Study combines climatic, tectonic models to explain Andean conundrum


Date:
December 14, 2021
Source:
University of Illinois at Urbana-Champaign, News Bureau
Summary:
The Andes Mountains are much taller than plate tectonic theories
predict they should be, a fact that has puzzled geologists for
decades. Mountain- building models tend to focus on the deep-seated
compressional forces that occur when tectonic plates collide and
send rocks skyward. A new study demonstrates how modern top-down
models that account for climate- related factors combined with
traditional bottom-up tectonic models can help uncover the
perplexing history of the Andes Mountains.



FULL STORY ==========================================================================
The Andes Mountains are much taller than plate tectonic theories
predict they should be, a fact that has puzzled geologists for
decades. Mountain-building models tend to focus on the deep-seated compressional forces that occur when tectonic plates collide and send
rocks skyward. A new study demonstrates how modern top-down models that
account for climate-related factors combined with traditional bottom-up tectonic models can help uncover the perplexing history of the Andes
Mountains.


==========================================================================
The study, led by former University of Illinois Urbana-Champaign graduate student Jiashun Hu, Illinois geology professor Lijun Liu and California Institute of Technology professor Michael Gurnis, is published in the
journal Nature Communications.

"The Andes are unique for their tectonic setting," Liu said. "The central portion of the range is abnormally tall for one formed by the relatively
low compressional stress and weak plate interface coupling we think
occurs when thin, dense oceanic crust subducts -- or slides under --
thick continental crust." Geochemical and chronological data indicate the
most recent Andean Mountain building phase began 40 million years ago,
and the most significant crustal shortening -- the process of mountain formation -- started at the central-to- northern part of the present-day
Andes and gradually expanded toward the south.

There is geologic evidence preserved along the Andean coast indicating
that the southward expansion of Andean Mountain growth continues today,
Liu said, but what is causing this migration and significant crustal deformation is still unclear.

Numerous studies show that higher erosion rates in the southern Andes,
due to a warmer and wetter climate than in the north, correspond chronologically with evidence of an increased influx of sediment into
the Andean Trench. The researchers said this sediment, which settled
along the bottom of the trench along the subducting edge Nazca plate,
may have acted as a lubricant atop the subducting plate by reducing compressional forces and resulting in lower mountains.



========================================================================== Liu's team has taken this climatic-tectonic relationship a step further
by uncovering the effect of a curious east-west trending feature known
as the Juan Fernandez Ridge -- a submerged volcanic hotspot chain that
still exists today.

"Today, where the Juan Fernandez Ridge intersects the coast of Chile,
it acts as a barrier to the northward-migrating sediments," said Hu, the
lead author of the study. "We hypothesize that this ridge has existed
for millions of years, slowly migrating southward with the subducting
Nazca Plate, starving the northern Andean Trench of sediments that
helped increase the plate coupling and mountain building behind the
migrating ridge." The team's new model accounts for the impact of the
Juan Fernandez Ridge on sediment transport through time.

"When we use our model to reverse time and reconstruct the subduction
history of the Nazca Plate in 3D space, the effects of including the
Juan Fernandez Ridge correspond remarkably well with geologic features
we see in the Andes today," Hu said.

The model has yet to be tested with the extensive range of hypotheses
that exist for the formation of the Andes Mountains -- some of which
include incredibly complex plate subduction geometries, the study reports.

"This study is a critical step forward to have the ability to
quantitatively link climate and tectonics -- something not well
represented in the past studies," Liu said.

The National Science Foundation, the National Natural Science Foundation
of China and the Center for Computational Science and Engineering at
the Southern University of Science and Technology supported this study.

========================================================================== Story Source: Materials provided by University_of_Illinois_at_Urbana-Champaign,_News_Bureau.

Original written by Lois Yoksoulian. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Jiashun Hu, Lijun Liu, Michael Gurnis. Southward expanding plate
coupling
due to variation in sediment subduction as a cause of Andean growth.

Nature Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-27518-8 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/12/211214134952.htm

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