Geophysical insights into the southern Lebombo volcanic sequences and its influence on the formation of the Zululand Basin

Abstract

The Lebombo sequence provides a glimpse into the past by studying its formation and the sequence of tectonic events that caused the mountainous monocline we see today. The Lebombo sequence is part of the failed triple junction, which had two arms that were sequences of the volcanic basalts and rhyolites and a third arm that was a dyke swarm. The cause of these structures was assumed to be due to a plume and/or lithospheric stretching. Each of these theories can be placed into the context of the Gondwana break-up. Once faulting and rifting of Gondwana began, the study area was also affected by additional crustal thinning and stretching, which led to the intrusions of the Lebombo sequence. As rifting then became drifting, the formation of the Zululand Basin occurred. The Zululand Basin formation is strongly tied to the Lebombo basement. The purpose of this study is to understand the subsurface structure and formation of the Lebombo sequence and the Zululand Basin using geophysical data. The geology of the region consists of the Lebombo sequence outcropping to the west of the Zululand Basin and dipping eastwards to form the basement of the Zululand Basin. The Lebombo is made up of the Jozini Rhyolites and the Sabie River Basalts. The sequences are overlain by the siltstones, sandstones, and reworked volcanics of the Zululand Basin. There are extensive intrusions in the Lebombo of dykes and the Bumbeni Complex. The Bumbeni Complex forms a ridge that divides the basin into two sub-basins. Most geological maps indicate the Bumbeni Complex as a single intrusion. In this study, the regional tectonics were interpreted from the Total Magnetic Intensity map. The regional interpretation of the area showed that the lineaments in the area do not have the same strike. The strike varies from NNE-SSW for the Jozini Formation to NW-SW for the Sabie River Basalt Formation. This change in strike could represent changes in the edge of the Kaapvaal Craton. Two 2D magnetic profiles were modelled using the open-source PyGMI software. One profile was in the northern section of the study area, where two seismic lines were used to constrain the model by determining the approximate depth of the Lebombo volcanic sequence. The observed data on iii profile 2 was not fitted well due to the resolution of the modelled area in PyGMI. The second 2D magnetic profile was in the area where the Bumbeni Complex intruded to see how it would affect the lithologies around. The results showed that the Bumbeni Complex was intruded after the volcanic sequences had dipped eastwards. The aim of these profiles was to assist in understanding the transition from the outcropping Lebombo volcanic sequences to the basin fill and to investigate the formation of the Bumbeni Complex. The profile models were not as successful as expected because fitting the data was difficult due to the presence of remanence and not enough physical property measurements to constrain the models. This made it difficult to determine the orientation and thickness of the intrusive dykes in the Lebombo sequence. A 3D inversion of the magnetic data was run, using ModelVision and UBC GIF-Tools, to determine whether the Bumbeni Ridge was a single intrusion or multiple intrusions across the basin. The results showed that the Lebombo sequence was intruded in three different places by the Bumbeni Complex to form the Bumbeni Ridge. Future work would include paleomagnetic studies and measurements of physical properties to assist with the modelling process.