Lack of porphyroblast rotation in the Otago schists, New Zealand: implications for crenulation cleavage development, folding and deformation partitioning

Porphyroblasts of garnet and plagioclase in the Otago schists have not rotated relative to geographic coordinates during non-coaxial deformation that post-dates their growth. Inclusion trails in most of the porphyroblasts are oriented near-vertical and near-horizontal, and the strike of near-vertical inclusion trails is consistent over 3000 km². Microstructural relationships indicate that the porphyroblasts grew in zones of progressive shortening strain, and that the sense of shear affecting the geometry of porphyroblast inclusion trails on the long limbs of folds is the same as the bulk sense of displacement of fold closures. This is contrary to the sense of shear inferred when porphyroblasts are interpreted as having rotated during folding.
Several crenulation cleavage/fold models have previously been developed to accommodate the apparent sense of rotation of porphyroblasts that grew during folding. In the light of accumulating evidence that porphyroblasts do not generally rotate, the applicability of these models to deformed rocks is questionable.
Whether or not porphyroblasts rotate depends on how deformation is partitioned. Lack of rotation requires that progressive shearing strain (rotational deformation) be partitioned around rigid heterogeneities, such as porphyroblasts, which occupy zones of progressive shortening or no strain (non-rotational deformation). Therefore, processes operating at the porphyroblast/matrix boundary are important considerations. Five qualitative models are presented that accommodate stress and strain energy at the boundary without rotating the porphyroblast: (a) a thin layer of fluid at the porphyroblast boundary; (2) grain-boundary sliding; (3) a locked porphyroblast/matrix boundary; (4) dissolution at the porphyroblast/matrix boundary, and (5) an ellipsoidal porphyroblast/shadow unit.     

The geochemistry of concretions from the Kimmeridge Clay Formation of southern and eastern England

Concretions from the Kimmeridge Clay Formation are of three types: calcareous concretions, septarian calcareous concretions and pyrite/calcite concretions and nodules, which occur within different mudstone facies. Isotopic and chemical analysis of the concretionary carbonates indicate growth in the Fe-reduction, sulphate-reduction and decarboxylation zones. The septarian concretions show a long and complex history, with early initiation of growth and development spanning several phases of burial, each often resulting in the formation of septaria. Growth apparently ceased in the transitional zone between the sulphate-reduction and the methanogenesis zones. Very early growth in the Fe-reduction zones is also seen in one sample. The non-septarian concretions began growth later within the sulphate-reduction zone and have had a simpler burial history while the pyrite/calcite concretions show carbonate cementation in the sulphate-reduction-methanogenesis transition zone. A ferroan dolomite/calcite septarian nodule with decarboxylation zone characteristics also occurs. Development of concretions appears to be indirectly controlled by the sedimentation rate and depositional environment, the latter determining the organic matter input to the sediments. Calcareous concretions predominate in swell areas and during periods of low sedimentation rate in the basins with poor organic matter preservation and deposition of calcareous mudstones. Pyrite/calcite concretions occur in organic-rich mudstones deposited under higher sedimentation rates in the basins, while the ferroan dolomite nodule grew under very high sedimentation rates.     

Microstructure and cleavage development in selected slates

A detailed microstructural study of three slates by high voltage transmission electron microscopy is reported. The slates are mineralogically similar, come from minor fold cores and exhibit differing degrees of cleavage intensity. All three slates have domains of orientated phyllosilicates (cleavage lamellae) which contain only a low percentage of quartz and carbonate. Between these lamellae are lenticular domains which contain deformed phyllosilicates and which are enriched in secondary minerals. The initiation of cleavage lamellae can be clearly observed in electron micrographs from one of the slates studied. It occurs along zones of intense deformation, viz. along kinks and microfolds, which form from initial crenulations that are difficult to detect in a petrological microscope.The important observed re-orientation mechanisms of the phyllosilicates during cleavage development in the three slates are strain induced crystallization and the growth of metamorphically stable phyllosilicates together with mechanical rotation. Microstructural evidence suggests that the cleavage lamellae once initiated can extend laterally into the lenticular domains as deformation proceeds. Interference between adjacent phyllosilicates during deformation is commonly observed and resultant extension sites are often enriched in secondary minerals. Chlorite rich pods occur in all of the slates studied and have complex microstructures consisting of both deformed and undeformed phyllosilicates. It is concluded that these pods may form after cleavage initiation.     

Growth and concentration of fibrous sillimanite related to heterogeneous deformation in K-feldspar-sillimanite metapelites

Abstract In foliated K-feldspar-sillimanite metapelites, fibrous sillimanite is commonly concentrated in folia that anastomose between lenticular pods of coarser-grained aggregates rich in quartz, feldspar and biotite, with or without garnet, cordierite and residual andalusite. Many of the folia appear to be limbs of crenulation microfolds. The sillimanite concentrations may be due largely to the ability of fibrous sillimanite aggregates to undergo strong non-coaxial deformation by grain-boundary sliding ('fibre sliding';) without appreciable build-up of dislocations, whereas other minerals are unstable in these zones and so concentrate in lower-strain interfolial zones. Initiation, and especially concentration of the sillimanite in folia, may be assisted by fluid flow and local base-cation leaching, whereby minerals unstable in zones of strong non-coaxial strain are dissolved and removed from these zones.     

川北地区下寒武统筇竹寺组钙质结核特征及成因机制

不同成因的钙质结核在研究沉积水体、早期成岩环境及泥页岩的压实程度方面具有重要意义。通过对川北地区旺苍县下寒武统筇竹寺组钙质结核抛光,发现结核内部具有层理、暗色圈层和张裂缝构造。结核和围岩的层理关系和具有漂浮状的微观结构特征表明,该地区结核是形成于压实作用之前的成岩早期结核。对结核中Al、Ti、Ca、K、Na元素分析表明,结核的元素分布受到结核内部裂缝的影响。结核内部富含生物碎屑和其形成深度为数十米的特征,推测结核主要是在硫酸盐还原带生长。结核具有暗色圈层之间层理不弯曲和元素移动的特征,揭示了结核为透入性生长,在整个结核生长阶段就开始大量同时结晶。Mg/Ca和Sr/Ca的比值说明,结核内部钙质成分为富镁方解石-生物成因方解石。结核中富含钙质成分的原因可能是围岩中的生物成因方解石大量向结核运移所致。裂缝的形态特征表明,裂缝为张裂缝。对结核上部地层的沉积特征研究表明,张裂缝的成因很可能与筇竹寺顶部的浊积岩和沧浪铺组底部的似瘤状灰岩快速搬运沉积有关。     

Chlorite-mica aggregates: morphology,orientation, development and bearing on cleavage formation in very-low-grade rocks

Chlorite-mica aggregates in slates from northern Spain have been investigated in very-thin thin sections. Specimens from various parts of a fold show different morphologies that are dependent on the operating deformation mechanism. From the limb to the hinge rigid-body rotation becomes less important and folding and intragranular kinking are more common. This is associated with an increasing aspect ratio (width/length) for the aggregates.Chlorite-mica aggregates were, in part, present before cleavage development and are statistically parallel to the bedding enveloping surface. Variation in degree of distortion of chlorite suggests continuous solution and growth during deformation. Muscovite, on the other hand, is generally more highly deformed, suggesting that it was all early. Splitting of micas along (001)-planes occurs, creating extension sites where chlorite growth can take place. Thus, an early diagenetic/metamorphic formation with mimetic growth of chlorite on detrital micas is favoured for the origin of the aggregates.The behaviour of the aggregates during deformation provides information on a mechanism for cleavage development. As a result of intragranular kinking, high angle boundaries are created, with parts of the deformed grains at a low angle to the cleavage plane. Mechanical rotation appears to be the important mechanism for cleavage development in these rocks. Modification by recrystallization, solution and growth processes can account for the observed microstructure.     

Tectonically dewatered slates in the Ludlovian of the Lake District,England

Multiple siltstone dykes intruded parallel to cleavage in the Ludlovian argillites and graywackes of the Lake District throw new light on the origin of slaty cleavage. Pore pressures equalling lithostatic pressure developed during the Caledonian orogeny and caused tectonic dewatering of the partially lithified sediments. Slaty cleavage was initiated as an essentially planar structure by the intrusion of thin pelitic folia during the escape of the pore water. Cleavage folia and fold axial planes may have a common geometric relationship to the deforming stresses, but are independent of each other in origin. Continued deformation after tectonic dewatering refracted the cleavage by rotation of competent layers. Critical examination of evidence supporting traditional theories of slaty-cleavage formation shows that all observed relationships are compatible with the tectonic-dewatering theory.     

Evaluating magnetic lineations (AMS) in deformed rocks

Magnetic lineation in rocks is given by a cluster of the principal axes of maximum susceptibility (K_max) of the Anisotropy of Magnetic Susceptibility (AMS) tensor. In deformed rocks, magnetic lineations are generally considered to be the result of either bedding and cleavage intersection or they parallel the tectonic extension direction in high strain zones. Our AMS determinations, based on a variety of samples that were taken from mudstones, slates and schists from the Pyrenees and Appalachians, show that strain is not the only factor controlling the development of magnetic lineation. We find that the development and extent to which the magnetic lineation parallels the tectonic extension direction depends on both the original AMS tensor, which in turn depends on the lithology, and the deformation intensity. Rocks having a weak pre-deformational fabric will develop magnetic lineations that more readily will track the tectonic extension.     

Discovery of a layer of probable impact melt spherules in the Late Archaean Jeerinah Formation,Fortescue Group,Western Australia

In the high‐grade (granulite facies) metamorphic rocks at Broken Hill the foliation is deformed by two groups of folds. Group 1 folds have an axial‐plane schistosity and a sillimanite lineation parallel to their fold axes; the foliation has been transposed into the plane of the schistosity by these folds. Group 2 folds deform the schistosity and distort the sillimanite lineation so that it now lies in a plane. Both groups of folds are developed as large folds. The retrograde schist zones are zones in which new fold structures have formed. These structures deform Group 1 and Group 2 folds and are associated with the formation of a new schistosity and strain‐slip cleavage. The interface between ore and gneiss is folded about Group 1 axial planes but about axes different from those in the foliation in the gneiss. On the basis of this, the orebody could not have been parallel to the foliation prior to the first recognizable structural and metamorphic events at Broken Hill. The orebody has been deformed by Group 2 and later structures.     

Relationship between growth of albite porphyroblasts and deformation in a sambagawa schist, Central Shikoku, Japan

Albite porphyroblasts in a basic schist of the Sambagawa metamorphic belt (Besshi—Bashi, Central Shikoku) show zonal variation of inclusions defined by inclusion-free mantles and inclusion-rich cores, though matrix minerals are partly incorporated in the outer zones of the mantles. From comparison of the crystallographic and dimensional fabrics of inclusion and matrix epidote, it has been concluded that the orientation of the principal axes of strain was different for the deformation of matrix fabrics and inclusion fabrics (fabrics produced before formation of cores). Inclusion amphibole in the cores belongs to the actinolite—common hornblende group, with Si content more than 7.0, whereas amphibole in the matrix and outer zones of mantles belongs to the common hornblende group, with Si content less than 7.1. This shows that the metamorphic temperature was higher during the phase of formation of the outer zones of the mantles than during that of the cores. The average direction of maximum growth of the mantles, which has been estimated from differences between the average shapes of porphyroblasts and those of cores, is parallel to the linear orientation of amphibole and epidote in the matrix. The average direction of minimum growth of the mantles coincides with the normal to the schistosity of the matrix. The mantles appear to be of the same generation as the matrix fabrics.     

Strain distribution within a km-scale,mid-crustal shear zone: The Kuckaus Mylonite Zone,Namibia

The subvertical Kuckaus Mylonite Zone (KMZ) is a km-wide, crustal-scale, Proterozoic, dextral strike-slip shear zone in the Aus granulite terrain, SW Namibia. The KMZ was active under retrograde, amphibolite to greenschist facies conditions, and deformed felsic (and minor mafic) gneisses which had previously experienced granulite facies metamorphism during the Namaqua Orogeny. Lenses of pre- to syn-tectonic leucogranite bodies are also deformed in the shear zone. Pre-KMZ deformation (D₁) is preserved as moderately dipping gneissic foliations and tightly folded migmatitic layering. Shear strain within the KMZ is heterogeneous, and the shear zone comprises anastomosing high strain ultramylonite zones wrapping around less deformed to nearly undeformed lozenges. Strain is localized along the edge of leucogranites and between gneissic lozenges preserving D₁ migmatitic foliations. Strain localization appears controlled by pre-existing foliations, grain size, and compositional anisotropy between leucogranite and granulite. The local presence of retrograde minerals indicate that fluid infiltration occurred in places, but most ultramylonite in the KMZ is free of retrograde minerals. In particular, rock composition and D₁ fabric heterogeneity are highlighted as major contributors to the strain distribution in time and space, with deformation localization along planes of rheological contrast and along pre-existing foliations. Therefore, the spatial distribution of strain in crustal-scale ductile shear zones may be highly dependent on lithology and the orientation of pre-existing fabric elements. In addition, foliation development and grain size reduction in high strain zones further localizes strain during progressive shear, maintaining the anastomosing shear zone network established by the pre-existing heterogeneity.     

Microstructural analysis of the classical spiral garnet porphyroblasts of south-east Vermont: evidence for non-rotation

New data strongly suggest that the classical spiral garnet porphyroblasts of south-east Vermont, USA, generally did not rotate, relative to geographical coordinates, throughout several stages of non-coaxial ductile deformation. The continuity of inclusion trails (S_i) in these porphyroblasts is commonly disrupted by planar to weakly arcuate discontinuities, consisting of truncations and differentiation zones where quartz–graphite S_i bend sharply into more graphitic S_i. Discontinuous, tight microfold hinges with relatively straight axial planes are also present. These microstructures form part of a complete morphological gradation between near-orthogonally arranged, discontinuous inclusion segments and smoothly curving, continuous S_i spirals. Some 2700 pitch measurements of well-developed inclusion discontinuities and discontinuous microfold axial planes were taken from several hundred vertically orientated thin sections of various strike, from specimens collected at 28 different locations around the Chester and Athens domes. The results indicate that the discontinuities have predominantly subvertical and subhorizontal orientations, irrespective of variations in the external foliation attitude, macrostructural geometry and apparent porphyroblast-matrix rotation angles. Combined with evidence for textural zoning, this supports the recent hypothesis that porphyroblasts grow incrementally during successive cycles of subvertical and subhorizontal crenulation cleavage development. Less common inclined discontinuities are interpreted as resulting from deflection of anastomosing matrix foliations around obliquely orientated crystal faces prior to inclusion. Most of the idioblastic garnet porphyroblasts have a preferred crystallographic orientation. Dimensionally elongate idioblasts also have a preferred shape orientation, with long axes orientated normal to the mica folia, within which epitaxial nucleation occurred. Truncations and differentiation zones result from the formation of differentiated crenulation cleavage seams against porphyroblast margins, in association with progressive and selective strain-induced dissolution of matrix minerals and locally also the porphyroblast margin. Non-rotation of porphyroblasts, relative to geographical coordinates, suggests that deformation at the microscale is heterogeneous and discontinuous in the presence of undeformed, relatively large and rigid heterogeneities, which cause the progressive shearing (rotational) component of deformation to partition around them. The spiral garnet porphyroblasts therefore preserve the most complete record of the complex, polyphase tectonic and metamorphic history experienced in this area, most of which was destroyed in the matrix by progressive foliation rotation and reactivation, together with recrystallization.     

The role of fluids in partitioning brittle deformation and ductile creep in auriferous shear zones between 500 and 700 °C

The fabric, mineralogy, geochemistry, and stable isotope systematics of auriferous shear zones in various hydrothermal gold deposits were studied in order to discuss the role of fluids in rock deformation at temperatures between 500 °C and 700 °C. The strong hydrothermal alteration and gold mineralization indicates that effective permeability development goes ahead with high-temperature rock deformation. The economic gold enrichment is often hosted by breccias and quartz veins in the ductile shear zones, which either formed at fast strain rates or by low strain continuous deformation at slow strain rates. Both processes require (1) a close-to lithostatic to supralithostatic fluid pressure and/or (2) a strong rheology contrast of the deformed lithologies that is often developed during progressive hydrothermal alteration. Compartments of high fluid pressure are sealed from the rest of the shear zones by high-temperature deformation mechanisms, e.g. intracrystalline plasticity and diffusion creep, and compaction. In contrast, in mylonites with heterogeneous crystal plastic and brittle deformation mechanisms for the various minerals, an interconnected network of a grain-scale porosity forms an effective fluid conduit, which hampers fluid pressure build-up and the formation of veins.The auriferous shear zones of the various gold mines represent fluid conduits in the deeper crust, 100 m along strike and up to 1000 m down-dip. The hydrothermal fluids infiltrated may be responsible for low magnitude earthquakes in the Earth's lower crust, which otherwise deforms viscously.     

Growth mechanisms and geochemistry of carbonate concretions from the Cambrian Wheeler Formation (Utah,USA)

Carbonate concretions provide unique records of ancient biogeochemical processes in marine sediments. Typically, they form in organic‐rich mudstones, where a significant fraction of the bicarbonate required for carbonate precipitation is supplied from the decomposition of organic matter in the sediments. As a result, carbonates that comprise concretions are usually characterized by broad ranges in δ¹³C and include values that are significantly depleted relative to seawater. This article reports results from a physical, petrographic and geochemical analysis of 238 concretions from the Wheeler Formation (Cambrian Series 3), Utah, USA, which are unusual in several respects. Most prominently, they formed in organic‐poor mudstones (total organic carbon = 0·1 to 0·5%) and are characterized by a narrow range of δ¹³C that onlaps the range of contemporaneous seawater values. Subtle centre to edge trends in δ¹³C demonstrate that concretion precipitation was initiated by local chemical gradients set up by microbial activity in the sediments, but was sustained during growth by a large pool of inorganic bicarbonate probably derived from alkaline bottom waters. The large inorganic pool appears to have been important in facilitating rapid precipitation of the concretion matrix, which occurred via both displacive and replacive carbonate precipitation during early diagenesis. Stable isotope data from cogenetic pyrite (δ³⁴S) and silica (δ¹⁸O) phases provide insight into the evolution of biogeochemical processes during concretion growth, and suggest that concretions were formed almost entirely during sulphate reduction, with only minor modification thereafter. Concretions of the Wheeler Formation appear to represent an end‐member system of concretion formation in which rapid growth was promoted by ions supplied from sea‐water. As such, they offer insight into the spectrum of processes that may influence the growth of carbonate concretions in marine sediments.     

Strain localization associated with channelized melt migration in upper mantle lithosphere: Insights from the Twin Sisters ultramafic complex,Washington, USA

We present results of field, microstructural, and textural studies in the Twin Sisters ultramafic complex (Washington State) that document localized deformation associated with the formation of dunite channels in naturally deformed upper mantle. The Twin Sisters complex is a well-exposed, virtually unaltered section of upper mantle lithosphere comprised largely of dunite and harzburgite (in cm- to m-scale primary compositional layers), and variably deformed orthopyroxenite and clinopyroxenite dikes. A series of ∼N–S striking, m-scale dunite bands (typically with porphyroclastic texture) occur throughout the study area and crosscut both the primary compositional layers and older orthopyroxenite dikes. Structural relationships suggest that these dunite bands represent former zones of channelized melt migration (i.e., dunite channels), and that strain localization was associated with melt migration. Early formed orthopyroxenite dikes are either absent within cross-cutting dunite channels, or have been displaced within channels relative to their position in the adjacent host rocks. These pre-existing orthopyroxenite dikes provide strain markers illustrating that displacement was localized primarily along channel margins, which have opposite senses of shear. In all cases where offsets were noted, the center of the channel was moved southward relative to its margins. Material flow and strain was, therefore, partitioned within channels during melt migration, and dunite channels did not accommodate net shear displacement of the adjacent host peridotites. Primary compositional layers adjacent to dunite channels document opposite rotation of olivine [100] crystallographic axes on either side of channel margins, consistent with the kinematic reversal inferred from offset markers at the outcrop scale, suggesting that the formation of dunite channels also induced host rock deformation proximal to channels. Strain localization that was focused at the margin of the bands was likely facilitated by melt-induced weakening. Channelized movement within the dunite bands may have resulted from matrix compaction within channels, pressure gradients during melt migration, or a combination of these processes during coaxial deformation.     

A new model for the formation of a spaced crenulation (shear band) cleavage in the Dalradian rocks of the Tay Nappe,SW Highlands,Scotland

The main conclusion of this study is that non-coaxial strain acting parallel to a flat-lying D₁ spaced cleavage was responsible for the formation of the D₂ spaced crenulation (shear band) cleavage in Dalradian rocks of Neoproterozoic-Lower Ordovician age in the SW Highlands, Scotland. The cm-dm-scale D₂ microlithons are asymmetric; have a geometrically distinctive nose and tail; and show a thickened central portion resulting from back-rotation of the constituent D₁ microlithons. The current terminology used to describe crenulation cleavages is reviewed and updated. Aided by exceptional 3D exposures, it is shown how embryonic D₂ flexural-slip folds developed into a spaced cleavage comprising fold-pair domains wrapped by anastomosing cleavage seams. The bulk strain was partitioned into low-strain domains separated by zones of high non-coaxial strain. This new model provides a template for determining the sense of shear in both low-strain situations and in ductile, higher strain zones where other indicators, such as shear folds, give ambiguous results. Analogous structures include tectonic lozenges in shear zones, and flexural-slip duplexes. Disputes over the sense and direction of shear during emplacement of the Tay Nappe, and the apparently intractable conflict between minor fold asymmetry and shear sense, appear to be resolved.     

Deformation history of the Hampden Synform in the Eastern Fold Belt of the Mt Isa terrane

The moderately metamorphosed and deformed rocks exposed in the Hampden Synform, Eastern Fold Belt, in the Mt Isa terrane, underwent complex multiple deformations during the early Mesoproterozoic Isan Orogeny (ca 1590–1500 Ma). The earliest deformation elements preserved in the Hampden Synform are first‐generation tight to isoclinal folds and an associated axial‐planar slaty cleavage. Preservation of recumbent first‐generation folds in the hinge zones of second‐generation folds, and the approximately northeast‐southwest orientation of restored L¹ ₀ intersection lineation suggest recumbent folding occurred during east‐west to northwest‐southeast shortening. First‐generation folds are refolded by north‐south‐oriented upright non‐cylindrical tight to isoclinal second‐generation folds. A differentiated axial‐planar cleavage to the second‐generation fold is the dominant fabric in the study area. This fabric crenulates an earlier fabric in the hinge zones of second‐generation folds, but forms a composite cleavage on the fold limbs. Two weakly developed steeply dipping crenulation cleavages overprint the dominant composite cleavage at a relatively high angle (>45°). These deformations appear to have had little regional effect. The composite cleavage is also overprinted by a subhorizontal crenulation cleavage inferred to have developed during vertical shortening associated with late‐orogenic pluton emplacement. We interpret the sequence of deformation events in the Hampden Synform to reflect the progression from thin‐skinned crustal shortening during the development of first‐generation structures to thick‐skinned crustal shortening during subsequent events. The Hampden Synform is interpreted to occur within a progressively deformed thrust slice located in the hangingwall of the Overhang Shear.     

Determination of finite strain in bedding surfaces using sedimentary structures and trace fossils: A comparison of techniques

Determination of the two-dimensional finite strain in turbidite bedding surfaces is important in both structural and sedimentological investigations of deformed sediments. The Meguma Group of Nova Scotia is one such sequence, in which the directional distribution of paleocurrent markers has been modified by strain. Previous strain determinations have used sand volcanoes as strain markers, which do not deform homogeneously with their matrix and, hence, may underestimate the true strain ratio. Intraclasts and concretions also suffer inhomogeneous deformation and are of limited use as strain markers. Hexagonal trace fossil networks can provide more reliable strain estimates but are rare. Widths of burrows exposed on bedding surfaces provide a new, straightforward means of measuring strain. Each burrow is represented by a pair of parallel lines. The separation of the lines is proportional to the burrow width, and their direction represents burrow orientation. The lines are tangential to the strain ellipse, which can be discerned despite initial scatter in burrow widths.     

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。