THE ROLE OF PRE-EXISTING BASEMENT STRUCTURES ON THE EVOLUTION OF THE MAKGADIKGADI RIFT ZONE, CENTRAL BOTSWANA: INSIGHTS FROM AEROMAGNETIC ANALYSIS AND 3D INVERSION

The propagation of rifting in Southern Africa continues to shape the landscape and influence paleo-lakes and rivers within the region. In this work, we used the Okavango Dike Swam as magnetic markers to map structures related to the rifting of the recently proposed Makgadikgadi Rift Zone (MRZ). A combination of filtering, Euler deconvolution, and 3D inversion of magnetic data along the strike of the dikes were used to map the depocenters, the relationship between pre-existing structures, the nascent rift-related structures, and the thermal structure below the proposed rif

Three-dimensional inversion of magnetic data shows relatively deep Curie point depths (CPD (>25 km) within the proposed Makgadikgadi Rift Zone (RZ). However, shallow CPD values (18-20) are mapped northwest of Ntwetwe Pans, where a series of faults overprinting the Okavango Dikes Swam is interpreted. The area, northwest of Ntwetwe, is coincident with a change in the direction of the Boteti River from a SE direction to a SW direction then back to a SE direction into Makgadikgadi Pans. The Makgadikgadi Pans area has little evidence of dikes being overprinted by younger rift-related structures except on the southern side of the Ntwetwe Pans, where a normal fault extending from the border with Zimbabwe is interpreted. The fault is consistent with the fault scarp extending from Lake Kariba RZ into Makgadikgadi Pans. The southern boundary of the lake and the basement fold are bounded by Lecha Fault to the northwest. Additionally, there are normal faults mapped that are concordant with the strike of the dikes. The Euler Deconvolution results show little evidence of the developed grabens and horsts within the Makgadikgadi Pans except for the area NW of the Ntwetwe Pans where values consistent with a ~300 m deep graben is interpreted. The identification of a rift-related basin northwest of Ntwetwe Pans shows that pre-existing structures play an important role in accommodating strain during rift initiation. Our results show little evidence of rifting in MRZ, if it does exist is at a early stage. The repetitive reactivation of older Proterozoic structures northwest of Ntwetwe Pans, constrained by the overprinting of the ODS, shows the importance of pre-existing structures in accommodating strain and changing the topography and flow direction of rivers.