發現出露於地表的斷層中最大者

原文網址：www.sciencedaily.com/releases/2016/11/161128132928.htm

發現出露於地表的斷層中最大者

地質學家首度能觀察並描述印尼東部的班達滑脫斷層(Banda Detachment fault)，並研究它的形成過程。

來自澳洲國立大學的研究第一作者Jonathan Pownall博士表示，這項發現有助於研究人員預測此區將來受到海嘯侵襲的風險高低。此區為火環(Ring of Fire)的一部分，也就是太平洋海盆周遭為人所知地震與火山爆發頻繁發生的地區。

「人們知道這處深淵的存在已有90年之久，但直到現在才有人可以解釋它的深度為何如此驚人。」Pownall博士說。

「我們的研究發現印尼東岸班達海下方7公里深的深淵形成原因，是海底沿著可能為地球上出露於地表的最大已知斷層面伸張而造成。」

國立澳洲大學和倫敦大學哈洛威學院的地質學家分析了班達海床的高解析度海底地形圖，發現位於海床的岩石被數百條互相平行的切痕劃開。

這些傷口顯示必定有塊比塔斯馬尼亞或比利時還大的地殼片段，沿著一道低角度的破裂面，也就是滑脫斷層(detachment fault)伸張了120公里而撕裂成現今所見的海底低地。

Pownall博士表示這條班達滑脫斷層出露在海床上的裂痕超過了60,000平方公里。

「這項發現有助於解釋地球海洋最深的區域之一何以達到如此驚人的深度。」他說。

同樣來自國立澳洲大學地球科學研究院的Gordon Lister教授表示，這是班達斷層首度能被研究人員觀察並且做出詳細紀錄。

「我們根據海底地形資料以及對此區域地質的相關知識，為這條斷層的存在做出詳細論述並命名為班達滑脫斷層。」Lister教授說。

Pownall博士表示在七月於印尼東部的航行中，他注意到此處特別顯著的地形跟地殼沿著斷層線伸張時會出現的地形之間彼此相符。

「我很震驚可以親眼見到我們假說中的斷層面，這次不是在電腦螢幕上，而是直接從海上探測出來。」Pownall博士說。

他說緊鄰斷層底部的岩石包含了源自於下方地函的岩石。

「這意味著必然發生了程度極大的拉張作用，使得海洋地殼變薄，在某些地方甚至被完全拉斷。」他說。

Pownall博士表示班達滑脫斷層的發現有助於科學家評估未來發生海嘯和地震的風險。

「班達滑脫斷層這類大型斷層滑移時會造成大地震。在海嘯發生風險極高的區域，對這些大型斷層有更深的認識是評估區域構造災害時的重要基礎。」他說。

Biggest exposed fault on Earth discovered

Geologists have for the first time seen and documented the Banda Detachment fault in eastern Indonesia and worked out how it formed.

Lead researcher Dr Jonathan Pownall from The Australian National University (ANU) said the find will help researchers assess dangers of future tsunamis in the area, which is part of the Ring of Fire -- an area around the Pacific Ocean basin known for earthquakes and volcanic eruptions.

"The abyss has been known for 90 years but until now no one has been able to explain how it got so deep," Dr Pownall said.

"Our research found that a 7 km-deep abyss beneath the Banda Sea off eastern Indonesia was formed by extension along what might be Earth's largest-identified exposed fault plane."

By analysing high-resolution maps of the Banda Sea floor, geologists from ANU and Royal Holloway University of London found the rocks flooring the seas are cut by hundreds of straight parallel scars.

These wounds show that a piece of crust bigger than Belgium or Tasmania must have been ripped apart by 120 km of extension along a low-angle crack, or detachment fault, to form the present-day ocean-floor depression.

Dr Pownall said this fault, the Banda Detachment, represents a rip in the ocean floor exposed over 60,000 square kilometres.

The discovery will help explain how one of Earth's deepest sea areas became so deep," he said.

Professor Gordon Lister also from the ANU Research School of Earth Sciences said this was the first time the fault has been seen and documented by researchers.

"We had made a good argument for the existence of this fault we named the Banda Detachment based on the bathymetry data and on knowledge of the regional geology," said Professor Lister.

Dr Pownall said he was on a boat journey in eastern Indonesia in July when he noticed the prominent landforms consistent with surface extensions of the fault line.

"I was stunned to see the hypothesised fault plane, this time not on a computer screen, but poking above the waves," said Dr Pownall.

He said rocks immediately below the fault include those brought up from the mantle.

"This demonstrates the extreme amount of extension that must have taken place as the oceanic crust was thinned, in some places to zero," he said.

Dr Pownall also said the discovery of the Banda Detachment fault would help assesses dangers of future tsunamis and earthquakes.

"In a region of extreme tsunami risk, knowledge of major faults such as the Banda Detachment, which could make big earthquakes when they slip, is fundamental to being able to properly assess tectonic hazards," he said.

原始論文：Jonathan M. Pownall, Robert Hall, Gordon S. Lister. Rolling open Earth’s deepest forearc basin. Geology, 2016; 44 (11): 947 DOI: 10.1130/G38051.1

引用自：Australian National University. "Biggest exposed fault on Earth discovered." ScienceDaily. ScienceDaily, 28 November 2016.