陸地如何分裂？對古典地函柱理論的質疑

原文網址：www.sciencedaily.com/releases/2015/08/150813092824.htm

How do continents break up? Classical theory of mantle plume is put in question

陸地如何分裂？對古典地函柱理論的質疑

New insights from South Africa

得自南非的新觀點

When the western part of the super-continent Gondwana broke up around 130 Million years ago, today's Africa and South-America started to separate and the South Atlantic was born. It is commonly assumed that enormous masses of magma ascended from the deep mantle up to higher levels, and that this hot mantle plume (the Tristan mantle plume) weakened the continental lithosphere, eventually causing the break-up of the continental plate of Gondwana.

當岡瓦納超大陸的西側於1億3000萬年前左右分裂時，今日的非洲與南美洲開始逐漸分離，而南大西洋也因此誕生。一般推測地函的大量岩漿從地函深部上升至非常淺處時，此熱地函柱(Tristan地函柱)會逐漸弱化大陸岩石圈，最終導致岡瓦納大陸板塊分離。

A group of German scientists are now questioning this theory. On the basis of seismic measurements published in the current issue of the journal Geology, scientists from Potsdam (GFZ German Research Centre for Geosciences), Bremerhaven (Alfred-Wegener-Institute, AWI) and Kiel (GEOMAR) show that impacts of the mantle plume on the continental crust are actually surprisingly small. This is by no means in agreement with a large plume playing an important role in the break-up process. Accordingly, a dominant or controlling role of a mantle plume for the break-up of the continent is thus questionable.

一群德國科學家現在對此理論提出質疑。根據他們發表在當期《地質》(Geology)期刊上的論文中的震測結果，來自波茨坦(德國地質科學研究中心, GFZ)、不來梅哈芬(阿爾弗雷德·韋格納研究所, AWI)以及基爾(萊布尼茨海洋科學研究所, GEOMAR)的科學家顯示事實上地函柱對大陸地殼的影響小得驚人。這跟在分裂過程中，大型地函柱具有重要腳色的理論毫無一致之處。因此，值得懷疑地函柱是否真在陸地分裂的過程中，扮演了主要腳色或是控制因素。

Hot ascending mantle plumes in the Earth's mantle are an important driving force in plate tectonics. With an assumed diameter of the plume heads of up to several thousand kilometers, the amount of hot material ascending from the core mantle boundary at 2900 km depth is sufficient to migrate through the continental lithosphere. This process leads to the eruption of large volcanic material (flood basalts) at the Earth's surface. This is also the case for Southern Africa and South America: the Parana/Etendeka/ flood basalt provinces are the direct consequences of the Gondwana break-up starting some 130 million years ago. Traces of the break-up process can be found on the newly formed ocean floor: the Walvis Ridge off the coast of Northern Namibia images the track of the mantle plume.

地函中的高熱上湧地函柱為板塊構造運動重要的驅動力之一。推測當地函柱頂端的直徑達數千公里寬時，這些從深達2900公里的核幔邊界湧上來的巨量高熱物質，會多到足以穿透大陸岩石圈。這種過程會導致地表噴發大規模的火山物質(洪流玄武岩, flood basalt)。而這就是發生在非洲南部與南美洲的現象：Parana/Etendeka洪流玄武岩省便是開始於1億3000萬年前，岡瓦納分裂的直接後果。分裂過程的蛛絲馬跡至今仍能在新生海床上發現：那米比亞北部外海的瓦維斯海脊(Walvis Ridge)即標記出了地函柱的軌跡。

In order to study these processes, German scientists investigated structures which are related to the break-up process of Gondwana in the South Atlantic. The upwelling of large amounts of hot mantle material produces regions of crustal and mantle rocks with different seismic velocities (with respect to the surrounding, unaffected regions). In cooperation with colleagues from AWI and GEOMAR, and with the support of the Geological Survey of Namibia, scientists from GFZ have carried out extensive seismic investigations on and offshore Northern Namibia. The crustal structure was investigated along several seismic lines.

為了研究這些過程，這群德國科學家調查了位於南大西洋中，與岡瓦納分裂的相關構造。上湧的大量熱地函物質會在地殼與地函的岩石中形成地震波速度不同的區域(相對於周遭未受影響的區域)。來自GFZ的科學家與AWI以及GEOMAR的同僚合作，並在納米比亞地質調查所的協助下，對納米比亞北部陸上以及外海進行大範圍的地震波調查。他們沿著數條地震測線(seismic line)來研究此區域的地殼構造。

"For the first time we could obtain images of the deeper crustal structure in the region where the Walvis Ridge joins the African continent, in order to study the impact of a mantle plume" explains Trond Ryberg from GFZ. "Our seismic investigations found a distinctive high-velocity anomaly in the lower crust between 20 and 40 km depth." This region of anomalously high seismic velocities can be related to the intrusion of magmatic material in the lower crust of the Earth. This was expected according to the current perception. But surprisingly, the size of the anomaly was far too small to be created by a large plume head playing an active role in the break-up process. Trond Ryberg: "The crustal structure in the investigated region reflects the general process of continental break-up rather than the immediate impact of the plume head on the lithosphere. In addition, we were able to reconstruct the direction of the mantle plume flow." It seems that the break-up of Gondwana in the South Atlantic and, in general, the role of mantle plumes during the continental break-up needs to be carefully re-evaluated.

「為了研究地函柱的影響，我們首度能得到瓦維斯海脊與非洲大陸相接處的深層地殼構造的影像。」來自GFZ的Trond Ryberg解釋。「我們的地震波調查發現在約20至40公里深的下部地殼有明顯的異常高速區。」這塊地震波速異常高的區域與岩漿物質侵入至下部地殼有關，以現今的觀點而言這算意料之中。但令人驚訝的是，對一個能在分裂過程中扮演要角的大型地函柱頂部來說，這個異常區顯然過於狹小。「我們調查區域的地殼構造反映出來的是一般的大陸分裂過程，而非地函柱頂部對岩石圈造成了直接影響。此外，我們也重建了地函柱流動的方向。」Trond Ryberg說。看來岡瓦納在南大西洋的分裂過程，以及一般而言地函柱在大陸分裂過程中的定位都有必要仔細地重新審視。

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "How do continents break up? Classical theory of mantle plume is put in question: New insights from South Africa." ScienceDaily. ScienceDaily, 13 August 2015.