大陸內部的構造活動或許不像地質學家認為的如此穩定
Lois Yoksoulian
南美和非洲內陸的構造活動相當穩定，但由伊利諾大學領導的團隊卻在此處下方辨識出意料之外的地球物理訊號。數據顯示在地球最淺層的穩定區塊發生的地質活動，也許比過往認為得還要接近現代。這項刊登在《自然―地質科學》(Nature
Geoscience)的發現挑戰了當今某些關於板塊構造的主流理論。
地球最古老的岩石位處大陸內部，距活動頻繁、岩石重新循環回地球內部之處―――板塊邊界十分遙遠。這些堅硬、上浮且根基深入地球內部的地體稱作穩定地塊，它們漂在地球表面已有數十億年之久，看似完全不受打擾。偶爾，它們彼此之間會在稱為超大陸循環的舞蹈中順著自身邊界相聚然後分開。
「通常我們認為穩定陸塊為低溫、穩定且地勢低平的地區。」研究共同作者，伊利諾大學的地質學教授Lijun Liu表示。「低溫是因為此處岩石距下方高溫的地函相當遙遠；穩定則是此處地殼長久以來都未受到斷層或者變形作用的影響；地勢低平是因為它們一直待在地表，歷經數十億年的侵蝕而導致。」
然而，穩定陸塊中卻有某些地方不符合這些規則。
研究主要作者，伊利諾大學的研究生Jiashun Hu表示：「舉例來說，在南美和非洲穩定陸塊的內部就有一些地勢高起的地區。」
研究人員利用伊利諾大學國家超級電腦應用中心的超級電腦「藍水」來處理地球物理數據，期望能夠更加瞭解這些地勢高起的地區。地質學家一直以來認為穩定陸塊深厚的根基因為組成礦物密度較低的關係，使得它們可以浮在底下的高溫地函之上。但是Liu表示新數據指出南美和非洲穩定陸塊(它們過去曾結合起來成為盤古超大陸的一部份)內部地勢高起的地區其下方的低溫地函呈現出分層構造，且下層的密度在過去比今日還要高。
密度差可能是由於一種稱為「地函脫層作用」(mantle
delamination)而造成的結果。研究人員表示當地函熱柱帶來的高溫岩漿和穩定陸塊的地殼作用之後，地殼下方密度較高的下層地函會從密度較輕的上層地函脫落下來。
Liu表示：「我們運用幾種不同類型的地震成像數據，可以看到我們認為是脫落下來的地函斷塊沉到高溫、黏稠的深部地函之中。」
「脫層作用發生之後，由於來自上方物質的冷卻作用，使得穩定陸塊根基附近的物質重新長回去。而這些新生物質的組成可能比之前在此處的物質密度還要低。」 伊利諾大學的地質學教授Craig
Lundstrom表示，「這些物質會帶來額外的浮力。由上浮所產生的推力可能形成了這些地勢特別高凸的區域。」
研究人員表示，團隊經由跨領域研究開始對地球構造運動的歷史得到相當合理――儘管也十分複雜的最新版本故事。
「非洲和南美的高凸地形不過是整個故事的一部份。」Hu表示，「有許多地質現象的所在位置，像是熱點的軌跡、大陸內部的火山活動、地表隆起與侵蝕，還有地震成像在穩地陸塊根基中呈現出的變形，似乎全都跟我們提出的脫層事件有深切的關聯，意味著它們之間可能存在著因果關係。」
也有證據支持在地球歷史上的其他時間，穩定地塊和地函柱之間曾在別處發生交互作用。
「岩石證據顯示在之前的超大陸循環中曾經發生抬升和侵蝕事件。」伊利諾大學地球、社會與環境學院的主任暨地質學教授Stephen
Marshak表示，「有個相關研究討論到類似事件，也就是陸地的抬升可能和穩定陸塊岩石圈的脫層有關。陸地抬升造成大規模侵蝕作用發生，使得前寒武紀基岩跟古生代的沉積岩地層的交界面成為『大不整合面』(Great Unconformity)。」
至今仍不清楚穩定地塊和地函柱之間的作用，可能會以何種方式影響被認為是地質活動不活躍的地區今日所發生的地震和火山活動。不過這項研究清楚表明地質學家對於所謂的穩定陸塊或許有了新的一層看法。
Continental interiors may not be as tectonically
stable as geologists think
A
University of Illinois-led team has identified unexpected geophysical signals
underneath tectonically stable interiors of South America and Africa. The data
suggest that geologic activity within stable portions of Earth’s uppermost
layer may have occurred more recently than previously believed. The findings,
published in Nature Geoscience,
challenge some of today’s leading theories regarding plate tectonics.
The most ancient rocks on Earth are located within continental
interiors, far from active tectonic boundaries where rocks recycle back into
the planet’s interior. These strong, buoyant and deeply rooted blocks of Earth,
called cratons, have been drifting on the surface for billions of years,
seemingly undisturbed. They occasionally join and break apart along their edges
in a dance called the supercontinent cycle.
“We usually think of cratons as being cold, stable and
low-elevation,” said professor of geology and study co-author Lijun Liu. “Cold because the rocks are far above the hot
mantle layers, stable because their crusts have not been disturbed
significantly by faulting or deformation, and their low elevation is because
they have been sitting there, eroding down for billions of years.”
However, there are places where cratons don’t follow these
rules.
“For example, there are regions of high topography within the
cratons of South America and Africa,” said graduate student and lead author
Jiashun Hu.
The researchers processed geophysical
data with the Blue Waters supercomputer at the National Center for
Supercomputing Applications at Illinois hoping to better understand these
high-elevation regions. The thick roots of cratons have been thought to be
buoyant due to their low-density mineral content, allowing them to float on top
of the hot underlying mantle. However, the new data indicate that the cold
mantle that lies below these regions in South America and Africa – once joined
as part of the supercontinent Pangea – has a layered structure and that the
lower layer was more dense in the past than it is today, Liu said.
This density difference could be the
result of a process called mantle delamination. During delamination, the denser
lower mantle layer peels away from the buoyant upper layer under the crust of
the craton after interacting with hot magma from mantle plumes, the researchers
said.
“From several types of seismic imaging
data, we can see what we think are delaminated mantle slabs sinking into the
hot, viscous deep mantle,” Liu said.
“The material that subsequently grows
back at the roots of the cratons after delamination, due to cooling from above,
is probably compositionally much less dense than what was there before,” said
geology professor Craig Lundstrom. “That adds buoyancy, and that force
from buoyancy could be what forms the anomalously high topography.”
This multidisciplinary study is
beginning to give the team a very logical – albeit complicated – update on the
story of Earth’s tectonic history, the researchers said.
“The high topography of Africa and
South America is only part of the story,” Hu said. “There are many geologic
phenomena such as the location of hotspot trajectories, continental volcanism,
surface uplift and erosion, as well as seismically imaged deformation within
the craton roots that all seem to correlate well with the proposed delamination
event, implying a potential causal relationship.”
There is also evidence to support
other locations of craton-plume interaction during other times in Earth’s
history.
“The rock record shows that uplift and
erosion events have taken place during previous supercontinent cycles,” said
geology professor and School of Earth, Society and Environment director
Stephen Marshak. “A related study discusses what might be a similar
event, namely continental uplift possibly related to delamination of cratonic
lithosphere that caused the period of global erosion resulting in the Great
Unconformity, which is the contact between Precambrian basement rock and
Paleozoic sedimentary strata.”
For now, it is not clear if and how
craton-plume interaction may affect modern-day earthquake activity and
volcanism in areas thought of as geologically inactive. However, the study
marks new thinking in how geologists may understand the so-called stable
cratons.
原始論文：Jiashun Hu, Lijun
Liu, Manuele Faccenda, Quan Zhou, Karen M. Fischer, Stephen Marshak & Craig
Lundstrom. Modification of the Western
Gondwana craton by plume–lithosphere interaction. Nature Geoscience, 2018; doi: 10.1038/s41561-018-0064-1
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