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Gravity modeling of Bushveld Complex connectivity supported by Southern African Seismic Experiment results

School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa e-mail:webbs{at}geosciences.wits.ac.za and cawthorng{at}geosciences.wits.ac.za

Teresia Nguuri

School of Geosciences, Private Bag 3, University of the Witwatersrand, Wits, 2050, South Africa CTBTO, Preparatory Commission, Vienna International Centre, P.O. Box 1200, 1400 Vienna, Austria e-mail:teresia.nguuri{at}ctbto.org

David James

Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Rd. N.W., Washington, D.C., 20015 USA e-mail: james{at}dtm.ciw.edu

Recent gravity modelling of the Bushveld Complex indicates that the western and eastern limbs of the Bushveld Complex are connected at depth. The model predicts a downwarp in the Moho beneath the Bushveld Complex, ensuring observed Airy isostatic balance is achieved. By constraining a new Bouguer gravity model with published Vibroseis results, crustal thicknesses determined using the receiver function method, and seismic velocity modelling of the crust from the Southern African Seismic Experiment; we demonstrate that the connected model of the Bushveld Complex is consistent with all available data. Crustal thicknesses determined from receiver functions indicate that the depth to the Moho thickens from a value of ~35 to ~40km in the southern Kaapvaal craton to ~50km beneath the central region of the Bushveld Complex. This seismologically determined Moho varies significantly from that calculated from Airy isostatic balance based solely on topography as a load in this region. The corresponding crustal velocity model, determined from inverting the receiver function results for Bushveld Complex stations, also indicates a thick crust and delimits a ~6km thick high velocity zone in the upper 10km of crust attributed to the presence of the Bushveld Complex. Comparison of the seismic crustal model with drill core data on the mafic rocks of the Bushveld Complex suggests a correspondence between high seismic velocities and high densities in the upper crust. Both the gravity model and the seismological results imply a density contrast of about 0.30mg/m³ at the crust/mantle boundary beneath the Bushveld Complex. We also find that the Moho transition beneath the Bushveld Complex is significantly broader than that beneath the rest of the Kaapvaal craton, outside of the Limpopo Belt. By constraining the modelling of the gravity data with these seismological results, outcropping geology and published Vibroseis profiles, we show that the dense mafic units of the western and eastern Bushveld Complex can be interpreted as having originally been emplaced as a connected sheet (or series of connected sheets), which has subsequently been deformed and faulted. The seismological results of the Kaapvaal project support the interpretation of a connected Bushveld Complex.

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