Geoelectric Structural Directions And Dimensionality Of The Damara Belt and Surrounding Cratons

Abstract

Magnetotelluric (MT) soundings were made along three transects, as part of the broader Southern African Magnetotelluric Experiment (SAMTEX), crossing the Kalahari Craton, the Neo-Proterozoic Ghanzi- Chobe/Damara belts (collectively termed the DMB) and the southern Congo/Angola craton in an effort to define the geo-electric structure across the region. The McNeice and Jones (2001) distortion decomposition technique was applied to the MT data and indicate significant depth and along-profile variations in geo-electric strike and dimensionality on all the three transects crossing these three tectonic units (i.e. Kalahari craton, Congo/Angola craton and the DMB). The geo-electric strikes are generally parallel to the North-East trending tectonic fabric as inferred from the magnetic data, but the significant strike variations with depth are expressions of heterogeneity in the lithospheric structure. The Kalahari craton, south of the DMB, exhibits a fairly one dimensional (1D) structure with preferred strike directions between 40-50 degrees for the entire period (i.e. depth) range, indicating little crust-mantle decoupling. The DMB appears to be strongly 2D at lower crustal and upper mantle depths (10-100s) with no consistent/preferred strike direction and significant phase differences between the conductive and resistive directions. North of the DMB and into the southern Congo/Angola craton there are significant variations in geo-electric strike direction and dimensionality at most sites for crust and upper mantle lithosphere. Although strike directions of between 50-60 degrees are observed on most sites for periods greater than 100s, the large phase differences indicate that the regional structure is 3D. These variations in strike directions and dimensionality suggest that sectional 2D inversion modelling of each profile is necessary and models with period dependent direction must be analyzed.