Some basic principles of strain analysis are discussed in relation to the interpretation of structural patterns in map view. 'Strike-slip' or wrench tectonic regimes show more complex patterns than extensional or contractional regimes because of two effects. The 'rotation effect' is well known - the progressive development and rotation of structures due to rotational strain, as illustrated using the simple shear model in many reviews and textbooks. Extensional and contractional structural patterns develop mainly under irrotational strain conditions. The 'strain path effect' is less generally recognized - in contrast to extensional or contractional tectonics, wrench tectonic regimes imply an approximately constant-area strain path in map view, along which the Y (short) axis of the strain ellipse shortens at the same time as the X (long) axis increases in length. The effect can best be understood with reference to the Ramsay (1967) 2D strain field diagram. Since irrotational constant-area strain regimes and rotational regimes with increasing or decreasing area ( transtension, transpression) also exist, the interpretation of map patterns in terms of tectonic regime requires a good knowledge of the basic rules of strain analysis.
A. G. Milnes, Geologisk Institutt, Universitetet i Bergen, Allégate 41, N-5007 Bergen, Norway.