Continental tapering of South America caused asymmetric non-uniform opening of the South Atlantic Ocean

As a cornerstone of the continental drift and seafloor spreading, the South Atlantic Ocean offers a field lab to build and test hypotheses regarding the dynamic mechanism of plate tectonics.
Continental tapering of South America caused asymmetric non-uniform opening of the South Atlantic Ocean
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The two-dimensional (2D) jigsaw puzzle of continents and oceans of the planet Earth is one of the most important discoveries in geoscience that is instrumental in proving whether continents have drifted and oceans opened; however, neither does the 2D jigsaw puzzle indicate how continents have drifted and oceans opened, nor does the puzzle answer what has been driving continents to drift and oceans to open.

 The planet Earth is 75% covered by a vast volume of ocean water and three quarters of the Earth surface is hidden underneath the ocean floor. The difficulty in accessing seafloor and subsurface geology has hampered our understanding of the underlying mechanism of continental drift and oceanic opening. Because of the lack of dimension in the 2D jigsaw puzzle, the underlying dynamic mechanism for continental drift and oceanic opening has been and still remains unclear.

In spite of the fact that geologists cannot see sub-seafloor geologic structures, geophysicists can “listen” to them by sending sound waves underneath the ocean and analyzing the echoed feedback at the surface. This is similar to using medical CT scan imagery to detect structures in human bodies. The strong acoustic (sonar or seismic) waves could penetrate through the seawater column and further beneath the seafloor before bouncing back to the receivers at the surface. The feed-back signals are then translated into digital images for computer vision and human interpretation. The process is somewhat similar to identifying people by listening to his or her voice without seeing his or her face.

 As a cornerstone of the continental drift and seafloor spreading, the South Atlantic Ocean offers a field lab to build and test hypotheses regarding the dynamic mechanism of plate tectonics. With the advance in geophysical technologies, investigation of the South Atlantic has been progressing from surface, through seafloor to subsurface, from shallow subsurface to deep basement, from local surveys to regional ones, and from one side to the other of the Ocean. The systematic progress in depth and breadth of geophysical investigation unravels a comprehensive and big picture, making it possible to build and test a three-dimensional (3D) jigsaw puzzle of the South Atlantic. Being advantageous over the 2D jigsaw puzzle, the 3D jigsaw puzzle recovers large-scale, deeply-rooted asymmetric rifts and transfer faults that are kinematically indicative of the asymmetry and non-uniformity in South Atlantic opening.

 Informed by the 3D jigsaw puzzle, analytical modeling adds an additional fourth dimension of time to the puzzle, making it possible to quantitatively simulate the dynamic phased opening of the South Atlantic Ocean. The dynamic opening created asymmetric non-linear trails of fossil hotspots (seamounts) as a track record of the along-axis migration of live hotspots, which turns out to be modulated by the southward tapering of the South American continent (Figure 1).

 Figure 1 Dynamic opening of the South Atlantic Ocean modulated by asymmetric rotational drift of the South American continent. The present-day outlines of the South American continent, South Atlantic Ridge and transforms are used to schematically demonstrate the concept because they have actually experienced intra-continental shear deformation along with rotation. a At approximately 137 Ma, South Atlantic opening began with an asymmetric graben, a gravitational slide and transfer faults. The rose diagram denotes the original orientations of rift faults (black) and transfer faults (red). b From approximately 137 Ma to 55 Ma, the southward tapering of the South American continent caused right-lateral shear along with clockwise rotation of the continent, leading to southward scissor opening of the Ocean. The southward increase in spreading rate induced southward propagation of decompression plume, driving active hotspots to migrate from north to south and leaving behind two split chains of fossil hotspots that cross over seafloor flowlines. The rose diagrams denote the split and rotated rift faults (black) and transfer faults (red). c Since approximately 55 Ma, as the effect of continental tapering has waned due to proportional and incremental accretion of the oceanic crust, continental drift and seafloor spreading have become non-rotational, causing migrating hotspots to stall and seamount chains to bend.

Advancement from the 2D, through 3D to 4D jigsaw puzzle leads to a long-sought-after analytical solution to the underlying mechanism for South Atlantic opening.  The mechanism points towards a chain reaction from continental drift, through seafloor spreading to ridge-hotspot interaction. The chain reaction poses challenges to classical models of stationary hotspots and of seamount trails as kinematic indicators of the movement direction of the South American and African plates. The findings undermine the classical theory of thermal convection as the active driver for continental drift and seafloor spreading, which will be instrumental in resolving the geodynamics of the Earth.

The South Atlantic Ocean is an integral component of the greater Atlantic Ocean and other oceans around the global. The integrated surface-subsurface investigation of the South Atlantic would help in-depth understanding of global marine environment, ecosystem, biologic productivity and energy resources. The asymmetry and non-uniformity in oceanic opening have been controlling the drift of mid-ocean ridges and migration of hotspots, which could in turn be associated with migration and evolution of hydrothermal vents, abnormal geochemical plumes and extreme biochemical environments. The ridge-hotspot interaction could play a role in modulating ocean water temperature, ocean current circulation and marine ecological and biogeographic heterogeneity. The asymmetry and non-uniformity in continental drifting and oceanic opening could affect spatial distributions of energy resources, both geothermal energy sourced from rifted mid-ocean ridges and conventional energy sourced from rifted continental margins.

Gao, D. Continental tapering of South America caused asymmetric non-uniform opening of the South Atlantic Ocean. Commun Earth Environ 3, 278 (2022). https://doi.org/10.1038/s43247-022-00587-3

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