Crust-mantle interactions beneath the Hangai Mountains in western Mongolia: Insights from 3D magnetotelluric studies and 4D thermo-mechanical modelling

蒙古西部的汉盖山脉呈圆顶状，是研究地壳-地幔相互作用在空间和时间上引起的正在发生和过去的陆内造山和岩浆作用的理想天然实验室。虽然这种陆内隆起和下沉长期以来被认为是大陆构造的重要组成部分，但它们的起源仍然是一个谜。对这一现象的解释多种多样且存在争议，包括热上升流地幔上的隆升、小规模软流圈上升流、岩浆底沉积、地壳剥离和下地壳流动。在地球物理和地质数据和模型的约束下，热力学建模现在可以模拟真实的构造过程，并测试不同的地球动力学假设。虽然过去和现在的地球物理研究主要集中在重力和地震学上，但杭井的大地电磁（MT）数据却缺失。然而，MT数据尤其重要，因为它们是估计地壳和上地幔电导率的唯一手段，而电导率对流体和部分熔融非常敏感。另一方面，水相和熔体为标定热力学模型提供了关键参数。在本项目中，我们建议使用3D MT对Hangai山脉下的地壳和上地幔电导率进行成像，并为4D高分辨率热力学建模研究提供约束条件。我们期望得到流体/熔体体积的估计，并从MT数据中识别出流变薄弱的区域（如果存在的话）。本研究旨在了解Hangai山脉动力地形的形成过程，并将其置于壳幔相互作用和动力地形的大框架中。该项目的关键组成部分是：1)根据三维电导率模型获取、处理和反演蒙古西部的大地电磁学数据；2)指导基于实验室电导率模型的约束条件下的四维热力学建模；3)进行4D热力学建模。拟议的项目是苏黎世联邦理工学院（ETHZ，瑞士）和德国梅恩斯特大学（UoM；德国）在德国-奥地利-瑞士（DACH）联合计划框架内的合作项目，并将与蒙古科学院天文学和地球物理研究中心（RCAG）合作开展。项目合作伙伴结合了大地电磁学数据采集、大地电磁学数据处理、大地电磁学正演和反演建模以及地球物理约束的四维正演热力学建模等领域的专业知识。该项目的主要预期产出为：1)蒙古国Hangai圆顶下的第一个三维地电模型；2)与地球物理和地质观测相一致的陆内岩石圈变形、岩浆活动和动态地形演化的地貌-热力学模型。

The dome-shaped Hangai Mountain range in western Mongolia is an ideal natural laboratory for studying on-going and past intra-continental orogenic and magmatic processes resulting from crust-mantle interactions in space and time. While such intra-continental uplift and subsidence have long been recognized as an important part of continental tectonics, their origin remains enigmatic. The explanations are diverse and controversial and include uplift above a hot upwelling mantle, small-scale asthenospheric upwelling, magmatic underplating, crustal delamination and lower crustal flow. Thermo-mechanical modelling, when constrained by geophysical and geological data and models, can nowadays simulate realistic tectonic processes and test for different geodynamic hypotheses. While past and on-going geophysical studies have focused on gravity and seismology, magnetotelluric (MT) data are missing from the Hangai. MT data are particularly important, however, as these are the sole means of estimating crustal and upper mantle electrical conductivity, which is very sensitive to fluids and partial melt. Hydrous phases and melt, on the other hand, provide critical parameters for calibrating thermo-mechanical models. In this project, we propose to image crustal and upper mantle electrical conductivity beneath Hangai Mountains using 3D MT and to derive constraints for 4D high-resolution thermo-mechanical modelling studies. We expect to derive estimates on fluid/melt volumes and to identify regions of rheological weakness from the MT data, if present. The study aims to understand the processes responsible for developing dynamic topography in the Hangai Mountains and to place them within the larger framework of crust-mantle interactions and dynamic topography. The key components of the project are to 1) acquire, process, and invert MT data in western Mongolia in terms of 3D conductivity models; 2) to guide 4D thermo-mechanical modelling with constraints that derive from laboratory-based electrical conductivity models; 3) to perform 4D thermo-mechanical modelling. The proposed project is a collaboration between ETH Zurich (ETHZ, Switzerland) and the University of Münster (UoM; Germany) in the framework of the joint German-Austrian-Swiss (DACH) program and will be carried out in cooperation with the Research Center of Astronomy and Geophysics (RCAG) of the Mongolian Academy of Sciences. The project partners combine expertise in the fields of MT data acquisition, MT data processing, MT forward and inverse modelling, and geophysically-constrained 4D forward thermo-mechanical modelling.The main expected outputs from the project are 1) the first 3D geo-electrical model beneath the Hangai dome in Mongolia and 2) a geomorphological-thermo-mechanical model of intra-continental lithospheric deformation, magmatic activity, and evolution of dynamic topography that is consistent with the geophysical and geological observations.