壓碎障礙：板塊沉入隱沒帶的過程

原文網址：https://ethz.ch/en/news-and-events/eth-news/news/2021/11/crushed-resistance.html

By Peter Rüegg

地球物理學家利用新的模型來解釋隱沒帶的板塊沉入地函過程中的行為：板塊底部的礦物顆粒體積變小，使得板塊強度變弱，因此更容易變形。

模型對於日本下方的隱沒板塊為何會分段提出了新觀點：板塊底部彎曲造成橄欖石晶體被壓碎。(圖片來源：Taras Gerya / ETH Zurich)

地球表面是由幾個大型板塊和無數的小型板塊組合而成，這些板塊彼此之間以極為緩滿的步調遠離或接近對方。在兩個板塊的交界，較重的海洋板塊會沉到較輕的大陸板塊之下，專家稱此過程為「隱沒」。然而，這些專家許久以來對潛入地函當中的板塊前緣，也就是隱沒板塊的遭遇感到十分疑惑。有些科學家推測隱沒板塊還是跟上方板塊一樣非常的堅硬，會向下彎折只是因為重力以及跟地函之間的力學作用。

嚴重變形的板塊前緣

然而，科學家透過地震層析成像建立出來的地球內部模型卻顯示了相反的結果：以美國西部為例，研究人員觀察到層析影像在不同深度有異常帶出沒，這暗示了沉到美洲下方的隱沒板塊可能分成了好幾段。因此科學家推斷地函裡的隱沒板塊勢必遭到了強烈變形，絕對不是堅硬且固定不變的。

其他研究人員，像是蘇黎世聯邦理工學院的教授Paul Tackley在電腦模型的幫助之下也確認了隱沒板塊真的會強度下降而容易變形。他們因此創建了「隱沒二元假說」(subduction dichotomy hypothesis)，簡單來說便是地表的板塊為堅硬且高強度的(也就是無法變形)，反之地函裡的板塊則是軟弱且低強度的。

尋找合理的機制

「但是目前為止，研究人員並沒有合理的機制來解釋下潛的隱沒板塊為什麼會彎曲，且變得較為軟弱，」蘇黎世聯邦理工學院的地球物理教授Taras Gerya表示。

觀測顯示隱沒板塊與另一個板塊接觸的頂面部分有無數個斷層。海水會經由這些斷層滲入板塊當中，事實上它們幾乎是被一股吸力給「吸」進去的。這會造成板塊頂部的強度降低，但是光憑此還不足以解釋隱沒板塊的分段現象――也就是層析影像中觀察到的異常帶。勢必還要有其他機制來讓隱沒板塊的底部弱化到一定程度，才能讓分段現象發生。

因此，Gerya和他美國的同僚David Bercovici與Thorsten Becker猜測在板塊向下彎折的地方，板塊底部會受到擠壓，使得其中堅硬、毫米大小的橄欖石晶體重新結晶成脆弱、微米大小的粒狀集合體(granular aggregate)。橄欖石晶體因而碎裂，造成板塊的強度變弱而能夠被彎折。

隱沒板塊前緣的分段

利用新的二維電腦模型並且引入上述的顆粒尺寸減小為核心機制，這三位研究人員可以在電腦上研究此過程。他們的成果最近發表在期刊《自然》(Nature)。

模擬確實顯示出隱沒板塊底部的橄欖石晶體遭到碾碎之後會造成板塊變形，並且漸漸分裂成好幾個斷塊。這些塊體本身還是堅硬的剛體，不過破碎的晶體就像柔軟的絞鍊一樣讓它們仍然連在一起。

在模擬當中，這些斷塊在板塊頂面的邊界會出現平行的裂縫，而在這些裂縫之下的區塊則是被壓碎的礦物晶體。

「想像你正在扳斷一條巧克力，」Gerya笑著說。巧克力棒也是只能沿著特定的脆弱之處來分成好幾塊。雖然巧克力塊本身非常硬，但是連接它們薄薄的地方卻很脆弱。「這就是隱沒板塊為什麼不會全體一致地彎曲或變形，而是分成不同段。」

以下可能是實際經過：當較重的板塊沉到較輕的板塊下方，隱沒板塊當中礦物顆粒較小的地方會成為弱點，使得板塊得以彎曲。彎曲造成的應力讓板塊底部更多地方的礦物碎裂，這些脆弱之處導致裂縫產生，使得板塊開始分成不同斷塊。隨著板塊前緣沉到地函越來越深的地方，彎曲之處就會形成更多的斷塊。結果便是隱沒板塊最終變成像是一塊塊堅硬的石板，彼此之間由可以彎折的鏈子串聯起來。到了地下大約600公里深的地方，分段的板塊前緣開始進入地函當中的「670公里不連續面」，從這裡開始它們的移動方向會轉為水平。

自然界的線索支持模擬結果

「我們模擬的結果也符合觀察到的自然現象，」Gerya解釋。許多研究在日本海溝探討自然界中的情況，此處是太平洋板塊沉到鄂霍次克板塊下方的地點。在這裡看到的斷層模式跟模型產生的十分符合。

研究人員利用最近對於隱沒的日本板塊所製作的高解析度地震層析模型，詳細探討其震波速度構造。他們發現地震發出的震波經過板塊中的某些點位會慢下來。這些現實當中出現的點位，分布模式和模擬中斷塊的邊界正好一樣。在自然情況以及電腦模型裡，這些區塊都含有非常小、寬度只有幾微米的晶體，它們的出現造成地震波的速度下降。

這些微小的晶體也造成板塊底部材料的黏滯性下降，換句話說，就是變得更容易流動。研究日本海溝的科學家也可以證明此點。「這代表我們的模型相當合理，而且可以為堅硬的板塊隱沒之後會變軟的假說提供紮實的物理背景，」Gerya表示。但是這項研究離結束還很遠：Bachelor的學生之一Simon Niggli首次利用三維模型模擬並描述板塊的特徵。研究人員接著想要探討板塊前緣的分段現象是否也可以解釋強烈地震為何發生。

 

Crushed resistance: Tectonic plate sinking into a subduction zone

Geophysicists can use a new model to explain the behaviour of a tectonic plate sinking into a subduction zone in the Earth’s mantle: the plate becomes weak and thus more deformable when mineral grains on its underside are shrunk in size.

The Earth’s surface consists of a few large plates and numerous smaller ones that are continuously moving either away from or towards each other at an extremely slow pace. At the boundaries of two plates, the heavier oceanic plate sinks below the lighter continental plate in a process that experts call subduction. For a long time, though, those experts have been puzzling over what happens to the plate margin that dives into the Earth’s mantle, known as the subducting slab. Some scientists assumed that the slab remains as rigid and strong as the plate itself and simply bends due to the gravity force and mechanical interaction with the Earth’s mantle.

Heavily deformed plate margin

However, models of the Earth’s interior constructed by scientists using seismic tomography revealed contradictory results: in the western United States, for example, the researchers observed anomalies at different depths on their tomographic images. These indicated that the slabs submerged beneath the Americas may be segmented. The scientists therefore concluded that the slabs in the mantle must be strongly deformed and are by no means rigid and immobile.

With the aid of computer models, other researchers, including ETH Professor Paul Tackley, confirmed that subducted slabs are indeed weak and deformable. And they formulated the subduction dichotomy hypothesis that can be expressed in simple terms: plates on the surface are rigid and strong (read: non-​deformable), while the slabs in the mantle are soft and weak.

Seeking a plausible mechanism

“Until now, however, research has lacked a plausible mechanism to explain how this bending occurs and why sinking plate margins (slabs) become soft and weak,” says Taras Gerya, Professor of Geophysics at ETH Zurich.

Observations revealed that numerous faults are found on the upper surface of a sinking plate where it meets the other plate. Seawater penetrates the plate through these faults and is in fact literally sucked in by suction forces. This weakens the plate on its upper side. Yet this alone is not sufficient to explain the segmentation of the slab – the anomalies observed on tomographic images. Another mechanism must also be at work to weaken the underside of the margin enough for segmentation to occur.

Gerya and his American colleagues David Bercovici and Thorsten Becker therefore suspected that compression of the underside of the plate at the point where it bends downward was “crushing” large and strong, millimetres size olivine crystals in the plate by forcing them to recrystallise into much weaker, micrometres size granular aggregate – thereby reducing the plate’s resistance and allowing it to bend.

Sinking plate margin divided into segments

Using a new two-​dimensional computer model that integrated this grain reduction as a central mechanism, the three researchers then studied the process in silico. Their study was recently published in the journal Nature.

And indeed, the simulations revealed that sinking plates deform due to the massive reduction of olivine grains on their undersides, splitting into individual segments over time. These segments are rigid and stiff, but remain connected to each other by weak hinges made of ground grains.

In the simulations, parallel cracks appear at the segment boundaries on the plate’s upper surface. Below these cracks are the zones with “crushed” mineral grains.

“Just imagine you’re breaking a bar of chocolate,” Gerya says with a grin. A bar of chocolate, too, can be divided into segments only along the specified weak points. The squares of chocolate are rigid, but the connecting pieces between them are weak. “That’s why a sinking plate isn’t uniformly bent or deformed, but segmented.”

And here’s how it might play out in reality: The heavier plate sinks under the lighter one. A weak spot with smaller mineral grains within the sinking plate allows it to bend. The bending stress causes the minerals to crumble in more places on the underside. The resulting weakness leads to a fracture, and a segment forms. As the plate margin sinks deeper and deeper into the mantle, it causes further segments to form at the bend. As a result, the slab eventually resembles a chain with rigid links and bendable connectors. At a depth of about 600 kilometres, the segmented plate margin slides onto what is known as the 670 km discontinuity in the Earth’s mantle, from which point it moves horizontally.

Clues from nature support simulation

“The results of our simulations are consistent with observations in nature,” Gerya explains. A great deal of research has been done on the natural situation along the Japan Trench, where the Pacific plate sinks below the Okhotsk plate. The pattern of faults found here is an exact match for the pattern produced in the simulations.

Researchers have also studied the seismic velocity structure of subducting Japan slab thoroughly using its recently produced high-​resolution seismic tomography model. They found that the velocity of the seismic waves sent out by earthquakes was reduced at some nodes inside the slab. The pattern with which these nodes occur in reality coincides with that of the segment boundaries from the simulations. And both in nature and in the computer model, it is zones with very small crystals only micrometres across that are responsible for reducing the velocity of the seismic waves.

These tiny crystal grains also make the underside plate material less viscous; in other words, it becomes runnier. Researchers at the Japan Trench were able to demonstrate this, too. “That means our model is very plausible and provides solid physical background for the hypothesis of rigid plates with weak slabs,” Gerya says. But the research is far from over: one of his Bachelor’s students, Simon Niggli, has modelled and described plate fractures in three dimensions for the first time. Next the researchers want to investigate whether the segmentation of plate margins can also be responsible for strong earthquakes.

原始論文：T. V. Gerya, D. Bercovici, T. W. Becker. Dynamic slab segmentation due to brittle–ductile damage in the outer rise. Nature, 2021; 599 (7884): 245 DOI: 10.1038/s41586-021-03937-x

引用自：ETH Zurich. "Crushed resistance."