Crustal Structure Beneath the Luangwa Rift, Zambia: Constraints from Potential Field Data

We used gravity and magnetic data to examine the thermal and crustal structure beneath the Luangwa Rift Valley (LRV) in Zambia in order to examine the geodynamic controls of its formation.. The LRV lies at the boundary between the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide orogenic belts between the Zimbabwe craton and the Bangwelu Block. We computed the Curie Point Depth (CPD) using two-dimensional (2D) power spectrum analysis of the aeromagnetic data, and these results were used to estimate heat flow beneath the LRV. We also inverted the aeromagnetic data for three-dimensional (3D) magnetic susceptibility distribution. We further determined the depths to the Moho using 2D power spectrum analysis of the satellite gravity data and 2D forward modeling of the terrestrial gravity data. We found that: (1) there is no consistent pattern of elevated CPD beneath the LRV, and as such no consistent pattern of elevated heat flow anomaly, (2) there are numerous 5-15 km wide magnetic bodies at shallow depth (5-20 km) beneath the LRV and the 2D forward gravity modeling suggests these to be dense intrusive bodies, (3) a thick crust (49-52 km) underlies the northwestern margin of the rift centered beneath the ~ 1 km high Muchinga escarpment which represents the main border fault of the LRV. This thick crust contrasts with the thinner crust (35-45 km) outside the rift, and (4) the thickened crust coincides with a NE-SE elongated belt of 1.05-1.0 Ga granitoids previously interpreted as manifestations of the metacratonization of the southeastern edge of the Bangweulu Block. Our 2D forward gravity model suggests that the thickened crust is due to the presence of possibly Karoo-aged magmatic under-plated mafic body (UPMB) whose thermal anomaly has since decayed. We suggest that the initiation of the LRV was associated with this deep magmatic activity that introduced rheological weaknesses that facilitated strain localization although it never breached the surface. It is also possible that metacratonization of the southeastern edge of the Bangwelu cratonic block might have facilitated the localization of the UPMB emplacement. New passive seismic and magnetotelluric data acquired as part of the NSF-funded PRIDE experiment will likely contribute to testing the validity of our interpretations.