Morarian Orogeny and Grenville Belt in Britain

Abstract

Now that the nature of the Morarian orogenesis (about 730 Myr BP) is becoming more apparent1,2 it seems appropriate to assess its significance in terms of late Proterozoic history of the northern Atlantic region. It has been argued3 that the event is related to subduction, with a suture partly coinciding with the Great Glen Fault. The event has also been associated with the closing of a Precambrian ocean during the time represented by the unconformity between the Stoer and Torridon groups4. The Proto-Atlantic opened after the deposition of the Torridon Group. In establishing the orogenic character of the Morarian Event several features are important. First, the event is known to have affected Moine rocks in a zone some 20 km wide. If, as seems likely, the Morarian fabric can be traced as far south as the Great Glen Fault then the belt is some 50 km wide after the Caledonian shortening. Until more is known about the geology of Sutherland, however, it is uncertain whether the entire length of the Moine outcrop (about 250 km) is involved. There is no reason to believe that the Morarian fabric is present to the south of the Great Glen Fault: the likelihood is that the Central Highland Granulites (often named the Moines nowadays) represent post-Moine sediments, but the possibility that they are Moines that were not influenced by the Morarian Orogeny cannot be excluded. Until this problem is resolved it would be prudent to revive the name Central Highlands Granulite for the sub-Dalradian rocks occurring to the south of the Great Glen Fault.