俯冲起始的机制、地质记录及其研究意义

Mechanism,Geological Record and Significance of Subduction Initiation

板块俯冲起始是固体地球科学的重大前沿领域之一,是板块构造理论发展的关键和新方向,主要包括俯冲起始时间和机制两个关键科学问题。目前,关于俯冲起始机制存在多种模型,但根据其驱动力的来源不同,可以分为自发俯冲起始和诱发俯冲起始两类,前者包括转换断层塌陷、被动陆缘塌陷和地幔柱头周缘垮塌,后者包括俯冲传递和极性反转。不论是自发还是诱发俯冲起始过程,均会产生相应的地质记录,主要包括俯冲带(SSZ)型蛇绿岩、前弧玄武岩、玻安岩和变质底板。深入研究俯冲起始的地质记录是解开俯冲起始机制的基础,更是检验俯冲起始机制最直接、最有效的手段。另外,通过俯冲起始的地质记录研究,可对地幔属性提供制约,揭示不同源区对大陆地壳增生的贡献,并有效恢复古大洋及相关构造单元的演化历史,进而对其动力过程提供制约。虽然地质记录是解决板块俯冲起始问题的关键,但这些记录具有碎片化、非系统性的特征。在今后的研究中,应将这些地质记录进行横向、大面积对比,从而形成系统性约束;另外,需要注意现有的地质记录和俯冲起始可能并不是一一对应的关系,即没有地质记录的地方不一定就没有俯冲起始。但无论如何,通过俯冲起始研究,可以深入理解特定板块的几何学、运动学及动力学过程,并预测板块行为。同时,对比类地行星(如火星等)的演化过程,将有助于揭开地球独特性和宜居性等未解之谜。

Initiation of new subduction zones is a fundamental process of Earth's plate tectonics,and it is the key and new direction for the development of plate tectonics theory.Although the subduction initiation has been a focus of research of many scientists during the last 50 years,triggers and mechanisms of subduction initiation remain controversial.Two end-member subduction initiation models have been proposed:spontaneous and induced.Spontaneous subduction initiation mainly includes three models,which are transform collapse,passive margin collapse and plume head margin collapse.Moreover,induced subduction initiation mainly includes subduction transference and polarity reversal.Natural examples of spontaneous and induced subduction initiation have representative geological records of supra-subduction zones(SSZ)ophiolites(ophiolitic mélanges),forearc basalts,boninites and metamorphic soles.These are rock records of upper plate extension and lower plate burial.This will open the door for geologists to contribute fundamentally for understanding subduction initiation.Therefore,a detailed study on geological records of subduction initiation is the basis for unraveling the subduction initiation mechanism,and it is also the most direct and effective means to test the mechanism.In addition,the study of the geological records of subduction initiation can provide constraints on the properties of the mantle,reveal the contribution of different sources to the continental crust accretion,and effectively recover the evolutionary history of the ancient oceanic and related tectonic units,thus providing constraints on their dynamical processes.Although geological records are the key to solve the problem of subduction initiation,these geological records are fragmented and non-systematic in character.In future studies,the geological records should be compared on a large scale so as to form systematic constraints for subduction initiation.Moreover,it should be noted that the existed geological records and subduction initiation may not correspond one by one,i.e.,where there is no geological record do not mean there is no subduction initiation.However,in any case,through studying on subduction initiation,it will help for deeply understanding the geometry,kinematics and dynamics of specific plate,and predict plate behavior.Moreover,comparing the evolution of Earth-like planets(such as Mars)will help to unravel the mystery of the uniqueness and habitability of Earth.