The Tibetan Plateau advanced in stages

The Tibetan Plateau - the largest and highest plateau, surrounded by the highest mountains in the world - challenged geologists for a long time trying to understand how and when This area can reach such a dizzying height. New evidence from an eight-year study by US and Chinese scientists shows that this plateau is gradually increasing in time and this elevated development occurs first in the central plateau and then that is to the north and the south.

According to Xixi Zhao, a scientist at the University of California, Santa Cruz (UCSC), 'The middle part of the plateau was first raised at least 40 million years ago while the Himalayas in the south and the mountains in the region The north is obviously only taller then. ' The team found that marine fossils show that the current Himalayas is still below sea level at the time when the central plateau has reached or nearly reached its present height. The average height of the plateau today is more than 4,500m.

Their results were published in the Proceedings of the National Academy of Sciences (online edition of the week from March 24 and then printed). Zhao, in partnership with the Institute of Planetary Physics and Geological Physics at UCSC, is the second author of the work. The first author Chengsham Wang of China University of Geosciences, Beijing cooperated with Zhao and other UCSC researchers since 1996.

Under the name of the 'roof of the world' , the Tibetan plateau was formed from the ongoing clashes of the strata when India drifted north towards Asia. Co-author Robert Coe, lecturer in planetary and planetary sciences at UCSC, said the idea of ​​how to lift the plateau has progressed before his first visit to Tibet in 1988.

'People often assume that the whole plateau rises at the same time, but now everything is clear that the individual parts of the plateau are raised at different times . Our work shows that the central part of the plateau is raised first, and this seems appropriate for other buildings. '

The rising process of the Tibetan plateau leads to profound changes in the region and the global climate. For climate researchers trying to understand the main stages of global climate change in the earth's past, the timing of lifting is an important source of information.

According to co-author Peter Lippert, a doctoral student at UCSC, 'one of the traditional notions of when Tibet became a plateau is that this phenomenon has only happened in the last 15 million years. The existence of a plateau at least 40 million years ago may have important climatic meanings. '

The US and Chinese geologists group made their findings based on a wide-ranging field study, mainly in a remote outlying area of ​​the Tibetan plateau. They focused on an area called the Hoh Xil Basin in the north-central part of the plateau. The geological history of this region is recorded in sedimentary rocks up to 5,000m thick. It is now a part of the plateau but it used to be a Basin on the northern edge of the central plateau.

According to Lippert 'The structure of the basin and the way the sediments accumulate, this is the type of basin that created the base of the big mountains. So we proved that at least 40 million years ago, south of the Hoh Xil Basin once had high terrain. '

Tibetan Plateau (Photo: chinatouching)

Other evidence supports the group's conclusions. In addition to field research, scientists use many other methods in the laboratory to determine the age of the stone. The past changes of the earth's magnetic field are recorded in the magnetization of the rock, thereby providing a dating method known as magnetic terrain method. This analysis was performed at Coe's laboratory at UCSC. Another method used in the study is called apatite mineral analysis, based on the traces of apatite crystals due to the decay of radioactive isotopes.

Scientists also discovered volcanic rocks in an area of ​​the central plateau south of the Hoh Xil Basin. This hard, flat lava layer lies on the top of layers of folded and sloping sedimentary rocks. Geologic methods determine its date to about 40 million years old. According to Lippert, ' the presence of flat volcanic rocks tells us that sedimentary rocks were devastated before the volcano, and it extended the life of volcanoes in this Tibetan region by 15 to 40 million. year.'

In the Himalayas, scientists discovered fossils of a kind of radiolarian named plankton that are actually younger than the sea fossils once recorded here 5 million years old. This discovery narrows the time period during which the Himalayas are raised. When the central part of the Tibetan plateau was raised more than 40 million years ago, Mount Everest and the rest of the Himalayas are still part of a pan deep in the ocean floor.

However, the Himalayas are complex and other groups are working to determine the timing of the lifting process more accurately.'Our main contribution is the data that we obtained from the north-central region of the plateau, which has not been thoroughly surveyed.'

Zhao noted that American researchers may not have been allowed to enter the area without the support of their Chinese colleagues. This long-term collaboration involves exchanging graduate students between UCSC and Chinese universities, as well as an opportunity for undergraduates to conduct field studies in Tibet.'This research collaboration is very good and has strong educational significance.'