The structural evolution of the Bumbeni Complex and Bumbeni Ridge and its relationship with Gondwana break-up: insights from high resolution aeromagnetic data and field mapping.

Abstract

The Bumbeni Complex is the remnant of a Cretaceous volcanic centre (~133 Ma) that is now exposed along the southern end of the Lebombo Mountain Range, adjacent to the cover sequences that infill the Zululand Basin in northern KwaZulu-Natal, South Africa. The Bumbeni Complex forms the western limit of the north-east trending Bumbeni Ridge and postdates the rocks of the Karoo Large Igneous Province. The comparison of field relationships, structural data, high-resolution aeromagnetic data and borehole logs are used to determine the formation and structural evolution of the Bumbeni Complex and Bumbeni Ridge by understanding the tectonic regimes responsible for the development of the brittle deformation structures in the study area. These events are then correlated with the regional tectonic events related to Gondwana break-up, thus further constraining the timing of deformation and possible formation mechanisms of the Bumbeni volcanism. The rhyolites of the Jozini Formation (Lebombo Group) provide a basement to the rocks of the Bumbeni Complex. These rhyolites are characterised by a N-S oriented eastward dipping normal and dextral strike-slip faults and associated planar systematic joint sets. These fractures are representative of a dextral strike-slip regime, interpretable on a dextral Riedel shear system. The presence of these dextral strike-slip faults suggests that a rotation in the paleo-stress regime from a vertical 1 to a horizontal NE-SW oriented 1 have occurred during deformation, which may be related to the second deformation event (175–155 Ma) attributed here to stage two of Gondwana break-up. These structural deformation patterns developed in the Jozini Formation are absent in the rocks of the Bumbeni Complex, suggesting the deformation in the Lebombo Group occurred prior and/or during the eruption and intrusion of the Bumbeni Complex. The aeromagnetic data defines four distinct magnetic domains (Domains 1–4), which are separated by distinct regional magnetic discontinuities and are defined by their structural framework. Domain 1 occurs in the SW of the study area and is characterised by N-S oriented wide lineaments (low frequency, deep seated), and correlate with the Lebombo Group and are likely related to the N-S oriented faults occurring in the Jozini Formation rhyolites. The formation of these structures is here associated with the E-W orientated spreading attributed to the initial stages of Gondwana break-up (~180–175 Ma). Domains 2, 3 and 4 mainly display E-W and NE-SW oriented high frequency shallow structures, which cross-cut each other suggesting that the shallow structures occurred at different times and post-date the occurrence of the deep structures. These structures are related to the second deformation event which is constrained between stages 2 and 3 (175–135 Ma). The field evidence reveals that the Bumbeni Complex comprises basaltic and rhyolitic rocks of the Mpilo and Fenda Formations, respectively, indicating bimodal volcanism typical of rift related extension. The bimodal volcanism is most likely related to local extension associated with the upwelling of the continental lithospheric mantle. The position of the Zululand Basin boreholes (ZD and ZG) in relation to the aeromagnetic anomalies and the geomagnetic timescale, reveal that the Bumbeni Complex comprise NE-striking remanent and non-remanently magnetised plutonic bodies. These NE-striking plutonic bodies, which are a result of ascended magma occur in the southeast and northeast of the study area, delineated from the aeromagnetic data, and are indicative of emplacement during positive and negative polarity periods which prevailed during the Cretaceous. The position of the Bumbeni Complex and extension of the volcanism along the Bumbeni Ridge is correlated with a once-active E-W trending spreading centre located in the Northern Natal Valley at ~133–125.3 Ma, based on its paleo-position. Thus, the Bumbeni event is attributed to a combination of these processes. This event is likely a volcanic centre that formed along a failed rift system.