The "Roof of the World" was originally a "Valley"! What happened to the Qinghai-Tibet Plateau?

Author: Han Yangmei

The Qinghai-Tibet Plateau, the third pole of the Earth, now has an average altitude of more than 4,000 meters and is known as the "Roof of the World." But before it became the "tall" it is today, the Qinghai-Tibet Plateau actually developed a low-altitude warm "valley."

How did the plateau evolve from a "valley" to a "roof of the world"? And what impact did its uplift process have on today's environment? These are mysteries about the Tibetan Plateau that have yet to be solved.

Over a period of more than 20 years, with the support of the second Qinghai-Tibet scientific expedition, the collision uplift and impact team led by Ding Lin, an academician of the Chinese Academy of Sciences and a researcher at the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences (hereinafter referred to as the Qinghai-Tibet Plateau Institute), has depicted the appearance and historical process of the low-altitude "Central Valley" before the uplift of the Qinghai-Tibet Plateau through comprehensive research in multiple fields and methods, including tectonic geological evolution, deep structure of the lithosphere, paleoaltitude, paleotemperature, paleovegetation analysis and paleoclimate simulation. The relevant research was recently published in Science Advances.

The “low point” before the rise

The uplift of the Qinghai-Tibet Plateau was complex and lasted for a long period of history, and the geodynamic process of its uplift remains controversial.

Some believe that during the Eocene Epoch, 53 to 36 million years ago, the southern part of the plateau first became the highest area, and then rose northward. Another view is that during the Eocene Epoch, the central part of the plateau was the highest area, forming the prototype of the Qinghai-Tibet Plateau, and then from the Miocene it expanded southward into the Himalayas and northward to the Kunlun Mountains and Qilian Mountains in the northern part of the Qinghai-Tibet Plateau.

Xiong Zhongyu, the first author of the paper and a doctor from the Qinghai-Tibet Plateau Institute, said that after the collision between the Indian Plate and the Eurasian Plate, a "low-altitude central valley" completely different from the current landform developed between the tall Gangdise orogenic belt and the Central Divide orogenic belt. It runs from west to east along the current Ritu-Gezhe-Nima-Bange-Nagqu-Dingqing line.

"But we don't know yet when did it uplift to its current height and what was the endogenous driving force that caused the uplift of the Central Valley." Ding Lin, the corresponding author of the paper, said that accurately quantifying the uplift process and characteristics is crucial to assessing its impact on the atmosphere and surface processes.

Since 1997, Ding Lin has led his team to conduct field surveys in the Lunpola Basin in the middle of the Central Valley in an attempt to solve this mystery.

The Lunpola Basin belongs to Bange County, with an area of ​​about 3,600 square kilometers, an altitude of about 4,700 meters, an annual average temperature of about 0°C, and an annual precipitation of 400-500 mm. It has a typical high-cold monsoon climate and is a hot spot for studying the uplift history, mechanism and environmental-biological effects of the Qinghai-Tibet Plateau.

Finding the "code" in volcanic ash

"The volcanic ash layer is volcanic ash from ancient volcanic eruptions deposited in the basin, which can be used to accurately determine the formation era and absolute age of the strata. The absolute age of the strata is like a 'ruler'. Only on the basis of this ruler can we accurately reconstruct the evolution process of the various spheres of the earth." Xiong Zhongyu told China Science Daily.

The research team discovered a total of 9 sets of volcanic ash in the basin. Using the zircon uranium-lead dating method, they determined the absolute age of the volcanic ash and established an absolute dating framework for the sedimentary strata in the Lunpola Basin 50 to 20 million years ago.

Research shows that the lower Niubao Formation was deposited between 50 and 29 million years ago, and the upper Dingqing Formation was deposited between 29 and 20 million years ago.

Based on this chronological framework, the research team collaborated with the paleoclimate simulation team of the University of Bristol in the UK and used the paleoclimate simulation method on the Qinghai-Tibet Plateau for the first time. They determined that the rainfall pattern in the central valley of the Qinghai-Tibet Plateau is a bimodal pattern in winter and summer.

At the same time, combined with rainfall, surface evaporation and soil moisture content, they revealed the formation season of paleosol calcareous nodules: the formation time of paleosol calcareous nodules in the lower Niubao Formation was March to June, while the formation time of paleosol calcareous nodules in the upper Niubao Formation was limited to two stages: May to June and September.

The deep circle is the "endogenous driving force"

Based on the ancient surface temperature determined by isotope data of ancient soil calcareous nodule clusters, the research team also creatively used the surface air wet-bulb temperature and the wet-bulb air temperature lapse rate to quantitatively restore the history of surface height changes in the Lunpola Basin.

The research results show that about 50 million to 38 million years ago, the Qinghai-Tibet Plateau showed the geomorphic features of "two mountains sandwiched by a basin", with the Gangdise Mountains at an altitude of about 4,500 meters and the Central Divide Mountains at an altitude of about 4,000 meters, sandwiched between them with the Central Valley at an altitude of about 1,700 meters. The Central Valley has a warm and humid climate, with precipitation dominated by westerly winds and monsoons, and subtropical flora and fauna flourishing, making it the "Shangri-La" inside the plateau.

About 38 to 29 million years ago, the central valley represented by the Lunpola Basin rapidly uplifted into a plateau with an altitude of more than 4,000 meters, which also marked the formation of the main part of the Qinghai-Tibet Plateau.

With the uplift of the Central Valley and the cooling of the global climate, the temperature in the central part of the plateau has dropped significantly, precipitation has decreased, and the monsoon effect in the south has been relatively strengthened. Climate change has caused the central part of the plateau to change from a warm and humid subtropical ecosystem to a cold and dry alpine ecosystem, with alpine meadows as the main surface vegetation.

Combined with previous research, the team further pointed out that the time when the orogenic belt to the north of the Yarlung Zangbo suture developed into the main body of the plateau was the late Eocene to early Oligocene (38 million to 29 million years ago), while the Himalayas to the south of the Yarlung Zangbo suture only reached its current height in the early Miocene (25 million to 15 million years ago).

Ding Lin introduced that the deep geodynamic mechanism that led to the uplift of the Central Valley was the subducting Lhasa mantle delamination, the upwelling of asthenosphere materials and the shortening of the upper crust. The uplift of the Central Valley marked the beginning of the great impact of the Qinghai-Tibet Plateau on the surface environment. This study has taken a solid step in the study of the spatiotemporal evolution of various spheres on the Qinghai-Tibet Plateau, and has an important demonstration role in the study of the Earth system science of the Qinghai-Tibet Plateau.

Related paper information:

https://science.org/doi/10.1126/sciadv.abj0944

China Science Daily (2022-03-02 Page 4, original title: How the "Valley" Becomes the "Roof of the World")

Editor | Zhao Lu

Typesetting | Zhihai