Orogenic plateau magmatism

这项研究将说明为什么地球上一些最高的地区有年轻的活火山。我们特别研究的区域是伊朗的高原，在一个板块活跃碰撞的地区，年轻的火山岩、已知的构造板块厚度和板块厚度变化有着独特的组合。结合该地区岩石类型和构造板块厚度的背景信息，它将告诉我们地球深处的哪个部分产生岩浆，为什么它正在融化，以及是什么控制着岩浆上升到地表。火山和山脉显示了地球是如何活跃的：从地球深处产生的力量和过程塑造了我们生活的景观。火山爆发、地震和山体滑坡带来的风险威胁着数百万人，但同时也控制着我们所有人赖以生存的土地、养分和水的分配。火山和山脉的形成有很多种方式，其中一些是完全独立的。但在某些构造环境下，它们会聚集在一起，在那里，移动的板块与大陆碰撞，形成大面积的变形区域，即碰撞带。其中许多是在地质时期形成的，现在已经不活跃了。苏格兰的山脉就是这些链条被侵蚀的根部，而火山曾经就在它们中间。但在西南亚，这一过程直到今天仍在继续，阿拉伯板块向北移动，向欧亚大陆推进。运动速度大约是20毫米/年，但在地质时间尺度上，这转化为相当大的变形：自从大约3500万年前最初的板块碰撞以来，已经发生了大约700公里的收敛。在碰撞之前，火山活动很常见，其环境类似于现代安第斯山脉。在最初的碰撞之后，它的强度减弱了，因为岩浆活动的主要触发因素之一在这个时候结束了——欧亚大陆下面的海洋板块下沉。后来，零星的岩浆活动可能代表了这一过程的终结。但在过去的几百万年里，在碰撞区出现了火山活动的激增，其原因尚不完全清楚，但可能与欧亚板块变形的总体重组有关。火山中心不是随机分布的。我们可以确定它们所在位置的模式，这为我们提供了地球深处为什么正在融化的线索。首先，它们集中在碰撞区内一个不再缩短的区域，即土耳其-伊朗高原。顾名思义，这是一块巨大的高地，地势平缓，不再有那种曾经使地球变形的造山活动。这种造山活动在阿拉伯板块和欧亚板块碰撞的广阔地带的边缘进行。其次，火山几乎总是在原来的欧亚板块上，这个板块曾经位于大洋板块的一块板块之上，而这块板块现在已经经过了它的下面（行话叫“俯冲”）。这块厚板是水和其他挥发性化合物的来源，这些化合物可能是导致地球深处融化和火山爆发的原因。但目前尚不清楚为什么海洋板块停止在欧亚大陆下移动数千万年后还会有水存在。第三，许多火山位于仍在断裂的地区，即使山脉没有形成。这表明断裂与岩浆活动的产生，或者至少是岩浆活动上升到地表之间存在联系。同样，细节不被理解，这是本研究的一部分。虽然我们一般都知道火山是在安第斯山脉、夏威夷和冰岛这样的地方产生的，但伊朗和西藏这样的地球高原代表了科学的前沿，也为我们提供了一个机会，让我们了解板块构造的最终结果是如何在地球上最高的地方产生火山的。

This study will show why some of the highest regions on Earth contain young and active volcanoes. Our particular study area is the high plateau of Iran, which has a unique combination of young volcanic rocks, known tectonic plate thickness and variation in plate thickness, in a region where the plates are actively colliding. Combined with background information on the rock types and thicknesses of the tectonic plates in this region, it will tell us which part of the deep Earth generates the magma, why it is melting, and what controls the ascent to the surface. Volcanoes and mountain ranges show how Earth is active: forces and processes arising from deep within the planet shape the landscapes on which we live. Risks from eruptions, earthquakes and landslides threaten millions of people, but at the same time control the distribution of land, nutrients and water on which we all depend. There are many ways to generate volcanoes and chains of mountains, some of which are entirely separate. But there are certain tectonic settings where they come together, where the moving plates collide continents to create vast regions of deformation known as collision zones. Many of these have occurred through geological time and are now inactive. The mountains of Scotland are the eroded roots of such chains, and the volcanoes once within their midst. But in SW Asia the processes continue at the present day, as the Arabian plate moves northward, pushing in to Eurasia. The rate of motion is roughly 20 mm/yr, but over geological timescales this translates to a considerable amount of deformation: ~700 km of convergence has occurred since initial plate collision ~35 million years ago. Volcanism was common before the collision, in a setting similar to the modern Andes. It reduced in intensity after the initial collision, because one of the main triggers for magmatism ended at this time - downgoing of oceanic plate beneath the Eurasian continent. Later, sporadic magmatism probably represented the tail-off from this process. But in the last few million years there has been an upsurge of volcanic activity across the collision zone, for reasons not entirely understood, but possibly related to a general re-organization of deformation of the Eurasian plate. The volcanic centres are not randomly distributed. We can determine patterns in their locations that give us clues about why the deep Earth is melting. First, they are concentrated in a region within the collision zone that is no longer shortening, known as the Turkish-Iranian plateau. As its name implies, this is a vast, high area with subdued relief, that no longer undergoes the kind of mountain building that once deformed the Earth in this area. That mountain building carries on at the margins of the broad zone that is the site of collision between the Arabian and Eurasian plates. Second, the volcanoes are almost always in the original Eurasian plate, which once lay above a slab of an oceanic plate that has now passed underneath it ('subducted' in the jargon). This slab is a source of water and other volatile compounds that could have been responsible for melting the deep Earth and causing the volcanoes. But it is unclear why water would be present tens of millions of years after the ocean plate stopped moving under Eurasia. Third, many of the volcanoes are located in regions which are still faulting, even if the mountains are not building up. This suggests a link between the faulting and either the generation of the magmatism, or at least its ascent to the surface. Again, the details are not understood and form part of this study. Whereas we generally understand the generation of volcanoes in settings like the Andes, Hawaii and Iceland, the high plateaux of the Earth like Iran and Tibet represent a frontier for science, and an opportunity to comprehend how the end result of plate tectonics can be volcanoes within the highest places on Earth.