Geoscientists Confirm Million-Year-Old Drip | Eurek alert!

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image: A simulation of the outermost rock layer of the earth’s shell using a silicone polymer fluid, modeling clay, and a sand-like layer made of ceramic and silica spheres demonstrates the process of lithospheric dripping.
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Credit: Julia Andersen, Tectonophysics Laboratory, University of Toronto

TORONTO, ON – Much like honey dripping slowly from a spoon, parts of the outermost rocky layer of the Earth’s shell continually sink into the more fluid layer of the planet’s mantle over millions of years. . Known as lithospheric dripping – named for the fragmentation of the rocky materials that make up the Earth’s crust and upper mantle – the process results in significant surface deformations such as basins, crustal folds and irregular uplifts .

Although the process is a relatively new concept in the decades-old field of plate tectonics, several examples of lithospheric dripping across the world have been identified – the Central Anatolian Plateau in Turkey and the Great Basin in the west. from the United States, for two. Now, a team of researchers led by geologists from the University of Toronto (U of T) have confirmed that several regions of the central Andes in South America formed in the same way.

And they did it using materials available at any hardware store and art supply outlet.

“We have confirmed that a deformation on the surface of an area of ​​the Andes results in an avalanche of much of the lithosphere below,” says Julia Andersen, a doctoral student in the Department of Earth Sciences at the Faculty of Arts and Sciences. of science at U of T and senior author of a study published in Earth & Environment Communications, which is part of the Nature family of journals. “Due to its high density, it flowed like cold syrup or honey deeper into the interior of the planet and is likely responsible for two major tectonic events in the central Andes – shifting the region’s surface topography by hundreds of kilometers and both crush and stretch the surface crust itself.

“Taken together, the results help define a new class of plate tectonics and may have implications for other terrestrial planets that do not have Earth-like plate tectonics, such as Mars and Venus.”

Lithospheric drip occurs when parts of the lowest layer of the Earth’s outer shell thicken and begin to drip into the mantle below when warmed to a certain temperature. As the fragments sink into the lower mantle, they first form a pool on the surface which later arises as the weight below breaks away and sinks deeper into the depths of the mantle. This results in an upward swing of the landmass over hundreds of kilometres.

The central Andean plateau is defined by the Puna and Altiplano highlands and was first formed when the Nazca plate slid under the South American plate during the well-documented process of subduction of the plate tectonics, during which part of the heavier of the two tectonic plates sinks. in the mantle when they converge. Previous studies have suggested, however, that the subsequent increase in topography of the central Andes was not uniform over time, but rather was built by sporadic pulses of uplift throughout the Cenozoic Era that started about 66 million years ago.

Furthermore, geological estimates indicate that the relative timing and mechanism of uplift in the region and the styles of tectonic deformation are different between the Puna and Altiplano plateaus. The Puna Plateau is characterized by a higher average elevation and includes several isolated inland basins, such as the Arizaro Basin and the Atacama Basin, and distinct volcanic centers.

“Various studies invoke lithosphere removal to explain widespread surface deformation and plateau evolution unrelated to subduction,” says Earth Science Professor Russell Pysklywec, study co-author and director. of Andersen’s thesis. “Furthermore, crustal shortening within the Arizaro Basin is well documented by folding and local thrust faulting, but the basin is not bounded by known tectonic plate boundaries, indicating that a more localized geodynamic process is occurring.”

Geoscientists have used sedimentary rock records to track surface elevation changes in the central Andes since Miocene times, about 18 million years ago. Seismic imaging provides a distant picture of the Earth’s interior much like an ultrasound for a human body, illuminating a new view of lithospheric drip structures.

Andersen and his colleagues claim that previous geological studies provide evidence for lithospheric drips in the region, but the dynamic processes of lithospheric drips and their role in driving local surface tectonics in these alleged geological cases are uncertain. For the most part, the predictions of geodynamic models have not been tested in the context of direct regional geological or geophysical observations.

So the team set out to develop analog 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 the basins of the Central Andes hinterland was caused by lithospheric drip processes.

“Recognizing the massive time and length scales involved in these processes – millions of years and hundreds of kilometers – we have designed innovative three-dimensional laboratory experiments using materials such as sand, clay and silicone to create scale analog models of drip processes,” says Andersen. “It was like creating and destroying tectonic mountain belts in a sandbox, floating on a simulated pool of magma – all under incredibly precise sub-millimeter measured conditions.”

The models were built inside a plexiglass tank with an array of cameras positioned above and beside the tank to capture any changes. The tank was first filled with polydimethylsiloxane (PDMS) – a silicone polymer fluid about 1,000 times thicker than table syrup – to serve as the Earth’s lower mantle. Next, the strongest section of the mantle was replicated using a mixture of PDMS and modeling clay and placed in the tank above the mantle. Finally, a sand-like layer made of a mixture of precision ceramic spheres and silica spheres was laid on top to serve as the earth’s crust.

The researchers activated the model by inserting a high-density seed into the PDMS and modeling clay layer, to initiate a blob that was then pulled down by gravity. Cameras on the outside of the tank operated continuously, capturing a high-resolution image roughly every minute.

“The dripping happens for hours, so you don’t see much happening from one minute to the next,” says Andersen. “But if you checked every few hours, you would clearly see the change – it just takes patience!” The study presents snapshots every ten hours to illustrate the progress of the drip.

The researchers then cross-referenced the size of the drop and the damage to the replica crust at selected time intervals to see how their scale processes matched sedimentary records of the areas in question over millions of years.

“We compared the results of our models to geophysical and geological studies conducted in the central Andes, particularly in the Arizaro Basin, and found that changes in crustal elevation caused by runoff in our models follow very closely. elevation changes in the Arizaro Basin,” Andersen explains. “We also observed crustal shortening with folds in the model as well as basin-like depressions on the surface, so we are confident that ‘a drip is very likely the cause of the deformations observed in the Andes.

The researchers suggest that the results aim to clarify the link between mantle processes and crustal tectonics, and how these geodynamic processes can be interpreted with observed or inferred episodes of lithospheric retreat. “The findings show that the lithosphere may be more volatile or fluid than we thought,” says Pysklywec.

Other contributors to the study include Tasca Santimano from the University of Toronto, Oguz Göğüş from Istanbul Technical University and Ebru Şengül Uluocak from Çanakkale Onsekiz Mart University in Turkey.

The study, titled “Symptomatic lithospheric drips triggering rapid topographic uplift and crustal deformation in the central Andes,” was published in Earth & Environment Communications. The research was made possible with support from a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada, the International Fellowship Program for Outstanding Researchers from the Council for Scientific and Technological Research of Turkey, a TUBITAK Fellowship for Visiting Scientists, as well as from Compute Ontario and the Digital Research Alliance of Canada.

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MEDIA CONTACTS:

Julia Anderson
Department of Earth Sciences, Faculty of Arts and Sciences
University of Toronto
julia.andersen@mail.utoronto.ca

Russell Pysklywec
Department of Earth Sciences
University of Toronto
russ@es.utoronto.ca

Sean Bettam
Communications and Public Affairs, Faculty of Arts and Sciences
University of Toronto
s.bettam@utoronto.ca


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