Published by the The Society of Rheology
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SynopsisThe slump test, initially developed to determine the flow properties of fresh concrete, has been adopted as a means of accurately measuring the yield stress of strongly flocculated suspensions. The slump test offers a quick and easy way of measuring yield stress without the need for sophisticated electronic equipment, thereby giving plant operators an effective tool for determining yield stress. The model used to predict the yield stress from the conical slump test was devised by Murata ͑1984͒ and corrected by Christensen ͑1991͒. In the present case the theory has been adapted for a cylindrical geometry. Yield stress measurements obtained with the vane for numerous mineral suspensions under known surface chemistry conditions are compared to the slump measurements and theoretical prediction. Good agreement is obtained.
A series of model experiments which were designed to simulate certain aspects of the development of large-scale folds in bedded sedimentary rocks are described and illustrated. The models used were multilayers made up of thin sheets of gelatine. Two types of experiment were carried out. In the first a section of the base of a horizontally layered model was slowly raised to simulate uplift and gravity gliding. In the second a horizontally layered model was laterally compressed by a slow moving piston. The initial development of lateral compressive stress in both types of model was achieved by the propagation of a stress front along the length of a model. In a number of experiments this stress front played an important role in localizing the initiation of folds. The existence and rate of propagation of the stress front were related to the presence of shear stresses along the base of a model. The development of folds in the majority of experiments resulted in well defined anticlines separated by broad, poorly defined synclinal zones. Individual anticlines were initiated at different times during the course of an experiment and this resulted either in the serial development of a train of folds or in the development of isolated anticlines. Asymmetric anticlines were, during the initial stages of amplification, upright symmetrical structures. Asymmetry tended to develop late in the amplification history of such folds. In three dimensions, folds tend to be non-cylindrical and adjacent structures form an array of
en echelon
periclines.
The experimental technique of centrifuge modelling has been used to elucidate patterns of total strain and progressive deformation associated with troughs that form by gravitational subsidence of a dense cover into a less dense basement. Model structures are analogous to synclines found between mantled gneiss domes, and the models provide data with which to test the hypothesis that Archean greenstone belts are products of vertical gravity tectonics.Experimental results include the following. (1) Strain within the main body of subsiding belts is dominated by strong horizontal contraction and vertical extension. In natural structures this strain pattern would produce vertical foliation and steeply plunging mineral lineation. (2) Within cover units near trough margins the flattening plane trends parallel with or at a low angle to the cover–basement interface at all levels in the trough. (3) Patterns of strain within subsiding troughs, at least as expressed in surface deformation, reflect the geometry of the cover–basement interface at depth. Flow within the cover units converges towards points of maximum trough subsidence to produce local vertical constrictive strain. Deformation above trough saddles is characterized by vertical flattening strain. (4) Horizontal shortening within a subsiding, stratified cover sequence produces early folds with horizontal axes and steep axial surfaces. Where the stratified sequence forms a relatively low-density unit within the cover, folds and thrusts verge consistently away from the trough axis. Continued subsidence and horizontal contraction tend to rotate initially shallowly dipping structures into steeply dipping attitudes.
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