Tectonophysics, 60 (1979) 263-277 263 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands STRAIN ANALYSIS USING DEFORMED PEBBLES: THE INFLUENCE OF INITIAL PEBBLE SHAPE RICHARD J. LISLE Geologisch Instituut der R~ksuniuersiteit, Leyden (The Netherlands) (Received July, 4,1978;revised version accepted November 10,1978) ABSTRACT Lisle, R.J., 1979. Strain analysis using deformed pebbles: the influence of initial pebble shape. Tectonophysics, 60: 263-277 Several methods exist for the determination of the finite strain ellipsoid from deformed pebble shapes. These methods are critically evaluated and others are proposed on the basis of calculations which predict both the sectional and three-dimensional shape of pebbles in simple deformed simulated conglomerates. In many cases it is found prefer- able to use an average pebble shape to estimate the tectonic strain and that the harmonic mean of the ratios of axial lengths yields an average pebble shape which is closest to the strain ellipsoid shape. INTRODUCTION As strain markers, deformed conglomerates are of unique importance. Their widespread distribution compared with other strain indicators, such as deformed fossils, allow the possibility of finite strain evaluation on a regional scale (e.g. Hossack, 1968). Their value is further enhanced by their ability to be recognised after intense deformation and high grade metamorphism and by their potential ability to indicate the total strain undergone by the rocks since their deposition. Unfortunately, strain analysis from deformed pebbles is not straightfor- ward. From previous work it has become clear that the shape and orientation of a deformed pebble does not represent the shape and orientation of the strain ellipsoid in the rock as a whole and that a number of factors including the initial pebble shape, initial orientation and the “competence” of the pebble relative to the rock as a whole and adjacent to material are likely to influence the pebble’s final shape and orientation. Ultimately strain methods using pebbles should aim at taking as many as possible of these factors into account. At the moment, however, work in this field is still at the stage of determining the extent of the role played by these factors.