Geologists have established the date of annexation of Sakhalin and Kamchatka to the Far East
It happened about fifty million years ago.
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Researchers from far East geological Institute Feb RAS together with colleagues from Taiwan and Germany have studied the breed Sakhalin related to the Okhotsk plate, and came to the conclusion that it collided with Eurasia already in the Cenozoic period. The new data will help to clarify the history of this mysterious plate. The corresponding article published in the Journal of Geophysical Research.
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The Okhotsk plate is called a continental lithospheric plate, which stretches from the center of the Japanese Islands (includes part) and to the North of the Kamchatka Peninsula. This plate plays an important role in volcanic activity in the Russian far East, so her research is needed for better understanding of seismic processes and the correct estimation of the risk of earthquakes and eruptions in different parts of the region. Her behavior is in many respects different from most tectonic plates — for example, it rotates clockwise about 0.2 degrees per million years, and the center of rotation is the Northern part of Sakhalin.
The main problem is the fact that the Northern and Western boundaries of the plate are still not entirely clear, and exact age and nature of the relationship with the North American continent.
According to one hypothesis, the authors of the work, the Okhotsk plate is a small ancient continent (microcontinent) that existed in the Pacific ocean in the Cretaceous period.
According to another hypothesis — is the last part of North America. To better understand what a hypothesis is correct, it is extremely important to gather as much factual data about the evolution of the Okhotsk plate.
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Researchers using uranium-lead method of Dating zircons from the Eastern part of Sakhalin island. The uranium-lead method is based on the fact that the decay of uranium-238 ends with lead-206, and another isotope of uranium (uranium-235) — lead-207, and the speed of both of these processes are measured quite accurately. Comparing the number of two isotopes of lead and two isotopes of uranium in an isolated crystal you can quite accurately determine its age. In the studied samples it amounted to at least 49 million years. Sakhalin and the age of granite intrusions (“jets” of granite, penetrating the lighter species), according to the same method, amounted to 36-38 million years.
The East Sakhalin accretionary complex began to be formed in a time when the Okhotsk plate collided with the Eurasian plate and one edge ducked beneath the Western border of Eurasia. But granitic intrusion in the Sakhalin area came when the Pacific plate ran into stopped earlier Eurasia plate Okhotsk and started to duck under it. Thus, the collision of the Okhotsk plate and Eurasia occurred about 49-38 million years ago.