Strain geometry,microstructure and mineral chemistry in metagabbro shear zones: a study of softening mechanisms during progressive mylonitization

The strain geometry, microstructure and metamorphism is described from two minor shear zones from the Chatelaudren metagabbro, N. Brittany. A serially slabbed shear zone reveals a strain geometry consistent with simple shear deformation. Strain calculations based on X trajectory angles coincide with those obtained from elliptical mineral clusters at high values of strain only. Strain profiles typically show a broad low-strain region with a narrow high-strain peak at the centre of the zones. Microstructures typically show distinct grain size reduction in both amphibole and feldspar towards the high strain region of shear zones, and this is discussed in terms of deformation mechanisms related to strain softening. A palaeostress estimate based on recrystallized feldspar grain sizes gives a differential stress of 32 MPa for the low strain region and 119 MPa in the shear zone centre. Electron probe analyses reveal chemical and mineralogical changes accompanying metamorphism within the shear zones. This suggests local conditions favourable for ionic diffusion and the activity of fluids is implied.     

Sustainable transpression: An examination of strain and kinematics in deforming zones with migrating boundaries

Most orogenic belts owe their development to oblique convergence and commonly have many orogen-parallel transpressional high-strain zones. To constrain the tectonic history of orogenic belts by structural and fabric analysis of rocks, it is desirable to understand quantitatively the relationship between the boundary conditions and the resulting strain distribution and kinematics in these zones. Most current models for transpression assume homogeneous deformation confined by boundaries that are fixed to material planes. This creates a strain compatibility problem at the margins of the active deforming zone and also requires that the strain rate normal to the zone boundaries increase to implausibly high values soon after the onset of oblique convergence (transpressional motion). The latter contradicts with the observation that transpressional motion can be sustained throughout an orogeny. The assumption that zone boundaries are fixed to material planes is unrealistic. The outstanding problems of current transpressional models are resolved in this paper by allowing the zone boundaries to migrate through the rock material. The consequence of zone boundary migration for the strain field and kinematics within a transpressional high-strain zone is investigated mathematically. The implications of the modeling for fabric interpretation are discussed. The modeling makes general predictions consistent with observed planar and linear fabric patterns in natural transpressional high-strain zones. It predicts that foliations in transpressional high-strain zones are subparallel to the zone boundaries regardless of variation in the imposed boundary conditions. Lineations cluster along the great circle girdle subparallel to the average foliation. The spread of the lineations may vary from point maxima to complete girdles.     

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

剪切带运动学：重点评述三斜模式和区分韧性滑动擦痕与拉伸线理的重要性

详细评述了剪切带运动学和内部几何学研究的新进展。剪切带内的递进变形一般为三斜对称,单斜剪切带(包括简单剪切带)是特例情形。理论模拟表明,如同许多天然剪切带中所见到的,在三斜剪切带中,拉伸线理的方位可从近水平到平行于倾向连续变化。过去一般将拉伸线理的方向当作剪切运动方向的做法是以简单剪切模式为基础的,不适合于一般剪切带。在三斜剪切带中,拉伸线理和剪切方向之间不存在简单关系。然而,C面上的韧性滑动擦痕平行于剪切方向发育,是剪切方向的可靠标志。因此,在剪切带运动学解释中区分韧性滑动擦痕与拉伸线理非常重要。     

Evolution of microstructures in Precambrian shear zones: An example from eastern India

Shear zones are areas of intense deformation in localized zones which can be used as natural laboratories for studying deformation characteristics. Metre to-micro scale structures that develop in response to a progressive simple shear in a shear zone are characterized by a protracted history of deformation and are immensely useful in delineating the history of progressive deformation. To decipher these localized zones of deformation and to establish the continuous non-coaxial character of deformation, detail microstructural studies are very useful. Singhbhum shear zone (SSZ), a regional Precambrian tectonic dislocation zone in eastern India, depicting a top-to-south thrust movement of the hanging wall provides a scope for studying microstructural characteristics developed in response to a progressive shear at mid-crustal level. SSZ is characterized by intense stretching lineation, isoclinal folds, shear planes, superposed schistosity and deformed quartz veins. Quasi-plastic (QP) deformation mechanisms were predominantly active in the SSZ. The overprinting relationship between the earlier and later schistosity with a consistent sense of shear indicates that earlier schistosity is transposed to later schistosity through the intermediate stages of crenulation cleavage during a progressive non-coaxial deformation. The recrystallization of quartz in mylonitic quartzite suggests protracted history of deformation. The analysis of the character of quartz grains of both the porphyroclasts and recrystallized grains suggests that strain was partitioned between the most intensely deformed central part of the shear zone and the shear-related deformation zone outside the central part of the shear zone.     

Testing the model of oblique transpression with oblique extrusion in two natural cases: Steps and consequences

Kinematic models of various types of transpression have been used to explain fabric features and strain in many natural deformation studies. Here, a mathematical model that encompasses all monoclinic and triclinic transpressional deformations including triclinic deformation with inclined simple shear (ϕ) and/or inclined extrusion orientations (υ) can be tested using a step-by-step approach with available field evidence. Two cases are presented. The first case from the Wabigoon–Quetico boundary in the Archean Superior Province utilizes both fabric orientation and quantified strain data. The best fit of the field evidence to the model indicates that deformation likely took place along subvertical shear zones via transpression with subhorizontal simple shear (ϕ = 0–20°) and variable inclined extrusion direction (extrusion can be either east or west and υ typically indicates extrusion orientations between 0 and 50° from vertical). The second case of the South Iberian shear zone has fabric orientation data, but no quantifiable strain possibilities. The best fit of the field evidence to the model indicates that deformation likely took place along a moderately inclined shear zone via transpression with subhorizontal simple shear (ϕ = 0–20°) and variable inclined extrusion direction (υ values between 0° and 80° from the true dip of the shear zone). Using this protocol in other examples of natural deformation will allow further constraints to be applied to kinematic models.     

P-T paths from high temperature shear zones beneath ophiolites

Abstract Mctamorphic rocks of the St Anthony Complex of north-western Newfoundland are best interpreted in terms of a high-temperature shear zone formed between down-going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High-grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta-morphism in high-strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high-strain zones. Mylonite zones in meta-basitcs at all levels in the complex contain the assemblage epidote-hornblende-albite-sodic oligoclase. These observations suggest that the 'inverted metamorphic gradient'within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re-equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub-ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, the P-T paths of which preserve records of burial and exhumation in subduction zones.     

Strain partitioning into dry and wet zones and the formation of Ca-rich myrmekite in syntectonic syenites: A case for melt-assisted dissolution-replacement creep under granulite facies conditions

The formation of Ca-rich myrmekites is described in syntectonic syenites crystallized and progressively deformed under granulite facies conditions. The syenites are found in high- and low-strain zones where microstructure and mineral composition are compared. Heterogeneously distributed water-rich, late-magmatic liquids were responsible for strain partitioning into dry and wet high-strain zones at outcrop scale, where contrasting deformation mechanisms are reported. In dry high-strain zones K-feldspar and clinopyroxene are recrystallized under high-T conditions. In wet high-strain zones, the de-stabilization of clinopyroxene and pervasive replacement of relatively undeformed K-feldspar porphyroclasts by myrmekite and subordinate micrographic intergrowths indicate dissolution-replacement creep as the main deformation mechanism. The reworking of these intergrowths is observed and is considered to contribute significantly to the development of the mylonitic foliation and banding. A model is proposed for strain partitioning relating a positive feedback between myrmekite-forming reaction, continuous inflow of late-magmatic liquids and dissolution-replacement creep in the wet zone at the expenses of original mineralogy preserved in the dry zones. Melt-assisted dissolution-replacement creep in syntectonic environments under granulite-facies conditions may extend the field of operation of dissolution-replacement creep, changing significantly the rheology of the lower continental crust.     

Sequence of Ductile Shear Zones in UHP Metamorphic Province within Dabie Massif, China

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阿尔金北缘地区韧性剪切带型金矿床构造控矿解析

韧性剪切带型金矿是一种成矿机制与控矿因素都与韧性剪切带及其演化密切相关的金矿床类型。阿尔金北缘地区不同层次剪切带发育,金矿床受韧性剪切带控制明显,在区域上,韧性剪切带控制金矿床(点)的分布;在矿床范围内,韧性剪切带及其演化过程中形成的韧脆性剪切带既是唯一的赋(含)矿构造,也对矿化带、矿体的形态、产状、规模及分布起决定性的控制作用;压扭性韧性剪切变形特点决定了金矿化类型以蚀变糜棱岩型为主,交代蚀变作用发育。含矿构造裂隙以P型为主,少量D型和R型,个别为R~1型和T型;构造变形所引发的动力分异作用及其形成的动力变质热液是金矿成矿流体的主要来源之一,这与地球化学和成矿流体包裹体研究显示的大平沟金矿床成矿物质主要来源于变质岩、成矿流体有相当部分来源于变质水的特征相吻合。结合糜棱岩磁组构研究,发现磁各向异性度P值与金元素含量呈负相关关系,说明构造变形早于金矿化蚀变作用,这种时序关系进一步佐证了韧性剪切带型金矿床的成矿模式。即大面积的韧性变形构造动力分异作用形成的含金热液不是就地原位矿化,而是向上迁移并集中到范围比较窄小的韧-脆性或脆性裂隙中才发生金元素富集,最终形成金矿床。     

Triassic high-strain shear zones in Hainan Island (South China) and their implications on the amalgamation of the Indochina and South China Blocks: Kinematic and Ar/Ar geochronological constraints

A kinematic and geochronological study has been carried out on the Triassic high-strain shear zones in Hainan Island, the southern South China Block. There are WNW- and NE-trending high-strain shear zones with greenschist- to amphibolite-facies metamorphism in this island. Kinematic indicators suggest a dextral top-to-the-NNE thrust shearing for the WNW-trending high-strain shear zones and a sinistral top-to-the-SE thrust shearing for the NE-trending shear zones. The quartz c-axis orientations of mylonitic rocks exhibit the domination of basal slip and some activation of a rhombohedra gliding system. The timing of shearing for these shear zones has been constrained by the ⁴⁰Ar/³⁹Ar dating analyses of synkinematic minerals. Middle Triassic (242–250 Ma) and late Triassic–early Jurassic (190–230 Ma) have been identified for the WNW- and NE-trending shear zones, respectively. A synthesis of these kinematic and thermogeochronological data points to a two-stage tectonic model for Hainan Island, that is, top-to-the-NNE oblique thrusting at 240–250 Ma followed by top-to-the-SE oblique thrusting at 190–230 Ma. In combination with the available data from the southern South China and Indochina Blocks, it is inferred that South Hainan and North Hainan have affinity to the Indochina and South China Blocks, respectively. The tectonic boundary between South Hainan and North Hainan lies roughly along the WNW-trending Changjiang–Qionghai tectonic zone probably linking to the Song Ma and Ailaoshan zones. The middle Triassic structural pattern of Hainan Island is spatially and temporally compatible with those of the South China and Indochina Blocks, and thus might be a derivation from the amalgamation of the Indochina with South China Blocks in response to the closure of the Paleotethys Ocean and subsequent subduction/collision.     

甘肃北山南带韧性剪切带型金矿床构造控矿解析

通过研究不同层次的韧性剪切带与金矿化的关系及含金裂隙类型。结合地质地球化学和磁组构分析。探讨了韧性剪切带对金矿化的控制作用。认为韧性剪切带的控矿作用表现为在区域上控制金矿床（点）的分布，在矿床范围内，它既是唯一的赋（含）矿构造，也控制着矿化带和矿体的形态，产状，规模和分布，在矿床成因上决定了金矿化以蚀变糜棱岩型为主，同时韧性剪切变形及其形成的动力变质热液是金矿成矿流体的来源之一。     

Displacement features associated with fault zones: a comparison between observed examples and experimental models

Fault zones commonly consist of discontinuous surfaces or are composed of different kinds of en echelon fractures (tension cracks, R or P-type fractures) which give the brittle shear zone a width. Translation along these faults and related features can occur by sliding on the elementary planes and/or by solution/deposition processes with opening of transtension zones and possible dilatation of the shear zone.In order to understand how fault planes develop and to investigate the mechanical conditions corresponding to natural configurations, displacement along an analogue fault model was examined under conditions of direct shear. The experimental fracture patterns were then compared with the natural features. The structural elements of certain faults created by testing, and showing transpression and transtension zones, were found to be similar to natural domino structures bounded by elementary shears, and could be compared with computed situations.It can be inferred that stresses are reoriented inside the shear zone; the angle between elementary fractures depends on their order of development; transpression and transtension zones occur systematically; and the shear zone undergoes dilatancy under low normal stresses.     

结构面力学性质的定量鉴定

变形带力学性质的鉴定是地质力学研究中的先行基础步骤.近来出现一些新的概念和方法,可用以定量表征变形带的力学性质.天然变形带通常是一般剪切作用的产物,是纯剪切(共轴缩短或伸展)和简单剪切的组合.为了定量说明两者间的相对贡献,提出了运动学涡度(W_k)这一物理量,并简单地定义为cos υ.υ是主变形面内两特征方向间的夹角.纯剪切的υ=90°,W_k=0;简单剪切的υ=0°,W_k=1.一般剪切的υ介于0°和90°之间,W_k为0到1.运动学涡度符号的正负分别代表变形带的减薄和增厚.υ可通过极摩尔圆法求出.主压应力(σ₁)方向与W_k的关系为W_k=sin2ξ.ξ是σ₁与变形带法线间的夹角.因此,可用以确定变形带的W_k和力学性质.根据最大有效力矩准则,韧性变形带与主压应力(σ₁)方向间的夹角为55°,可用以确定古应力轴的方向,并可能确定变形时差应力的大小.      

Deformation conditions, kinematics, and displacement history of shallow crustal ductile shearing in the Norumbega fault system in the Northern Appalachians, eastern Maine

The Norumbega fault system in the Northern Appalachians in eastern Maine experienced complex post-Acadian ductile and brittle deformation from middle through late Paleozoic times. Well-preserved epizonal ductile shear zones in Fredericton belt metasedimentary rocks and granitic batholiths that intrude them provide valuable information on the nature, geometry, and evolution of orogen-parallel strike-slip Norumbega faulting. Metasedimentary rocks were ductilely sheared into phyllonite schistose mylonite, whereas granite into mylonite within the ductile shear zones. Ductile shearing took place at conditions of the lower greenschist facies with peak temperatures on the order of 300–350° based on comparison of plastic quartz and brittle feldspar microstructures, confirming a shallow crustal environment during faulting.Ductile shear strain was partitioned into two major shear zones in easternmost Maine—the Waite and Kellyland zones—but these zones converge toward the southwest. Megascopic, mesoscopic, and microscopic kinematic indicators confirm that fault motion in both zones was dominantly dextral strike-slip. Detailed mapping, especially in the plutonic rocks, reveals a complex ductile deformation history in the area where the Waite and Kellyland zones converge. Shear strain is broadly distributed in the rocks between Kellyland and Waite zones, and increases toward their junction. Multiple dextral high-strain zones oblique to both zones resemble megascopic synthetic c′ shear bands. Together with the Kellyland and Waite master shear zones, these define a megascopic S–C′ structure system produced in a regional-scale dextral strike-slip shear duplex that developed in the transition zone between the deeper (south-central Maine) and shallower (eastern Maine) segments of the Norumbega fault system.Granite plutons caught within the strike-slip shear duplex were intensely sheared and progressively smeared into long and narrow slivers identified by this study. The western lobe of the Deblois pluton and the Lucerne pluton have been recognized as the sources, respectively of the Third Lake Ridge and Morrison Ridge granite slivers. Restoration of both granite slivers to their presumed original positions yields approximately 25 km of dextral strike-slip displacement along only the Kellyland and synthetic ductile shear zones.     

Small-scale ductile shear zones as transposed rheologically weak domains: A numerical modeling investigation and practical application

We test the hypothesis that small ductile shear zones are developed from initial rheologically weak domains. We regard weak domains as ellipsoidal inhomogeneities and apply Eshelby's formalism extended for power-law viscous materials to investigate the kinematics and finite strain evolution of the partitioned flow fields in weak domains. We show that, under an imposed bulk flow field, weak domains, regardless of their initial shapes and orientations, deform into zone-like features at relatively small bulk strains and the deformation paths inside weak domains have all characteristics expected in ductile shear zones. We apply our model to the Cap de Creus area Spain, where abundant small ductile shear zones exist. To relate the fabrics inside these shear zones with the regional deformation, we take a multi-scale approach. We assume that the area contained many weak domains which were randomly shaped and oriented initially and were deformed into shear zones eventually in response to the regional flow. We constrain the regional flow field by the fabric patterns across the area and compute numerically the partitioned flow fields in individual weak domains. The latter are related to fabrics inside shear zones. Our model reproduces first-order features of field-observed fabrics. Although the deformation path of each shear zone was close to simple shearing, the deformation of the whole belt was not. Our approach also resolves the strain compatibility problem for a finite-sized shear zone embedded in a far less deformed country rock.     

Strain compatibility and shear zones: is there a problem?

In analyzing deformation in rocks, it is important to ensure that the solutions obtained satisfy strain compatibility. This creates a challenge to understanding patterns of strain associated with shear zones, in which measured strain may appear incompatible with the strain in the shear zone walls. Flattening strains are common in natural shear zones with locally straight and parallel boundaries: to satisfy compatibility conditions such strains require volume loss across the shear zone or deviations from plane strain, with or without discontinuities between the shear zone and the wall rock. In the case of shear zones for which there is no evidence of volume loss or discontinuities along the shear zone walls, problems of strain compatibility may be resolved if individual shear zones are linked together in an appropriate fashion. Shear zones commonly occur in anastomosing arrays, and simple configurations of such arrays and the strains associated with them are examined. It is shown that local transpression with strain compatibility can be accounted for in this way. Quite complex local strain patterns can develop in simple arrays.     

Microstructural study of a natural slip zone: quantification and deformation history

Microstructure of a natural slip zone was comprehensively examined using a combination of images captured systematically by optical microscopy (OPM) and backscattered electron microscopy (BEM) techniques. Microstructural features identified on these images were processed and evaluated using an advanced image analysis system, which proved that quantitative analyses could considerably enhance the understanding of shear behavior of slip zones. It was found that variations of porosity, abundance of platy clay particles and alignments of particles are significant indicators revealing nature of deformation processes. These indicators show that global mechanical behavior of the investigated slip zone can be conceptualized as that of normally consolidated clayey soils under drained conditions.

The geometric patterns of the microstructure of the slip zone are similar to the S–C fabrics seen in tectonic shear zones. It is suggested that combined progressive bulk simple shear and pure shear modes enable to realistically reconstruct the kinematic history of the slip zone, through which particle movements and microstructural evolution were accomplished via various types of particulate flows. The results of this study show that clay mineralogy plays a more important role in the development of the slip zone than abundance of clay-size particles, while both clay mineralogy and relative proportions of each particle size fraction control the response of particles to shear deformation. Among the fractions present in the slip zone, fine silts are the strongest indicator of global shear stress characterized by their highest degree of alignment, whereas clay particles are the weakest. Highest degree of shape preferred orientation is also found within fine silt domains.     

Strain and Crystallographic Fabric in Mesoscopic Ductile Shear Zones of Garhwal Himalaya

The MCT Zone of Bhagirathi valley of Garhwal Himalaya is characterized by numerous mesoscopic ductile shear zones. These shear zones are developed in response to nearly NNE-SSW maximum horizontal compression and provide an opportunity to study the variation in strain and crystallographic fabrics within the ductile shear zones.The grain shape and orientation of quartz under microscope reflect that strain is higher in the center and it progressively decreases towards the shear zone boundary. The preferred orientation of quartz c-axes across the shear zone suggests that the single girdle of the quartz c-axes are probably first developed at the shear zone boundary and become prominent in the center of shear zone with increase in the intensity of deformation. The strong crystallographic preferred orientation normal to foliation suggests that the internal deformation of the quartz might have taken place by dislocation creep mechanism exhibiting a non-coaxial deformation history.     

太行山南段自立庄韧性剪切带变形特征

太行山南段临城自立庄地区古元古界甘陶河群中低级变质岩中发育一条左行逆冲型韧性剪切带。自立庄韧性剪切带出露长约10 km,宽约1 km,走向NNE,往西缓倾,在EW方向上由若干条强变形带与其间的弱变形域或岩块组成,平面上呈现平行式的组合特征。该韧性剪切带内发育糜棱岩、拉伸线理和皱纹线理、不对称褶皱、石香肠构造和构造透镜体、S-C面理和旋转碎斑等宏观和微观构造。S-C面理、旋转碎斑、不对称褶皱等宏微观变形特征一致表明自立庄韧性剪切带上盘由西往东逆冲的运动学性质。在对韧性剪切带宏观、微观构造特征研究基础上,结合区域资料,认为自立庄韧性剪切带的形成与华北克拉通古元古代末期西部陆块与东部陆块的EW向碰撞拼合有关,是18.5 Ga吕梁运动的产物。自立庄韧性剪切带的厘定为太行山南段古元古代构造演化提供了基础资料。