Smith and Hallam’s (1970) fit of the southern continents
Smith and Hallam (1970) proposed a fit of South America and Africa (Fig. 3) as part of a reconstruction of “Gondwana”. They based their reconstruction on similar methods to Bullard et al. (1965), using the 914-m contour as the edge of the continent, and rotation data for their best fit is shown in Table 1. The most important feature of the Smith and Hallam reconstruction is in the utilisation of geological and geophysical evidence to support their reconstruction. Geological evidence cited includes the continuation of the West African shield into the Sao Luis of northern Brazil based on age determinations, the extension of a Pre-Cambrian sequence from the Bahia of Brazil into Gabon, the close match of structures and sedimentary sequences between Argentina and South Africa, and the cross-cutting relationship of the Palaeozoic “Brazilides” fold belt and the Palaeo – Mesozoic “Gondwanide” fold belt.
Figure 3. Smith and Hallam, 1973.
Pplates reconstruction using the Smith and Hallam (1970) rotation parameters for South America relative to Africa. Africa rotated according to the Global Isochron Chart (Royer et al. 1992)
Smith and Hallam also utilised palaeontological, sedimentary, and igneous evidence to postulate when the break-up of Gondwana could have initiated. Based primarily on evidence of the Serra Geral volcanism in the Parana Basin, thought to be related to break-up because it occurs primarily along the continental margin, they concluded break-up began between the Upper Jurassic and Lower Cretaceous. Neritic sediments along the African and South American margins suggested a slightly later, Middle Cretaceous time for break-up. Palaeomagnetic evidence based on differences in apparent polar wander paths suggested dispersal occurred at 100 Ma, but this was not conclusive in the case of Africa and South America as sea-floor spreading rates increased in the South Atlantic at about 120 Ma, subsequent to initial Jurassic rifting.
This reconstruction gathered evidence from multiple sources, and incorporated it into a complex model for the timing of break-up. The advent of large geological datasets and improved computing facilities has subsequently enabled better-constrained, more complex reconstructions.