The South America, Africa, Northern America Connection – A Removal - Restoration Project
Removing post-rift sedimentary sections and restoring crustal extension allows us to approximate the pre-rift shapes of continental margins. Applied to the equatorial margins of Africa and South America, we can derive a pre-Aptian reconstruction of the equatorial Atlantic.
Measured thickness of rift and post-rift sediments with geophysical and paleobathymetry data is used to estimate present-day thickness of underlying crust and how much stretching has taken place. In the equatorial Atlantic, this results in more reliable shapes for Africa and South America than the 2,000 metre isobath used in the ‘Bullard reconstruction’. We must also restore the effects of extension between north-western and southern Africa (rift systems extend through central Africa) not accounted for by Bullard. The result is that, prior to the Aptian, the two continents are seen to be a single block, juxtaposing the Demerara and Guinea Plateaus. In contrast, the Bullard fit has a 500 km gap between these two areas.
Euler rotation poles, defined by marine magnetic anomalies and fracture zones in the central Atlantic, are then used to rotate the reconstructed shape of Africa/South America back towards North America, with the aim of showing the Pangea Supercontinent prior to the formation of the Gulf of Mexico. Figure 3a shows reconstructions of the region for Aptian and older times. Because continental reassembly in the Gulf of Mexico region is dependent on the results of rotating the Africa-South America reconstruction back towards North America, the difference between Bullard fit and the fit used in Figure 3a will affect the final reassembly more than any other kinematic parameter. The reconstructed shape of the north-western Andes, after restoring the effects of Jurassic rifting and accretion of Caribbean terranes, is important when we start to analyse the finer details of Gulf opening.
The relationship of North and South America before the Jurassic defines a line separating two parts of Mexico. The south, overlapped during the Triassic by South America, must have migrated into today’s position as the Gulf evolved, Cordilleran terrane migrated and thrustbelts shortened. Northern Mexico, not overlapped by South America, may have been more or less in place relative to today’s geography. These kinematic constraints can now be used to reconstruct western Pangea and to trace the Mesozoic plate-kinematic evolution of the Gulf of Mexico and Proto-Caribbean regions.
Figures 3a and b
Putting it all Together Again – The Opening of the Gulf of Mexico
Figures 3b–3d show primary developmental stages in the Gulf’s evolution: after opening was finished, post-rift but prior to formation of any oceanic crust and, finally, early during rifting. The key is that Yucatán moved independently of the larger continents as the Gulf opened. Prior to the Jurassic, Yucatán fit snugly against the north-east Mexico- Texas-north-west Florida paleo-margin, avoiding overlapping other continental areas. This fit does not, however, close the south-eastern Gulf where southern Florida must be retracted north-westward against Yucatán and out of overlap with Africa. The geology of eastern Mexico and the occurrence of two separated salt basins suggests that the Gulf opened in two stages. The syn-rift stage involved intracontinental stretching between Yucatán and North America. By Early Oxfordian, the Louann and Campeche salt provinces were about to separate; there was no significant salt deposition on crust and open marine conditions were quickly established. The subsequent migration of Yucatán then required that eastern Mexico became a transform rather than a rifted margin, with Yucatán motion relative to North America about ‘pole C’ (see Figure 3b). Similarly, the ‘Proto- Caribbean Basin’ involved a synchronous rotational opening between Yucatán and Venezuela- Trinidad. Figures 3b–3d show the approximate flowlines along which these basins opened. Only by ca. 130 Ma had South America moved sufficiently far from North America to get Yucatán into its present position, and the Gulf could not have finished opening before then.
Figure 3c and d
This model has interesting implications for exploration. Heat flow, subsidence history, occurrence of salt, distribution/thickness of source rocks and basement structural fabric depend on whether margins are strike-slip or rift-dominated. Venezuela’s passive margin section is predicted to have existed from the end of Middle Jurassic, but the predicted, buried, Late Jurassic shelf section has not received much attention – yet.
In summary, regional kinematic analysis is extremely cost-effective and deserves an important role in the exploration process in many other parts of the world. When applied properly to appropriate areas, it is not ‘arm-waving’. Much can be gleaned about basement and basin formation and evolution, leading to the creation or dismissal of numerous play concepts, allowing explorationists with a comprehensive kinematic framework to work more confidently and more efficiently. Finally, in frontier evaluation programmes, regional kinematic analysis may not tell you exactly where to drill, but it can tell you where not to drill.
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Geosciences
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