THE ROLE OF SHALLOW DIKES IN EARLY-STAGE CONTINENTAL BREAKUP: EVIDENCE FROM VENT ALIGNMENTS IN THE EAST AFRICAN RIFT
Cones within >10 Ma rift basins in the northern Main Ethiopian Rift (MER) show rift-parallel dikes along the full length of the central axis. In contrast, cones are largely absent in the centers of <7 Ma rifts in Tanzania, as well as magmatically-starved basins of the EAR western branch (e.g., Kivu rift, Democratic Republic of Congo). In these examples, volcanic cones cluster in oblique transfer zones between adjacent rift basins, with cone alignments showing both radial and rift-parallel patterns that extend into the distal tips of adjacent rift basins.
Young cones in immature or magma-poor rifts thus indicate that eruptive dikes have primarily focused into transfer zones over the past 1 Myr, not the central parts of the rift basins. The centers of rift basins have nonetheless experienced voluminous eruptive activity at time scales of ~1-2 Myr (e.g., 1.4-0.8 Ma lavas in the Magadi basin, Kenya; ~1 Ma lavas in the Natron basin, Tanzania), implying infrequent fissure eruptions. Activity clustered around volcanic centers in transfer zones in the interim (e.g., Suswa, Magadi basin; Gelai and Oldoinyo Lengai, Natron basin; Nyamuragira and Nyiragongo, Kivu basin). The combination of rift-parallel and radial dikes indicates that high magma pressures sometimes locally dominate over far-field stresses in transfer zones. Inherited crustal structure also appears to influence magma conduits in these zones. Magma-rich transfer zones may thus be centers for shallow magma storage that contribute to the rifting process in both immature and magma-poor rifts, with dikes propagating laterally from transfer zones into the tips of rift basins, facilitating rift propagation. In more evolved rifts (MER), eruptive, rift-parallel dikes intrude more readily across the full length of the basin, providing a significant contribution to the rifting process.