Researchers have just discovered the remnants of an ocean floor 250 million years old beneath the depths of the Pacific.
This discovery could challenge our understanding of tectonic processes. Indeed, these remnants are linked to the Earth's internal movements, which are much more complex than previously thought.
A map of the East Pacific Rise region where the discovery of an ancient sea floor was made.
Credit: Jingchuan Wang.
By analyzing the structure of the Earth's mantle, the team led by Jingchuan Wang from the University of Maryland used seismic tomography. This technique allows visualization of the Earth's internal structure through seismic waves generated by distant earthquakes.
The scientists revealed a significant thickening in the mantle transition zone, which lies between 255 and 410 miles (410 and 660 km) below the Earth's surface. This zone, which separates the upper mantle from the lower mantle, is essential for Earth's dynamics.
The data collected revealed the presence of cold material in this region, indicating that this material likely originated from the surface. The researchers believe these are remnants of an ancient oceanic crust, submerged during a period when the first dinosaurs were beginning to appear.
In other words, the observed thickness indicates the presence of a layer of material that has slipped beneath another, resulting in a slow descent toward the lower mantle. This subduction process contributes to the movement of tectonic plates and the complex interactions within the Earth, thus influencing geological phenomena such as earthquakes and volcanism.
A: Beginning of a double divergent subduction beneath the Phoenix plate (to the west) and western Gondwana (to the east) during the Triassic, followed by an inversion of subduction of the western branch in the early Cretaceous, with the consumption of the ocean basin. The Phoenix plate breaks up around 120 Ma, and fragments are captured by the Farallon plate, which begins to subduct beneath South America.
B: The model predicts an intra-oceanic subduction zone where the Phoenix plate is subducted eastward beneath the Farallon plate. Towards the end of the Cretaceous, the subducted oceanic plate detaches from the surface, coinciding with the start of the Nazca plate's subduction beneath South America. The interaction between the Phoenix plate and the Pacific LLSVP anomaly leads to the separation of superswell and East Pacific anomalies.
The dotted black lines mark the major mantle discontinuities (255, 410, and 620 miles; 410, 660, and 1000 km depth).
Surprisingly, this oceanic crust seems to have sunk more slowly than expected. This suggests that the transition zone may slow down the descent of materials to the lower mantle, acting as a brake on tectonic dynamics. Additionally, this crust could explain the peculiar shape of a velocity anomaly observed at the base of the mantle, known as the Pacific LLSVP. This anomaly is a massive region of the lower mantle that influences geologic phenomena on the surface.
The study, published in the journal
Science Advances, highlights the influence of surface elements on deep dynamics. These discoveries could have implications for our understanding of other rocky planets and their evolution. Researchers now plan to explore other regions of the Pacific to find other ancient structures and enhance our knowledge of tectonic movements over time.
What is seismic tomography?
Seismic tomography is a geophysical technique that allows visualization of the Earth's internal structure. By using seismic waves generated by earthquakes, scientists can create three-dimensional images of the Earth's different layers, including the crust, mantle, and core.
The data collected using this method is essential for understanding Earth's dynamics, particularly the movements of tectonic plates and geological processes. By analyzing how seismic waves propagate through the various layers, researchers can deduce the composition, density, and temperature of the materials within them.
What is the importance of subduction in plate tectonics?
Subduction is a tectonic process where one lithospheric plate sinks beneath another, descending into the Earth's mantle. This phenomenon enables the recycling of surface materials as it transports oceanic crust and sediments to great depths.
Subduction also plays a fundamental role in forming mountains, volcanoes, and earthquakes. Subduction zones are often associated with deep ocean trenches and intense volcanic activity. By studying these zones, scientists can better understand plate interactions and the mechanisms affecting Earth's dynamics.
Article author: Cédric DEPOND