image: The distribution of U-Pb zircon ages defines a series of tightly folded Carboniferous-Jurassic magmatic belts around the Mongol-Okhotsk Orocline, providing key evidence for the rollback-scissor closure of the Mongol-Okhotsk Ocean. view more
Credit: ©Science China Press
Plate tectonics is a paradigm based on the generation and extinction of oceans and on the subduction of water, which accordingly is the diagnostic tectonic feature of a hydrous Earth. The traditional subduction model concerns oceanic plates with regular boundaries, but today we know there are many oceans with irregular boundaries. What is the fate of these oceans? How do they subduct before terminal closure? These issues are critical for a holistic understanding of the plate movements of the Earth from past to present, and we can only find the answers by studying the history of fossil oceans. Suture zones contain a record of the history of oceans on the continents, and curved orogenic belts (oroclines) provide clues to resolve the irregular boundary problems.
To provide diagnostic and definitive constraints, the authors studied the unique Mongol-Okhotsk orocline and suture in the east of the Central Asian Orogenic Belt (CAOB), the world's largest accretionary orogen. The study has been published in the National Science Review (NSR).
The Mongol-Okhotsk orogenic belt with its adjacent areas contains one of the largest Phanerozoic felsic magmatic provinces on Earth. Wang's team has established over many years an extensive archive of isotopic age data and geochemical analyses of magmatic rocks from which they have compiled a set of digital maps that provide the definitive constraints to help resolve the critical issues. They used 2660 new and previously published zircon ages to identify several giant Carboniferous to Jurassic magmatic magmatic belts that have been tightly folded around the orocline. These results demonstrate that these magmatic belts underwent both earlier oceanward and later suture-parallel migration, which led to to a rollback scissor/zipper-like closure of the ocean. This plate tectonic model provides a robust explanation of the formation of the Mongolian-Okhotsk Orocline and its irregular, isoclinal magmatic belt and plate boundary. Accordingly, the authors have produced a new model for the irregular closure of an ocean: rollback of subducted plates and rotation of convergent blocks.
This study defines the temporal and spatial extent and scope of the Mongolian-Okhotsk belt, through the spatial extent of the magmatic belts. The spatial extent varied from Carboniferous to Jurassic. This further confirms that the Central Asian orogenic belt was formed in two oceanic plate tectonic systems, one is the Paleo-Asian oceanic system, mainly in the west and south of the Central Asian orogenic belt; and the other is the Mongolia-Okhotsk oceanic system, mainly in the east. The author proposes that this is like the present analog of Southeast Asia, controlled by the Indian Ocean plate on the west side and the Pacific plate on the east side. Therefore, the Central Asian orogenic belt is a typical complex orogenic belt, called the Central Asian orogenic system.
This research's critical discovery and highlight are based on digital mapping by big data. They are marked by identifying a series of giant “tightly folded” magmatic belts and north oceanic-toward and suture-parallel magmatic migrations, and restoration of both retreat and scissor-like subduction-closure of the Mongol-Okhotsk Ocean. This is rarely in previous studies.
This study demonstrates how coeval oceanward and suture-parallel magmatic migrations can reveal a rollback and scissor/zipper-like closure of an ocean, which gives rise so many magmatic rocks. These processes help understand the complex closure of an ocean with curved and irregular margins and the formation of an orocline. Determining the magmatism of an ancient suture zone is a useful approach to unravel the relationships of arc folding, oroclinal development, and ocean closure (suturing).
Peter Cawood, a tectonic geologist of the Australian Academy of Sciences wrote the following comment: "This approach reflects the impressive developments in mass spectrometry and micro-analysis over the last quarter of a century, notably the growth of rapid laser ICP analysis. The generation and assembly of such large data sets have become increasingly important in untangling the history of mountain belts by providing information of high spatial and temporal fidelity that enables the testing of models for the generation and evolution of the Earth's outer rigid layer, the lithosphere". He also opined: "Of course, new data and resultant models such as those presented by Wang et al. for the Mongol-Okhotsk orocline, like all good science, are not the end of the story but rather open up new avenues for future research. In particular, these expanding data sets, whether specific (e.g., geochemistry) or multifaceted (e.g., paleomagnetic, geochemistry, basin analysis, and geophysics) allow us to understand better the 4-dimensional evolution of mountain belts as well as the whole Earth system".
See the article:
Rollback, scissor-like closure of the Mongol-Okhotsk Ocean and formation of an orocline: Magmatic migration based on a large archive of age-data
https://doi.org/10.1093/nsr/nwab210
Commentary by Peter Cawood：
Untangling the history of oroclines and mountain belts
https://doi.org/10.1093/nsr/nwab211
Journal
National Science Review