Plume-Generated Triple Junctions: Key Indicators in Applying Plate Tectonics to Old Rocks

Continental lithosphere-especially where stationary with respect to mantle plumes-is marked by plume-generated uplifts typically crested by volcanoes that rupture in three rifts at angles of about 120° to each other, perhaps because this configuration requires the least work. It is proposed that since the plate tectonic regime began, about years B.P., divergent plate motion has commonly begun at axial dikes emplaced in rifts formed in this way. A normal course of events is that two of the rifts meeting at a junction to open by plate accretion while the third rift becomes inactive as a failed arm. The evolution of 45 selected junctions, with ages ranging back to years B.P., illustrates a variety of ways in which triple junctions may develop. Bends in rifted Atlantic-type continental margins reflect the distribution of triple junctions at the time continents parted and plume traces on ocean floors lead away from these former triple junctions. Where oceans have closed by continental collision, rifts (failed arms) (aulacogens of Soviet authors), striking at high angles into orogenic belts, mark the location of former triple junctions. Reactivation of old rifts is common and new rifts have frequently developed on the sutures along which oceans have closed. Base metal mineralization, especially in the form of syngenetic copper ores, is a feature of some failed arms (Montana, Zambia, Coppermine) and others, which contain up to 10 km of marine sediment, possess some of the world's major petroleum deposits (Northern North Sea, Niger Delta, Gippsland Basin, Gulf of Suez, and Gulf of Sirte). Many of the world's great rivers flow down failed arms (Mississippi, Amazon, Niger, Zambezi, Limpopo, Rhine).