A Discussion on the measurement and interpretation of changes of strain in the Earth - Patterns and magnitudes of natural strain in rocks

Abstract

Strain values based upon the shape of the deformation ellipsoid as determined from measurements of deformed objects are described from five different tectonic environments. Patterns of strain are shown to vary significantly and systematically depending upon the geological structure. The studied regions are: ( a ) The Cambrian slate belt of W ales: simple upright and symmetrical folded structures. ( b ) The Taconic slate belt of New York and Verm ont: overturned and asymmetrical folded structures. ( c ) The Rhodesian Archaean shield: intensely deformed upright and isoclinally folded structures. ( d ) The Helvetic nappes of the A lps: recumbently folded structures resulting from gravitational gliding. ( e ) The Moine nappe of northwest Scotland: recumbently folded structures of an orogenic marginal thrust zone. The finite permanent strains measured in these various environments cover a wide field of possible deformational modes, from almost pure flattening to almost pure constriction. Strain magnitudes are over 1000 % in extreme cases. Deformation by flattening, involving shortening of original dimensions by up to 75 % in one principal direction and concomitant extension of up to 250 % in another principal direction, is characteristic of high-level upright structures and the basal parts of superficial gravitational gliding structures. On the other hand, constrictional deformation involving extensions of between 250 % and 1500 % is characteristic of deep-level upright structures and marginal orogenic thrust fronts. From the intensities of measured strain and the position occupied by the deformation ellipsoid in the field of possible strains, it should be possible to deduce the mode of origin and depth of formation of many major geological structures.