Insights into incipient rifting in thick lithosphere from a GNSS survey of the southern Malawi Rift

Rift initiation is the first stage of continental breakup, which is a fundamental component of the cycle of plate tectonics. However, there are many unanswered questions regarding the kinematics of deformation during early stages of rifting. Furthermore, forces from mantle flow and dynamic topography appear insufficient to break the cold, thick, cratonic lithosphere in stable continental areas such as southern Africa. We explore early-rift kinematics by conducting a GNSS survey across the southern Malawi rift, at the southern end of the amagmatic western-branch of the East African Rift System.

We established 14 GNSS stations in southern Malawi and combined these with existing regional stations in southern Africa from the GeoPRISMS community geodetic solution. We show how these data are compatible with a revised plate configuration for southern Africa. In this configuration, the Nubian plate is divided into three separate plates, with the new plate boundaries following existing bands of seismicity and active faulting. A line of active rifts extending southwest from Tanzania, through the Luano and Luangwa rifts in Zambia and along the Okavango Rift in Botswana forms the boundary between the Nubia and San plates. The boundary of the triangular shaped Angoni plate is the Malawi rift in the east, the Zambezi rift in the south and the Luano and Luangwa rifts in the west. Extension rates across the boundary between the Nubia and San plates range from ~1.5 mm yr^-1 across the Luangwa rift to <1 mm yr^-1 across the Okavango delta.

Across the southern Malawi rift, our survey indicates an extension rate of 2.6 ± 0.4 mm yr^-1. Although the surface expression of the Malawi rift is <150 km wide, in our geodetic profile across the rift, 75% of the relative velocity is contained within a region 890 km wide. The lithosphere beneath the southern Malawi rift is 130-170 km thick, with no evidence for contemporary magmatism. In contrast, across the magmatic rifts of the northern East African Rift system, which are hosted in 50-80 km thick lithosphere, 75% of the relative velocity is restricted to zones ~200 km wide. This highlights that strain accumulating within thick continental lithosphere is much more distributed that the surface expression of active tectonics.