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1.
A statistical analysis for the Rf/φ method is developed in the paper. The technique provides estimates of Rs, Ri and θ according to Rf and φ. Moreover, we can restore the pattern of initial orientations of marker objects by means of their frequency histogram, provided the strain is homogeneous. It is unnecessary to postulate random initial orientations if the principal directions of strain ellipsoid are known.  相似文献   

2.
A recent note by De Paor (1980) suggests that finite strain cannot be decomposed into tectonic and compaction components on the basis of asymmetry of the Rfφ data about the mean φ trace. This is true if only the shape of the Rfφ data field is considered, but shown to be incorrect when the distribution of data within this field is considered.  相似文献   

3.
The strain associated with the Horn Head Slide, a major tectonic break in the Dalradian rocks of NW Ireland, is recorded by pebbles in an adjacent quartzite horizon. Mean X/Y ratios of the deformed pebble shapes in excess of 8.0 are seen closest to the slide and the field of three-dimensional shapes lies along the K = 1 line. The usual methods of separating initial shape ratio (Ri) and tectonic strain ratio (Rs) from the deformed shape ratio (Rf) of ellipsoidal markers are based on measurements of variation in fluctuation (e.g., the (Rf/φ technique). However, due to the high X/Y strains in this situation and since the pebbles initially lay parallel to bedding and to a principal plane of the finite strain ellipsoid, fluctuation is generally very low. Thus, except for the least deformed X/Y data, the Rf/φ technique is inapplicable and other methods are used. For X/Y data with mean (Rf > 4.0: Rs is calculated as the harmonic mean of Rf; maximum Ri values only are obtained from the range of Rf data. For all Y/Z and X/Z data: Ri is calculated from the logarithmic range (ωlog) of Rf; Rs is simply obtained from the geometric mean of Rf modified by Ri. It is concluded from this that a varying prolate tectonic strain (K - 1.5) reaching X/Y values in excess of 8.00 was coaxially superimposed on an initial oblate shape fabric to produce the present field of deformed pebbles in the quartzite near the slide.  相似文献   

4.
Shear strain γ in brittle fault zones is related to final and initial grain size parameters, df and di, respectively, by an expression of the form: where μk the coefficient of kinetic friction of crushed rock and σn the normal stress across the zone at the time of faulting.Technological literature suggests that ƒ(df, di) may be given by 10 , where Wi is a material constant. The resulting relationship between shear strain and grain size seems to be compatible with existing experimental data.  相似文献   

5.
Measuring strain from deformed xenoliths is problematic due mainly to the large initial shape variations of these markers. A method is described which allows mean initial shape to be determined for a number of xenolith populations by displaying their logarithmic ranges (log Rfmax — log Rfmax) on a Range diagram. The diagram contains a check on validity and allows bad samples to be recognised. Xenolith data from a deformed granite in northwest Ireland is analysed using the method.  相似文献   

6.
Measurements of total, incremental and progressive strains associated with the development of small scale crenulation cleavage in some low-grade metamorphic rocks from Australia and Switzerland are applied to a discussion of the mechanical significance of the cleavage.Limits are placed on the amount of incremental and total slip or simple shear possible along the cleavage by the observation that the XY principal plane trace of bulk total crenulation strain coincides within 4° of the crenulation cleavage trace in all cases where this strain has been measured or estimated. The measurements are made on eight specimens using deformed porphyroblasts, crystal fibres in pressure-shadows around pyrite and flattened folds and include deformations with coaxial and non-coaxial histories.Further measurements derived from pressure-shadow fibres (eight specimens) show that the style and orientation of incremental deformation are essentially independent of the crenulation cleavage, except for a limit (43°) to the obliquity of the principal incremental extension axis during a given cleavage episode. The only special deformation related to the cleavage is the coaxial one. An indication of passive cleavage behaviour at high strain is shown by the progressive strain history of one specimen. Evidence for passive rotation of a transected axial plane is shown by another. A model is proposed to account for these observations, especially the conditions necessary for initiation and continued development of a new cleavage fabric.Some further applications of existing strain measurement techniques are described: of the Rf/Øf method to heterogeneously superposed tectonic strains and of an improved procedure of t′α/α flattening analysis.  相似文献   

7.
A new diagram for the representation of stress states is proposed and compared with Nadai's stress diagram. The diagram is a graph whose axes are labelled as the differences of the principal stresses (σ2-σ3 as ordinate axis, σ1-σ2 as the abscissa; where σ1 > σ2 > σ3 are the principal stresses). The design of the plot has been deliberately modelled on that of the ‘log Flinn’ diagram which is used to represent finite strain ellipsoids. The position of the plotted stress state on this diagram depends on the nature of the deviatoric (non-hydrostatic) component of the stress tensor. The distance of the plotted stress from the origin corresponds broadly to the departure of the stress from a hydrostatic state and the parameter R, defined as the gradient of the line joining the plotted state to the origin, expresses the type of symmetry possessed by the stress tensor.It is explained how the diagram can be used to represent calculated palaeostresses and, in particular, how the parameter R can be found directly from some existing methods of stress analysis currently in use.Besides its proposed function to represent the results of such analyses it is felt that the use of the diagram may make clear the essential elements of the definitions of well-known terms used to describe particular stress states (e.g. plane stress, triaxial stress, axial compression, etc.).  相似文献   

8.
Marine, organic-rich rock units commonly contain little for vitrinite reflectance (VR0) measurement, the most commoly used method of assessing thermal maturity. This is true of the Lower Jurassic “Nordegg Member”, a type I/II, sulphur-rich source rock from the Western Canada Sedimentary Basin. This study examines the advantages and pitfalls associated with the use of Rock-Eval Tmax and solid bitumen reflectance (BR0) to determined maturity in the “Nordegg”. Vitrinite reflectance data from Cretaceous coals and known coalification gradients in the study area are used to extrapolate VR0 values for the “Nordegg”.Tmax increases non-linearly with respect to both BR0 and extrapolated VR0 values. A sharp increase in the reflectaance of both solid bitumen and vitrinite occurs between Tmax 440–450°C, and is coincident with a pronounced decrease in Hydrogen Index values and the loss of solid bitumen and telalginite fluorescence over the same narrow Tmax interval. This Tmax range is interpreted as the main zone of hydrocarbon generation in the “Nordegg”, and corresponds to extrapolated VR0 values of 0.55–0.85%. The moderate to high sulphur contents in the kerogen played a significant role in determining the boundaries of the “Nordegg” oil window.A linear relationship between BR0 and extrapolated VR0, as proposed elsewhere, is not true for the “Nordegg”. BR0 increases with respect to extrapolated VR0 according to Jacob's (1985) formula (VR0=0.618×(BR0)+0.40) up to VR0≈0.72% (BR0≈0.52%). Beyond this point, BR0 increases sharply relative to extrapolated VR0, according to the relatioship VR0 = 0.277 × (BR0) + 0.57 (R2 = 0.91). The break in the BR0−VR0 curve at 0.72%VR0 is thought to signifiy the peak of hydrocarbon generation and represents a previously unrecognized coalification jump in the solid bitumen analogous to the first coalification jump of liptinites.  相似文献   

9.
Simple shear of deformable square objects   总被引：1，自引：0，他引：1  
Finite element models of square objects in a contrasting matrix in simple shear show that the objects deform to a variety of shapes. For a range of viscosity contrasts, we catalogue the changing shapes and orientations of objects in progressive simple shear. At moderate simple shear (γ=1.5), the shapes are virtually indistinguishable from those in equivalent pure shear models with the same bulk strain (RS=4), examined in a previous study. In theory, differences would be expected, especially for very stiff objects or at very large strain. In all our simple shear models, relatively competent square objects become asymmetric barrel shapes with concave shortened edges, similar to some types of boudin. Incompetent objects develop shapes surprisingly similar to mica fish described in mylonites.  相似文献   

10.
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 σa-types, in which the porphyroclast is isolated in a relatively homogeneous matrix, and σb-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 σa-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 σa-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 σa- 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.  相似文献   

11.
In normal faulting regimes, the magnitudes and orientations of the maximum and minimum principal compressive stresses may be known with some confidence. However, the magnitude of the intermediate principal compressive stress is generally much more difficult to constrain and is often not considered to be an important factor. In this paper, we show that the slip characteristics of faults and fractures with complex or nonoptimal geometry are highly sensitive to variation or uncertainty in the ambient effective intermediate principal stress (σ′2). Optimally oriented faults and fractures may be less sensitive to such variations or uncertainties. Slip tendency (Ts) analysis provides a basis for quantifying the effects of uncertainty in the magnitudes and orientations of all principal stresses and in any stress regime, thereby focusing efforts on the most important components of the system. We also show, for a normal faulting stress regime, that the proportion of potential surfaces experiencing high slip tendency (e.g., Ts ≥ 0.6) decreases from a maximum of about 38% where σ′2 = σ′3, to a minimum of approximately 14% where σ′2 is halfway between σ′3 and σ′1, and increases to another high of approximately 29% where σ′2 = σ′1. This analysis illustrates the influence of the magnitude of σ′2 on rock mass strength, an observation previously documented by experimental rock deformation studies. Because of the link between fault and fracture slip characteristics and transmissivity in critically stressed rock, this analysis can provide new insights into stress-controlled fault transmissivity.  相似文献   

12.
This study defines the Mio-Pliocene to present-day stress regime acting at the northeastern corner of the eastern Mediterranean region along the Karasu Valley (i.e., the Amanos Range), taking in the Antakya, Osmaniye and Kahramanmaras provinces. The inversion slip vectors measured on fault planes and chronologies between striations indicate that the stress regime varied from transpressional initially to transtensional, having consistent NW- and NE-trending σHmax (σ1) and σHmin (σ3) axes, respectively; there are significantly different mean stress-ratio (Rm) values however. The older mean stress state is characterized by N151±11°E-trending σ1 and N59±12°E-trending σ3 axes, and by a mean arithmetic Rm value of 0.76, indicating that the regional stress regime is transpressional. The younger stress regime is characterized by N154±8°E-trending σ1 and N243±8°E-trending σ3 axes, and by a mean arithmetic Rm value of 0.17, indicating a transtensional character for this regional stress regime. The low R values of the stress deviators related to the recent stress state reflect normal-component slips. The earthquake focal mechanism inversions confirm that the younger stress regime continues into the Recent. The inversion identifies a transtensional stress regime representing strike-slip and an extensional stress state with a consistent NE-trending σHmin (σ3) axis. These stress states are characterized by N66°E and N249°E-trending σ3 axes, respectively. Both significant regional stress regimes induce left-lateral displacement along the southern part of the East Anatolian Fault (EAF, or Amanos Fault). The temporal change, probably in Quaternary time, within the regional stress regime—from transpression to transtension—resulted from the coeval influences of subduction processes in the west–southwest (i.e., along the Cyprus arc), continental collision in the east, and westward escape of the Anatolian block.  相似文献   

13.
In order to study the Pyrenean tectonic phase, a quantitative method of stress analysis using microfault measurements is used on a calcareous plateau located in southern France. The method developed here allows the determination of several tectonic events and the evaluation of (with σ σ2 σ3).The Pyrenean compression is seen to occur in two stages, confirming previous geological studies.A recent canyon allows the study of the variation with depth of the R ratio on a vertical cross-section (300 m). With a simple model of gravity and tectonic stresses, the vertical variation of R can be used to estimate quantitatively the Pyrenean tectonic stresses. For realistic values of the parameters in this model, the horizontal tectonic stresses are obtained in the following range: 50–200 bar for the maximum horizontal principal stress, 10–25 bar for the other horizontal stress.These results seem to be consistent with in situ stress measurements, but they are much lower than those predicted by experimental rock mechanics.  相似文献   

14.
The bulk properties and bitumen molecular compositions of a rank-series of 38 humic coals from the New Zealand Coal Band (Cretaceous–Cenozoic) have been analysed to investigate early maturation processes affecting coaly organic matter through diagenesis to moderate catagenesis (Rank(Sr) 0.0–11.8, Ro 0.23–0.81%). The samples comprise a relatively restricted range of vitrinite rich coal types formed largely from higher land plant material under relatively oxic conditions, but with a significant contribution from microbial biomass. With increasing rank, total organic carbon contents show a general increase, whereas moisture and asphaltene contents decrease. Bitumen yields also decrease through the stages of diagenesis and early catagenesis (Rank(Sr) < 9, Ro < 0.55%), indicating partial loss of initial bitumen during early maturation. Thermal generation of hydrocarbons begins slowly at Rank(Sr)  5–6 (Ro  0.40%) as indicated by the constant occurrence and gradual increase of isoprenoids (e.g., pristane and phytane) and hopanoids in their more mature αβ configuration. This early phase of catagenesis, not previously recognised in New Zealand coals, is followed at Rank(Sr)  9 (Ro  0.55%) by the main catagenesis phase characterised by a more rapid increase in the generation of hydrocarbons, including total n-alkanes, isoprenoids and αβ-hopanes. Changes in the maturity of New Zealand coals can be traced by the Carbon Preference Index and several hopane maturity parameters, including 22S/(22S + 22R), αβ/(αβ + βα) and ββ/(αβ + βα + ββ).  相似文献   

15.
A ‘soft’ carbon-based high-volatile bituminous (Ro max=0.68%) coal and a ‘hard’ carbon-based Pennsylvania anthracite (Ro max=5.27%) were deformed in the steady state at high temperatures and pressures in a series of coaxial and simple shear deformation experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Tests were carried out in a Griggs-t type solid (NaCl) medium apparatus at T=400–900°C, constant displacement rates of 10-5−10-6 s−1, at confining pressures of 0.6 GPa (coaxial) or 0.8 and 1.0 GPa (simple shear). Coaxial samples were shortened up to 50%, whereas shear strains up to 4.9 were attained in simple shear tests. Experiments lasted up to 118 h. Deformed, high-volatile bituminous coal was extensively coked and no correlation between strain and Ro max, bireflectance or coal texture was observed in any samples. With increasing temperature, Ro max and bireflectance increase in highly anisotropic, coarse mosaic units, but remain essentially constant in the fine granular mosaic, which becomes more abundant at higher temperatures. Graphite-like reflectances are observed locally only in highly reactive macerals and in pyrolytic carbon veins. The degree of molecular ordering attained in deformed bituminous coal samples appears to be determined by the heating-pressurization path rather than by subsequent deformation.Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T>700°C, with average bireflectance values increasing from 1.68% at 700°C to 6.36% at 900°C. Anisotropy is greatest in zones of high strain at all temperatures. In anthracite samples deformed in simple shear over the 600–900°C range at 1.0 GPa, the average Ro max values increase up to 11.9%, whereas average bireflectance values increase up to 10.7%. Bireflectance increases with progressive bedding rotation and, thus, with increasing shear strain. Graphitization occurs in several anthracite samples deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy of highly anisotropic material in one sample confirms the presence of graphite with d002=0.3363 nm. These data strongly suggest that shear strain, through its tendency to align basic structural units, is the factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 2200°C required in hydrostatic heating experiments at ambient pressure.  相似文献   

16.
The magnetic susceptibility anisotropy of 275 specimens comprising 38 sites from the Cambrian slate belt in North Wales was measured to determine the magnetic fabric of the slates. The susceptibility ellipsoid is oblate for all sites, and the maximum/intermediate susceptibility plane always coincides with the cleavage plane of the slates which has a Caledonian strike and is nearly vertical. The maximum axes align sub-vertically and the intermediate axes sub-horizontally, trending NE-SW. The minimum susceptibility axes are normal to this foliation plane and coincide with the poles to the slaty cleavage. The orientations of the principal susceptibility axes are found to be in excellent agreement with the orientations of the principal strain directions, determined by X-ray goniometry on one of the samples from almost all of the sites. Correlation of the magnetic susceptibility anisotropy with predicted March strains (March, 1932) shows that the principal magnitudes of susceptibility can be related to those of the strain by: (for i = 1, 2, 3. The orthogonal principal axes), where χf and χ0 are the final and initial susceptibilities along a given axis i and lf and li are final and initial axial dimensions in the same direction i of a principal strain axis. The exponent a for the North Wales slates was found to be 0.145 ± 0.005. Knowledge of such a relationship may permit rapid approximate determinations of a petrofabric in similar rocks from their magnetic fabrics. However, the exponent a will probably have to be recalibrated for each rock type.  相似文献   

17.
Bonini (2009, Structural controls on a carbon dioxide-driven mud volcano field in the Northern Apennines (Pieve Santo Stefano, Italy): relations with pre-existing steep discontinuities and seismicity. Journal of Structural Geology 31, 44–54) presents a 2D mechanical analysis to infer the failure conditions responsible for the seismicity distribution during an Mw = 4.6 seismic sequence nucleating during 2001 in the Northern Apennines. In my view the mechanical analysis presented in this paper has some weakness or is not well constrained, in particular: 1) the assumption of a dip angle of 50°, is not consistent with the activated structures; 2) the Pf = σ3 condition, difficult to be attained along a cohesionless fault dipping at 50°; 3) the isotropic stress state, i.e. σ2 = σ3, that is not consistent with the active or recent stress field in the area.  相似文献   

18.
The influence of vorticity and rheology of matrix material on the development of shape-preferred orientation (SPO) of populations of rigid objects was experimentally studied. Experiments in plane strain monoclinic flow were performed to model the fabric development of two populations of rectangular rigid objects with object aspect ratios (Rob) 2 and 3. The density of the rigid object populations was 14% of the total area. Objects were dispersed in a Newtonian and a non-Newtonian, power law matrix material with a power law exponent n of 1.2. The kinematic vorticity number (Wn) of the plane strain monoclinic flow was 1, 0.8 and 0.6 with finite simple shear strain of 4.6, 3.0 and 0.9, respectively. In experiments with Rob=3, the SPO is strongly influenced by Wn and the material properties of the matrix. Deformation of a power law matrix material and low Wn resulted in a stronger SPO than deformation of a linear viscous matrix and high Wn. Strain localization coupled with particle interaction plays a significant role in the development of a shape-preferred orientation. High strain simple shear zones separate trains of rigid objects that are surrounded by low strain zones with Wn lower than the bulk Wn. In fabrics involving populations of objects with Rob=2, rheology of the matrix materials does not systematically influence the intensity of the SPO.  相似文献   

19.
“Hard” carbon-based Pennsylvania anthracite was deformed in the steady-state at high temperatures and pressures in a series of coaxial and simple shear experiments designed to constrain the role of shear strain and strain energy in the graphitization process. Graphitization did not occur in coaxially deformed anthracite. Nonetheless, dramatic molecular ordering occurs at T 700°C, with average bireflectance values (%) increasing from 1.68 at 700°C to 6.36 at 900°C. Romin is lowest and bireflectance is highest in zones of high strain (e.g., kink bands) at all temperatures.In anthracite samples deformed in simple shear over the 600°–900°C range at 1.0 GPa, average Romax (%) values increase up to 11.9, whereas average bireflectance (%) values increase up to 10.7. Bireflectance increases with increasing shear strain and locally exceeds 12.5%. Graphitization occurs in several anthracite sample deformed in simple shear at 900°C. X-ray diffraction and transmission electron microscopy confirms the presence of graphite with d002=0.3363 nm. These data strongly suggest that shear strain is the dominant factor responsible for the natural transformation of anthracite to graphite at temperatures far below the 1600°C required for graphitization of other hard carbons in earlier hydrostatic heating experiments at 0.5 GPa pressure.  相似文献   

20.
The chromites from the alpine type ultramafic intrusive of Sukinda, India, display a typical partly inverse spinel form and occur in two distinct zones: Brown Ore Zone (BOZ) and Grey Ore Zone (GOZ). The host ultramafites are mostly altered and are represented by the serpentinite, tremolite-talc(chlorite) schist, talc-serpentine schist and chlorite rock. The less altered variants are dunite, harzburgite and websterite. A dyke of orthopyroxenite runs through the main ultramafic body.The composition of olivine (Fo92), orthopyroxene (En92–89) and Al2O3 contents of the parental liquid (10.40–11.45%) determined from chromites, suggest that the parent melt is of boninitic affinity. The chemical plot of TiO2 content against cr# of chromites corroborates a boninitic parental melt. The Fe–Mg partitioning in olivine and chromite depicts the temperature for chromitites as 1200 °C. A compositional plot of mg# and cr# suggests crystallization at high pressure conditions, corresponding to the kimberlite xenolith field. From the P–T diagram of pyrolite melting and mineral assemblage, the pressure of crystallization is stipulated to be ≥1.2 GPa. The fO2 values estimated from Fe3+/Cr+Al+Fe3+ ratios range from 10−8.3 to 10−9.3 for the GOZ and 10−7.1 to 10−7.3 for the BOZ. The fO2 values together with the pressure range suggest crystallization at upper mantle conditions. The heterogeneity in chemical composition and fO2 conditions for the GOZ and BOZ could be linked to heterogeneity in the upper mantle.  相似文献   

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