Asthenospheric flow and plate tectonics in the Antarctic

The 800-km wide body of water known as Drake Passage lies between the tip of South America and the Antarctica. Connecting the Atlantic, Pacific and Southern Ocean, this remote region is where powerful currents collide, giving the Drake Passage its reputation as one of the roughest sea passages in the world.

Until the opening of the Panama Canal, the Drake Passage was an important but testing trade route, which, as the name suggests, was first discovered by 16th century sailors. Half a millennium later, it is the site of new discoveries by geophysicists who are using advanced technologies and plate tectonics’ theory to study the evolution of the Earth.

Drake Passage was formed when an opening developed along the junction of the South American and Antarctic tectonic plates. This allowed the oceans to flow in and separated South America from Antarctica. The opening is believed to have taken place at a critical moment in history, around 40 million years ago, when the greenhouse warmth of the Eocene Epoch gave way to the icy conditions of the Oligocene.

The Alvarez hypothesis
The American geologist Walter Alvarez is best known for his theory that an asteroid impact was responsible for the mass extinction of dinosaurs. Prompted by another interest in finding geological evidence for why the Earth’s tectonic plates move, Alvarez’s research and the ideas he developed about convection within the Earth’s mantle still intrigue scientists today.

Alvarez proposed that if upper mantle return flow existed, the flow would be conditioned by continental masses, depending on whether they separate or join. His ideas assumed the existence of ‘asthenospheric currents’ or upper mantle outflow from the Pacific to the Atlantic through three oceanic gateways, including Drake Passage and the neighbouring Scotia Sea.

Asthenospheric flow
Alvarez’s hypothesis is the starting point of research currently being carried out by geophysicists Manuel Catalán of the Spanish Royal Naval Observatory, and Yasmina M. Martos of NASA GSFC and the University of Maryland, USA. Using advanced technologies and plate tectonic theory, they are interested in what Drake Passage, or rather the Earth below its seabed, can tell us about asthenospheric flow and its effects on the surface and above.

Earth’s surface is mainly covered
by oceans and knowledge of their evolution is essential for understanding global changes.

Lying immediately below the Earth’s solid outer shell or ‘lithosphere’, the ‘asthenosphere’ extends approximately from 100 to 300 km below its surface. The asthenosphere is the layer most involved in the movement of the tectonic plates, which ‘glide’ above it, thanks to the convective currents that rise through it from the deep mantle.

Asthenospheric currents suppose an injection of heat that affects geophysical features sensitive to temperature. Crustal’s magnetic properties provide a view of the Earth’s crust and provide information regarding plate tectonic history. Additionally, magnetic anomalies provide information about the thermal structure of the Earth’s lithosphere: where the temperature is high, magnetic anomaly amplitudes weaken.

Barriers in the Scotia Sea
Martos and Catalán’s early research published in 2014 (Martos et al., 2014) looked at barriers to outflow between the Pacific and Atlantic. It also set out to determine the main features of the lithosphere below the Scotia Sea to the east of Drake Passage, based on the compilation and analysis of global satellite and gravity data.

In particular, for the first time they identified significant variation in the depth of the lithosphere below the Scotia Sea, with the western area having a thinner lithosphere. They interpreted this as a consequence of asthenospheric current flows and argued that the thinned lithosphere of the newly born Scotia Sea facilitated mantle flows between the Pacific and Atlantic, supporting Alvarez’s hypothesis.

Geothermal structure
In 2019, Catalán and Martos published research (Martos et al., 2019), which studied the geothermal structure of the lithosphere of the Scotia Sea and the relationships of the Pacific and Atlantic upper mantle. Using magnetic anomalies to build a thermal model to create a geothermal heat flux map of the ocean floor, they also used multichannel seismic and satellite depth measurement or ‘bathymetry’ data to determine thermal subsidence.

Their study detected the presence of two high geothermal heat flow branches, one to the south of South America and the other to the north of the Antarctic Peninsula. As proposed in Alvarez’s hypothesis, the results further corroborated the existence of a mantle outflow that poured into the Atlantic from the Pacific.

The data also showed that the seafloor under the West Scotia Sea reaches thermal equilibrium more quickly than other oceans do and behaves like old oceanic crust does in larger oceans. Catalán and Martos attributed the anomalous thermal evolution detected to both high geothermal heat flow branches, which acted as an out-of-the-ordinary heat source causing this anomalous thermal state and evolution.

Atlantic asthenospheric flow
In 2019, Catalán and Martos also published research (Catalán et al., 2019) into asthenospheric currents further north in the Earth’s mantle below the Atlantic Ocean, between the Gulf of Cadiz and Canary Archipelago. This area links an active volcanic environment – the Canary Archipelago – and an area where two tectonic plates meet- the Eurasia and Africa plates.

The thinned lithosphere of the newly born Scotia Sea facilitated mantle flows between the Pacific and Atlantic.

Here Catalán and Martos’s objectives were to analyse and integrate different geophysical techniques and data, including geothermal heat flow, total tectonic subsidence and seismicity, in order to understand the effects of the still debated hotspot track that was responsible for the creation of the Canary Archipelago. In this way they hoped to establish a plausible geodynamic connection between the two locations: Canary Islands and the Gulf of Cadiz.

Their research detected a branch of mantle material that migrates from the Canary Islands towards the northeast, reaching the Gulf of Cadiz more than 1000 km away. They also found a correlation between the maximum depth at which earthquakes occur in the area and the thermal structure of the lithosphere.

This study found evidence of lithospheric thinning between the Canary Islands and the Gulf of Cadiz and in turn, the presence of an asthenospheric channel which connects both scenarios feeding and altering locally the Eurasia-Africa Plate boundary. It suggests that the present plate boundary is affected thermally and in turn mechanically, due to the influence of the underlying asthenospheric flow.

Powell Basin
Most recently Catalán and Martos’s published research (Catalán et al., 2020) has taken them back to the Scotia Sea and surrounding areas to study the unusual magnetic anomalies in the Powell Basin, to the east of the Antarctic Peninsula. The creation of this small ocean basin was a by-product of the earlier opening of Drake Passage and continental fragmentation of the northeastern extremity of the Antarctic Peninsula.

Using magnetic, gravity, multichannel seismic profiles and bathymetry data, they found that the oceanic seafloor extension appears to be smaller than previously thought. In addition, they detected an area where magnetic anomaly amplitudes are almost a half smaller than in other areas of the basin. They attribute this weakened magnetic response to raised temperature in the lithosphere caused by a branch of the Pacific mantle outflow that penetrates into the Powell Basin.

As a result, Catalán and Martos proposed that asthenospheric flow from the Scotia Sea had a major impact in the evolution of the crustal magnetic properties of the Powell Basin.

Conclusion
As Catalán and Martos note, “Earth’s surface is mainly covered by oceans and knowledge of their evolution is essential for understanding global changes.”

Their extensive work in the area of the Drake Passage and the Scotia Sea has helped to support Alvarez’s hypothesis about the existence of mantle outflow and establish the area as one of the most important asthenospheric gateways on Earth.

Their research suggests further that asthenospheric flow affects not only the dynamics which controls plate tectonics, but also crustal magnetic properties such as those seen in the Powell Basin. In addition, asthenospheric flows affect the seismicity of a region such is the case in the Gulf of Cadiz, an area where two tectonic plates meet.