Porphyroclast systems as kinematic indicators

Porphyroclasts of relatively strong minerals in mylonites commonly have an internal monoclinic shape symmetry defined by tails of dynamically recrystallized material. The geometry of a porphyroclast and its tails, called a ‘porhyroclast system’, can serve as a valuable indicator of the sense of vorticity. Porphyroclast systems have been divided into σ- and δ-types on the basis of the geometry of the tails. σ-Types have wedge-shaped recrystallized tails whose median lines lie on opposite sides of a reference plane parallel to the tails and containing the symmetry axis for the system. σ-Types are further subdivided into a σ_a-types, in which the porphyroclast is isolated in a relatively homogeneous matrix, and σ_b-types, in which the porphyroclast system is associated with a shear band foliation in the matrix. δ-Types typically have narrow recrystallized tails whose median lines cross the reference plane adjacent to the porphyroclast. Consequently, embayments of matrix material occur adjacent to the porphyroclasts and the tails display characteristic bends.A porphyroclast system in a mylonite develops when the relatively weak dynamically recrystallized grain aggregate in the porphyroclast mantle changes its shape due to non-coaxial flow in the adjacent matrix. This behaviour has been modelled in shear box experiments. Passive marker lines around rigid cylinders embedded in silicone putty were subjected to simple shear. The experiments were modified to simulate a change in recrystallization rate (R) with respect to rate of deformation (γ) by decreasing the diameter of the rigid cylinder during deformation at variable rates. The ratio R/γ appears to be one of the most important factors in determining which porphyroclast system will develop. At high R/γ values, flow of recrystallized material away from the porphyroclast is continuously appended by the production of new grains and wedge-shaped σ_a-type tails develop. At low R/γ values, relatively few new grains are added to the tails which become thinned and deflected by drag due to the spinning motion of the porphyroclast. In addition, most porphyroclast systems at low shear strains are of σ_a-type or lack monoclinic symmetry, whereas δ-types are only developed at high shear strain values. Complex porphyroclast systems, characterized by two generations of tails, are observed in many of the natural and model shear zones studied and may form due to fluctuating R/γ. Conditions that allow isolated σ_a- and δ-type porphyroclast systems to be used as sense of vorticity indicators are: the systems should have a monoclinic shape symmetry; matrix grain size should be small with respect to porphyroclast size; matrix fabric should be homogeneous; deformation history should be simple, and observations should be made on sections normal to the inferred bulk vorticity vector for the mylonite.     

Vein arrays as kinematic indicators in kinked anisotropic materials

Vein arrays associated with natural kink bands have been, even recently, used by some workers as kinematic markers to distinguish between the migration and the rotation models of kink band mechanics.In an outcrop of schistose meta-agglomerate, conjugate kinks and synthetic sinistral shears are associated with a component of sinistral slip along the foliation. Discordant scalariform vein arrays form in association with the shears, but not the kinks. Tension gash elements of the arrays form by combined layer-parallel slip and layer-parallel extension along the rotated schistosity. They are only kinematically significant at the scale of the length of the rotated segment anisotropy.Applying this analysis to kink bands elsewhere, it is concluded that discordant vein arrays are not diagnostic of the rotational kink band model, or of high resolved shear stress on kink boundaries.     

Mineral fish: their morphological classification,usefulness as shear sense indicators and genesis

Mineral fish are sheared and commonly asymmetric mineral grains or clusters of grains. This work reports 11 sub-types of mineral fish showing a top-to-SE sense of ductile shearing in the Karakoram Metamorphic Complex (KMC). The mineral fish are of three broad geometries: sigmoid, lenticular and parallelogram. Reliable senses of shear are indicated by the overall asymmetry and inclination of mineral fish. On the other hand, the true shear sense is not always indicated by either the orientations of their cleavage planes or those of the individual grains in composite mineral fish. The ranges of local orientations of single sigmoid mineral fish that include the lower values (<23°) in the KMC indicate their extensive ductile shearing. The studied mineral fish were products of a range of deformation mechanisms including homogeneous deformation, simple shear, intra-granular slip, crystal-plastic deformation, fracturing and synthetic shearing. Additionally, some examples might have undergone duplex slips and a few nucleated and grew either prior to or during the top-to-SE shearing.     

伸展褶劈理(C’面理)在递进变形过程中的行为及其对剪切带面理(S面理)的影响

伸展褶劈理（extensional crenulation cleavage,简称C’面理）作为韧性剪切带内常见的构造面理之一,在递进变形过程中是否旋转以及其如何影响剪切带面理（S面理）是尚未解决的构造地质前沿问题。本文统计了不同剪切带中C’面理、S面理和C面理（糜棱岩面理）的夹角。结果发现,从剪切带边缘向中心随着应变逐渐增大,C’面理与剪切平面（C面理）的夹角（α）呈近正态单峰式分布,表明C’面理在递进变形过程中不旋转。在C’面理发育的情况下,S面理与C面理的夹角（β）随着应变的增加逐渐增大,即S面理在C’面理形成后发生反向旋转,因此原用于计算剪应变的公式：tan 失效。强弱互层材料的应变分配结果显示,一般剪切条件下,粘性分层与剪切平面的夹角（β）减小到20°时,强硬层的运动学涡度接近于0,即表现为纯剪切,而软弱层表现为近简单剪切;β小于10°时,强硬层发生反向旋转。由此可见,伸展褶劈理是不均匀岩层在递进变形过程中应变分配（strain partitioning）的结果。随着递进变形的进行,S面理与C面理的夹角逐渐减小,强硬层被分配以更多的纯剪切而布丁化,随后内部产生微型剪切条带并扩展形成C’面理,承担应变分配的简单剪切。至此,随着应变分配的完成,C面理上的剪切作用自行停止,因此C’面理不发生旋转,而S面理在C’面理的制约下开始反向旋转。     

Sterols as ecological indicators

Plots of the C-27, C-28 and C-29 sterol contents of marine plankton, higher plants, soils, and lacustrine and marine sediments form discrete areas in a triangular diagram. Because the positions of these plots of the various samples are relatable to biological sources, sterol analyses may be used to define ecological systems.     

Tectonic transport directions of the Lycian nappes in southwest Turkey constrained by kinematic indicators

The orientation, asymmetry and cross-cutting relationships of the structures along the contact zone between the Lycian nappes and the Menderes Massif suggest the presence of three deformation phases in the Milas region of southwest Turkey. The first deformation phase (D1) is characterized by a ductile deformation with top-to-the-NE sense of shear. Structural data of the first deformation measured along the uppermost part of the Menderes Massif and the base of the Lycian nappes suggest that the lowermost unit of the Lycian nappes was emplaced initially from southwest to northeast onto the Menderes Massif during the Early Eocene. The second deformation phase (D2) is also ductile in nature and is characterized by an E–W-trending stretching lineation with a bivergent sense of shear, which is probably related to the load of the overlying nappes. A third deformation phase (D3) is characterized by south-dipping normal faults with top-to-the-S sense of movement. This third deformation phase can be related to southward movement of the Lycian nappes along a low-angle décollement zone. The tectonic contact between the Menderes Massif and the Lycian nappes and their strongly-deformed rocks are unconformably covered by approximately flat-lying, coal-bearing Early–Middle Miocene sedimentary rocks, which constrains the upper time limit for all three deformation phases.     

Biomarker Compounds as Indicators of Paleoenvironments

In an attempt to assess the paleoenvironments of terrestrial sediments,some twenty-two representa-tive Chinese non-marine sediment samples were studied using the molecular organic geochemistry method.The sediment samples stuied include oilshale,shale,mudstone and glauber salt from Tertiary to Cretaceous in age.Judging from geological/geochemical data and paleosalinity data,the samples studied are of lacustrine sedimentary origin and can be divided into three different types:fresh water,brackish and saline/hypersaline lake sediments.The aliphatic fractions were separated from the extracts of the samples and analysed by means of GC and GC/MS instruments,giving a number of parameters such as relative abundances of alkanes and cycloalkanes as shown in the mass chromatograms.     

粘土矿物对古气候指示作用浅析

由于粘土矿物的成分、组合、含量以及结晶度的变化可以反映母源区气候冷、暖周期性变化，因此利用粘土矿物重建古气候环境近年来得到广泛的应用。然而，由于粘土矿物的敏感性，其气候记录的信息往往会被其他影响因素(如母源区、沉积环境、成岩作用等)掩盖。因此，在利用粘土矿物重建古气候环境时，需要综合考虑气候以及非气候的各种影响因素。     

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.     

Carbonate lithofacies as paleolatitude indicators: problems and limitations

Detailed study of Miocene carbonates in the Mediterranean region and their analogues on modern carbonate shelves (in the Mediterranean Sea, Brazil and other areas) reveals at least three major types of carbonate platform lithofacies in addition to the classic tropical coral reef (chlorozoan) lithofacies: (a) chloralgal lithofacies, similar to the chlorozoan, but without hermatypic corals; (b) rhodalgal lithofacies, characterized by abundant encrusting coralline algae; and (c) molechfor lithofacies, consisting of benthic foraminifers, molluscs, echinoids, bryozoans and barnacles. These carbonate lithofacies present complex distribution patterns seemingly related primarily to latitude and depth that control water temperature, although other factors (e.g., water circulation, river discharge, suspended sediment) controlling water salinity and temperature, nutrient content, light penetration, etc., also play important roles. Chloralgal and rhodalgal lithofacies can be considered two transitional terms between the two end-members: the chlorozoan lithofacies, which characterizes shallow tropical shelves; and the molechfor lithofacies, which characterizes colder and/or deeper areas. Detailed textural and sequential analysis are required for satisfactory interpretation of these lithofacies in ancient rocks.     

Porphyroblast nucleation, growth and dissolution in regional metamorphic rocks as a function of deformation partitioning during foliation development

Abstract In regional metamorphic rocks, the partitioning of deformation into progressive shearing and progressive shortening components results in strain and strain-rate gradients across the boundaries between the partitioned zones. These generate dislocation density gradients and hence chemical potential gradients that drive dissolution and solution transfer. Phyllosilicates and graphite are well adapted to accommodating progressive shearing without necessarily building up large dislocation density gradients within a grain, because of their uniquely layered crystal structure. However, most silicates and oxides cannot accommodate strain transitions within grains without associated dislocation density gradients, and hence are susceptible to dissolution and solution transfer. As a consequence, zones of progressive shearing become zones of dissolution of most minerals, and of concentration of phyllosilicates and graphite. Exceptions are mylonites, where strain-rates are commonly high enough for plastic deformation to dominate over diffusion rates and therefore over dissolution and solution transfer. Porphyroblastic minerals cannot nucleate and grow in zones of active progressive shearing, as they would be dissolved by the effects of shearing strain on their boundaries. However, they can nucleate and grow in zones of progressive shortening and this is aided by the propensity for microfracturing in these zones, which allows rapid access of fluids carrying the material presumed to be necessary for nucleation and growth. Zones of progessive shortening also have a number of characteristics that help to lower the activation energy barrier for nucleation, this includes a build up of stored strain-energy relative to zones of progressive shearing, in which dissolution is occuring. Porphyroblast growth is generally syndeformational, and previously accepted criteria for static growth are not valid when the role of deformation partitioning is taken into account. Porphyroblasts in a contact aureole do not grow statically either, as microfracturing, associated with emplacement, allows access of fluids in a fashion that is similar to microfracturing in zones of progressive shortening. The criteria used for porphyroblast timing can be readily accommodated in terms of deformation partitioning, reactivation of deforming foliations, and a general lack of rotation of porphyroblasts, with the spectacular exception of genuinely spiralling garnet porphyroblasts.     

Trace elements in magnetite as petrogenetic indicators

We have characterized the distribution of 25 trace elements in magnetite (Mg, Al, Si, P, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Ge, Y, Zr, Nb, Mo, Sn, Hf, Ta, W, and Pb), using laser ablation ICP-MS and electron microprobe, from a variety of magmatic and hydrothermal ore-forming environments and compared them with data from the literature. We propose a new multielement diagram, normalized to bulk continental crust, designed to emphasize the partitioning behavior of trace elements between magnetite, the melt/fluid, and co-crystallizing phases. The normalized pattern of magnetite reflects the composition of the melt/fluid, which in both magmatic and hydrothermal systems varies with temperature. Thus, it is possible to distinguish magnetite formed at different degrees of crystal fractionation in both silicate and sulfide melts. The crystallization of ilmenite or sulfide before magnetite is recorded as a marked depletion in Ti or Cu, respectively. The chemical signature of hydrothermal magnetite is distinct being depleted in elements that are relatively immobile during alteration and commonly enriched in elements that are highly incompatible into magnetite (e.g., Si and Ca). Magnetite formed from low-temperature fluids has the lowest overall abundance of trace elements due to their lower solubility. Chemical zonation of magnetite is rare but occurs in some hydrothermal deposits where laser mapping reveals oscillatory zoning, which records the changing conditions and composition of the fluid during magnetite growth. This new way of plotting all 25 trace elements on 1 diagram, normalized to bulk continental crust and elements in order of compatibility into magnetite, provides a tool to help understand the processes that control partitioning of a full suit of trace elements in magnetite and aid discrimination of magnetite formed in different environments. It has applications in both petrogenetic and provenance studies, such as in the exploration of ore deposits and in sedimentology.     

The use of hydroxyacids as geochemical indicators

The distributions of hydroxyacids in a variety of Recent and ancient sediments have been determined by gas chromatography-mass spectrometry. A lacustrine sediment (Rostherne Mere, U.K.) and two marine situations (Cariaco Trench, Black Sea) were analyzed as examples of Recent depositional environments. β-hydroxyacids occurred in all of the Recent sediments, consistent with their presumed microbial origin, whereas di- and trihydroxyacids were present in the sediments with a significant higher plant input (Black Sea and Rostherne Mere sediments). The two ancient (Eocene) sediments examined, viz. the Greene River (U.S.) and Messel (Germany) oil-shales, contained only trace amounts of hydroxyacids. These results provide evidence that hydroxyacids are in general not well preserved over geological time.     

Periglacial phenomena and their significance as climatic and edaphic indicators

As formed by frost-action and the presence of intensely frozen ground periglacial phenomena, especially microrelief features, are good examples for geomorphic phenomena as being clearly dependent upon specific climatic conditions. Whereas climate controls and delimits the formation and occurrence of periglacial phenomena at a zonal and regional scale, non-climatic vegetational, topographical and edaphic factors are significant within the climatically defined boundary conditions at a smaller scale. It can be shown that each type of periglacial phenomenon is dependent on specific climatic and edaphic conditions. Data on these conditions are compiled in several tables. On the other hand, because of this dependence on specific environmental conditions periglacial phenomena are at the same time diagnostic indicators for such conditions. As to climate they are indicators for specific climatically defined types of frozen ground and for thermal conditions in terms of mean values, such as mean air temperatures, freezing and thawing indexes. Spatial associations of genetically different types of periglacial phenomena indicate the extent and regional subdivision of the periglacial environment and its climatic limits. The definition of the climate-diagnostic value of present periglacial phenomena is an essential perequisite for the palaeoclimatic interpretation phenomena are sensitive to climatic variations of small amplitude and as such they are also indicators of recent short-term climatic changes. As to edaphic conditions they indicate frost-action potential of the groud and various soil properties which have practical implications. As edaphic indicators periglacial phenomena are of significance for terrain evaluation programmes in the Arctic for practical purposes.     

Calcite strains, kinematic indicators, and magnetic flow fabric of a Proterozoic pseudotachylyte swarm, Minnesota River valley, USA

Near Granite Falls, Minnesota sub-parallel pseudotachylyte, mafic dikes, and calcite veins crosscut Archean granulite facies rocks in the Minnesota River valley adjacent to the north-dipping Yellow Medicine Shear Zone (YMSZ; N80°E) that separates the Montevideo and Morton tectonic terranes. The docking of these two Archean terranes occurred prior to intrusion of the 2.067 Ga Kenora-Kabetogama dike swarm as demonstrated by aeromagnetic anomalies (correlated with field exposures) that cross the YMSZ without offset. Tectonic adjustments along the YMSZ associated with the Penokean Orogeny ( 1.8 Ga) are likely responsible for pseudotachylyte formation.Pseudotachylyte is exposed in 22 sub-parallel veins ( N80°E, 90°) each less than 2 cm wide across an outcrop width of 45 m. The pseudotachylyte matrix is commonly banded, and contains crystal fragments (quartz, plagioclase, amphibole, rutile, apatite, ilmenite, ulvöspinel), magnetite microlites, flow banding swirls, amygdules (filled with calcite, ankerite and siderite), collapsed vesicles, and abundant lithic clasts. Pseudotachylyte formed in a number of phases. Kinematic reconstruction is complex, utilizing winged porphyroclasts, S-C structures in the country rock, and fault drag indicators along the pseudotachylyte zones. Dextral motion along the YMSZ is the most common observation. Mechanically twinned calcite within amygdules in the pseudotachylyte preserves horizontal shortening normal to the pseudotachylyte strike. Calcite veins are apparently contemporaneous with the pseudotachylyte; one set preserves twinning strains identical to the calcite amygdule strains, and the second set contains a horizontal, vein-parallel (N70°E) shortening strain. The pseudotachylyte contains a flow fabric, as determined by AMS techniques, that is a proxy for vertical flow (K_max is vertical). The Kenora-Kabetogama dikes, identified geochemically, are locally parallel to the pseudotachylyte and the adjacent YMSZ tectonic suture and preserve a vertical-to-horizontal, dike-parallel AMS fabric from east (Franklin) to west (Granite Falls). Hornblende andesite dikes (055°, 1.8 Ga) are not found south of the suture, are not associated with pseudotachylyte and have a different paleopole and AMS fabric.     

Marine terraces as indicators of global geoidal and hydrological changes

Spatial pattern analysis of marine terrace elevations from 40–30 thous. years BP was used to reconstruct sea level/geoid surface and geoid parameters during that time. The polar flattening of geodetic ellipsoid was lower than its present value (1/298.81 and 1/298.26) respectively because of glacial-induced mass redistribution. Increase in polar stress occurred during the last 30 thous. years was possibly driven by pertubation brought to the gravitational field by disintegration of polar ice sheets in the Northern hemisphere. But the polar flattening value becomes only half-restored during the last deglacial hemicycle. So repetitive glacial advances during Pleistocene acted as a global “pump” for uncondensed zones at the upper/lower mantle boundary. Dissipation of tidal energy is an order of magnitude less intensive in its Earth's rotation effect. A contribution to the International Geological Correlation Programme Project JGCP — 274 “Coastal Evolution in the Quaternary”.     

Omphacite microstructures as time-temperature indicators of blueschist- and eclogite-facies metamorphism

Omphacites from a wide range of geological environments have been examined by transmission electron-microscopy. Their microstructures are sufficiently variable as to be potential indicators of thermal history for blueschist and eclogite metamorphism. In particular, the average size of equiaxed antiphase domains (APD's) arising from cation ordering appears to be a characteristic feature of each environment and increases in the sequence: Franciscan, blueschist (1) ≈ Turkey, blueschist (2) < Guatemala, jadeitic blocks in serpentinite (3) < Syros, blueschist (9) ≈ Red Wine Complex, Canada, amphibolite (1) < Maksyutov Complex, Urals, blueschist (3) ≈ Zermatt-Saas, blueschist (5) ≈ Allalin, metagabbro (4) < Tauern, eclogite (1) ≈ Franciscan, eclogite (5) < Nybö, Norway, eclogite (2) (numbers in brackets indicate the number of hand specimens for which omphacite microstuctures are known). A relationship between APD size, annealing time and temperature has been derived by analogy with the known APD coarsening behaviour in other systems where: (APD size)ⁿ $$({\text{APD size)}}^{\text{n}} \propto {\text{e}}^{{\text{(}} - {\text{Q/RT)}}} \cdot {\text{ }}time{\text{.}}$$ . Most omphacites fit into a self-consistent scheme with n=8±2 if the activation energy (Q) is assumed to be that of cation disordering (75 kcal mole^?1), available estimates of peak metamorphic temperature (T) are used, and a reasonable geological time-scale is taken as 10⁴–10⁸ years. According to this model, APD sizes are set in a relatively short interval of the total history of a rock when its temperature is close to its peak value. APD sizes are much more sensitive to temperature than to time and may be used as a geothermometer which has the advantage of not being reset by re-equilibration at low temperatures. Petrological implications arising from the model are that Allalin metagabbros were metamorphosed at a similar peak temperature to Zermatt-Saas blueschists, Franciscan eclogites reached higher temperatures than has been previously supposed and that the microstructures in some Sesia-Lanzo omphacites are consistent with a high temperature, pre-blueschist origin. Deviation from an ideal coarsening law with n=2 implies that the APD's are not simply stacking mistakes but have some associated structural or compositional modification locally. Excess titanium concentrated at APD's in Red Wine Complex omphacites may account for their anomalously low observed APD size.