Influence of pre-existing lithospheric stress on the expression and propagation of mantle plumes

Mantle plumes form a vital component of mantle circulation resulting from the up-welling of hot mantle material from as low as the the core/mantle boundary. Plumes that rise from deep in the mantle will interact with the lithosphere regardless of whether this is continental or oceanic, or the regional tectonic stresses. Plumes have long been associated with volcanic rifted margins, with the heat transferred from the upper mantle into the lithosphere lowering the lithospheric yield stress and allowing for increased rates of extension. It remains unclear if extension is driven by the arrival of the plume under lithosphere already under tension, or if the plume alters lithospheric stresses and induces tension. Evidence of plumes interacting with orogensis remains sparse, with the Grenville Orogen / Mid-Continent Rift displaying the clearest evidence for such an event. Mafic rocks of the Keweenawan Large Igneous Province (LIP) were emplaced during a discrete period of extension between the Shawinigan and Ottawan phases of the Grenville Orogeny. Whether the arrival of a mantle plume triggered a switch from shortening to tension in the lithosphere, or plume material was allowed to rise when lithospheric stresses changed remains unclear. To explore how plumes interact with regional lithospheric stresses to alter melt volume, and location, topography and lithospheric strain we present 2D numerical models of the upper mantle where plumes arrive at the base of a lithosphere under imposed stress. The models indicate that plumes themselves generate low stresses and rates of extension within the lithosphere and only transition the lithosphere from shortening to extension at very low rates of shortening. Under higher rates of lithopsheric shortening melt volume is depressed and may travel significant distance beneath lithosphere. Regional lithospheric extension results in increased melt volume that can be focused into the rifted axis as the lithosphere necks.