A preferred orientation of intrusion of Precambrian dykes

AN analysis of the original orientation of Precambrian dykes has produced a nonrandom pattern that implies the existence of a Precambrian stress field of global extent. In general, dyke intrusion tends to occur in a north–south orientation, and is concentrated in low latitudes. This distribution has many similarities to the present distribution of sheeted diabase complexes of present day oceanic ridge crests, and may reflect the persistence of the same stress field throughout geological time. Other evidence in favour of the existence of this stress field can be found in tectonic motions or geological features that exhibit a symmetrical distribution relative to the Earth's rotational axis. For example, Moore1 showed that the preferred orientation of spreading axes is directed north–south with a secondary maximum orientated east–west. Active transform faults exhibit a unimodal east–west distribution. A former study of recent continental rift valleys has revealed a preferred north–south orientation of fracturing. Cainozoic grabens, most of which have not been significantly rotated by plate movements following their formation, exhibit an especially strong peak within 10° of north2. Here we describe our analysis of preferred fracture orientations using the trends and directions of Precambrian dykes. A central point of this study is the assumption that regional dyke patterns record fractures which result from regional stress fields operative at the time of dyke intrusion. Escher et al.3 have confirmed the above assumption using geological evidence from the Amitsoq dykes of Greenland.