Geoscientist confirms millions of years old crustal drop beneath the Andes Mountains

Just as honey slowly drips from a spoon, parts of the rocky outermost layer of Earth’s shell have been continuously sinking into the more fluid layers of the planet’s mantle over millions of years. Known as lithospheric dripping – named for the fragmentation of the rocky material that makes up the Earth’s crust and upper mantle – this process results in significant deformation of the surface such as basins, crustal folds and irregular elevations.

Although the process is a relatively new concept in the field of decades-old plate tectonics, several examples of lithospheric droplets around the world have been identified – the Central Anatolian Plateau in Turkey and the Great Basin in the western US, to name two. Now, a research team led by Earth scientists at the University of Toronto has confirmed that several areas of the central Andes Mountains in South America formed in a similar way.

And they do so using materials available at hardware stores and art supply outlets.

“We have confirmed that the deformation on the surface of the Andes Mountains area has a large portion of the lithosphere beneath it that collapses,” said Julia Andersen, PhD candidate in the department of Earth sciences at U of T and lead author of a study published in Communications Earth & Environment, part of the journal Nature family. “Due to its high density, it drips like cold syrup or honey deeper into the planet’s interior and is likely responsible for two major tectonic events in the Middle Andes – shifting the region’s surface topography by hundreds of kilometers and both rattling and stretching the surface crust itself.

“Overall, the results help define a new class of plate tectonics and may have implications for other terrestrial planets that lack Earth-like plate tectonics such as Mars and Venus.”

A geological map of the Arizaro Basin, showing folding faults and thrust faults within the basin, compared to the surface view of the experimental simulation of lithospheric droplets. The fold and the direction of shortening are depicted by red arrows. Photo credit: Left: DeCelles, et al. (2015). Right: Julia Anderson et al. (2022).

Lithosphere dripping occurs when part of the lowest layer of Earth’s outer shell thickens and begins to drip into the mantle below when it is warmed to a certain temperature.

As the fragments sink into the lower mantle, they first form depressions on the surface which then emerge as the weight beneath breaks apart and sinks further into the deeper mantle depths. This causes the land mass to swing upward for hundreds of kilometers.

The Central Andean Plateau is defined by the Puna and Altiplano plateaus and was first formed when the Nazca plate slid under the South American plate during a well-documented tectonic plate subduction process, in which part of the two heavier tectonic plates sank. into the mantle when they meet.

Previous studies have suggested, however, that the subsequent ascent of the Middle Andean topography was not uniform in time but rather built up through sporadic pulses of uplift throughout the Cenozoic Era which began about 66 million years ago.

Geological estimates suggest that the relative timing and mechanism of uplift in the region and the forces of tectonic deformation differ between the Puna and Altiplano plateaus. The Puna Plateau is characterized by higher mean elevations and includes several isolated inland basins, such as the Arizaro Basin and Atacama Basin, and distinct volcanic centers.

“Various studies call for lithosphere removal to explain widespread non-subduction-associated surface deformation and plateau evolution,” said Russell Pysklywec Professor of earth sciences, study co-author and Andersen’s PhD supervisor. Furthermore, crustal shortening in the interior of the Arizaro Basin is well documented by local thrust folds and faults but the basin is not bounded by known tectonic plate boundaries, suggesting there are more localized geodynamic processes taking place.

Geoscientists have used sedimentary rock records to track changes in the surface elevation of the Middle Andes since the Miocene epoch about 18 million years ago. Seismic imaging provides long-range ultrasound-like images of the Earth’s interior for the human body, illuminating a new view of the droplet structure of the lithosphere.

Simulations of the rocky outer layer of the Earth’s shell using liquid silicon polymer, modeling clay, and a sand-like layer made of ceramic balls and silica demonstrate the lithospheric droplet process. (photo by Julia Andersen/Tectonophysics Lab/University of Toronto)

Andersen and his colleagues say past geological studies advance evidence for lithospheric droplets in the region, but the dynamic processes of lithospheric droplets and their role in driving local surface tectonics in this recognized geological case are uncertain. For the most part, geodynamic model predictions have not been tested in the context of direct regional geological or geophysical observations.

So, the team set out to develop analogue laboratory models with geological and geophysical constraints to recreate what happened over thousands of centuries and test their hypothesis that the topographic and tectonic evolution of inland basins in the Middle Andes was caused by lithospheric drip processes.

“Knowing the enormous time and length scales involved in this process – millions of years and hundreds of kilometers – we designed innovative three-dimensional laboratory experiments using materials such as sand, clay and silicon to create scaled analog models of the droplet process,” Andersen say. “It’s like creating and destroying a tectonic mountain belt in a sandbox, floating in a simulated magma pool — all under very precise sub-millimeter measurement conditions.”

The model is built in a Plexiglass tank with a set of cameras positioned above and to the side of the tank to capture any changes. The tank was first filled with polydimethylsiloxane (PDMS) – a liquid silicone polymer roughly 1,000 times thicker than table syrup – to serve as the earth’s lower mantle. Next, the densest part of the mantle was replicated using a mixture of PDMS and modeling clay and fed into a tank over the mantle. Finally, a sand-like layer made of a mixture of precision ceramic balls and silica balls is laid on top to serve as the earth’s crust.

The researchers activated the model by feeding high-density seeds into the PDMS and modeling the clay layer, to initiate droplets which were then pulled down by gravity. The camera outside the tank runs continuously, capturing high-resolution images every minute or so.

“Drips go on for hours so you won’t see a lot going on from one minute to the next,” Andersen said. “But if you check every few hours, you’ll clearly see the changes – it just takes patience.” The study presents a snapshot of every 10 hours to illustrate the progress of the droplets.

The researchers then cross-referenced droplet size and damage to the replica’s crust at specific time intervals to see how their scale processes matched the sedimentary records of the area in question over millions of years.

Artist’s impression of the two types of lithospheric droplets, supported by surface views from experimental simulations of the process. One results in the thickening and lifting of the earth’s crust, while the other results in the formation of depressions on the surface. (photo by Julia Andersen/Tectonophysics Lab/University of Toronto)

“We compared the results of our model with geophysical and geological studies conducted in the Central Andes, specifically in the Arizaro Basin, and found that the drop-induced crustal elevation changes in our model track very well with the Arizaro elevation changes. Basin,” Anderson said. “We also observed shortening of the crust with folds in the model as well as depressions like depressions on the surface so we believe that droplets are very likely to be the cause of the deformation observed in the Andes.”

The researchers suggest the findings aim to clarify the relationship between mantle processes and crustal tectonics, and how those geodynamic processes can be interpreted by observed or inferred episodes of lithospheric deletion. “The findings suggest that the lithosphere could be more volatile or liquid-like than we believed,” said Pysklywec.

Additional contributors to the study include Tasca Santimano, from the U of T department of earth sciences, and Oguz Göğüş at Istanbul Technical University and Ebru engül Uluocak at Anakkale Onsekiz Mart University in Turkey.

This research was made possible thanks to the support of the Discovery Grant from the Natural Sciences and Engineering Research Council of Canada, the International Fellowship for Outstanding Research Program of the Scientific and Technological Research Council of Turkey, the TUBITAK Fellowship for Visiting Scientists, as well as Compute Ontario and the Digital Research Alliance of Canada.

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