MAGMATIC UNDER-PLATING BELOW THE KAROO AGED LUANGWA RIFT VALLEY?

We used aeromagnetic data, moderate resolution satellite and high resolution ground gravity data to examine the thermal and crustal structure beneath the Luangwa Rift Valley (LRV) in Zambia in order to determine the geodynamic controls of its formation. We computed Curie Point Depths (CPD) using two-dimensional (2D) power spectrum analysis and three-dimensional (3D) inversion of the aeromagnetic data, and the results were used to calculate heat flow under the LRV. We also determined the crustal thickness under the LRV by calculating the depths to the Moho using 2D power spectrum analysis of the satellite gravity data. We found that (1) There is no elevated CPD beneath LRV, and as such no high heat flow. (2) 3D inversion of aeromagnetic data suggests the presence of magnetic bodies below the LRV. (3) Moho depth results suggests a thicker crust and deeper Moho (~50 km) beneath the rift, compared to 35-45 km depths away from the rift. The gravity-Moho depths differ from Moho depths determined from passive seismic studies especially beneath the rift but are comparable at locations outside the rift. (4) The high resolution ground gravity data suggests the presence of a broad long-wavelength positive anomaly, possibly related to deep dense material at the Moho. This anomaly is modified by shorter-wavelength positive anomalies at the rift shoulders and floor (possibly related to denser bodies in agreement with the 3D inversion results of aeromagnetic data), and negative short-wavelength anomalies (related to rift sediment infill). We subsequently developed a 2D forward model from the high resolution gravity data to reconcile the thermal and crustal characteristics of the LRV. Our model suggests the deeper Moho below the rift to be the result of magmatic under-plating. The difference between the gravity and passive seismic Moho depths estimates may be because the passive seismic data imaged the Moho as the top of the under-plated mafic body whereas the gravity data considered the base of the under-plated mafic body as the Moho. Magmatic under-plating may have introduced a rheological weakness that may have helped to localize strain during rift initiation in Karoo times. It is possible that other palaeo-rifts may have similar thermal and crustal characteristics as the LRV.