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.     

Transpressive Structures in the Ghadir Shear Belt,Eastern Desert,Egypt: Evidence for Partitioning of Oblique Convergence in the Arabian‐Nubian Shield during Gondwana Agglutination

Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt. The Ghadir Shear Belt is a 35 km-long, NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic. Within this shear belt, strain is highly partitioned into shortening, oblique, extensional and strike-slip structures at multiple scales. Moreover, strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains. In the East Ghadir and Ambaut shear belts, the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated. These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones. The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones. The earlier fabric(S1), is locally recognized in low strain areas and SW-ward thrusts. S2 is associated with a shallowly plunging stretching lineation(L2), and defines ～NW-SE major upright macroscopic folds in the East Ghadir shear belt. F2 folds are superimposed by ～NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation. F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt. The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones. Dextral ENEstriking shear zones were subsequently active at ca. 595 Ma, coeval with sinistral shearing along NW-to NNW-striking shear zones. The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt. Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments. Upright folds, fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning. The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.     

Charts for estimating strength parameters from slips in homogeneous slopes

When the slope geometry, unit weight and pore water pressure distribution in a homogeneous soil slope are given, the location of the critical slip surface for a particular method of slices is related only to the ratio of c′ (cohesion) and tan ϕ′ (internal friction angle). To utilize this relationship in back calculating c′ and ϕ′ values, two slope models with circular slip mechanism are presented to represent shear failures in simple fill or cut slopes and long natural slopes, and for each of the models, two dimensionless parameters regarding c′/tan ϕ′ and maximum depth of the critical slip surface are introduced. Based on the new findings from limit equilibrium analysis of slope models, charts are plotted to show the relationships of the two dimensionless parameters and ϕ′ values for a safety factor of unity. Using the charts can make a rapid estimate of the magnitude of both c′ and ϕ′ from a known position of the failure surface in a homogeneous slope.     

Structural and P-T Evolution of a Major Cross Fold in the Central Zone of the Limpopo High-Grade Terrain, South Africa

The Central Zone (CZ) of the Limpopo Complex of southern Africais characterized by a complex deformational pattern dominatedby two types of fold geometries: large sheath folds and crossfolds. The sheath folds are steeply SSW-plunging closed structureswhereas the cross folds are north–south-oriented withnear-horizontal fold axes. In the area south of Messina thiscomplexly folded terrain grades continuously towards the southinto a crustal-scale ENE–WSW-trending ductile shear zonewith moderate dip towards the WSW. All sheath folds documentconsistent top-to-the-NE thrust movement of high-grade material.The timing of this shear deformational event (D₂) and thus ofthe gneissic fabric (S₂) is constrained (at     

Strain analysis of deformed granitic rocks (Helikian), Muskoka District,Ontario

Three-dimensional analysis of irrotational, longitudinal, finite strain was carried out on samples from a crescentic sheet which intruded and was deformed with a host gneiss unit of probable Helikian age. Analytical methods were compared using deformed feldspar grains representing four ideal degrees of strain intensity observed in the porphyritic sheet. The polar plot and R_f/φ, R_s methods proved most reliable and sensitive.Data derived from fabric and strain analysis at 38 sites in the units suggest a two-stage deformational sequence. The first stage produced recumbent, isoclinal, similar (class 2) folds with northwest-trending hinge surface traces. This fold form was modified during the second stage to produce an overall type 2 fold interference pattern. The second stage produced upright, open buckle folds as well as the resultant strain fabric currently observed. Strain analysis confirms the general fold geometries of the model, and also documents competency contrasts between the matrix and feldspar grains with increased strain intensity and magnitude. Deformation of feldspar grains in the sheet involved modification of a fabric of low strain magnitude (?_s = 0.3) and a k value near unity to magnitudes of ?_s = 2.6 and k = 0.6. Matrix strain intensities and magnitudes are consistently higher than those of the feldspar markers in the sheet. This variation is related to competency differences between the matrix and the feldspar grains. Fabric anisotropy accounts for the strain gradient observed between the sheet and gneiss.     

Kilometre-scale sheath folds in the western Betics (south of Spain)

The three-dimensional exposures of the Sierra Blanca marbles and neighbouring rocks in the western Betic cordillera of the south of Spain show tight to isoclinal F₃ folds re-folded by a larger sheath-like structure. The relationships between both structures are as follows: (1) The mean axial surface for Sierra Blanca sheath fold can be defined as the great circle of best fit through the ß-axes of sub-cylindrical F₃ folding domains. (2) The asymmetric F₃-folds indicate a single mainly eastward normal sense of shear. (3) The mean attitude of the axial-plane crenulation foliation, Sc, is also sub-parallel to the mean axial surface for Sierra Blanca sheath fold. The regional context of the Sierra Blanca sheath fold is finally discussed, and a model of a heterogeneous high-strain extensional zone proposed.     

The development of sheath folds in viscously stratified materials in simple shear conditions: An analogue approach

Sheath folds are highly non-cylindrical folds occurring in a variety of geological settings, and have been studied using different approaches. With the present work, we provide a quantitative analysis of the generation and development of sheath folds in a viscously layered system in simple shear conditions. The sheath folds develop from an initial non-cylindrical deflection located on the highly viscous layer. The analogue experiments investigated the influence of (1) variations in the viscosity ratio between the high viscosity layer and the matrix (η_hvl/η_m), (2) variations in the ratio between the amplitude of the initial deflection and the thickness of the high viscosity layer (A_f/T_hvl), and (3) progressive simple shear (γ). The results show that increases in η_hvl/η_m will produce progressively less elongated sheath folds, while increases in A_f/T_hvl will result in more elongated sheath folds. We present regime diagrams with η_hvl/η_m and A_f/T_hvl for different shear strains illustrating under which conditions sheath folds form. In case the original deflection amplitude and layer thickness as well as γ can be retrieved for sheath folds in nature, then their geometry can be used to quantify the effective η_hvl/η_m.     

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe,north-west Ireland

Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S₃ schistosity is developed axial planar to mesoscopic and minor F₃ folds. In areas of low D₃ strain, F₃ fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F₃ orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.     

Extensional shear band development on the outer margin of the Alpine mylonite zone,Tatare Stream,Southern Alps,New Zealand

Schistose mylonitic rocks in the central part of the Alpine Fault (AF) at Tatare Stream, New Zealand are cut by pervasive extensional (C′) shear bands in a well-understood and young, natural ductile shear zone. The C′ shears cross-cut the pre-existing (Mesozoic—aged) foliation, displacing it ductilely synthetic to late Cenozoic motion on the AF. Using a transect approach, we evaluated changes in geometrical properties of the mm–cm-spaced C′ shear bands across a conspicuous finite strain gradient that intensifies towards the AF. Precise C′ attitudes, C′-foliation dihedral angles, and C′–S intersections were calculated from multiple sectional observations at both outcrop and thin-section scales. Based on these data the direction of ductile shearing in the Alpine mylonite zone during shear band activity is inferred to have trended >20° clockwise (down-dip) of the coeval Pacific-Australia plate motion, indicating some partitioning of oblique-slip motion to yield an excess of “dip-slip” relative to plate motion azimuth, or some up-dip ductile extrusion of the shear zone as a result of transpression, or both. Constant attitude of the mylonitic foliation across the finite strain gradient indicates this planar fabric element was parallel to the shear zone boundary (SZB). Across all examined parts of the shear zone, the mean dihedral angle between the C′ shears and the mylonitic foliation (S) remains a constant 30 ± 1° (1σ). The aggregated slip accommodated on the C′ shear bands contributed only a small bulk shear strain across the shear zone (γ = 0.6–0.8). Uniformity of per-shear slip on C′ shears with progression into the mylonite zone across the strain gradient leads us to infer that these shears exhibited a strain-hardening rheology, such that they locked up at a finite shear strain (inside C′ bands) of 12–15. Shear band boudins and foliation boudins both record extension parallel to the SZB, as do the occurrence of extensional shear band sets that have conjugate senses of slip. We infer that shear bands nucleated on planes of maximum instantaneous shear strain rate in a shear zone with W_k < 0.8, and perhaps even as low as <0.5. The C′ shear bands near the AF formed in a thinning/stretching shear zone, which had monoclinic symmetry, where the direction of shear-zone stretching was parallel to the shearing direction.     

鄂东南铁山不对称鱼嘴状石香肠构造基质层中的应变分析

利用惯量椭圆法对鄂东南铁山不对称鱼嘴状石香肠构造基质层中不同位置的方解石进行二维有限应变测量。运动学涡度以及相关参数的等值线分布图表明:不对称鱼嘴状石香肠构造基质层中的应变分布是不均匀的。基质层中,靠近断开的能干层处的剪切变形中纯剪切占优势,伸长方向趋向于平行剪切方向;而靠近连续的能干层处的剪切变形中简单剪切占优势,伸长方向趋向于垂直剪切方向。     

Progressive development of structures in a ductile shear zone

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

The structural evolution of sheath folds: A case study from Cap de Creus

It has long been recognised that within zones of intense non-coaxial deformation, fold hinges may rotate progressively towards the transport direction ultimately resulting in highly curvilinear sheath folds. However, there is a surprising lack of detailed and systematic field analysis of such “evolving” sheath folds. This case study therefore focuses on the sequential development of cm-scale curvilinear folds in the greenschist-facies El Llimac shear zone, Cap de Creus, Spain. This simple shear-dominated dextral shear zone displays superb three dimensional exposures of sheath folds defined by mylonitic quartz bands within phyllonite. Increasing amounts of fold hinge curvature (δ) are marked by hinge segments rotating into sub-parallelism with the mineral lineation (Lm), whilst the acute angle between the axial-planar hinge girdle and foliation (ω) also displays a sequential reduction. Although Lm bisects the noses of sheath folds, it is also clearly folded and wrapped-around the sheath hinges. Lm typically preserves a larger angle (θ) with the fold hinge on the lower limb (L) compared to the upper (U) limb (θL > θU), suggesting that Lm failed to achieve a steady orientation on the lower limb. Adjacent sheath fold hinges forming fold pairs may display the same sense of hinge arcing to define synthetic curvature, or alternatively opposing directions of antithetic curvature. Such patterns reflect original buckle fold geometries coupled with the direction of shearing. The ratio of long/short fold limbs decreases with increasing hinge curvilinearity, indicating sheath folds developed via stretching of the short limb, rather than migrating or rolling hinge models. This study unequivocally demonstrates that both hinges of fold pairs become curvilinear with sheaths closing in the transport direction recording greater hinge-line curvilinearity compared to adjacent return hinges. This may provide a useful guide to bulk shear sense.     

Side-stepping of axial surface traces in superposed folding

During the refolding of an early non-isoclinal fold (say,F ₁) we may find an offset or side-stepping of the axial surfaces of the later folds (say,F ₂). The offsets can be seen in both type 2 and type 3 interference patterns. An analysis of the shear fold model shows that there is a maximum limit for the magnitude of side-stepping. The side-stepping is larger for larger interlimb angles ofF ₁. It decreases with progressive tightening ofF ₂. By recognizing such side-stepping we can predict on which side the F₁ hinge should lie even if the hinge is unexposed or lies outside the domain of observation. The general rule for the sense of side-stepping is the same for shear folds, flexural slip folds and buckling folds. However, the side-stepping in buckling folds should be used with caution, sinceF ₂ folds on buckled single-layers may show an offset whose sense is opposite to that predicted by the general rule.     

Geometry of sheath folds and related fabrics at the Luikonlahti mine,Svecokarelides, eastern Finland

The Luikonlahti Cu-Co-Zn sulphide ore deposit is hosted by metasediments associated with serpentinites in the 1.97 Ga old Outokumpu assemblage in the Svecokarelides of eastern Finland. Polyphase deformation of the host rocks, a history shared by the ore body, includes a phase of sheath fold propagation. A modified vergence rule, utilizing only the intersection geometry of planar fabric elements, permits recognition of these extremely curvilinear folds in poorly exposed terrain. The detailed geometry of these rocks is independently resolved from borehole and underground stope records. Sheath fold propagation occurred during D₂, the second phase of regional deformation. In the Kaavi district D₂ major structures are either thrusts or thrust-related. The Luikonlahti sheaths are located in a steeply dipping shear zone formed during this deformation episode.     

Parallel stretching lineations and fold axes oblique to a shear displacement direction—a model and observations

Spatial variations in shear strain rate are expected in ductile shear zones. Where the variation is a change in shear strain perpendicular to the displacement direction, the effect is to rotate the shear slip planes. This is a mechanism for giving a rotation of fold axes into parallelism with the slip and extension direction in a rock. If such a variation in shear strain affects rocks with a strong planar anisotropy it is possible to produce a fabric with an apparent stretching lineation parallel to fold axes, but both significantly oblique to the slip direction. A possible example of this is seen in strongly deformed quartz-mica schists from Syros, Greece, where a stretching lineation is seen parallel to fold hinges over a range of fold axes orientations of at least 40°.     

Evolution of Eastern Ghats granulite belt of India in a compressional tectonic regime and juxtaposition against Iron Ore Craton of Singhbhum by oblique collision — transpression

Three major episodes of folding are evident in the Eastern Ghats terrain. The first and second generation folds are the reclined type; coaxial refolding has produced hook-shaped folds, except in massif-type charnockites in which non-coaxial refolding has produced arrow head folds. The third generation folds are upright with a stretching lineation parallel to subhorizontal fold axes. The sequence of fold stylesreclinedF ₁and coaxialF ₂, clearly points to an early compressional regime and attendant progressive simple shear. Significant subhorizontal extension duringF ₃folding is indicated by stretching lineation parallel to subhorizontal fold axes. In the massif-type charnockites low plunges ofF ₂folds indicate a flattening type of deformation partitioning in the weakly foliated rocks (magmatic ?). The juxtaposition of EGMB against the Iron Ore Craton of Singhbhum by oblique collision is indicative of a transpressional regime.     

Structural evolution of a briançonnais cover nappe,the caprauna-armetta unit (ligurian alps,Italy)

The Caprauna-Armetta Unit (CAU) is a Briançonnais cover nappe emplaced on the external margin of the Ligurian Briançonnais Zone. A structural analysis of the nappe indicates that there are four superposed deformations (D₁-D₄). D₁ produced large recumbent isoclinal folds associated with a strong axial-plane cleavage and a SW-trending lineation. These folds can be related to a SW-directed overthrust shear. D₂ produced open to moderately tight folds with subvertical axial planes, overturned towards the northeast. D₃ and D₄ are represented by large wavelength open folds affecting only the large-scale setting of the nappe.A finite strain map of the nappe has been compiled using data from an oolitic limestone layer. The measured strains appears to be essentially the product of the D₁ phase. The measured ellipsoids are generally triaxial. The trend of the finite strain X axes is towards the southwest. Prolate ellipsoids with very high R_xz ratios occur on the inverted limbs and sometimes near the hinge zones of the anticlinal F₁ folds. Oblate ellipsoids are prevalent on the normal limbs. This pattern of finite strain resulted from deformation in a ductile shear zone generated within the tectonic units trailed at the base of the huge Helminthoid Flysch Nappe during its motion towards the foreland.     

Interaction of flexural shear,S–C fabrics,and oblique shear during folding of micaceous quartzite

This paper investigates the geometry, microstructure, and c-axis fabrics of an outcrop scale, micaceous quartzite fold produced under greenschist facies metamorphic conditions in the Moeda quartzite, Quadrilátero Ferrı́fero granite–greenstone terrain, southeastern Brazil. The fold limbs show development of opposed S–C fabrics and asymmetric quartz c-axis fabrics compatible with flexural slip along the fold surface. Towards the fold hinge, there is an increasing presence of oblique shear bands (here named S-bands) which gradually change to crenulations within the hinge zone. The oblique S-bands are interpreted to have formed through connection of several S-planes, increasing accommodation of antithetical shear along these S-planes and offset of the initial C-planes at intermediate stages of folding. This mechanism represents a kinematic inversion in the role played by the two sets of foliations in S–C structures. Our observations support flexural slip for early stages of folding. However, with progressive closure of the fold, the flexural slip mechanism involves increasing contributions from oblique shear on the S-bands, thus approximating an intermediate situation between flexural slip and passive folding (shear parallel to the axial plane).     

斜向逆冲断层相关褶皱的正演模型与实例分析

斜向逆冲作用在自然界普遍存在,研究斜向逆冲断层相关褶皱的构造几何学特征,识别断层相关褶皱是否存在斜向逆冲有重要意义。文章采用Trishear 4.5、Gocad以及Trishear3D软件构建一系列不同滑移量的断层转折褶皱和断层传播褶皱的二维正演剖面,通过连接一系列不同排列方式的二维剖面建立了三种不同逆冲滑移方向的断层转折褶皱和断层传播褶皱的假三维模型,通过不同假三维模型的比较分析来探讨斜向逆冲断层相关褶皱的构造几何学特征。研究发现,斜向逆冲断层相关褶皱区别于正向逆冲断层相关褶皱的特征主要有两点：① 正向逆冲断层相关褶皱层面等高线图上的最高点与后翼等高线中点的连线以及水平切面上的核心点与后翼中点的连线方向均与断层走向垂直,而斜向逆冲断层相关褶皱的最高点以及核心点与后翼中点的连线方向均与断层走向斜交,并且最高点与后翼等高线中点的连线方向或者核心点与后翼中点的连线方向均与逆冲滑移方向一致;② 在褶皱平行断层走向纵剖面上,正向逆冲断层相关褶皱各个层面最高点的连线是直立的,而斜向逆冲断层相关褶皱各个层面最高点的连线发生倾斜。通过这两个特征可以判别褶皱是否存在斜向逆冲以及逆冲的方向。将模型分析结果运用到四川盆地西南部三维地震勘探资料所覆盖的邛西背斜和大兴西背斜的实例中。研究结果表明,两个背斜均存在右旋斜向逆冲,逆冲方向与各自断层走向的夹角均为70°左右,邛西背斜和大兴西背斜的逆冲方向分别是NE79°和NE77°左右,这与龙门山南段晚上新世以来的主应力方向以及反演的汶川地震最大主应力方向一致。