Kinematics of the South Atlantic rift

For decades, every plate reconstruction of the South Atlantic included a structural escape valve buried in Patagonia. The Gastre Shear Zone — a large, inferred fault system cutting across southern South America — was a fixture in textbook models. The logic was simple: move Patagonia along that line, and the two continents fit back together. Both Torsvik and colleagues and Macdonald and colleagues relied on it. It made the geometry work. The problem, as Christian Heine, J. Zoethout, and R. Dietmar Müller showed in their 2013 plate kinematic study, is that it doesn't exist. Not in the geological record. Not in the Late Jurassic or Early Cretaceous. A detailed field study by von Gosen and Loske had already indicated as much. Heine and colleagues went further: they built a reconstruction that doesn't need it at all. To understand why that matters, you have to grasp what a tight-fit reconstruction actually demands. When you reverse the clock on continental breakup, you're attempting to restore two coastlines to their original joined position — undoing hundreds of millions of years of stretching, rifting, and seafloor spreading, until the conjugate margins sit flush against each other, as they did before the ocean existed between them. If your reconstruction leaves overlaps of one hundred kilometers or gaps of fifty kilometers, those aren't minor errors. They're telling you that something in your model of how the plates moved is wrong. Previous models covered over those mismatches with the Gastre Shear Zone. Heine and colleagues had to find a different answer. Their approach was to construct the reconstruction from the ground up, combining structural restorations of the conjugate passive margins, extension estimates from intracontinental rift basins across Africa and South America, and crustal-scale seismic data — including newly interpreted ION GXT CongoSPAN lines and published sediment isopachs. They utilized the open-source plate kinematic software GPlates and the Generic Mapping Tools as their integration platform, tying everything to a carefully chosen timescale: the Gee and Kent polarity timescale, with the base of magnetic chron M0r placed at 121 million years ago. Ocean-floor magnetic isochrons from Seton and colleagues constrained the seafloor-spreading stages — isochrons M7n at approximately 127.23 million years ago, M4 at 126.57 million years ago, and M0r at 120.6 million years ago anchored the key transition moments. The central methodological challenge was addressing the deforming zones — the Central African Rift System and the West African Rift System — where the crust wasn't moving as a rigid block but stretching internally. The team used sediment-thickness-derived subsidence to estimate how much the crust had thinned, expressed through what's called a stretch factor: the ratio of final to original crustal thickness. Translated into distance, estimates for basins like the Termit ranged from roughly 60 to 85 kilometers of orthogonal extension. By accounting for this distributed deformation quantitatively, they could back-calculate what the pre-rift configuration of the west African and Brazilian margins actually looked like — without invoking any shear zones that the geological record does not support. They also introduced the LaLOC — the landward limit of oceanic crust — as a practical boundary separating extended continental crust from true oceanic crust, giving the reconstruction a clean tie point in the distal margin. What emerged from all of this was a three-phase model for how the South Atlantic rift actually evolved. It's worth walking through each phase because each one leaves a specific fingerprint in the geology we can still see today. Phase One runs from 140 million years ago to about 126 million years ago — a roughly 14 million-year episode of very slow, broadly east-west extension. The key word is slow. In the Campos and Jatobá sectors, relative extension velocities ran at about 7 to 9 millimeters per year. In the northernmost equatorial segments near Potiguar and Rio Muni, some parts were moving at only 2 millimeters per year. That's slower than a fingernail grows. Yet this quiet, unhurried stretching accomplished something critical: it generated the full width of what geologists call the pre-salt sag basin. A pre-salt basin is a broad, shallow sedimentary depression that formed before massive layers of evaporite salt were deposited on top of it — and in Brazil's case, that basin is the geological address of some of the largest offshore oil discoveries of the 21st century. Heine and colleagues state it plainly: by the time extension rates shifted around 127 million years ago, the pre-salt basin width had already been fully generated. The oil field geometry was set in those 14 quiet million years. Phase Two, from about 126 million years ago to the base of the Aptian at around 121 million years ago, is brief but dramatic. Strain localizes. The lithosphere weakens rapidly in the equatorial Atlantic. The extension direction rotates — by as much as 75 degrees in the northern segments and about 30 degrees in the southern Pelotas and Walvis segment. And the pace accelerates: plate velocities between South America and Africa increase by roughly a factor of three during this interval, a jump recorded in the spacing between magnetic anomalies M4 and M0. This is also the interval that explains the São Paulo High, a long-puzzling structural feature on the Brazilian margin. As the extension direction shifted and deformation reorganized, strain jumped asymmetrically toward the African side, effectively rifting the São Paulo High away from its earlier position and producing the fault trends — including the Cabo Frio counterregional trend — that are visible in the margin today. Heine and colleagues tie the High's origin directly to the timing and vector change in extension during the late Hauterivian to early Aptian. It's not an anomaly that needs a special explanation. It's a predictable product of the kinematic shift. Phase Three begins around 120 million years ago and is defined by diachronous breakup — meaning the continents didn't separate all at once along a clean line, but in pieces, at different times, at different latitudes. Breakup initiated first in the northernmost central South Atlantic and then propagated outward. By 120 million years ago, continental breakup had already occurred in several northern segments. Final separation in the outer Santos and Benguela conjugate margins happened around 113 to 112 million years ago. In the equatorial Atlantic, oceanic accretion in the Deep Ghanaian Basin began around 117 million years ago, with final equatorial breakup not until roughly 103 million years ago. The spreading ridge didn't clear the Côte d'Ivoire-Ghana Ridge until about 99 to 100 million years ago. This is one ocean opening in many pulses, over roughly 40 million years of progressive disconnection. So what does the model actually explain that previous ones couldn't? Two things, clearly. The pre-salt basin width — previously treated as a somewhat mysterious given — emerges directly from the geometry of slow Phase One stretching. The São Paulo High — previously an enigmatic structural high requiring ad hoc explanations — falls out naturally from the Phase Two kinematic reorganization. And the reconstruction achieves all of this without the Gastre Shear Zone. Removing that feature isn't just a bookkeeping correction. It changes how geologists should think about intraplate deformation in southern South America: large, long-lived shear zones are not required by the fit, and they're not supported by the field evidence. The authors are candid about what the model doesn't resolve. Absolute timing remains uncertain, because coherent stratigraphy across both conjugate margin systems is lacking, and crustal-scale seismic data in several African rift domains are sparse and old. The structural details of some basin boundaries are still imprecise. What Heine, Zoethout, and Müller have provided is not a final answer but a testable baseline — a self-consistent, quantitative kinematic framework against which new seismic surveys, drilling results, and regional mapping can be checked. They made their data and high-resolution reconstructions publicly available to accelerate exactly that process. The South Atlantic, as they frame it, is an ideal laboratory for understanding how large-scale plate tectonics and lithospheric dynamics interact. The model sets the stage. The next round of data will tell us how well it holds. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.