Controls on the spatio-temporal distribution of plume-related excess melting during continental rifting

The complexity in the relationship between mantle and lithosphere processes may be most directly exemplified in the coupling between upwelling plumes and extending lithosphere at rifted margins, and the distribution of excess melting across these regions through space and time. Rifted margins are often described in two end-member classes; magma-rich and magma-poor, typified by the emplacement of seaward dipping reflector sequences (SDRs) and high velocity lower crustal bodies (HVLC) or the exhumation of serpentinised mantle with little extrusive melt, respectively. Previous studies have linked margin architecture and magmatic budget to extension velocity, lithosphere thickness, and rheology. The role of mantle plumes remains poorly constrained, with plumes associated with both magma-poor and magma-rich margins, implying that their influence on excess melt production is not straightforward. Our study aims to better constrain the relationship between mantle plumes and excess melting at rifted margins by exploring the interaction of plumes originating from the mantle transition zone and rifting.

We present two-dimensional numerical simulations to investigate how mantle plumes interact with lithosphere extension during continental rifting. Rifting is simulation using the ALE finite-element code FANTOM, incorporating a thermal anomaly at the base of the upper mantle to represent a stalled plume source. We systematically vary velocity, plume temperature anomaly, and plume position relative to the rift axis to explore how these parameters control the timing, magnitude, and spatial distribution of excess melting during breakup.

Our results indicate that excess melting associated with mantle plumes is both transient and spatially distributed. The timing, magnitude and lateral distribution of excess melting depends non-linearly on the interaction between plume buoyancy and lithospheric extension rate, with the strongest plume influence occurring at intermediate extension velocities. Plumes residing directly beneath the rift axis focus melt, producing temporally concentrated, focussed melt zones that promote earlier rift breakup whereas plumes which lie adjacent to the rift axis produce spatially offset and temporally delayed melt focussing, resulting in narrower but less efficiently coupled melt zones. These results demonstrate that plume-driven excess melting may be highly time-dependent with an evolving spatial distribution that reflects the efficiency of melt focussing relative to the thinning lithosphere.