Dike intrusion under shear stress: Effects on magnetic and vesicle fabrics in dikes from rift zones of Tenerife (Canary Islands)

A theoretical model predicting how anisotropy of magnetic susceptibility (AMS) and vesicle fabrics are modified by shear stress resolved on the dike walls prior to the final cooling of magma is developed for vertical dikes. The resulting fabrics are asymmetric with respect to initial fabrics assumed to be symmetric. Application of this model together with collected data on magma flow direction, dike propagation direction and mechanism, and shear sense, allow us to interpret dike fabrics in terms of shear resolved on the dike walls during intrusion (en echelon arrangement, offsetting, and dike curvature). The interpretation of AMS and vesicle fabrics of the margins of four dikes shows a reasonable agreement with the proposed theoretical models, suggesting that asymmetric fabrics can be used to infer magma flow and may provide valuable information on the shear resolved on the dike walls during intrusion.     

Unraveling magnetic fabrics

The anisotropy of magnetic susceptibility has been proven to be an excellent indicator for mineral fabrics and therefore deformation in a rock or sediment. Low-field anisotropy is relatively rapid to measure so that a sufficient number of samples can be measured to obtain a good statistical representation of the magnetic fabric. The physical properties of individual minerals that contribute to the observed magnetic fabric include bulk susceptibility and intrinsic anisotropy of the mineral phase, its volume concentration, and its degree of alignment. Several techniques have been developed to separate magnetic subfabrics arising from magnetization types, i.e., ferrimagnetism, antiferromagnetism, paramagnetism, and diamagnetism. Susceptibility anisotropy can be measured in low or high fields and at different temperatures in order to isolate a particular subfabric. Measuring the anisotropy of a remanent magnetization can also isolate ferrimagnetic fabrics. A series of case studies are presented that exemplify the value of isolating magnetic subfabrics in a geological context. It is particularly useful in rocks that carry a paramagnetic and diamagnetic subfabric of similar magnitude, such that they negate one another. Further examples are provided for purely paramagnetic subfabrics and cases where a ferrimagnetic subfabric is also identified.     

广东河台金矿田糜棱岩磁组构对剪切带变形与成矿的约束

河台金矿田是广东省内规模最大的金矿,由于金矿床都产于糜棱岩带中,因而被认为是典型的与剪切带有关的金矿。然而,对于Au矿化与剪切带变形条件之间的关系目前仍不十分清楚。高村和云西是河台金矿田两个代表性的金矿床,分别受ML11和ML9两条剪切带的控制。本文在对两条剪切带中的含矿和无矿糜棱岩的变形特征进行野外调查和室内岩（矿）相学观察的基础上,分别开展岩石磁学和磁化率各向异性（AMS）特征研究。糜棱岩x- T曲线显示了磁黄铁矿的居里温度,结合磁滞回线、等温剩磁（IRM）以及矿相学特征,表明河台金矿田糜棱岩几乎不含磁铁矿,载磁矿物主要为黑云母和磁黄铁矿,但是磁黄铁矿的贡献要远大于黑云母。两条糜棱岩带中不含矿糜棱岩的平均磁化率Km＜500×10-6 (SI)、磁化率各向异性度Pj、椭球体形状参数T、磁面理F和磁线理L没有明显差别。并且形成两条剪切带的主应力方向也近于一致,最大主应力方向σ1（K3）为近水平NNW- SSE向,最小主应力σ3（K1）为NEE- SWW向,倾伏角较缓。运动学涡度Wk值也近似,在0. 12~0. 65之间,小于0. 71。上述特征暗示两条含金剪切带都是在印支期NNW- SSE向近水平的强烈挤压作用下形成的,其中发育的糜棱岩为S型和SL型构造岩,具有强烈压扁的特征,并且在剪切变形过程中纯剪切作用所占的比重要明显大于简单剪切。然而,两条剪切带中含矿糜棱岩与无矿糜棱岩的AMS特征有明显的区别。含矿糜棱岩的K1近于直立,且Km＞500×10-6 (SI)和Pj值都明显大于无矿糜棱岩。因此,糜棱岩的变形强度与含矿性具有一定的正相关关系,而在燕山期叠加的脆韧性变形使得最大磁化率主轴（即最小主应力轴）变陡,更有利于热液的运移和最终成矿。     

Secondary cleavages in ductile shear zones

Secondary cleavages developed at late stages in ductile shear zones show several features that are inconsistent with progressive simple shear in the zone. These are: the orientation of a single secondary cleavage oblique to the shear zone boundaries; conjugate sets with opposite senses of shear, and multiple sets with the same sense of shear. These features can be explained if the bulk flow is partitioned into slip along discrete failure planes parallel to the primary foliation (S), coaxial stretching along the foliation, and spin.     

Progressive refolding in ductile shear zones

The foliation formed in a ductile shear zone can become folded by continuing shear in the zone without the foliation having to enter the shortening sector of the flow field. If the foliation lies parallel to the shear plane, infinitesimal variations in the rate of shear strain cause compensating rotations that may amplify into folds. Small increases in the rate cause backward rotations (counter to the sense of shear): these do not amplify. Small decreases cause forward rotations that do amplify.     

Fabric asymmetry and shear sense in movement zones

Asymmetry between lattice preferred orientation and shape fabrics is commonly observed in microstructural studies of many deformed mineral aggregates (e. g. ice, carbonate rocks, peridotites and quartzites). Theoretical predictions are limited by the nature of the models available and more can be gained by discussion of the mechanisms which give rise to such asymmetry in specific natural and experimental situations. Asymmetry is often related to the sense of shear but there are other factors to be considered as well. It is concluded that fabric asymmetry can be a useful tool for the analysis of flow regimes in naturally deformed rocks, as long as the conditions prerequisite to successful application of this technique can be shown to have existed, and cognizance is taken of the limitations of the technique.     

Study of ductile shear zones (Galicia, Spain) using texture goniometry and magnetic fabric methods

A joint magnetic and mica fabric study has been carried out on rocks from recognised shear zones in Galicia (Spain). The two main aims of the study are to determine strain gradients in a foliated material without any significant strain markers and to obtain a quantitative relationship between the magnetic susceptibility anisotropy ellipsoid and the mica fabric axes. The intensity parameters of the fabric on profiles intersecting the studied areas indicate the dominant influence of two major shear zones on the fabric development. The regional fabric shows less significant variations across minor (less developed) shear zones.The empirical relationship between magnetic fabric and strain reported by Rathore (1979, 1980) and Rathore and Henry (1982) is found to hold in the magnetic and mica fabrics comparison. The relationship found in this study is:     

A theory of geological faults and shear zones

Concepts of materials science are applied in an attempt to describe the growth and properties of shear or fault zones which are associated with deep focus earthquakes. Aspects of fracture toughness, dynamic recrystallisation and superplasticity are used to model the observed characteristics of such zones. The fault or shear zone is considered to be composed of very fine grained rock which is capable of deformation by the mechanism of “superplasticity” such that it has little resistance to shear as compared with the surrounding rock mass. This “superplastic crack” will therefore have an associated region of concentrated shear stress around its perimeter which, at typical rock temperatures of greater than half the melting point, will continuously promote dynamic recrystallisation. In this way the fault is self-propagating until growth velocities are reached which do not allow sufficient time for the recrystallisation process to occur. The intermittent activity of earthquakes is thus in qualitative agreement with the theory.     

西藏阿里雅鲁藏布江缝合带韧性剪切带的磁组构特征

应用磁组构测量方法,厘定、划分了札达地区印度河—雅鲁藏布江缝合带内的韧性剪切带,两条强韧性带分别位于缝合带北缘和南缘,均具有南盘(下盘)俯冲、平面右旋扭动运动特征和压扁型应变特征。韧性剪切发生于65Ma以前,它们应是印—欧两大板块俯冲-碰撞剪切应变产物,是板块缝合带的重要组成部分。     

Influence of shear heating on microstructurally defined plate boundary shear zones

In plate-boundary scale ductile shear zones defined by microstructural weakening, shear heating may lead to a temperature increase over 5 m.y. of up to 80 °C just below the brittle ductile transition, up to 120 °C just below the Moho, and to thermal boundary zones tens of km wide on either side of the shear zone. Where rock strength is highest, shear zones are narrow (∼1 km), and thermal gradients within the shear zone itself are low, so there is no tendency for increased localization. Heating results in thermal weakening, but this is partly offset by grain growth related to the decrease in stress. In shear zones of the order of 10 km width, shear stress, strain rate, and hence heat generation are lower, and thermal gradients are insufficient to cause additional strain localization. Temperature increases in the mid-crust are of the order of 10 °C, insufficient to cause partial melting or an increase in metamorphic grade. In the upper mantle, shear zones may be 50 km or more wide, and the temperature increase is less than 20 °C in 5 m.y., but temperature differences between center and margin may enhance the strain rate at the center by up to 18%.     

Sheath fold development in monoclinic shear zones

We use numerical simulations to investigate the evolution of sheath folds around slip surfaces in simple‐shear‐dominated monoclinic shear zones. A variety of sheath fold shapes develops under general shear, including tubular folds with low aspect ratio eye patterns and tongue‐like structures showing bivergent flanking structures in sections normal to the sheath elongation, which may potentially lead to confusing shear sense interpretations. Not all investigated monoclinic flow end‐members lead to the development of sheath folds sensu stricto (folds with apical angle <90°). The aspect ratio of the eye patterns, R_yz, correlates with the ratio between the principal strain in the Y‐direction and the smaller of the principal strains in the X–Z plane, and thus it could be used in strain analysis.     

Large shear zones with no relative displacement

In the Oman ophiolite, the large scale Makhibiyah shear zone, in Wadi Tayin massif was generated with no or little relative motion between the two adjacent blocks, in contrast with what is reported from otherwise similar shear zones in deep crust and upper mantle. This shear zone is asymmetrical with, along one margin an asthenospheric mantle (～1200 °C) and along the adjacent margin, a lithospheric mantle (～1000 °C). Within the hotter side and with increasing shear strain, horizontal flow lines smoothly swing towards the shear zone direction before abutting against the wall of the lithosphere side. Profuse mafic melts issued from the hotter mantle are frozen in the shear zone by cooling along this lithospheric wall. Tectonic and magmatic activities are entirely localized within the asthenospheric compartment. Mantle flow lines were rotated, during their channelling along this NW‐SE shear zone, in the NW and SE opposite directions. Depending on whether the flow lines are deviated NW or SE, dextral or sinistral shear sense is recorded in the shear band mylonitic peridotites. This demonstrates that the shear zone was not generated by strike‐slip motion, a conclusion supported by regional observations.     

Analysis of deformation paths in shear zones

The geometry of ductile shear zones can be used to solve problems of regional tectonics if the deformation path of material in the zones is sufficiently understood. Flow in many shear zones may have deviated from simple shear and consequently data on the final deformation state such as finite strain and volume change are insufficient for reconstruction of the deformation path, even if flow parameters were constant during progressive deformation. Additional information on the flow vorticity number is also needed and can be obtained from fabric elements such as sets of folded-boudinaged veins, rotated porphyroblasts and blocked rigid objects. Mohr circle constructions are presented as a tool to calculate deformation parameters from fabric data, to represent the deformation path graphically and to reconstruct flow parameters from the shape of this path. If the vorticity number or the volume change rate changed during progressive deformation, the deformation path can be partly reconstructed using sets of fabric elements which register mean and final values of these parameters.

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Zusammenfassung Die Geometrie duktiler Scherzonen kann dazu verwendet werden, regionaltektonische Probleme zu lösen, sofern der im Material abgebildete Deformationsweg dieser Zonen genügend verstanden wird. Der Flow in vielen Scherzonen mag sich aus einfacher Scherung ableiten lassen, daher genügen Daten über den letzten Deformationszustand wie z. B. der finite Strain und die Volumenänderung nicht für die Rekonstruktion des Deformationsweges, auch nicht bei konstanten Fließparametern während der fortschreitenden Deformation. Zusätzliche Daten über die Verwirbelungszahl (Rotationszahl) sind nötig. Sie lassen sich ableiten aus verschiedenartigen Gefügeelementen wie Gruppen gefaltetboundinierter Gänge, rotierten Porphyroblasten und »verklemmten«, starren Objekten. Vorgestellt werden anhand von Gefügedaten Konstruktionen am Mohr'schen Spannungskreis zur Bestimmung der Deformationsparameter, um damit den Deformationsweg graphisch darzustellen und daraus die Fließparameter abzuleiten. Änderungen in der Rotationszahl (Vorticity) oder der Geschwindigkeit in der Volumenänderung während progressiver Deformation erlauben den Deformationsweg zumindest teilweise zu rekonstruieren. Verwendung finden hierbei diejenigen Gefügeelemente, die sowohl die durchschnittliche Deformation als auch die letzten Deformationsereignisse registriert haben.

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Résumé La géométrie des shear zones ductiles peut être utilisée pour résoudre des problèmes de tectonique régionale, pour autant que l'histoire de la déformation des matériaux de ces zones soit suffisamment bien comprise. Il peut arriver, dans beaucoup de shear zones, que le processus ductile se soit écarté du modèle du glissement simple et qu'en conséquence, les caractères finals de la déformation, tels que l'ellipsoïde de la déformation finie, ou le changement de volume, s'avèrent insuffisants pour pouvoir reconstruire l'histoire de la déformation et ce, même si les paramètres de fluage sont restés constants au cours du processus. Il est alors nécessaire de disposer d'informations supplémentaires quant à la vorticité; ces informations peuvent être fournies par certains éléments structuraux tels que des groupes de veines plissées-boudinées, des porphyroblastes qui ont toruné et des objets rigides bloqués. Au moyen de constructions appliquées au cercle de Mohr, il est possible de calculer les paramètres de la déformation à partir des données structurales, de représenter graphiquement l'histoire de la déformation et de retrouver les paramètres de fluage à partir de la forme de cette représentation. Si, au cours de la déformation progressive, la vorticité ou le taux de variation de volume se modifient, l'histoire de la déformation peut être reconstituée partiellement par l'utilisation de groupes d'éléments structuraux qui enregistrent les valeurs moyenne et finale de ces paramètres.

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含金剪切带的类型划分及成矿机理

含金剪切带型金矿床是一种重要的新型矿床类型。本文系统地总结了含金剪切带的特征,提出了含金剪切带的几何学和运动学分类方案,并以成矿源,成矿场、成矿期和成矿相等方面论述了含金剪切带型金矿床的剪切动力成矿机制,建立了过渡型和叠加型两类动力成矿模式。     

Strain and shape-fabric variations associated with ductile shear zones

The foliated and compositionally-banded granitic orthogneisses in the central core of the Maggia Nappe, a Lower Pennine basement nappe of the Central Swiss Alps, are shown to be the sheared equivalent of late-Hercynian age granitic intrusions. These ductile shear zones show mineral assemblages in amphibolite facies, are Alpine in age and form an anastomosing network enclosing remnant lozenge-shaped pods of relatively undeformed rock.The foliation developed within the shear zones concomitantly with a change in shape of quartz grain aggregates from initially equidimensional, through ‘tear-drop’ shapes, to ribbon-like aggregates. These shape changes occurred by intracrystalline glide together with intercrystalline slip on deformation-induced planar surfaces.     

A structural-geomorphological interpretation of Southern California shear zones

The neotectonic pop-up structures of Southern California were studied using the structural-geomorphological method, which included the analysis of topographic maps that were constructed on the basis of digital elevation models. The data we obtained are compared with the geological structure and the kinematic model of the block structure of the region based on the analysis of the current movements in accordance with the GPS data. The applicability of this method for neotectonic reconstructions of shear zones is considered. It is shown that the structures that are identified this way may be correlated with the kinematic model of the Southern California block structure that was developed using the GPS data.     

Geological aspects of deformation in continental shear zones

A method of kinematic analysis of structures, microstructuresand mineral preferred orientations, initially devised in the study of peridotites, has been applied to crustal rocks bearing evidence of large strains produced in metamorphic environments. Three tectonic lineaments (Angers-Lanvaux, Montagne Noire and Maydan) were selected. They illustrate a general situation arising in continental crusts when they are deformed by ductile transcurrent fault systems.The Angers-Lanvaux structure is bilaterally symmetric; its dominant feature is the horizontal stretching lineation which is parallel to the fold axes. The foliation and slaty cleavage in the most surficial formations wrap around the axis of the whole structure. The folds in the slates away from the axis also exhibit axes parallel to the general trend, but no stretching lineations. These folds are attributed to crustal shortening in a direction normal to the ductile fault. In the Montagne Noire recumbent folds are thrusted away from the axis of the structure over at least 25 km. The metamorphism is also centered on the structure and symmetrically reduced away from it. The core of the structure is occupied by a strongly lineated orthogneiss, cut by a late intrusive granite. The Maydan axial zone displays clear evidence of partial melting at various scales within the deformed gneisses: (1) in gashes perpendicular to the stretching lineation which in these anatectic formations tends to plunge at more than 45°; (2) in bands of deformed pegmatites; and (3) possibly in granites which on the one hand intrude the surrounding formations and on the other converge with increasing deformation on the fault zone. The quartz preferred orientations and microstructures in quartzite layers from Angers indicate that the plastic flow plane and direction lie, respectively, close to the foliation and lineation, the slight departure is ascribed to a flow with a rotational shear component.All this suggests a general model for the origin of such ductile zones. The horizontal relative displacement of crustal blocks along a ductile band is responsible for its overall steeply dipping foliation and horizontal lineations. Viscous heating progressively tends to concentrate the plastic flow along its axis. It is also responsible for the development of metamorphism and of anatexis at depth; the partially melted rocks tend to rise, building at shallower depth the arched structure in the axis of the ductile zone, with a continuing flowage parallel to this axis probably now in the solid state; they can also intrude the surrounding terrain as undeformed batholiths. The folds parallel to the stretching lineation in the axial zone are explained by the fact that, due to the escape of anatectic melts, the formations at depth flow in a narrowing channel. The upwelling of the axial structure induces a compression with folding in the surrounding sedimentary formations and gravity nappe sliding away from the axis.     

Neoformations and transformations of clay minerals in tectonic shear zones

Summary Neoformations and transformations of clay minerals in tectonic shear zones of the Penninikum (Hohe Tauern/Austria) were investigated by X-ray diffraction, optical and X-ray microanalytical methods.The shear zones developed in granite-gneisses, gneiss-phyllonites and mica-schists. Low-temperature solution transfers led to chemical alteration of the tectonically crushed rocks and to the formation of clay minerals in shear zones.The distribution of clay minerals and the alteration sequences were evidence of three successive alteration stages. These alteration stages can be characterized by kaolinization, illitization and montmorillonitization.The argillation processes, together with tectonic compression, caused a decrease in permeability and were in this way the main controlling factor for the environment during the development of tectonic shear zones.

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Neubildungen und Umwandlungen von Tonmineralen in tektonischen Scherzonen

Zusammenfassung Neubildungen und Umwandlungen von Tonmineralen in tektonischen Scherzonen aus dem Penninikum (Hohe Tauern/Österreich) wurden röntgendiffraktometrisch, lichtoptisch und röntgenmikroanalytisch untersucht.Die Scherzonen entwickelten sich in Granitgneisen, Gneisphylloniten und Glimmerschiefern. Durch niedrigtemperierten Lösungsumsatz erfolgten in den Scherzonen eine chemische Umwandlung des tektonischen Gesteinszerreibsels und die Bildung von Tonmineralen.Aus der Tonmineralverteilung und der Untersuchung von Umwandlungsreihen ließen sich drei aufeinanderfolgende Umwandlungsstadien erkennen. Diese können durch Kaolinisierung, Illitisierung und Montmorillonitisierung charakterisiert werden.Die Tonbildungsprozesse bewirkten gemeinsam mit der tektonischen Einengung eine Verringerung der Permeabilität und steuerten hauptsächlich auf diese Weise die Milieubedingungen während der Entwicklung von tektonischen Scherzonen.

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With 14 Figures     

Theoretical and natural strain patterns in ductile simple shear zones

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

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

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

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

Stretch in shear zones: implications for section balancing

The stretch, S, of a line in a zone of general shear deformation is given by S = Wsinθ /sinθ', where the shear zone's final to initial width ratio is W and the line's inclination to the zone boundary is θ before deformation and θ' after. For the special cases of simple shear and pure shear, S = sinθ / sinθ', and S² = sin 2θ / sin 2θ', respectively. Fisher's and Sorby's formulae may be combined in a form appropriate to general shear strain, γ = cotθ' - Rcotθ, where R is the axial ratio of the irrotational component of deformation and γ is the shear strain; for simple shear and pure shear the equation reduces to γ = cotθ' - cotθ, and tanθ = Rtanθ, respectively. These relationships may be used to estimate strain in exposed faulted strata, or to restrict the possible geometries of inferred fault ramps in balanced geological cross-sections.