Spatial patterns of volcanism between adjacent rift segments

Volcanic activity between adjacent rift segments remains a critical, yet poorly understood aspect of volcano-tectonic processes in extensional regions. Here, we investigate how crustal stresses, particularly the interplay between tensional and unloading (removal of mass) stresses, influence magma propagation and the spatial distribution of volcanic centers in regions between spatially separate rift segments. Using the Copernicus Global Digital Elevation Model, we quantified asymmetries in crustal unloading and examined spatial correlations between volcanic centers and rift segment terminations in three exemplary study regions, including the Rhenish Massif (Germany), the Virunga Volcanic Complex (Western Branch of the East African Rift System), and the Adda'do Magmatic Segment (Southern Afar, Ethiopia). We additionally conducted experiments with analog materials to simulate the roles of hydrostatic and unloading stresses, as well as tensional stresses on magma trajectories. We found that magma pathways, and therefore the distribution of volcanic centers, are shaped by the competition between tensional and unloading stresses within and between separate rift segments. Minor extension and low-magnitude asymmetric crustal unloading result in scattered monogenic volcanoes (Rhenish Massif). Moderate extension and pronounced asymmetric rifting redirect magma laterally toward the tectonically less active, gentler side of half-grabens (Virunga Volcanic Complex). High extension rates and moderate rift asymmetry cause a concentration of both polygenic and monogenic volcanoes along rift axes (Adda'do Magmatic Segment). In all of these settings, diking constitutes a complementary mechanism, in addition to tectonic faulting, to accommodate regional extension.