Is magmatic underplating the cause of post-rift uplift and erosion within the Cabora Bassa Basin, Zambezi Rift, Zimbabwe?

The Cabora Bassa Basin in northern Zimbabwe is an Upper Paleozoic Karoo bas in trending almost east-west. It has clearly recognisable gravity and magne tic signatures from which its dimensions are estimated to be approximately 150 km long and at least 62 km wide. Its southern boundary is marked by a n orth dipping, listric, master fault of possibly a ramp-flat-ramp geometry. Within the basin there is erosional truncation of over 2 km of sediment at the top of the stratigraphy and an absence of a post-rift thermal subsidenc e phase. Modern and major river channels are characteristically narrow, dee p and without considerable amounts of silt on the river beds, whilst their valley sides are marked by multiple terraces. These ongoing erosional proce sses are evidence for sustained and possibly episodic uplift of the basin s ince the end of rifting in the Late Cretaceous/Early Tertiary. An evaluation of possible uplift mechanisms for the basin and its surroundi ngs lends support to lithospheric thickening as the most likely mechanism. Compression and magmatic underplating and/or intrusion are two common ways of thickening the lithosphere. The absence of major compressional structure s within the basin suggests that magmatic underplating and intrusion may ha ve played a major role in lithospheric thickening. Major element data for t he mantle-derived Jerama basalts indicate substantial hidden cumulates, whi ch possibly thickened the lithospheric column. Modelling of gravity data, constrained by both seismic reflection results, and the densities of the surface rocks, show that the crustal thickness ben eath the basin is in the range 19-23 km. The stretching factors from seismi c and gravity models range from 1.6 to 1.9. There is up to 5 km difference between the expected and modelled crustal thicknesses beneath the basin, wh ich could be explained by magmatic underplating. (C) 1999 Elsevier Science Limited. All rights reserved.