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|Title:||Strain Rate and Temperature Controls on Strain Heterogeneity: Its Significance for Deformational Concepts|
|Author(s):||Holm, Paul Eric|
|Department / Program:||Geology|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Natural rock deformation is characteristically heterogeneous on all scales. Through experimental deformation of a typical oolitic limestone at upper crustal conditions, an attempt is made to evaluate the relative influence of temperature and strain rate upon the promotion of strain homogeneity. The experimental results are used to help evaluate the important analytical techniques of shape factor analysis and the use of strain heterogeneity to establish a path of progressive deformation.
Samples from the Oolite Series of the British Jurassic were collected in order to investigate the deformational response of relatively young, tectonically undeformed limestones. These rocks may presently be in a similar diagenetic condition as were many naturally deformed limestones at the onset of deformation. Specimens were deformed at 100 MPa confining pressure, room temperature, and 10('-4)/sec. Most specimens deformed permanently at the onset of differential loading and variations in strength correlated well with diagenetic factors such as initial porosity and degree of compaction.
The sample from the Westington Quarry was selected for further study because of its large number of spherical coids and known high ductility at the imposed environmental conditions. Specimens were deformed at a common confining pressure of 200 MPa, temperatures of 25(DEGREES), 100(DEGREES), and 200(DEGREES)C, and strain rates of 10('-4)/sec., 10('-6)/sec. and 10('-7)/sec. Subsequent strain analyses were made on large negative prints of central axial thin sections of the deformed specimens.
At 25(DEGREES)C or 10('-4)/sec. the mode of deformation at high strains was ductile faulting. In contrast, brittle failure occurred at 100(DEGREES)C and 200(DEGREES)C for the two slower rates at axial strains of nearly 40%. The resultant faults were very sharp and were preceded by fine microfaults. Differences in the deformational behavior and strengths of the specimens at the various temperatures and strain rates are explained by the relative contribution of the deformation mechanisms of cataclasis and intracrystalline slip.
A measurable increase in strain homogeneity on the scale of individual ooids was observed with increasing temperature and decreasing strain rate. A qualitative view of the variation in strain homogeneity is obtained by contouring the calculated shortening strains of the individual ooids. A fully quantitative measure is obtained by expressing the degree of shape variation as a coefficient of determination obtained from the least squares fit of the ooid axial data. This homogeneity coefficient varied systematically for specimens deformed to 40% strain from 0.38 at 25(DEGREES)C and 10('-4)/sec. to 0.73 to 200(DEGREES)C and 10('-6)/sec. and 10('-7)/sec. Perfectly homogeneous strain would be represented by a value of 0.80 for this sample.
A comparison of simulated, natural, and experimental deformation of ooids showed that although a certain amount of spread of axial ratios can be attributed to initial shape variation, heterogeneity of strain would impart sufficient additional spread to define the deformation path. The variation in shape and orientation of the ooids that results from heterogeneous stain is attributed by the various shape factor methods, all of which assume homogeneous strain, to variations in initial shape and orientation. This imparts a degree of inaccuracy into the analysis, particularly for those methods that rely on a graphical determination of the strain ratio.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2014-12-14|