Relative timing of rifting and volcanism on Earth and its tectonic implications

Abstract

Rifts occur in diverse tectonic environments that result from the continuous two‐dimensional evolution of the multi‐plate mosaic of the Earth and also from the interaction between the mantle processes and the overlying lithosphere. Most rifts have associated volcanics that are mainly basaltic. In continental rifts these basalts are predominantly alkaline and the relative timing of rifting and volcanism is variable; however, despite this variation there seem to be two basic types of rifting‐volcanism relative timing: in one, volcanism and usually local doming predates major rift formation whereas, in the other, rifts form first and volcanism (? and doming) follow thereafter. These two basic types of rifting‐volcanism relative timing may be related to two basic modes of rifting. In the first the mantle plays an active role, convection ‘plumes’ dome up and crack the lithosphere, whereas, in the second, the horizontal movements of plates give rise to extension of the lithosphere and induce rifting. In this latter case, the mantle is passive. Numerous local conditions complicate this simple pattern and result in extremely complicated rifting‐volcanism relationships that make geophysical/geochemical modelling difficult. Petrologic/geochemical studies of rift volcanics do not provide unique solutions for our understanding of rift environments. Detailed stratigraphic/structural analysis of individual rifts are still the best methods for rift analysis.