原文網址：https://news.illinois.edu/view/6367/1692853846
By Lois
Yoksoulian
伊利諾大學厄巴納—香檳分校的新研究指出，地球大陸板塊內部年代久遠、看似穩定的區域(稱為「穩定地塊」)在形成之後，地殼以下的部分曾經反覆受到變形作用。這道假說不只否定了數十年來板塊構造理論的傳統看法，也有望回答為什麼穩定地塊大部分區域在構造上都保持穩定的同時，底部卻經歷過劇烈變化的問題。
這張假想的橫剖面顯示了羅迪尼亞超大陸分裂時的地殼與地函，一道上湧的地函柱引發了下部地函的脫落作用。圖片來源：Lijun Liu
地球最外面由地殼和地函組成的堅硬岩層稱為岩石圈。在伊利諾大學的地質學教授Lijun
Liu主持的研究中，研究人員透過先前取得的岩石圈密度資料，探討穩定陸塊表面的地形跟下方岩石圈的厚度之間有什麼樣的關係。
研究結果發表在期刊《自然—地球科學》(Nature
Geosciences)。
研究寫出穩定陸塊的內部自行成後就幾乎沒有變形的特性，使其成為地球上最長壽的構造單位。它們活過了數次超大陸循環——像是盤古超大陸，以及知名度較低、更加古老的羅迪尼亞超大陸的形成和分裂。
「科學家普遍接受穩定地塊下方位在地函的深厚根部，又稱為『龍骨』，可以保護它們。一般認為龍骨上浮且堅固的特性使其可以維持長時間的穩定狀態，」Lui表示。
最近Liu的研究團隊發表的幾篇論文直接挑戰了這項觀念。他們指出地函龍骨的密度其實非常高。
在2022的一篇研究中，團隊證實穩定地塊的龍骨具有高浮力的傳統觀點，意味著地球大多數的穩定地塊都會比海平面高3公里左右，但實際上它們的海拔高度通常都只有100公尺。Liu表示地殼下方的岩石圈地函要有足夠的密度，才能把露在表面的部分拉低大約2公里。
另一篇研究中，團隊運用重力場的測量結果精準描繪了穩定地塊龍骨的密度構造。他們發現高密度物質最有可能存在於地函龍骨的下部，意味穩定地塊的密度有隨著深度而增加的趨勢。
在這篇最新的論文中，團隊得出超大陸開始分裂時上湧的地函(地函柱)，往往會把地函龍骨下部的高密度部分從上方的岩石圈剝除。這些剝落下來(又稱拆層，delaminated)的龍骨在熾熱的地函受熱之後，又會回到岩石圈的底部。
「這整個過程就像熔岩燈一樣，靠近表面的低溫物質會下沉，而靠近底部的高溫物質則會上升，」Liu表示。
研究寫說在岩石圈觀察到的某些奇特的地球物理性質，正是體現了這種變形歷史。
「比方說，讓岩石垂直震動的地震波在地函龍骨下半部的行進速度比上半部還要快，因為下半部在垂直方向曾屢次受到變形，而上半部受到的變形則較少，」Liu表示。
團隊也確認地函拆層能讓穩定地塊的地表抬升，導致侵蝕作用發生。
「這反映在地殼厚度與岩石圈厚度的強烈關係之上，在此研究之前沒有人做出這項觀察結果，」Liu表示。「在過去兩次大型的抬升與侵蝕事件中特別明顯：分別是羅迪尼亞與盤古超大陸分崩離析的時候。前者造成了知名的大不整合面——這道地球岩石紀錄中的特點顯示出當時沒有新的沉積物堆積的跡象，只有穩定地塊被侵蝕得非常深。這是今天我們看到穩定地塊表面有古老的下部地殼碎片出露的原因。」
團隊借助數值模擬得出穩定地塊龍骨底部間歇性的變形，正是穩定地塊的地殼可以在漫長的地質歷史中存活下來的原因。
「我們對於穩定地塊的生存方式得出了新的假說，相信它將會大幅改變人們對於陸地演化，以及地球的板塊構造運動如何運作的看法，」Liu表示。
參與這項研究的包括伊利諾大學的地質學教授Craig
Lundstrom；伊利諾大學的研究生Yaoyi
Wang、
Zebin Cao、Lihang
Peng、Diandian
Peng；還有中國科學院的教授Ling
Chen。
研究經費來自美國國家科學基金會以及中國國家自然科學基金委員會。
Geologists challenge conventional view
of Earth's continental history, stability with new study
The seemingly stable regions of the Earth’s
continental plates – the so-called stable cratons – have suffered repetitive
deformation below their crust since their formation in the remote past,
according to new research from the University of Illinois Urbana-Champaign.
This hypothesis defies decades of conventional plate tectonics theory and begs
to answer why most cratons have remained structurally stable while their
underbellies have experienced significant change.
In a study led by Illinois geology professor Lijun
Liu, researchers used previously collected density data from the Earth’s
uppermost rigid layers of crust and mantle – known as the lithosphere – to
examine the relationship between craton surface topography and the thickness of
their underlying lithosphere layer.
The results of the study are published in the journal
Nature Geosciences.
The lack of deformation within the cratons since
their formation makes them the longest-lived tectonic units on Earth –
surviving supercontinent cycles like the formation and breakup of the
supercontinent Pangea, as well as the lesser-known and more ancient
supercontinent Rodina, the study reports.
“It is generally accepted that the cratons are
protected by their thick underlying mantle roots, or keels, which are believed
to be buoyant and strong and thus stable over time,” Lui said.
Several recent papers from Liu’s research group
directly challenge this wisdom by showing that these mantle keels are actually
quite dense.
In a 2022 study, the team demonstrated that the
traditional view of buoyant craton keels implies that most of the Earth’s
cratons would be sitting about 3 kilometers above the sea surface, while in
reality, their elevation is only a few 100 meters. This requires the
lithospheric mantle below the crust to be of high enough density to pull the
surface down by about 2 kilometers, Liu said.
In another study, the team used gravity field
measurements to pinpoint the density structure of the craton keels to find that
the lower portion of the mantle keel is most likely where the high-density
material resides, implying a depth-increasing density profile below the
cratons.
In the new paper, the team shows that the lower
portion of the mantle keel that has a high density and tends to repeatedly peel
away from the lithosphere above when mantle upwellings, called plumes, initiate
supercontinent breakup. The peeled-off – or delaminated – keels could return to
the base of the lithosphere after they warm up inside the hot mantle.
“The whole process is like what happens in a lava
lamp, where the cool material near the surface sinks and the warm material near
the bottom rises,” Liu said.
This deformation history is expressed in some of the
more puzzling geophysical properties observed in the lithosphere, the study
reports.
“For example, the repetitive vertical deformation of
the lower half of the mantle keel allows the seismic waves that vibrate the
rock vertically to travel faster, compared to the upper half of the keel, which
experienced less vertical deformation,” Liu said.
The team also determined that mantle delamination
will cause the craton surface to rise, leading to erosion.
“This is reflected in the strong dependence of
crustal thickness on lithospheric thickness, an observation never made before
this study,” Liu said. “In particular, there are two major uplift and erosion
events in the past, when supercontinents Rodinia and Pangea each separated, the
former causing what is known as the Great Unconformity – a feature in the
Earth’s rock record shows no evidence of new deposition, only deep craton
erosion. This is the reason why we see pieces of ancient lower crust exposed at
the craton’s surface today.”
With the help of numerical simulations, the team said
that this episodic deformation style of the lower craton keels is how the
craton crusts survived the long geological history.
“We believe this newly hypothesized lifestyle of
cratons will significantly change people’s view on how continents evolve and
how plate tectonics operate on Earth,” Liu said.
Illinois geology professor Craig Lundstrom and
Illinois graduate students Yaoyi Wang, Zebin Cao, Lihang Peng and Diandian
Peng; and Chinese Academy of Sciences professor Ling Chen contributed to this
study.
The National Science Foundation and the National
Natural Science Foundation of China supported this research.
原始論文：Yaoyi Wang,
Zebin Cao, Lihang Peng, Lijun Liu, Ling Chen, Craig Lundstrom, Diandian Peng,
Xiaotao Yang. Secular craton evolution due to cyclic deformation of underlying
dense mantle lithosphere. Nature Geoscience, 2023; DOI: 10.1038/s41561-023-01203-5
引用自：University of Illinois at Urbana-Champaign,
News Bureau. "Geologists challenge conventional view of Earth's
continental history, stability with new study."
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