Researchers Discover Previously Unknown Process in Tectonics that Turns Seafloor into Mountains

Scientists use tiny minerals called zircons as geological clocks. These crystal grains, often the size of sand, record the chemical signals of the geological environment they formed in. In a new study led by scientists at the University of Texas at Austin, researchers used them to describe what may be a missing step in a fundamental tectonic process that lifts seafloors up to become mountains.

The study, published in the journal Geology, describes zircons from the Andes mountains in Patagonia. While the zircons formed as tectonic plates collided, they displayed a chemical signature associated with the time when the plates were separating. The researchers believe this unexpected signature could be explained by underlying tectonic plate mechanics not yet described in other models.

This missing step involves a kind of geological squeezing in a magma chamber where the zircons form before reaching the surface, with oceanic crust entering the chamber before continental crust. If you put any ocean basin underneath this magma, you get a change in the composition of this magma as it incorporates, said lead study author Fernando Rey, a Ph.D. student at UT’s Jackson School of Geosciences. This is something that was not documented prior to this study.

This theory of ocean magma mixing is important as it may represent a transitional step in forming post-arc basins, an important geological structure that shapes landscapes, geological records, and helps regulate the planet’s climate. These basins form between separating oceanic and continental tectonic plates, opening as plates pull apart and closing as they come back together.

The opening and closing of the Rocas Verdes basin, a backarc basin in Patagonia, described by researchers at the University of Texas at Austin in a study published in Geology. Panels B and C illustrate the process of basin closure, in which an underlying portion of oceanic crust is thrust into a magma chamber (B) and detaches ahead of continental crust (C). The last two panels (C and D) show the collision of the oceanic plate and the continental plate, which compressed the basin to form the Andes mountain range of Patagonia that we see today. Credit: Fernando Rey et al.

While basin opening creates oceanic crust, closure squeezes it to form mountains, bringing a geological history record to the surface where humans can access it more easily. Ocean crust erosion is also a major driver of natural carbon dioxide storage. It’s how Earth sequesters carbon. It works very effectively on its own but can take hundreds of thousands, if not millions, of years, says coauthor Matt Malkowski.

Malkowski collected the zircons examined in the study from rock and sediment samples in a Patagonia deposit. The samples spanned the complete record of the post-arc basin, called the Rocas Verdes Basin, from its opening to closure. When Rey began analyzing the zircon chemical signatures, at first nothing seemed amiss.

But zircons associated with basin closure did not show the expected chemical shift, known to scientists as a “pull down” pattern. Rey and collaborators hypothesized the data could be explained if the change did not appear until 200 million years later, in zircons that formed 30 million years ago when the basin was well into its closing phase.

Their paper proposes a model where the same tectonic forces squeezing oceanic crust to form mountains could be undermining parts of that crust and pushing it into the magma chamber influencing recorded crystal signatures during initial and middle closure stages.

As continents continue compressing, oceanic crust eventually gets replaced by continental crust, the source of the pull-down signal. The researchers believe this transitional phase where zircons receive oceanic crustal juice could be part of post-arc basins worldwide.

Sources

The University of Texas at Austin | F.M. Rey, M.A. Malkowski, et al., Detrital isotopic record of a retreating accretionary orogen: An example from the Patagonian Andes. Geology 2024; doi: doi.org/10.1130/G51918.1