夏威夷―天皇海山鏈的神祕轉彎

原文網址：www.sciencedaily.com/releases/2017/06/170608123615.htm

夏威夷―天皇海山鏈的神祕轉彎

位於太平洋海床上，有道年代為8000萬年且將近6000公里長的海山鏈，其中最年輕的那端為夏威夷火山諸島。包含數十座火山的夏威夷―天皇海山鏈以中間有道60度的奇異轉折而聞名，數十年來科學家對其成因一直有相當激烈的討論。一種解釋為太平洋板塊的移動方向突然發生改變，另一種模型則主張從8000萬年前就開始形成海山鏈的地函柱，往南移動而使海山鏈轉向。由奧斯陸大學、德國波茨坦地質科學研究中心(GFZ)和烏特勒支大學的研究團隊在《自然通訊》(Nature Communications)刊登的新研究中，則認為這兩種作用顯然都扮演了某種重要腳色。

許多火山島是由稱作地函柱的高熱柱狀地函上湧而成，它們的起源為3000公里深處接近地函底部的地方。地函柱不太會受到在其上方，於地表緩緩移動的板塊影響。因此，以地函柱為來源的火山長鏈隨著它們距正在活動的熱點越來越遠，年代也會跟著變老，並綿延長達數百至數千公里。在夏威夷熱點形成的軌跡中，最年輕的是夏威夷火山諸島，整個火山鏈可以一路延伸至今日位於太平洋西北部的底特律海底山，它在8000萬年前由火山活動形成。夏威夷―天皇海山鏈的特徵是有一道絕無僅有的60度彎折，將其分成較老的天皇海山鏈跟較年輕的夏威夷海山鏈。此道彎折的定年結果為4700萬年前。

此篇新研究的主要作者，當時在GFZ擔任訪問學者的奧斯陸大學研究人員Trond Torsvik表示：「形成夏威夷―天皇海山鏈轉向的根本原因是，在4700萬年前太平洋板塊的移動方向發生了顯著改變。」團隊證實了美國地球物理學家Jason Morgan在1970年代早期就已經提出來的理論。Torsvik表示：「但整個過程當然不會像40年前提出時那麼單純。」

Jason Morgan是第一位用熱點來當作全球板塊移動參考點的科學家。在他的模型中，他認為地函柱是固定在地函之中，而地表的熱點標明了地函柱的位置。夏威夷―天皇海山鏈轉向可以單純歸因至太平洋板塊運動方向的改變。但是，他以固定熱點所做的板塊模型從1980年代起就開始受到挑戰，

論文共同作者之一，GFZ的Bernhard Steinberger表示：「自1990年代晚期，有越來越多證據顯示熱點並非完全固定不動。」他繼續說現在這個現象已經被廣為接受，而地函流動模型預測夏威夷的熱點一直在慢慢地往南方漂移。「但某些近期研究主張轉向可以完全不需要太平洋板塊的運動方向發生改變，僅憑早於4700萬年前熱點迅速往南方移動就可解釋。」他說，「這種情境顯得相當吸引人，因為太平洋周圍的板塊呈現出來的地質證據，並未清楚顯示太平洋板塊的運動方向曾發生改變。」

新研究清楚顯示為何如此簡單的詮釋是行不通的。因為它必須要讓熱點以超乎現實的速度移動，也就是每年移動大約42公分，比板塊移動的平均速度還要快上許多。此外，這也意謂著天皇海山鏈僅在500萬年之間就形成了，而底特律海底山的年代應該只有5200萬年。但底特律海底山的年代紀錄顯示此島嶼的年紀約有8000萬年，清楚證明了這項預測是錯誤的。

Steinberger表示：「反之，熱點往西南西方向以較慢的速度移動可以同時解釋天皇海山鏈的幾何型態和年代。」然而，熱點若往該方向移動，會跟地函對流模型有所牴觸。

另一位共同作者，奧斯陸大學的Pavel Doubrovine表示：「我們的論文是個絕佳範例，顯示出如何利用相當簡單的動力學模型來模擬板塊和熱點的運動，來探討在夏威夷―天皇海山鏈轉向的形成過程中，哪些地球動力學狀是可能的情境，又有哪些是不可能的。我們的結論無法避免60度的轉向主要是由太平洋板塊的運動方向改變所造成。」但仍需要地函柱往南移動一定程度，否則夏威夷―天皇海山鏈會縮短將近800公里。

Torsvik主張：「要解釋夏威夷―天皇海山鏈的幾何形狀、長度和年代變化，板塊移動方向改變和熱點移動兩者都是不可或缺的。板塊運動改變和熱點漂移，這兩個位處兩端的情境孰對孰非已經爭辯長達20餘年，若之後地球物理學家終於能一致同意兩者都無法滿足現實情況，那我們就可以進一步地提出一項更加有趣的問題：太平洋板塊的運動在4700萬年前發生改變的背後原因究竟是什麼？」他接著說，希望這項問題不會再花上另一個40年來回答。

The mysterious bend in the Hawaiian-Emperor chain

The volcanic islands of Hawaii represent the youngest end of a 80 million years old and roughly 6,000 kilometres long mountain chain on the ground of the Pacific Ocean. The so-called Hawaiian-Emperor chain consisting of dozens of volcanoes is well known for its peculiar 60 degrees bend. The cause for this bend has been heavily debated for decades. One explanation is an abrupt change in the motion of the Pacific tectonic plate, the opposite model states southward drift of the mantle plume that has sourced the chain since its beginning 80 million years ago. Apparently both processes play an important role, shows a new study in Nature Communications, published by a group of scientists from the University of Oslo, German Research Centre for Geosciences GFZ Potsdam, and Utrecht University.

Many volcanic ocean islands are created by columnar shaped hot upwellings called mantle plumes that originate near the ~3000 km deep base of Earth's mantle. Mantle plumes are not much influenced by surface motions of the tectonic plates that slowly move over them. Hence, long linear chains of plume-sourced volcanoes that get older and older with increasing distance from active hotspots can be tracked for hundreds to thousands of kilometres. In the Hawaiian hotspot trail, the Hawaii islands are the youngest in the chain that stretches nearly 6,000 km to Detroit seamount in the northwest Pacific, where volcanism occurred about 80 million years ago. An unprecedented 60 degrees bend characterizes the Hawaiian-Emperor Chain, dividing it into the older Emperor Chain and the younger Hawaiian Chain. The bend has been dated to 47 Ma.

"The ultimate cause for the formation of the Hawaiian-Emperor Bend (HEB) was a prominent change in the Pacific plate motion at 47 Ma," says the lead author of the new study, Trond Torsvik from the University of Oslo and visiting researcher at GFZ at the moment. The team affirms a hypothesis by the US-geophysicist Jason Morgan who proposed that already in the early 1970s. "But it is not that simple as it was suggested forty years ago," says Torsvik.

Jason Morgan was the first to use hotspots as a reference frame for global plate motions. In his model mantle plumes -- which are manifested by hotspots at the surface -- were considered fixed in the mantle, and the Hawaiian-Emperor Bend was attributed to a simple directional change of the Pacific plate motion. But his plate model with fixed hotspots became challenged from the 1980s.

"Since the late 1990s it has become clear that hotspots are not totally fixed," says GFZ´s Bernhard Steinberger, one of the co-authors of the paper. That is now generally accepted, he adds, and mantle flow models predict that the Hawaiian hotspot has drifted slowly to the south. "But some recent studies have argued that rapid southward motion of the hotspot before 47 Ma can explain the formation of the bend without requiring Pacific plate motion change," he says. "Such a scenario has become attractive because the geology of the plates surrounding the Pacific shows no clear evidence for a Pacific plate motion change."

The new study shows clearly why this simply does not work. It would require an unrealistically high rate of hotspot motion of about 42 cm/year which would be much faster than the average speed of tectonic plates. Moreover, this would imply that the Emperor Chain was created in just five million years and Detroit Seamount should only be 52 million years old. This prediction is obviously falsified by the recorded Detroit Seamount island ages of about 80 Ma.

"Alternatively, a slower hotspot motion towards the WSW could explain both geometry and ages of the Emperor chain," says Steinberger. However, such a direction of motion is inconsistent with mantle convection models.

"Our paper is a good example of how very simple simulations of plate and hotspot kinematics can be used to explore which geodynamic scenarios for the formation of the Hawaiian-Emperor Bend are possible, and which ones are not," says Pavel Doubrovine from the University of Oslo, another co-author on the paper. "We cannot avoid the conclusion that the 60 degrees bend is predominantly caused by a directional change in the Pacific plate motion." Yet, some southward plume motion is required, otherwise the Hawaiian-Emperor Chain would be around 800 kilometres shorter.

"Explaining the geometry, length and age progression of the Hawaiian-Emperor Chain, requires both: the change in the direction of plate motion and the movement of the hotspot," states Torsvik. "If, after more than two decades of debating the end-member scenarios of plate motion change versus hotspot drift, geophysicists will be able to agree that neither of the two is satisfactory -- then we can move forward and address a more interesting question: what actually drove the Pacific plate motion to change at about 47 million years ago?" Hopefully, it will not take further 40 years to get an answer to this, he adds.

原始論文：Trond H. Torsvik, Pavel V. Doubrovine, Bernhard Steinberger, Carmen Gaina, Wim Spakman, Mathew Domeier. Pacific plate motion change caused the Hawaiian-Emperor Bend. Nature Communications, 2017; 8: 15660 DOI: 10.1038/ncomms15660

引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz Centre. "The mysterious bend in the Hawaiian-Emperor chain." ScienceDaily. ScienceDaily, 8 June 2017.