Deformation in the Cambrian Slate Belt of Wales

Abstract

Understanding of the deformation which gives rise to geological structures is greatly enhanced by knowledge of the total strains to which their component parts have been subjected. Strains may be measured directly provided that some suitable indicators are present. The Cambrian Slate Belt of Wales contains ideal strain indicators in the form of perfect triaxial ellipsoidal bodies at a limited number of localities. These indicators enable fully quantitative evaluation of the strain involved in the production of slaty cleavage. Variations in three-dimensional deformation also explain aspects of large scale structure, such as changes in fold dihedral angles and the positioning and magnitude of major plunge variations in non-cylindrical folds on all scales. Strains measured from these indicators are compared in detail with strains measured from intensities of preferred orientation of phyllosilicates obtained by x-ray transmission goniometry. The results obtained by both methods cover an appreciable range of principal strains. They are consistent in their close relationship to each other and also to the local and regional geological structure. The natural strains, $$l_{n}(l + \varepsilon _{i})$$, obtained by x-ray goniometry ($$^{p}N_{i}$$) are usually a little lower than the natural strains obtained from the ellipsoidal indicators ($$^{s}N_{i}$$). Their comparison reveals a correlation coefficient ($$r^{2}$$) of 0.992 and establishes the following relationship between the results of the two methods: $$^{p}N_{i} = -0.002 \pm 0.011 + (0.92 \pm 0.02)^{s}N_{i}$$ From the established relationships, strains are measured from preferred orientation and appropriately corrected for a cross section of the regional structure in an area where no ellipsoidal indicators are available.