Title: Random point distributions and strain measurement in rocks
Abstract: Established methods of “nearest neighbour” or neighbour “centre-to-centre” studies of objects in rocks can only give a true finite strain ratio in the peculiar circumstance that it is possible to identify in the strained state which objects were closest before deformation. The theoretical possibility of measuring strain ratios from object distributions exists more generally. For “random” distributions in which the object positions are mutually independent, there can be no possibility of strain measurement. If objects depart from this special case in the direction of either clustering or anticlustering, any strain suffered should be determinable, but the confidence of the determination increases the further the original departure from independence. Without knowledge of pre-strain neighbours, studies of present nearest neigbours or centre-to-centre distances cannot show whether this possibility of strain determination exists. They produce ellipses with little or no correspondence to the strain ellipse. A method, illustrated here for both simulated and actual object distributions, produces an ellipse of correct strain ratio by use of the centre-to-centre distances separating every object from every other, within a limiting distance empirically determined for each sample. This method assumes an original isotropic distribution on the sample surface, and will not work unless this assumption is approximately correct. Of the practical limitations, the severest by far is the need for a planar homogeneous sample bearing preferably 300 or more objects. Heterogeneities are likely to cause difficulties with strain measurement for strain ratios greater than about 6 : 1. If close rigid objects have jostled each other aside during bulk strain, this method is no more able than other centre-to-centre methods to give a correct strain ratio. Visual interpretation both of petrographie features of the rock and of the pattern this method produces, remains an essential part of the strain determination.
Publication Year: 1979
Publication Date: 1979-11-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 523
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