A multidisciplinary study on the emplacement mechanism of the Qingyang–Jiuhua Massif in Southeast China and its tectonic bearings. Part I: Structural geology,AMS and paleomagnetism

During the Cretaceous, the South China Block (SCB) experienced a widely distributed extensional event including numerous plutons emplacement and basin opening. Investigations on the tectonic regime coeval with pluton emplacement, and emplacement mechanism of the pluton remain relatively rare in the SCB. In order to address these questions, a multidisciplinary approach, including field structural and petrographic observations, anisotropy magnetic susceptibility (AMS) and paleomagnetic analyses, was carried out on the Qingyang–Jiuhua granitic massif which intrudes into the Lower Yangtze fold belt in the northeastern part of the SCB. The Qingyang–Jiuhua massif is composed of the granodioritic Qingyang and monzogranitic Jiuhua plutons dated by zircon U–Pb method at ca. 142 Ma, and ca. 131 Ma, respectively. Our structural observations show that the intrusion of the Qingyang–Jiuhua massif does not modify the fold strike. A weak ductile deformation of the country rocks and granitoid can be only observed in the boundary zone with limited contact metamorphism. In the contact aureole of the massif, the foliation follows the pluton contour, and the mineral lineation is rare. When present, it exhibits a down-dip attitude. Field and microstructural observations indicate isotropic magmatic textures in most parts of the massif. The AMS analysis of 93 sites reveals weak values for the anisotropy degree (P_J < 1.2) and oblate magnetic fabric dominance (T > 0) for most of the measured samples. Two principal foliation patterns are identified: horizontal foliations in the center of the plutons, and vertical foliations on the boundaries. Magnetic lineation strike is largely scattered, and weakly inclined at the scale of the entire massif. The paleomagnetic investigations indicate that (a) the younger Jiuhua pluton did not produce a remagnetization in the older Qingyang pluton, (b) no relative movement can be observed between these two plutons, (c) the entire massif did not experience any important relative movement with respect to South China, considering the paleomagnetic uncertainties. Integrating the newly obtained results with previous observations, our study favors a permissive emplacement mechanism for the two plutons, i.e. vertical magma intrusion into an opening space controlled by the NW–SE brittle stretching of the upper crust, which is in agreement with a weak extensional regional tectonic framework of the SCB.     

Internal structure of a collapsed terrain: The Lújar syncline and its significance for the fold- and sheet-structure of the Alborán Domain (Betic Cordilleras, Spain)

Detailed structural analysis of the Sierra de Lújar in the western Alpujarras region (Betic Cordilleras, S Spain), a very representative area of the terrain known as the Alborán Domain, has revealed the existence of a very large N-vergent recumbent syncline which involves the whole mountain massif and neighbouring areas. The Lújar syncline and, probably, the associated recumbent anticline which crops out southeast of Sierra de Lújar show a great variation in the orientation of the hinge line. Although having a curved shape, the hinge line is contained in a plane whose attitude coincides with the main attitude of the axial-plane crenulation foliation (Sc), suggesting that it is a sheath fold.The strongly deformed overturned limb of the syncline is cut by two low-angle normal faults displacing towards the north. Similarity in the kinematics between the faults and the fold, and the association between the faults and the high-strain zone in the overturned limb of the fold, suggest that they are related. Regional constraints on the age of the crenulation cleavage and the low-angle normal faults indicate that they formed during the early Miocene late-orogenic extensional event in the Alborán Domain.We propose an alternate explanation for the structure of the Alpujarras region in which the Lújar syncline forms part of a recumbent syncline–anticline pair that extends along much of the Alpujarride outcrop in the southern Betic Cordillera. In several places, the fold is disrupted by low-angle normal faults, and it is overlain by an upper Alpujárride extensional sheet mainly composed of medium- to high-grade metamorphic rocks. We suggest that all these structures arose from the extensional deformation under decreasing temperature conditions of a previously thickened and metamorphosed orogenic crust.     

Early formed regional antiforms and synforms that fold younger matrix schistosities: their effect on sites of mineral growth

In the metamorphic cores of many orogenic belts, large macroscopic folds in compositional layering also appear to fold one or more pervasive matrix foliations. The latter geometry suggests the folds formed relatively late in the tectonic history, after foliation development. However, microstructural analysis of four examples of such folds suggests this is not the case. The folds formed relatively early in the orogenic history and are the end product of multiple, near orthogonal, overprinting bulk shortening events. Once large macroscopic folds initiate, they may tighten further during successive periods of sub-parallel shortening, folding or reactivation of foliations that develop during intervening periods of near orthogonal shortening. Reactivation of the compositional layering defining the fold limbs causes foliation to be rotated into parallelism with the limbs.Multiple periods of porphyroblast growth accompanied the multiple phases of deformation that postdated the initial development of these folds. Some of these phases of deformation were attended by the development of large numbers of same asymmetry spiral-shaped inclusion trails in porphyroblasts on one limb of the fold and not the other, or larger numbers of opposite asymmetry spirals on the other limb, or similar numbers of the same asymmetry spirals on both limbs. Significantly, the largest disparity in numbers from limb to limb occurred for the first of these cases. For all four regional folds examined, the structural relationships that accompanied these large disparities were identical. In each case the shear sense operating on steeply dipping foliations was opposite to that required to originally develop the fold. Reactivation of the folded compositional layering was not possible for this shear sense. This favoured the development of sites of approximately coaxial shortening early during the deformation history, enhancing microfracture and promoting the growth of porphyroblasts on this limb in comparision to the other. These distributions of inclusion trail geometries from limb to limb cannot be explained by porphyroblast rotation, or folding of pre-existing rotated porphyroblasts within a shear zone, but can be explained by development of the inclusion trails synchronous with successive sub-vertical and sub-horizontal foliations.     

Foliation development and refraction in metamorphic rocks: reactivation of earlier foliations and decrenulation due to shifting patterns of deformation partitioning

Abstract Reactivation of early foliations accounts for much of the progressive strain at more advanced stages of deformation. Its role has generally been insufficiently emphasized because evidence is best preserved where porphyroblasts which contain inclusion trails are present. Reactivation occurs when progressive shearing, operating in a synthetic anastomosing fashion parallel to the axial planes of folds, changes to a combination of coarse- and finescale zones of progressive shearing, some of which operate antithetically relative to the bulk shear on a fold limb. Reactivation of earlier foliations occurs in these latter zones. Reactivation decrenulates pre-existing or just-formed crenulations, generating shearing along the decrenulated or rotated pre-existing foliation planes. Partitioning of deformation within these foliation planes, such that phyllosilicates and/or graphite take up progressive shearing strain and other minerals accommodate progressive shortening strain, causes dissolution of these other minerals. This results in concentration of the phyllosilicates in a similar, but more penetrative manner to the formation of a differentiated crenulation cleavage, except that the foliation can form or intensify on a fold limb at a considerable angle to the axial plane of synchronous macroscopic folds. Reactivation can generate bedding-parallel schistosity in multideformed and metamorphosed terrains without associated folds. Heterogeneous reactivation of bedding generates rootless intrafolial folds with sigmoidal axial planes from formerly through-going structures. Reactivation causes rotation or ‘refraction’of axial-plane foliations (forming in the same deformation event causing reactivation) in those beds or zones in which an earlier foliation has been reactivated, and results in destruction of the originally axial-plane foliation at high strains. Reactivation also provides a simple explanation for the apparently ‘wrong sense’, but normally observed ‘rotation’of garnet porphyroblasts, whereby the external foliation has undergone rotation due to antithetic shear on the reactivated foliation. Alternatively, the rotation of the external foliation can be due to its reactivation in a subsequent deformation event. Porphyroblasts with inclusion trails commonly preserve evidence of reactivation of earlier foliations and therefore can be used to identify the presence of a deformation that has not been recognized by normal geometric methods, because of penetrative reactivation. Reactivation often reverses the asymmetry between pre-existing foliations and bedding on one limb of a later fold, leading to problems in the geometric analysis of an area when the location of early fold hinges is essential. The stretching lineation in a reactivated foliation can be radically reoriented, potentially causing major errors in determining movement directions in mylonitic schistosities in folded thrusts. Geometric relationships which result from reactivation of foliations around porphyroblasts can be used to aid determination of the timing of the growth of porphyroblasts relative to deformation events. Other aspects of reactivation, however, can lead to complications in timing of porphyroblast growth if the presence of this phenomenon is not recognized; for example, D₂-grown porphyroblasts may be dissolved against reactivated S₁ and hence appear to have grown syn-D₁.     

Quartz fabrics and recumbent folds in the Sierra de Los Filabres (SE-Spain)

Abstract

An important generation of recumbent folds can be recognized in the Nevado-Filabride nappe complex in the Sierra de los Filabres in SE Spain.

Folding post-dates an initial phase of flattening and is prior to the main phase of non-coaxial deformation and thrusting in the upper part of the complex, involved in a large-scale movement zone with dominant sense of shear to the northwest.

Axial planes and axes of these folds are aligned sub-parallel to the plastic flow plane in quartz and the shear direction respectively. Relict quartz fabrics can be found however, which strongly suggest that some fold axes originated in this position and did not rotate towards parallelism with the shear direction. This apparently contradicts the generally accepted model of development of folds in mylonite zones.

The redistribution of linear structures of the older flattening fase over the folds, confirms that the original orientation of fold axes was close to the present and indicates that an important component of flexural slip was involved in fold formation.     

华北克拉通太古宙上壳岩早前寒武纪构造变形与变质作用关系——以北京密云沙厂地区为例

沙厂铁矿区上壳岩在太古宙时期经历了麻粒岩相、角闪岩相和绿片岩相的多相变质作用,这种多相变质作用的发生及其分布特点是与本区台怀运动旋回的两幕构造变形的形成有密切关系。本文根据不同变质相的矿物组合与不同变形幕构造片理的相互关系的研究得出角闪岩相退变是与台怀变形序列的第一幕变形相对应,而绿片岩相退变是与第二幕变形有联系的结论。退变质作用的不均匀性,即其强弱的表现与不同变形幕的构造变形有直接关系。     

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.     

Two-phase mechanism of the formation of platinum-metal basites of the Fedorova Tundra intrusion on the Kola Peninsula: New data on geology and isotope geochronology

This report contains the results of the authors’ studies on geology and isotope geochronology, which allowed them to formulate the thesis of a two-phase mechanism of the formation of rock associations in the Fedorova Tundra massif. During the former phase (2526–2507 My), a laminated series of the massif was formed, with shows of platinum-metal mineralization of reef type. During the later phase (2493–2485 My), a taxitic zone of the massif appeared, with a highly developed basal platinum-metal mineralization.     

Zambales ophiolite,Philippines

The Acoje massif is part of a mafic-ultramafic complex, the Zambales ophiolite, and is a fragment of Mesozoic oceanic crust. This paper documents the occurrence and phase relations of sulfides and associated phases in the critical zone of the Acoje massif. The Acoje critical zone (ACZ) forms the basal cumulate sequence of the massif and consists of a variably serpentinized lower ultramafic zone and a relatively less altered upper mafic zone. Two distinct sulfide associations have been identified: (1) a troilite (±pyrrhotite)-dominated group hosted by the mafic zone and (2) a pentlandite-dominated group hosted by the ultramafic zone. Troilite-dominated assemblages represent the original mineralogy of magmatically precipitated sulfides in the entire cumulate sequence. The pentlandite-dominated group appears to have evolved from the primary magmatic sulfides during low-temperature re-equilibration. The paragenetic evolution from the magmatic assemblage to the low-temperature assemblage appears to have proceeded as follows: (1) S-rich hexagonal pyrrhotite+pentlandite+chalcopyrite (or cubanite)+magnetite, (2) S-poor hexagonal pyrrhotite+pentlandite+intermediate solid solution (iss) phase (and/or cubanite)+magnetite, (3) troilite (or mackinawite)+pentlandite+iss+magnetite, (4) troilite (or mackinawite)+pentlandite+iss+native Cu+magnetite, (5) pentlandite+native Cu+magnetite, and (6) pentlandite+native Cu+Fe-Ni alloy+magnetite. This evolutionary trend, in conjunction with the observed textural, chemical, and sulfur-isotopic relations, indicates that the native metal and alloy phases in the ACZ were produced by low-temperature reduction of the primary magmatic sulfides. Correlations between sulfide assemblages and coexisting silicate-hydrosilicate-oxide assemblages further indicate that this alteration occurred during retrograde serpentinization of the Acoje massif. Two end-member models that could explain the inferred low-temperature mineralogic evolution of the ACZ sulfides are described: (1) an isothermal reduction model and (2) a non-isothermal equilibration model. Both isothermal and non-isothermal effects apparently were involved in the development of variably reduced sulfide-oxide-metal assemblages from the initial magmatic sulfides.     

平卧式褶皱推覆—巢湖煤田的基本构造型式

本文从构造分析的角度出发，指出安徽巢湖煤田的基本构造型式为大型的平卧褶皱，以及在此基础上，沿倒转翼及其以下构造部位发育起来的推覆构造。     

The establishment of recumbent folds in the Lower Palaeozoic near Queanbeyan,New South Wales

Lower Palaeozoic sedimentary and volcanic rocks east of Queanbeyan, N.S.W., have undergone multiple deformation resulting in four systems of folds. The first of these consists of large isoclinal, recumbent folds (F1). The second generation folds (F2) are the most pronounced; they consist of flattened flexural‐slip folds with well developed axial‐plane slaty cleavage. Minor variants of this system are associated with meridionally‐trending faults. Third and fourth generation folds are minor kink systems.

The existence of first generation folds was established on the basis of F2 fold‐facing determinations, and their likely form was deduced from the geometrical variations of F2 folds. It is thought that all fold phases developed during the Late Silurian Bowning Orogeny.     

An interpretation of the structure of the Blatherskite Nappe,Alice springs,Northern Territory

The basal unit of the Amadeus Basin sequence is the Heavitree Quartzite, and this formation usually forms a single east‐west ridge along the northern side of the MacDonnell Ranges. However, at Alice Springs there are two such ridges. Basement rocks crop out on the northern side of each ridge, and dolomite and shale of the Bitter Springs Formation crop out on their southern sides. The northern outcrop of dolomite and shale is tightly folded, and is separated from the southern outcrop of basement by a major fault. The bedding of the sediments, the axial plane of the fold, and the fault all dip south at about 45°. Inverted facings on parasitic folds indicate that the northern outcrop of quartzite and dolomite plus shale is an antiform in inverted rocks. Hence the southern outcrop of basement and quartzite is synformal, and is interpreted as the frontal part of a fold nappe. The nappe started as a recumbent anticline whose middle limb of quartzite sheared out as the anticline travelled several kilometres southwards relative to the dolomite and shale below, which formed a tight recumbent syncline. Later monoclinal uplift of the northern half of the area tilted the nappe into its present south‐dipping attitude, thus converting the recumbent anticline into a synform and the recumbent syncline into an antiform.     

Formation of the Yakuno ophiolite; accretionary subduction under medium-pressure-type metamorphic conditions

The notion that the Yakuno ophiolite and overlying Maizuru Group represents an accretionary prism formed during the Permian evolution of Japan on the Yakuno eruptive sequence, association of hemipelagic mudstone with silicic tuff, exotic fossiliferous limestones derived from previously accreted sea-mounts, upward coarsening of sequences terrigenous sandstone and conglomerate, and mildly deformed Permian and Triassic forearc basin formations. The most important indicator, however, is the seaward imbrication and repetition observed in both the Maizuru Group and the ophiolite itself. D1 deformation structures include axial–planar foliations (pressure-solution cleavage for the Maizuru Group and granulite–amphibolite metamorphic layering in the ophiolite), flattening type strain, symmetric pressure shadows and fringes, and isoclinal folds showing axial–planar foliations and thrust faulting at their overturned limb. The exceptional asymmetry observed indicates seaward-directed shearing near the thrust, while D1 structures in the Maizuru zone are explained by off-scraping, above the basal decollement. The later Jurassic D2 kink fold structure includes a first-order asymmetric kink with a brittle thrust at its overturned limb, more-or-less coeval with M2 retrograde metamorphism. Medium-pressure M1 prograde metamorphism in the Yakuno ophiolite produced layering of granulite and amphibolite, and in the Maizuru Group, formation of illite along pressure-solution cleavage of mudstones. The metamorphic grade is controlled by the stratigraphic relationships and appears typical of that in ocean floor regions. However, there was only one episode of M1 prograde metamorphism which occurred contemporaneously with D1 off-scraping. Given that subduction zones are normally characterized by high P/T metamorphic regimes, the observed P/T history appears to reflect relatively unusual conditions. Such high thermal gradients may plausibly reflect the approach of a young, hot oceanic plate which continued subducting beneath the Japanese arc. Accordingly, the Yakuno ophiolite was probably formed at the trench–trench–ridge triple junction.     

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.     

Neue Daten zur Biostratigraphie und zur tektonischen Lagerung der Phyllit-Gruppe und der Trypali-Gruppe auf der Insel Kreta (Griechenland)

Based on many new fossil discoveries mainly conodonts and ostracodes an attempt has been made to determine the stratigraphic range of the high pressure/low temperature metamorphic Phyllite Group in West Crete (Greece). The numerous conodont findings indicate — in spite of strong supply of clastic material — a fully marine environment from the Upper Carboniferous up to the end of the Lower Triassic. The prevailing part of the Middle Triassic cannot yet covered by fossils. The upper Triassic shows in the lower (invers) part of the Phyllite Group a shallow water facies with ostracodes, bivalves and at the Triassic/Lias boundary a saliniferous facies, however in the upper (in original position lying) part a conodonts-foraminiferes bearing littoral facies. The present biostratigraphic data point out that the lower part of the Phyllite Group lies in inverted order and the upper part in original position. Thus the Phyllite Group on the whole seems (in West Crete) to form a huge recumbent isoclinal fold analogous to the isoclinal folding on a smaller scale as frequently exposed. The carbonate Trypali Group can be interpreted as either the recumbent limb of this fold structure or as part of the underlying Talea Ori Group. The Trypali Group seems not to be a particular unit. For the reconstruction of the paleogeographic and geodynamic evolution of the South Aegean region the following evidence may be important:

1. the marine sedimentation in the Upper Palaeozoic and the Lower Triassic, especially the marine Upper Permian (Dorashamian), an isolated occurrence 2000 km away from comparable sediments further east and
2. the obvious termination of deposition at the Triassic/Lias boundary. A future interpretation of these results may be the key to a better understanding of the geodynamic process which led to the high pressure/low temperature metamorphism of the Phyllite Group.

     

The relationship between foliation and strain: an experimental investigation

A multilayered salt/mica specimen with embedded strain markers was shortened to produce a fold and the distribution of strain was subsequently mapped out over the profile plane. On a fine scale the initial foliation, which is parallel to the undeformed layers, is folded by tight kinks to produce two new foliations; one is defined by the preferred orientation of kink boundaries and the other by the preferred orientation of (001) of mica. In the hinge region of the fold the first of these new foliations is parallel to the local λ₁λ₂-principal plane of strain whereas the preferred orientation of mica is bimodal and is symmetrical about the λ₁λ₂-plane. Elsewhere the two new foliations are not parallel to the principal plane of strain and angular divergencies of up to 30–35° are measured. If a March model with initial random mica orientation is assumed for the development of mica preferred orientation then the correct value of strain is predicted but the orientation of the principal plane of strain can be grossly in error. A theoretical analysis of the angular relationships to be expected between kink boundaries and the λ₁λ₂-plane of strain confirms that for the type of geometries experimentally developed, large divergences of up to 35° should be common. In rocks where the foliation has developed by processes similar to those recorded here, large angular divergencies between the foliation and the λ₁λ₂-principal plane of strain should be expected as the rule.     

Dehydration-related deformation during regional metamorphism, NW Sardinia, Italy

The relationship between deformation and dehydration has been investigated in Hercynian regionally metamorphosed rocks exposed on NW Sardinia. Two episodes of prograde mineral growth (M₁ & M₂) involving dehydration are recognized: growth of chlorite/phengite porphyroblasts at anchizone metamorphic conditions, contemporaneous with the first phase of deformation, D₁, and growth of biotite from chlorite and phengite coincident with the second phase of deformation, D₂. Deformation during both episodes of dehydration is characterized by penetrative axial planar foliations defined by well-developed phyllosilicate preferred orientations quantified by XRD textural goniometry, tight to isoclinal similar folds (interlimb angles <40°), and mineral-filled veins (hydrofractures) orientated parallel to axial planar foliations, that formed contemporaneously with the development of the penetrative foliations. No prograde mineral growth occurred during D₂ at chlorite-zone conditions. D₂ deformation in the absence of dehydration is characterized by non-penetrative crenulation cleavages, poorly developed phyllosilicate preferred orientations, relatively open (interlimb angles >40°), low-strain similar folds and minor brittle deformation. Systematic variations in macrofold interlimb angles, with respect to the timing of mineral growth, indicate that enhanced shortening (c. 80%) occurred during dehydration. Microfabrics show that the onset of dehydration is associated with the transition from a crenulation cleavage to a penetrative foliation. The presence of axial planar hydrofractures that formed coevally with dehydration and fabric development requires that supralithostatic fluid pressures and low differential stresses (<c. 20 MPa) accompanied dehydration. These features demonstrate a connection between the timing of dehydration and the style of deformation.     

Modern geochemistry in light of D.I. Mendeleyev's periodic system of chemical elements

A geological interpretation of the gravitational and magnetic fields is a method based on Poisson's relation between the gravitational and magnetic potentials. Within the northern and middle Caspian there are parts of the crust with different types of spatial relation between the density and magnetic-action boundaries, corresponding to different structural-tectonic elements of platforms of different ages. The authors locate the southern boundary of the East European Platform; they distinguish the Scythian and Turanian plates: in the Paleozoic base of the Scythian plate they distinguish a graben of the axial zone and a limb of the Donets- Caspian fold system; they determine the position of the upper Paleozoic downwarps filled with volcanic-clastic strata at the edges of the Rostov -Kochubeyev zone of the pre-Permian basement and on the slopes of the middle Caspian massif. —Authors.     

Micromechanical control of rheological anisotropy in quartz mylonite

The Moine nappe mylonites of the area of Loch Eriboll are inferred to have deformed predominantly by simple shear. Samples were taken from each limb of a late recumbent fold and the quartz crystallographic preferred orientation and optical microstructure were investigated. The pattern of preferred orientation was found to be related to the local folded orientation of the foliation and stretching lineation for the overturned limb and to the inferred imposed deformation for the normal limb. The mechanical development of the fold was modelled as a similar fold forming in a homogeneous mechanically anisotropic material with linear viscous rheology. A very high mechanical anisotropy was required for active amplification of small initial perturbations under simple shear applied at 10 ° to the initial enveloping surface. The high anisotropy also constrained the local kinematics to approximate simple shear parallel to the plane of easy shear, which contributes to the maintenance of the preferred orientation patterns relative to the local macroscopic fabric elements. The degree of mechanical anisotropy calculated from the preferred orientation assuming the operation of the common quartz slip systems is lower than that required for mechanical amplification of the folds. Consequently, it is suggested that the mechanical anisotropy derives from sliding on preferentially aligned grain boundaries.     

Complex microstructures preserved in rocks with a simple matrix: significance for deformation and metamorphic processes

Schists from the foothills of the Central Sierra Nevada contain one dominant matrix foliation and yet four phases of growth of both cordierite and andalusite porphyroblasts can be distinguished. These occurred early during four separate deformation events that formed successive steep and shallow foliations. A fifth deformation event pre-dates the growth of all porphyroblasts studied. The multiple phases of porphyroblast growth allow correlation of structures across and along the region. A repeated pattern of deformation, in terms of the curvature of earlier foliations against the overprinting one, allows samples containing porphyroblasts with simpler inclusion trail geometries to be interpreted with confidence. The large-scale fold structures in this region formed before or during the second of the five deformation events recorded by the porphyroblasts. However, the matrix foliation is predominantly a product of the fourth deformation, which has commonly reactivated or re-used older foliations, and is dominated by east-side-up shear. The intervening third deformation produced locally intense foliations and was accompanied by top-to-the-east shear. The very weak fifth deformation produced weak crenulations with subhorizontal axial planes and was coaxial. Multiple phases of episodic but synchronous growth of cordierite and andalusite were produced by the KFMASH univariant equilibrium Ms+Chl+Qtz=And+Crd+Bt+H₂O. The rocks crossed this reaction at a pressure just below the intersection with the KFMASH divariant equilibrium Ms+Chl+Qtz=Crd+Bt+H₂O; the latter being overstepped in favour of the former as there is no evidence for cordierite growth prior to andalusite in these rocks. Subsequent multiple episodes of synchronous growth of cordierite and andalusite indicate that the possible variation in P–T during subsequent deformations was not large. This requires the high-amplitude macroscopic fold to form prior to porphyroblast growth and then be simply tightened and modified by the younger deformations.