RELATIONSHIP BETWEEN JOINTS IN PRE-CAMBRIAN, LOWER PALAEOZOIC AND CARBONIFEROUS ROCKS IN THE WEST MIDLANDS OF ENGLAND

Abstract

Summary Joint measurements were made in three northerly trending fold structures in the English Midlands, with the principal object of studying the relationships between joints in Pre-Cambrian, Lower Palaeozoic and Carboniferous rocks which had been subjected to similar folding processes. It was noted that the joints were nearly always perpendicular to bedding except in the Pre-Cambrian volcanic rocks, and, further, it was found that the joints were related to local rather than regional structures and tended to change orientation in sympathy with changes in bedding dip. It is concluded that wrench type joints tend to develop at deeper structural levels whereas at higher levels in the same fold tensional strike and dip joints are formed. There is no direct evidence from these areas that the joints above an unconformity are inherited from those below, and joint parallelism, where it occurs, can easily be explained by posthumous fold movements in which younger rocks have the same fold axes and directions of dip as the older rocks. It is suggested that the north trending Midland structures are part of a wider pattern, extending over much of England and Wales, which was initiated in Pre-Cambrian times.