Andean Back Arc Processes in Argentina

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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.