Folding, strain and Graham's fold test in palaeomagnetic investigations

Abstract

Summary. Graham's fold test is an important test of stability of magnet- ization and is widely used in palaeomagnetic studies. The change in dispersion of palaeomagnetic directions achieved by 'returning' layers to their original orientation can give an indication of the relationship between the time of deformation and of magnetization acquisition. However, McElhinny's criteria for application of the test are regarded by McFadden & Jones to be inappro- priate. Additionally, simple rotation of directions about the strike of the layer may not be a sufficient procedure. The rock type and style (and amount) of deformation may often appreciably affect the orientation of a vector or magnetic direction during deformation. Hence detailed observations are necessary to provide the basis of the steps used in applying the fold test. Apparent failure of the test may not indicate the absence of pre-deformation stable magnetization in a rock. Although Graham's fold test is important in palaeomagnetic (and related tectonic) investigations of deformed rocks, application of the test may be very complex.