What controls inter-rift volcanism?

Volcanic activity in extensional geodynamic settings manifests itself in in-rift, off-rift, and inter-rift areas; the latter coinciding with structurally complex transfer zones between two rift segments. While in- and off-rift volcanism has been associated with the competition between tensional crustal stresses and gravitational unloading pressure, volcanism in inter-rift zones is still not adequately understood. Here, we combine modelling and data from natural cases to test whether topographic highs in the inter-rift zone may control magma trajectories. We performed six analogue experiments tracking the propagation of air-filled cracks in gelatine by imposing three boundary conditions: differential tensile stresses, pre-existing anisotropies, and asymmetric and symmetric inter-rift topography. Next, we compared our results with Digital Elevation Model (DEM) analyses of young volcanic provinces such as the Virunga complex (Rwanda), the Adda’do magmatic segment (Ethiopia), and the Eifel (Germany). Analogue results show different outcomes depending on the imposed boundary conditions. When only a differential tensile stress is applied, the air-filled crack trajectories tend to align perpendicular to the least principal stress (σ3) and focus on the centre of the gelatine box. In contrast, where inter-rift topography exists, the high sectors progressively attract the trajectories along the strike of the rift. Therefore, when injection occurs below the rift centre, magma is deflected across-rift, toward an off-rift location. Conversely, when magma injection occurs below the rift tip, magma is deflected along-rift, in the inter-rift zone. Finally, the spatial patterns of the air-filled cracks obtained by our modelling are consistent with the results of the DEM analysis of the three study areas, where volcanic activity focuses on the inter-rift segments and shifts away from the rift tips and axis as the rift widens and deepens.