原文網址：https://news.illinois.edu/blog/view/6367/534594
板塊強度也許可以解釋西藏高原的外型
地質學家長久以來對形成青藏高原的機制感到不解。青藏高原的抬升是由板塊碰撞促成，一項新研究發現此地體的歷史或許主要受控於板塊的強度。由於此區是地球上地震活動最活躍的地區之一，瞭解青藏高原的地質史也能讓科學家更加認識現今發生的地震活動。
這項新發現刊登於期刊《自然通訊》(Nature Communications)。
即使是從太空看下來，青藏高原仍顯得相當龐大。這座巨大無比的高地由兩座大陸板塊――印度和亞州聚合而成，它讓世上其他山脈不論是高度或寬度都相形見絀。其他山脈大多數看起來不過是微微隆起的狹長肉疤，而喜馬拉雅高原則像是一大片不對稱的瘡痂，被崎嶇的崚峰包圍。
青藏高原形成過程的古典模型顯示大約5000萬年前，快速移動的印度大陸板塊迎頭撞上相對固定的亞洲板塊。Liu表示此聚合過程可能把地殼擠成一大團，成為今日我們所見的喜馬拉雅山脈和青藏高原，但此模型卻無法解釋為什麼青藏高原的外觀並不對稱。
主要作者，中國科學院的Lin Chen表示：「青藏高原的寬度並非一致。它西側的寬度非常狹窄而東側則非常寬廣――過去許多的模型都無法解釋這點。」
Liu說過去那些模型許多都把重點放在高原區表層現今的地質特性，但是故事的真實情節也許要在地下深處亞州和印度板塊的交界處才能發現。
Liu表示：「高原地區所處的亞洲板塊地形有很大的變化；但碰撞帶周圍印度板塊的地形和移動速度從西到東幾乎都一模一樣。所以為什麼亞洲板塊有如此大的變化？」
為了解開這項謎題，Liu和他的共同作者探討當岩石組成強度不同的板塊碰撞時會發生什麼事情。他們利用一套三維電腦模型模擬大陸碰撞以測試這項想法。
Liu表示：「我們觀察兩種情況――分別是脆弱的亞州板塊和堅硬的亞洲板塊。在兩種模型當中我們都設定印度板塊由堅硬的岩石組成。」
當研究人員讓模型開始運作，他們發現在亞洲板塊為堅硬的情況下，結果會形成狹窄的高原。脆弱的亞洲板塊模型則會產生寬闊的高原，就像我們今日所看到的。
Liu表示：「接著我們執行了第三種情況，此模型中的亞洲板塊同時有堅硬和脆弱的部份。一座西邊堅硬而東邊脆弱的亞洲板塊形成的高原方位跟今日我們看到的情況非常相似。」
此模型除了可以預測表面地形之外，也有助於解釋利用地震波觀測技術看到的某些複雜地下構造。
「令人興奮的是利用如此簡單的模型就能導出跟我們今日觀察到的現象相當接近的結果。」Liu表示，「而且現今發生的地震活動位置和陸地移動方向也跟我們用模型預測出來的相當一致。」
Strength of tectonic plates may explain shape of
Tibetan Plateau
Geoscientists have long puzzled over
the mechanism that created the Tibetan Plateau, but a new study finds that the
landform’s history may be controlled primarily by the strength of the tectonic
plates whose collision prompted its uplift. Given that the region is one of the
most seismically active areas in the world, understanding the plateau’s
geologic history could give scientists insight to modern day earthquake
activity.
The new findings are published in the journal Nature Communications.
Even from space, the Tibetan Plateau appears huge. The massive highland,
formed by the convergence of two continental plates, India and Asia, dwarfs
other mountain ranges in height and breadth. Most other mountain ranges appear
like narrow scars of raised flesh, while the Himalaya Plateau looks like a
broad, asymmetrical scab surrounded by craggy peaks.
“The asymmetric shape and complex subsurface structure of the
Tibetan Plateau make its formation one of the most significant outstanding
questions in the study of plate tectonics today,” said University of Illinois geology professor and study co-author Lijun Liu.
In the classic model of Tibetan Plateau formation, a fast-moving Indian
continental plate collides head-on with the relatively stationary Asian plate
about 50 million years ago. The convergence is likely to have caused the
Earth’s crust to bunch up into the massive pile known as the Himalaya Mountains
and Tibetan Plateau seen today, but this does not explain why the plateau is
asymmetrical, Liu Said.
“The Tibetan Plateau is not uniformly wide,” said Lin Chen, the lead
author from the Chinese Academy of Sciences. “The western side is very narrow
and the eastern side is very broad – something that many past models have
failed to explain.”
Many of those past models have focused on the surface geology of the
actual plateau region, Liu said, but the real story might be found further
down, where the Asian and Indian plates meet.
“There is a huge change in topography on the plateau, or the Asian plate,
while the landform and moving speed of the Indian plate along the collision
zone are essentially the same from west to east,” Liu said. “Why does the Asian
plate vary so much?”
To address this question, Liu and his co-authors looked at what happens
when tectonic plates made from rocks of different strengths collide. A series
of 3-D computational continental collision models were used to test this idea.
“We looked at two scenarios – a weak Asian plate and a strong Asian
plate,” said Liu. “We kept the incoming Indian plate strong in both models.”
When the researchers let the models run, they found that a strong Asian
plate scenario resulted in a narrow plateau. The weak Asian plate model
produced a broad plateau, like what is seen today.
“We then ran a third scenario which is a composite of the strong and weak
Asian plate models,” said Liu. “An Asian plate with a strong western side and
weak eastern side results in an orientation very similar to what we see today.”
This model, besides predicting the surface topography, also helps explain
some of the complex subsurface structure seen using seismic observation
techniques.
“It is exciting to see that such a simple model leads to something close
to what we observe today,” Liu said. “The location of modern earthquake
activity and land movement corresponds to what we predict with the model, as
well.”
原始論文：Lin Chen, Fabio A. Capitanio, Lijun Liu, Taras V.
Gerya. Crustal rheology controls on the Tibetan plateau formation
during India-Asia convergence. Nature Communications, 2017; 8:
15992 DOI: 10.1038/ncomms15992
引用自：University
of Illinois at Urbana-Champaign. "Strength of tectonic plates may explain
shape of the Tibetan Plateau."
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