研究人員利用地磁資料發現地中海底下有年齡為3.4億年的海洋板塊

原文網址：www.sciencedaily.com/releases/2016/08/160815114933.htm

Researcher uncovers 340 million year-old oceanic crust in the Mediterranean Sea using magnetic data

研究人員利用地磁資料發現地中海底下有年齡為3.4億年的海洋板塊

A researcher at Ben-Gurion University of the Negev (BGU) has identified that the eastern Mediterranean Sea contains the world's oldest oceanic crust still in place and could be almost 340 million years-old.

一名內蓋夫本-古里安大學(BGU)的研究人員發現地中海東側保有全世界最古老的海洋板塊，其年代可能有將近3.4億年 。

The study reported in Nature Geoscience was conducted by Dr. Roi Granot, a senior lecturer in the BGU Department of Geology and Environmental Sciences.

這篇發表於《自然—地質科學》(Nature Geoscience)的研究是由BGU地質和環境科學系的資深講師 Roi Granot博士指導。

Some of the fundamental tectonic characteristics of the eastern Mediterranean remain unresolved due to the extremely thick sedimentary cover (10 to 15 km) and the lack of accurate magnetic anomaly data. The researchers towed magnetic sensing equipment to collect 7,000 km (4,300 miles) of marine magnetic profiles across the Herodotus and Levant Basins, eastern Mediterranean, to study the nature and age of the underlying igneous crust.

地中海東部海底由於被一層極端深厚的沉積物覆蓋(10至15公里)，加上缺乏精確的地磁異常(magnetic anomaly)測量數據，使得這個區域構造的基礎性質有些仍然不太清楚。研究人員為了瞭解地中海東部下方火成岩地殼的性質和年代，他們在當地的希羅多德海盆及黎凡特海盆中牽引磁力偵測儀，來繪製出遍及整個海盆，總長度達到7000公里的海洋地磁剖面，。

Dr. Granot used the magnetic data to analyze the nature of the crust in the Herodotus Basin, and found that the rocks are characterized by magnetic stripes -- the hallmark of oceanic crust formed at a mid-ocean ridge. As magma at a mid-ocean ridge axis cools, magnetization of the minerals in the newly forming rocks align with the direction of Earth's magnetic field.

Granot博士利用地磁數據來分析希羅多德海盆地殼組成的特性，結果發現這些岩石地磁方向的分布具有像磁條似的特徵，這是於中洋脊產生的海洋地殼的註冊商標。當中洋脊軸部噴出的岩漿冷卻時，新生岩石中的礦物會磁化而跟當時地球磁場方向排列一致。

"Changes in the magnetic field's orientation over time are recorded in the ocean floors, creating a unique barcode that provides a time stamp for crust formation," Dr. Granot says. "The results shed new light on the tectonic architecture and evolution of this region and have important implications on various geodynamic processes."

「海床岩石會記錄地球磁場方向隨時間的變化，不同年代會形成獨一無二的條碼而成為可讓我們追溯地殼何時形成的時間戳記。」 Granot博士說。「研究結果讓我們更加瞭解這個區域地質構造的形成和演化過程，且對地質動力學各方面的研究也有相當重大的啟發。」

By using this principle and identifying skewed patterns in the magnetic stripes, Dr. Granot showed that the oceanic crust in the Herodotus Basin could be as much as 340 million years old.

利用這項原理來辨認地磁條帶中折線的模式跟哪個年代相符， Granot博士表示希羅多德海盆的海洋地殼年代可遠達3.4億年。

Oceanic crust is typically recycled back into Earth's mantle relatively rapidly at subduction zones due to its high density, thus most oceanic crust is less than 200 million years old.

由於海洋地殼的密度較高，因此在隱沒帶相較於大陸板塊，它們通常會較快循環回到地函內部，造成多數海洋地殼的年紀不會超過2億年。

"The area is covered by thick sedimentary coverage, making it unclear precisely how old the crust is and whether it is even oceanic at all," Dr. Granot says. With the new geophysical data, we could make a big step forward in our geological understanding of the area."

「由於這個區域被相當厚重的沉積物覆蓋，因此過往我們無法精確得知下方地殼的年代，甚至連它是否為海洋地殼都無法確定。」Granot 博士說。「新的地球物理數據讓我們對此區域地質的瞭解有了長足進展。」

He suggests that the crust might be a remnant of the ancient Tethys Ocean, which existed long before the Atlantic and Indian Oceans. If correct, this implies the ocean formed much earlier than previously thought.

他認為這塊地殼是古代特提斯洋(Tethys Ocean)殘存下來的一部分，特提斯洋是遠在大西洋和印度洋出現之前存在的海洋。如果這項假說為真，代表特提斯洋形成的年代較過往認為的早了許多。

引用自：American Associates, Ben-Gurion University of the Negev. "Researcher uncovers 340 million year-old oceanic crust in the Mediterranean Sea using magnetic data." ScienceDaily. ScienceDaily, 15 August 2016. 

原始論文：Roi Granot. Palaeozoic oceanic crust preserved beneath the eastern Mediterranean. Nature Geoscience, August 2016.