Ancient silicic volcanism and incipient rift propagation during Gondwana breakup; geophysical insights from the Bumbeni ridge, South Africa

Abstract

A detailed understanding of the association between magmatism and rift initiation is key to defining the prevailing tectonic conditions active during the initial break-up of Gondwana. The Bumbeni Complex in northern KwaZulu-Natal, South Africa represents an early Cretaceous silicic volcanic centre exposed at the southwestern termination of a linear zone of magnetic anomalies which are buried beneath sedimentary cover. Newly acquired high resolution aeromagnetic data combined with legacy seismic and borehole data is analysed to define the structural architecture of the basement beneath the Zululand Basin in northern KwaZulu-Natal. Multiple prominent positive and negative anomalies ranging from −200 nT to +300 nT based on International Geomagnetic Reference Field (IGRF) and corrected Reduced to Pole (RTP) data are resolved and form a distinct geophysical lineament. Geophysical and borehole core data indicate that the lineament represents a chain of silicic central volcanoes developed as a ridge feature, the Bumbeni Ridge, along the axial zone of an incipient rift structure. Detailed geological mapping and high resolution geophysical data are analysed and compared with analogous contemporary continental rift environments, as well as published data within the adjacent offshore Northern Natal Valley. Results are presented which propose a continental rift evolutionary model for Gondwana breakup in the Zululand Basin in northern KwaZulu-Natal.