大陆动力学数值模拟:问题、进展与展望

大陆动力学是地球动力学的基本组成部分,是板块构造理论的重要拓展,是固体地球科学的核心命题之一。在系统的地质、地球物理和地球化学观测的基础上,数值模拟是探讨大陆动力学过程和机制的有效手段。本文主要基于大陆动力学数值模拟,围绕大陆形成和演化过程的四个关键科学问题进行总结和探讨。(1)大陆起源与早期地球动力学演化。现今观测到的最早的大陆地壳岩石来自冥古宙,说明地球早期就已经开始大陆地壳的形成;关于当时的构造体制,存在多种不同的模式及其过渡和转换(岩浆洋、热管、滞盖等),该问题的约束甚少,是一个地球动力学的前沿科学问题。(2)大陆岩石圈的稳定性与破坏。大陆岩石圈形成之后经历几十亿年尺度的长时间演化,有些克拉通可以保持大致的稳定性直至现今,而有的克拉通却在显生宙期间经历显著的改造和破坏,无论其稳定存在还是改造破坏,都是值得深入探讨的科学问题。(3)大陆深俯冲与极限折返。超高压岩石折返代表了地球表层物质由浅入深而又由深及浅的物质循环,系统的观测和模拟对该过程和机制已有比较清晰的理解;而近年来观测到的200~350 km的超深折返岩石又提出新的挑战,其模式和机制有待进一步探索。(4)大陆碰撞造山差异性。造山带解析是大陆动力学研究的核心内容,伴随日益精细化的地学观测,也对动力学模拟提出了新的挑战,多地体的复合造山动力学值得进一步探究;同时,青藏高原作为最重要的碰撞造山带,其一级动力学驱动力仍不明确。本文在系统探讨这些科学问题的基础上,进一步提出大陆动力学数值模拟的未来研究方向:一方面是问题导向型,针对上述仍未解决的重要科学问题,尤其是一级控制机制问题,通过创新思路和精细模拟,并与观测进行系统对比分析,最终给出答案;另一方面是技术导向型,极力发展新的数值模拟方法和技术(例如双相流乃至多相流),以适应复杂地质演化中的相变、流体、熔体、地表过程等的模拟需求,这两个研究方向的发展将共同提高大陆动力学过程和机制的认知水平,促进大陆动力学理论的发展。

Numerical modeling of continental dynamics:questions,progress and perspectives

Continental dynamics,a fundamentalfield of geodynamics,is an important extension of plate tectonics and thus a key research direction in solid Earth sciences.Based on the systematic geological,geophysical and geochemical observations,numerical modeling is an efficientway for studying the processes and mechanism of continental dynamic evolution.This study focuses on the numerical modeling of continental dynamics,with discussion onfour key scientific issues of the continental formation and evolution.(1)The early Earth geodynamics and the origin of continents.The observed oldest continental crustal rocks have a Hadean age,which means that the continental crust started to form in the very early stage of the Earth.The tectonic regimes of the early Earth are far from well constrained,thatmay include the magma ocean,the heat pipe,the stagnant lid,etc.Thus,it is still an unresolved,cutting-edge geodynamic problem.(2)Continental lithospheric stability and destruction.The continental lithosphere can experience a long-time evolution of billions of years,during which some cratons keep their general stability,however some others are significantly modified and/or destroyed in the Phanerozoic.Either the long-time-scale stability or the destruction of continental lithosphere is the critical scientific question for furtherstudy.(3)Continental deep subduction and extreme depth exhumation.The exhumation of ultra-high-pressure(UHP)rock is a typical process representing the material circulation in the subduction channel.The general mode and dynamics arealready understood throughsystematic observations and numerical models.However,the exhumation of super-high pressure rocks from 200~350 km depths challenges the previous models and thus requires further study.(4)Continental collision and contrasting mountain building.The deciphering of mountain belt is a key issue of continental dynamics.Sophisticatednumerical models are required for comparison with detailed observations tobetter understand the composite mountain building processes with multiple terranes.In addition,as the most important mountain building process,the first-order driving force of Tibetan plateau evolution is still not well understood,requiring further studies.Based on the discussion of these key scientific issues,future studies incontinental dynamic modeling in two major directions are discussed.The first is problem-driven,requiring new strategies and ideas to solve the above-mentioned key continental dynamic problems,especially the first-order mechanisms.The second direction is technique-driven,which calls for the new numerical modeling methods and technologies,e.g.two-phase flow or even multi-phase flow,to meet the requirements of complex geological processes,for example,the phase change,the aqueous fluid flow,the magmatism and the surficial processes.The progresses of both directions are going to improve the understanding of continental dynamicsand further promote the development of continental dynamics theory.