Andean Back Arc Processes in Argentina

这项工作的总体目标是了解形成地球表面的主要构造活动之一：海洋板块在大陆边缘下的俯冲。更直接的焦点是在大陆边缘附近形成的火山和稳定的大陆内部之间鲜为人知的弧后。当海洋板块异常浮力而不立即下沉到内部深处时，这个区域变得特别活跃。大约8000万年前，这种情况造就了美国的大部分落基山脉，今天在南美洲也很活跃。纳斯卡板块在智利下方约100公里处变平，并在向东600公里处的阿根廷下方急剧下降。之前的工作已经利用一种叫做大地电磁（MT）方法的电磁技术，对纳斯卡平板的地下电导率进行了成像，深度可达600公里。令人惊讶的是，有证据表明，当下行板块进入400公里深度的地震过渡带时，可能会产生部分熔融；平板的南缘是被撕裂的，而不是像地震学家通常认为的那样，不断地扭曲回一个更正常的倾斜几何形状；平板上的水控制着它的地震活动性，而且这些水正在向上泄漏，如果平板在地质未来变陡，可能会导致大规模的火山爆发。这些初步解释依赖于二维（2D）结构假设。然而，数据和结构推断表明，现实不是二维的。在一个由地球物理计划和国际科学与工程办公室资助的新项目中，所有数据将使用三维（3D）和各向异性二维反演算法重新检查，并收集大约20个额外的MT站点以增强3D覆盖。三维反演是使用一个72节点的计算机集群和一家勘探公司提供的软件进行的。这项工作需要来自美国和阿根廷的科学家和研究生之间的密切合作。这项工作的广泛影响与了解地球的化学分异、山带的形成、矿床的产生以及地震和火山风险有关。此外，解决正在讨论的地质问题所需的技术进步可直接应用于改进矿物、能源和水资源的勘探和评价。

The overall goal of this work is to understand one of the main types of tectonic activity that has shaped Earth's surface: subduction of an ocean plate under the edge of a continent. The more immediate focus is the poorly understood back-arc between the volcanoes that form near the edge of the continent and the stable continental interior. This zone becomes especially active when the ocean plate is unusually buoyant and does not immediately descend into the deep interior. This circumstance produced much of the U.S. Rocky Mountains about 80 million years ago and is active in South America today.The Nazca Plate flattens out under Chile at about 100 km depth and descends steeply under Argentina 600 km to the east. Prior work has imaged electrical conductivity to depths of up to 600 km associated with this Nazca flat-slab using an electromagnetic technique for sensing sub-surface electrical conductivity called the magnetotelluric (MT) method. Surprising implications include evidence that partial melt can be generated when the down-going slab enters the seismic transition zone at a depth of 400 km; that the southern edge of the flat-slab is torn rather than warping continuously back to a more normal dipping geometry as commonly argued by seismologists; that water in the flat slab is controlling its seismicity and that this water is leaking upward where it may result in a massive volcanic flare-up if the slab steepens in the geologic future.These preliminary interpretations rely on two-dimensional (2D) structure assumptions. However the data and structural inferences made argue that the reality is not 2D. In a new project funded by the Geophysics Program and the Office of International Science and Engineering, all the data will be re-examined using three-dimensional (3D) and anisotropic 2D inversion algorithms and collecting about 20 additional MT sites to enhance 3D coverage. 3D inversion is being carried out using a 72-node computer cluster and software provided by an exploration company. This work involves close collaboration between scientists and graduate students from both the U.S. and Argentina. Broader impact of this work is related to understanding the chemical differentiation of Earth, formation of mountain belts, the generation of mineral deposits, and earthquake and volcanic risk. Furthermore, the technical advances required to solve the geological problems being addressed can be directly applied to improving mineral, energy and water resource exploration and assessment.