Orientation Of Principal Stress Axes Research Articles

Selected geomorphic method applied to neotectonic studies in the Northern Carpathians

A number of geomorphic techniques have been applied to study morphotectonic pattern of the Northern Carpathians. Apart from classical ones, a wide range of morphometric and statistical methods has also been used. Among these, the construction of the base-level, goniobase and isolong maps require most serious consideration. Neotectonic implications can also be drawn from careful analyses of geometries of the valley network. The highest bifurcation ratios are usually confined to areas showing most intensive Quaternary uplift. The same applies to increased drainage density measures and the ratio of first-order valleys to the total number of valleys. Parameters of longitudinal river bed profiles are sometimes indicative of tectonic tendencies. The North Carpathian rivers reveal an eastward increase in order, length and relief, being accompanied by a decrease in the mean river bed gradient. The highest concavity measures express the river profile maturity while the position of inflexion point delimits sectors modelled by erosional and accumuiationai processes. Another opportunity for morphotectonic reasoning is provided by the studies of orientation of principal stress axes, inferred from the drainage pattern. All these techniques should be verified by detailed geological mapping, aided by interpretation of aerial photographs and radar images, as well as by geophysical soundings.

From orientation to magnitudes in paleostress determinations using fault slip data

Determinations of reduced stress tensors using fault slip data yield the orientation of principal stress axes and the ratio Φ of the differences between principal stress magnitudes. The use of rupture and friction laws allows determination of the two remaining unknowns, that is, the reconstruction of the complete stress tensor. Taking into account the depth of overburden brings an additional constraint. The method is applied and discussed in the case of the Hoover Dam site (western U.S.A.), where large data sets and rock mechanics information are available. Differences between intact sample and rock mass properties account for apparent disagreements between paleostress levels determined in similar tectonic environments. Pore pressure plays an important role; where information about pore pressure is absent, zero and hydrostatic pore pressure cases should be considered as limits.

Petrofabrics: A critical review

Petrofabric contributions are here classified into seven categories, namely: (1) descriptive techniques; (2) kinematic analyses; (3) geometric analyses of folds; (4) absolute strain analyses; (5) studies of experimentally deformed single crystals and rocks; (6) dynamic analyses of deformation features in naturally deformed rocks; and (7) studies for engineering rock mechanics. The nature of each subfield is given, and items (2) and (6) are discussed in detail.The principles of kinematic analysis through symmetry are reviewed with the aid of simple models of brittle and ductile deformations in which componental displacements are specified and the symmetry of the initial and final fabric is known. The models illustrate the degree of similarity among the symmetries of the fabric, componental displacements, deformation (strain and rigid-body rotation), and states of stress that exist in rocks at the time a fabric element forms. Both isotropic and anisotropic models are considered. In the more realistic cases the symmetry of the pattern of componental displacements is identical to the symmetry of the resulting fabric. If the body behaves isotropically during the deformation, symmetry elements in the final fabric will be common to corresponding elements in the symmetries of the states of stress and deformation. If, however, the body behaves anisotropically one cannot relate symmetry elements in the final fabric to the symmetry of the states of stress and deformation without making assumptions about the symmetry of the anisotropy. The study of fabric symmetry may contribute some detail to the geometric pattern of the strain or displacement fields, but is otherwise of little use in furthering understanding of the mechanics of geologic deformation.Results of dynamic analysis, which involve determination of the orientations and relative magnitudes of principal stresses in naturally deformed rocks, are critically reviewed. The principal axes inferred from fractures, twin lamellae, deformation lamellae, etc., are those within the material at the instant the fabric element forms. As such they correspond to the incremental stresses which are coincident with the incremental strains in linear materials. Certain generalizations are drawn from previous work on slightly and moderately deformed rocks in which the orientation of the incremental stresses and strains are probably coincident with the total stresses and strains. The critique leads to a working philosophy to guide future studies, namely: dynamic petrofabric analyses provide the means for testing hypotheses generated through theoretical and experimental studies of rock deformation. A study should be designed to test specific hypotheses. Samples should be collected from critical domains where the results may provide a basis of choice between alternative hypotheses. The techniques need to be refined to distinguish small differences in the orientations of principal stress axes deduced from fabric data. The chronological sequence in the development of the fabric elements requires attention. Several suggestions along these lines are offered.

Principal Stress Directions from Plastic Flow in Crystals

Methods for determining orientations of principal stress axes in deformed rocks involve dynamic analysis of twin-gliding and of extinction bands produced by inhomogeneous translation gliding in crystals. The methods, beginning with Turner9s (1953) technique for dynamic analysis of calcite twins, have been developed using as guides the results from experiments under controlled laboratory conditions. Structures induced by intragranular flow in calcite, dolomite, quartz, micas, orthopyroxenes, clinopyroxenes, olivine, and other common rock-forming materials, may now be used to derive orientations of principal stresses causing the deformation. The various methods, some new, are discussed in detail and examples of their application to tectonites are given. The usefulness of such studies is illustrated by evaluating the observed orientations of principal stresses around folds in light of new data from a theoretical analysis of large amplitude folding of viscous layers in a less viscous matrix. Other areas of research in structural geology in which these methods should prove useful have also been outlined.