocratic material (e.g., Carvalho et al., 2016Carvalho et al., , 2017Daczko et al., 2016;Ghatak et al., 2022;Lee et al., 2020;Meek et al., 2019;Piazolo et al., 2020;Silva et al., 2022;Stuart et al., 2018aStuart et al., , 2018b. ...
Melt transfer and migration occurs through both supra- and sub-solidus rocks. Mechanisms of melt transfer include dyking, mobile hydrofracturing and diffuse porous melt flow where melt flow may or may not be channelized via instabilities or into high-strain zones of active deformation. Here, we highlight the microstructural- and outcrop-scale signatures of syn-deformational melt-migration pathways through high-strain zones that cut sub-solidus rocks. High-strain zones with high proportions (>10%) of macroscopic, internally undeformed, felsic or leucocratic material are readily interpreted as important melt-migration pathways and are most common in supra-solidus host rocks. However, it is challenging to recognise high-strain melt-migration pathways through sub-solidus rocks; these pathways may lack noticeable felsic or leucocratic components at the outcrop scale and share many macroscopic features in common with ‘classic' sub-solidus mylonite, such that the two are generally conflated. We contrast field and microstructural characteristics of ‘classic' mylonite originating from solid-state deformation with those of high-strain zones that also cut sub-solidus rocks yet have microstructural indicators of the former presence of melt. We compile several features allowing one to distinguish solid-state from melt-present deformation in high-strain zones that cut sub-solidus rocks. Our aim is to encourage geologists to assess such high-strain zones on a case-by-case basis, in view of sub-solidus (i.e., mylonitic) versus melt-present deformation. Such assessment is crucial as (1) rocks deformed in the presence of melt, even small percentages of melt, are orders of magnitude weaker than their solid-state equivalents, (2) melt-rock interaction in such zones may result in metasomatism, and (3) such zones may sustain long-lived melt migration and ascent enabling chemical differentiation at a crustal scale. With this contribution we aim to increase the ease of recognising this important subset of melt-migration pathways by assisting in clarity of description and interpretation of high-strain rocks.
... The surface energy of olivine and by extension of other silicates is often assumed to be of the order of 1 N m − 1 ; alternatively, the surface tension of FeS melt was measured to be a few hundred mN m − 1 (Terasaki et al., 2009). Therefore, the capillary number may be an important factor in deformation-induced melt segregation, such as filter pressing (Brown et al., 1995;van der Molen, 1985), dynamic wetting of grain boundaries (Rosenberg and Riller, 2000), or general deformation, mostly along a network of shear zones (Brown and Solar, 1998;Davidson et al., 1992;Mancktelow, 2002;Vanderhaeghe, 1999). As stated in the introduction section, our experiments considered the materials as a continuum; therefore, our results cannot be directly extended to the grain framework systems. ...
... However, large magmatic bodies form by incremental assembly on a million-year time scale and will consequently have complex shapes, obscuring the relation between tectonics and magmatism (de Saint Blanquat et al., 2011). The style of pluton construction will be moreover controlled by the rate of arrival of melt versus the rate of crystallization of the accumulated melt (Brown and Solar, 1998). A composite structure may develop if an individual batch of melt crystallizes before the arrival of a subsequent batch of melt, whereas the successive arrival of melt batches before extensive crystallization is conducive to mixing and more homogeneous magmatic body shapes. ...
... A composite structure may develop if an individual batch of melt crystallizes before the arrival of a subsequent batch of melt, whereas the successive arrival of melt batches before extensive crystallization is conducive to mixing and more homogeneous magmatic body shapes. For example, in the granite complexes of western Maine, which were emplaced during Devonian Acadian transpression tectonics (Brown and Solar, 1998), the smaller bodies are sub-circular in map view. They were interpreted to have been emplaced beneath the effective solidus level in the crust (Brown and Solar, 1998), and their regular shape was attributed to the effect of the exposed crustal level (Brown and Solar, 1998). ...
Traditional strain analysis assumes that a particle deforms affinely with its matrix and that the magnitude and geometry of particle deformation are representative of the bulk strain. However, there are forces, lumped together under the terms of competency or viscosity contrast, which oppose particle deformation. For small weak particles, surface tension may be important in resisting particle deformation. We performed analogue model experiments to investigate the effect of surface tension on the deformation of a viscous particle (bubble or droplet) enclosed in a viscous matrix. The experiments were performed using a ring-shear apparatus allowing large magnitudes of plane strain, simple shear. The results demonstrate the controlling influence of capillary number on particle deformation. The particles did not continuously elongate with increasing shear strain but achieved a steady-state value dependent on capillary number. Because capillary number represents the ratio of shear stresses, which deform particles, to the surface (interfacial) stresses restraining elongation, there should be a strong dependence of particle strain on the deformed particle size and bulk strain rate. The most important geological implications of this study are in the field of melt segregation in deforming anatectic systems.
... Les conditions expérimentales pourraient donc surestimer l'impact du liquide sur la perte de viscosité. Cependant, de nombreuses études documentent l'initiation et la propagation de fractures dans des migmatites en raison du changement de volume positif qui accompagne réactions de fusion (vapeur absente) (Bons et al., 2004(Bons et al., , 2008Brown, 2004Brown, , 2013Brown & Solar, 1998Clemens & Mawer, 1992 ;Clemens et al., 1997 ;Petford et al., 2000 ;Yakymchuk et al., 2013). Lorsque la quantité de liquide dans la roche atteint 20 à 40% (RCMP, « Rheological Critical Melt Percentage ») une autre chute brutale de près de dix ordre de grandeur est observée qui correspond à la perte du réseau solide, on passe alors des métatexite au diatexites (Sawyer et al., 2011) lesquelles ont un comportement rhéologique proche de celui Chapitre 1. Problématique scientifique : les relations entre fusion partielle, déformation et transferts de matière au cours de l'orogénèse. ...
... En raison des forces gravitaires et tectoniques, le drainage du liquide silicaté via des processus de compaction ou de cisaillement est inévitable et peut s'opérer à des échelles de temps très courtes ( Fig. 3 ; Jackson et al., 2003 ;Scaillet & Searle, 2006). La fusion partielle et la déformation sont des mécanismes intimement liés (Hollister 1993 ;Brown & Solar, 1998) L'effet du paramètre temps sur la production et la ségrégation de liquide peut aussi dépendre de la tectonique à grande échelle. Plus la roche va stagner longtemps aux conditions P-T favorables, plus la quantité de liquide produit sera grande. ...
... Continent-scale strike-slip shear zones are very efficient conduit along which ductile and brittle deformation drives the segregation and transport of melt from deep to shallow crustal levels where syn-kinematic plutons emplaced (e.g. D' Lemos et al., 1992;Tikoff and Teyssier, 1994;Tommasi et al., 1994;Brown and Solar, 1998;Brown, 2013). Large(continental)-scale shearing may last over tens of m.y. ...
Partial melting, melt migration and strain partitioning during the Variscan orogeny.
Examples of the Aiguilles-Rouges (Alpes) and Agly (Pyrenees) massifs.
... Due to the increased porosity and permeability that accompanies deformation, high-strain zones are ideal pathways for fluid migration, either by aqueous fluids or melts (e.g., Brown and Solar, 1998;Etheridge et al., 2020;Gonçalves et al., 2012Gonçalves et al., , 2021Hutton, 1988;McCaig and Knipe, 1990;Menegon et al., 2008Menegon et al., , 2015Rosenberg, 2004;Stuart et al., 2018a;White and Knipe, 1978;D'lemos et al., 1992). Both types of fluid migration in high-strain zones are well understood and easily recognised if, in the case of melt, a certain threshold of interconnected fluid (> 10%) is present during deformation (e.g., Brown & Solar, 1998;Collins & Sawyer, 1996;Weinberg & Hasalová, 2015;Závada et al., 2018). ...
... Due to the increased porosity and permeability that accompanies deformation, high-strain zones are ideal pathways for fluid migration, either by aqueous fluids or melts (e.g., Brown and Solar, 1998;Etheridge et al., 2020;Gonçalves et al., 2012Gonçalves et al., , 2021Hutton, 1988;McCaig and Knipe, 1990;Menegon et al., 2008Menegon et al., , 2015Rosenberg, 2004;Stuart et al., 2018a;White and Knipe, 1978;D'lemos et al., 1992). Both types of fluid migration in high-strain zones are well understood and easily recognised if, in the case of melt, a certain threshold of interconnected fluid (> 10%) is present during deformation (e.g., Brown & Solar, 1998;Collins & Sawyer, 1996;Weinberg & Hasalová, 2015;Závada et al., 2018). Such high proportions are suggested in the field by the high percentage of leucosome (Sawyer, 2008). ...
... A key feature of the Earth's crust is its layered character mainly based on chemical composition, with the upper crust having a higher silicic bulk composition and concentration of granitic material compared with the more ferromagnesian bulk composition of the lower crust (Vigneresse, 1995; Rushmer, 2006). Deeply seated high-strain zones in the lower to middle crust are considered part of the crustal system for melt migration (Brown & Solar, 1998;Daczko et al., 2016). Zones of dominantly flattening strain function as major pathways for the ascent of melt originating from anatectic processes in the lower to middle crust and are partially responsible for batch mass transfer of melt to the upper crust (Hutton, 1988;Brown & Solar, 1999;Sawyer et al., 2011;Daczko et al., 2016;Etheridge et al., 2020), as exemplified by the proposed transfer of melt throughout high-strain zones in our model of glimmerite schist belt formation (Fig. 15). ...
The paradigm for hydrous high-strain zones that cut dry host rocks is for fluid-rock interaction to have involved aqueous fluids. However, the role of silicate melt is increasingly recognised. This contribution examines the formation of glimmerite (biotitite) bands during melt migration in the Gough Dam shear zone, a high-strain zone in central Australia that was active during the Alice Springs Orogeny (c. 450–300 Ma). The glimmerite bands cut and replace a range of quartzo-feldspathic protoliths, including granitic gneiss and quartzite. Melt that migrated through the high-strain zone is interpreted to have penetrated relict layers along a network of fractures, enhancing dissolution of the precursor rock and causing replacement by glimmerite crystallisation. Microstructures indicative of the former presence of melt in the high-strain zone include: pseudomorphs of former melt pockets of granitic composition; small dihedral angles of interstitial phases; elongate grain boundary melt pseudomorphs; neighbourhoods of grains connected in three dimensions; and localised static grain growth and recovery. Other microstructures indicative of melt-present deformation include randomly oriented neosome grains, and evidence of activation of multiple slip systems during deformation. The degree of quartzite modification to glimmerite is recorded by an increase in biotite mode, and correlated with higher Ti concentrations in biotite (higher apparent temperature) and changes to trace element and REE compositions. Melt-assisted coupled dissolution-precipitation reactions during melt flux are interpreted to partially reset Proterozoic monazite U-Pb ages inherited from the protolith (> 1630 Ma) to younger Palaeozoic ages, with a complex age pattern partially congruent with the Alice Springs Orogeny (apparent ages range from c. 606–371 Ma, with a dominant age peak at c. 451 Ma). We propose that the glimmerite formed during dynamic melt migration of an externally-derived hydrous peraluminous melt, driving reaction replacement of various felsic protoliths during this orogenic event.
... Inside the MPSZ, muscovite fish exists in the sheared paragneiss, calcsilicate rocks, and pegmatitic leucogranite (Lacassin et al. 1997) but has not been dated previously. It is noted that the occurrence of leucogranite could be associated with shear activity spatially and temporally (Brown and Solar 1998;Schneider et al. 1999). Thus, a deformed pegmatitic leucogranite with S-C fabrics showing sinistral shearing was examined in this study. ...
... The crust can be weakened by partial melting in metamorphism or by pluton emplacement, which further assists the development of a shear zone (Neves et al. 2000). A shear activity would further help the segregation of melts from the deeper crust (Brown and Solar 1998). Such a scenario fits well with the MPSZ. ...
... Leucogranite should be a nice thermal history recorder of a shear zone. The occurrence of leucogranites is common in shear zones, metamorphic belts and orogens because their melts could be derived from partial melting or fractionation of magma (Brown 2001;Brown and Solar 1998;Deniel et al. 1987;Milord et al. 2001;Neves et al. 1996;Neves et al. 2000;Sawyer 1998;Schneider et al. 1999;Searle et al. 2012;Solar and Brown 2001). Leucogranites usually contain zircon which may provide the crystallization age. ...
The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying 40Ar/39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S–C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ 40Ar/39Ar dating on muscovite yield ages mainly within 42–38 Ma with calculated closure temperatures of 435–330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset 40Ar/39Ar ages. The K-feldspar 40Ar/39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42–30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology.
... Shear zones are strain localization features that occur at all scales in the lithosphere and significantly control magma emplacement, fluid circulation, rift development and orogenic evolution, thus being one of the main controls on lithosphere rheology (Ramsay, 1980;Sibson, 1990;Brown and Solar, 1998;Micklethwaite et al., 2010;Vauchez et al., 2012;Montési, 2013;Clerc et al., 2015;Smeraglia et al., 2016;Fossen and Cavalcante, 2017;Précigout et al., 2017). Assessing the timing of shear zone activity is, therefore, critical to understand crustal deformation behavior in different tectonic settings. ...
... All, pre-, syn-, interand post-kinematic intrusions are of interest, as they provide independent constrains on the timing of shearing. Hence, a detailed analysis of magmatic fabrics (Paterson et al., 1989) is critical to discriminate between pre-, syn-and post-kinematic igneous bodies (Fig. 11), based on either field mapping of magmatic foliations and lineations or anisotropy of magnetic susceptibility data (e.g., Hutton et al., 1990;Brown and Solar, 1998;Paterson et al., 1998;Steenken et al., 2000;Rosenberg, 2004). In particular, intrusions do not only provide constraints on the timing of shear zone activity, but also additional valuable information on kinematics and strain (Ramsay, 1980;Wheeler, 1987;Oyhantçabal et al., 2001;Vitale and Mazzoli, 2010;Lisle, 2014). ...
... Numerous field-based investigations of largescale exhumed strike-slip fault systems have demonstrated a close spatial relationship between melting and shear zones (e.g., Brown and Solar, 1998;Misra et al., 2014;Rosenberg and Handy, 2005;Rutter and Neumann, 1995;Vigneresse et al., 1996;Vigneresse and Tikoff, 1999;Cao et al., 2011a;Cao and Neubauer, 2016). Studies have proposed that deep-seated shear zones originate from the meltbearing crust and serve as conduits for the channeled ascent of melts to higher crustal levels (e.g., Cao and Neubauer, 2016;Rosenberg and Handy, 2005;Vanderhaeghe, 2009 and references therein). ...
... Shear zones and anatexis are in a positive feedback loop (Brown and Solar, 1999). Numerous field-based investigations of large-scale transpressive fault systems have demonstrated a close spatial relationship between melting and mylonitic shear zones (e.g., Brown and Solar, 1998;Misra et al., 2014;Rosenberg and Handy, 2005;Rutter and Neumann, 1995;Vigneresse et al., 1996;Vigneresse and Tikoff, 1999). Based on the U-Pb dating of zircons (ca. ...
Studies of crustal anatexis have provided valuable insights into the evolution of metamorphism, deformation, and tectonic processes at convergent plate margins during the orogeny. The transition of metatexite to diatexite migmatites records crucial information about the tectonothermal evolution and rheology of the deep crust. Along the Ailao Shan–Red River shear zone, metatexite migmatites, diatexite migmatites, and leucogranites are widely distributed within the upper amphibolite and granulite facies zones of the Diancang Shan metamorphic complex. The high-pressure granulite-facies metamorphism, with the mineral assemblage comprising garnet + kyanite + K–feldspar + plagioclase + biotite + quartz + melt, is first recognized from the patch of metatexite migmatites in the complex. Detailed petrographic evidence and thermodynamic modeling revealed that the migmatite underwent nearly isothermal decompression metamorphism, presenting a clockwise P–T path. The peak metamorphic P–T conditions are constrained at approximately 11 kbar and 810 ℃ using thermodynamic modeling, and the amount of melt generated during heating is up to 18 mol%. The extraction and segregation of the melts are evidenced by the presence of leucosomes within the migmatites and leucogranite dikes, which record the melt flow network through the crust. Zircon and monazite geochronology of migmatites record the timing of the melting episode that began at approximately 36 Ma and lasted until 20 Ma. All these results are consistent with orogenic crust thickening accompanied by pervasive anatexis during the Late Eocene to the Early Oligocene in the Ailao Shan–Red River shear zone. Combined with the available data related to the other continental–exhumed shear zone, we propose that the crustal anatexis had a significant effect on the thermal state of deep–seated shear zones, thus controlling the rheological behavior of the lithosphere and playing an essential role in the initial localization of shearing in the lower crust.
... Alternatively, the assumed structural control may simply be conditioned by the injection of magma and the consequent modification of the stress field in the host rocks. If we choose the intermediate alternative, that is, that there is positive feedback between magmatic and tectonic stresses (e.g., Hutton, 1997;de Saint Blanquat et al., 1998;Brown and Solar, 1998), and assuming that the available magmatic structures and fabrics are very scarce, we will next evaluate the role of regional structures in the study area during the emplacement process of both intrusive bodies, which ultimately also respond to the influence of the emplacement of magma batches. An arcuate, NE-SW-trending low gravity anomaly, 5.7 km wide, coincide with the ubication and orientation of the set of the brittle structures and is aligned with the major axes of the intrusive bodies, which it includes (Fig. 5). ...
... A: Modified from Brown and Solar (1998) B: Modified from Osberg et al. (1985), Nicholson et al (2006), Wise and Brown (2010) primary lithiophilite, triplite, dickinsonite, and/or montebrasite); with minor Cs-rich beryl and accessory cassiterite, Nb-Ta oxides, and apatite. ...
Apatite is an accessory phase in all the units of the internally zoned Berry-Havey complex pegmatite. This body presents a highly fractionated core zone, enriched in Li, F, B, Be, and P, which hosts three different types of pockets, some of them often containing tens to hundreds of gemmy euhedral Li-rich tourmaline crystals, together with other mineral phases such as lepidolite. Processes involved in the complex internal evolution of pegmatitic melts that give rise to zoned bodies containing pockets are not completely understood. To shed light on these processes, apatite from all the different units of the Berry-Havey pegmatite (wall zone, intermediate zone, core margin, and core zone pods) and from the three pocket types (Li-poor, Li-rich, and apatite seams) has been characterized petrographically and later analyzed for major (electronic microprobe) and trace elements (LA-ICP-MS). Results indicate that apatite chemistry changed significantly during the crystallization of the Berry-Havey pegmatite, reflecting the conditions at each stage and mainly depending on the fractionation degree, fO2, and paragenetic association.
Fluorapatite is found in all the units except the core margin, the Li-poor pockets, and the seams, where Mn-bearing fluorapatite is present. A gradual increase of the Mn content in apatite from the pegmatite border (wall zone) inward, up to the formation of subrounded masses of Mn-Fe phosphate in the core zone pods, parallels the increasing fractionation of the melt. Phosphate crystallization would deplete the residual melt in Mn, probably causing the significant Mn-decrease observed in apatite from the core zone pods and Li-rich pockets. The late depletion of Mn could also be related to an increase of fO2 in the melt during the later stages of its evolution.
Main trace element variations in apatite at both pegmatite and crystal scales correspond to REE, Y, and Sr. Yttrium and REE behave in a very similar way, decreasing inward, i.e., with fractionation of the pegmatitic melt (ΣREE from 1796 ppm in the apatite from the wall zone to 0 ppm in the core zone; and Y from 1503 ppm in the apatite from the wall zone to 0 ppm in the core zone); which could be due to early crystallization of REE-bearing phosphates such as monazite and xenotime. Strontium shows a more complex trend, with an initial depletion in apatite from the wall zone (52 ppm) to the intermediate zone (3 ppm) and a pronounced increase from the core margin (23–87 ppm) up to the core zone and pockets (up to 2.87 wt%). This increase of Sr at the latest fractionation stages of the pegmatite is interpreted to be associated with a late, incompatible character of this element in highly fractionated melts, related to the composition of feldspars from the core margin (mainly pure albite). The lack of Ca in feldspars would decrease affinity for Sr incorporation into their structure and, consequently, Sr would go preferentially to apatite in the core zone pods and, more markedly, in the pockets.
Apatite also records changes in the redox conditions during crystallization, with the highest fO2 at the end of the crystallization, mainly reflected in the Eu and Ce anomalies. The chemistry of apatite also reflects the evolution of the pegmatitic melt during crystallization regarding the fluids saturation and pockets generation. Accordingly, at least two exsolution events took place during the Berry-Havey crystallization history: (1) at the beginning of the core zone crystallization, giving rise to the Li-poor pockets, and (2) after the crystallization of the Li-rich pods of the core zone, resulting in the Li-rich pockets. The apatite-rich seams may have crystallized between these two exsolution events or later, at a subsolidus stage, after a Na-autometasomatism episode.
This study shows how a detailed petrographic and chemical characterization of apatite associated with different units of a highly fractionated, internally zoned pegmatite may help understand the crystallization history of pegmatitic melts. It is also evidenced that during the internal evolution of pegmatites, apatite chemistry records variations in the fO2, elemental fractionation, interaction with competing mineral phases, fluids activity and exsolution events. In addition, it is shown how apatite chemistry may be useful as an exploration tool for pegmatites.
... The outcrop clearly shows that the granitic material has also intruded litpar-lit along the intrafolial spaces of the folded bedding planes (Figure 7(c)) (e.g. Brown and Solar 1998). It gives an apparent impression of a folded (pre-to syn-D 2 ) granitic vein. ...
Structural, metamorphic, and geochronological study of the Delhi Supergroup rocks near the western margin of the Aravalli-Delhi Mobile Belt (ADMB) elucidate the Neoproterozoic tectonics of the South Delhi Fold Belt (SDFB) and its implication for the evolution of the Greater Indian Landmass (GIL). In this study, we establish three deformational events in the SDFB. While the D1 event produced isoclinal, reclined F1 folds synchronous with prograde amphibolite facies metamorphism (~5 kbar, 650°C), D2 formed outcrop to map-scale upright F2 folds. D3 comprised ‘partitioned transpression’ along two subvertical shear zones. Oblique dextral-reverse movement (D3a) along one shear zone formed an outcrop-scale positive flower structure, while sinistral-reverse movement along the other rotated and steepened the hinges of the F2 folds (D3b). U-Pb zircon dating of post-tectonic (post-D2, but pre-D3) and syntectonic (syn-D3) granite intrusives suggests >844 Ma age for D2 and ca. 820 Ma for D3. An older magmatic event (ca. 990 Ma) is recorded from sheared pink granites (pre- to syn-D1 intrusion). We suggest that the amalgamation of the ADMB and the Marwar Craton was initiated by the Grenville-age (ca. 1000 Ma) subduction-collision, while the younger events (ca. 844 Ma and 820 Ma) mark the final stitching of the ADMB – Marwar Craton along the Phulad-Ranakpur palaeo-suture zone to form the GIL. The distribution of early (ca. 1000–900 Ma) and late (ca. 800–700 Ma) Tonian magmatic and metamorphic ages in the SDFB suggest that the final growth of the GIL took place through multiple stages of oblique collision in a pulsating orogenesis through the Early Neoproterozoic.
... The relationship between shearing and magma emplacement has been extensively studied and documented. Many studies have demonstrated that shearing is likely the primary process controlling magma ascent, emplacement, and the melting and expulsion of magmas from their source regions (Dlemos et al., 1992;Vigneresse, 1995;Brown and Solar, 1998;Druguet and Hutton, 1998;Leitch and Weinberg, 2002;Rosenberg, 2004;Denele et al., 2008;Burda and Gaweda, 2009;Passarelli et al., 2010). On the contrary, several authors suggested that magma can influence the development of shear zones (Neves and Vauchez, 1995;Neves et al., 1996) and that the presence of magma at depth in extensional settings can control strain distribution and structural evolution (Corti et al., 2002). ...
The Guanpo pegmatite field in the North Qinling orogenic belt (NQB), China, hosts the most abundant LCT pegmatites. However, their emplacement conditions and structural control remain unexplored. In this contribution, we investigated it combining pegmatite orientation measurement with oxygen isotope geothermometry and fluid inclusion study. The orientations of type A1 pegmatites ( P f < σ 2 ) are predominantly influenced by P‐ and T‐fractures due to simple shearing in Shiziping dextral thrust shear zone during D 2 deformation, whereas type A2 pegmatites (contemporaneous with D 4 ) are governed by hydraulic fractures aligned with S 0 and S 0+1 stemming from fluid pressure ( P f < σ 2 ). Additionally, type B pegmatites ( P f ≤ σ 2 ) exhibit orientations shaped by en echelon extensional fractures in local ductile shear zones (contemporaneous with D 3 ). The albite‐quartz oxygen isotope geothermometry and microthermometric analysis of fluid inclusions in elbaites from the latest pegmatites (including types B and A2) suggest that the crystallization P‐T for late magmatic and hydrothermal stages are 527.5–559.2°C, 320°C, 3.1–3.6 kbar and 2.0 kbar, respectively. Our observations along with previous studies suggest that the genesis of the LCT pegmatites was a long‐term, multi‐stage event during early Paleozoic orogeny (including the collision stage) of the NQB, and was facilitated by various local fractures.
... Deformation-assisted melt migration involves deformation-induced dilatancy that forms pressure gradients during granular flow such that melt is drawn towards sites of active deformation (Etheridge et al., 2021;Fusseis et al., 2009;Menegon et al., 2015;. The most understood deformationassisted melt migration mechanisms are dyke-formation in the brittle regime and flow through zones of high strain deformation (i.e., shear zones) in the ductile regime, as they can be easily recognized in outcrop (Brown & Solar, 1998). However, the currently identified number and volume of these channelled melt pathways are not enough to accommodate the mass and heat transfer needed for the chemical differentiation of the crust (Rudnick, 1995). ...
Recent studies of the Cretaceous lower arc crust exposed in Fiordland, New Zealand, conclude that shear zones are sites of melt migration and mass transfer through the deep crust. Here, we investigate the 4–10 km‐wide George Sound Shear Zone, which cuts the Western Fiordland Orthogneiss, comprising two main rock types: two‐pyroxene gneiss and hornblende gneiss. Previous studies infer a predominantly igneous origin for the two types of gneiss. However, this study finds that melt‐rock interaction within the George Sound Shear Zone formed the hornblende gneiss from the precursor two‐pyroxene gneiss. Petrographic analyses of samples collected in transects across the shear zone show hydration reaction textures ranging from rims of hornblende + quartz around pyroxene grains to complete replacement of pyroxene grains. Plagioclase is recrystallized and partially replaced by clinozoisite. Additionally, biotite mode increases towards the higher strain rocks in the shear zone. Backscatter images and polarized light microscopy show microstructures indicative of former melt‐present deformation, including (a) interconnected mineral films of quartz and K‐feldspar along grain boundaries, (b) grains that terminate with low apparent dihedral angles, (c) interstitial grains, with some (d) undulose extinction in plagioclase and (e) serrated grain boundaries. In addition, zircon microstructures are consistent with Zr mobility, further supporting the former presence of melt; geochemical data show enrichment of Zr in the hornblende gneiss as compared to the two‐pyroxene gneiss. From the above observations, it is inferred that a felsic to intermediate hydrous melt migrated through the George Sound Shear Zone reacting with the host two‐pyroxene gneiss of the Western Fiordland Orthogneiss. Melt migration along grain boundaries was deformation assisted, (i) causing hydration of pyroxene to hornblende + quartz, and plagioclase to clinozoisite, (ii) increasing proportions of biotite within the shear zone and (iii) causing depletion of Cr + Ni and Zr enrichment in the hydrated rock. Our interpretation is supported by published observations of pegmatite dyke swarms that intruded into the George Sound Shear Zone, the P‐T conditions of deformation and characterization of microstructures that contrast sharply with those typically found in mylonitic rocks formed under solid‐state metamorphic conditions. Our results confirm that hydrous shear zones within otherwise anhydrous country rock are retrogressive and may represent evidence of melt migration through zones of deformation.
... Ductile shear zones play a critical role in displacement accommodation, melt/fluid migration and the burial-exhumation process during orogenesis (Brown and Solar 1998;Xypolias and Doutsos 2000;Alsop and Holdsworth 2004;Raimondo et al. 2011;Larson et al. 2015;Fossen and Cavalcante 2017). Quantifying the timing of shear zone activity, therefore, is crucial to understanding the evolution of an orogenic system (e.g. ...
Dating the timing of deformation within shear zones is critical to quantifying orogenic processes and developing time-resolved regional tectonic frameworks. Such work requires the integration of detailed microstructural analysis with in-situ geochronology to quantify when deformation mechanisms were active at the microscale. The Proterozoic, granulite-facies Strangways Metamorphic Complex was exhumed during the Devonian to Carboniferous intracontinental Alice Springs orogeny. New microstructural observations, quartz c -axis orientation analyses, mica chemistry, Ti-in-biotite thermometry, and in-situ mica Rb-Sr and titanite U-Pb geochronology outline a detailed history of ductile shearing across the complex. Movement along the north boundary shear zone of Strangways Metamorphic Complex appears to have initiated ca. 382 Ma, preceding peak metamorphism in the area during the Alice Springs orogeny. Widespread reverse-sense ductile shearing occurred within the Strangways Metamorphic Complex between ca. 365 and 355 Ma and correlates with rapid cooling of the region. Late-stage ductile deformation is recorded at ca. 335 Ma, likely reflecting the terminal exhumation of the Strangways Metamorphic Complex. Finally, the new in situ muscovite and biotite Rb-Sr data collected herein permit comparison with previous two-point mica Rb-Sr isochrons and ⁴⁰ Ar- ³⁹ Ar dates from the same specimens. In the rocks analyzed, the biotite Rb-Sr system returned dates similar to the previous ⁴⁰ Ar- ³⁹ Ar white mica dates, perhaps indicating a similar effective closure temperature.
Supplementary material: https://osf.io/yjp24/?view_only=0bf5bec9f5c94a1aa4fb413320ec24e8
... Shear zones in tangential, transcurrent, or transpressional systems (sensu Twiss and Moore, 2007) act as important conduits in the transport and accumulation of anatectic melt along the crust (Brown and Rushmer, 1997;Brown and Solar 1998a, 1998b, 1999Brown, 2004). In contact with regions of anatexis, these zones operate as anatectic melt migration engines, creating dilational spaces that act as traps for the expulsion and accommodation of molten material (Weinberg et al., 2009(Weinberg et al., , 2010. ...
The Minas-Bahia Orogen, which is Siderian-Orosirian age, is exposed in the northern sector of the Sao Francisco Craton and in its African counterpart, the Congo Craton. In this sector, two orogenic domains outcrops with geological evolution and terrains with different trends: Western and Eastern Bahia. The advance of scientific knowledge in the northeast of the West Bahia Orogenic Domain has revealed the existence of more tectonic terrains than those traditionally delimited, and reveal the presence of juvenile terranes and cratonic crust separated complex zones where they interacted. The region of interaction between the Gaviao and Bom Jesus da Lapa Paleoplate has been the subject of debates about the tectonic significance of granulitic rocks with Meso-
Neoarchean protoliths, which are intruded by Rhyacian - Orosirian granitoids. The set of geological data presented in this article demonstrates that the metatexite migmatites with felsic granulitic paleosome and charnockitic neosome were generated during the initial phases of the collision between these two paleoplates, between 2068 and 2058 Ma. The structural mass transport is from NW to SE. The youngest migmatization event, in high amphibolite facies, occurred during NNW-SSE trending sinistral strike-slip tectonics as suggested by the emplacement of Dn+3 neosomes controlled by S/C/C’ structures, in between 2049–2000 Ma. The interpreted tectonic model is complex and involves the accretion and collision among Bom Jesus da Lapa, Gaviao and Jequie paleoplates during the Rhyacian-Orosirian, contributing to the advancement of knowledge of tectonic pieces and their interactions that contributed to the formation of Columbia supercontinent.
... Las zonas de cizalla son estructuras fundamentales para comprender el rol que desempeñan los eventos de deformación ocurridos en la corteza en diferentes contextos tectónicos. Estas estructuras se caracterizan por: i) canalizar deformación durante la formación de los orógenos, ii) permitir la migración de fluidos, iii) favorecer el emplazamiento de cuerpos ígneos y, iv) en ocasiones, actuar como límites tectónicos separando dominios corticales con diferente evolución (Ramsay 1980, Sibson 1990, van der Pluijm et al. 1994, Brown y Solar 1998, Searle et al. 1998, Whitmeyer y Simpson 2003, Basei et al. 2005, Passarelli et al. 2011, Finch et al. 2015, Oriolo et al. 2015, Fossen y Cavalcante 2017, Dragone et al. 2017, Oriolo et al. 2018, Druguet et al. 2021. Estudios recientes han demostrado que, para acotar la edad de la deformación ocurrida en estas zonas de cizalla, es necesario un análisis estructural y geocronológico complementario que aborde las rocas metamórficas de los bloques adyacentes (Oriolo et al. 2016, 2018, van der Pluijm et al. 1994). ...
RESUMEN La porción norte del bloque de piso de la faja de cizalla Guacha Corral en la Sierra de Comechingones se encuentra dominada por una secuencia migmática clasificada como metatexitas metasedimentarias y metatexitas máficas, cuyos contactos generalmente son transicionales y se caracterizan por presentar distintas morfologías en función del contenido de fundido presente. Los resultados obtenidos del análisis estructural de las
... Many previous studies (e.g., field analysis, laboratory experiments, numerical modeling, seismology, hydrogeology) have focused on describing and discussing the architecture, initiation mechanisms, and rock failure processes of shear zones (Liu et al., 2022;Fagereng and Beall, 2021;Vannucchi, 2019;Hong et al., 2018;Fossen and Cavalcante, 2017;Cao and Neubauer, 2016;Mancktelow and Pennacchioni, 2013;Mancktelow, 2008;Wibberley et al., 2008;Rosenberg, 2004;Brown and Solar, 1998;Wintsch et al., 1995;Scholz, 1989Scholz, , 1980Sibson, 1977). The shear zone is known that strain localizes into the tabular zone from small outcropsize individual zone to large composite structure in the largescale in the lithosphere (Fossen and Cavalcante, 2017). ...
A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level. However, the deformation evolution of strain localization at a small-scale remains unclear. This study investigated <10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone. The small-scale ductile shear zones exhibit a typical transition from protomylonite, mylonite to extremely deformed ultramylonite, and decreasing mineral size from coarse-grained aggregates to extremely fine-grained mixed phases. Shearing sense indicators such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite. The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions. Quartz aggregates suggested a consistent temperature with an irregular feature, exhibiting a dominated high-temperature prism slip system. Additionally, coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep, while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained (∼50 µ m) mixed-phases in the ultramylonite. Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10−15 to 10−13 s−1 from low-strain mylonite to high strained ultramylonite. The localization and strain rate-limited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation. Furthermore, the potential occurrence of the small-scale shear zone was initiated at a middle-deep crust seated crustal condition dominated by the temperature-controlled formation and rheological weakening.
... Stuart et al., 2017Stuart et al., , 2018Weinberg et al., 2013). However, low percentages of preserved crystallized melt volume are expected if deformation is synchronous with melt presence, as deformation will markedly enhance movement of melt (Brown & Solar, 1998;Collins & Sawyer, 1996;Etheridge et al., 2020;Rosenberg & Handy, 2005;Van der Molen & Paterson, 1979). Therefore, the signature of the former presence of melt in high-strain zones may be cryptic at the outcrop scale and requires in-depth microstructural analysis to identify it . ...
Melt migration through high-strain zones in the crust fundamentally influences their rheological behaviour and is important for the transfer of fluids to upper crustal regions. The inference of former melt-present deformation, based on field observations, may be hampered if the high-strain zone experience a low time-integrated melt flux or high melt volume expulsion during deformation. In these cases, typical macro-scale field evidence of former melt presence limits interpretations. In this contribution, we investigate igneous field evidence ranging from obvious to cryptic in the Gough Dam shear zone (central Australia), a 2–4 km wide high-strain zone shown to have acted as a significant melt pathway during the Alice Springs Orogeny. Within bands of the high-strain zone, granitic lenses are easily discernible in the field and are inferred to have formed during melt present deformation. Related coarse K-feldspar is observed in biotite-rich (> 75 vol%) schist (glimmerite) as either isolated grains, forming trails (sub)parallel to the main foliation, or in aggregates with subordinate quartz. Detailed characterisation of the granitic lenses shows that pockets of phenocrysts may be entrained in the shear zone. If melt expulsion and melt-rock interaction is severe, isolated K-feldspar grains in glimmerite may form. These grains exhibit (i) partially preserved crystal faces; (ii) a lack of internal grain deformation; (iii) reaction textures preferentially formed along the main crystallographic axes showing dissolution of K-feldspar and precipitation of dominantly biotite; (iv) low-strain domains between multiple K-feldspar grains are inferred to enclose crystallised melt pockets, with some apparently isolated grains showing connectivity in three dimensions; and (v) a weak quartz and K-feldspar crystallographic preferred orientation. These observations suggest an igneous phenocrystic origin for the isolated K-feldspar grains hosted in glimmerite which is consistent with the observed REE concentration patterns with positive Eu anomaly. We propose that the K-feldspar phenocrysts are early-formed crystals that were entrained into the glimmerite rocks as reactive melt migrated through the actively deformating high-strain zone. Previously entrained K-feldspar phenocrysts were trapped during the collapse of the melt pathway when melt flux-related fluid pressure waned while confining pressure and tectonic stress were still significant. The active deformation facilitated expulsion or loss of the melt phase but retainment and trapping of phenocrysts. Hence, the presence of isolated or “trains” of K-feldspar phenocrysts are a cryptic signature of syndeformational melt transfer. If melt transfer occurs in an open chemical system, phenocrysts will be entrained within the reaction product of melt rock interaction. We suggest that these so-called “trapped phenocrysts” are a viable indicator of former syntectonic melt passage through rocks.
... Stuart et al., 2017Stuart et al., , 2018Weinberg et al., 2013). However, low percentages of preserved crystallized melt volume are expected if deformation is synchronous with melt presence, as deformation will markedly enhance movement of melt (Brown & Solar, 1998;Collins & Sawyer, 1996;Etheridge et al., 2020;Rosenberg & Handy, 2005;Van der Molen & Paterson, 1979). Therefore, the signature of the former presence of melt in high-strain zones may be cryptic at the outcrop scale and requires in-depth microstructural analysis to identify it . ...
Melt migration through high-strain zones in the crust fundamentally influences their rheological behaviour and is important for the transfer of fluids to upper crustal regions. The inference of former melt-present deformation, based on field observations, may be hampered if the high-strain zone experience a low time-integrated melt flux or high melt volume expulsion during deformation. In these cases, typical macro-scale field evidence of former melt presence limits interpretations. In this contribution, we investigate igneous field evidence ranging from obvious to cryptic in the Gough Dam shear zone (central Australia), a 2-4 km wide high-strain zone shown to have acted as a significant melt pathway during the Alice Springs Orogeny. Within bands of the high-strain zone, granitic lenses are easily discernible in the field and are inferred to have formed during melt present deformation. Related coarse K-feldspar is observed in biotite-rich (> 75 vol%) schist (glimmerite) as either isolated grains, forming trails (sub)parallel to the main foliation, or in aggregates with subordinate quartz. Detailed characterisation of the granitic lenses shows that pockets of phenocrysts may be entrained in the shear zone. If melt expulsion and melt-rock interaction is severe, isolated K-feldspar grains in glimmerite may form. These grains exhibit (i) partially preserved crystal faces; (ii) a lack of internal grain deformation; (iii) reaction textures preferentially formed along the main crystallographic axes showing dissolution of K-feldspar and precipitation of dominantly biotite; (iv) low-strain domains between multiple K-feldspar grains are inferred to enclose crystallised melt pockets, with some apparently isolated grains showing connectivity in three dimensions; and (v) a weak quartz and K-feldspar crystallographic preferred orientation. These observations suggest an igneous phenocrystic origin for the isolated K-feldspar grains hosted in glimmerite which is consistent with the observed REE concentration patterns with positive Eu anomaly. We propose that the K-feldspar phenocrysts are early-formed crystals that were entrained into the glimmerite rocks as This article is protected by copyright. All rights reserved. reactive melt migrated through the actively deformating high-strain zone. Previously entrained K-feldspar phenocrysts were trapped during the collapse of the melt pathway when melt flux-related fluid pressure waned while confining pressure and tectonic stress were still significant. The active deformation facilitated expulsion or loss of the melt phase but retainment and trapping of phenocrysts. Hence, the presence of isolated or "trains" of K-feldspar phenocrysts are a cryptic signature of syndeformational melt transfer. If melt transfer occurs in an open chemical system, phenocrysts will be entrained within the reaction product of melt rock interaction. We suggest that these so-called "trapped phenocrysts" are a viable indicator of former syntectonic melt passage through rocks.
... As a result, adequate heat supply may be linked during the exhumation stage of the Sandmata rocks along the shear zones ( Fig. 10; stage-III). These zones/channels acted as the route for fluid transportation and caused partial melting of the existing country rocks (TTG-like; sodic gneisses (this study), which promoted the generation of anatectic melt (Brown and Solar, 1998;Carvalho et al., 2016;Douce, 1996;Menegon et al., 2015;Watkins et al., 2007), leading to the reworking of the crustal rocks and producing potassium-rich granitoids. ...
The Aravalli Craton of the Indian shield constitutes heterogeneous basement lithologies (Banded Gneissic Complex; BGC), and among them, the granitoids are the most voluminous lithology. The BGC comprises two lithotectonic units, viz., BGC-I and BGC-II. The BGC-II has been further classified as amphibolite facies Mangalwar and granulite facies Sandmata Complexes. In the present study, the gneisses of the Mangalwar Complex are geochemically categorized into (i) low-and high-pressure sodic gneisses and (ii) potassic gneisses. The sodic gneisses are metaluminous and characterized by high Sr/Y and LaN/YbN ratios; and exhibit subduction-related negative anomalies of Nb and Ti. The εNd (t=2992 Ma) ranges from +2.3 to +3.1, with average Nd-depleted mantle model age (TDM) of 3.06 Ga. The whole-rock Sm-Nd isochron age is ∼3.0 Ga (2992±340 Ma). Genetically, the sodic gneisses originated from melting of an enriched precursor (oceanic plateau) in an arc environment. These gneisses show strong correlations with the gneisses from BGC-I depicting similar geochemical signatures. In contrast, the potassic gneisses are characterized by slightly higher SiO2 along with high K2O and high large-ion lithophile elements and negative Eu anomalies along with negative εNd (t=1.7 Ga) (-13.2 to -3.9), higher initial ⁸⁷Sr/⁸⁶Sr isotopic ratios and average TDM = 2.87 Ga. These geochemical features of the potassic gneisses are indicative that they were derived from the reworking of the pre-existing TTG-like (sodic gneisses) crust during the Paleoproterozoic Era.
... Within the model of fractured networks, emplacement occurs through a fractured network system (Clemens and Droop, 1998), wherein the percolation rate is dependent on the drain rate (Vigneresse and Clemens, 2000;Bons et al., 2009). This mechanism of magma-emplacement may explain how the smallest fractures can feed larger dykes and, finally, plutons (Brown and Solar, 2009). Bons et al. (2009) suggested that the mechanism of magma emplacement would stop because of sealing, preventing stable connectivity as seen in the experiments (Fig. 7). ...
A 2D Hele-Shaw cell was built to study microfracture nucleation, growth, and network formation during internal fluid production. Fluid is slowly produced into a low permeability solid, which leads to a local fluid pressure increase that controls the nucleation of microfractures that grow and then connect to create flow pathways. This process occurs during the primary migration of hydrocarbons in source rocks, which is the main topic of our study. It may also occur in other geological systems, such as the expulsion of water during dehydration of clay-rich sediments in sedimentary basins or serpentinite rocks in subduction zones and the transport of magmatic melts. Our system consists of a transparent, brittle gelatin material mixed with yeast and sugar. The consumption of sugar by yeast leads to CO2 formation, resulting in microfracture nucleation and growth. We varied three parameters, (1) anisotropy (i.e., number of layers), (2) lateral sealing, and (3) rate of fluid production. We tracked fluid movement through the opening and closing of microfractures within the system. Microfracture nucleation density is similar in a layered system to previous studies (0.45 microfracture per cm²). However, we observed that lateral confinement (0.31 microfracture per cm²) and rate of expulsion (0.99 microfracture per cm²) affect nucleation density and the geometrical characteristics of the microfracture network. The size, extent, and geometry of the microfracture network are dependent on all three parameters investigated, where lateral confinement and a higher rate of expulsion result in greater microfracture network connectivity. Layers control the angle of intersection between microfractures. Furthermore, layering and sealing have an impact on fracture topology. Results also show that the microfracture pattern significantly influences the fluid expulsion rate. Our results have direct applications to understanding how fluid migration occurs in low-permeability rocks through the development of a connected microfracture network produced by internal fluid generation.
... The selvedges are inferred to be crystallised products of local reactions between melt and the host rock (e.g. Collins and Sawyer, 1996;Brown and Solar, 1998). These field relationships focus attention on structurally controlled, melt-rock interaction as the most likely mechanism for reaction replacement of intermediate gneiss by mafic gneiss in the southern high strain zone. ...
Across all scales, fluid-rock interactions facilitate metamorphism, increase chemical mobility, disturb geochronological systems and change the rheological properties of rocks. In regions affected by fluid activity, determining the nature of fluid-rock interaction, i.e., fluid type (aque- ous fluid or silicate melt), scale of fluid migration and the proceeding reactions, is required to better understand the region’s geological history. The Entia Dome in central Australia is well-studied, but lack of a comprehensive understanding of fluid-rock interaction has limited previous interpretations. This thesis uses microstructures to implicate silicate melt as the key fluid present during dome formation. Syn-tectonic melt migration, reaction and metasomatism in major shear zones modified the chemistry of precursor rocks. Melt-rock interaction at the mineral scale modified zircons via coupled dissolution-precipitation and disturbed their U-Pb ages. Although, new apatite U-Pb dates („312 Ma) confirm the Carboniferous age of dom- ing. This study finds tectonic extrusion of melt-weakened lower crust is critical in forming the extensional dome structure during the contractional Alice Springs Orogeny. We further demon- strate that fluid-rock interaction is intimately tied to all aspects of the Entia Dome’s formation. Therefore, it is essential that fluid-rock interactions at all scales are considered when building robust histories of geological regions.
... Most layers of leucosome seem to be parallel to migmatitic foliation while others cross the compositional layering. This discordance can be interpreted as evidence that leucosome was derived from the anatectic melt and was later injected in C"-type extensional shear bands (Brown, 2007) and dilatant sites surrounding disrupted resisters or pelitic restites (Fig. 6c, d), both representing flow channels synchronous with deformation (Brown and Solar, 1998). The syn-tectonic ca. ...
U-Pb ages of zircon from samples of tonalite and migmatite from the Central Iberian Zone (Spain), previously assigned to the Carboniferous were obtained using a sensitive high mass-resolution ion microprobe. These ages indicate a close temporal relationship between Ordovician tonalitic magmatism and high-grade (low-P) metamorphism in Iberia. The age obtained for Santa Cruz tonalite (476 ± 1 Ma) represents a constraint on the partial melting event experienced by Ediacaran Schist-Greywacke Complex siliciclastic rocks, while Risco Chico leucogranite that alternates with tonalites and migmatites yielded a younger age (468 ± 3 Ma). The age range of Neoproterozoic relict zircon grains preserved in Ordovician plutonic rocks coincides with that of host Ediacaran metasedimentary rocks (Puerto de Santa Cruz migmatites), which are the most plausible source of leucogranites. Altogether, the component of inherited zircon, peraluminous calc-alkaline composition, and an abundance of pelitic restites found in Ordovician tonalites provide evidence of a mixed contribution from old Ediacaran magmatic arc rocks and siliciclastic sequences. Our findings supply a missing piece in the Lower-Middle Ordovician tectonic puzzle of the northern Gondwana margin, revealing that tonalitic magmatism, ductile deformation, and high-grade metamorphism all occurred within a relatively restricted period simultaneously with siliciclastic deposition in active extensional basins.
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5976757
... Fifthly, the Permian A-type granites are commonly associated with coevally emplaced tholeiitic mafic rocks, and alkaline silica undersaturated syenites have also been identified in the Kokshaal area, southern Kyrgyzstan Tianshan, which were attributed to upwelling of the asthenosphere mantle induced by the large-scale shearing (Konopelko et al., 2007(Konopelko et al., , 2009. Therefore, the shear zones may not only provide the pathways for diverse magmas/fluids, but also control the temporal and spatial distributions of the magmatic rocks (Brown and Solar, 1998;D'lemos et al., 1992;Konopelko et al., 2007Konopelko et al., , 2009). (Cao et al., 2017;Chai et al., 2008;Chen et al., 2018;Deng et al., 2015;Gao and Zhou, 2013;Jiang et al., 2017;Long et al., 2020;Luo et al., 2015;Mao et al., 2021b;Mao et al., 2014a;Mao et al., 2019;Mao et al., 2014b;Song et al., 2013;Song et al., 2011;Su et al., 2011b;Tang et al., 2013;Zhang et al., 2020;Zhao et al., 2015). ...
Multiple arc systems were developed in response to Neoproterozoic to Mesozoic consumption of the Paleo-Asian Ocean and their magmatic evolution is crucial for understanding the arc-arc amalgamation in the Central Asian Orogenic Belt. Here, we report whole-rock geochemical data as well as simultaneous in situ zircon U-Pb and Lu-Hf isotope results for the late Paleozoic magmatic rocks associated with the shear zones in the Gangou section of the Eastern Tianshan to constrain their petrogenesis and arc-arc amalgamation processes. We obtained ∼307 Ma and ∼270 Ma zircon U-Pb ages for the Late Carboniferous diorites and Middle Permian andesites, respectively. These rocks have high Mg# (∼65 and ∼47, respectively) values, and are enriched in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), with depleted zircon εHf(t) values (+12.1 to +18.1), suggesting an origin from partial melting of the depleted mantle with various degree of differentiation. Furthermore, our results also provided ∼276 Ma zircon U-Pb ages for the Middle Permian granitic porphyries and ∼256 Ma zircon U-Pb ages for the Late Permian diabase rocks. The granitic porphyries are geochemically similar to A2-type granites, i.e., high K2O+Na2O, FeOT/MgO, Ga/Al and low CaO, Sr and Ba. They have high zircon εHf(t) (+9.0 to +13.7) values, indicating a possible derivation from juvenile lower crust. The diabase rocks show depletion of light rare earth elements (LREEs), resembling normal mid-ocean ridge basalt. These rocks have depleted zircon Hf isotopic compositions (εHf(t) = +8.9 to +15.8), demonstrating a possible derivation from partial melting of the depleted mantle. The available geochemical data from the Eastern Tianshan show that the Permian mafic rocks along the shear zones possess higher Nb/La and lower Ba/La ratios than their counterparts in the Carboniferous. These contrasting features imply that the Carboniferous mafic rocks originated from a metasomatic mantle wedge, while the Permian mafic rocks were likely derived from the depleted mantle with addition of enriched asthenosphere components. The high zircon saturation temperatures (mostly > 800 ℃) of the Permian A-type granitoids and diverse magma sources of coeval mafic rocks also imply possible asthenosphere upwelling. Therefore, we propose that the Late Carboniferous magmatic rocks were likely resulted from subduction of the North Tianshan oceanic plate, while the spatial and temporal evolution of the Permian magmatic rocks were probably attributed to large-scale dextral transcurrent tectonics associated with arc-arc amalgamation.
... (cf.Brown and Solar, 1998b;Marchildon and Brown, ...
The Ersfjord Granite is part of a suite of c.1.80–1.75 Ga syeno-granites in the West Troms Basement Complex, northern Norway, presumed to belong to the Transscandinavian Igneous Belt (TIB-1) in the Fennoscandian Shield. Previous data suggest the granite formed post-collisional and ascended as a batholith pluton from a source generated by delamination of mafic-intermediate lower crust. We argue that the Ersfjord Granite was emplaced initially (c. 1.80 Ga) as multiple tabular sills in an extensional setting, then as successive melt injections (c. 1.78–1.75 Ga) in an evolving Andean/Cordilleran type accretionary orogen at the waning stages of the Svecofennian orogen. Field observations indicate melt ascent initiated as successive sills (EG-I) into well-foliated Meso/Neoarchaean TTG gneisses. Some sills preserved a magmatic layering, others injected and assimilated the host rock gneisses leaving pendants of mafic gneiss/migmatite residuum in between the granite sills. The first tectonic patches of melts (EG-II) ascended into the middle/upper crust along regional shear zones and injected into ductile imbricate thrust stacks (D1 event) during NE-SW directed crustal shortening and medium grade P-T conditions, using the sills and ancestor migmatite pendants as melt pathways. Then the tabular EG-I and II granite sheets and adjacent gneisses were coaxially folded by upright macro-folds (D2 event) and steep, granitic pegmatite dyke swarms (EG-III) intruded parallel to the fold axial surface and in related D2 thrusts, at low grade metamorphic conditions. The final melt emplacement (D3 event) included granite pegmatite dykes and sills (EG-IV) along subvertical D3 fold limbs and steep strike-slip shear zones. Our provisional extension, and successive advancing accretionary orogenic emplacement model for the Ersfjord Granite may explain ascent of many other TIB-1 magmas in the Fennoscandian Shield.
... The mountain-building thrust faults of Io, and associated extensional features, may counterintuitively serve as direct conduits of melt to the surface (Bland & McKinnon, 2016). These faults, or, if appropriate, their downward extension as a ductile shear zone, may serve as conduits for melt propagation, as is commonly observed on Earth in the form of melt-rich shear zones and dikes deviated by preexisting fractures and faults (Brown & Solar, 1998;Hollister & Crawford, 1986;Hutton, 1988;Le Corvec et al., 2013;Valentine & Krogh, 2006). ...
Melt from the presumed magma ocean in Io's interior reaches the surface at well‐documented paterae and hotspots. To do so, melt needs to cross the thermal lithosphere of Io, even though, as it loses heat, it may stall inside the lithosphere. Permeability barriers form when melt crystallization is rapid and further prevents melt ascent unless a different melt transport mechanism develops. The heat pipe model of Io provides such a mechanism, allowing the melt to travel from the molten asthenosphere to the surface at discrete points. However, unless these heat pipes are billions of years old and constant in both location and flux, melt must ascend through the cold lithosphere at other locations and form new melt conduits. We model here the crystallization sequence of melts as they rise through the lithosphere of Io and determine under what conditions a permeability barrier may form. The barrier is generally deep, but can be elevated 100s of meters to several kilometers in areas of high strain rate or low resurfacing rate. We propose a feedback mechanism where regions closer to a heat pipe experience a higher resurfacing rate, driving the permeability barrier deeper, while regions away from a heat pipe experience a lower resurfacing rate allowing the permeability barrier to rise. A new heat pipe may develop in these regions where the melt is focused by variations of permeability barrier elevation while the old heat pipe closes, potentially leading to changes of heat pipe location over a time scale of 100,000 years.
... Such a fast cooling may not be viable through cooling by erosion, rather could result from the transport of rocks up a thrust ramp (Storm and Spear, 2005). Brown and Solar (1998) argued that the deformation and failure of rocks in such a situation are augmented by a certain critical percentage of melt. In the present case, incipient melting during M 2 could facilitate thrusting and shearing which has been inferred in an earlier study . ...
This study characterizes two metamorphic cycles from two samples of an aluminous granulite of the Eastern Ghats Belt, India using petrological, phase equilibria, geospeedometric, and, petrochronological data. The first metamorphic cycle ensued through dehydration melting of F-biotite along a shallow dP/dT prograde path reaching ultrahigh temperature (>950 °C) at 7–8 kbar pressure. The peak metamorphic assemblage is represented by sapphirine + spinel + Al-orthopyroxene + cordierite + ilmenite + plagioclase + quartz which constitute the coarse granoblastic assemblage. Reaction corona comprising orthopyroxene + sillimanite + garnet ± F-biotite over the peak phases imply near-isobaric cooling from the peak with the termination of the first cycle. The melt produced during the prograde stage has been lost to a large extent to stabilize the peak assemblage. The second metamorphic cycle ensued with the decomposition of cordierite by a skeletal intergrowth of orthopyroxene + sillimanite + quartz and a symplectic intergrowth of garnet + quartz between orthopyroxene and cordierite. A delicate symplectite comprising cordierite + quartz + K-feldspar ± plagioclase also formed at this stage within the leucocratic layers. These intergrowth textures resulted from reworking of the cooled granulite (increasing P-T) through incipient melting of F-biotite subsequent decompression, and melt-solid interaction during the terminal stage of the second metamorphic cycle. Geospeedometric data involving garnet and biotite suggest the nonlinear nature of cooling with an initial fast cooling from the peak condition of the first metamorphic cycle. Monazite petrochronological data yield 1002 ± 3 Ma and 944 ± 6 Ma ages that belong to the peaks of first (ca. 1030–990 Ma) and the second (ca. 950–900 Ma) metamorphic cycles respectively of the central crustal province of the belt. This two-cycle evolution is related to several alternate extension (pull) and compression (push) cycles in an accretionary tectonic setting during ca. 1030–900 Ma.
... % that is sufficient to create an interconnected system of melt films in the partially molten felsic protolith (e.g. Rosenberg & Handy, 2005), but is not enough for further melt segregation (e.g. Brown et al., 1995;Brown & Solar, 1998). Nevertheless, presence of a leucosome in sample LP19-11 ( Fig. 3a, b), which is clear evidence for the melt segregation, suggests that amount of the melt during the metamorphic peak and uplift was higher than the modelled values. ...
Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean-Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810-830°C) exhumation from pressure of 10.0 – 10.2 kbar to 7.5-7.0 kbar during the Neoarchean event (2.68 – 2.62 Ga). The exhumation mediated by anatectic melt supports interpretation of the Ha-Tshanzi structure as a diapir-related granite-gneiss dome. During upwelling of the dome, the melt segregated into leucosome, while growth of cordierite sequestered water from the melt assisting to its crystallization at the end of the sub-isothermal decompression stage. As the rheology of the rock changed, melt-dominated deformation was transformed to solid-dominated ductile shear deformation. In contrast to earlier sub-isothermal decompression P-T path, the gentler slope ~75°/kbar of the decompression-cooling path marks the exhumation from pressures ~ 7 kbar down to pressures 5 - 4.5 kbar and cooling down to 600-550°C. Dating of zircon, monazite and rutile shows that the Neoarchean evolution of the metapelite was strongly overprinted by the Paleoproterozoic event at ca. 2.01 Ga. The results of the study highlight the significance of domical structures related to granitic diapirs in the exhumation of the Central Zone of the Limpopo Complex.
... (2) (Ramsay, 1980). Field evidence, laboratory measurements and modeling studies suggest that these shear zones correspond to regions of intense permeability driving the migration of fluids (e.g., Patriat and Jolivet, 1998;Brown and Solar, 1998;Manatschal, 1999;Gottardi et al., 2015;Roche et al., 2018). In turn, the presence of fluids within extensional shear zones fosters the deformation and facilitates the rupturing of the continental lithosphere (e.g., Kendall et al., 2005;Lavier and Manatschal, 2006;Gillard et al., 2016). ...
Asthenosphere-lithosphere interactions modulated by surface processes generate outstanding topographies and sedimentary basins, but the nature of these interactions and the mechanisms through which they control the evolution of extensional tectonic settings are elusive. Basal lithospheric shearing due to plume-related mantle flow leads to extensional lithospheric rupturing and associated magmatism, rock exhumation, and topographic uplift away from the plume axis by a distance inversely correlated to the lithospheric elastic thickness. When moisturized air encounters a topographic barrier, it rises, decompresses, and saturates, leading to enhanced erosion on the windward side of the uplifted terrain. Orographic precipitation and asymmetric erosional unloading facilitate strain localization and lithospheric rupturing on the wetter and more eroded side of an extensional system. This simple analytical model is validated against thermo-mechanical numerical experiments where a rheologically stratified lithosphere above an asthenospheric plume is subject to fluvial erosion proportional to stream power during extension. Our modeling results are consistent with Paleogene mantle upwelling and flood basalts in Ethiopia synchronous to distal initiation of lithospheric stretching/rupturing in the Gulf of Aden, which progressively propagates into the Red Sea. The present-day asymmetric topography and extensional structures in the Main Ethiopian Rift may also be an effect of a Neogene-to-present orographic erosional gradient. Although inherently related to the lithosphere rheology, the evolution of continental rifts appears even more conditioned by the mantle and surface dynamics than previously thought.
... Vaughan and Scarrow (2003) postulated that metasomatized portions of the lithospheric mantle may control the localization and initiation of strike-slip faulting. Once generated, transcurrent shear zones subvertical foliation favor the transport of magmas through penetrative flow mechanism (Brown and Solar, 1998), allowing interaction of mantle melts with crustal melts of different compositions. We propose that these processes generated the progenitor magmas of the Puxinanã and Areial plutons, which had their composition slightly modified by fractional crystallization during the ascent and emplacement. ...
Expressive granitic magmatism and a complex system of transcurrent shear zones are the main features associated with the Ediacaran evolution of the Borborema Province (northeastern Brazil). Their petrogenesis and relationships with tectonic structures can provide important insights into lithospheric processes during the collisional to post-collisional stages of the Brasiliano Orogeny. The Esperança Complex is a NE-trending elongated granitic batholith with ca. 650 km2 comprising five plutons emplaced between the Patos and Campina Grande dextral shear zones in the central–eastern part of the Borborema Province. An integrated study of the complex was carried out using Usingle bondPb zircon SHRIMP and LA-ICPMS geochronology, mineral and whole-rock geochemistry, and Ndsingle bondHf isotope geochemistry (whole-rock and zircon). The plutons are metaluminous to slightly peraluminous with geochemical and isotopic signature reflecting different sources. The Remigio and Areial plutons have Paleoproterozoic to Neoarchean Nd and Hf TDM model ages and strong negative εNd(t) (−15.29 to −16.71) and εHf (−12.20 to −21.03) values. Their geochemistry is compatible with derivation through partial melting of mafic to intermediate metaigneous sources like the Paleoproterozoic basement (Areial pluton) and metasedimentary sources (Remígio pluton). The Puxinanã, Serrote da Cobra and Pocinhos plutons show higher ɛNd (−5.90 to −7.35) and ɛHf (−2.83 to −6.91) values, and Statherian to Calymmian Nd and Hf TDM model ages, ranging from 1.4 to 1.7 Ga and from 1.6 to 1.8 Ga, respectively. They have geochemical affinity with A-type ferroan granites. The geochemical and isotopic signature are similar to those of Tonian orthogneiss that constitute part of the plutons' host rocks, suggesting derivation from Tonian crustal sources. Evidence of magma mixing and the presence of inherited zircon grains with ages like those of metasedimentary host rocks suggest contribution of the lithospheric mantle and sedimentary sources in different proportions in their petrogenesis. The reported data are compatible with building of the Esperança Complex by multiple intrusive events. The Pocinhos and Serrote da Cobra plutons are the oldest (~600 Ma) and their genesis is probably associated with heat input resulting from crustal thickening at the end of a long contractional period (ca. 630–600 Ma). The genesis of the Puxinanã and Areial plutons (~585 to 565 Ma) involved interactions of crustal and mantle mafic melts, with magma transport and emplacement controlled by the Campina Grande shear zone at the beginning of the transcurrent regime. The Remígio pluton is the youngest intrusion (~565 Ma) and its genesis can be related to a high geothermal gradient resulting from emplacement of the older plutons, possibly enhanced by strain heating related to displacement along the Patos shear zone.
... In turn, intrusions favor additional deformation partitioning by the advection of hot and weak material within the shear zones D'Lemos et al., 1992;Gébelin et al., 2009;Hutton, 1997;Le Pourhiet et al., 2012). Second, magma migration profits kinematically controlled conduits, where melt is preferentially pumped along the maximum principal finite elongation direction (Brown and Solar, 1998a;. At the same time, movement of magma creates a thermal antiform providing a low-viscosity zone where strain rate increases and ductile deformation is enhanced; the thermal corridor propagates upward leading to melt-enhanced embrittlement at its top Solar, 1998a, b, 1999;Gébelin et al., 2009). ...
Oblique convergent margins, like the subduction of Phoenix beneath the South American plate during Jurassic and Early Cretaceous times, are characterized by strain partitioning and a positive feedback loop between strike-slip deformation and magma ascent along the magmatic arc.
Located in the Coastal batholith of northern Chile, the Flamenco pluton is one of the youngest Andean intrusives emplaced in the western active margin of South America. Besides an older SW domain of granodioritic rocks (c.a. 213 Ma), the NW and E domains of the Flamenco pluton were emplaced between 194 and 186 Ma. They present a normally zoned structure constituted by external gabbroic to Qtz-dioritic magmatic facies and tonalites and granodiorites located in inner areas of the intrusive body. These domains are separated by a central strip of stretched coarse-grained Crd-schists that presents ductile asymmetrical folding and S–C structures that point to the NW-directed displacement of the E domain of the pluton. This syn-emplacement shear zone shows kinematic compatibility and continuity to the north and south with folded and mylonitic metasediments out of the contact aureole of the pluton. Together, these segments constitute a large, steeply dipping sigmoidal structure of average N–S direction; the called here Chañaral transcurrent shear zone.
Contemporary to the emplacement of the Flamenco pluton, slight variations in the trend of the crustal-scale structure generated strike-slip and transpressive sectors along the Chañaral shear zone, which favored the access of intruding magmas to the final emplacement level. As a paradigmatic example, the curviplanar Flamenco shear zone, an internal, magmatic branch of the main structure that traverses the E domain of the pluton, is defined by the sinistral and reverse shearing under magmatic conditions of the previously mingled mafic and felsic batches. Consequently, the transpressive Flamenco shear zone is interpreted as an ascent conduit where gabbroic and granodioritic liquids interacted during the building of the intrusive body. In addition, these sheared rocks were affected by late textural coarsening processes that evidence the slow and cyclical cooling of the growing magma reservoir. In contrast with the steeply dipping contacts and structures found to the east, the NW domain of the pluton shows sharp and gently dipping contacts between almost horizontal magmatic layers. We suggest that the western block of the Chañaral shear zone was a relatively passive footwall dominated by horizontal flow trajectories and lower replenishment rates according to the inverse emplacement sequence, i.e., late external mafic batches intruded along the margins of the felsic core. The variable structural arrangement of the crustal rocks that hosted the Flamenco pluton was the result of the complex interaction between far-field and local, magmatic forces during the emplacement process, besides the interference with pre-Andean structures.
The presence of the Chañaral shear zone favoring the emplacement of the Flamenco pluton demonstrates that the Late Jurassic to latest Early Cretaceous Atacama Fault System had earlier precursors and both the magmatic arc axis and the transcurrent shear zones migrated landward during Jurassic times.
A dominantly NW-SE directed extensional tectonics in the Early Cretaceous significantly reworked the Late Permian-Triassic orogenic framework of the Dabie orogenic belt. The North Dabie complex (NDC) is the principal domain recording this tectonic event. However, the precise structure-kinematic architectures, particularly those observed in the ductile regime, along with the respective time scales for different extensional stages, have not been adequately established. This significantly impedes our comprehensive understanding of the extensional style and deformation history in the North Dabie complex. To better address these issues, we conducted a systematic structural study and LA-ICP-MS zircon U-Pb dating of the pre-, syn-, and post-kinematic intrusions and syn-kinematically metamorphosed high-grade gneisses/migmatites of the NDC. Our results demonstrate that the extensional deformation in the NDC may initiate at ca. 144 Ma, which is characterized by a pervasive NW-SE oriented coaxial plastic flow in the ductile regime of the middle-lower crust. A large-scale detachment processing zone subsequently started activating at ca. 140 Ma at the upper-middle level of the middle crust, and concentratedly accommodated the extensional strain by top-to-NW ductile shearing. Locally, there was uprising of sub-magmatic flow in the atatexite-diatexite from the deeper lower crust taking place in the manner of top-to-outward shearing as early as ca. 137 Ma. This composite process of extension manifests vertical strain partitioning across the ductile middle-lower crusts and progressive strain localization during the lithospheric thinning. The NW-SE orientation dominated extensional tectonics was strongly driven by the westward subduction of the Paleo-Pacific oceanic plate during the Late Mesozoic.
Structural and tectono-metamorphic analysis of strain partitioning
during late-orogenic oblique deformation : insights from the Variscan
Tanneron Massif and the Terre Adélie Craton
In migmatitic environments the behaviour of the system is controlled by the generation
and amount of the anatectic melt. Accordingly, migmatites typically show a genetic
linkage between the tectonic deformation and melt migration. We investigated this
relationship in Olkiluoto (SW Finland) and identified four phases of ductile deformation,
which are distinguished by the multiple folding phases, ductile shear events and
cross-cutting features associated with pegmatitic leucosomes and/or a specific type
of diatexitic migmatite with feldspar megacrysts. U–Pb LA-MC-ICPMS data on zircon
cores and rims from migmatites and cross-cutting pegmatites indicate two distinct
metamorphic events associated with melt generation and migration at 1.87–1.84 Ga
and 1.82–1.78 Ga. These two migmatitic events suggest that the orogenic evolution of
the area was long-lasting and characterized by slow cooling. The structural data and the
age constraints presented in this paper support the idea of similar tectonic evolution
and metamorphic environment in SW Finland and central E Sweden.
The Aiguilles-Rouges and Mont-Blanc massif represent a segment of a crustal-scale transpressional shear zone named the East Variscan Shear Zone (EVSZ) along which two plutonic pulses occurred during Early and Late Carboniferous times. The aim of this study is to constrain the relationships between the dynamics of the crustal-scale shear zone, the mechanisms of pluton emplacement at different structural levels within the crust and the magma sources. A detailed structural analysis of the whole massif highlights the crustal-scale anastomosed network of the EVSZ. Microstructural observations and LA-ICPMS U–Th–Pb zircon ages from large plutons constrain the beginning of the transpression at ca. 340 Ma. From 340 to 305 Ma, the EVSZ broadened and formed a 25 km-wide dextral S–C–C’ anastomosed shear zone network with dilation zones acting as preferential pathways for melt migration and pluton growths. Moreover, LA-ICPMS U–Th–Pb zircon ages from small magmatic bodies (i.e. pegmatite, aplite and microgranite) indicate an Ordovician-age inheritance component. Field evidences and zircon inheritance indicate that Late Carboniferous granitic melts are mainly derived from water-fluxed melting of Ordovician orthogneiss with the input of mantellic-source derived magmas. Over time, the growth of the dextral anastomosed network enhanced water transfer through the shear zones and water-fluxed melting to produce more anatectic melts.
In this study, we integrate anisotropy of magnetic susceptibility (AMS) and microstructural data including quartz crystallographic preferred orientation (CPO) from SEM-EBSD from the Manchi Pan-African granitic pluton with the objective of deciphering the time relationship between fabric development, emplacement and regional tectonics. The Manchi pluton, located in the western Cameroon domain of the Pan-African Fold Belt of Central Africa in Cameroon, is made of hornblende-biotite granite (HBtG), biotite granite (BtG) and protomylonites of HBtG. The pluton intruded the high grade country rocks made of banded gneiss and amphibolites. AMS of the pluton is controlled dominantly by paramagnetic phases such as, Fe-bearing silicates (biotite and hornblende). This is inferred from the low Km values (<500 × 10⁻⁶ SI units) as well as from hysteresis data. The AMS ellipsoid shape is mostly oblate, implying that flattening component of the strain was important during granite emplacement. Microstructural observations reveal presence of magmatic to sub-magmatic and solid-state deformation textures developed at moderate-to low-temperatures. This is consistent with the presence of rhomb and basal slip systems established from quartz c-axis data through EBSD analysis. The pluton recorded top-toward SW to top-toward south sense of shear as established by the kinematic markers. This indicates a component of simple shear during tectonic evolution of the pluton. We infer that the magnetic fabrics recorded in the Manchi granitic pluton is a manifestation of the D2 regional deformation. The dominance of flattening strain and the concentric pattern of magnetic data are indicative of ballooning being an important mechanism under transpressive regime.
Mafic igneous rocks are volumetrically limited in collisional orogens but are significant for revealing specific geological processes. The mafic intrusive rocks in the eastern Lhasa batholiths along the Parlung fault (the easternmost branch of the Jiali fault zone), southern Tibet, are commonly regarded as the easternmost mafic components of an inferred Shiquanhe–Nam Tso–Jiali–Parlung suture zone, which is assumed to mark a back-arc oceanic basin related to the southward subduction of the Bangong-Nujiang Meso-Tethyan Ocean. However, the petrogenesis and tectonic setting of the mafic intrusive rocks have been poorly constrained. In this paper, we present petrological, geochemical and geochronological data for this mafic suite and evaluate the tectonic implications. The mafic intrusive rocks can be approximately divided into pyroxene-amphibole cumulate and medium-grained and pegmatitic hornblende gabbros that are characterized by the occurrence of very calcic plagioclase (An = 80–92) and abundant amphibole crystals, indicating an H2O-enriched magmatic signature. Their whole-rock compositions, which include negative Nb and Ta anomalies, also suggest a subduction-modified setting. Zircon LuHf isotope data show that their magmas were formed by magmatic mixing between melts derived from enriched subcontinental lithospheric mantle and liquids derived from upwelling asthenosphere. Zircon UPb dating results reveal crystallization ages of ~114–110 Ma for this mafic suite, corresponding to the late stage of regional granitic magmatism (~135–110 Ma) that was related to the Lhasa-Qiangtang collision after the closure of the Bangong-Nujiang Meso-Tethyan Ocean. Given that coeval mafic and felsic rocks are distinctly controlled by the Parlung fault, we suggest that asthenospheric upwelling due to the break-off of the subducted Bangong-Nujiang oceanic slab not only induced partial melting of the enriched subcontinental lithospheric mantle but also resulted in regional rifting/extension and the initial development of the eastern Jiali (Parlung) fault.
The granulite-complexes of the Sierras de Córdoba, inside context of Sierras Pampeanas, expose the deeply-uplifted internal metamorphic zone of an early Cambrian orogen. One of the largest granulite-complexes in the north-central of Sierra de Comechingones consists mainly of a set of stromatic and diatexitic migmatites all derived from metasedimentary progenitors. Migmatites vary over a spectrum of morphologies ranging from patch, net, and stromatic metatexites to schollen-schlieren and nebulite diatexites with widespread interspersed anatectic granites. The granulite-complex subordinately includes amphibolites, marbles, serpentinized ultramafic rocks, and mafic plutonic rocks. Petrographic analysis, textural and structural interpretation, thermobarometric estimates, and inherited and metamorphic zircon dating reveal the metamorphic history of migmatites in the north-central part of the Sierra de Comechingones. Metastable preservation of kyanite reflects that migmatites were buried (0.69–0.78 GPa) and heated (∼650 °C) along a prograde trajectory before the onset of extensive anatexis. Then migmatites were further heated to granulite-facies peak temperatures (≥800 °C) at similar pressures (0.77 ± 0.06 GPa) through the sillimanite stability field. Widespread partial melting of the metasedimentary sequence occurred during peak metamorphism. The entire granulite-complex cooled to temperatures below 550 °C and ascended to middle crustal depths (0.3–0.5 GPa). The prograde heating trajectory and decompression during uplift overcoming cooling and tectonic exhumation are compatible with a predominantly compressive deformation. Coeval widespread partial melting with contractional non-coaxial deformation triggered self-reorganization of anatectic melts giving rise syn-deformational migmatites. In all the analyzed migmatites, U–Pb zircon geochronology yields inherited ages corresponding to those of orogenic systems that characterize the West Gondwana supercontinent. Inherited ages show a polymodal distribution pattern with contributions from Neoproterozoic (551–734 Ma; i.e., Brasiliano - Pan-African) and Mesoproterozoic detrital zircons (850–1182 Ma; i.e., Grenvillian), and minor proportions of Paleoproterozoic zircon grains (1751–1839 Ma). Ages interpreted as metamorphism and anatexis suggest the development of a single metamorphic event and restricted anatexis to a relatively short period of about 15 Ma (ca. 540-525 Ma). Metamorphic ages associated with the metamorphic peak trajectory are related to the Cambrian Pampean Orogeny.
Granite intrusion in the upper crust along a crustal-scale shear zone has been modelled by injecting a Newtonian fluid (low-viscosity silicone putty) into a sandpack containing a ductile layer of silicone putty which acted as a weak level along which the injected material could spread. The strike-slip regime was obtained using two mobile rigid basal plates sliding horizontally. Boundary conditions were chosen in order to analyse the influence of thicknesses of the brittle cover and of the volume of injected material on the pattern of the intrusion. Experiments showed that (1) the presence of a soft layer between two competent units allowed the formation of laccolithic intrusions by lateral expansion in the emplacement site, (2) intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime, (3) this direction controls the emplacement beginning with the first stages of injection, (4) the lateral expansion of the intrusion is locally controlled by arrays of Riedel faults formed in the overburden, (5) the control of faults on the intrusion pattern increases with increasing brittle/ductile ratio, i.e. with increasing depth and (or) decreasing thickness of the weak layer, (6) intrusions are sigmoidal or lozenge shaped in horizontal view, (7) the greater the brittle/ductile ratio, the less sigmoidal are the intrusions, (8) feeding pipes shifted with respect to the strike-slip zone result in asymmetric intrusions, with the development of a sheared tail trailing behind the intrusion. Examples of leucogranites of the South Armorican Shear Zone (South Brittany, France) emphasize that our experiments can explain the geometry of many syntectonic granites emplaced along strike-slip zones. They further shed some light on mechanisms of pluton intrusion in the upper crust.
The permeability of a partially molten rock is one of the primary factors controlling the melt segregation rate. With decreasing melt percentage, permeability becomes increasingly sensitive to the grain-scale geometry of partial melt. At low melt percentages, the ratio of grain-boundary energy to solid-melt interfacial energy, [γ ss ]/[γ sl ], is the fundamental physical property that determines the equilibrium melt geometry, including the wetting angle θ at a solid-solid-melt triple junction, the area-tovolume ratio s/v of melt pockets at grain corners, and the permeability threshold φ c (φ c , is the volume melt percentage at which melt interconnection is established). The trends of increasing θ and φ c or decreasing s/v with decreasing [γ ss ]/[γ sl ] are well established in the case of a monomineralic system with isotropic interfacial energies. We argue that these general trends must apply as well in natural systems where solid-melt interfacial energies are essentially anisotropic. Measurements of wetting angles at quartz-quartz-melt, feldspar-feldspar-melt and amphibole-amphibole-melt triple junctions consistently yielded low to very low median values, in the range 10°–60°. These low angles result from high values of [γ ss ]/[γ sl ], up to 2.0 for the lowest angles. They indicate that the permeability thresholds of partially molten crustal protoliths should be very low: The major implication of the experimental studies is to show that melt segregation may potentially operate at very low degrees of melting (theoretically, any value ≥ φ c ). Because of the high viscosities of granitic melts, melt segregation is presumed to be inefficient at such low degrees of melting. There may exist therefore a range of melt percentages above (φ c over which the partial melt is interconnected but nearly stagnant.
Transpression is a process that thickens the crust and therefore in
obliquely convergent orogens as in normally convergent orogens there is
the potential to generate granitic melts. Individual transcurrent shear
zones may not only control the ascent paths, siting, and emplacement
mechanisms of plutons, but they may also cause the genesis of the
granitoids themselves. Two contrasting situations are examined. In the
Hercynian shear zones of Iberia, thickening together with hydrous
fluxing along shear zones created intracrustal wet melting of fertile
Gondwanan sediments to produce syntectonic granites. In the northern
part of the British Caledonides, the association of compositionally
expanded granitoids with a major mantle component and transcurrent shear
zones may be explained by melting of continental crust at the lower
limits of crustal transpressional faults detaching into the Moho.
Continental plates represent rheologically heterogeneous media in which complex finite strain fields may develop due to interaction of plate tectonic processes with intraplate heterogeneities. Such a deformation pattern is displayed by the Borborema shear zone system in northeastern Brazil. It involves continental-scale, curvilinear, E-W trending right-lateral transcurrent shear zones. This strain field results from the compressional deformation of a highly heterogeneous continental lithosphere composed of a stiff domain (craton) and rheologically weaker domains (basins). The interaction between imposed boundary conditions and the internal structure of the plate may give rise to highly heterogeneous strain fields, as exhibited by the Borborema shear zone system, in which intraplate rheological heterogeneities induce the development of branched or sinuous shear zones. -from Authors
Material strain softening is commonly taken as a necessary and sufficient condition for localization in deforming rocks. However, there is a wide range of experimental and theoretical information which shows that localization can occur in sands, brittle rocks and ductile metals under strain-hardening conditions. This paper aims to bring these two contrasted views together. Three separate criteria are necessary in order to understand localization behaviour. The first involves the stability of the deforming system. The second determines whether a deforming system will undergo bifurcation so as to cease deforming in a homogeneous mode and instead deform in an inhomogeneous mode such as barrelling or localization. The stability and bifurcation criteria are independent of each other since barrelling is a stable mode whereas localization is unstable. The third criterion establishes if the unstable bifurcation mode is one of localization or of some other kind. Localization may arise from the presence of vertices on the yield surface (as in the case of pressure insensitive, rate dependent metals and in brittle rocks due to the development of preferred microfractures for slip) or from the constitutive relation being such that the plastic strain-rate vector is not normal to the yield surface (as in the cases of pressure sensitive, dilatant rocks, of materials deforming by crystal-plastic processes involving dislocation cross-slip and/or climb, and of visco-plastic materials in which voids are forming due to diffusive processes). It is important to distinguish between material and system softening (or hardening) behaviour. The theory for a kinematically unconstrained shortening experiment (that is, rigid, frictionless platens) indicates that localization can occur in strain-hardening materials but the system must strain-soften from then on; that is, localization occurs at peak stress for the system even though the material may continue to harden (or soften). However, the addition of kinematic constraints (such as friction at elastic platens, a constraint to deform in plane strain or at constant volume) means that localization may occur in a system that is monotonically strain hardening. Shear zones in naturally deformed rocks show ample evidence of dilatant behaviour in that evidence for the passage of large volumes of fluid during localization is common as is the development of dilatant vein systems. As such, since shear zones are strongly constrained by the elastic and (limited) plastic response of the relatively undeformed rocks surrounding the shear zones, strain-hardening behaviour of the system is to be expected as the norm, even if the rocks within the shear zones are undergoing material strain-softening.
In a lithospheric-scale, orogen-parallel transcurrent shear zone of the Pan-African Dom Feliciano belt of southern Brazil, two successive generations of magmas, an early calc-alkaline and a late peraluminous, have been emplaced during deformation. Microstructures show that these granitoids experienced a progressive deformation from magmatic to solid state under decreasing temmperature conditions. The geochemical evolution of the synkinematic magmatism suggests an early lower crustal source or a mixed mantle/crustal source, followed by middle to upper crustal source for the melts. Shearing in lithospheric faults may induce partial melting in the lower crust by shear heating in the upper mantle, but, whatever the process in initiating partial melting, lithospheric transcurrent shear zones may collect melt at different depths. These zones may act as heat conductors (by advection), promoting an upward propagation of partial melting in the crust. Synkinematic granitoids localize most, if not all, deformation in the studied shear zone. -from Authors
The Maclaren Glacier metamorphic belt is an exhumed portion of a deep-crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalite sill (the Valdez Creek tonalite) was emplaced into the shear zone while it was active. A one-dimensional thermal model for sill-shaped plutons of varying thicknesses, which are emplaced into country rocks at different intial temperatures, indicates that melt can be present for long periods of time in the deep crust (>1 m.y.) If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. -from Authors
In the Borborema province of northeast Brazil, neoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. In some cases magma emplacement was clearly favored by shear zone activity, but for several plutons this model is not satisfactory. In these plutons, pre-full crystallization strike-slip deformation, evidenced by parallelism of magmatic foliations and lineations with the solid-state mylonitic fabric, and by a transition from magmatic to solid-state flow, is restricted to the vicinity of the shear zones. Evidence of shear zone activity prior to magma emplacement is lacking, and the magmatic foliation away from the shear zones is in most cases shallowly dipping and concordant with the slightly older, gently-dipping, regional gneissic foliation. Field and anisotropy of magnetic susceptibility mapping, together with petrographic and geochemical studies performed in one of the magmatic complexes of the Borborema province have revealed a structure and a magmatic fabric incompatible with the shear zone-controlled emplacement model. Away from the shear zones, this complex has retained a stratification inherited from the mixing of crystal-poor magmas of contrasting composition, and a magmatic fabric characterized by low-to moderate-dip magmatic foliations bearing a NW-trending lineation, which contrast with vertical foliations bearing NE- or E-W-trending stretching lineations in the shear zones and indicates that crystallization started prior to shear zone development.
A sequence of distinctive stratigraphic units has been defined in a succession of sillimanite-grade metasedimentary schists, gneisses, and granulites in eastern and central New Hampshire north of the 44th parallel. The units are correlated and continuous with the formations of the Rangeley area, Maine, to the northeast. The same sequence of units has been mapped immediately south of latitude 44 degree and is firmly correlated southward with a succession of units in south-central New Hampshire.
During the Late Proterozoic (Brioverian), a large number of plutonic and volcanic units were emplaced in the N part of the Armorican Massif (France); all these series are of calc-alkaline affinities and are now considered as the strongest argument for the existence of a subduction zone, at that time, in this area. Two domains can be distinguished, based on the petrographic and geochemical characters of the rocks series: 1) the N domain is interpreted as an island arc where the magmas are of primary (mantellic) origin (M); 2) the S domain where the magmas are mainly of secondary (crustal) origin (C). This variation in origin is coupled with a decrease in the age of the magmatic units (from the N to the S) according to a classic scheme in the subduction zones.-English summary
The purpose of this chapter is to describe briefly and summarize changes in the mineral assemblages and mineral compositions that are observed in various rock types along the metamorphic field gradient in the metamorphic high of central Massachusetts and southwestern New Hampshire, USA. The progressive metamorphism of rocks of basaltic composition is discussed here more extensively than for rocks of common pelitic compositions, because detailed descriptions of the progressive metamorphism of basaltic rocks at these metamorphic conditions are less common. This chapter is aimed, in part, at non-petrologists and postgraduate students. As a consequence, some basic subjects are discussed, including some general comments concerning P-T-t (pressure-temperature-time) paths and metamorphic field gradients. However, some aspects should interest petrologists looking for meatier topics.
An analytical expression for the steady state thermal structure of thrust belts and subduction systems shows that surface erosion rate, basal accretion rate, and upper plate heat production are extremely important in controlling the temperature and metamorphic history of orogenic belts and accretionary prisms, especially where upper plate thickness (h) is large. For sufficiently large magnitudes of upper plate heat production or sufficiently rapid erosion rates inverted (negative) geothermal gradients are produced in the lower part of the upper plate and the upper part of the lower plate. The potential importance of erosion and accretion is illustrated by two examples: beneath the Himalayas and within the upper plate of the Peru trench. Shear heating along thrust faults or subduction boundaries is not necessary to produce the high metamorphic temperatures or surface heat flows observed in these two examples, and the generation of large amounts of heat by shear stresses on the faults appears to be inconsistent with some of the observations made in these areas. -from Author
An integrated geochronological/microstructural investigation at the Willimantic dome, Connecticut, has determined the age of gneissic and mylonitic deformation fabrics in Avalonian basement rocks and two generations of metamorphism and deformation in overlying metamorphic cover cover rocks. Geochronological data define a sharp discontinuity in isotopic ages about the basement-cover contact and indicate that basement and cover record different tectonic and thermal histories until they were juxtaposed along extensional shear zones in the late Paleozoic. Combined with other evidence it is proposed that Avalonian crust was emplaced beneath the Acadian metamorphic belt in the Carboniferous during tectonic convergence. The subsequent heating and gravitational collapse of the thickened, composite structural section led to crustal melting (270-285 Ma), extensional faulting (265-280 Ma), and possibly the removal of structural section between basement and cover as these rock groups were juxtaposed in the Permian for the first time. -from Authors
Following the Middle Devonian Acadian deformation an extensive belt of high grade metamorphism was formed in New England. In south-western Maine, at the northern end of this belt, there occurs a transition along the strike from regional low-pressure/high-temperature metamorphism to contact metamorphism in low-grade rocks. Petrological studies indicate that this transition occurs along a surface plunging to the north-east at about 3.5°, with respect to the Middle-to-Late Devonian erosion surface. In addition, detailed petrological mapping has defined a history of temporally separate, localized metamorphic events associated with plutonism and occurring at increasingly deeper levels to the south-west. Geochronological studies constrain ambient temperatures in the transition zone at the time of metamorphism to be less than 300° C in the north-east and between 350° C and 500° C in the south-west. They also establish a pattern of diachronous cooling due to differential uplift and erosion, with cooling occurring later and most rapidly to the south-west. Geophysical evidence suggests that along with this spatial variation in metamorphic style the shapes of the plutons in Maine undergo a transition from laterally extensive sheet-like bodies in the high grade terrane to more equant-shaped bodies in the low-grade terrane. Using the results of these petrological, geochronological and geophysical studies, as well as those of stratigraphical and structural studies we construct a thermal model for the transition zone. The model suggests that the Acadian metamorphism in south-western Maine is a result of deep-level contact metamorphism near laterally extensive granitic sills dipping to the north-east with respect to the present erosion surface. The plutons themselves are interpreted to be a result of lower crustal melting in response to crustal thickening in the presence of normal or slightly augmented mantle heat flux.
Major Cadomian structures of northern Brittany, France, are reviewed from the Léon region in the west to the Mancellian region in the east. The whole region is dominated by a NE trending structural grain. From the NW to the SE three main tectonic units have been distinguished: (a) the Trégor volcanic and plutonic belt; (b) the Baie de St Brieuc metamorphic-thrust belt and © the Guingamp-St Malo high temperature belt. Principal strain trajectories, finite strain measurements, shear criteria and geochronological data are used to establish a kinematic model of Cadomian deformation. The three units have been thrust over each other and towards the SW at 590-580 Ma. An extrapolation at a regional scale is made using available geophysical data (gravimetry, magnetism and SWAT seismic lines). A new model for oblique thrusting toward the SW along an ENE trending plate boundary is proposed.
This review concerns intrusive magmatism related to the Cadomian orogeny in the northeastern part of the Armorican Massif of France. The Cadomian orogeny is interpreted to represent tectono-thermal activity in a continental margin above a subduction zone. The North Armorican Shear Zone (NASZ) separates two major Cadomian terranes, the North Armorican Composite Terrane (NACT) and the Central Armorican Terrane (CAT). The amalgamation of three terranes within the NACT (the St Brieuc Terrane (SBT), the St Malo Terrane (SMT) and the Mancellian Terrane (MT)) around 540 Ma by sinistral transpression is a key element in the Cadomian orogeny. Cadomian plutonic complexes intrude the c. 2000 Ma Icartian Gneisses and form the local basement to, and are emplaced into the Brioverian succession. Cadomian magmatism spans approximately 275 Ma, from c. 700 Ma to c. 425 Ma.
Early Cadomian foliated plutonic complexes occur within the Guernsey-La Hague structural block of the SBT; they represent volcanic arc granites derived in a subduction zone environment. Late Cadomian post-tectonic plutonic complexes within the Guernsey—La Hague structural block were emplaced c. 500 Ma, exhibit a variety of ‘mixed magma’ features and have a normal to mature continental arc geochemistry. To the southeast, the Jersey structural block within the SBT represents a higher structural level as well as a greater distance away from the probable trench site. Here, the late Cadomian posttectonic plutonic complexes were emplaced during Cambrian and Ordovician-Silurian times and represent mature continental arc activity, probably associated with the decay of the Cadomian subduction zone system after transpressional terrane accretion.
The St Malo Terrane, to the southeast, represents an inverted intra- or behind-arc basin. The migmatite belts within this terrane developed by crustal anatexis. Emplacement of homogeneous diatexites and leucogranites synkinematically with sinistral strike-slip movement along shear belts generated by the transpressive terrane accretion allows this event to be dated at c. 540 Ma.
The Mancellian Terrane, further to the southeast, comprises low-grade Brioverian metasediments into which has been emplaced the Mancellian Batholith, a late Cadomian post-tectonic series of predominantly granite complexes. These complexes have the geochemical features of volcanic arc granites.
Overall, Cadomian magmatism represents c. 275 Ma of plutonic activity broadly related to subduction beneath a continental margin. The early part of the Cadomian orogenic cycle includes volcanic-arc plutonic suites and basaltic volcanic rocks with associated feeder dykes. The peak of the Cadomian orogeny involves intra- or behind-arc basin inversion with concomitant mid-crustal anatexis and sinistrally transpressive terrane accretion c. 540 Ma. The late evolution begins with andesite-rhyolite volcanism and calcalkaline plutonism occurs over a period of c. 100 Ma during the decay of the Cadomian subduction zone system.
The St Cast shear zone, situated in Northern Brittany, is a crustal-scale sinistral wrench zone of Cadomian age. We describe the associated deformation history on the basis of a detailed analysis of structures, microstructures and quartz fabrics of rocks outcropping at the Pointe de St Cast.
The study area shows rocks of dominantly granodioritic composition which are intruded by a leucogranite (the Roche de St Cast). Strain measurements, foliation trajectories and outcrop-scale shear zones reveal a heterogeneous pattern of plane-strain type, associated with a bulk sinistral transcurrent shearing within both intrusion and country rocks. rocks.
Local shear zone patterns are however different within the two units. Country-rocks show decimetre scale shear zones anastomosed around low strain domains. In contrast, the intrusion shows a more homogeneous structure with a single set of parallel pervasive millimetre scale shear bands (sinistral C-S structures). These differences are interpreted in terms of different local kinematics and bulk thermal history as expected between a magmatic intrusion and its surrounding country-rocks during syntectonic emplacement. The overall structural pattern emphasizes that the Roche de St Cast is a syntectonic granite emplaced within a major zone of Cadomian sinistral wrenching.
The Precambrian basement of NE Brittany contains a number of NNE-trending migmatite belts of Cadomian (c. 540 Ma) age. These are heterogeneously reworked by subvertical, ductile shear zones which occur both within individual migmatite belts, and along their boundaries with low-grade Brioverian metasediments. The shear zones are characterized by blastomylonitic fabrics, penetrative sub-horizontal or gently-plunging stretching lineations, and shear criteria which indicate a consistent subhorizontal, sinistral strike slip sense of movement. The migmatite belts, and the greenschist facies Brioverian metasediments imbricated with them along the strike-slip zones, form the St Malo Terrane deformed at c. 540 Ma. This terrane is bounded to the NW and SE by two lithologically and metamorphically distinct terranes, the St Brieuc and Mancellian terranes respectively. Boundaries between the three terranes are inferred to be crustal scale sinistral strike-slip zones. The St Brieuc Terrane was also deformed and metamorphosed in a transpressive setting either prior to or during amalgamation with the St Malo Terrane at c. 540 Ma. The Cadomian belt of northern Brittany is interpreted as a sinistrally transpressive strike-slip belt related to deformation in the hanging-wall of an oblique subduction zone located to the north, and across which there is likely to have been substantial lateral displacement.
The finite difference code FLAC is used to examine the distribution of regions of high and low mean normal stress (or pressure) and of maximum dilation around deforming, periodic shear zones. The assumption is made here that the fluid pressure is equal to the mean normal stress. Fluid flow is favoured by large pressure gradients, and is enabled by regions of dilatancy. It is commonly assumed that regions of high dilation are necessarily associated with regions of low pressure. However, it is shown here that this need not be the situation. Cases in which maximum dilation is associated with the maximum pressure may be useful for understanding the presence of periodic melt segregations whereas cases in which maximum dilation is associated with minimum pressure may be useful for understanding metamorphic differentiation during crenulation cleavage development.
The age of regional metamorphism and partial melting in the late Proterozoic succession of the St Malo area has been established at around 540 Ma. Whole-rock homogenization of the Sr isotope systems has been achieved only in the anatectic granites, whereas the associated metatexitic and diatexitic gneisses remain highly heterogeneous. At lower grade than the sillimanite isograd, metasedimentary gneisses contain detrital zircons which record the regional metamorphic event as a U-Pb lower intercept age (536 + or - 14 Ma). In the sillimanite zone U-Pb zircon lower intercept ages are around 500 + or - 10 Ma, and suggest a recent loss of radiogenic Pb from the observed overgrowths. In their regional context, the St Malo migmatites can be considered as resulting from late stage crustal melting in the Cadomian orogenic belt, following genesis of calc-alkaline magma at an active margin around 600 Ma ago. An age of 540 Ma for partial melting in the St Malo migmatites places the end of the Cadomian orogeny near the Precambrian-Cambrian boundary.- from Author
The North Armorican composite terrane, NW France, is a collage of displaced terranes which result from the amalgamation of Cadomian continental arcs and marginal basin complexes by sinistral transpression along a continental margin above a subduction zone. Early Cadomian arc activity occurred at c. 700-650 Ma, but terrane accretion did not occur until c. 540 Ma, and post-tectonic magmatism persisted well into the Palaeozoic. Cadomian events thus span a considerably greater period than previously supposed.
Enhanced ductility is expected if deformation occurs during phase changes in minerals or during metamorphic reactions. Transformation-enhanced ductility largely arises from stresses generated by the transformation and may be an important consideration during mantle deformation. Reaction-enhanced ductility is a more complex process reflecting softening due to fluid release, increased diffusivities and the formation of small, strain-free grains. It is suggested that deformation should preferentially occur during the development of new minerals and may be inhibited by subsequent grain growth.
The Late Cretaceous to early Tertiary Great Tonalite Sill is a very long (~1000km) and thin (<25km), orogen-parallel, composite batholith, which lies between the Insular superterrane and the Intermontane superterrane. The remarkable narrowness of the Great Tonalite Sill is probably the result of melting at the base of a very localized zone of thicknened crust produced by the associated narrow contractional shear zone extending along the orogen length. Such a shear zone of Late Cretaceous to early Tertiary age, lying along 800km of the possible boundary between the Insular and Intermontane superterranes, implies that it may represent the actual boundary between them. If this is correct, it implies that the large-scale tectonic regime during emplacement of the Great Tonalite Sill was predominantly orthogonal and not obliquely dextral as has been indicated from paleomagnetic data. -from Authors
Experiments at 900 °C and 1.4 GPa show that the distribution of aqueous fluid in fine-grained feldspar aggregates changes from isolated pores under hydrostatic conditions to mostly wetted grain boundaries during deformation; the isolated pore distribution is rapidly regained during annealing following deformation. The deformation-enhanced fluid distribution causes a switch from dislocation creep to diffusion creep accompanied by a significant decrease in strength; it also increases the bulk transport rate through the aggregate by more than an order of magnitude. A change in fluid distribution is not observed in fine-grained quartz aggregates deformed at similar conditions. If deformation-enhanced fluid distribution occurs in naturally deformed feldspathic rocks, it could help to explain the localization of strain and enhancement of bulk transport in ductile shear zones.
The interplay among mechnical, chemical, hydrological, and thermal
processes in the evolution of crustal shear zones makes an analytical
approach to their study difficult. As an alternative, a stochastic
description (using percolation theory) is used to gain insight into
reaction softening and volume loss in ductfle deformation zones.
Directed percolation is preferred to ordinary percolation as a model
because, in common with natural shear zones, directed percolation
clusters have high length/width ratios and anisotropic permeability. In
addition, transport along the cluster length (for p
> p c is linear with time (modeling
fluid advection). The process of strain softening is modeled by
subjecting a critical cluster to variable amounts of simple shear,
resulting in a geometry similar to that of natural shear zones. A
stochastic model for the evolution of porosity with time on a directed
lattice displays fixed-point behavior for a range of pore-collapse
rates. The observation that natural shear zones from a variety of
tectonic settings display mean volume losses of 60%-70%; suggests that
the system naturally evolves toward a critical state. This can be
explained by assuming that pore collapse is regulated by the tendency
for fluid pressure to remain close to lithostatic. Volume loss in
crustal shear zones appears to be an example of hydromechanical-chemical
self-organized critical behavior.
Granitic rocks are here described as mixtures of weak and more resistant fractions whose proportions vary with strain and temperature and control some macroscopic characteristics of shear structures produced during deformation. Three main structural fields are distinguished on a strain/temperature diagram: (1) discrete shear zones anastomosed around low-strain domains, (2) microscale associations of pervasive foliation and ductile shear bands (e.g., C-S structures), and (3) "steady state" homogeneous foliations. We outline the following: a sharp drop in bulk strength is expected to occur around 500 °C within wet granites; the widespread occurrence of pervasive ductile shear bands is diagnostic of retrograde deformation histories, especially as encountered within syntectonic plutons; and most prograde deformation histories, as expected during reworking of pretectonic basement, should produce heterogeneous strain patterns associated with discrete shear zones.
Fabrics within a 1-km-wide shear zone along the eastern margin of the Bronson Hill anticlinorium in central Massachusetts provide important information on the regional tectonic development during the Devonian Acadian orogeny. Two distinct mineral lineations observed within the zone are each defined by alignment of coarse sillimanite needles and elongated quartz and feldspar grains and aggregates. A progressive change between the two deformation fabrics is suggested by evidence of overlap between the two, by their striking similarity in style and metamorphic conditions of formation, and by the similarity in orientation of their bulk strain axes, with a change only in the transport direction between them. The kinematics and relationships between them suggest a revised tectonic model for the later part of the Acadian orogeny and for development of orogen-parallel lineations in this region. -from Authors
The mechanical behavior of partially molten Westerly granite was
investigated in the temperature range 800°-1100°C, 250 MPa
confining pressure, by means of constant strain rate, creep, and stress
relaxation tests. The only water in the samples came from the breakdown
of hydrous phases, biotite, minor chlorite and muscovite and alteration
products of feldspars. Thus the amount of melt was controlled by the
test temperature and ranged from 3% at 800°C to 50% at 1100°C.
Over that temperature range, strength decreased from ≈500 MPa to less
than 1 MPa, and a preliminary constitutive flow law for the partially
molten rock was obtained to allow extrapolation to low strain rates. The
comparative viscosity of the melt alone was estimated at 950° and
1000°C from the distance it could be made to penetrate into a porous
sand under a known pressure gradient. Under all conditions, deformation
of the matrix of solid grains was by brittle fracture only. Samples
containing up to 10 vol % melt failed with the formation of a shear
fault zone. At higher melt fractions, melt-filled "pores" collapsed by
shear-enhanced compaction, squeezing the melt into axial cracks. Above
40 vol % melt, unfractured solid grains were carried about passively in
the flowing liquid. There was no evidence of a "rheologically critical
melt percentage" in this system. By analogy with the uniaxial compaction
of water-saturated soils, a simple model is erected to describe a
two-stage process for the extraction of granitic melts from their
protoliths with the aid of nonhydrostatic stress. Shear-enhanced
compaction is inferred to drive melt into a network of melt-filled
veins, whereupon porous flow through the high-permeability vein network
allows rapid drainage of melt to higher crustal levels.