Convective partial melting: 1. A model for the formation of thick basaltic sequences during the initiation of spreading

Abstract

Stretching and thinning of continental lithosphere during rifting and initiation of seafloor spreading cause upwelling of mantle rocks into a pressure/temperature environment where partial melting occurs. The amount of melt liberated is, however, insufficient to account for the thickness of igneous crust emplaced near the continent‐ocean boundary at some passive margins. These igneous structures are known from seismic reflection and deep refraction studies on numerous passive margins worldwide. Seismic studies also suggest that the transition from continental to oceanic crust on these “volcanic margins” is relatively abrupt. Their evolution apparently does not include a prolonged extensional phase preceding the inception of spreading. To account for these observations, we propose a model for the generation of partial melt in which the lateral temperature contrasts evoked by rifting and asthenospheric upwelling drive convection in the upper mantle. Deep, hot mantle material is transported upward by the convection, and hence much larger volumes flow through the region where partial melting occurs than during passive upwelling. Numerical calculations show that “convective partial melting” can provide substantial additions to melting due to passive upwelling alone. Using this model, we propose an evolutionary scheme for “volcanic” passive margins, emphasize the contrast with “nonvolcanic” margins, and describe conditions which might lead to the development of both types.