Plate 1] The evaluation of finite strain in naturally deformed rocks is restricted by the limited occurrence of good n atu ral strain indicators which are also homogeneous with respect to the m atrix. This problem is overcome by establishing the relation between measured finite strain and those physical behaviour characteristics of rocks th at are dependent upon the anisotropy resulting from deformation. Accordingly, the strain measured from natural indicators is calibrated against degree of

more »

... ferred orientation, magnetic susceptibility anisotropy, and seismic anisotropy. This will perm it three approaches to be used independently for the evaluation of strain, provided th at a minim al num ber of actual strains are available. The relation between measured strain and the degree of preferred orientation of layer silicates as revealed by X -ray transmission goniometry is established for a group of fine grained tectonites of dom inantly planar fabric which have an average defor m ation ellipsoid of form 1.6 : 1 : 0.26. The strains measured from the degree of preferred orientation are in rem arkable agreem ent with those measured from natural strain indicators. T he measured deformation ellipsoids for a wide range of strains are also com pared to the correlative ellipsoids of magnetic susceptibility anisotropy. The axes of both sets of ellipsoids are coincidental and the shape relationship between deforma tion and magnetic susceptibility ellipsoids is established by linear regression. Finally, the anisotropy of seismic velocities is determ ined by measuring the pseudocompressional velocity and two orthogonally polarized pseudo shear wave velocities for each of a m inim um of nine non-coplanar directions. The velocity surfaces thus obtained define an elastic or seismic velocity anisotropy ellipsoid, the axes of which are also precisely coincidental with those of the finite deformation ellipsoid. The influence of rock fabric upon seismic velocities is such th at for a rock which has undergone a principal finite extension of 135 % and a finite shortening of 65 %, the difference of com pressions and shear wave velocities between these two directions is in the ratio 1.26:1 for Pw aves and 1.33:1 for Sw aves. I ntroduction The Cam brian Slate Belt of Wales provides an ideal opportunity for making a first attem pt to apply four independent methods to the investigation of strain in highly anisotropic rocks characterized by slaty cleavage. Lower C am brian slates, approxim ately 1 km in vertical thickness occur in a strongly deformed zone that is 24 km in length and up to 2J km in width,