An international team of scientists headed by a geologist from Novosibirsk Inna Safonova reports to have found ancient oceanic sediments in the South Tien Shan, Kyrgyzstan. They have also managed to evaluate the size of the ancient ocean, which gave birth to the islands. The researchers consider that the paleo-islands discovered were similar to the modern Hawaiian Islands.
The results have been published in an high-rank authoritative journal Gondwana Research, in the top three of world best journals in Earth’s Sciences (Impact Factor 8.24). The team included researchers from Russia, Germany, the UK and Japan.
Inna Safonova works as a senior researcher at the Laboratory of experimental geochemistry and petrology of the Earth's Mantle of the Geology and Geophysics Department of Novosibirsk State University and the Laboratory of Magmatic Petrology and Ore Formation at the Institute of Geology and Mineralogy, SB RAS. She also leads a multi-disciplinary UNESCO-IUGS IGCP Project #592 "Continental construction of the Altaids (Central Asian Orogenic Belt) compared to actualistic examples from the Western Pacific". The participants and experts of the project come from more than 50 countries.
There has been much controversy about the tectonic history of the Tien Shan, especially about the lifetime of ancient oceans, which closures formed mountain ranges, and their sizes. There has been obtained a lot of evidence, such as certain types of sediments and other rock formations, which suggest that there was either a big marginal sea or an ocean in the study area. Now many researchers believe that that ancient Turkestan Ocean (also referred to as South Tianshan paleo-ocean or paleo-sea) was located between the Kazakhstan and Tarim paleo-continents, which are parts of present-day Eurasia.
Some geologists think that the Turkestan Ocean was an isolated oceanic realm and had no connections to other oceans of similar ages. The others suggest that it was a big marginal sea rather than an ocean, an example of which is the present-day Sea of Japan located between the Eurasian continent and the Japanese archipelago. “Different hypothesis cause heated debates about the size, the age and the paleogeographic location of this object,” says Inna Safonova.
How can we answer these questions without witnessing these ancient seas and oceans, which are a thing of a distant past? Scientists study the history of paleo-oceans by analyzing their “remains”, fragments of the deep-sea floor and oceanic rises, such as islands. Ancient oceanic plates typically subduct in the mantle while topographically high oceanic islands are more likely to remain on the surface, and thus can reveal a lot of details about their geological past.
“We were lucky to find some fragments of oceanic islands in the South Tien Shan, Kyrgyzstan. It is not just oceanic sediments that could cause confusion belonging either to a very deep marginal sea or an ocean. They are fragments of islands, and paleontological data analysis explains their age. The fragments used to be a chain of volcanic islands which existed during the whole the Devonian period, i.e. from 415 to 360 mln years ago, that is about 55 mln years. It is a really long term!” says Inna Safonova. She emphasizes that this estimation refers only to the islands, whose structure, composition and age closely resemble those of the biggest known objective of intraplate volcanism, the Hawaiian Islands. The ocean was presumably old… and big.
“The size and the age of the ocean are directly related. The bigger the ocean, the thicker its crust and the older its age. Given an approximate velocity of the subduction of an oceanic plate into the mantle, we can estimate the minimal size of the ocean. We assume that the Turkestan Ocean can be compared to the Indian Ocean. It used to be quite a big ocean surrounded by two or more continents,” says the researcher.
Where would fragments of oceanic islands come from to appear in the mountains? The Paleo-Asian or Turkestan ocean was, as geologists say, reducing and closing as the continents surrounding were approaching each other. As a result, most of the oceanic plate subducted to the deep mantle, but some parts, in particular, the oceanic islands, were “cut off” the plate by the margins of continental plates and appeared on the surface. The continents went on moving and thrusting, with the mountains rising and pushing the fragments island upwards.
In order to discover, identify and study the conditions under which the fragments of certain ocean segments formed, geologists used a new approach, the model of Oceanic Plate Stratigraphy, OPS, suggested by the Japanese colleagues. Inna Safonova together with Prof. Maruyama Shigenori (Tokyo Institute of Technology) and Prof. Kojima Satoru (University of Gifu, Japan) proved the model to be universal and working in different Pacific-type mountain belts , e.g. in Central Asia. The OPS model allows scientists to find fragments of paleo-oceans on modern continents, as well as to estimate the age, size and the direction of the movement and subduction of oceanic plates. The model is based on investigation of certain formations or associations of various sedimentary and volcanic rocks taking into account their major geological, petrologic and geochemical characteristics.
“Each segment of an oceanic island,” says Inna Safonova, “such as the foot, shelf, carbonate «cap» reef limestones”, is characterized by a certain association of sedimentary rocks. This is the first step of diagnostics, which is followed by analysis of volcanic rocks, usually basalts. Most samples were taken at the Atbashi–Kokshaal accretionary belt of the South Tianshan orogen in Kyrgyzstan. We described and carefully studied the basalts while our colleagues dated them according to the microfossils, mainly radiolarians, brachiopods, algae, corals, etc.. Almost all field in Earth Sciences, including geology, paleontology, geochemistry, geophysics, petrogenesis, tectonics and so on, contributed to our research.”
The origin of those volcanic oceans in the middle of an ocean is often attributed to “hot spots” such as the Hawaiian hotspot, which is related to mantle plumes.
“Mantle plumes are columns of hot material ascending from Earth’s deep,” explains Inna. “they can melt the Earth and induce volcanic activity, which results in the appearance of new volcanoes and eruption of lava of specific composition. As continental plate moves over the plume, new volcanos, like those of the Hawaiian Chain, appear.”
The researchers used the mantle plume model while studying the South Tien Shan and concluded that a similar chain of paleo-islands formed under the influence of manlte plume within the ancient Turkestan Ocean plate.