地壳放射性生热效应对大陆俯冲过程影响的数值模拟研究

Effects of Crustal Radiogenic Heat Production on Continental Subduction: Insights from Numerical Modelling

岩体中的放射性生热是地幔对流和地壳变质作用的关键热源之一,但地壳放射性生热率是如何影响大陆俯冲-碰撞的动力学过程,尤其是大陆碰撞区域的热结构演化,尚未获得共识。本文使用热-力学数值模拟方法对上、下地壳放射性生热率进行系统的模拟实验,以研究其对大陆俯冲动力学演化过程的影响。模型实验表明,由于大陆上地壳富集U、Th和K等主要放射性生热元素,且放射性生热率的变化区间较大(1.0~3.0μW/m^3),导致其对大陆俯冲碰撞动力学演化过程的影响较为显著,主要包括进入俯冲通道内的上地壳体积大小、碰撞区域内地壳熔融范围、俯冲下地壳物质折返的规模和两大陆的耦合程度等四个方面。而大陆下地壳则以中-基性岩为主,相对亏损U、Th、K等主要放射性生热元素,且放射性生热率的变化区域较小(0.2~0.8μW/m^3),致使其对大陆俯冲演化过程的影响相对有限,主要通过控制俯冲下地壳以及大陆板片的粘滞度和流变强度的大小,进而制约大陆俯冲过程下地壳物质折返的规模以及板片倾角的大小。

Radiogenic heat production is a key energy source for convection in the Earth’s mantle and for metamorphic reactions in the continental crust.However,its effects on continental subduction remain a poorly resolved aspect of plate tectonics.Based on2D thermo-mechanical modeling,the sensitivity of model results to radiogenic heat production rates in the continental upper crust and lower crust is thoroughly investigated.The granodioritic upper crust usually concentrates radioactive isotopes(e.g.,U,Th and K)with a wide range of radiogenic heat production rate,i.e.,roughly from1.0μW/m3to3.0μW/m3.The radiogenic heat significantly effects the continental subduction evolution including the volume of subducted continental upper crust,the distribution and evolution of crustal melting in the collision zone,exhumation of subducted lower crust along the subduction channel,and coupling of the subducting continent with the overriding continent.In contrast,the mafic lower crust is depleted in radioactive isotopes(e.g.,U,Th and K),resulting in a low radiogenic heat production rate(0.2?0.8μW/m3)and relatively limited influences on the geodynamic processes of continual subduction.Specifically,the radiogenic heating of the continental lower crust mainly regulates the viscosity and rheological strength of the continental slab to further affect the degree of the lower crust exhumation and the continental slab dip angle.