REVISED SCENARIO FOR THE BUSHVELD MEGA-IMPACT(S)

Hypothesis: The 2.06 Ga Bushveld Complex (BC) occupies a heat-dominated multiring impact structure. In 1925, Daly & Molengraaff (J. Geol. 32, 1-35) recognized it as surficial; persistent myths of an intrusion (classic: lopolith) trace back to misinterpretations of its supposed roof, the Rooiberg “felsite” Group (part of the BC, though the S.A. Committee for Stratigraphy officially ruled otherwise in 1980). Hatton & Schweitzer (J. Af. Earth Sci. 21, 579-594. 1995) documented a Rooiberg mafic-to-felsic suite encompassing the entire BC, but Eales (Council Geosci. Ser. 2, 25-44, 2001) still called it “a classic example” of rhyolite and part of the pre-BC Transvaal Supergroup. Buchanan & Reimold (E&PSL 155, 149-165, 1998) and Buchanan et al., (Contr. Min. Petrol. 137, 133-146, 1999) modeled Rooiberg meltrocks as conventional volcanics with sedimentary interbeds, but textures and minerals differ from known volcanic rocks and no eruptive centers are known. Textural evidence favors impact-triggered superheated and quenched emulsions, as at Sudbury (Zieg & Marsh, GSA Bull. 117, 1427-1450, 2005). Tsunami-type interbeds suggest that water influxes triggered explosive overflows from a Sudbury-type meltpool (only upper granophyre is exposed). Inflated overflows overrode the collapsing peak ring and pooled (up to 4 km) in the outer ring. Above partly melted quartzose debris (detached from collapsed central uplift), the transition from mafic to felsic meltrocks is enriched in Fe, Ti, P (Twist, Econ Geol. 80, 1153-1165, 1885), as at Sudbury.

Meanwhile, decompression melts from an impact-induced upwelling (Jones et al., E&PSL 202, 551-556) formed massive mafic and granitic sills (> 10 km; horizontal from paleomagnetic evidence; Hattingh, S. Af. Geophys. Rev. 2, 75-77, 1998) beneath accumulating Rooiberg melts in the outer ring. Simultaneous subsidence explains their survival without collapse. Crustal melting eventually led to caldera-like collapse of the entire BIC, forming a lobate 400-km basin ringed by megabreccia. Sills in the outer ring, tilted basinward by collapse, became the dipping sheets modeled by geophysics (Meyer and de Beer, Nature 325, 610-612, 1987). As the meltpool equilibrated with the crust, the upper zones of Rooiberg overflow (easily accessible to visitors) came to resemble conventional rhyolite.