THE INTERPLAY BETWEEN PRE-EXISTING STRUCTURES AND MANTLE FLUIDS IN STRAIN LOCALIZATION DURING CONTINENTAL RIFT INITIATION: THE OKAVANGO RIFT ZONE TESTIMONY

We used aeromagnetic and gravity data to investigate the crustal and thermal structures beneath the Quaternary-age NE-trending, Okavango Rift Zone (ORZ) in northwestern Botswana in order to understand their role in strain localization during rift initiation. The ORZ is amagmatic rift and it represents the southwestern-most continuation and the youngest segment of the East African Rift System. It extends within the Mesoproterozoic-Neoproterozoic Ghanzi-Chobe-Damara orogenic belt sandwiched between the Archean-Paleoproterozoic Congo craton in the northwest and the Magondi block to the southeast. The absence of surface volcanism within the ORZ makes it difficult to explain its initiation through the widely-accepted model of “magma-assisted rifting”. We used three-dimensional (3D) inversion of aeromagnetic data to estimate the Curie Point Depth (CPD) and heat flow under the rift and surrounding Precambrian basement areas. We also used two-dimensional (2D) power-density spectrum analysis of gravity data to estimate the Moho depth. Our results reveal the presence of shallow CPDs (8-15 km) and high heat flow (60-90 mWm^-2) beneath a ~60 km wide NE-trending zone correlated with the ORZ border faults and the boundary between the Precambrian orogenic belts. This is accompanied by thin crust (<30 km) in the northeastern and southwestern parts of the rift zone. Within the Precambrian basement areas, the CPDs are deeper and vary between 16 and 30 km, the heat flow estimates are lower and vary between 30 and 50 mWm^-2, and these are associated with thicker crust (~40-50 km). We interpret the thermal structure under the ORZ as due to upward migration of hot mantle fluids through the lithospheric column that utilized the presence of Precambrian lithospheric structures as conduits. These fluids weaken the crust enhancing rift nucleation. Our interpretation is supported by 2D forward modeling of gravity data suggesting the presence of a wedge of altered lithospheric mantle centered beneath the ORZ. Our model can be tested by the results of magnetotelluric and seismic studies currently being conducted in the ORZ. If our interpretation is correct, it may result in a paradigm shift in which strain localization in continental rift initiation could be achieved through lithospheric processes without asthenospheric involvement.