The future of failure: stress or strain?

Strains in rocks can be observed but ancient stresses can only be inferred. We should re-examine the potential of strain geometry as the key to understanding and interpreting common shear structures ranging from faults to plastic shear zones. The concept of failure along zero extension directions can be applied to natural structures in rocks and is predicated on strain compatibility between differently strained volumes. Zero extension directions are considered for two strain configurations, plane strain (k=1) and uniaxial shortening (k=0). The crucial difference between shear fractures, or faults, and plastic yield zones is that the former are preceded by dilatation while the latter are isovolumetric. Volume changes during deformation affect the orientations of zero extension directions and hence of the resulting structures. With isovolumetric strain, yield occurs on planes at 45° to the principal shortening direction in plane strain and at 54.7° to this axis in uniaxial shortening. Uniaxial shortening experiments on rock samples allow estimation of the relative volumetric strains when yield zones initiate. When this volumetric strain is used to estimate the orientation of shear fractures in plane strain, ca 70° dips are predicted for normal faults at high crustal levels, decreasing downwards to 45°.     

Strain refraction in layered systems

Strain refraction across competence contrasts is presented as a simple model consisting of two components, a homogeneous strain and a heterogeneous simple shear. For Newtonian materials, the ratio of the layer-parallel simple shear component in adjacent layers is the inverse of their viscosity ratio. Strong changes in ellipsoid size, shape and orientation are predicted across viscosity contrasts.The geological implications of strain refraction theory are considered within the context of the ‘cleavage/strain debate’. The particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies (and kinematic histories) should appear to be subparallel to the XY planes of measured strain ellipsoids. Competent rocks should develop dominantly layer-orthogonal strain, and incompetent layers shear-dominated deformation. A variety of structural features ranging from cleavage refraction, changing lineation orientations, folds transected by cleavage, changes from coaxial to non-coaxial deformation, and ramp-flat fault geometry may be the result of stress and strain refraction in rocks.     

Relationships between gold concentration and structure in quartz veins from the Hodgkinson Province, northeastern Australia

The Hodgkinson Province is a tract of␣multiply deformed Silurian-Devonian rocks in north␣Queensland, Australia. Gold-bearing quartz veins from the West Normanby Goldfield in the northern Hodgkinson Province were emplaced during the Permian D₄ event, broadly coeval with regional granite emplacement. Taylors Fault, a major structure that formed during D₂, hosts the veins which infill dilatational jogs opened during sinistral-normal reactivation of the fault in D₄. Veins contain graphitic laminations that formed when fault planes segmented wallrocks adjacent to the veins, producing tabular clasts that were tectonically sliced into the reefs. Laminations are the result of progressive shear strain, associated with continued movement on the faults, which caused strain-enhanced dissolution of silicate minerals and residual graphite enrichment in the clasts. This process produced graphite-coated shear planes that delimit zones of grain size reduction in the veins. Laminations commonly contain stylolites, which nucleated on pronounced sinuosities of the shear planes due to progressive shortening during D₄. Gold particles have preferentially nucleated in zones of relatively coarser-grained quartz adjacent to the shear planes, where shortening strain caused microfracturing and allowed fluid access. Gold may have been introduced with the quartz, but was redistributed within the reefs and localized along the laminations by the effects of synchronous, progressive deformation. Regionally, gold deposits show close spatial relationships with granite plutons of the Permian Whypalla Supersuite. Relationships in the West Normanby Gold Field support a regional model of reef emplacement and gold mineralization during the Permian D₄ event. Received: 24 August 1997 / Accepted: 14 October 1997     

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.     

Shear criteria in granite and migmatite deformed in the magmatic and solid states

Microstructural criteria for the determination of the sense of shear in rocks homogeneously deformed in the partially melted state are similar to those which apply to solid-state deformation. Sense of shear determination is either direct, deduced from the sense of rotation of markers, or indirect, involving the obliquity between the shear and foliation planes, or between the successive foliations imprinted at different stages of progressive deformation.This study is a by-product of the detailed structural and microstructural investigation of a high-grade metamorphic rock pile (Variscan Vosges Massif, France) which underwent subhorizontal shearing during partial melting and further solidification. Depending on the rock chemistry, on the position in the pile and the relative timing of progressive deformation, layered migmatites and homogeneous granites were variously deformed in the partially melted and solid states. The sense of shear obtained from these rock types, using the criteria presented here, consistently gives a top to SW direction.     

Shear bands,crenulations and differentiated layering in ice-mica models

Thin sheets of composite ice-mica have been deformed in order to simulate the development of cleavages in quartz-mica rocks. A strong initial mica preferred orientation was variably oriented to the shortening direction. Deformation parallel to the foliation results in a crenulation type cleavage developing from shear bands initiated after a component of pure shear. Deformation oblique to the foliation produces a differentiated cleavage and involves a large component of shear strain subparallel to the original anisotropy. The strain is accommodated by intra- and intercrystalline processes that produce extensive grain elongation and rearrangement of the ductile matrix, thereby forming ice vs mica rich regions. On the other hand, there is no drastic morphological change when a sample is shortened perpendicular to an original foliation: that is, where the micas lie in the plane of no shear strain. Instead, the mica fabric is strengthened and the grains in the ductile matrix are flattened.Two models are presented for the initiation, propagation and evolution of the observed crenulation versus differentiated cleavage types. These depend on mica stacking and orientation relative to the transverse properties of the sample and also on the direction of anisotropy to the XY plane of the bulk strain ellipsoid. The models invoke shear on planes of high shear strain and rotation of the shear bands and rigid mica grains into a direction approximately parallel to the bulk extension direction.     

Finite and incremental deformations recorded by planar subfabrics in S-tectonites

Analysis of relative componental movements in foliated rocks is formulated in terms of space-continuous deformations assuming that a portion of the strain recorded by planar subfabrics results from differential movements on closely spaced shear surfaces (i.e. fiducial planes). Continuous and discontinuous velocity boundary conditions controlling deformation patterns within subdomains of folded layers are analyzed by combining the spatial velocity and finite deformation gradients. Within each subdomain internal rotations cause material elements oblique to the principal strain rate directions to undergo a series of complex strain transfers resulting in their compensatory lengthening and shortening during finite intervals. Equations are derived which continuously monitor successive variations in the logarithmic strain rates, ?(N,t), for fabrics whose rotation axes are parallel to an intermediate principal axis. Values of ?(N,t) at an angle N to the shear plane are numerically equal at time t, to the magnitude of the Hencky strain rate vector ($\text{dh}{}_{\mathrm{i}}\text{dt}$) referred to natural strain coordinates and used in conjunction with $\text{e}\text{?}\text{(N,t)}$ and the finite stretch, evaluate contemporary strain profiles for groups of planar fabrics replacing passive material planes. Applications to rectilinear shearing modes reveal that the most significant changes in local extensional rates are located between the maximum shearing and principal stretching directions. Assuming sectional continuity and constant material properties of the subfabrics and their matrix, these variations are correlated with systematic spacings between boudin structures suggesting that recognition of multiple orders of boudinage with respect to a potential shear surface in natural S-tectonites can be useful in deciphering local finite and incremental deformation coefficients as well as differentiate continuous ($\text{dh}{}_{\mathrm{i}}\text{dt}\text{= ?(t)}$ or constant) from pulsatory ($\text{dh}{}_{\mathrm{i}}\text{dt}$ is undefined at t) overprinting of the subdomain.     

Effects of crack inclination on shear failure of brittle geomaterials under compression

A micromechanics-based approach is proposed to predict the shear failure of brittle rocks under compression. Formulation of this approach is based on an improved wing microcrack model, the Mohr-Coulomb failure criterion, and a micro-macro damage model. The improved wing microcrack model considers the effects of crack inclination angle on mechanical behaviors of rocks. The micro-macro damage model describes the relation between crack growth and axial strain. Furthermore, comparing experimental and theoretical relations between crack initiation stress and confining pressure, model parameters (i.e., μ, a, β, and φ) hardly measured by test are solved. Effects of crack inclination angle, crack size, and friction coefficient on stress-strain relation, compressive strength, internal friction angle, cohesion, shear failure plane angle, and shear strength are discussed in details. A most disadvantaged crack angle is found, which is corresponding to the smallest compressive strength, cohesion, internal friction angle, and shear strength of rocks. Rationality of the theoretical results is verified by the published experimental results. This approach provides a theoretical prediction for effects of microcrack geometry on macroscopic shear properties in brittle rocks under compression.     

The texture of cross-micas in rocks affected by schistosity-parallel displacements

Cross-micas are described in a rock for which relative displacements from sliding on schistosity planes have previously been demonstrated. Individual cross-micas in these rocks have incurred similar displacements along (001) surfaces. Shear strain on planes parallel to (001) in each cross-mica is determined and plotted on a map of the surface being investigated. The distribution illustrates that higher shear strains are found in cross-micas that are close to translation surfaces along which garnet porphyroblasts have been sliced into tabular sections.The study illustrated in this paper could be extended to other types of cross-micas and may prove to be a useful method for the investigation of strain history in foliated rocks.     

The indigenous origin of Witwatersrand “carbon”

In the Witwatersrand approximately 40% of the gold is intimately associated with so-called “carbon” in “carbon seam reefs”, which occur in over a dozen paleoplacers, many of them concentrated at two stratigraphic levels in the 7000-m-thick succession of Archean siliciclastic sedimentary rocks. This is reduced carbon, present as kerogen admixed in various proportions with derivative (now solid) bitumen(s). Oil generation and migration were active geological processes during Early Earth history. Numerous possible source rocks for oil generation, including the carbon seams themselves, occur within the Witwatersrand basin. In the Witwatersrand ore, oil-bearing fluid inclusions are also present, derived like the bitumen, by thermal maturation of the kerogen. The presence of kerogen and bitumen in the Witwatersrand sedimentary rocks, together with a wealth of observations on the spatial distribution of the carbon seams confirm that the carbon originated in situ from living organisms in microbial mat cover, as opposed to flowing in from elsewhere as liquid hydrocarbons as some researchers have suggested. Paleochannels, which truncated auriferous carbon seams early in the depositional history, are of widespread occurrence, and micro-synsedimentary faults offset carbon seams. The carbon seams are thus indigenous biogenic markers that grew contemporaneously with placer development. The various features highlighting the nature and spatial distribution of Witwatersrand carbon seams provide a classic case where field evidence trumps laboratory data in the reconstruction of geological processes.     

Pure shear to simple shear-dominated transpression during the Eburnean major D2 deformation,Daléma sedimentary basin,eastern Senegal

Field studies in the Palaeoproterozoïc Daléma basin, Kédougou-Kéniéba Inlier, reveal that the main tectonic feature comprises alternating large shear zones relatively well-separated by weakly deformed surrounding rock domains. Analysis of the various structures in relation to this major D₂ phase of Eburnean deformation indicates partitioning of sinistral transpressive deformation between domains of dominant transcurrent and dominant compressive deformation. Foliation is mostly oblique to subvertical and trending 0–30° N, but locally is subhorizontal in some thrust-motion shear zones. Foliation planes of shear zones contain a superimposed subhorizontal stretching lineation which in places cross-cuts a steeply plunging stretching lineation which is clearly expressed in the metasedimentary rocks of weakly deformed surrounding domains. In the weakly deformed domains, the subhorizontal lineation is absent, whereas the oblique to subvertical lineation is more fully developed. Finite strain analyses of samples from surrounding both weakly deformed and shearing domains, using finite strain ratio and the Fry method, indicate flattened ellipsoid fabrics. However, the orientation of the long axis (X) of the finite strain ellipsoid is horizontal in the shear zones and oblique within the weakly deformed domains. Exceptionally, samples from some thrust zones indicate a finite strain ellipsoid in triaxial constriction fabrics with a subhorizontal long axis (X). In addition, the analysis of the strain orientation starting from semi-ductile and brittle structures indicates that a WNE–ESE (130° N to 110° N) orientation of strain shortening axis occurred during the Eburnean D₂ deformation.     

Distinctive diamagnetic fabrics in dolostones evolved at fault cores,the Dead Sea Transform

We resolve the anisotropy of magnetic susceptibility (AMS) axes along fault planes, cores and damage zones in rocks that crop out next to the Dead Sea Transform (DST) plate boundary. We measured 261 samples of mainly diamagnetic dolostones that were collected from 15 stations. To test the possible effect of the iron content on the AMS we analyzed the Fe concentrations of the samples in different rock phases. Dolostones with mean magnetic susceptibility value lower than −4 × 10⁻⁶ SI and iron content less than ∼1000 ppm are suitable for diamagnetic AMS-based strain analysis. The dolostones along fault planes display AMS fabrics that significantly deviate from the primary “sedimentary fabric”. The characteristics of these fabrics include well-grouped, sub-horizontal, minimum principal AMS axes (k₃) and sub-vertical magnetic foliations commonly defined by maximum and intermediate principal AMS axes (k₁ and k₂ axes, respectively). These fabrics are distinctive along fault planes located tens of kilometers apart, with strikes ranging between NNW-SSE and NNE-SSW and different senses of motion. The obtained magnetic foliations (k₁–k₂) are sub-parallel (within ∼20°) to the fault planes. Based on rock magnetic and geochemical analyses, we interpret the AMS fabrics as the product of both shape and crystallographic anisotropy of the dolostones. Preferred shape alignment evolves due to mechanical rotation of subordinate particles and rock fragments at the fault core. Preferred crystallographic orientation results from elevated frictional heating (>300 °C) during faulting, which enhances c-axes alignment in the cement-supported dolomite breccia due to crystal-plastic processes. The penetrative deformation within fault zones resulted from the local, fault-related strain field and does not reflect the regional strain field. The analyzed AMS fabrics together with fault-plane kinematics provide valuable information on faulting characteristics in the uppermost crust.     

Are S–C structures,duplexes and conjugate shear zones different manifestations of the same scale-invariant phenomenon?

By measuring S spacing, C spacing and the S–C angle (α) in deformed rocks, this paper investigates the geometry of previously published examples of S–C and S–C-like structures on a scale range between micrometres and several hundred kilometres. The results indicate that common S–C fabrics of thin-section, hand-specimen and outcrop scale, and conjugate fault/mylonite zones of map scale define a simple function C_spacing=2S_spacing, which depicts a scale-invariant geometry over ten orders of magnitude. Logarithmic plots of cumulative frequency suggest that the S–C fractal set (D=0.13) is restricted to the scale range between 600–800 μm and 1 km where genuine S–C structures, characterized by antithetic shear on the S planes, can be formed. Below 600–800 μm, grain scale processes seem to influence the development of S–C structures. Above the upper limit (1 km), only S–C-like structures with duplex kinematics (synthetic shear on S planes) occur. The S–C and S–C–C′ fractals are envisaged as self-similar structures where the foliations work as both S or C planes, depending on which scale is considered.     

Calcite texture development in experimentally induced ductile shear zones

Specimens of fine grained micritic limestone were deformed in plane strain geometry in pure shear, a combination of simple and pure shear, and in simple shear. Temperatures were 400° C and 500° C, confining pressure was 100 MPa. In the experiments with a simple shear component strain is concentrated and approximately homogeneous in a 2–3 mm wide shear zone. Shear displacement is documented by marker lines and circles. Shear strain γ varies between 0.84 and 1.56. Strain is recorded by flattening of individual grains, defining a foliation normal to the axis of principal finite shortening ε ₁. No twinning is observed on a macroscopic scale. X-ray and neutron diffraction techniques were used to characterize texture before and after deformation. All specimens display strong preferred orientation as documented by 0006, 10¯14 and 11¯220 pole-figures, c axes pole-figures display three maxima in the ε₁–ε₃ plane. If the axes of the strain ellipsoids are used as a coordinate system textures in pure and simple shear are similar but there is considerable monoclinic distortion in simple shear which is attributed to the noncoaxial strain path.     

Localization of deformation and kinematic shift during the hot emplacement of the Ronda peridotites (Betic Cordilleras,southern Spain)

The Ronda peridotites form the largest mass of subcontinental mantle outcropping on land. Unlike other orogenic lherzolite massifs, the two main bodies of Ronda (the Sierra Bermeja and Sierra Alpujata massifs) are unique cases where ductile shear zones linked to the hot thrusting of mantle over continental crustal rocks are well exposed. We present a new insight into the deformation localization in these shear zones based on structural, fabric and petrological data. The Ronda peridotites show increasing deformation towards the continental footwall rocks, from porphyroclastic rocks to ultramylonites. Garnet-pyroxenites from the basal shear zone of the Alpujata massif yield ca. 1100 °C and 1.4 GPa for the mylonitization. Such conditions promoted partial melting and the formation of felsic dynamothermal aureoles from the underlying crustal rocks. Subsequent deformation is mainly localized in the dynamothermal aureoles, since they are weaker than the peridotites. Both aureoles show marked strain gradients towards the contact but record different kinematics. In Sierra Alpujata, kinematic criteria indicate a top-to-the ENE shear sense, whereas in Sierra Bermeja the felsic mylonites provide a top-to-the NNW motion. A transpressional setting is proposed to explain such kinematic shift.     

A quantitative study of deformation mechanisms and finite strain in quartzites

The Weverton quartzites in the Maryland Blue Ridge are deformed by one major period of greenschist-grade deformation. The components of finite strain due to different independent mechanisms have been measured for these rocks. The total strain is split up into two major components: $$\varepsilon ^t = \varepsilon ^p + \varepsilon ^d .$$ The finite natural strain caused by dislocation creep (? ^d ) is measured by a new technique using folded and stretched rutile needles which are good strain markers within the quartz crystals. Pressure solution strain (? ^p ) is measured from the ratio of the area of new crystals and fibers to the whole rock area in principal sections. Grain boundary sliding is a dependent process which accompanies both mechanisms. Pressure solution obeys a linear Newtonian flow law, \(\left| {\dot \gamma _0^p } \right| = A_p \left| {\tau _0 } \right|\) , while dislocation creep obeys a power law of the form \(\left| {\dot \gamma _0^d } \right| = A_d \left| {\tau _0 } \right|^n \) where \(\dot \gamma _0^p ,\dot \gamma _0^d \) are octahedral shear strain rates, τ₀ is the octahedral shear stress and A _p , A _p and n are constants. A direct correlation between finite strain measurements and the operating flow laws can be made. Application of these methods and principles to a few field examples indicates that the rocks obey a flow law partly governed by each mechanism. Any set of physical conditions defines a unique flow law and there is a transition in creep behavior from dominantly Newtonian to a power law with increasing strain rate.     

Structural geometry and kinematics of the Ailao Shan shear zone: insights from integrated structural,microstructural, and fabric studies of the Yao Shan complex,Yunnan, Southwest China

ABSTRACT

The Yao Shan complex, a massif near the southern segment of the Ailao Shan–Red River (ASRR) shear zone, bears important information on the structural framework of the massif and the kinematics of ductile shearing along the ASRR shear zone. In this contribution, structural, microstructural, quartz c-axis fabric, magnetic fabric, and geochronologic data are used to determine the structural framework of the Yao Shan massif and its tectonic implications for the ASRR shear zone. The Yao Shan complex is characterized by an overall linear A-type antiform that contains a core of high-grade metamorphic rocks with Palaeoproterozoic to Mesozoic protoliths and a mantle of Permo-Triassic low-grade rocks. Both the high-grade metamorphic core and low-grade Permo-Triassic rocks have experienced progressive ductile shearing. Anisotropy of magnetic susceptibility (AMS) results from 17 samples collected along the Xinjie–Pingbian section across the complex show that magnetic lineation (K_max) and foliation (K_max–K_int) are generally subparallel to the corresponding structural elements in the sheared rocks. The shape parameter E values of the magnetic ellipsoids are indicative of dominantly oblate and plane strain, but vary with protolith type and degree of strain among the various rock types. In agreement with the field and microstructural observations, the corrected degree of anisotropy (Pj) values reflect high shear strain in the core rocks and relatively low shear strain in the low-grade strata. A kinematic analysis based on structural and magnetic fabric data shows that both left- and right-lateral shear occurred during the deformation of the Yao Shan complex. Therefore, instead of being an element of the ASRR shear zone, the Yao Shan complex constitutes a crustal-scale inharmonic A-type fold with a fold axis parallel to the stretching lineation. Geochronologic data reveal that the folding occurred coevally with ductile shearing of the middle to lower crust between ca. 30 and 21 Ma.     

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.     

P-T paths and problems of high-temperature polymetamorphism

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the T _min of the older P-T trajectory corresponded to T _max of the younger trajectory, and often T _max–T _min > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.     

折劈发育条件的分析——以鞍山附近元古宙地体之折劈(S2)为例

 本文以辽宁省鞍山附近元古宙千枚岩和片岩中的折劈S₂为论述的基础。按照简单剪切原理计算出发育折劈的岩石中的γ(剪应变)值。通过γ等值线图及断面图、T_M/T_QF-α相关图和变形标志(石英)形态比的研究,初步认为,折劈岩石中矿物组成、结构、微构造和α角等的明显“分异”现象,主要由剪应变和伴随发生的物质迁移所造成。有限应变状态的特点是:剪应变高的带(M)和剪应变较低的带(QF)相间排列。相邻带剪应变差异控制着扩散物质迁移机制,对微构造(如微褶皱)的生成,有重要作用。折劈生成于T低于500℃,P 约为5kb 左右的绿片岩相变质环境,它标志着地壳处于区域性抬升状态,相继产生的共轭折劈和膝折带(属于 D,构造),则表明已抬升到足以引起岩石总体体积扩张的高度。