The geological record of the movement of Earth's "tectonic plates" is preserved in the magnetic orientation of old rocks found in the interior of these tectonic plates. As these rocks form, the magnetic orientation of their magnetic minerals aligns in a direction that depends on their position with the Earth's magnetic field. As the tectonic plates move and their position with the Earth's magnetic poles shift, the magnetic orientation of the new, younger rocks changes accordingly. By tracking these changes in the magnetic orientation, apparent polar wander paths or APWPs can be generated, which gives a picture of the continent's position or the movement of Earth's tectonic plates at different geological time scales.
From the APWPs, geologists can trace the movement of continents dating back millions of years. One important event was the opening of the Japan Sea in the Miocene epoch of the current geological era (Cenozoic), whereby southwest Japan drifted away from the Asian continent. However, not much is known regarding the tectonic history of the region for the preceding Mesozoic era.
In a study published in Earth, Planets and Space, researchers from Okayama University, Japan aimed to fill this gap, by constructing the Mesozoic APWP for southwest Japan. This information is useful to understand the tectonic activity in East Asia, as Professor Koji Uno, the lead scientist on the study, explains, "The construction of the Mesozoic APWP for southwest Japan would contribute to elucidating the intracontinental deformation history along the eastern margin of East Asia since the Mesozoic."
The researchers initially conducted paleomagnetic analysis on sandstone and mudstone samples taken from southwest Japan. By measuring the remnant magnetization in the rock samples, they determined the 110 Ma paleomagnetic pole position. In addition to this, they derived the paleomagnetic pole positions based on data from other studies to construct an APWP for southwest Japan during the mid to Late Cretaceous every 10 million years i.e. 90, 80, and 70 Ma. Combining their data with data on the well-established Miocene paleomagnetic poles, the researchers obtained the APWPs that highlighted the movement of southwestern Japan from the Cretaceous in the Mesozoic era to the Cenozoic era (110 million years to 12 Ma).
Comparing the APWPs of southwest Japan to that of East Asia, the researchers found the pole positions to be stationary between 110 Ma and 70 Ma implying that southwest Japan was a stable part of East Asia during the Cretaceous. However, post-Cretaceous, in the Cenozoic era, two clockwise rotations in the pole positions were found. The researchers interpret these as tectonic rotations of southwest Japan. "The earlier rotation occurred during the Paleogene (between 70 and 20 Ma), when southwest Japan was attached to the Korean Peninsula, as part of the East Tan-Lu Block. During the Neogene (between 20 and 12 Ma), the later rotation occurred, and southwest Japan detached from the East Tan-Lu Block to form the Japan Sea", elaborates Prof. Uno.
These findings highlight the interaction of southwest Japan with East Asia and improve the understanding of the tectonic history of the region. Prof. Uno observes, "It is suggested that the interior of southwestern Japan was stably preserved, despite it experiencing a large tectonic event, the formation of the Japan Sea. Previous studies have shown that Kibi Plateau, the area where new data was obtained in our study, was a stable continental ground; the results of our study also support this idea. This is an important piece of evidence for the relative geological stability of the Japanese islands."
Truly, unraveling history, even if it is the geological history of Earth, needs treading on the paths wandered along previously by the planet.
About Okayama University, Japan
As one of the leading universities in Japan, Okayama University aims to create and establish a new paradigm for the sustainable development of the world. Okayama University offers a wide range of academic fields, which become the basis of the integrated graduate schools. This not only allows us to conduct the most advanced and up-to-date research, but also provides an enriching educational experience.
Website: https://www.okayama-u.ac.jp/index_e.html
About Professor Koji Uno from Okayama University, Japan
Professor Koji Uno is a Professor at the Department of Earth Sciences at Okayama University. His research areas include paleomagnetism, tectonics geology, and rock magnetism.
