Effect of lithospheric root on decompression melting in plume-lithosphere interaction models

Global distribution of large igneous provinces (LIPs) shows that volcanic events associated with diapiric plume heads have often occurred near the margins of cratonic lithosphere. Previous numerical models of plume–lithosphere interaction and decompression melting assumed the lithosphere as a rigid lid of mostly uniform thickness. Here we present a 3-D numerical model of a starting mantle plume, consisting of a diapiric head and a trailing conduit, which interacts with a moving lithospheric plate of non-uniform thickness. We study in particular the effect of a lithospheric root on plume flow and partial melting. The plume is deflected away from the root, preferentially in the upstream direction. The presence of a root facilitates more than three-fold enhancement in the melting rate by restricting the horizontal spreading of plume material and the diffusion of thermal anomaly in comparison to the model having no root. The large melting rate is sustained for longer duration in the presence of depletion buoyancy of the residue. Our model does not require an excessively high plume temperature or compositionally heterogeneous plume head and reproduces the characteristics of melting rates and melt volumes seen in flood basalt volcanic provinces and the trailing hotspot tracks. The model also supports occurrence of volcanism of different ages at a given location as a result of initial melting in the upstream region and its subsequent passage over the plume conduit due to plate motion. Inferences for the occurrence of such late-stage volcanism are discussed for the Deccan Volcanic Province. Cenozoic volcanism of eastern Australia is also discussed as an example of deflection of a plume due to the presence of thick lithosphere.