科學家重建出許久以前消失在安地斯山下方的板塊
安地斯山沿著南美洲西岸綿延將近7000公里(約4300英里)，是世界上最長的山脈。
John Suppe、吳恩霖和陳奕維(由左至右)發表在《自然》的論文中，描述了安地斯山脈的形成過程。
安地斯山脈的邊緣是兩個板塊交會之處，此處的隱沒作用造成其中一個板塊下潛至另一板塊之下。長久以來，教科書常以安地斯山脈當作範例來解釋穩定而持續的隱沒作用，最終讓我們見到這座綿延不絕的山脈。。
重建安地斯山脈的形成過程
在近日發表於期刊《自然》(Nature)的論文中，美國休士頓大學的地質學家團隊重建了納斯卡海板塊的隱沒過程。他們發現在地表之下1500公里(約900英里)深的地方，還可以見到納斯卡海板塊的殘餘蹤跡。
研究結果顯示安地斯山脈的形成過程，比過往提出來的模型還要複雜許多。
這篇論文被《自然》選為1月24日當期的封面。作者之一，休士頓大學自然科學學院的地質學助理教授吳恩霖博士說：「安地斯山脈的形成過程一直是板塊構造理論的經典範例。」論文主要作者是吳博士指導，主修地質學的研究生陳奕維；另一位共同作者是該校地球與大氣科學系的特聘教授John Suppe。
殘存在地函中的板塊
板塊滑入地殼下方，接著進入地函之後並不會完全消失。反之，板塊會繼續沉往地核，就跟樹葉緩緩沉到湖底一樣。在下沉過程中，板塊原有的樣貌不會被徹底抹滅，使我們有機會一窺數百萬年前的地表形貌。
就像醫生運用電腦斷層掃描技術可以得到患者體內的影像，地質學家利用從地震波中蒐集到的資訊，也能描繪出這些殘存在地函的板塊樣貌。
「我們重建過去所用的方法比任何人都更加精準，使結果的精細程度超越以往認為的極限。」吳恩霖說，「我們成功回溯至恐龍生存的年代。」
納斯卡板塊的隱沒過程
在這篇剛發表的論文中，研究人員重建殘留於地函的納斯卡板塊，結果顯示板塊的年代最老可以到達白堊紀。這是殘留在地函的板塊中，迄今發現最老也最深的。
陳奕維說：「我們發現的跡象顯示板塊到達過渡帶(transition
zone)的時候，會在板塊表面留下記號。」過渡帶是地函內部的一個不連續帶，板塊沉入地函時如果碰到過渡帶動作就會慢下來，造成上方繼續往下沉的板塊堆積於此。
他們也發現證據顯示納斯卡板塊並非一直穩定地隱沒到地函當中，有時候納斯卡板塊會被拉離安地斯山脈的邊緣，造成火山活動。為了證實這個想法，他們模擬了沿著安地斯山脈發生的火山活動。
吳恩霖表示：「為了驗證我們的模型，我們觀察了安地斯山脈沿線，超過14000筆火山活動紀錄中呈現的模式。」
構造與層析成像中心
這項研究是波士頓大學構造與層析成像中心進行的計畫之一，此機構的主任為Suppe。
「層析成像(tomography)是利用地震波來得到地球內部的影像。波士頓大學構造與層析成像中心集結了來自不同領域的專家，目標是把層析成像用於構造相關的研究。」吳恩霖說，「比方說，我們在探測這些消失的板塊時用的方法，便是改良自探勘石油的技術。」
Scientists reconstruct
ancient lost plates under Andes mountains
The Andes Mountains are the longest continuous mountain range
in the world, stretching about 7,000 kilometers, or 4,300 miles, along the
western coast of South America.
The Andean
margin, where two tectonic plates meet, has long been considered the textbook
example of a steady, continuous subduction event, where one plate slipped under
another, eventually forming the mountain range we see today.
Reconstructing Andes Mountain Formation
In a recent
paper, published in the journal Nature, a group of
University of Houston geologists reconstructed the subduction of the Nazca
Ocean plate, the remnants of which are currently found down to 1,500
kilometers, or about 900 miles, underneath the Earth’s surface.
Their
results show that the formation of the Andean mountain range was more
complicated than what previous models suggested.
“The Andes
Mountain formation has long been a paradigm of plate tectonics,” said Jonny Wu,
assistant professor of geology in the College of Natural Sciences and
Mathematics, and one of the authors of the paper. The lead author is Wu’s
advised Ph.D. student in geology Yi-Wei Chen. Distinguished Professor of Earth
and Atmospheric Sciences John Suppe is also a co-author. This article was
selected for the cover of the Jan. 24 issue.
Plate
Remnants in the Earth’s Mantle
When
tectonic plates move under the Earth’s crust and enter the mantle, they do not
disappear. Rather, they sink down toward the core, like leaves sinking to the
bottom of a lake. As these plates sink, they retain some of their shape,
offering glimpses of what the Earth’s surface looked like millions of years
ago.
These
plate remnants can be imaged, similar to the way CT scans allow doctors to see
inside of a patient, using data gleaned from earthquake waves.
“We have
attempted to go back in time with more accuracy than anyone has ever done
before. This has resulted in more detail than previously thought possible,” Wu
said. “We’ve managed to go back to the age of the dinosaurs.”
Nazca
Plate Subduction
This
latest paper represents the deepest and oldest plate remnants reconstructed to
date, with these plates dating back to the Cretaceous Period.
“We found
indications that when the slab reached the transition zone, it created signals
on the surface,” Chen said. A transition zone is a discontinuous layer in the
Earth’s mantle, one which, when a sinking plate hits it, slows down the plate’s
movement, causing a build-up above it.
They also
found evidence for the idea that, instead of a steady, continuous subduction,
at times the Nazca plate was torn away from the Andean margin, which led to
volcanic activity. To confirm this, they modeled volcanic activity along the
Andean margin.
“We were
able to test this model by looking at the pattern of over 14,000 volcanic
records along the Andes,” Wu said.
The Center
for Tectonics and Tomography
This work
was conducted as part of the UH Center for Tectonics and Tomography, which is
directed by Suppe.
“The
Center for Tectonics and Tomography brings together experts from different
fields, in order to relate tomography, which is the imaging of the Earth’s
interior from seismology, to the study of tectonics,” Wu said. “For example,
the same techniques we use to explore for these lost plates are adapted from
petroleum exploration techniques.”
原始論文：Yi-Wei Chen,
Jonny Wu & John Suppe. Southward propagation of Nazca subduction
along the Andes. Nature, 2019 DOI: 10.1038/s41586-018-0860-1
引用自：University of Houston. "Scientists reconstruct ancient lost plates
under Andes mountains: Reconstruction offers glimpse of how the Earth looked
millions of years ago."
沒有留言:
張貼留言