Deep Crustal Profile Across the Southern Karoo Basin, Cape Fold Belt Front and Beattie Magnetic Anomaly, South Africa
Ansa S. Lindeque, Maartin J. de Wit, and T. Ryberg
University of Cape Town, Cape Town, South Africa
Tectonically, southernmost Africa is dominated by the Paleozoic Cape Fold Belt (CFB) and the Paleozoic-Mesozoic Karoo Basin, underlain by a Precambrian basement that hosts the world’s largest terrestrial magnetic anomaly, the E-W Beattie Magnetic Anomaly (BMA). This research presents the first near vertical reflection (NVR) seismic data acquisition, -processing and -interpretations of a ~100 km northeast-southwest line across the southernmost Karoo Basin, the BMA and the tectonic front of the CFB. The line is part of the Inkaba yeAfrica geophysical Agulhas-Karoo Transect across the southern margin of Africa (www.inkaba.org). The NVR seismic profile reveals a complex circa 42 to 45 km thick crust, consistent with previous teleseismic receiver function analyses and wide angle refraction onshore/offshore modelling. In the upper crust (<13 km), surface geology and two deep boreholes link a seismic stratigraphic package of variably dipping reflectors above a section of sub-horizontal reflectors to the Karoo- and Cape Supergroups, respectively. Folds and faults previously mapped at surface correlate downwards with distinct features in the upper 1 to 1.5 km of the seismic image, and with lateral velocity variation in shallow P- and S- wave velocity models. High S-wave velocities (2.82 - 3.30 km/s) correspond to major anticlinal hinges and low velocities to synclinal hinges. seismic profile also images low-angle thrust faults rooted in a zone of local décollements at 2 to 3 km depth. This zone corresponds to velocity changes in the coincident refraction data, and a ~150 meter thick high conductivity/low resistivity (< 2 Ωm) band in the coincident magnetotelluric (MT) data.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery