DYKES EMPLACED INTO BASEMENT AND COVER ROCKS AT THE INCIPIENT NAMIBIAN MARGIN – STRUCTURE AND MAGNETIC FABRICS

The major Henties Bay-Outjo dyke swarm (HOD) in coastal and inland NW Namibia at the African continental margin is part of the South Atlantic Large Igneous Province, which originated during the early Cretaceous break-up of Africa-South America. The HOD is oriented NE-SW and intruded the crystalline basement of the NE-SW trending, Neoproterozoic Damara mobile belt. There are only a few known locations, where the dykes of the HOD are associated with erosional remnants of sedimentary cover rocks of lower Jurassic age. The results of detailed magnetic and fabric studies of these latter dykes are compared with earlier results on the HOD dykes in the crystalline basement, in order to determine potential differences in flow fabrics and emplacement history.

The samples were studied microscopically (microstructure, opaque mineralogy) and magnetic properties determined (bulk susceptibility, anisotropy of magnetic susceptibility (AMS), hysteresis, NRM). Microscopic studies and kappa-T measurements identify magnetite and titanomagnetite as the dominant magnetic minerals. Two main AMS fabric types (normal and inverse) can be recognized, which are comparable to those reported earlier from mafic dykes of the Ponta Grossa dyke swarm in Brasil. At least the normal fabric type provides information on flow fabrics. Steep AMS long axes indicate subvertical magma flow, and shallow long axes subhorizontal flow. Dyke emplacement was a complex and localized process with multiple sites of vertical magma flow and lateral spreading. The analysis of the recently sampled dykes will allow a broader view and better-founded conclusion about magma flow directions and emplacement mechanisms in the dyke swarm.